publicationDate stringlengths 10 10 | title stringlengths 17 233 | abstract stringlengths 20 3.22k | id stringlengths 9 12 |
|---|---|---|---|
1999-03-15 | Metal-insulator transition in CMR materials | We report on resistivity measurements in La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ and
Nd$_{0.7}$Sr$_{0.3}$MnO$_{3}$ thin films in order to elucidate the underlying
mechanism for the CMR behavior. The experimental results are analyzed in terms
of quantum phase transition ideas to study the nature of the metal-insulator
transition in manganese oxides. Resistivity curves as functions of
magnetization for various temperatures show the absence of scaling behavior
expected in a continuous quantum phase transition, which leads us to conclude
that the observed metal-insulator transition is most likely a finite
temperature crossover phenomenon. | 9903238v2 |
2000-01-06 | Time Dependent Effects and Transport Evidence for Phase Separation in La_{0.5}Ca_{0.5}MnO_{3} | The ground state of La_{1-x}Ca_{x}MnO_{3} changes from a ferromagnetic
metallic to an antiferromagnetic charge-ordered state as a function of Ca
concentration at x ~ 0.50. We present evidence from transport measurements on a
sample with x = 0.50 that the two phases can coexist, in agreement with other
observations of phase separation in these materials. We also observe that, by
applying and then removing a magnetic field to the mainly charge-ordered state
at some temperatures, we can "magnetically anneal" the charge order, resulting
in a higher zero-field resistivity. We also observe logarithmic time dependence
in both resistivity and magnetization after a field sweep at low temperatures. | 0001064v1 |
2000-02-14 | Transition Temperature and Magnetoresistance in Double-Exchange Compounds with Moderate Disorder | We develop a variational mean-field theory of the ferromagnetic transition in
compounds like Lanthanum-Manganite within the framework of the Double-Exchange
Model supplemented by modest disorder. We obtain analytical expressions for the
transition temperature, its variation with the valence electron-density and its
decrease with disorder.
We derive an expression for the conductivity for both the paramagnetic and
the ferromangetic metallic phases, and study its dependence on the temperature
and magnetic field. A simple relation between the resistivity in the
ferromagnetic phase and the spontaneous magnetization is found. Our results are
in a good agreement with the experimental data on transition temperatures and
resistivity in the manganite compounds with relatively small disorder. We
comment on the effects of increased disorder. | 0002191v1 |
2000-03-11 | Effect of γ-irradiation on superconducting transition temperature and resistive transition in polycrystalline YBa_{2}Cu_{3}O_(7-δ) | A bulk polycrystalline sample of YBa_(2)Cu_(3)O_(7-\delta) (\delta \approx
0.1) has been irradiated by \gamma-rays with ^{60}Co source. Non-monotonic
behavior of T_{c} with increasing irradiation dose \Phi (up to 220 MR) is
observed: T_{c} decreases at low doses (\Phi < 50 MR) from initial value
(\approx 93 K) by about 2 K and then rises, forming a minimum. At higher doses
(\Phi > 120 MR) T_{c} goes down again. The temperature width of resistive
transition increases rather sharply with dose below 75 MR and drops somewhat at
higher dose. The results observed are discussed, taking into account the
granular structure of sample studied and the influence of \gamma-rays on
intergrain Josephson coupling. | 0003192v1 |
2000-06-07 | Step-wise Behavior of Vortex-Lattice Melting Transition in Tilted Magnetic Fields in Single Crystals Bi2Sr2CaCu2O8+d | The vortex lattice melting transition in single crystals Bi2Sr2CaCu2O8+d was
studied by the in-plane resistivity measurements in magnetic fields tilted away
from the c-axis to the ab-plane. In order to avoid the surface barrier effect
which hinders the melting transition in the conventional transport
measurements, we used the Corbino geometry of electric contacts. For the first
time, the complete Hc-Hab phase diagram of the melting-transition in
Bi2Sr2CaCu2O8+d is obtained. The c-axis melting field component Hc-melt
exhibits the novel, step-wise dependence on the in-plane magnetic fields Hab
which is discussed on the base of the crossing vortex lattice structure. The
sharp change of resistance behavior observed near the ab-plane suggests
transformation from first-order to second-order phase transition. | 0006095v1 |
2000-08-07 | Large two-level magnetoresistance effect in doped manganite grain boundary junctions | We performed a systematic analysis of the tunneling magnetoresistance (TMR)
effect in single grain boundary junctions formed in epitaxial
La(2/3)Ca(1/3)MnO(3) films deposited on SrTiO(3) bicrystals. For magnetic
fields H applied parallel to the grain boundary barrier, an ideal two-level
resistance switching behavior with sharp transitions is observed with a TMR
effect of up to 300% at 4.2 K and still above 100% at 77 K. Varying the angle
between H and the grain boundary results in differently shaped resistance vs H
curves. The observed behavior is explained within a model of magnetic domain
pinning at the grain boundary interface. | 0008105v1 |
2000-10-20 | Composite Spin Waves, Quasi-Particles and Low Temperature resistivity in Double Exchange Systems | We make a quantum description of the electron low temperature properties of
double exchange materials. In these systems there is a strong coupling between
the core spin and the carriers spin. This large coupling makes the low energy
spin waves to be a combination of ion and electron density spin waves. We study
the form and dispersion of these composite spin wave excitations. We also
analyze the spin up and down spectral functions of the temperature dependent
quasi-particles of this system. Finally we obtain that the thermally activated
composite spin waves renormalize the carriers effective mass and this gives
rise to a low temperature resistivity scaling as T ^{5/2}. | 0010312v1 |
2000-12-13 | Phase-coherence transition in granular superconductors with $π$ junctions | We study the three-dimensional XY-spin glass as a model for the resistive
behavior of granular superconductors containing a random distribution of $\pi$
junctions, as in high-$T_c$ superconducting materials with d-wave symmetry. The
$\pi$ junctions leads to quenched in circulating currents (chiralities) and to
a chiral-glass state at low temperatures, even in the absence of an external
magnetic field. Dynamical simulations in the phase representation are used to
determine the nonlinear current-voltage characteristics as a function of
temperature. Based on dynamic scaling analysis, we find a phase-coherence
transition at finite temperature below which the linear resistivity should
vanish and determine the corresponding critical exponents. The results suggest
that the phase and chiralities may order simultaneously for decreasing
temperatures into a superconducting chiral-glass state. | 0012238v1 |
2000-12-19 | Resistance Spikes at Transitions between Quantum Hall Ferromagnets | We report a new manifestation of first-order magnetic transitions in
two-dimensional electron systems. This phenomenon occurs in aluminum arsenide
quantum wells with sufficiently low carrier densities and appears as a set of
hysteretic spikes in the resistance of a sample placed in crossed parallel and
perpendicular magnetic fields, each spike occurring at the transition between
states with different partial magnetizations. Our experiments thus indicate
that the presence of magnetic domains at the transition starkly increases
dissipation, an effect also suspected in other ferromagnetic materials.
Analysis of the positions of the transition spikes allows us to deduce the
change in exchange-correlation energy across the magnetic transition, which in
turn will help improve our understanding of metallic ferromagnetism. | 0012367v1 |
2001-01-30 | Stress-induced metallic behavior under magnetic field in Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ (x = 0.5 and 0.4) thin films | We have investigated the role of the stress-induced by the presence of the
substrate in thin films of colossal magnetoresistive manganites on structural,
resistive and magnetic properties. Because of the strong coupling between the
small structural distortions related to the charge-ordering (CO) and the
resistive properties, the presence of the substrate prevents the full
developpement of the charge ordering in Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$,
especially in the very thin films. For thicker films, the CO state exists, but
is not fully developped. Correlatively, the magnetic field which is necessary
to suppress the CO is decreased drastically from 25 Tesla to about 5 Tesla on
SrTiO$_{3}$ substrates. We have also investigated the influence of the doping
level by studying the case of Pr$_{0.6}$Ca$_{0.4}$MnO$_{3}$. | 0101448v1 |
2001-04-18 | Electron Transport in Diborides: Observation of Superconductivity in ZrB2 | We report on syntheses and electron transport properties of polycrystalline
samples of diborides (AB2) with different transition metals atoms (A=Zr,Nb,Ta).
The temperature dependence of resistivity, \rho(T), and ac susceptibility of
these samples reveal superconducting transition of ZrB2 with Tc=5.5 K, while
NbB2 and TaB2 have been observed nonsuperconducting up to 0.37 K. Hc2(T) is
linear in temperature below Tc, leading to a rather low Hc2(0)= 0.1 T. At T
close to Tc, Hc2(T) demonstrates a downward curvature. We conclude that these
diborides as well as MgB2 samples behaves like a simple metals in the normal
state with usual Bloch-Gr\"uneisen temperature dependence of resistivity and
with Debye temperatures: 280 K, 460 K and 440 K, for ZrB2, NbB2 and MgB2,
respectively, rather than T^2 and T^3 as previously reported for MgB2. | 0104323v1 |
2001-05-02 | In-plane Hall effect in c-axis-oriented MgB2 thin films | We have measured the longitudinal resistivity and the Hall resistivity in the
ab-plane of highly c-axis-oriented MgB2 thin films. In the normal state, the
Hall coefficient (R_H) behaves as R_H ~ T with increasing temperature (T) up to
130 K and then deviates from that linear T-dependence at higher temperatures.
The T^2 dependence of the cotangent of the Hall angle is only observed above
130 K. The mixed-state Hall effect reveals no sign anomaly over a wide range of
current densities from 10^2 to 10^4 A/cm^2 and for magnetic fields up to 5 T. | 0105024v2 |
2001-05-04 | A New Method of Probing the Phonon Mechanism in Superconductors including MgB$_{2}$ | Weak localization has a strong influence on both the normal and
superconducting properties of metals. In particular, since weak localization
leads to the decoupling of electrons and phonons, the temperature dependence of
resistance (i.e., $\lambda_{tr}$) is decreasing with increasing disorder, as
manifested by Mooij's empirical rule. In addition, Testardi's universal
correlation of $T_{c}$ (i.e., $\lambda$) and the resistance ratio (i.e.,
$\lambda_{tr}$) follows. This understanding provides a new means to probe the
phonon mechanism in superconductors including MgB$_{2}$. The merits of this
method are its applicability to any superconductors and its reliability because
the McMillan's electron-phonon coupling constant $\lambda$ and $\lambda_{tr}$
change in a broad range, from finite values to zero, due to weak localization.
Karkin et al's preliminary data of irradiated MgB$_{2}$ show the Testardi
correlation, indicating that the dominant pairing mechanism in MgB$_{2}$ is the
phonon-mediated interaction. | 0105091v1 |
2002-01-25 | Degradation of LaMnO{3-y} surface layer in LaMnO{3-y}/ metal interface | We report electrical measurements showing the degradation processes of
LaMnO$_{3-y}$ (LaMnO) in LaMnO/normal metal interface in both point contact and
planar-type junctions. Immediately after the preparation of the interface, the
degradation process was followed by measuring the evolution of the junction
resistance versus time. This process is characterized by the appearance of a
second maximum in the resistance vs. temperature (R-T) dependence at
temperatures lower than the Curie temperature T$_c$, at which the
metal-insulator transition occurs in the bulk. These effects are explained in
terms of the formation of a depleted interface layer in LaMnO caused by an
out-diffusion of oxygen from the manganite surface to the normal metal. This
assumption is confirmed by XPS measurement. Similar results on LaSrMnO$_{3-y}$
interfaces are also obtained. | 0201460v1 |
2002-02-06 | Thermoelectric properties of the brownmillerite oxide Ca_{2-y}La_yCo_{2-x}Al_xO_5 | We prepared the brownmillerite oxide Ca_{2-y}La_yCo_{2-x}Al_xO_5, and found
that it was an n-type conductor. The thermopower and the resistivity of the
single crystal are -90 microV/K and 68 mOhm cm along the ab direction at 440 K,
which suggest relatively good thermoelectrical properties, compared with other
transition-metal oxides. Their temperature dependences are of activation type,
and the activation energies are 0.2 eV for the resistivity and 0.04 eV for the
thermopower. These energies differ by one order in magnitude, which implies
that a polaron dominates the charge transport. A sign of the thermopower of the
polycrystals changes from negative to positive at 500 K, indicating that holes
are excited thermally to decrease the magnitude of thermopower. | 0202087v1 |
2002-04-29 | Concentration of Charge Carriers and Anomalous Gap Parameter in the Normal State of High-$T_c$ Superconductors | Fermi-Dirac statistics has been utilized by introducing the average
ionization energy ($E_I$) as an additional anomalous energy gap in order to
derive the two-dimensional concentration of charge carriers and the
phenomenological resistivity model for the superconducting polycrystalline
materials. The best fitted values of $E_I$ and the charge carriers'
concentration ranges in the vicinity of 4 to 9 meV and 10$^{16}$ m$^{-2}$
respectively for the superconducting single crystal samples and polycrystalline
compounds synthesized with various compositions via solid-state reactions. The
phenomenological resistivity model is further redefined here based on the
gapless nature of charge-carriers' dynamics within the Cu-O$_2$ planes that
corresponds to anomalous Fermi liquid behavior, which is in accordance with the
nested Fermi liquid theory. | 0204601v1 |
2002-08-09 | Quasi-one-dimensional superconductivity above 300 K and quantum phase slips in individual carbon nanotubes | A great number of the existing data for electrical transport, the Altshuler
Aronov Spivak and Aharonov Bohm effects, as well as the tunneling spectra of
individual carbon nanotubes can be well explained by theories of the quantum
phase slips in quasi-one-dimensional superconductors. The existing data
consistently suggest that the mean-field superconducting transition temperature
T_{c0} in both single-walled and multi-walled carbon nanotubes could be higher
than 600 K. The quantum phase slip theories naturally explain why the on-tube
resistances in the closely packed nanotube bundles or in the individual
multi-walled nanotubes with large diameters approach zero at room temperature,
while a single tube with a small diameter has a substantial resistance. | 0208198v4 |
2002-08-13 | Characterization of one-dimensional quantum channels in InAs/AlSb | We report the magnetoresistance characteristics of one-dimensional electrons
confined in a single InAs quantum well sandwiched between AlSb barriers. As a
result of a novel nanofabrication scheme that utilizes a 3nm-shallow wet
chemical etching to define the electrostatic lateral confinement, the system is
found to possess three important properties: specular boundary scattering, a
strong lateral confinement potential, and a conducting channel width that is
approximately the lithography width. Ballistic transport phenomena, including
the quenching of the Hall resistance, the last Hall plateau, and a strong
negative bend resistance, are observed at 4K in cross junctions with sharp
corners. In a ring geometry, we have observed Aharonov-Bohm interference that
exhibits characteristics different from those of the GaAs counterpart due to
the ballistic nature of electron transport and the narrowness of the conducting
channel width. | 0208265v1 |
2002-08-27 | Anomalous Hall Effect of Calcium-doped Lanthanum Cobaltite Films | The Hall resistivity, magnetoresistance, and magnetization of
La_{1-x}Ca_{x}CoO_{3} epitaxial films with x between 0.25 and 0.4 grown on
lanthanum aluminate were measured in fields up to 7 T. The x=1/3 film, shows a
reentrant metal insulator transition. Below 100 K, the x=1/3 and 0.4 films have
significant coercivity which increases with decreasing temperature. At low
temperature the Hall resistivity remains large and essentially field
independent in these films, except for a sign change at the coercive field that
is more abrupt than the switching of the magnetization. A unique
magnetoresistance behavior accompanies this effect. These results are discussed
in terms of a percolation picture and the mixed spin state model for this
system. We propose that the low-temperature Hall effect is caused by
spin-polarized carriers scattering off of orbital disorder in the spin-ordered
clusters. | 0208530v1 |
2003-01-24 | Semiconductive and Photoconductive Properties of the Single Molecule Magnets Mn$_{12}$-Acetate and Fe$_8$Br$_8$ | Resistivity measurements are reported for single crystals of
Mn$_{12}$-Acetate and Fe$_8$Br$_8$. Both materials exhibit a
semiconductor-like, thermally activated behavior over the 200-300 K range. The
activation energy, $E_a$, obtained for Mn$_{12}$-Acetate was 0.37 $\pm$ 0.05
eV, which is to be contrasted with the value of 0.55 eV deduced from the
earlier reported absorption edge measurements and the range of 0.3-1 eV from
intramolecular density of states calculations, assuming $2E_a$= $E_g$, the
optical band gap. For Fe$_8$Br$_8$, $E_a$ was measured as 0.73 $\pm$ 0.1 eV,
and is discussed in light of the available approximate band structure
calculations. Some plausible pathways are indicated based on the crystal
structures of both lattices. For Mn$_{12}$-Acetate, we also measured
photoconductivity in the visible range; the conductivity increased by a factor
of about eight on increasing the photon energy from 632.8 nm (red) to 488 nm
(blue). X-ray irradiation increased the resistivity, but $E_a$ was insensitive
to exposure. | 0301497v1 |
2003-02-12 | Avoided Antiferromagnetic Order and Quantum Critical Point in CeCoIn$_5$ | We measured specific heat and resistivity of heavy fermion CeCoIn5 between
the superconducting critical field $H_{c2} = 5 T$ and 9 T, with field in the
[001] direction, and at temperatures down to 50mK. At 5T the data show Non
Fermi Liquid behavior down to the lowest temperatures. At field above 8T the
data exhibit crossover from the Fermi liquid to a Non Fermi Liquid behavior. We
analyzed the scaling properties of the specific heat, and compared both
resistivity and the specific heat with the predictions of a spin-fluctuation
theory. Our analysis leads us to suggest that the NFL behavior is due to
incipient antiferromagnetism (AF) in CeCoIn5, with the quantum critical point
in the vicinity of the $H_{c2}$. Below $H_{c2}$ the AF phase which competes
with the paramagnetic ground state is superseded by the superconducting
transition. | 0302226v1 |
2003-02-21 | Anomalous Hall Effect and Magnetoresistance of SrFe1-xCoxO3-d | Transport and magnetic studies on polycrystalline samples of SrFe1-xCoxO3-d
have been carried out to investigate the relationship between the magnetic
structure and the anomalous Hall resistivity rH. The hysteretic behavior of the
magnetization observed in the measurements with varying temperature T up and
then down after zero field cooling indicates that the system has the reentrant
spin-glass phase, which is supported by the increasing width of the magnetic
reflections observed by neutron diffraction with decreasing T below the Curie
temperature TC. Detailed analyses of the observed Hall resistivity rH indicate
that the anomalous Hall coefficient exhibits unusual behavior in the reentrant
spin-glass phase. The magnetic field (H)- and T-dependence of the
magnetoresistance of the present system can be understood by a spin dependent
tunneling model. | 0302436v1 |
2003-02-24 | Spin-wave scattering at low temperatures in manganite films | The temperature $T$ and magnetic field $H$ dependence of the resistivity
$\rho$ has been measured for La$_{0.8-y}$Sr$_{0.2}$MnO$_{3}$ (y=0 and 0.128)
films grown on (100) SrTiO$_{3}$ substrates. The low-temperature $\rho$ in the
ferromagnetic metallic region follows well $\rho (H,T)=\rho
_{0}(H)+A(H)\omega_{s}/\sinh (\hbar \omega_{s}/2k_{B}T)+B(H)T^{7/2}$ with $\rho
_{0}$ being the residual resistivity. We attribute the second and third term to
small-polaron and spin-wave scattering, respectively. Our analysis based on
these scattering mechanisms also gives the observed difference between the
metal-insulator transition temperatures of the films studied. Transport
measurements in applied magnetic field further indicate that spin-wave
scattering is a key transport mechanism at low temperatures. | 0302486v1 |
2003-04-07 | Thickness dependence of the properties of epitaxial MgB2 thin films grown by hybrid physical-chemical vapor deposition | We have studied the effect of deposition rate and layer thickness on the
properties of epitaxial MgB2 thin films grown by hybrid physical-chemical vapor
deposition on 4H-SiC substrates. The MgB2 film deposition rate depends linearly
on the concentration of B2H6 in the inlet gas mixture. We found that the
superconducting and normal-state properties of the MgB2 films are determined by
the film thickness, not by the deposition rate. When the film thickness was
increased, the transition temperature, Tc, increased and the residual
resistivity, rho0, decreased. Above about 300 nm, a Tc of 41.8 K, a rho0 of
0.28 mikroOhm.cm, and a residual resistance ratio RRR of over 30 were obtained.
These values represent the best MgB2 properties reported thus far. | 0304164v1 |
2003-06-05 | Variable-range-hopping conductivity of half-doped bilayer manganite LaSr$_{2}$Mn$_{2}$O$_{7}$ | We report measurements of in-plane $\rho_{ab}$ and out-of-plane $\rho_{c}$
resistivities on a single crystal of the half-doped bilayer manganite
LaSr$_{2}$Mn$_{2}$O$_{7}$. In the temperature $T$ range 220 to 300 K, the
resistive anisotropy $\rho_{c}/\rho_{ab}=A+B/T$ ($A$ and $B$ constants), which
provides evidence for the variable-range-hopping conduction in the presence of
a Coulomb gap. This hopping mechanism also accounts for the quadratic magnetic
field $H$ and $\sin^{2}\phi$ dependences of the negative magnetoresistivity
$\ln [\rho_{i}(T,H,\phi)/\rho_{i}(T,H=0)]$ ($i=ab,c$), where $\phi$ is the
in-plane angle between the magnetic field and the current. | 0306133v1 |
2003-06-11 | Enhanced electrical resistivity before Néel order in the metals, RCuAs$_2$ (R= Sm, Gd, Tb and Dy | We report an unusual temperature (T) dependent electrical resistivity($\rho$)
behavior in a class of ternary intermetallic compounds of the type RCuAs$_2$
(R= Rare-earths). For some rare-earths (Sm, Gd, Tb and Dy) with negligible
4f-hybridization, there is a pronounced minimum in $\rho$(T) far above
respective N\'eel temperatures (T$_N$). However, for the rare-earths which are
more prone to exhibit such a $\rho$(T) minimum due to 4f-covalent mixing and
the Kondo effect, this minimum is depressed. These findings, difficult to
explain within the hither-to-known concepts, present an interesting scenario in
magnetism. | 0306266v1 |
2003-07-14 | Interplay between disorder, quantum and thermal fluctuations in ferromagnetic alloys: The case of UCu2Si(2-x)Ge(x) | We consider, theoretically and experimentally, the effects of structural
disorder, quantum and thermal fluctuations in the magnetic and transport
properties of certain ferromagnetic alloys.We study the particular case of
UCu2Si(2-x)Ge(x). The low temperature resistivity, rho(T,x), exhibits Fermi
liquid (FL) behavior as a function of temperature T for all values of x, which
can be interpreted as a result of the magnetic scattering of the conduction
electrons from the localized U spins. The residual resistivity, rho(0,x),
follows the behavior of a disordered binary alloy. The observed non-monotonic
dependence of the Curie temperature, Tc(x), with x can be explained within a
model of localized spins interacting with an electronic bath whose transport
properties cross-over from ballistic to diffusive regimes. Our results clearly
show that the Curie temperature of certain alloys can be enhanced due to the
interplay between quantum and thermal fluctuations with disorder. | 0307328v2 |
2003-10-20 | Studies of Current-Driven Excitations in Co/Cu/Co Trilayer Nanopillars | We measure the dynamic resistance of a Co/Cu/Co trilayer nanopillar at varied
magnetic field $H$ and current $I$. The resistance displays the usual behavior,
almost symmetric in $H$, both when magnetization switching is hysteretic at
small $I,H$, and reversible at larger $I,H$. We show differences in the $I,H$
magnetization stability diagram measured by holding $I$ fixed and varying $H$
and vice versa. We also show how the peak in $dV/dI$ associated with telegraph
noise in the reversible switching regime, is calculated from the telegraph
noise variations with $I$. Lastly, we show data for a similar sample that
displays behavior asymmetric in $H$, and a negative reversible switching peak
instead of a usual positive one. | 0310472v1 |
2003-11-17 | The Normal State Resistivity of Grain Boundaries in YBa2Cu3O7-delta | Using an optimized bridge geometry we have been able to make accurate
measurements of the properties of YBa2Cu3O7-delta grain boundaries above Tc.
The results show a strong dependence of the change of resistance with
temperature on grain boundary angle. Analysis of our results in the context of
band-bending allows us to estimate the height of the potential barrier present
at the grain boundary interface. | 0311386v1 |
2004-02-09 | Molecular electronics exploiting sharp structure in the electrode density-of-states. Negative differential resistance and Resonant Tunneling in a poled molecular layer on Al/LiF electrodes | Density-functional calculations are used to clarify the role of an ultrathin
LiF layer on Al electrodes used in molecular electronics. The LiF layer creates
a sharp density of states (DOS), as in a scanning-tunneling microscope (STM)
tip. The sharp DOS, coupled with the DOS of the molecule leads to negative
differential resistance (NDR). Electron transfer between oriented molecules
occurs via resonant tunneling. The I-V characteristic for a thin-film of tris
(8-hydroxyquinoline)- aluminum (AlQ) molecules, oriented using electric-field
poling, and sandwiched between two Al/LiF electrodes is in excellent agreement
with theory. This molecular device presents a new paradigm for a convenient,
robust, inexpensive alternative to STM or mechanical break-junction structures. | 0402257v1 |
2004-05-06 | The antiferromagnetic transition of UPd2Al3 break-junctions: A new realization of N-shaped current-voltage characteristics | We have investigated metallic break junctions of the heavy-fermion compound
UPd2Al3 at low temperatures between 0.1K and 9K and in magnetic fields up to
8T. Both the current-voltage I(V) characteristics and the dV/dI (V) spectra
clearly showed the superconducting ($T_{\rm c}\simeq$ 1.8K) as well as the
antiferromagnetic ($T_{\rm N}\simeq$14K) transition at low temperatures when
the bias voltage is raised. The junctions with lateral size of order 200nm had
huge critical current densities around $5\times 10^{10} A/m^2 at the
antiferromagnetic transition and hysteretic I(V) characteristics. Degrading the
quality of the contacts by in situ increasing the local residual resistivity
reduced the hysteresis. We show that those hysteretic I(V) curves can be
reproduced theoretically by assuming the constriction to be in the thermal
regime. It turns out that these point contacts represent non-linear devices
with N-shaped I(V) characteristics that have a negative differential resistance
like an Esaki tunnel diode. | 0405118v1 |
2004-05-11 | Weak links and phase slip centers in superconducting MgB2 wires | MgB2 superconducting wires were produced by the Mg diffusion method. Scanning
electron microscopy (SEM), optical microscopy, dispersive x-ray analysis (EDS)
and XRD diffraction were used to study the physical structure and content of
the wires. Magnetic properties (Tcm, Hc1, Hc2, Jc by the Bean model) were
obtained with a SQUID magnetometer, and transport properties (Tcr, Hc2,
resistivity and residual resistivity ratio) were measured using a standard
four-lead configuration. The V-I characteristics of the wires close to the
critical temperature showed a staircase response, which was attributed to the
presence of weak links, creating phase slip centers. The origin of those weak
links is discussed in relation to their formation and structure. | 0405219v1 |
2004-05-27 | Magnetic, electrical resistivity, heat-capacity and thermopower anomalies in CeCuAs2 | The results of magnetic susceptibility, electrical resistivity ($\rho$),
heat-capacity (C) and thermopower (S) measurements on CeCuAs2, forming in
ZrCuSi2-type tetragonal structure, are reported. Our investigations reveal that
Ce is trivalent and there is no clear evidence for long range magnetic ordering
down to 45 mK. The $\rho$ behavior is notable in the sense that (i) the
temperature (T)-coefficient of $\rho$ is negative in the entire range of
measurement (45 mK to 300 K) with large values of $\rho$, while S behavior is
typical of metallic Kondo lattices, and (ii) $\rho$ is proportional to T-0.6 at
low temperatures, without any influence on the exponent by the application of a
magnetic field, which does not seem to classify this compound into
hither-to-known non-Fermi liquid (NFL) systems. In contrast to the logarithmic
increase known for NFL systems, C/T measured down to 0.5 K exhibits a fall
below 2 K. The observed properties of this compound are unusual among Ce
systems. | 0405638v1 |
2004-06-24 | On Electron Transport in ZrB12, ZrB2 and MgB2 | We report on measurements of the temperature dependence of resistivity,
$\rho(T)$, for single crystal samples of ZrB$_{12}$, ZrB$_{2}$ and
polycrystalline samples of MgB$_{2}$. It is shown that cluster compound
ZrB$_{12}$ behaves like a simple metal in the normal state, with a typical
Bloch -- Gr\"uneisen $\rho(T)$ dependence. However, the resistive Debye
temperature, $T_{R}=300 K$, is three times smaller than $T_{D}$ obtained from
specific heat data. We observe the $T^{2}$ term in $\rho(T)$ of these borides,
which could be interpreted as an indication of strong electron-electron
interaction. Although the $\rho (T)$ dependence of ZrB$_{12}$ reveals a sharp
superconductive transition at $T_{c}=6.0 K$, no superconductivity was observed
for single crystal samples of ZrB$_{2}$ down to $1.3 K$. | 0406615v1 |
2004-07-07 | Kondo Behavior of U in CaB$_6$ | Replacing U for Ca in semiconducting CaB$_6$ at the few at.% level induces
metallic behaviour and Kondo-type phenomena at low temperatures, a rather
unusual feature for U impurities in metallic hosts. For
Ca$_{0.992}$U$_{0.008}$B$_6$, the resistance minimum occurs at $T$ = 17 K. The
subsequent characteristic logarithmic increase of the resistivity with
decreasing temperature merges into the expected $T^2$ dependence below 0.8 K.
Data of the low-temperature specific heat and the magnetization are analyzed by
employing a simple resonance-level model. Analogous measurements on LaB$_6$
with a small amount of U revealed no traces of Kondo behavior, above 0.4 K. | 0407158v1 |
2004-08-11 | Conductivity, weak ferromagnetism and charge instability in $α-MnS$ single crystal | The temperature dependence of resistivity, magnetization and electron-spin
resonance of the $\alpha- MnS $ single crystal were measured in temperature
range of $5 K < T < 550 K$. Magnetization hysteresis in applied magnetic field
up to 0.7 T at $T=5 K, 77 K, 300 K$, irreversible temperature behavior of
magnetization and resistivity were found . The obtained data were explained in
terms of degenerate tight binding model using random phase approximation. The
contribution of holes in $t_{2g}$ and $e_g$ bands of manganese ions to the
conductivity, optical absorbtion spectra and charge instability in $\alpha -MnS
$ were studied. Charge susceptibility maxima resulted from the competition of
the on-site Coulomb interaction between the holes in different orbitals and
small hybridization of sub-bands were calculated at $T=160 K, 250 K, 475 K$. | 0408232v1 |
2004-08-26 | Deviations from plastic barriers in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ thin films | Resistive transitions of an epitaxial Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ thin
film were measured in various magnetic fields ($H\parallel c$), ranging from 0
to 22.0 T. Rounded curvatures of low resistivity tails are observed in
Arrhenius plot and considered to relate to deviations from plastic barriers. In
order to characterize these deviations, an empirical barrier form is developed,
which is found to be in good agreement with experimental data and coincide with
the plastic barrier form in a limited magnetic field range. Using the plastic
barrier predictions and the empirical barrier form, we successfully explain the
observed deviations. | 0408555v2 |
2004-08-27 | Examination of the c-axis resistivity of Bi_{2}Sr_{2-x}La_xCuO_{6+δ} in magnetic fields up to 58 T | We measure the magnetic-field dependence of the c-axis resistivity,
\rho_c(H), in a series of Bi_{2}Sr_{2-x}La_xCuO_{6+\delta} (BSLCO) single
crystals for a wide range of doping using pulsed magnetic fields up to 58 T.
The behavior of \rho_c(H) is examined in light of the recent determination of
the upper critical field H_{c2} for this material using Nernst effect
measurements. We find that the peak in \rho_c(H) shows up at a field H_p that
is much lower than H_{c2} and there is no discernable feature in \rho_c(H) at
H_{c2}. Intriguingly, H_p shows a doping dependence similar to that of T_c, and
there is an approximate relation k_{B}T_c \simeq {1/2}g\mu_{B}H_p. Moreover, we
show that the data for the lowest-T_c sample can be used to estimate the
pseudogap closing field H_{pg}, but the method to estimate H_{pg} proposed by
Shibauchi {\it et al.} [Phys. Rev. Lett. {\bf 86}, 5763 (2001)] must be
modified to apply to the BSLCO system. | 0408603v1 |
2004-10-06 | Ballistic resistivity in aluminum nanocontacts | One of the major industrial challenges is to profit from some fascinating
physical features present at the nanoscale. The production of dissipationless
nanoswitches (or nanocontacts) is one of such attractive applications.
Nevertheless, the lack of knowledge of the real efficiency of electronic
ballistic/non dissipative transport limits future innovations. For multi-valent
metallic nanosystems -where several transport channels per atom are involved-
the only experimental technique available for statistical transport
characterization is the conductance histogram. Unfortunately its interpretation
is difficult because transport and mechanical properties are intrinsically
interlaced. We perform a representative series of semiclassical molecular
dynamics simulations of aluminum nanocontact breakages, coupled to full quantum
conductance calculations, and put in evidence a linear relationship between the
conductance and the contact minimum cross-section for the geometrically favored
aluminum nanocontact configurations. Valid in a broad range of conductance
values, such relation allows the definition of a transport parameter for
nanomaterials, that represents the novel concept of ballistic resistivity. | 0410138v1 |
2004-12-01 | Magnetoresistive Effects in Ferromagnet-Superconductor Multilayers | We consider a nanoscale system consisting of Manganite-ferromagnet and
Cuprate-superconductor multilayers in a spin valve configuration. The
magnetization of the bottom Manganite-ferromagnet is pinned by a
Manganite-antiferromagnet. The magnetization of the top Manganite-ferromagnet
is coupled to the bottom one via indirect exchange through the superconducting
layers. We study the behavior of the critical temperature and the
magnetoresistance as a function of an externally applied parallel magnetic
field, when the number of Cuprate-superconductor layers are changed. There are
two typical behaviors in the case of a few monolayers of the Cuprates: a) For
small magnetic fields, the critical temperature and the magnetoresistance
change abruptly when the flipping field of the top Manganite-ferromagnet is
reached. b) For large magnetic fields, the multilayered system re-enters the
zero-resistance (superconducting) state after having become resistive (normal). | 0412005v1 |
2004-12-12 | Effect of oxygen content on the transport properties and magnetoresistance in [Ca$_{2}$CoO$_{3-δ}$]$_{0.62}$[CoO$_{2}$] single crystals | Transport property is investigated in
[Ca$_{2}$CoO$_{3-\delta}$]$_{0.62}$[CoO$_{2}$] single crystals obtained by
varying annealing conditions. The $\rho_{ab}(T)$ exhibits a resistivity
minimum, and the temperature corresponding to this minimum increases with the
loss of oxygen content, indicative of the enhancement of spin density wave
(SDW). Large negative magnetoresistance (MR) was observed in all single
crystals [Ca$_{2}$CoO$_{3-\delta}$]$_{0.62}$[CoO$_{2}$], while a
magnetic-field-driven insulator-to-metal (IM) transition in oxygen annealed
samples. These results suggest a ferromagnetic correlation in system enhanced
by oxygen content. In addition, a low temperature thermal activation
resistivity induced by fields was observed in single crystals annealed in
oxygen atmosphere. | 0412298v1 |
2004-12-27 | Ferromagnetism and possible heavy fermion behavior in single crystals of NdOs$_4$Sb$_{12}$ | Single crystals of the filled-skutterudite compound NdOs$_4$Sb$_{12}$ have
been investigated by means of electrical resistivity, magnetization, and
specific heat measurements. The NdOs$_4$Sb$_{12}$ crystals have the
LaFe$_4$P$_{12}$-type cubic structure with a lattice parameter of 9.3 \AA.
Possible heavy-fermion behavior is inferred from specific heat measurements,
which reveal a large electronic specific heat coefficient $\gamma \approx 520$
mJ/mol-K$^2$, corresponding to an effective mass $m^* \sim$ 98 $m_e$. Features
related to a ferromagnetic transition at {$\sim$ 0.9 K} can be observed in
electrical resistivity, magnetization and specific heat. Conventional
Arrott-plot analysis indicates that NdOs$_4$Sb$_{12}$ conforms to mean-field
ferromagnetism. | 0412713v3 |
2005-02-03 | Transverse "resistance overshoot" in a Si/SiGe two-dimensional electron gas in the quantum Hall effect regime | We investigate the peculiarities of the "overshoot" phenomena in the
transverse Hall resistance R_{xy} in Si/SiGe. Near the low magnetic field end
of the quantum Hall effect plateaus, when the filling factor \nu approaches an
integer i, R_{xy} overshoots the normal plateau value h/ie^2. However, if
magnetic field B increases further, R_{xy} decreases to its normal value. It is
shown that in the investigated sample n-Si/Si_{0.7}Ge_{0.3}, overshoots exist
for almost all \nu. Existence of overshoot in R_{xy} observed in different
materials and for different \nu, where splitting of the adjacent Landau bands
has different character, hints at the common origin of this effect. Comparison
of the experimental curves R_{xy}(\nu) for \nu = 3 and \nu = 5 with and without
overshoot showed that this effect exist in the whole interval between plateaus,
not only in the region where R_{xy} exceeds the normal plateau value. | 0502094v1 |
2005-03-03 | Transport properties of diluted magnetic semiconductors: Dynamical mean field theory and Boltzmann theory | The transport properties of diluted magnetic semiconductors (DMS) are
calculated using dynamical mean field theory (DMFT) and Boltzmann transport
theory. Within DMFT we study the density of states and the dc-resistivity,
which are strongly parameter dependent such as temperature, doping, density of
the carriers, and the strength of the carrier-local impurity spin exchange
coupling. Characteristic qualitative features are found distinguishing weak,
intermediate, and strong carrier-spin coupling and allowing quantitative
determination of important parameters defining the underlying ferromagnetic
mechanism. We find that spin-disorder scattering, formation of bound state, and
the population of the minority spin band are all operational in DMFT in
different parameter range. We also develop a complementary Boltzmann transport
theory for scattering by screened ionized impurities. The difference in the
screening properties between paramagnetic ($T>T_c$) and ferromagnetic ($T<T_c$)
states gives rise to the temperature dependence (increase or decrease) of
resistivity, depending on the carrier density, as the system goes from the
paramagnetic phase to the ferromagnetic phase. The metallic behavior below
$T_c$ for optimally doped DMS samples can be explained in the Boltzmann theory
by temperature dependent screening and thermal change of carrier spin
polarization. | 0503077v1 |
2005-04-22 | Blue light-emitting diode based on ZnO | A near-band-edge bluish electroluminescence (EL) band centered at around 440
nm was observed from ZnO p-i-n homojunction diodes through a semi-transparent
electrode deposited on the p-type ZnO top layer. The EL peak energy coincided
with the photoluminescence peak energy of an equivalent p-type ZnO layer,
indicating that the electron injection from the n-type layer to the p-type
layer dominates the current, giving rise to the radiative recombination in the
p-type layer. The imbalance in charge injection is considered to originate from
the lower majority carrier concentration in the p-type layer, which is one or
two orders of magnitude lower than that in the n-type one. The current-voltage
characteristics showed the presence of series resistance of several hundreds
ohms, corresponding to the current spread resistance within the bottom n-type
ZnO. The employment of conducting ZnO substrates may solve the latter problem. | 0504587v1 |
2005-05-17 | Transport properties of moderately disordered UCu$_4$Pd | We present a detailed study on the (magneto)transport properties of as-cast
and heat treated material UCu$_4$Pd. We find a pronounced sample dependence of
the resistivity $\rho$ of as-cast samples, and reproduce the annealing
dependence of $\rho$. In our study of the Hall effect we determine a metallic
carrier density for all samples, and a temperature dependence of the Hall
constant which is inconsistent with the Skew scattering prediction. The
magnetoresistive response is very small and characteristic for spin disorder
scattering, suggesting that overall the resistivity is controlled mostly by
nonmagnetic scattering processes. We discuss possible sources for the
temperature and field dependence of the transport properties, in particular
with respect to quantum criticality and electronic localization effects. | 0505418v1 |
2005-07-02 | Drastic ground state changes induced by Ni substitution in NaxCoO2 | We report on the effect of Ni substitution at the Co site on the physical
properties of NaxCoO2 system by investigating the series NaxCo1-yNiyO2 (x=0.75,
0<y<0.15). An upturn in the resistivity is observed in all Ni substituted
samples as the temperature is lowered, suggestive of the occurrence of a
Metal-Insulator Transition (MIT). The temperature at which this transition
occurs increases with Ni content. The temperature dependence of the resistivity
in the metallic region in the Ni substituted samples shows a T2 dependence,
which is qualitatively different from that observed in the pristine sample. The
evolution of the Fano asymmetry parameter, extracted by analyzing the lineshape
of the IR active in-plane Co-O mode, both as a function of Ni concentration and
temperature corroborates the occurrence of the MIT. It is argued that the
progressive substitution of the Co4+ ions with Ni increases the probability of
double occupancy and therefore the on-site Coulomb interaction energy leading
to a shift in the thermodynamically driven MIT to higher temperatures. | 0507045v1 |
2005-08-19 | Determination of the intrinsic anomalous Hall effect of SrRuO$_3$ | The anomalous Hall effect (AHE) of epitaxial SrRuO$_3$ films with varying
lattice parameters is investigated, and analyzed according to the Berry-phase
scenario. SrRuO$_3$ thin films were deposited on SrTiO$_3$ substrates directly,
or using intermediate buffer layers, in order to finely control the epitaxial
strain. The AHE of the different films exhibits intrinsic features such as the
sign change of the Hall resistivity with the temperature, even for small
thicknesses of SrRuO$_3$. However, the anomalous Hall conductivity is greatly
reduced from its intrinsic value as the carrier scattering is increased when
the epitaxial strain is released. We argue that the AHE of fully strained
SrRuO$_3$ film with low residual resistivity represents the intrinsic AHE of
SrRuO$_3$. | 0508443v1 |
2005-09-05 | Magnetic and electronic transport percolation in epitaxial GeMn films | Electronic transport and magnetic properties of Ge1-xMnx/Ge(100) films are
investigated as a function of Mn dilution. Depending on x, characteristic
temperatures separate different regimes in both properties. Resistivity
exhibits an insulator-like behavior in the whole temperature range and, below
about 80 K, two distinct activation energies are observed. At a higher
temperature value, TR, resistivity experiences a sudden reduction. Hall
coefficient shows a strong contribution from the anomalous Hall effect and, at
TR, a sign inversion, from positive to negative, is recorded. The magnetic
properties, inferred from magneto-optical Kerr effect, evidence a progressive
decrease of the ferromagnetic long range order as the temperature is raised,
with a Curie temperature TC not far from TR. The transport and magnetic results
are qualitatively consistent with a percolation mechanism due to bound magnetic
polarons in a GeMn diluted magnetic semiconductor, with localized holes [A.
Kaminski and S. Das Sarma, Phys. Rev. B 68, 235210 (2003)]. | 0509111v1 |
2005-10-07 | Size Dependent Breakdown of Superconductivity in Ultranarrow Nanowires | Below a certain temperature Tc (typically cryogenic), some materials lose
their electric resistance R entering a superconducting state. Folowing the
general trend toward a large scale integration of a greater number of
electronic components, it is desirable to use superconducting elements in order
to minimize heat dissipation. It is expected that the basic property of a
superconductor, i.e. dissipationless electric current, will be preserved at
reduced scales required by modern nanoelectronics. Unfortunately, there are
indications that for a certain critical size limit of the order of 10 nm, below
which a "superconducting" wire is no longer a superconductor in a sense that it
acquires a finite resistance even at temperatures close to absolute zero. In
the present paper we report an experimental evidence for a superconductivity
breakdown in ultranarrow quasi-1D aluminum nanowires. | 0510181v1 |
2005-10-07 | Evidence of ratchet effect in nanowires of a conducting polymer | Ratchet effect, observed in many systems starting from living organism to
artificially designed device, is a manifestation of motion in asymmetric
potential. Here we report results of a conductivity study of Polypyrrole
nanowires, which have been prepared by a simple method to generate a variation
of doping concentration along the length. This variation gives rise to an
asymmetric potential profile that hinders the symmetry of the hopping process
of charges and hence the value of measured resistance of these nanowires become
sensitive to the direction of current flow. The asymmetry in resistance was
found to increase with decreasing nanowire diameter and increasing temperature.
The observed phenomena could be explained with the assumption that the spatial
extension of localized state involved in hopping process reduces as the doping
concentration reduces along the length of the nanowires. | 0510188v1 |
2005-10-11 | Highly oriented VO2 thin films prepared by sol-gel deposition method | Highly oriented VO2 thin films were grown on sapphire substrates by the
sol-gel method that includes a low pressure annealing in an oxygen atmosphere.
This reduction process effectively promotes the formation of the VO2 phase over
a relatively wide range of pressures below 100 mTorr and temperatures above
400oC. X-ray diffraction analysis showed that as-deposited films crystallize
directly to the VO2 phase without passing through intermediate phases. VO2
films have been found to be with [100]- and [010]-preferred orientations on
Al2O3(1012) and Al2O3(1010) substrates, respectively. Both films undergo a
metal-insulator transition with an abrupt change in resistance, with different
transition behaviors observed for the differently oriented films. For the
[010]-oriented VO2 films a larger change in resistance of 1.2x10^4 and a lower
transition temperature are found compared to the values obtained for the
[100]-oriented films. | 0510267v2 |
2005-11-10 | Thermoelectric properties of the layered Pd oxide R_2PdO_4 (R = La, Nd, Sm and Gd) | We prepared polycrystalline samples of R$_2$PdO$_4$ (R = La, Nd, Sm and Gd)
using a NaCl-flux technique. The measured resistivity is of the order of
10$^3-10^4$ $\Omega$cm at room temperature, which is two orders of magnitude
smaller than the values reported so far. We further studied the substitution
effects of Ce for Nd in Nd$_{1.9}$Ce$_{0.1}$PdO$_4$, where the substituted Ce
decreases the resistivity and the magnitude of the thermopower. The activation
energy gap of 70-80 meV and the effective mass of 15 evaluated from the
measured data are suitable for thermoelectric materials, but the mobility of
10$^{-6}$ cm$^2$/Vs is much lower than a typical value of 1-10 cm$^2$/Vs for
other thermoelectric oxides. | 0511245v1 |
2005-11-10 | Anderson localization in carbon nanotubes: defect density and temperature effects | The role of irradiation induced defects and temperature in the conducting
properties of single-walled (10,10) carbon nanotubes has been analyzed by means
of a first-principles approach. We find that di-vacancies modify strongly the
energy dependence of the differential conductance, reducing also the number of
contributing channels from two (ideal) to one. A small number of di-vacancies
(5-9) brings up strong Anderson localization effects and a seemly universal
curve for the resistance as a function of the number of defects. It is also
shown that low temperatures, around 15-65 K, are enough to smooth out the
fluctuations of the conductance without destroying the exponential dependence
of the resistivity as a function of the tube length. | 0511265v1 |
2005-11-17 | Electrical Switching in Metallic Carbon Nanotubes | We present first-principles calculations of quantum transport which show that
the resistance of metallic carbon nanotubes can be changed dramatically with
homogeneous transverse electric fields if the nanotubes have impurities or
defects. The change of the resistance is predicted to range over more than two
orders of magnitude with experimentally attainable electric fields. This novel
property has its origin that backscattering of conduction electrons by
impurities or defects in the nanotubes is strongly dependent on the strength
and/or direction of the applied electric fields. We expect this property to
open a path to new device applications of metallic carbon nanotubes. | 0511447v1 |
2005-11-28 | Laser microscopy of tunneling magnetoresistance in manganite grain-boundary junctions | Using low-temperature scanning laser microscopy we directly image electric
transport in a magnetoresistive element, a manganite thin film intersected by a
grain boundary (GB). Imaging at variable temperature allows reconstruction and
comparison of the local resistance vs temperature for both, the manganite film
and the GB. Imaging at low temperature also shows that the GB switches between
different resistive states due to the formation and growth of magnetic domains
along the GB. We observe different types of domain wall growth; in most cases a
domain wall nucleates at one edge of the bridge and then proceeds towards the
other edge. | 0511662v1 |
2006-03-16 | Coordinate transformation in the model of long Josephson junctions: geometrically equivalent Josephson junctions | The transition from the model of a long Josephson junction of variable width
to the model of a junction with a coordinate-dependent Josephson current
amplitude is effected through a coordinate transformation. This establishes the
correspondence between the classes of Josephson junctions of variable width and
quasi-one-dimensional junctions with a variable thickness of the barrier layer.
It is shown that for a junction of exponentially varying width the barrier
layer of the equivalent quasi-one-dimensional junction has a distributed
resistive inhomogeneity that acts as an attractor for magnetic flux vortices.
The curve of the critical current versus magnetic field for a Josephson
junction with a resistive microinhomogeneity is constructed with the aid of a
numerical simulation, and a comparison is made with the critical curve of a
junction of exponentially varying width. The possibility of replacing a
distributed inhomogeneity in a Josephson junction by a local inhomogeneity at
the end of the junction is thereby demonstrated; this can have certain
advantages from a technological point of view. | 0603439v1 |
2006-03-28 | Depolarizing-Field Effect in Strained Nanoscale Ferroelectric Capacitors and Tunnel Junctions | The influence of depolarizing field on the magnitude and stability of a
uniform polarization in ferroelectric capacitors and tunnel junctions is
studied using a nonlinear thermodynamic theory. It is predicted that, in
heterostructures involving strained epitaxial films and metal electrodes, the
homogeneous polarization state may remain stable against transformations into
the paraelectric phase and into polydomain states down to the nanoscale. This
result supports the possibility of depolarizing-field-related resistive
switching in ferroelectric tunnel junctions with dissimilar electrodes. The
resistance on/off ratio in such junctions is shown to be governed by the
difference between the reciprocal capacitances of screening space charges in
the electrodes. | 0603762v2 |
2006-03-31 | Mechanism and Scalability in Resistive Switching of Metal-Pr0.7Ca0.3MnO3 Interface | The polarity-dependent resistive-switching across metal-Pr0.7Ca0.3MnO3
interfaces is investigated. The data suggest that shallow defects in the
interface dominate the switching. Their density and fluctuation, therefore,
will ultimately limit the device size. While the defects generated/annihilated
by the pulses and the associated carrier depletion seem to play the major role
at lower defect density, the defect correlations and their associated hopping
ranges appear to dominate at higher defect density. Therefore, the switching
characteristics, especially the size-scalability, may be altered through
interface treatments. | 0603832v1 |
2006-04-12 | Influence of Domain Wall on Magnetocaloric Effect in GdPt$_{2}$ | The resistivity, magnetoresistance and in-field heat capacity measurements
were performed on GdPt$_{2}$ intermetallic compound. The magnetocaloric
parameters $\Delta T_{ad}$ and $-\Delta S$ were derived from the in-field heat
capacity data. Comparison has been made between the magnetocaloric effect
$-\Delta S$ and difference in resistivity $-\Delta \rho$ $(=\rho(H)-\rho(0))$
as a function of temperature. There is distinct difference in the temperature
dependence of $-\Delta S$ and $-\Delta \rho$ below the ferromagnetic transition
temperature. However after removing the domain wall contribution from $-\Delta
\rho$, the nature of $-\Delta S$ and $-\Delta \rho$ dependence as a function of
temperature are similar. Our observation indicates that the domain wall
contribution in magnetocaloric effect is negligible in spite of the fact that
it has significant contribution in magnetotransport. | 0604297v1 |
2006-06-08 | Weak localization correction to the anomalous Hall effect in polycrystalline Fe films | In situ transport measurements have been made on ultrathin ($<$100 {\AA}
thick) polycrystalline Fe films as a function of temperature and magnetic field
for a wide range of disorder strengths. For sheet resistances $R_{xx}$ less
than $\sim 3k\Omega$, we find a logarithmic temperature dependence of the
anomalous Hall conductivity $\sigma_{xy}$ which is shown for the first time to
be due to a universal scale dependent weak localization correction within the
skew scattering model. For higher sheet resistance, granularity becomes
important and the break down of universal behavior becomes manifest as the
prefactors to $\sigma_{xx}$ and $\sigma_{xy}$ decrease at different rates with
increasing disorder. | 0606215v2 |
2006-07-24 | Magnetoresistance in Thin Permalloy Film (10nm-thick and 30-200nm-wide) Nanocontacts Fabricated by e-Beam Lithography | In this paper we show spin dependent transport experiments in
nanoconstrictions ranging from 30 to 200nm. These nanoconstrictions were
fabricated combining electron beam lithography and thin film deposition
techniques. Two types of geometries have been fabricated and investigated. We
compare the experimental results with the theoretical estimation of the
electrical resistance. Finally we show that the magnetoresistance for the
different geometries does not scale with the resistance of the structure and
obtain drops in voltage of 20mV at 20Oe. | 0607608v1 |
2006-07-28 | Magnetic hysteresis in the microwave surface resistance of Nb samples in the critical state | We discuss the hysteretic behavior of the field-induced variations of the
microwave surface resistance in superconductors in the critical state.
Measurements have been performed in a bulk sample of Nb and a powdered one at
different values of the temperature. We discuss a model, based on the Coffey
and Clem theory, in which we take into account the flux distribution inside the
sample, due to the critical state. The experimental results are justified
quantitatively in the framework of our model. We show that by fitting the
experimental data it is possible to determine the value of the critical current
density and its field dependence. | 0607754v1 |
2006-09-25 | Doping Dependence of Polaron Hopping Energies in La(1-x)Ca(x)MnO(3) (0<= x<= 0.15) | Measurements of the low-frequency (f<= 100 kHz) permittivity at T<= 160 K and
dc resistivity (T<= 430 K) are reported for La(1-x)Ca(x)MnO(3) (0<= x<= 0.15).
Static dielectric constants are determined from the low-T limiting behavior of
the permittivity. The estimated polarizability for bound holes ~ 10^{-22}
cm^{-3} implies a radius comparable to the interatomic spacing, consistent with
the small polaron picture established from prior transport studies near room
temperature and above on nearby compositions. Relaxation peaks in the
dielectric loss associated with charge-carrier hopping yield activation
energies in good agreement with low-T hopping energies determined from
variable-range hopping fits of the dc resistivity. The doping dependence of
these energies suggests that the orthorhombic, canted antiferromagnetic ground
state tends toward an insulator-metal transition that is not realized due to
the formation of the ferromagnetic insulating state near Mn(4+) concentration ~
0.13. | 0609634v1 |
2006-10-10 | Spin-polarized transport in ferromagnetic multilayered semiconductor nanostructures | The occurrence of inhomogeneous spin-density distribution in multilayered
ferromagnetic diluted magnetic semiconductor nanostructures leads to strong
dependence of the spin-polarized transport properties on these systems. The
spin-dependent mobility, conductivity and resistivity in
(Ga,Mn)As/GaAs,(Ga,Mn)N/GaN, and (Si,Mn)/Si multilayers are calculated as a
function of temperature, scaled by the average magnetization of the diluted
magnetic semiconductor layers. An increase of the resistivity near the
transition temperature is obtained. We observed that the spin-polarized
transport properties changes strongly among the three materials. | 0610275v1 |
2006-10-12 | Domain wall magnetoresistance in a nanopatterned La(2/3)Sr(1/3)MnO3 track | We have measured the contribution of magnetic domain walls (DWs) to the
electric resistance in epitaxial manganite films patterned by electron-beam
lithography into a track containing a set of notches. We find a DW
resistance-area (RA) product of ~2.5 10^(-13) Ohm/m^2 at low temperature and
bias, which is several orders of magnitude larger than the values reported for
3d ferromagnets. However, the current-voltage characteristics are highly linear
which indicates that the DWs are not phase separated but metallic. The DWRA is
found to increase upon increasing the injected current, presumably reflecting
some deformation of the wall by spin-transfer. When increasing temperature, the
DWRA vanishes at ~225K which is likely related to the temperature dependence of
the film anisotropy. | 0610338v1 |
2006-10-25 | Origin of negative differential resistance in molecular junctions of Rose Bengal | Negative differential resistance (NDR) is tuned at junctions of
electronically different dimer and trimer of Rose Bengal on an atomic flat gold
(111) surface. Isolated molecule did not show any NDR. But it was induced to
show double NDR with large peak to valley ratio (1.8~3.1) in room temperature
via charging its neighbor reproducibly by an electrical pulse. In some sections
of junction by applying pulse one could destroy the phenomenon or regenerate it
by STM manipulation of molecules. NDR was also independent of polaronic nature.
It was possible to write bits 1 and 0 for cationic NDR (in dimer) and 00, 01,
10, 11 for di-anionic NDR (trimer) which generated 2/4 bit memory in a atomic
scale junction showing importance of junction electronics in future of
moletronics. | 0610683v1 |
2006-11-13 | Weak antilocalization in epitaxial graphene: evidence for chiral electrons | Transport in ultrathin graphite grown on silicon carbide is dominated by the
electron-doped epitaxial layer at the interface. Weak anti-localization in 2D
samples manifests itself as a broad cusp-like depression in the longitudinal
resistance for magnetic fields 10 mT$< B <$ 5 T. An extremely sharp
weak-localization resistance peak at B=0 is also observed. These features
quantitatively agree with graphene weak-(anti)localization theory implying the
chiral electronic character of the samples. Scattering contributions from the
trapped charges in the substrate and from trigonal warping due to the graphite
layer on top are tentatively identified. The Shubnikov-de Haas oscillations are
remarkably small and show an anomalous Berry's phase. | 0611339v2 |
2006-12-19 | Semiconductors between spin-polarized source and drain | Injecting spins into a semiconductor channel and transforming the spin
information into a significant electrical output signal is a long standing
problem in spintronics. Actually, this is the prerequisite of several concepts
of spin transistor. In this tutorial article, we discuss the general problem of
spin transport in a nonmagnetic channel between source and drain. Two problems
must be mastered: i) In the diffusive regime, the injection of a spin polarized
current from a magnetic metal beyond the ballistic transport zone requires the
insertion of a spin dependent and large enough interface resistance. ii) In
both the diffusive and ballistic regimes, and whatever the metallic or
semiconducting character of the source/drain, a small enough interface
resistance is the condition to keep the dwell time shorter than the spin
lifetime and thus to conserve the spin accumulation-induced output signal at an
optimum level. Practically, the main difficulties come from the second
condition. In our presentation of experimental results, we show why the
transformation of spin information into a large electrical signal has been more
easily achieved with carbon nanotubes than with semiconductors and we discuss
how the situation could be improved in the later case. | 0612495v1 |
2007-01-17 | Anomalous Hall effect in anatase Ti1-xCoxO2 at low temperature regime | Anomalous Hall effect (AHE) of a ferromagnetic semiconductor anatase \cotio
thin film is studied from 10K to 300K. Magnetic field dependence of anomalous
Hall resistance is coincident with that of magnetization, while the anomalous
Hall resistance decreases at low temperature in spite of nearly
temperature-independent magnetization. Anomalous Hall conductivity sigma_AHE is
found to be proportional to the square of Hall mobility, suggesting that charge
scattering strongly affects the AHE in this system. The anatase Ti1-xCoxO2 also
follows a scaling relationship to conductivity sigma_xx as sigma_AHE ~
sigma_xx^1.6, which was observed for another polymorph rutile Ti1-xCoxO2,
suggesting an identical mechanism of their AHE. | 0701395v2 |
2007-02-17 | Room-Temperature Quantum Hall Effect in Graphene | The quantum Hall effect (QHE), one example of a quantum phenomenon that occur
on a truly macroscopic scale, has been attracting intense interest since its
discovery in 1980 and has helped elucidate many important aspects of quantum
physics. It has also led to the establishment of a new metrological standard,
the resistance quantum. Disappointingly, however, the QHE could only have been
observed at liquid-helium temperatures. Here, we show that in graphene - a
single atomic layer of carbon - the QHE can reliably be measured even at room
temperature, which is not only surprising and inspirational but also promises
QHE resistance standards becoming available to a broader community, outside a
few national institutions. | 0702408v1 |
2007-02-20 | Anomalous electric conductions in KSbO3-type metallic rhenium oxides | Single crystals of KSbO3-type rhenium oxides, La4Re6O$19, Pb6Re6O19, Sr2Re3O9
and Bi3Re3O11, were synthesized by a hydrothermal method. Their crystal
structures can be regarded as a network of three-dimensional orthogonal-dimer
lattice of edge-shared ReO6 octahedra. All of them exhibit small magnitude of
Pauli paramagnetism, indicating metallic electronic states without strong
electron correlations. The resistivity of these rhenates, except Bi3Re3O11,
have a temperature dependence of $rho(T)=\rho_{0}+AT^{n}$ $(n \approx 1.6)$ in
a wide temperature range between 5 K and 300 K, which is extraordinary for
three-dimensional metals without strong electron correlations. The resistivity
of Bi3Re3O11 shows an anomaly around at 50 K, where the magnetic susceptibility
also detects a deviation from ordinary Pauli paramagnetism. | 0702451v1 |
2007-03-14 | Electrical transport and percolation in magnetoresistive manganite / insulating oxide composites: case of La0.7Ca0.3MnO3 / Mn3O4 | We report the results of electrical resistivity measurements carried out on
well-sintered La0.7Ca0.3MnO3 / Mn3O4 composite samples with almost constant
composition of the magnetoresistive manganite phase (La0.7Ca0.3MnO3). A
percolation threshold (fc) occurs when the La0.7Ca0.3MnO3 volume fraction is ~
0.19. The dependence of the electrical resistivity as a function of
La0.7Ca0.3MnO3 volume fraction (fLCMO) can be described by percolation-like
phenomenological equations. Fitting the conducting regime (fLCMO > fc) by the
percolation power law returns a critical exponent t value of 2.0 +/- 0.2 at
room temperature and 2.6 +/-0.2 at 5 K. The increase of t is ascribed to the
influence of the grain boundaries on the electrical conduction process at low
temperature. | 0703367v1 |
2007-03-25 | Origin of superconductivity in nominally "undoped" T'-La$_{2-x}$Y$_{x}$CuO$_{4}$ films | We have systematically studied the transport properties of the
La$_{2-x}$Y$_{x}$CuO$_{4}$(LYCO) films of T'-phase ($0.05\leq x \leq 0.30$). In
this nominally "undoped" system, superconductivity was acquired in certain Y
doping range ($0.10\leq x \leq 0.20$). Measurements of resistivity, Hall
coefficients in normal states and resistive critical field ($H^\rho_{c2}$)in
superconducting states of the T'-LYCO films show the similar behavior as the
known Ce-doped n-type cuprate superconductors, indicating the intrinsic
electron-doping nature. The charge carriers are induced by oxygen deficiency.
Non-superconducting Y-doped Pr- or Nd-based T'-phase cuprate films were also
investigated for comparison, suggesting the crucial role of the radii of A-site
cations in the origin of superconductivity in the nominally "undoped" cuptates.
Based on a reasonable scenario in the microscopic reduction process, we put
forward a self-consistent interpretation of these experimental observations. | 0703643v1 |
2005-11-13 | Non-radiating and radiating configurations driven by left-handed metamaterials | It is shown that a pair of identical emitters (e.g. wire dipole antennas) in
the focal points of a disc, made of left-handed metamaterial (a "perfect"
lens), form a non-radiating electromagnetic configuration. The emitters are fed
with voltages of equal magnitude and pi-out-of-phase. Detailed
finite-difference time-domain (FDTD) modeling shows that there are
non-propagating electromagnetic fields generated - fields that remain confined
within the region between the emitters and the lens. The energy balance of the
system shows that the radiation resistance of the system is very low. This
means that the input power is converted to heat in the volume of the lens and
only a small fraction of it is radiated. The system performance shows that
disturbing the configuration of the non-propagating electromagnetic fields with
the presence of an externally introduced object stimulates radiation. This
suggests possible detector applications. In-phase feeding voltages are also
studied with the consequence that the radiation resistance of the antennae is
increased. | 0511113v1 |
2007-04-16 | Memory function formalism approach to electrical conductivity and optical response of dilute magnetic semiconductors | A combination of the memory function formalism and time-dependent
density-functional theory is applied to transport in dilute magnetic
semiconductors. The approach considers spin and charge disorder and
electron-electron interaction on an equal footing. Within the weak disorder
limit and using a simple parabolic approximation for the valence band we show
that Coulomb and exchange scattering contributions to the resistivity in GaMnAs
are of the same order of magnitude. The positional correlations of defects
result in a significant increase of Coulomb scattering, while the suppression
of localized spin fluctuations in the ferromagnetic phase contributes
substantially to the experimentally observed drop of resistivity below T_c. A
proper treatment of dynamical screening and collective excitations is essential
for an accurate description of infrared absorption. | 0704.2061v1 |
2007-05-15 | Magnetic, magneto-thermal and magneto-transport properties in SmMn2Si2-xGex compounds | The effect of Ge substitution for Si in SmMMn2Si2-xGex compounds has been
studied. The Sm ordering temperature is found to be much larger in the compound
with x=2, as compared to the compounds with x=0 and 1. The increase in the
intra layer Mn-Mn distance is found to be responsible for this increase. Among
these three compounds, SmMn2Ge2 is found to show re-entrant ferromagnetism at
low temperatures. The magnetic contribution to the heat capacity has been found
in all the three compounds. The splitting of the ground state multiplet has
been estimated by fitting the magnetic part of the heat capacity data using the
Schottky formula. The isothermal magnetic entropy change is found to remain the
same for x=0 and 1, but decrease in the compound with x=2, though the nature of
magnetic transition changes from second order to first order, as x is increased
from 0 to 2. The electrical resistivity increases with Ge concentration. The
excess resistivity in the antiferromagnetic region has been calculated. | 0705.2237v1 |
2007-06-01 | Anomalous magnetoresistance of EuB$_{5.99}$C$_{0.01}$: Enhancement of magnetoresistance in systems with magnetic polarons | We present results of measurements of electrical, magnetic and thermal
properties of EuB$_{5.99}$C$_{0.01}$. The observed anomalously large negative
magnetoresistance as above, so below the Curie temperature of ferromagnetic
ordering $T_C$ is attributed to fluctuations in carbon concentration. Below
$T_C$ the carbon richer regions give rise to helimagnetic domains, which are
responsible for an additional scattering term in the resistivity, which can be
suppressed by a magnetic field. Above $T_C$ these regions prevent the process
of percolation of magnetic polarons (MPs), acting as "spacers" between MPs. We
propose that such "spacers", being in fact volumes incompatible with existence
of MPs, may be responsible for the decrease of the percolation temperature and
for the additional (magneto)resistivity increase in systems with MPs. | 0706.0091v1 |
2007-07-03 | Spatial correlation of linear and nonlinear electron transport in superconducting microwave resonator: laser scanning microscopy analysis | Spatially-resolved techniques of laser scanning microscopy (LSM) have been
used to image simultaneously the spatial variations of (i) rf current flow,
J$_RF$(x,y), of (ii) areas of resistive dissipation and (iii) the sources of
microwave nonlinearity (NL) in an operating superconducting resonator. The RF
power dependent spatial evolution of these linear and NL microwave properties
in the meander strip YBCO/LAO superconducting resonator have been LSM probed at
different temperatures below Tc. The influence of both topologies of the
twin-domain YBCO structure and of J$_RF$(x,y) peaks at the edges of
superconducting strip line on its NL properties was analyzed in detail with a
micron-scale spatial resolution. Result shows the resistive origin of the
dominant sources of microwave NLs | 0707.0358v1 |
2007-07-04 | Resistive Switching in Cr doped SrTiO3: An X-ray Absorption Spectroscopy study | X-ray absorption spectroscopy was used to study the microscopic origin of
conductance and resistive switching in chromium doped strontium titanate
(Cr:SrTiO3). Differences in the x-ray absorption near edge spectroscopy (XANES)
at the Cr K-edge indicate that the valence of Cr changes from 3+ to 4+
underneath the anode of our sample device after the application of an electric
field. Spatially resolved x-ray fluorescence microscopy ($\mu$-XRF) maps show
that the Cr4+ region retracts from the anode-Cr:SrTiO3 interface after a
conducting state has been achieved. This interface region is studied with
extended x-ray absorption fine structure (EXAFS) and the results are compared
with structural parameters obtained from density functional theory (DFT)
calculations. They confirm that oxygen vacancies which are localized at the
octahedron with a Cr at its center are introduced at the interface. It is
proposed that the switching state is not due to a valence change of chromium
but caused by changes of oxygen vacancies at the interface. | 0707.0655v1 |
2007-08-21 | Negative differential resistance of Styrene on an ideal Si[111] surface: dependence of the I-V characteristics on geometry, surface doping and shape of the STM-tip | We study the electron transport properties through a supported organic
molecule styrene (C8H8) on an ideal silicon surface Si[111] and probed by a
STM-tip. The I-V characteristics and the differential conductance of the
molecule are calculated using a self consistent approach based on non
equilibrium Green's functions. Two different adsorption configurations for the
molecule on the surface were considered which corresponds to a global and a
local minimum of the total energy. In both cases we find a negative
differential resistance (NDR) in a given interval of bias voltages. This effect
is controlled by the states available close to the Fermi level of the surface
and can be manipulated by properly doping the substrate. We also analyze the
influence of the tip-shape on the I-V characteristics. | 0708.2834v1 |
2007-12-21 | Vibronic polarons: comments on a model for the colossal field-resistance effects in manganites | In addition to mechanisms already proposed to account for the formation in
manganites of a small-polaron superlattice above the Curie temperature Tc and
to a metallic-like sea of large polarons below Tc, we now consider other
observed colossal-resistance inducing fields, such as magnetic, electric,
photon, or strain fields. We attribute the charge-ordered phase formation to
the occurrence of strong dipolar binding of vibronic small polarons arising
from the phonon coupling of highly polarizable two-level orbital systems. These
species having associated inherent electric and magnetic off-center dipoles,
they couple to the external fields leading to the observed colossal effects.
The random phase appears due to polaron band widening in the external field. | 0712.3803v1 |
2008-03-05 | The behavior of magnetic ordering and the KOndo effect in the alloys, Ce2Rh(1-x)Co(x)Si3: Evidence from bulk studies for Fermi-surface change during magnetic ordering - QCP transformation and applicability of SDW pictur | The results of magnetic susceptibility, electrical resistivity (rho), and
heat capacity measurements as a function of temperature are reported for the
alloys, Ce2Rh(1-x)Co(x)Si3, crystallizing in an AlB2-derived hexagonal
strcture. Ce2RhSi3 exhibits antiferromagnetic ordering at 7 K. The Neel
temperature decreases gradually with the increase in Co concentration. For x
greater than 0.6, no magnetic ordering is observed down to 0.5 K.
Interestingly, the x= 0.6 alloy exhibits signatutes of non-Fermi liquid
behavior, while the Co end member is a Fermi liquid. Thus, a transformation of
magnetic ordering state to non-magnetism via non-Fermiliquid state by
isoelectronic chemical doping is evident in this solid solution. The electrical
resistivity data for x= 0.2 and 0.3 alloys show an upturn at respective Neel
temperatures, establishing the formation of a magnetism-induced pseudo-gap for
these intermediate compositions alone as though there is a gradual Fermi
surface transformation as the quantum critical point is approached. | 0803.0652v1 |
2008-03-09 | Upper critical field, Hall effect and magnetoresistance in the iron-based layered superconductor LaFeAsO_{0.9}F_{0.1-δ} | By using a two-step method, we successfully synthesized the iron based new
superconductor LaFeAsO_{0.9}F_{0.1-\delta}$. The resistive transition curves
under different magnetic fields were measured, leading to the determination of
the upper critical field Hc2(T) of this new superconductor. The value of Hc2 at
zero temperature is estimated to be about 50 Tesla roughly. In addition, the
Hall effect and magnetoresistance were measured in wide temperature region. A
negative Hall coefficient R_H has been found, implying a dominant conduction
mainly by electron-like charge carriers in this material. The charge carrier
density determined at 100 K is about 9.8E20cm^{-3}, which is close to the
cuprate superconductors. It is further found that the magnetoresistance does
not follow Kohler's law. Meanwhile, the different temperature dependence
behaviors of resistivity, Hall coefficient, and magnetoresistance have
anomalous properties at about 230 K, which may be induced by some exotic
scattering mechanism. | 0803.1288v2 |
2008-03-12 | Electron transport and thermoelectric properties of layered perovskite LaBaCo2O5.5 | We have investigated the systematic transport properties of the layered
112-type cobaltite LaBaCo2O5.5 by means of electrical resistivity,
magnetoresistance, electroresistance and thermoelectric measurements in various
conditions. In order to understand the complex conduction mechanism of
LaBaCo2O5.5, the transport data have been analyzed using different theoretical
models. The system shows semiconductor-semiconductor like transition (TSC)
around 326K, corresponding to ferromagnetic transition and in the low
temperature region resistivity data follows the Motts variable range hopping
model. Interestingly, near and below the room temperature this compound depicts
significant change in electro- and magnetoresistance behavior, the latter one
is noteworthy near the magnetic phase boundary. The temperature dependence of
thermopower, S(T), exhibits p-type polaronic conductivity in the temperature
range of 60-320K and reaches a maximum value of 303 uV/K (at 120K). In the low
temperature AFM region, the unusual S(T) behavior, generally observed for the
cobaltite series LnBaCo2O5.5 (Ln = Rare Earth), is explained by the electron
magnon scattering mechanism as previously described for perovskite manganites. | 0803.1806v1 |
2008-03-14 | From Ohmic to Ballistic Transport in Oriented Graphite | In this work we show that for a quasi-2D system of size $\Omega$ and
thickness $t$ the resistance goes as $(2\rho/\pi t)\ln(\Omega/W)$, diverging
logarithmically with the size. Measurements in highly oriented pyrolytic
graphite (HOPG) as well as numerical simulations confirm this relation.
Furthermore, we present an experimental method that allows us to obtain the
carriers mean free path $l(T)$, the Fermi wavelength $\lambda(T)$ and the
mobility $\mu(T)$ directly from experiments without adjustable parameters.
Measuring the electrical resistance through microfabricated constrictions in
HOPG and observing the transition from ohmic to ballistic regime we obtain that
$0.2 \mu$m $\lesssim l \lesssim 10 \mu$m, $0.1 \mu$m $\lesssim \lambda \lesssim
2 \mu$m and a mobility $5 \times 10^4$ cm$^2$/Vs $ \lesssim \mu \lesssim 4
\times 10^7$ cm$^2$/Vs when the temperature decreases from 270K to 3K. A
comparison of these results with those from literature indicates that
conventional, multiband Boltzmann-Drude approaches are inadequate for oriented
graphite. The upper value obtained for the mobility is much larger than the
mobility graphene samples of micrometer size can have. | 0803.2203v2 |
2008-03-19 | Theory of quantum metal to superconductor transitions in highly conducting systems | We derive the theory of the quantum (zero temperature) superconductor to
metal transition in disordered materials when the resistance of the normal
metal near criticality is small compared to the quantum of resistivity. This
can occur most readily in situations in which ``Anderson's theorem'' does not
apply. We explicitly study the transition in superconductor-metal composites,
in an s-wave superconducting film in the presence of a magnetic field, and in a
low temperature disordered d-wave superconductor. Near the point of the
transition, the distribution of the superconducting order parameter is highly
inhomogeneous. To describe this situation we employ a procedure which is
similar to that introduced by Mott for description of the temperature
dependence of the variable range hopping conduction. As the system approaches
the point of the transition from the metal to the superconductor, the
conductivity of the system diverges, and the Wiedemann-Franz law is violated.
In the case of d-wave (or other exotic) superconductors we predict the
existence of (at least) two sequential transitions as a function of increasing
disorder: a d-wave to s-wave, and then an s-wave to metal transition. | 0803.2902v2 |
2008-03-23 | Origin of negative differential resistance in a strongly coupled single molecule-metal junction device | A new mechanism is proposed to explain the origin of negative differential
resistance (NDR) in a strongly coupled single molecule-metal junction. A
first-principles quantum transport calculation in a Fe-terpyridine linker
molecule sandwiched between a pair of gold electrodes is presented. Upon
increasing applied bias, it is found that a new phase in the broken symmetry
wavefunction of the molecule emerges from the mixing of occupied and unoccupied
molecular orbital. As a consequence, a non-linear change in the coupling
between molecule and lead is evolved resulting to NDR. This model can be used
to explain NDR in other class of metal-molecule junction device. | 0803.3342v1 |
2008-04-13 | Conductance of p-n-p graphene structures with 'air-bridge' top gates | We have fabricated graphene devices with a top gate separated from the
graphene layer by an air gap--a design which does not decrease the mobility of
charge carriers under the gate. This gate is used to realise p-n-p structures
where the conducting properties of chiral carriers are studied. The band
profile of the structures is calculated taking into account the specifics of
the graphene density of states and is used to find the resistance of the p-n
junctions expected for chiral carriers. We show that ballistic p-n junctions
have larger resistance than diffusive ones. This is caused by suppressed
transmission of chiral carriers at angles away from the normal to the junction. | 0804.2081v3 |
2008-06-16 | Metal-insulator transition and giant anisotropic magnetoresistance in ultra thin (Ga,Mn)As | MBE-grown, 5 nm-thick annealed Ga0.95Mn0.05As films with Tc~90K demonstrate
transition from metallic to insulating state below To~10K, where sheet
resistances Rsh~h/e2 and both longitudinal Rxx and transverse Rxy components
become comparable. Below metal-insulator transition we found giant anisotropic
magnetoresistance (GAMR), which depends on orientation of magnetization to
crystallographic axes and manifests itself in positive magnetoresistance near
50% for Rxx at T=1.7K, H//[110] crystallographic direction and parallel to
current in contrast to smaller and negative magnetoresistance for H//
direction. We connect GAMR with anisotropic spin-orbit interaction resulting in
formation of high- and low- resistance states with different localization along
non-equivalent easy axes. | 0806.2590v1 |
2008-06-28 | Theory of spin magnetohydrodynamics | We develop a phenomenological hydrodynamic theory of coherent magnetic
precession coupled to electric currents. Exchange interaction between electron
spin and collective magnetic texture produces two reciprocal effects:
spin-transfer torque on the magnetic order parameter and the Berry-phase gauge
field experienced by the itinerant electrons. The dissipative processes are
governed by three coefficients: the ohmic resistance, Gilbert damping of the
magnetization, and the "beta coefficient" describing viscous coupling between
magnetic dynamics and electric current, which stems from spin mistracking of
the magnetic order. We develop general magnetohydrodynamic equations and
discuss the net dissipation produced by the coupled dynamics. The latter in
particular allows us to determine a lower bound on the magnetic-texture
resistivity. | 0806.4656v2 |
2008-07-28 | Transport anomalies across the quantum limit in semimetallic Bi$_{0.96}$Sb$_{0.04}$ | We report on a study of electronic transport in semi-metallic
Bi$_{0.96}$Sb$_{0.04}$. At zero field, the system is a very dilute Fermi liquid
displaying a T$^{2}$ resistivity with an enhanced prefactor. Quantum
oscillations in resistivity as well as in Hall, Nernst and Seebeck responses of
the system are detectable and their period quantifies the shrinking of the
Fermi surface with antimony doping. For a field along the trigonal axis, the
quantum limit was found to occur at a field as low as 3T. An ultraquantum
anomaly at twice this field was detected in both charge transport and Nernst
response. Its origin appears to lie beyond the one-particle picture and linked
to unidentified many-body effects. | 0807.4432v1 |
2008-07-30 | Ultra-Thin Silver Films obtained by Sequential Quench-Anneal Processing | We have used the two-step growth technique, quench condensing followed by an
anneal, to grow ultra thin films of silver on glass substrates. As has been
seen with semiconductor substrates this process produces a metastable
homogeneous covering of silver. By measuring the in situ resistance of the film
during growth we are able to see that the low temperature growth onto
substrates held at 100 Kelvin produces a precursor phase that is insulating
until the film has been annealed. The transformation of the precursor phase
into the final, metallic silver film occurs at a characteristic temperature
near 150K where the sample reconstructs. This reconstruction is accompanied by
a decrease in resistance of up to 10 orders of magnitude. | 0807.4948v1 |
2008-08-09 | Electronic transport and specific heat of 1T- VSe2 | The results of low temperature thermoelectric power and the specific heat of
1T-VSe2 (Vanadium diselenide) have been reported along with the electrical
resistivity, and Hall coefficient of the compound. The Charge Density Wave
(CDW) transition is observed near 110K temperature in all these properties. The
Thermoelectric power has been measured from 15K to 300K spanning the
incommensurate and commensurate CDW regions. We observed a weak anomaly at the
CDW transition for the first time in the specific heat of VSe2. The linear
temperature dependence of resistivity and thermoelectric power at higher
temperatures suggests a normal metallic behavior and electron-phonon scattering
above the CDW transition. The positive thermoelectric power and negative Hall
coefficient along with strongly temperature dependent behavior in the CDW phase
suggest a mixed conduction related to the strongly hybridized s-p-d bands in
this compound. | 0808.1332v2 |
2008-08-11 | Non-trivial length dependence of the conductance and negative differential resistance in atomic molecular wires | We study the electronic and transport properties of two novel molecular wires
made of atomic chains of carbon atoms (polyynes) capped with either,
benzene-thiols or pyridines. While both molecules are structurally similar, the
electrical conductance of benzene-thiol-capped chains attached to gold
electrodes is found to be much higher than that of pyridine-capped chains. We
predict that the conductance is almost independent of molecular length, which
suggests that these molecules could be ideal molecular wires for sub-10 nm
circuitry. Both systems exhibit negative differential resistance (NDR) but its
origin and characteristics depend on the type of molecule. We find a novel type
of NDR mechanism produced by the movement of the LUMO resonance with bias. We
also show that by gating the pyridine-capped molecules it is possible to make
the NDR disappear and dramatically modify the $I$-$V$ characteristics and the
length dependence. | 0808.1494v2 |
2008-08-21 | Electrons and holes in Si quantum well: a room-temperature transport and drag resistance study | We investigate carrier transport in a single 22 nm-thick double-gated Si
quantum well device, which has independent contacts to electrons and holes.
Conductance, Hall density and Hall mobility are mapped in a broad double-gate
voltage window. When the gate voltage asymmetry is not too large only either
electrons or holes occupy the Si well and the Hall mobility shows the
fingerprints of volume inversion/accumulation. At strongly asymmetric
double-gate voltage an electric field induced electron-hole (EH) bi-layer is
formed inside the well. The EH drag resistance R_{he} is explored at balanced
carrier densities: R_{he} decreases monotonically from 860 to 37 Ohms when the
electron and hole density is varied between ~0.4-1.7x10^{16} m^{-2}. | 0808.2914v1 |
2008-10-27 | Abrupt Emergence of Pressure-Induced Superconductivity of 34 K in SrFe2As2: A Resistivity Study under Pressure | We report resistivity measurement under pressure in single crystals of
SrFe_2As_2, which is one of the parent materials of Fe-based superconductors.
The structural and antiferromagnetic (AFM) transition of T_0 = 198 K at ambient
pressure is suppressed under pressure, and the ordered phase disappears above
P_c ~ 3.6-3.7 GPa. Superconductivity with a sharp transition appears
accompanied by the suppression of the AFM state. T_c exhibits a maximum of 34.1
K, which is realized close to the phase boundary at P_c. This T_c is the
highest among those of the stoichiometric Fe-based superconductors. | 0810.4856v2 |
2008-12-11 | Difference of Oxide Hetero-Structure Junctions with Semiconductor Electronic Devices | Charge carrier injection performed in Pr0.7Ca0.3MnO3 (PCMO) hetero-structure
junctions exhibits stable without electric fields and dramatic changes in both
resistances and interface barriers, which are entirely different from behaviors
of semiconductor devices. Disappearance and reversion of interface barriers
suggest that the adjustable resistance switching of such hetero-structure oxide
devices should associate with motion of charge carriers across interfaces. The
results suggested that injected carriers should be still staying in devices and
resulted in changes in properties, which guided to a carrier self-trapping and
releasing picture in strongly correlated electronic framework. Observations in
PCMO and oxygen deficient CeO2 devices show that oxides as functional materials
could be used in microelectronics with some novel properties, in which
interface is very important. | 0812.2071v1 |
2008-12-11 | Evidence for Coexistence of Superconductivity and Magnetism in Single Crystals of Co-doped SrFe$_2$As$_2$ | In order to investigate whether magnetism and superconductivity coexist in
Co-doped SrFe$_2$As$_2$, we have prepared single crystals of
SrFe$_{2-x}$Co$_x$As$_2$, $x$ = 0 and 0.4, and characterized them via X-ray
diffraction, electrical resistivity in zero and applied field up to 9 T as well
as at ambient and applied pressure up to 1.6 GPa, and magnetic susceptibility.
At $x$ = 0.4, there is both magnetic and resistive evidence for a spin density
wave transition at 120 K, while $T_c$ = 19.5 K - indicating coexistent
magnetism and superconductivity. A discussion of how these results compare with
reported results, both in SrFe$_{2-x}$Co$_x$As$_2$ and in other doped 122
compounds, is given. | 0812.2091v1 |
2009-01-10 | Wafer-Scale, Sub-5 nm Junction Formation by Monolayer Doping and Conventional Spike Annealing | We report the formation of sub-5 nm ultrashallow junctions in 4 inch Si
wafers enabled by the molecular monolayer doping of phosphorous and boron atoms
and the use of conventional spike annealing. The junctions are characterized by
secondary ion mass spectrometry and non-contact sheet resistance measurements.
It is found that the majority (~70%) of the incorporated dopants are
electrically active, therefore, enabling a low sheet resistance for a given
dopant areal dose. The wafer-scale uniformity is investigated and found to be
limited by the temperature homogeneity of the spike anneal tool used in the
experiments. Notably, minimal junction leakage currents (<1 uA/cm2) are
observed which highlights the quality of the junctions formed by this process.
The results clearly demonstrate the versatility and potency of the monolayer
doping approach for enabling controlled, molecular-scale ultrashallow junction
formation without introducing defects in the semiconductor. | 0901.1396v1 |
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