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Intrinsic giant Stark effect of boron-carbon-nitride nanoribbons with
zigzag edges: Electronic properties of zigzag boron-carbon-nitride (BCN) nanoribbons, where
the outermost C atoms on the edges of graphene nanoribbons are replaced by B or
N atoms, are theoretically studied using the first-principles calculations. We
show that BCN nanoribbons are metallic, since several bands cross the Fermi
level. For BCN nanoribbons in a rich H$_2$ environment, the so-called nearly
free electron state appears just above the Fermi level because of the intrinsic
giant Stark effect due to the internal electric field of a transverse dipole
moment. The position of the nearly free electron state can be controlled by
applying an electric field parallel to the dipole moment. The hydrogenation of
the nitrogen atom is necessary for the appearance of the giant Stark effect in
BCN nanoribbons. We also discuss the effect of stacking order on the intrinsic
giant Stark effect in bilayer BCN nanoribbons. | cond-mat_mes-hall |
Kondo effect in coupled quantum dots with RKKY interaction: Finite
temperature and magnetic field effects: We study transport through two quantum dots coupled by an RKKY interaction as
a function of temperature and magnetic field. By applying the Numerical
Renormalization Group (NRG) method we obtain the transmission and the linear
conductance. At zero temperature and magnetic field, we observe a quantum phase
transition between the Kondo screened state and a local spin singlet as the
RKKY interaction is tuned. Above the critical RKKY coupling the Kondo peak is
split. However, we find that both finite temperature and magnetic field restore
the Kondo resonance. Our results agree well with recent transport experiments
on gold grain quantum dots in the presence of magnetic impurities. | cond-mat_mes-hall |
Skyrmion dynamics in quantum Hall ferromagnets: Exploring a classical solution of the non-linear sigma model for a quantum
Hall ferromagnet, a skyrmion-magnon effective hamiltonian is obtained via the
collective coordinates method. Using the Feynman-Vernon functional integral
formalism for this model we find the temperature dependent transport
coefficients which characterize a single skyrmion dynamics. | cond-mat_mes-hall |
Electronic transport of folded graphene nanoribbons: We investigate the electronic transport properties of a folded graphene
nanoribbon with monolayer nanoribbon contacts. We consider two possible
foldings: either the nanoribbon can be folded onto itself in the shape of a
hairpin with the nanoribbon leads at a $0^\circ$ angle, or the monolayer
contacts have different directions, forming a $60^\circ$ angle. The system is
described by a single $\pi$-band nearest-neighbor tight-binding Hamiltonian
taking into account curvature effects. We have found that for the case of a
nanoribbon folded over itself the conductance oscillates from almost zero and a
finite value depending on the coupling between contacts, whereas in the
$60^\circ$ angle folding the conductance is only slightly perturbed, allowing
for the connection of graphene nanoelectronic components in a variety of
geometries. | cond-mat_mes-hall |
Intrinsic spin-orbit torque in an antiferromagnet with a weakly
noncollinear spin configuration: An antiferromagnet is a promising material for spin-orbit torque generation.
Earlier studies of the spin-orbit torque in an antiferromagnet are limited to
collinear spin configurations. We calculate the spin-orbit torque in an
antiferromagnet whose spin ordering is weakly noncollinear. Such
noncollinearity may be induced spontaneously during the magnetization dynamics
even when the equilibrium spin configuration is perfectly collinear. It is
shown that deviation from perfect collinearity can modify properties of the
spin-orbit torque since noncollinearity generates extra Berry phase
contributions to the spin-orbit torque, which are forbidden for collinear spin
configurations. In sufficiently clean antiferromagnets, this modification can
be significant. We estimate this effect to be of relevance for fast
antiferromagnetic domain wall motion. | cond-mat_mes-hall |
Magnetoplasmon excitations in arrays of circular and noncircular quantum
dots: We have investigated the magnetoplasmon excitations in arrays of circular and
noncircular quantum dots within the Thomas-Fermi-Dirac-von Weizs\"acker
approximation. Deviations from the ideal collective excitations of isolated
parabolically confined electrons arise from local perturbations of the
confining potential as well as interdot Coulomb interactions. The latter are
unimportant unless the interdot separations are of the order of the size of the
dots. Local perturbations such as radial anharmonicity and noncircular symmetry
lead to clear signatures of the violation of the generalized Kohn theorem. In
particular, the reduction of the local symmetry from SO(2) to $C_4$ results in
a resonant coupling of different modes and an observable anticrossing behaviour
in the power absorption spectrum. Our results are in good agreement with recent
far-infrared (FIR) transmission experiments. | cond-mat_mes-hall |
Theoretical Study of Electrical Conduction Through a Molecule Connected
to Metallic Nanocontacts: We present a theoretical study of electron transport through a molecule
connected to two metallic nanocontacts. The system investigated is 1,4
benzene-dithiolate (BDT) chemically bonded to two Au contacts. The surface
chemistry is modeled by representing the tips of the Au contacts as two atomic
clusters and treating the molecule-cluster complex as a single entity in an
extended Huckel tight binding scheme. We model the tips using several different
cluster geometries. An ideal lead is attached to each cluster, and the lead to
lead transmission is calculated. The role of the molecule-cluster interaction
in transport is analyzed by using single channel leads. We then extend the
calculations to multi-channel leads that are a more realistic model of the
tip's environment. Using the finite-voltage, finite temperature Landauer
formula, we calculate the differential conductance for the different systems
studied. The similarities and differences between the predictions of the
present class of models and recent experimental work are discussed. | cond-mat_mes-hall |
Kondo physics in tunable semiconductor nanowire quantum dots: We have observed the Kondo effect in strongly coupled semiconducting nanowire
quantum dots. The devices are made from indium arsenide nanowires, grown by
molecular beam epitaxy, and contacted by titanium leads. The device
transparency can be tuned by changing the potential on a gate electrode, and
for increasing transparencies the effects dominating the transport changes from
Coulomb Blockade to Universal Conductance Fluctuations with Kondo physics
appearing in the intermediate region. | cond-mat_mes-hall |
Peierls-type Instability and Tunable Band Gap in Functionalized Graphene: Functionalizing graphene was recently shown to have a dramatic effect on the
electronic properties of this material. Here we investigate spatial ordering of
adatoms driven by the RKKY-type interactions. In the ordered state, which
arises via a Peierls-instability-type mechanism, the adatoms reside mainly on
one of the two graphene sublattices. Bragg scattering of electron waves induced
by sublattice symmetry breaking results in a band gap opening, whereby Dirac
fermions acquire a finite mass. The band gap is found to be immune to the
adatoms' positional disorder, with only an exponentially small number of
localized states residing in the gap. The gapped state is stabilized in a wide
range of electron doping. Our findings show that controlled adsorption of
adatoms or molecules provides a route to engineering a tunable band gap in
graphene. | cond-mat_mes-hall |
Terahertz lasing from intersubband polariton-polariton scattering in
asymmetric quantum wells: Electric dipole transitions between different cavity polariton branches or
between dressed atomic states with the same excitation number are strictly
forbidden in centro-symmetric systems. For doped quantum wells in semiconductor
microcavities, the strong coupling between an intersubband transition in the
conduction band and a cavity mode produces two branches of intersubband cavity
polaritons, whose normal-mode energy splitting is tunable and can be in the
terahertz region. Here, we show that, by using asymmetric quantum wells, it is
possible to have allowed dipolar transitions between different polaritonic
branches, leading to the emission of terahertz photons. We present a quantum
field theory for such a system and predict that high-efficiency, widely tunable
terahertz lasing can be obtained. | cond-mat_mes-hall |
Detection of quantum interference without interference: Quantum interference is typically detected through the dependence of the
interference signal on certain parameters (path length, Aharonov-Bohm flux,
etc.), which can be varied in a controlled manner. The destruction of
interference by a which-path measurement is a paradigmatic manifestation of
quantum effects. Here we report on a novel measurement protocol that realizes
two objectives: (i) certifying that a measured signal is the result of
interference avoiding the need to vary parameters of the underlying
interferometer, and (ii) certifying that the interference signal at hand is of
quantum nature. In particular, it yields a null outcome in the case of
classical interference. Our protocol comprises measurements of
cross-correlations between the readings of which-path weakly coupled detectors
positioned at the respective interferometer's arms and the current in one of
the interferometer's drains. We discuss its implementation with an
experimentally available platform: an electronic Mach-Zehnder interferometer
(MZI) coupled electrostatically to "detectors" (quantum point contacts). | cond-mat_mes-hall |
Thermodynamics of a Single Mesoscopic Phononic Mode: In recent decades, the laws of thermodynamics have been pushed down to
smaller and smaller scales, within the field of stochastic thermodynamics and
state-of-art experiments performed on mesoscopic systems. These measurements
concern electrons, photons, and mesoscopic mechanical objects. Here we report
on the measurements of thermal fluctuations of a single mechanical mode
in-equilibrium with a heat reservoir. The device under study is a
nanomechanical beam with a first flexure resonating at 3.8MHz, cooled down to
temperatures in the range from 100mK to 400mK. The technique is constructed
around a microwave opto-mechanical setup using a cryogenic High Electron
Mobility Transistor, and is based on two parametric amplifications implemented
in series: an in-built opto-mechanical 'blue-detuned' pumping plus a Traveling
Wave Parametric Amplifier stage. We demonstrate our ability to resolve energy
fluctuations of the mechanical mode in real-time up to the fastest relevant
speed given by the mechanical relaxation rate. The energy probability
distribution is then exponential, matching the expected Boltzmann distribution.
The variance of fluctuations is found to be $(k_B T)^2$ with no free
parameters. Our microwave detection floor is about 3 Standard Quantum Limit at
6GHz; the resolution of our fastest acquisition tracks reached about 100
phonons, and is related to the rather poor opto-mechanical coupling of the
device ($g_0/2\pi\approx 0.5~$Hz). This result is deeply in the classical
regime, but shall be extended to the quantum case in the future with systems
presenting a much larger $g_0$ (up to $2\pi\times 250~$Hz), potentially
reaching the resolution of a single mechanical quantum. We believe that it will
open a new experimental field: phonon-based quantum stochastic thermodynamics,
with fundamental implications for quantum heat transport and macroscopic
mechanical quantum coherence. | cond-mat_mes-hall |
Unexpected Gaussian line shapes reveal electron-adsorbate interaction as
dominant broadening mechanism in quantum corrals: Understanding the factors influencing the lifetime of electronic states in
artificial quantum structures is of great significance for advancing quantum
technologies. This study focuses on CO-based quantum corrals on Cu(111).
Tunneling spectroscopy measurements reveal a strong correlation between the
size of the quantum corral and spectral width, characterized by a predominant
Gaussian line shape. We attribute this dominant Gaussian-shaped lifetime
broadening to the interaction of surface state electrons with the corral
boundary. To further investigate this phenomenon, we constructed corrals of
varying wall densities. Our findings indicate that elastic processes, such as
tunneling, are more sensitive to wall density than coupling to the bulk. | cond-mat_mes-hall |
Fast, low-current spin-orbit torque switching of magnetic tunnel
junctions through atomic modifications of the free layer interfaces: Future applications of spin-orbit torque will require new mechanisms to
improve the efficiency for switching nanoscale magnetic tunnel junctions
(MTJs), while also controlling the magnetic dynamics to achieve fast,
nanosecond scale performance with low write error rates. Here we demonstrate a
strategy to simultaneously enhance the interfacial magnetic anisotropy energy
and suppress interfacial spin memory loss by introducing sub-atomic and
monatomic layers of Hf at the top and bottom interfaces of the ferromagnetic
free layer of an in-plane magnetized three-terminal MTJ device. When combined
with a beta-W spin Hall channel that generates spin-orbit torque, the
cumulative effect is a switching current density of 5.4 x 106 A/cm2, more than
a factor of 3 lower than demonstrated in any other spin-orbit-torque magnetic
memory device at room temperature, and highly reliable switching with current
pulses only 2 ns long. | cond-mat_mes-hall |
Onset of optical-phonon cooling in multilayer graphene revealed by RF
noise and black-body radiation thermometries: We report on electron cooling power measurements in few-layer graphene
excited by Joule heating by means of a new setup combining electrical and
optical probes of the electron and phonon baths temperatures. At low bias,
noise thermometry allows us to retrieve the well known acoustic phonon cooling
regimes below and above the Bloch Gr\"uneisen temperature, with additional
control over the phonon bath temperature. At high electrical bias, we show the
relevance of direct optical investigation of the electronic temperature by
means of black-body radiation measurements that provide higher accuracy than
noise thermometry. In this regime, the onset of new efficient relaxation
pathways involving optical modes is observed | cond-mat_mes-hall |
Demonstration of Entanglement of Electrostatically Coupled
Singlet-Triplet Qubits: Quantum computers have the potential to solve certain interesting problems
significantly faster than classical computers. To exploit the power of a
quantum computation it is necessary to perform inter-qubit operations and
generate entangled states. Spin qubits are a promising candidate for
implementing a quantum processor due to their potential for scalability and
miniaturization. However, their weak interactions with the environment, which
leads to their long coherence times, makes inter-qubit operations challenging.
We perform a controlled two-qubit operation between singlet-triplet qubits
using a dynamically decoupled sequence that maintains the two-qubit coupling
while decoupling each qubit from its fluctuating environment. Using state
tomography we measure the full density matrix of the system and determine the
concurrence and the fidelity of the generated state, providing proof of
entanglement. | cond-mat_mes-hall |
Confinement and Fermion Doubling Problem in Dirac-like Hamiltonians: We investigate the interplay between confinement and the fermion doubling
problem in Dirac-like Hamiltonians. Individually, both features are well known.
First, simple electrostatic gates do not confine electrons due to the Klein
tunneling. Second, a typical lattice discretization of the first-order
derivative $k \rightarrow -i\partial_x$ skips the central point and allow
spurious low-energy, highly oscillating solutions known as fermion doublers.
While a no-go theorem states that the doublers cannot be eliminated without
artificially breaking a symmetry, here we show that the symmetry broken by the
Wilson's mass approach is equivalent to the enforcement of hard-wall boundary
conditions, thus making the no-go theorem irrelevant when confinement is
foreseen. We illustrate our arguments by calculating the following: (i) the
band structure and transport properties across thin films of the topological
insulator Bi$_2$Se$_3$, for which we use ab-initio density functional theory
calculations to justify the model; and (ii) the band structure of zigzag
graphene nanoribbons. | cond-mat_mes-hall |
Ion-beam nanopatterning of silicon surfaces under co-deposition of
non-silicide-forming impurities: We report experiments on surface nanopatterning of Si targets which are
irradiated with 2 keV Ar + ions impinging at near-glancing incidence, under
concurrent co-deposition of Au impurities simultaneously extracted from a gold
target by the same ion beam. Previous recent experiments by a number of groups
suggest that silicide formation is a prerequisite for pattern formation in the
presence of metallic impurities. In spite of the fact that Au is known not to
form stable compounds with the Si atoms, ripples nonetheless emerge in our
experiments with nanometric wavelengths and small amplitudes, and with an
orientation that changes with distance to the Au source. We provide results of
sample analysis through Auger electron and energy-dispersive X-ray
spectroscopies for their space-resolved chemical composition, and through
atomic force, scanning transmission electron, and high-resolution transmission
microscopies for their morphological properties. We discuss these findings in
the light of current continuum models for this class of systems. The
composition of and the dynamics within the near-surface amorphized layer that
ensues is expected to play a relevant role to account for the unexpected
formation of these surface structures. | cond-mat_mes-hall |
Lateral plasmonic crystals: Tunability, dark modes, and weak-to-strong
coupling transition: We study transmission of the terahertz radiation through a two-dimensional
electron gas with a concentration controlled by grating gate electrodes.
Voltage applied to these electrodes creates a lateral plasmonic crystal with a
gate-tunable band structure. We find that only a part of plasmonic modes of
such a crystal is seen in the transmission spectrum for the case of homogeneous
excitation (so-called bright modes), while there also exist dark modes which
show up only in a case of inhomogeneous excitation. We develop a theory that
describes both weak- to strong- coupling transition in the crystal with
increasing depth of the density modulation and a transition from resonant to
super-resonant regime with increasing quality factor of the structure. We
discuss very recent experiment, where transmission of the terahertz radiation
through GaN/AlGaN based grating gate periodic structures was studied. We argue
that this experiment represents an evidence of formation of the lateral
plasmonic crystal with the band structure fully controlled by the gate
electrodes and magnetic field, in a full agreement with developed theory. | cond-mat_mes-hall |
On the possibility of the fractional ac Josephson effect in
non-topological conventional superconductor-normal-superconductor junctions: Topological superconductors supporting Majorana Fermions with non-abelian
statistics are presently a subject of intense theoretical and experimental
effort. It has been proposed that the observation of a half-frequency or a
fractional Josephson effect is a more reliable test for topological
superconductivity than the search for end zero modes. Low-energy end modes can
occur accidentally due to impurities. In fact, the fractional Josephson effect
has been observed for the semiconductor nanowire system. Here we consider the
ac Josephson effect in a conventional s-wave superconductor-normal
metal-superconductor junction at a finite voltage. Using a Floquet-Keldysh
treatment of the finite voltage junction, we show that the power dissipated
from the junction, which measures the ac Josephson effect, can show a peak at
half (or even incommensurate fractions) of the Josephson frequency. A similar
conclusion is shown to hold for the Shapiro step measurement. The ac fractional
Josephson peak can also be understood simply in terms of Landau-Zener processes
associated with the Andreev bound state spectrum of the junction. | cond-mat_mes-hall |
Phononic bandgap nano-acoustic cavity with ultralong phonon lifetime: We present measurements at millikelvin temperatures of the
microwave-frequency acoustic properties of a crystalline silicon nanobeam
cavity incorporating a phononic bandgap clamping structure for acoustic
confinement. Utilizing pulsed laser light to excite a co-localized optical mode
of the nanobeam cavity, we measure the dynamics of cavity acoustic modes with
single-phonon sensitivity. Energy ringdown measurements for the fundamental
$5$~GHz acoustic mode of the cavity shows an exponential increase in phonon
lifetime versus number of periods in the phononic bandgap shield, increasing up
to $\tau \approx 1.5$~seconds. This ultralong lifetime, corresponding to an
effective phonon propagation length of several kilometers, is found to be
consistent with damping from non-resonant two-level system defects on the
surface of the silicon device. Potential applications of these ultra-coherent
nanoscale mechanical resonators range from tests of various collapse models of
quantum mechanics to miniature quantum memory elements in hybrid
superconducting quantum circuits. | cond-mat_mes-hall |
Electron cooling with graphene-insulator-superconductor tunnel junctions
and applications to fast bolometry: Electronic cooling in hybrid normal metal-insulator-superconductor junctions
is a promising technology for the manipulation of thermal loads in solid state
nanosystems. One of the main bottlenecks for efficient electronic cooling is
the electron-phonon coupling, as it represents a thermal leakage channel to the
phonon bath. Graphene is a two-dimensional material that exhibits a weaker
electron-phonon coupling compared to standard metals. For this reason, we study
the electron cooling in graphene-based systems consisting of a graphene sheet
contacted by two insulator/superconductor junctions. We show that, by properly
biasing the graphene, its electronic temperature can reach base values lower
than those achieved in similar systems based on metallic ultra-thin films.
Moreover, the lower electron-phonon coupling is mirrored in a lower heat power
pumped into the superconducting leads, thus avoiding their overheating and
preserving the cooling mechanisms. Finally, we analyze the possible application
of cooled graphene as a bolometric radiation sensor. We study its main figures
of merit, i.e. responsivity, noise equivalent power and response time. In
particular, we show that the built-in electron refrigeration allows reaching a
responsivity of the order of 50 nA/pW and a noise equivalent power of order of
$\rm 10^{-18}\, W\, Hz^{-1/2}$ while the response speed is about 10 ns,
corresponding to a thermal bandwidth in the order of 20MHz. | cond-mat_mes-hall |
Phonon wave packet emission during state preparation of a semiconductor
quantum dot using different schemes: The carrier-phonon interaction in semiconductor quantum dots can greatly
affect the optical preparation of the excited state. For resonant excitation
used in the Rabi preparation scheme, the polaron is formed accompanied by the
emission of a phonon wave packet, leading to a degradation of preparation
fidelity. In this paper, phonon wave packets for different coherent excitation
schemes are analyzed. One example is the adiabatic rapid passage scheme relying
on a chirped excitation. Here, also a phonon wave packet is emitted, but the
preparation fidelity can still be approximately unity. A focus is on the phonon
impact on a recently proposed swing-up scheme, induced by two detuned pulses.
Similar to the Rabi scheme, a degradation and a phonon wave packet emission is
found, despite the detuning. If the swing-up frequency coincides with the
maximum of the phonon spectral density, a series of wave packets is emitted
yielding an even stronger degradation. The insight gained from our results will
further help in designing an optimal preparation scheme for quantum dots. | cond-mat_mes-hall |
Imaging electron flow from collimating contacts in graphene: The ballistic motion of electrons in graphene encapsulated in hexagonal boron
nitride (hBN) promises exciting opportunities for electron-optics devices. A
narrow electron beam is desired, with both the mean free path and coherence
length exceeding the device size. One can form a collimating contact in
graphene by adding zigzag contacts on either side of the electron emitter that
absorb stray electrons to form a collimated electron beam [23]. Here we provide
images of electron flow from a collimating contact that directly show the width
and shape of the electron beam, obtained using a Scanning Gate Microscope (SGM)
cooled to 4.2 K. The device is a hBN-encapsulated graphene hall bar with narrow
side contacts on either side of the channel that have an electron emitter at
the end and absorbing zig-zag contacts at both side. To form an image of
electron flow, the SGM tip is raster scanned at a constant height above the
sample surface while the transmission to a receiving contact on opposite sides
of the channel is measured. By displaying the change {\Delta}T vs. tip
position, an image of ballistic flow is obtained. The angular width of the
electron beam leaving the collimating contact is found by applying a
perpendicular magnetic field B that bends electron paths into cyclotron orbits.
SGM images reveal that electron flow from a collimating contact disappears
quickly at B = 0.05T while the flow from a non-collimating contact persists up
to B = 0.19 T. Ray tracing simulations agree well with the experimental images
over a range of B and electron density n. By fitting the half-width at half-max
(HWHM) of the magnitude of electron flow in the experimental SGM images, we
find a narrow half angular width {\Delta}{\theta} = 9.2{\deg} for the electron
flow from the collimating contact, compared with a wide flow {\Delta}{\theta} =
54{\deg} from the non-collimating contact. | cond-mat_mes-hall |
Electronic bandstructure and optical gain of lattice matched III-V
dilute nitride bismide quantum wells for 1.55 $μ$m optical communication
systems: Dilute nitride bismide GaNBiAs is a potential semiconductor alloy for near-
and mid-infrared applications, particularly in 1.55 $\mu$m optical
communication systems. Incorporating dilute amounts of Bismuth (Bi) into GaAs
reduces the effective bandgap rapidly, while significantly increasing the
spin-orbit-splitting energy. Additional incorporation of dilute amounts of
Nitrogen (N) helps to attain lattice matching with GaAs, while providing a
route for flexible bandgap tuning. Here we present a study of the electronic
bandstructure and optical gain of the lattice matched
GaN$_x$Bi$_y$As$_{1-x-y}$/GaAs quaternary alloy quantum well (QW) based on the
16-band k$\cdot$p model. We have taken into consideration the interactions
between the N and Bi impurity states with the host material based on the band
anticrossing (BAC) and valence band anticrossing (VBAC) model. The optical gain
calculation is based on the density matrix theory. We have considered different
lattice matched GaNBiAs QW cases and studied their energy dispersion curves,
optical gain spectrum, maximum optical gain and differential gain; and compared
their performances based on these factors. The thickness and composition of
these QWs were varied in order to keep the emission peak fixed at 1.55 $\mu$m.
The well thickness has an effect on the spectral width of the gain curves. On
the other hand, a variation in the injection carrier density has different
effects on the maximum gain and differential gain of QWs of varying
thicknesses. Among the cases studied, we found that the 6.3 nm thick
GaN$_3$Bi$_{5.17}$As$_{91.83}$ lattice matched QW was most suited for 1.55
$\mu$m (0.8 eV) GaAs-based photonic applications. | cond-mat_mes-hall |
Sideband ground-state cooling of graphene with Rydberg atoms via vacuum
forces: We present a scheme leading to ground-state cooling of the fundamental
out-of-plane (flexural) mode of a suspended graphene sheet. Our proposal
exploits the coupling between a driven Rydberg atom and the graphene resonator,
which is enabled by vacuum forces. Thanks to the large atomic polarizability of
the Rydberg states, the Casimir-Polder force is several orders of magnitude
larger than the corresponding force achieved for atoms in the ground state. By
playing with the distance between the atom and the graphene membrane, we show
that resolved sideband cooling is possible, bringing the occupation number of
the fundamental flexural mode down to its quantum limit. Our findings are
expected to motivate physical applications of graphene at extremely low
temperatures. | cond-mat_mes-hall |
Decoupling of the many-body effects from the electron mass in GaAs by
means of reduced dimensionality: Determining the (bare) electron mass $m_0$ in crystals is often hindered by
many-body effects since Fermi-liquid physics renormalises the band mass, making
the observed effective mass $m^*$ depend on density. Here, we use a
one-dimensional (1D) geometry to amplify the effect of interactions, forcing
the electrons to form a nonlinear Luttinger liquid with separate holon and
spinon bands, therefore separating the interaction effects from $m_0$.
Measuring the spectral function of gated quantum wires formed in GaAs by means
of magnetotunnelling spectroscopy and interpreting them using the 1D
Fermi-Hubbard model, we obtain $m_0=(0.0525\pm0.0015)m_\textrm{e}$ in this
material, where $m_\textrm{e}$ is the free-electron mass. By varying the
density in the wires, we change the interaction parameter $r_\textrm{s}$ in the
range from $\sim$1-4 and show that $m_0$ remains constant. The determined value
of $m_0$ is $\sim 22$% lighter than observed in GaAs in geometries of higher
dimensionality $D$ ($D>1$), consistent with the quasi-particle picture of a
Fermi liquid that makes electrons heavier in the presence of interactions. | cond-mat_mes-hall |
Probing topological protected transport in finite-sized
Su-Schrieffer-Heeger chains: In order to transport information with topological protection, we reveal and
demonstrate experimentally the existence of a characteristic length $L_c$,
coined as the transport length, in the bulk size for edge states in
one-dimensional Su-Schrieffer-Heeger (SSH) chains. In spite of the
corresponding wavefunction amplitude decays exponentially, characterized by the
penetration depth $\xi$, the transport between two edge states remains possible
even when the lattice size $L$ is much larger than the penetration depth, i.e.,
$\xi \ll L \le L_c$. Due to the non-zero coupling energy in a finite-size
system, the supported SSH edge states are not completely isolated at the two
ends, giving an abrupt change in the wave localization, manifested through the
inverse participation ratio to the lattice size. To verify such a
non-exponential scaling factor to the system size, we implement a chain of
split-ring resonators and their complementary ones with controllable hopping
strengths. By performing the measurements on the group velocity from the
transmission spectroscopy of non-trivially topological edge states with pulse
excitations, the transport velocity between two edge states is directly
observed with the number of lattices up to $20$. Along the route to harness
topology to protect optical information, our experimental demonstrations
provide a crucial guideline for utilizing photonic topological devices. | cond-mat_mes-hall |
A study on the universality of the magnetic-field-induced phase
transitions in the two-dimensional electron system in an AlGaAs/GaAs
heterostructure: Plateau-plateau (P-P) and insulator-quantum Hall conductor (I-QH) transitions
are observed in the two-dimensional electron system in an AlGaAs/GaAs
heterostructure. At high fields, the critical conductivities are not of the
expected universal values and the temperature-dependence of the width of the
P-P transition does not follow the universal scaling. However, the semicircle
law still holds, and universal scaling behavior was found in the P-P transition
after mapping it to the I-QH transition by the Landau-level addition
transformation. We pointed out that in order to get a correct critical
exponent, it is essential that the scaling analysis must be performed near the
critical point. And with proper analysis, we found that the P-P transition and
the insulator quantum Hall conductor transitions are of the same universal
class. | cond-mat_mes-hall |
Current noise of a superconducting single electron transistor coupled to
a resonator: We analyze the current and zero-frequency current noise properties of a
superconducting single electron resonator (SSET) coupled to a resonator,
focusing on the regime where the SSET is operated in the vicinity of the
Josephson quasiparticle resonance. We consider a range of coupling strengths
and resonator frequencies to reflect the fact that in practice the system can
be tuned to quite a high degree with the resonator formed either by a
nanomechanical oscillator or a superconducting stripline fabricated in close
proximity to the SSET. For very weak couplings the SSET acts on the resonator
like an effective thermal bath. In this regime the current characteristics of
the SSET are only weakly modified by the resonator. Using a mean field
approach, we show that the current noise is nevertheless very sensitive to the
correlations between the resonator and the SSET charge. For stronger couplings,
the SSET can drive the resonator into limit cycle states where self-sustained
oscillation occurs and we find that regions of well-defined bistability exist.
Dynamical transitions into and out of the limit cycle state are marked by
strong fluctuations in the resonator energy, but these fluctuations are
suppressed within the limit cycle state. We find that the current noise of the
SSET is strongly influenced by the fluctuations in the resonator energy and
hence should provide a useful indicator of the resonator's dynamics. | cond-mat_mes-hall |
Magnetotransport in graphene on silicon side of SiC: We have studied the transport properties of graphene grown on silicon side of
SiC. Samples under study have been prepared by two different growth methods in
two different laboratories. Magnetoresistance and Hall resistance have been
measured at temperatures between 4 and 100 K in resistive magnet in magnetic
fields up to 22 T. In spite of differences in sample preparation, the field
dependence of resistances measured on both sets of samples exhibits two periods
of magneto-oscillations indicating two different parallel conducting channels
with different concentrations of carriers. The semi-quantitative agreement with
the model calculation allows for conclusion that channels are formed by
high-density and low-density Dirac carriers. The coexistence of two different
groups of carriers on the silicon side of SiC was not reported before. | cond-mat_mes-hall |
Resonant single and multi-photon coherent transitions in a detuned
regime: We performed quantum manipulations of the multi-level spin system S=5/2 of a
Mn$^{2+}$ ion, by means of a two-tone pulse drive. The detuning between the
excitation and readout radio frequency pulses allows one to select the number
of photons involved in a Rabi oscillation as well as increase the frequency of
this nutation. Thus detuning can lead to a resonant multi-photon process. Our
analytical model for a two-photon process as well as a numerical generalization
fit well the experimental findings, with implications in the use of multi-level
spin systems as tunable solid state qubits. | cond-mat_mes-hall |
Self-Focusing Skyrmion Racetracks in Ferrimagnets: We theoretically study the dynamics of ferrimagnetic skyrmions in
inhomogeneous metallic films close to the angular momentum compensation point.
In particular, it is shown that the line of the vanishing angular momentum can
be utilized as a self-focusing racetrack for skyrmions. To that end, we begin
by deriving the equations of motion for the dynamics of collinear ferrimagnets
in the presence of a charge current. The obtained equations of motion reduce to
those of ferromagnets and antiferromagnets at two special limits. In the
collective coordinate approach, a skyrmion behaves as a massive charged
particle moving in a viscous medium subjected to a magnetic field. Analogous to
the snake orbits of electrons in a nonuniform magnetic field, we show that a
ferrimagnet with the nonuniform angular momentum density can exhibit snake
trajectories of skyrmions, which can be utilized as racetracks for skyrmions. | cond-mat_mes-hall |
Dynamics of hole singlet triplet qubits with large g-factor differences: The spin-orbit interaction is the key element for electrically tunable spin
qubits. Here we probe the effect of cubic Rashba spin-orbit interaction on
mixing of the spin states by investigating singlet-triplet oscillations in a
planar Ge hole double quantum dot. By varying the magnetic field direction we
find an intriguing transformation of the funnel into a butterfly-shaped
pattern. Landau-Zener sweeps disentangle the Zeeman mixing effect from the
spin-orbit induced coupling and show that large singlet-triplet avoided
crossings do not imply a strong spin-orbit interaction. Our work emphasizes the
need for a complete knowledge of the energy landscape when working with hole
spin qubits. | cond-mat_mes-hall |
Bilayer graphene: gap tunability and edge properties: Bilayer graphene -- two coupled single graphene layers stacked as in graphite
-- provides the only known semiconductor with a gap that can be tuned
externally through electric field effect. Here we use a tight binding approach
to study how the gap changes with the applied electric field. Within a parallel
plate capacitor model and taking into account screening of the external field,
we describe real back gated and/or chemically doped bilayer devices. We show
that a gap between zero and midinfrared energies can be induced and externally
tuned in these devices, making bilayer graphene very appealing from the point
of view of applications. However, applications to nanotechnology require
careful treatment of the effect of sample boundaries. This being particularly
true in graphene, where the presence of edge states at zero energy -- the Fermi
level of the undoped system -- has been extensively reported. Here we show that
also bilayer graphene supports surface states localized at zigzag edges. The
presence of two layers, however, allows for a new type of edge state which
shows an enhanced penetration into the bulk and gives rise to band crossing
phenomenon inside the gap of the biased bilayer system. | cond-mat_mes-hall |
Manipulation of edge states in microwave artificial graphene: Edge states are one important ingredient to understand transport properties
of graphene nanoribbons. We study experimentally the existence and the internal
structure of edge states under uniaxial strain of the three main edges: zigzag,
bearded, and armchair. The experiments are performed on artificial microwave
graphene flakes, where the wavefunctions are obtained by direct imaging. We
show that uniaxial strain can be used to manipulate the edge states: a single
parameter controls their existence and their spatial extension into the ribbon.
By combining tight-binding approach and topological arguments, we provide an
accurate description of our experimental findings. A new type of zero-energy
state appearing at the intersection of two edges, namely the corner state, is
also observed and discussed. | cond-mat_mes-hall |
Direct electronic measurement of the spin Hall effect: The generation, manipulation and detection of spin-polarized electrons in
nanostructures define the main challenges of spin-based electronics[1]. Amongst
the different approaches for spin generation and manipulation, spin-orbit
coupling, which couples the spin of an electron to its momentum, is attracting
considerable interest. In a spin-orbit-coupled system, a nonzero spin-current
is predicted in a direction perpendicular to the applied electric field, giving
rise to a "spin Hall effect"[2-4]. Consistent with this effect,
electrically-induced spin polarization was recently detected by optical
techniques at the edges of a semiconductor channel[5] and in two-dimensional
electron gases in semiconductor heterostructures[6,7]. Here we report
electrical measurements of the spin-Hall effect in a diffusive metallic
conductor, using a ferromagnetic electrode in combination with a tunnel barrier
to inject a spin-polarized current. In our devices, we observe an induced
voltage that results exclusively from the conversion of the injected spin
current into charge imbalance through the spin Hall effect. Such a voltage is
proportional to the component of the injected spins that is perpendicular to
the plane defined by the spin current direction and the voltage probes. These
experiments reveal opportunities for efficient spin detection without the need
for magnetic materials, which could lead to useful spintronics devices that
integrate information processing and data storage. | cond-mat_mes-hall |
Exciton-phonon-scattering: A competition between bosonic and fermionic
nature of bound electron-hole pairs: The question of macroscopic occupation and spontaneous emergence of coherence
for exciton ensembles has gained renewed attention due to the rise of van der
Waals heterostructures made of atomically thin semiconductors. The hosted
interlayer excitons exhibit nanosecond lifetimes, long enough to allow for
excitonic thermalization in time. Several experimental studies reported
signatures of macroscopic occupation effects at elevated exciton densities.
With respect to theory, excitons are composite particles formed by fermionic
constituents, and a general theoretical argument for a bosonic thermalization
of an exciton gas beyond the linear regime is still missing. Here, we derive an
equation for the phonon mediated thermalization at densities above the
classical limit, and identify which conditions favor the thermalization of
fermionic or bosonic character, respectively. In cases where acoustic,
quasielastic phonon scattering dominates the dynamics, our theory suggests that
transition metal dichalcogenide (TMDC) excitons might be bosonic enough to show
bosonic thermalization behaviour and decreasing dephasing for increasing
exciton densities. This can be interpreted as a signature of an emerging
coherence in the exciton ground state, and agrees well with the experimentally
observed features, such as a decreasing linewidth for increasing densities. | cond-mat_mes-hall |
Spin-orbit coupling and the static polarizability of single-wall carbon
nanotubes: We calculate the static longitudinal polarizability of single-wall carbon
nanotubes in the long wavelength limit taking into account spin-orbit effects.
We use a four-orbital orthogonal tight-binding formalism to describe the
electronic states and the random phase approximation to calculate the
dielectric function. We study the role of both the Rashba as well as the
intrinsic spin-orbit interactions on the longitudinal dielectric response, i.e.
when the probing electric field is parallel to the nanotube axis. The
spin-orbit interaction modifies the nanotube electronic band dispersions, which
may especially result in a small gap opening in otherwise metallic tubes. The
bandgap size and state features, the result of competition between Rashba and
intrinsic spin-orbit interactions, result in drastic changes in the
longitudinal static polarizability of the system. We discuss results for
different nanotube types, and the dependence on nanotube radius and spin-orbit
couplings. | cond-mat_mes-hall |
4$π$ and 8$π$ dual Josephson effects induced by symmetry defects: In topological insulator edges, the duality between the Zeeman field
orientation and the proximitized superconducting phase has been recently
exploited to predict a magneto-Josephson effect with a 4$\pi$ periodicity. We
revisit this latter Josephson effect in the light of this duality and show that
the same 4$\pi$ quantum anomaly occurs when bridging two spinless Thouless
pumps to a p-wave superconducting region that could be as small as a single and
experimentally-relevant superconducting quantum dot - a point-like defect. This
interpretation as a dual Josephson effect never requires the presence of
Majorana modes but rather builds on the topological properties of adiabatic
quantum pumps with Z topological invariants. It allows for the systematic
construction of dual Josephson effects of arbitrary periodicity, such as 4$\pi$
and 8$\pi$, by using point-like defects whose symmetry differs from that of the
pump, dubbed symmetry defects. Although adiabatic quantum pumps are typically
discussed via mappings to two-dimensional geometries, we show that this
phenomenology does not have any counterpart in conventional two-dimensional
systems. | cond-mat_mes-hall |
Strain-induced pseudomagnetic and scalar fields in symmetry-enforced
Dirac nodes: It is known that Dirac nodes can be present at high-symmetry points of
Brillouin zone only for certain space groups. For these cases, the effect of
strain is treated by symmetry considerations. The dependence of strain-induced
potentials on the strain tensor is found. In all but two cases, the
pseudomagnetic field potential is present. It can be used to control valley
currents. | cond-mat_mes-hall |
TSTG II: Projected Hartree-Fock Study of Twisted Symmetric Trilayer
Graphene: The Hamiltonian of the magic-angle twisted symmetric trilayer graphene (TSTG)
can be decomposed into a TBG-like flat band Hamiltonian and a high-velocity
Dirac fermion Hamiltonian. We use Hartree-Fock mean field approach to study the
projected Coulomb interacting Hamiltonian of TSTG developed in C\u{a}lug\u{a}ru
et al. [Phys. Rev. B 103, 195411 (2021)] at integer fillings $\nu=-3, -2, -1$
and $0$ measured from charge neutrality. We study the phase diagram with
$w_0/w_1$, the ratio of $AA$ and $AB$ interlayer hoppings, and the displacement
field, which introduces an interlayer potential $U$ and hybridizes the TBG-like
bands with the Dirac bands. At small $U$, we find the ground states at all
fillings $\nu$ are in the same phases as the tensor products of a Dirac
semimetal with the filling $\nu$ TBG insulator ground states, which are
spin-valley polarized at $\nu=-3$, and fully (partially) intervalley coherent
at $\nu=-2,0$ ($\nu=-1$) in the flat bands. An exception is $\nu=-3$ with
$w_0/w_1 \gtrsim 0.7$, which possibly become a metal with competing orders at
small $U$ due to charge transfers between the Dirac and flat bands. At strong
$U$ where the bandwidths exceed interactions, all the fillings $\nu$ enter a
metal phase with small or zero valley polarization and intervalley coherence.
Lastly, at intermediate $U$, semimetal or insulator phases with zero
intervalley coherence may arise for $\nu=-2,-1,0$. Our results provide a simple
picture for the electron interactions in TSTG systems, and reveal the
connection between the TSTG and TBG ground states. | cond-mat_mes-hall |
Quantum pumping in graphene: We show that graphene-based quantum pumps can tap into evanescent modes,
which penetrate deeply into the device as a consequence of Klein tunneling. The
evanescent modes dominate pumping at the Dirac point, and give rise to a
universal response under weak driving for short and wide pumps, in close
analogy to their role for the minimal conductivity in ballistic transport. In
contrast, evanescent modes contribute negligibly to normal pumps. Our findings
add a new incentive for the exploration of graphene-based nanoelectronic
devices. | cond-mat_mes-hall |
Localized surface plasmons in a continuous and flat graphene sheet: We derive an integral equation describing surface-plasmon polaritons in
graphene deposited on a substrate with a planar surface and a dielectric
protrusion in the opposite surface of the dielectric slab. We show that the
problem is mathematically equivalent to the solution of a Fredholm equation,
which we solve exactly. In addition, we show that the dispersion relation of
the localized surface plasmons is determined by the geometric parameters of the
protrusion alone. We also show that such system supports both even and odd
modes. We give the electrostatic potential and the stream plot of the
electrostatic field, which clearly show the localized nature of the surface
plasmons in a continuous and flat graphene sheet. | cond-mat_mes-hall |
Stabilization of single-electron pumps by high magnetic fields: We study the effect of perpendicular magnetic fields on a single-electron
system with a strongly time-dependent electrostatic potential. Continuous
improvements to the current quantization in these electron pumps are revealed
by high-resolution measurements. Simulations show that the sensitivity of
tunnel rates to the barrier potential is enhanced, stabilizing particular
charge states. Nonadiabatic excitations are also suppressed due to a reduced
sensitivity of the Fock-Darwin states to electrostatic potential. The
combination of these effects leads to significantly more accurate current
quantization. | cond-mat_mes-hall |
Even-odd effects in NSN scattering problems: Application to graphene
nanoribbons: We study crossed Andreev reflection (CAR) of electrons or holes in normal
metal-superconductor-normal metal junctions and highlight some very strong
effects of the underlying lattice. In particular, we demonstrate that for sharp
interfaces and under certain, albeit generic, symmetry conditions, the CAR
probability exactly vanishes for an even number of atoms in the superconducting
region. This even-odd effect applies notably to NSN junctions made of graphene
nano-ribbons with armchair edges and for zigzag edges with somewhat more
restrictive conditions. We analyze its robustness towards smoothing of the
boundaries or doping of the sample. | cond-mat_mes-hall |
Effect of picosecond magnetic pulse on dynamics of electron's subbands
in semiconductor bilayer nanowire: We report on possibility of charge current generation in nanowire made of two
tunnel coupled one-dimensional electron waveguides by means of single magnetic
pulse lasting up to 20 ps. Existence of interlayer tunnel coupling plays a
crucial role in the effect described here as it allows for hybridization of the
wave functions localized in different layers which can be dynamically modified
by applying a time changeable in-plane magnetic field. Results of
time-dependent DFT calculations performed for a bilayer nanowire confining many
electrons show that the effect of such magnetic hybridization relies on tilting
of electrons' energy subbands, to the left or to the right, depending on a sign
of time derivative of oscillating magnetic field due to the Faraday law.
Consequently, the tilted subbands become a source of charge flow along the
wire. Strength of such magneto-induced current oscillations may achieve even
$0.6\mu\textrm{A}$ but it depends on duration of magnetic pulse as well as on
charge density confined in nanowire which has to be unequally distributed
between both transport layers to observe this effect. | cond-mat_mes-hall |
Control of Spin Dynamics of Excitons in Nanodots for Quantum Operations: This work presents a step furthering a new perspective of proactive control
of the spin-exciton dynamics in the quantum limit. Laser manipulation of
spin-polarized optical excitations in a semiconductor nanodot is used to
control the spin dynamics of two interacting excitons. Shaping of femtosecond
laser pulses keeps the quantum operation within the decoherence time.
Computation of the fidelity of the operations and application to the complete
solution of a basic quantum computing algorithm demonstrate in theory the
feasibility of quantum control. | cond-mat_mes-hall |
Coupling single photons from discrete quantum emitters in WSe$_2$ to
lithographically defined plasmonic slot-waveguides: We report the observation of the generation and routing of single plasmons
generated by localized excitons in a WSe$_2$ monolayer flake exfoliated onto
lithographically defined Au-plasmonic waveguides. Statistical analysis of the
position of different quantum emitters shows that they are $(3.3 \pm
0.7)\times$ more likely to form close to the edges of the plasmonic waveguides.
By characterizing individual emitters we confirm their single-photon character
via the observation of antibunching of the signal ($g^{(2)}(0) = 0.42$) and
demonstrate that specific emitters couple to the modes of the proximal
plasmonic waveguide. Time-resolved measurements performed on emitters close to,
and far away from the plasmonic nanostructures indicate that Purcell factors up
to $15 \pm 3$ occur, depending on the precise location of the quantum emitter
relative to the tightly confined plasmonic mode. Measurement of the point
spread function of five quantum emitters relative to the waveguide with <50nm
precision are compared with numerical simulations to demonstrate potential for
higher increases of the coupling efficiency for ideally positioned emitters.
The integration of such strain-induced quantum emitters with deterministic
plasmonic routing is a step toward deep-subwavelength on-chip single quantum
light sources. | cond-mat_mes-hall |
Floquet multi-gap topology: Non-Abelian braiding and anomalous Dirac
string phase: Topological phases of matter span a wide area of research shaping fundamental
pursuits and offering promise for future applications. While a significant
fraction of topological materials has been characterized using symmetry
requirements of wave functions, the past two years have witnessed the rise of
novel multi-gap dependent topological states, the properties of which go beyond
these approaches and are yet to be fully explored. Thriving upon these
insights, we report on uncharted anomalous phases and properties that can only
arise in out-of-equilibrium Floquet settings. In particular, we identify
Floquet-induced non-Abelian braiding mechanisms, which in turn lead to a phase
characterized by an anomalous Euler class, the prime example of a multi-gap
topological invariant. Most strikingly, we also retrieve the first example of
an `anomalous Dirac string phase'. This gapped out-of-equilibrium phase
features an unconventional Dirac string configuration that physically manifests
itself via anomalous edge states on the boundary. Our results therefore not
only provide a stepping stone for the exploration of intrinsically dynamical
and experimentally viable multi-gap topological phases, but also demonstrate a
powerful way to observe these non-Abelian processes notably in quantum
simulators. | cond-mat_mes-hall |
Optical Probing of the Spin Polarization of the nu=5/2 Quantum Hall
State: We apply polarization resolved photoluminescence spectroscopy to measure the
spin polarization of a two dimensional electron gas in perpendicular magnetic
field. In the vicinity of filling factor nu=5/2, we observe a sharp
discontinuity in the energy of the zero Landau level emission line. We find
that the splitting between the two circular polarizations exhibits a sharp drop
at nu=5/2 and is equal to the bare Zeeman energy, which resembles the behavior
at even filling factors. We show that this behavior is consistent with filling
factor nu=5/2 being unpolarized. | cond-mat_mes-hall |
Anisotropy of spin relaxation and transverse transport in metals: Using first principles methods we explore the anisotropy of the spin
relaxation and transverse transport properties in bulk metals with respect to
the direction of the spin quantization axis in paramagnets or of the
spontaneous magnetization in ferromagnets. Owing to the presence of the
spin-orbit interaction the orbital and spin character of the Bloch states
depends sensitively on the orientation of the spins relative to the crystal
axes. This leads to drastic changes in quantities which rely on interband
mixing induced by the spin-orbit interaction. The anisotropy is particularly
striking for quantities which exhibit spiky and irregular distribution in the
Brillouin zone, such as the spin-mixing parameter or the Berry curvature of the
electronic states. We demonstrate this for three cases: (i) the Elliott-Yafet
spin-relaxation mechanism in paramagnets with structural inversion symmetry;
(ii) the intrinsic anomalous Hall effect in ferromagnets; and (iii) the spin
Hall effect in paramagnets. We discuss the consequences of the pronounced
anisotropic behavior displayed by these properties for spin-polarized transport
applications. | cond-mat_mes-hall |
Magnetotransport in multi-Weyl semimetals: A kinetic theory approach: We study the longitudinal magnetotransport in three-dimensional multi-Weyl
semimetals, constituted by a pair of (anti)-monopole of arbitrary integer
charge ($n$), with $n=1,2$ and $3$ in a crystalline environment. For any $n>1$,
even though the distribution of the underlying Berry curvature is anisotropic,
the corresponding intrinsic component of the longitudinal magnetoconductivity
(LMC), bearing the signature of the chiral anomaly, is insensitive to the
direction of the external magnetic field ($B$) and increases as $B^2$, at least
when it is sufficiently weak (the semi-classical regime). In addition, the LMC
scales as $n^3$ with the monopole charge. We demonstrate these outcomes for two
distinct scenarios, namely when inter-particle collisions in the Weyl medium
are effectively described by (a) a single and (b) two (corresponding to inter-
and intra-valley) scattering times. While in the former situation the
contribution to LMC from chiral anomaly is inseparable from the non-anomalous
ones, these two contributions are characterized by different time scales in the
later construction. Specifically for sufficiently large inter-valley scattering
time the LMC is dominated by the anomalous contribution, arising from the
chiral anomaly. The predicted scaling of LMC and the signature of chiral
anomaly can be observed in recently proposed candidate materials, accommodating
multi-Weyl semimetals in various solid state compounds. | cond-mat_mes-hall |
Giant spin-orbit torque in a single ferrimagnetic metal layer: Antiferromagnets and compensated ferrimagnets offer opportunities to
investigate spin dynamics in the 'terahertz gap' because their resonance modes
lie in the 0.3 THz to 3 THz range. Despite some inherent advantages when
compared to ferromagnets, these materials have not been extensively studied due
to difficulties in exciting and detecting the high-frequency spin dynamics,
especially in thin films. Here we show that spin-obit torque in a single layer
of the highly spin-polarized compensated ferrimagnet Mn2RuxGa is remarkably
efficient at generating spin-orbit fields \mu_0H_eff, which approach 0.1x10-10
T m2/A in the low-current density limit -- almost a thousand times the Oersted
field, and one to two orders of magnitude greater than the effective fields in
heavy metal/ferromagnet bilayers. From an analysis of the harmonic Hall effect
which takes account of the thermal contributions from the anomalous Nernst
effect, we show that the antidamping component of the spin-orbit torque is
sufficient to sustain self-oscillation. Our study demonstrates that spin
electronics has the potential to underpin energy-frugal, chip-based solutions
to the problem of ultra high-speed information transfer. | cond-mat_mes-hall |
Growth and Optical Properties Investigation of Pure and Al-doped SnO2
Nanostructures by Sol-Gel Method: SnO2 nanoparticles with different percentage of Al (5%, 15%, and25%) were
synthesized by sol-gel method. The structure and nature of nanoparticles are
determined by of X-ray diffraction analysis. Also morphology of the samples is
evaluated by SEM. Moreover, the optical properties of the samples are
investigated with UV-Visible and FT-IR. The XRD patterns are indicated that all
samples and incorporation aluminum ions into the SnO2 lattice have tetragonal
rutile structure. The crystalline size of nanoparticles is decreased with
increasing the Al percentage. The SEM results confirmed that the size of
nanoparticles decreases with increasing the Al percentage. Also, FT-IR and
UV-Visible results showed that the optical band gap of nanoparticles increases
with the increasing the Al percentage. Finally, we have used the EDX analysis
to study the chemical composition of the products. Pure tin and oxygen have
been observed. The doped samples showed the existence of Al atoms in the
samples of the crystal structure of SnO2. | cond-mat_mes-hall |
Gate-induced magneto-oscillation phase anomalies in graphene bilayers: The magneto-oscillations in graphene bilayers are studied in the vicinity of
the K and K' points of the Brillouin zone within the four-band continuum model
ased on the simplest tight-binding approximation involving only the nearest
neighbor interactions. The model is employed to construct Landau plots for a
variety of carrier concentrations and bias strengths between the graphene
planes. The quantum-mechanical and quasiclassical approaches are compared. We
found that the quantum magneto-oscillations are only asymptotically periodic
and reach the frequencies predicted quasiclassically for high indices of Landau
levels. In unbiased bilayers the phase of oscillations is equal to the phase of
massive fermions. Anomalous behavior of oscillation phases was found in biased
bilayers with broken inversion symmetry. The oscillation frequencies again tend
to quasiclassically predicted ones, which are the same for $K$ and $K'$, but
the quantum approach yields the gate-tunable corrections to oscillation phases,
which differ in sign for K and K'. These valley-dependent phase corrections
give rise, instead of a single quasiclassical series of oscillations, to two
series with the same frequency but shifted in phase. | cond-mat_mes-hall |
Nonabelian magnonics in antiferromagnets: We present a semiclassical formalism for antiferromagnetic (AFM) magnonics
which promotes the central ingredient of spin wave chirality, encoded in a
quantity called magnonic isospin, to a first-class citizen of the theory. We
use this formalism to unify results of interest from the field under a single
chirality-centric formulation. Our main result is that the isospin is governed
by unitary time evolution, through a Hamiltonian projected down from the full
spin wave dynamics. Because isospin is SU(2)-valued, its dynamics on the Bloch
sphere are precisely rotations - which, in general, do not commute.
Consequently, the induced group of operations on AFM spin waves is nonabelian.
This is a paradigmatic departure from ferromagnetic magnonics, which operates
purely within the abelian group generated by spin wave phase and amplitude. Our
investigation of this nonabelian magnonics in AFM insulators focuses on
studying several simple gate operations, and offering in broad strokes a
program of study for interesting new logic families in antiferromagnetic spin
wave systems | cond-mat_mes-hall |
A Compact Approximate Solution to the Friedel-Anderson Impuriy Problem: An approximate groundstate of the Anderson-Friedel impurity problem is
presented in a very compact form. It requires solely the optimization of two
localized electron states and consists of four Slater states (Slater
determinants). The resulting singlet ground state energy lies far below the
Anderson mean field solution and agrees well with the numerical results by
Gunnarsson and Schoenhammer, who used an extensive 1/N_{f}-expansion for a spin
1/2 impurity with double occupancy of the impurity level.
PACS: 85.20.Hr, 72.15.Rn | cond-mat_mes-hall |
Coherent phonon Rabi oscillations with a high frequency carbon nanotube
phonon cavity: Phonon-cavity electromechanics allows the manipulation of mechanical
oscillations similar to photon-cavity systems. Many advances on this subject
have been achieved in various materials. In addition, the coherent phonon
transfer (phonon Rabi oscillations) between the phonon cavity mode and another
oscillation mode has attracted many interest in nano-science. Here we
demonstrate coherent phonon transfer in a carbon nanotube phonon-cavity system
with two mechanical modes exhibiting strong dynamical coupling. The
gate-tunable phonon oscillation modes are manipulated and detected by extending
the red-detuned pump idea of photonic cavity electromechanics. The first- and
second-order coherent phonon transfers are observed with Rabi frequencies 591
kHz and 125 kHz, respectively. The frequency quality factor product
fQ_m~2=10^12 Hz achieved here is larger thank k_B T_base/h, which may enable
the future realization of Rabi oscillations in the quantum regime. | cond-mat_mes-hall |
Decoherence of two entangled spin qubits coupled to an interacting
sparse nuclear spin bath: application to nitrogen vacancy centers: We consider pure dephasing of Bell states of electron spin qubits interacting
with a sparse bath of nuclear spins. Using the newly developed two-qubit
generalization of cluster correlation expansion method, we calculate the spin
echo decay of $|\Psi\rangle$ and $|\Phi\rangle$ states for various interqubit
distances. Comparing the results with calculations in which dephasing of each
qubit is treated independently, we identify signatures of influence of common
part of the bath on the two qubits. At large interqubit distances, this common
part consists of many nuclei weakly coupled to both qubits, so that decoherence
caused by it can be modeled by considering multiple uncorrelated sources of
noise (clusters of nuclei), each of them weakly affecting the qubits.
Consequently, the resulting genuinely two-qubit contribution to decoherence can
be described as being caused by classical Gaussian noise. On the other hand,
for small interqubit distances the common part of the environment contains
clusters of spins that are strongly coupled to both qubits, and their
contribution to two-qubit dephasing has visibly non-Gaussian character. We show
that one van easily obtain information about non-Gaussianity of environmental
noise affecting the qubits from the comparison of dephasing of $|\Psi\rangle$
and $|\Phi\rangle$ Bell states. Numerical results are obtained for two nitrogen
vacancy centers interacting with a bath of $^{13}$C nuclei of natural
concentration, for which we obtain that Gaussian description of correlated part
of environmental noise starts to hold for centers separated by about 3 nm. | cond-mat_mes-hall |
Zero-point fluctuations in the ground state of a mesoscopic normal ring: We investigate the persistent current of a ring with an in-line quantum dot
capacitively coupled to an external circuit. Of special interest is the
magnitude of the persistent current as a function of the external impedance in
the zero temperature limit when the only fluctuations in the external circuit
are zero-point fluctuations. These are time-dependent fluctuations which
polarize the ring-dot structure and we discuss in detail the contribution of
displacement currents to the persistent current. We have earlier discussed an
exact solution for the persistent current and its fluctuations based on a Bethe
ansatz. In this work, we emphasize a physically more intuitive approach using a
Langevin description of the external circuit. This approach is limited to weak
coupling between the ring and the external circuit. We show that the zero
temperature persistent current obtained in this approach is consistent with the
persistent current calculated from a Bethe ansatz solution. In the absence of
coupling our system is a two level system consisting of the ground state and
the first excited state. In the presence of coupling we investigate the
projection of the actual state on the ground state and the first exited state
of the decoupled ring. With each of these projections we can associate a phase
diffusion time. In the zero temperature limit we find that the phase diffusion
time of the excited state projection saturates, whereas the phase diffusion
time of the ground state projection diverges. | cond-mat_mes-hall |
Entanglement genesis by ancilla-based parity measurement in 2D circuit
QED: We present an indirect two-qubit parity meter in planar circuit quantum
electrodynamics, realized by discrete interaction with an ancilla and a
subsequent projective ancilla measurement with a dedicated, dispersively
coupled resonator. Quantum process tomography and successful entanglement by
measurement demonstrate that the meter is intrinsically quantum non-demolition.
Separate interaction and measurement steps allow commencing subsequent data
qubit operations in parallel with ancilla measurement, offering time savings
over continuous schemes. | cond-mat_mes-hall |
Graphene Plasmonics: a Novel Fully Atomistic Approach for Realistic
Structures: We demonstrate that the plasmonic properties of realistic graphene and
graphene-based materials can effectively and accurately be modeled by a novel,
fully atomistic, yet classical, approach, named $\omega$FQ. Such model is able
to reproduce all plasmonic features of these materials, and their dependence on
shape, dimension and fundamental physical parameters (Fermi energy, relaxation
time and two-dimensional electron density). Remarkably, $\omega$FQ is able to
accurately reproduce experimental data for realistic structures of hundreds of
nanometers ($\sim$ 370.000 atoms), which cannot be afforded by any
\emph{ab-initio} method. Also, the atomistic nature of $\omega$FQ permits the
investigation of complex shapes, which can hardly be dealt with by exploiting
widespread continuum approaches. | cond-mat_mes-hall |
Spin-Orbit Based Coherent Spin Ratchets: The concept of ratchets, driven asymmetric periodic structures giving rise to
directed particle flow, has recently been generalized to a quantum ratchet
mechanism for spin currents mediated through spin-orbit interaction. Here we
consider such systems in the coherent mesoscopic regime and generalize the
proposal of a minimal spin ratchet model based on a non-interacting clean
quantum wire with two transverse channels by including disorder and by
self-consistently treating the charge redistribution in the nonlinear
(adiabatic) ac-driving regime. Our Keldysh-Green function based quantum
transport simulations show that the spin ratchet mechanism is robust and
prevails for disordered, though non-diffusive, mesoscopic structures. Extending
the two-channel to the multi-channel case does not increase the net ratchet
spin current efficiency but, remarkably, yields a dc spin transmission
increasing linearly with channel number. | cond-mat_mes-hall |
Decoherence in qubits due to low-frequency noise: The efficiency of the future devices for quantum information processing is
limited mostly by the finite decoherence rates of the qubits. Recently a
substantial progress was achieved in enhancing the time, which a solid-state
qubit demonstrates a coherent dynamics. This progress is based mostly on a
successful isolation of the qubits from external decoherence sources. Under
these conditions the material-inherent sources of noise start to play a crucial
role. In most cases the noise that quantum device demonstrate has 1/f spectrum.
This suggests that the environment that destroys the phase coherence of the
qubit can be thought of as a system of two-state fluctuators, which experience
random hops between their states. In this short review we discuss the current
state of the theory of the decoherence due to the qubit interaction with the
fluctuators. We describe the effect of such an environment on different
protocols of the qubit manipulations - free induction and echo signal. It turns
out that in many important cases the noise produced by the fluctuators is
non-Gaussian. Consequently the results of the interaction of the qubit with the
fluctuators are not determined by the pair correlation function only.
We describe the effect of the fluctuators using so-called spin-fluctuator
model. Being quite realistic this model allows one to evaluate the qubit
dynamics in the presence of one fluctuator exactly. This solution is found, and
its features, including non-Gaussian effects are analyzed in details. We extend
this consideration for the systems of large number of fluctuators, which
interact with the qubit and lead to the 1/f noise. We discuss existing
experiments on the Josephson qubit manipulation and try to identify
non-Gaussian behavior. | cond-mat_mes-hall |
Perspective on Coupled Colloidal Quantum Dot Molecules: Electronic coupling and hence hybridization of atoms serve as the basis for
the rich properties of the endless library of naturally occurring molecules.
Colloidal quantum dots (CQDs) manifesting quantum strong confinement, possess
atomic like characteristics with s and p electronic levels, which popularized
the notion of CQDs as artificial atoms. Continuing this analogy, when two atoms
are close enough to form a molecule so that their orbitals start overlapping,
the orbitals' energies start to split into bonding and anti-bonding states made
out of hybridized orbitals. The same concept is also applicable for two fused
core-shell nanocrystals in close proximity. Their band-edge states, which
dictate the emitted photon energy, start to hybridize changing their electronic
and optical properties. Thus, an exciting direction of artificial molecules
emerges leading to a multitude of possibilities for creating a library of new
hybrid nanostructures with novel optoelectronic properties with relevance
towards diverse applications including quantum technologies. In a model fused
core-shell homodimer molecule, the hybridization energy is strongly correlated
with the extent of structural continuity, the delocalization of the exciton
wavefunction, and the barrier thickness as calculated numerically. The
hybridization impacts the emitted photon statistics manifesting a faster
radiative decay rate, photon bunching effect, and modified Auger recombination
pathway compared to the monomer artificial atoms. Future perspectives for the
nanocrystals chemistry paradigm are highlighted. | cond-mat_mes-hall |
Quantum Coherence at Low Temperatures in Mesoscopic Systems: Effect of
Disorder: We study the disorder dependence of the phase coherence time of quasi
one-dimensional wires and two-dimensional (2D) Hall bars fabricated from a high
mobility GaAs/AlGaAs heterostructure. Using an original ion implantation
technique, we can tune the intrinsic disorder felt by the 2D electron gas and
continuously vary the system from the semi-ballistic regime to the localized
one. In the diffusive regime, the phase coherence time follows a power law as a
function of diffusion coefficient as expected in the Fermi liquid theory,
without any sign of low temperature saturation. Surprisingly, in the
semi-ballistic regime, it becomes independent of the diffusion coefficient. In
the strongly localized regime we find a diverging phase coherence time with
decreasing temperature, however, with a smaller exponent compared to the weakly
localized regime. | cond-mat_mes-hall |
Quasi-Periodic Nanoripples in Graphene Grown by Chemical Vapor
Deposition and Its Impact on Charge Transport: The technical breakthrough in synthesizing graphene by chemical vapor
deposition methods (CVD) has opened up enormous opportunities for large-scale
device applications. In order to improve the electrical properties of CVD
graphene grown on copper (Cu-CVD graphene), recent efforts have focussed on
increasing the grain size of such polycrystalline graphene films to 100
micrometers and larger. While an increase in grain size and hence, a decrease
of grain boundary density is expected to greatly enhance the device
performance, here we show that the charge mobility and sheet resistance of
Cu-CVD graphene is already limited within a single grain. We find that the
current high-temperature growth and wet transfer methods of CVD graphene result
in quasi-periodic nanoripple arrays (NRAs). Electron-flexural phonon scattering
in such partially suspended graphene devices introduces anisotropic charge
transport and sets limits to both the highest possible charge mobility and
lowest possible sheet resistance values. Our findings provide guidance for
further improving the CVD graphene growth and transfer process. | cond-mat_mes-hall |
Numerical Analysis of the Anderson Localization: The aim of this paper is to demonstrate, by simple numerical simulations, the
main transport properties of disordered electron systems. | cond-mat_mes-hall |
Quantum Phenomena in Low-Dimensional Systems: A brief summary of the physics of low-dimensional quantum systems is given.
The material should be accessible to advanced physics undergraduate students.
References to recent review articles and books are provided when possible. | cond-mat_mes-hall |
Observation of charged excitons in hole-doped carbon nanotubes using
photoluminescence and absorption spectroscopy: We report the first observation of trions (charged excitons), three-particle
bound states consisting of one electron and two holes, in hole-doped carbon
nanotubes at room temperature. When p-type dopants are added to carbon nanotube
solutions, the photoluminescence and absorption peaks of the trions appear far
below the E11 bright exciton peak, regardless of the dopant species. The
unexpectedly large energy separation between the bright excitons and the trions
is attributed to the strong electron-hole exchange interaction in carbon
nanotubes. | cond-mat_mes-hall |
Giant Interaction-Induced Gap and Electronic Phases in Rhombohedral
Trilayer Graphene: Due to their unique electron dispersion and lack of a Fermi surface, Coulomb
interactions in undoped two-dimensional Dirac systems, such as single, bi- and
tri-layer graphene, can be marginal or relevant. Relevant interactions can
result in spontaneous symmetry breaking, which is responsible for a large class
of physical phenomena ranging from mass generation in high energy physics to
correlated states such as superconductivity and magnetism in condensed matter.
Here, using transport measurements, we show that rhombohedral-stacked trilayer
graphene (r-TLG) offers a simple, yet novel and tunable, platform for study of
various phases with spontaneous or field-induced broken symmetries. Here, we
show that, contrary to predictions by tight-binding calculations,
rhombohedral-stacked trilayer graphene (r-TLG) is an intrinsic insulator, with
a giant interaction-induced gap {\Delta}~42meV. This insulating state is a
spontaneous layer antiferromagnetic with broken time reversal symmetry, and can
be suppressed by increasing charge density n, an interlayer potential, a
parallel magnetic field, or a critical temperature Tc~38K. This gapped
collective state can be explored for switches with low input power and high
on/off ratio. | cond-mat_mes-hall |
Quantum Hall system in Tao-Thouless limit: We consider spin-polarized electrons in a single Landau level on a torus. The
quantum Hall problem is mapped onto a one-dimensional lattice model with
lattice constant $2\pi/L_1$, where $L_1$ is a circumference of the torus (in
units of the magnetic length). In the Tao-Thouless limit, $L_1\to 0$, the
interacting many-electron problem is exactly diagonalized at any rational
filling factor $\nu=p/q\le 1$. For odd $q$, the ground state has the same
qualitative properties as a bulk ($L_1 \to \infty$) quantum Hall hierarchy
state and the lowest energy quasiparticle exitations have the same fractional
charges as in the bulk. These states are the $L_1 \to 0$ limits of the
Laughlin/Jain wave functions for filling fractions where these exist. We argue
that the exact solutions generically, for odd $q$, are continuously connected
to the two-dimensional bulk quantum Hall hierarchy states, {\it ie} that there
is no phase transition as $L_1 \to \infty$ for filling factors where such
states can be observed. For even denominator fractions, a phase transition
occurs as $L_1$ increases. For $\nu=1/2$ this leads to the system being mapped
onto a Luttinger liquid of neutral particles at small but finite $L_1$, this
then develops continuously into the composite fermion wave function that is
believed to describe the bulk $\nu=1/2$ system. The analysis generalizes to
non-abelian quantum Hall states. | cond-mat_mes-hall |
Quantum manipulation in a Josephson LED: We access the suitability of the recently proposed Josephson LED for quantum
manipulation purposes. We show that the device can both be used for on-demand
production of entangled photon pairs and operated as a two-qubit gate. Besides,
one can entangle particle spin with photon polarization and/or measure the spin
by measuring the polarization. | cond-mat_mes-hall |
Model for Topological Phononics and Phonon Diode: The quantum anomalous Hall effect, an exotic topological state first
theoretically predicted by Haldane and recently experimentally observed, has
attracted enormous interest for low-power-consumption electronics. In this
work, we derived a Schr{\"o}dinger-like equation of phonons, where
topology-related quantities, time reversal symmetry and its breaking can be
naturally introduced similar as for electrons. Furthermore, we proposed a
phononic analog of the Haldane model, which gives the novel quantum (anomalous)
Hall-like phonon states characterized by one-way gapless edge modes immune to
scattering. The topologically nontrivial phonon states are useful not only for
conducting phonons without dissipation but also for designing highly efficient
phononic devices, like an ideal phonon diode, which could find important
applications in future phononics. | cond-mat_mes-hall |
Scaling of intrinsic domain wall magneto-resistance with confinement in
electromigrated nanocontacts: In this work we study the evolution of intrinsic domain wall
magnetoresistance (DWMR) with domain wall confinement. Clean permalloy notched
half-ring nanocontacts are fabricated using a special ultra-high vacuum
electromigration procedure to tailor the size of the wire in-situ and through
the resulting domain wall confinement we tailor the domain wall width from a
few tens of nm down to a few nm. Through measurements of the dependence of the
resistance with respect to the applied field direction we extract the
contribution of a single domain wall to the MR of the device, as a function of
the domain wall width in the confining potential at the notch. In this size
range, an intrinsic positive MR is found, which dominates over anisotropic MR,
as confirmed by comparison to micromagnetic simulations. Moreover, the MR is
found to scale monotonically with the size of the domain wall, $\delta_{DW}$,
as 1/$\delta_{DW}^b$, with $b=2.31\pm 0.39 $. The experimental result is
supported by quantum-mechanical transport simulations based on ab-initio
density functional theory calculations. | cond-mat_mes-hall |
Plasmons enhance near-field radiative heat transfer for graphene-covered
dielectrics: It is shown that a graphene layer on top of a dielectric slab can
dramatically influence the ability of this dielectric for radiative heat
exchange. Effect of graphene is related to thermally excited plasmons.
Frequency of these resonances lies in the terahertz region and can be tuned by
varying the Fermi level through doping or gating. Heat transfer between two
dielectrics covered with graphene can be larger than that between best known
materials and even much larger at low temperatures. Moreover, high heat
transfer can be significantly modulated by electrical means that opens up new
possibilities for very fast manipulations with the heat flux. | cond-mat_mes-hall |
Exciton Polaritons in a Two-Dimensional Lieb Lattice with Spin-Orbit
Coupling: We study exciton-polaritons in a two-dimensional Lieb lattice of
micropillars. The energy spectrum of the system features two flat bands formed
from $S$ and $P_{x,y}$ photonic orbitals, into which we trigger bosonic
condensation under high power excitation. The symmetry of the orbital wave
functions combined with photonic spin-orbit coupling gives rise to emission
patterns with pseudospin texture in the flat band condensates. Our work shows
the potential of polariton lattices for emulating flat band Hamiltonians with
spin-orbit coupling, orbital degrees of freedom and interactions. | cond-mat_mes-hall |
Quantum thermodynamics in a single-electron box: This chapter provides an overview of the methods and results for quantum
thermodynamic experiments with single-electron devices. The experiments with a
single-electron box on Jarzynski equality and Crooks relation, two-temperature
fluctuation relations, and Maxwell's demon performed over the past few years
are reviewed here. We further review the first experimental realization of an
autonomous Maxwell's demon using a single-electron box as the demon. | cond-mat_mes-hall |
$Γ(2)$ modular symmetry, renormalization, group flow and the
quantum Hall effect: We construct a family of holomorphic $\beta$-functions whose RG flow
preserves the $\Gamma(2)$ modular symmetry and reproduces the observed
stability of the Hall plateaus. The semi-circle law relating the longitudinal
and Hall conductivities that has been observed experimentally is obtained from
the integration of the RG equations for any permitted transition which can be
identified from the selection rules encoded in the flow diagram. The generic
scale dependance of the conductivities is found to agree qualitatively with the
present experimental data. The existence of a crossing point occuring in the
crossover of the permitted transitions is discussed. | cond-mat_mes-hall |
Origin of Discrepancies in Inelastic Electron Tunneling Spectra of
Molecular Junctions: We report inelastic electron tunneling spectroscopy (IETS) of multilayer
molecular junctions with and without incorporated metal nano-particles. The
incorporation of metal nanoparticles into our devices leads to enhanced IET
intensity and a modified line-shape for some vibrational modes. The enhancement
and line-shape modification are both the result of a low lying hybrid metal
nanoparticle-molecule electronic level. These observations explain the apparent
discrepancy between earlier IETS measurements of alkane thiolate junctions by
Kushmerick \emph{et al.} [Nano Lett. \textbf{4}, 639 (2004)] and Wang \emph{et
al.} [Nano Lett. \textbf{4}, 643 (2004)]. | cond-mat_mes-hall |
Influence of MgO tunnel barrier thickness on spin-transfer ferromagnetic
resonance and torque in magnetic tunnel junctions: Spin-transfer ferromagnetic resonance (ST-FMR) in symmetric magnetic tunnel
junctions (MTJs) with a varied thickness of the MgO tunnel barrier (0.75 nm <
$t_{MgO}$ < 1.05 nm) is studied using the spin-torque diode effect. The
application of an RF current into nanosized MTJs generates a DC mixing voltage
across the device when the frequency is in resonance with the resistance
oscillations arising from the spin transfer torque. Magnetization precession in
the free and reference layers of the MTJs is analyzed by comparing ST-FMR
signals with macrospin and micromagnetic simulations. From ST-FMR spectra at
different DC bias voltage, the in-plane and perpendicular torkances are
derived. The experiments and free-electron model calculations show that the
absolute torque values are independent of tunnel barrier thickness. The
influence of coupling between the free and reference layer of the MTJs on the
ST-FMR signals and the derived torkances are discussed. | cond-mat_mes-hall |
Hole Flying Qubits in Quantum Dot Arrays: Quantum information transfer is fundamental for scalable quantum computing in
any potential platform and architecture. Hole spin qubits, owing to their
intrinsic spin-orbit interaction (SOI), promise fast quantum operations which
are fundamental for the implementation of quantum gates. Yet, the influence of
SOI in quantum transfer protocols remains an open question. Here, we
investigate hole flying qubits using shortcuts to adiabaticity protocols, i.e.,
the long-range transfer of hole spin states and the quantum distribution of
entangled pairs in semiconductor quantum dot arrays. We show that electric
field manipulation allows dynamical control of the SOI, enabling simultaneously
the implementation of quantum gates during the transfer, with the potential to
significantly accelerate quantum algorithms. By harnessing the ability to
perform quantum gates in parallel with the transfer, we employ dynamical
decoupling schemes to focus and preserve the spin state, leading to higher
transfer fidelity. | cond-mat_mes-hall |
Tunable microwave impedance matching to a high impedance source using a
Josephson metamaterial: We report the efficient coupling of a $50\,\Omega$ microwave circuit to a
high impedance conductor. We use an impedance transformer consisting of a
$\lambda/4$ co-planar resonator whose inner conductor contains an array of
superconducting quantum interference devices (SQUIDs), providing the resonator
with a large and tunable lineic inductance $\mathcal{L}\sim 80 \mu_0$,
resulting in a large characteristic impedance $Z_C\sim 1\,\mathrm{k}\Omega$.
The impedance matching efficiency is characterized by measuring the shot noise
power emitted by a dc biased high resistance tunnel junction connected to the
resonator. We demonstrate matching to impedances in the $15$ to
$35\,\mathrm{k}\Omega$ range with bandwidths above $100\,\mathrm{MHz}$ around a
resonant frequency tunable in the $4$ to $6\,\mathrm{GHz}$ range. | cond-mat_mes-hall |
Elementary Charge Transfer Processes in Mesoscopic Conductors: We determine charge transfer statistics in a quantum conductor driven by a
time-dependent voltage and identify the elementary transport processes. At zero
temperature unidirectional and bidirectional single charge transfers occur. The
unidirectional processes involve electrons injected from the source terminal
due to excess dc bias voltage. The bidirectional processes involve
electron-hole pairs created by time-dependent voltage bias. This interpretation
is further supported by the charge transfer statistics in a multiterminal beam
splitter geometry in which injected electrons and holes can be partitioned into
different outgoing terminals. The probabilities of elementary processes can be
probed by noise measurements: the unidirectional processes set the dc noise
level while bidirectional ones give rise to the excess noise. For ac voltage
drive, the noise oscillates with increasing the driving amplitude. The
decomposition of the noise into the contributions of elementary processes
identifies the origin of these oscillations: the number of electron-hole pairs
generated per cycle increases with increasing the amplitude. The decomposition
of the noise into elementary processes is studied for different time-dependent
voltages. The method we use is also suitable for systematic calculation of
higher-order current correlators at finite temperature. We obtain current noise
power and the third cumulant in the presence of time-dependent voltage drive.
The charge transfer statistics at finite temperature can be interpreted in
terms of multiple charge transfers with probabilities which depend on energy
and temperature. | cond-mat_mes-hall |
Anomalous zero-temperature magnetopolaronic blockade of resonant
electron tunneling in Majorana-resonant-level single-electron transistor: The magnetopolaronic generalization of a Majorana-resonant-level (MRL) model
is considered for a single-level vibrating quantum dot coupled to two
half-infinite $g=1/2$ Tomonaga-Luttinger liquid (TLL) leads. A qualitatively
new non-trivial formula for the effective transmission coefficient and
differential conductance for resonant magnetopolaron-assisted tunneling is
obtained under the assumption about a fermion-boson factorization of
corresponding averages. This approach is valid for the case of weak
magnetopolaronic coupling in a system. Surprisingly, it is found that despite a
supposed weakness of interaction between fermionic and bosonic subsystems in
that case, a strongly correlated electron transport in the system reveals
features of strong (and, hence, anomalous) magnetopolaronic blockade at zero
temperature if the energy of a vibrational quantum is the smallest (but
nonzero) energy parameter in the system. Such an effect should be referred to
as magnetic phase-coherent magnetopolaron-assisted resonant tunneling of
Andreev type, that originates from a special, Majorana-like, symmetry of
magnetopolaron-coupled tunnel Hamiltonian. The effect predicted in this paper
can be used as an experimental fingerprint of Majorana-resonant level situation
in single-electron transistors as well as for detection of ultra-slow
zero-point oscillations of suspended carbon nanotubes in the Majorana-resonant
level regime of electron tunneling through corresponding single-electron
transistors. | cond-mat_mes-hall |
Alternating currents and shear waves in viscous electronics: Strong interaction among charge carriers can make them move like viscous
fluid. Here we explore alternating current (AC) effects in viscous electronics.
In the Ohmic case, incompressible current distribution in a sample adjusts fast
to a time-dependent voltage on the electrodes, while in the viscous case,
momentum diffusion makes for retardation and for the possibility of propagating
slow shear waves. We focus on specific geometries that showcase interesting
aspects of such waves: current parallel to a one-dimensional defect and current
applied across a long strip. We find that the phase velocity of the wave
propagating along the strip respectively increases/decreases with the frequency
for no-slip/no-stress boundary conditions. This is so because when the
frequency or strip width goes to zero (alternatively, viscosity go to
infinity), the wavelength of the current pattern tends to infinity in the
no-stress case and to a finite value in a general case. We also show that for
DC current across a strip with no-stress boundary, there only one pair of
vortices, while there is an infinite vortex chain for all other types of
boundary conditions. | cond-mat_mes-hall |
Cyclotron Resonance Assisted Photocurrents in Surface States of a 3D
Topological Insulator Based on a Strained High Mobility HgTe Film: We report on the observation of cyclotron resonance induced photocurrents,
excited by continuous wave terahertz radiation, in a 3D topological insulator
(TI) based on an 80 nm strained HgTe film. The analysis of the photocurrent
formation is supported by complimentary measurements of magneto-transport and
radiation transmission. We demonstrate that the photocurrent is generated in
the topologically protected surface states. Studying the resonance response in
a gated sample we examined the behavior of the photocurrent, which enables us
to extract the mobility and the cyclotron mass as a function of the Fermi
energy. For high gate voltages we also detected cyclotron resonance (CR) of
bulk carriers, with a mass about two times larger than that obtained for the
surface states. The origin of the CR assisted photocurrent is discussed in
terms of asymmetric scattering of TI surface carriers in the momentum space.
Furthermore, we show that studying the photocurrent in gated samples provides a
sensitive method to probe the effective masses and the mobility of 2D Dirac
surface states, when the Fermi level lies in the bulk energy gap or even in the
conduction band. | cond-mat_mes-hall |
Dirac Theory and Topological Phases of Silicon Nanotube: Silicon nanotube is constructed by rolling up a silicene, i.e., a monolayer
of silicon atoms forming a two-dimensional honeycomb lattice. It is a
semiconductor or an insulator owing to relatively large spin-orbit interactions
induced by its buckled structure. The key observation is that this buckled
structure allows us to control the band structure by applying electric field
$E_z$. When $E_z$ is larger than a certain critical value $E_{\text{cr}}$, by
analyzing the band structure and also on the basis of the effective Dirac
theory, we demonstrate the emergence of four helical zero-energy modes
propagating along nanotube. Accordingly, a silicon nanotube contains three
regions, namely, a topological insulator, a band insulator and a metallic
region separating these two types of insulators. The wave function of each zero
mode is localized within the metallic region, which may be used as a quantum
wire to transport spin currents in future spintronics. We present an analytic
expression of the wave function for each helical zero mode. These results are
applicable also to germanium nanotube. | cond-mat_mes-hall |
Experimental demonstrations of high-Q superconducting coplanar waveguide
resonators: We designed and successfully fabricated an absorption-type of superconducting
coplanar waveguide (CPW) resonators. The resonators are made from a Niobium
film (about 160 nm thick) on a high-resistance Si substrate, and each resonator
is fabricated as a meandered quarter-wavelength transmission line (one end
shorts to the ground and another end is capacitively coupled to a through
feedline). With a vector network analyzer we measured the transmissions of the
applied microwave through the resonators at ultra-low temperature (e.g., at 20
mK), and found that their loaded quality factors are significantly high, i.e.,
up to 10^6. With the temperature increases slowly from the base temperature
(i.e., 20 mK), we observed the resonance frequencies of the resonators are blue
shifted and the quality factors are lowered slightly. In principle, this type
of CPW-device can integrate a series of resonators with a common feedline,
making it a promising candidate of either the data bus for coupling the distant
solid-state qubits or the sensitive detector of single photons. | cond-mat_mes-hall |
Nano-ironing van der Waals Heterostructures Towards Electrically
Controlled Quantum Dots: Assembling two-dimensional van der Waals layered materials into
heterostructures is an exciting development that sparked the discovery of rich
correlated electronic phenomena and offers possibilities for designer device
applications. However, resist residue from fabrication processes is a major
limitation. Resulting disordered interfaces degrade device performance and mask
underlying transport physics. Conventional cleaning processes are inefficient
and can cause material and device damage. Here, we show that thermal scanning
probe based cleaning can effectively eliminate resist residue to recover
pristine material surfaces. Our technique is compatible at both the material-
and device-level, and we demonstrate the significant improvement in the
electrical performance of 2D WS2 transistors. We also demonstrate the cleaning
of van der Waals heterostructures to achieve interfaces with low disorder. This
enables the electrical formation and control of quantum dots that can be tuned
from macroscopic current flow to the single-electron tunnelling regime. Such
material processing advances are crucial for constructing high-quality vdW
heterostructures that are important platforms for fundamental studies and
building blocks for quantum and nano-electronics applications. | cond-mat_mes-hall |
Operation of graphene quantum Hall resistance standard in a cryogen-free
table-top system: We demonstrate quantum Hall resistance measurements with metrological
accuracy in a small cryogen-free system operating at a temperature of around
3.8K and magnetic fields below 5T. Operating this system requires little
experimental knowledge or laboratory infrastructure, thereby greatly advancing
the proliferation of primary quantum standards for precision electrical
metrology. This significant advance in technology has come about as a result of
the unique properties of epitaxial graphene on SiC. | cond-mat_mes-hall |
High thermoelectric performance in excitonic bilayer graphene: We consider the excitonic effects on the thermal properties in the AB-stacked
bilayer graphene. The calculations are based on the bilayer generalization of
the usual Hubbard model at the half-filling. The full interaction bandwidth is
used without any low-energy assumption. We obtain the unusually high values for
the electronic figure of merit even at the room-temperatures which is very
promising for the thermoelectric applications of the AB-bilayer structure. We
discuss the effects of the interlayer Coulomb interaction and temperature on
different thermal parameters in the bilayer graphene and we emphasize the role
of the charge neutrality point in the thermal properties and within the
excitonic insulator transition scenario. The calculated values of the rate of
thermoelectric conversion efficiency suggest the possibility of
high-performance device applications of AB-bilayer graphene. | cond-mat_mes-hall |
Exotic plasma as classical Hall Liquid: A non-relativistic plasma model endowed with an ``exotic'' structure
associated with the two-parameter central extension of the planar Galilei group
is constructed. Introducing a Chern-Simons statistical gauge field provides us
with a self-consistent system; when the magnetic field takes a critical value
determined by the extension parameters, the fluid becomes incompressible and
moves collectively, according to the Hall law. | cond-mat_mes-hall |
Majorana edge modes of topological exciton condensate with
superconductors: I study the edge states of the topological exciton condensate formed by
Coulomb interaction between two parallel surfaces of a strong topological
insulator. When the condensate is contacted by superconductors with a {\pi}
phase shift across the two surfaces, a pair of counter-propagating Majorana
modes close the gap at the boundary. I propose a nano-structured system of
topological insulators and superconductors to realize unpaired Majorana
fermions. The Majorana signal can be used to detect the formation of the
topological exciton condensate. The relevant experimental signatures as well as
implications for related systems are discussed. | cond-mat_mes-hall |
Sign changes and resonance of intrinsic spin Hall effect in
two-dimensional hole gas: The intrinsic spin Hall conductance shows rich sign changes by applying a
perpendicular magnetic field in a two-dimensional hole gas. Especially, a
notable sign changes can be achieved by adjusting the characteristic length of
the Rashba coupling and hole density at moderate magnetic fields. This sign
issue may be easily realized in experiments. The oscillations of the intrinsic
spin Hall conductance as a function of 1/$B$ is nothing else but Shubnikov-de
Haas oscillations, and the additional beatings can be quantitatively related to
the value of the spin-orbit coupling parameter. The Zeeman splitting is too
small to introduce effective degeneracy between different Landau levels in a
two-dimensional hole gas, and the resonant intrinsic spin Hall conductance
appears in high hole density and strong magnetic field due to the transition
between mostly spin-$-{1/2}$ holes and spin-3/2 holes is confirmed. Two likely
ways to establish intrinsic spin Hall effect in experiments and a possible
application are suggested. | cond-mat_mes-hall |
Ripples in a graphene membrane coupled to Glauber spins: We propose a theory of ripples in suspended graphene sheets based on
two-dimensional elasticity equations that are made discrete on the honeycomb
lattice and then periodized. At each point carbon atoms are coupled to Ising
spins whose values indicate the atoms local trend to move vertically off-plane.
The Ising spins are in contact with a thermal bath and evolve according to
Glauber dynamics. In the limit of slow spin flip compared to membrane
vibrations, ripples with no preferred orientation appear as long-lived
metastable states for any temperature. Numerical solutions confirm this
picture. | cond-mat_mes-hall |
Electric generation of spin in crystals with reduced symmetry: We propose a simple way of evaluating the bulk spin generation of an
arbitrary crystal with a known band structure in the strong spin-orbit coupling
limit. We show that, in the presence of an electric field, there exists an
intrinsic torque term which gives rise to a nonzero spin generation rate. Using
methods similar to those of recent experiments which measure spin polarization
in semiconductors, this spin generation rate should be experimentally
observable. The wide applicability of this effect is emphasized by explicit
consideration of a range of examples: bulk wurtzite and strained zincblende
(n-GaAs) lattices, as well as quantum well heterojunction systems. | cond-mat_mes-hall |
Designer quantum states of matter created atom-by-atom: With the advances in high resolution and spin-resolved scanning tunneling
microscopy as well as atomic-scale manipulation, it has become possible to
create and characterize quantum states of matter bottom-up, atom-by-atom. This
is largely based on controlling the particle- or wave-like nature of electrons,
as well as the interactions between spins, electrons, and orbitals and their
interplay with structure and dimensionality. We review the recent advances in
creating artificial electronic and spin lattices that lead to various exotic
quantum phases of matter, ranging from topological Dirac dispersion to complex
magnetic order. We also project future perspectives in non-equilibrium
dynamics, prototype technologies, engineered quantum phase transitions and
topology, as well as the evolution of complexity from simplicity in this newly
developing field. | cond-mat_mes-hall |
Energy Spectrum and Quantum Hall Effect in Twisted Bilayer Graphene: We investigate the electronic spectra and quantum Hall effect in twisted
bilayer graphenes with various rotation angles under magnetic fields, using a
model rigorously including the interlayer interaction. We describe the spectral
evolution from discrete Landau levels in the weak field regime to the fractal
band structure in the strong field regime, and estimate the quantized Hall
conductivity for each single gap. In weak magnetic fields, the low-energy
conduction band of the twisted bilayer is quantized into electron-like Landau
levels and hole-like Landau levels above and below the van Hove singularity,
respectively, reflecting a topological change of the Fermi surface between
electron pocket and hole pocket. Accordingly the Hall conductivity exhibits a
sharp drop from positive to negative at the transition point. In increasing
magnetic field, the spectrum gradually evolves into fractal band structure
so-called Hofstadter's butterfly, where the Hall conductivity exhibits a
nonmonotonic behavior varying from a minigap to a minigap. The magnetic field
strength required to invoke the fractal band structure is more feasible in
smaller rotating angle. | cond-mat_mes-hall |
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