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2003-09-29 | A common trend in the chemical evolution of Local Group dwarf spheroidals and Damped Ly-alpha systems | We compare chemical abundances of Local Group dwarf spheroidals, obtained
from recent UVES/VLT observations, and of high redshift Damped Lyman alpha
systems (DLAs), corrected for dust effects. We focus, in particular, on the
abundance ratio between alpha-capture elements and iron, alpha/Fe, a well known
indicator of chemical evolution. Comparison of the data in the plane alpha/Fe
versus Fe/H shows a remarkable similarity between the dwarf spheroidals and the
DLAs, suggestive of a common trend in their chemical evolution. At any given
metallicity these two distinct types of astronomical targets show alpha/Fe
ratios systematically lower than those of Milky Way stars. In terms of chemical
evolution models, this suggests that, on average, dSph galaxies and DLA systems
are characterized by lower, or more episodic, star formation rates than the
Milky Way. | 0309765v1 |
2003-10-15 | H_2-bearing damped Lyman-alpha systems as tracers of cosmological chemical evolution | The chemical abundances in damped Lyman-alpha systems (DLAs) show more than 2
orders of magnitude variation at a given epoch, possibly because DLAs arise in
a wide variety of host galaxies. This could significantly bias estimates of
chemical evolution. We explore the possibility that DLAs in which H_2
absorption is detected may trace cosmological chemical evolution more reliably
since they may comprise a narrower set of physical conditions. The 9 known H_2
absorption systems support this hypothesis: metallicity exhibits a faster, more
well-defined evolution with redshift than in the general DLA population. The
dust-depletion factor and, particularly, H_2 molecular fraction also show rapid
increases with decreasing redshift. We comment on possible observational
selection effects which may bias this evolution. Larger samples of H_2-bearing
DLAs are clearly required and may constrain evolution of the UV background and
DLA galaxy host type with redshift. | 0310409v2 |
2004-01-21 | Constraints on Early Nucleosynthesis from the Abundance Pattern of a Damped Ly-alpha System at z = 2.626 | We have investigated chemical evolution in the young universe by analysing
the detailed chemical enrichment pattern of a metal-rich galaxy at high
redshift. The recent detection of over 20 elements in the gas-phase of a damped
Lyman-alpha absorber (DLA) at z = 2.626 represents an exciting new avenue for
exploring early nucleosynthesis. Given a strict upper age of ~2.5 Gyr and a
gas-phase metallicity about one third solar, we have shown the DLA abundance
pattern to be consistent with the predictions of a chemical evolution model in
which the interstellar enrichment is dominated by massive stars with a small
contribution from Type Ia supernovae. Discrepancies between the empirical data
and the models are used to highlight outstanding issues in nucleosynthesis
theory, including a tendency for Type II supernovae models to overestimate the
magnitude of the "odd-even" effect at subsolar metallicities. Our results
suggest a possible need for supplemental sources of magnesium and zinc, beyond
that provided by massive stars. | 0401413v1 |
2004-03-23 | The clustering of galaxies around three damped Ly-alpha absorbers at Redshift Three | [Abridged] We present out results on the cross-correlation of Lyman break
galaxies (LBGs) around three damped \Ly absorbers (DLAs) at redshift three from
deep multi-band MOSAIC images obtained at the KPNO 4m. The large area of the
MOSAIC images, 0.31 $\deg^2$ allows us to probe the clustering of LBGs on
scales up to 20 Mpc co-moving to test whether DLA halos are more or less
massive than LBG halos. . We present our survey that covers a total of 1
deg$^2$ and contains $\sim$3,000 LBGs with photometric redshifts between 2.8
and 3.5.
Within a redshift slice of width $W_z=0.15$, we find that the DLA-LBG
cross-correlation is $1.6\pm 1.3$ times the LBG-LBG autocorrelation. This
corresponds to a correlation length of $r_o=5\pm4.5h^{-1}$ (co-moving). The
cross-correlation is most significant on scales 5-10 Mpc, and is significantly
greater than zero at the $>95$% level from Monte Carlo simulations. | 0403544v2 |
2004-03-31 | Cold Neutral Gas in a z=4.2 Damped Lyman-alpha System: The Fuel for Star Formation | We discuss interstellar temperature determinations using the excitation
equilibrium of the ^2P levels of Si II and C II. We show how observations of
the ^2P_3/2 fine structure levels of Si II and C II (which have significantly
different excitation energies, corresponding to ~413 and 92 K, respectively)
can be used to limit gas kinetic temperatures. We apply this method to the
z=4.224 damped Lyman-alpha system toward the quasar PSS1443+27. The lack of
significant absorption out of the SiII ^2P_3/2 level and the presence of very
strong C II ^2P_3/2 provides an upper limit to the temperature of the C
II*-bearing gas in this system. Assuming a solar Si/C ratio, the observations
imply a 2-sigma limit T<954 K for this absorber; a super-solar Si/C ratio gives
stricter limits, T<524 K. The observations suggest the presence of a cold
neutral medium; such cold gas may serve as the fuel for star formation in this
young galaxy. | 0404005v2 |
2004-04-27 | Molecular fraction limits in damped Lyman-alpha absorption systems | We have used the Green Bank Telescope (GBT) and Berkeley-Illinois-Maryland
Association (BIMA) array to search for redshifted millimetre absorption in a
sample of damped Lyman-alpha absorption systems (DLAs). This brings the number
of published systems searched from 18 to 30. In 17 cases we reach $3\sigma$
limits of $\tau\leq0.1$, which is a significant improvement over the previous
searches and more than sufficient to detect the 4 known redshifted millimetre
absorbers ($\tau\gapp1$). While the CO rotational (millimetre) column density
limits obtained are weaker than the electronic (optical) limits, they may
provide useful limits below the atmospheric cut-off for the Lyman and Werner
\MOLH-bands in the UV ($z_{\rm abs}\lapp1.8$). Using a model for the DLA
metallicity evolution combined with assumed HCO$^+$/\MOLH ~and CO/\MOLH
\~conversion ratios, we use the molecular column density limits to calculate
plausible \MOLH ~molecular fraction limits. Finally, we use these results to
discuss the feasibility of detecting rotational CO transitions in DLAs with the
next generation of large radio telescopes. | 0404516v1 |
2004-08-07 | Metals and Dust in Intermediate-redshift Damped Ly-alpha Galaxies | We report spectroscopic observations with the Multiple Mirror Telescope for
11 damped Lyman-alpha absorbers (DLAs) or strong DLA candidates at 0.1 < z <
1.5, including several absorbers discovered in the Sloan Digital Sky Survey. In
particular, we have measured absorption lines of Zn II, Cr II, Ni II, Fe II, Mn
II, Ti II, Ca II, and Si II. These measurements have doubled the sample of Zn
and Cr measurements at z < 1. The average relative abundance patterns in these
objects are very similar to those found for high-redshift DLAs reported in the
literature. Our observations suggest that the dust content, as determined by
[Cr/Zn], does not show much change with redshift. We also examine the sample
for correlation of [Cr/Zn] with estimates of the quasar reddening. Our data
suggest that the global mean metallicity of DLAs, as measured by the gas phase
abundance of Zn, at best shows a weak evolution with redshift over the range
0.4 < z <3.9. | 0408139v1 |
2004-09-22 | On Detecting the X-ray Silhouette of a Damped Lyman alpha System | We explore the possibility of resolving an image of a damped Lyman alpha
(DLA) system in absorption against an extended, diffuse background X-ray
source. Typical columns of neutral hydrogen in DLAs are high enough to block
out up to ~30% of the soft X-ray flux at an observed photon energy of 0.5 keV,
and we find that ~ 1% of the area of extended X-ray sources at z > 1 have their
0.5 keV flux reduced by at least 20%. We discuss the observability of such
absorption and find that < 2 arcsecond resolution, and > 300 photons per
angular resolution element are required in the 0.3-8 keV band for its
detection, and in order to distinguish it from intrinsic surface brightness
fluctuations. For the surface brightness of the currently known high-redshift
extended X-ray sources, this requires an integration time of a few Msec on
Chandra. The detection will be within the reach of a routine observation with a
next generation X-ray telescope such as XEUS or Generation X. | 0409516v2 |
2004-10-11 | Is the solar corona nonmodally self-heated? | Recently it was pointed out that nonmodally (transiently and/or
adiabatically) pre-amplified waves in shear flows, undergoing subsequent
viscous damping, can ultimately heat the ambient flow. The key ingredient of
this process is the ability of waves to grow, by extracting energy from the
spatially inhomogeneous mean flow. In this paper we examine this mechanism in
the context of the solar coronal plasma flows. "Self-heating" (SH) processes
are examined when both viscous damping and magnetic resistivity are at work. We
show that if the plasma viscosity is in the favorable range of values the
asymptotic SH rate in these flows can be quite substantial. | 0410279v1 |
2004-10-23 | Extended Neutral Gas Around z ~ 0.5 Galaxies: Properties of Damped Lya Absorbing Galaxies | I review current results from searching for galaxies giving rise to damped
Lya absorbers (DLAs) at z<1. Using 14 confirmed DLA galaxies, I further show
that intermediate-redshift galaxies possess large HI envelope out to 24-30
h^{-1} kpc radius. The photometric and spectral properties of these galaxies
confirm that DLA galaxies are drawn from the typical field population, and not
from a separate population of low surface brightness or dwarf galaxies.
Comparisons of the ISM abundances of the DLA galaxies and the metallicities of
the absorbers at large galactic radii suggest that some DLAs originate in the
relatively unevolved outskirts of galactic disks. | 0410558v1 |
2004-11-25 | Survey for Galaxies Associated with z~3 Damped Lyman alpha Systems I: Spectroscopic Calibration of u'BVRI Photometric Selection | We present a survey for z~3 Lyman break galaxies (LBGs) associated with
damped Lyman alpha systems (DLAs) with the primary purpose of determining the
DLA-LBG cross-correlation. This paper describes the acquisition and analysis of
imaging and spectroscopic data of 9 quasar fields having 11 known z~3 DLAs
covering an area of 465 arcmin^2. Using deep u'BVRI images, 796 LBG candidates
to an apparent R_AB magnitude of 25.5 were photometrically selected from 17,343
sources detected in the field. Spectroscopic observations of 529 LBG candidates
using Keck LRIS yielded 339 redshifts. We have conservatively identified 211
z>2 objects with <z>=3.02+/-0.32. We discuss our method of z~3 LBG
identification and present a model of the u'BVRI photometric selection
function. We use the 339 spectra to evaluate our u'BVRI z~3 Lyman break
photometric selection technique. | 0411681v1 |
2005-01-06 | Observations of Solar Flare Doppler Shift Oscillations with the Bragg Crystal Spectrometer on Yohkoh | Oscillations in solar coronal loops appear to be a common phenomenon.
Transverse and longitudinal oscillations have been observed with both the
Transition Region and Coronal Explorer and Extreme Ultraviolet Imaging
Telescope imaging experiments. Damped Doppler shift oscillations have been
observed in emission lines from ions formed at flare temperatures with the
Solar Ultraviolet Measurements of Emitted Radiation Spectrometer. These
observations provide valuable diagnostic information on coronal conditions and
may help refine our understanding of coronal heating mechanisms. I have
initiated a study of the time dependence of Doppler shifts measured during
flares with the Bragg Crystal Spectrometer (BCS) on Yohkoh. This Letter reports
the detection of oscillatory behavior in Doppler shifts measured as a function
of time in the emission lines of S XV and Ca XIX. For some flares, both lines
exhibit damped Doppler shift oscillations with amplitudes of a few km/s and
periods and decay times of a few minutes. The observations appear to be
consistent with transverse oscillations. Because the BCS observed continuously
for almost an entire solar cycle, it provides numerous flare data sets, which
should permit an excellent characterization of the average properties of the
oscillations. | 0501093v1 |
2005-03-09 | Abundances in Damped Ly-alpha Galaxies | Damped Ly_alpha galaxies provide a sample of young galaxies where chemical
abundances can be derived throughout the whole universe with an accuracy
comparable to that for the local universe. Despite a large spread in redshift,
HI column density and metallicity, DLA galaxies show a remarkable uniformity in
the elemental ratios rather suggestive of similar chemical evolution if not of
an unique population. These galaxies are characterized by a moderate, if any,
enhancement of alpha-elements over Fe-peak elemental abundance with [S/Zn]
about 0 and [O/Zn] about 0.2, rather similarly to the dwarfs galaxies in the
Local Group. Nitrogen shows a peculiar behaviour with a bimodal distribution
and possibly two plateaux. In particular, the plateau at low N abundances
([N/H] < -3), is not observed in other atrophysical sites and might be evidence
for primary N production by massive stars. | 0503214v1 |
2005-03-10 | Pulsar: repeatable Lagrangian singularity | In general, the interior of radially symmetric self-gravitating sphere is
considered in terms of hydrostatic equilibrium (HSE). This approach implies the
possibility of the static being of a body. Such a static state is assumed to be
the result of asymptotic damping of the process of formation. It is shown here
that the damping of this process is impossible: if a sphere vibrates radially,
then compressional wave is singular at the centre; dynamical singularity has no
intermediate stages of the fading; the HSE-state is unachievable.
Self-gravitating sphere perpetually vibrates in essentially singular way, it
contains dynamical central region -- pulsatile Lagrangian cavity. Theoretical
properties of this cavity indicate that this is a pulsar. A pulsar is common
structural feature for every self-gravitating structure. | 0503231v2 |
2005-03-14 | Quasi-Periodic Oscillations in Relativistic Tori | Motivated by recent interesting work on p-mode oscillations in axisymmetric
hydrodynamic black-hole tori by Rezzolla, Zanotti, and collaborators, I explore
the robustness of these oscillations by means of two and three-dimensional
relativistic hydrodynamic and MHD simulations. The primary purpose of this
investigation is to determine how the amplitudes of these oscillations are
affected by the presence of known instabilities of black-hole tori, including
the Papaloizou-Pringle instability (PPI) and the magneto-rotational instability
(MRI). Both instabilities drive accretion at rates above those considered in
Rezzolla's work. The increased accretion can allow wave energy to leak out of
the torus into the hole. Furthermore, with the MRI, the presence of turbulence,
which is absent in the hydrodynamic simulations, can lead to turbulent damping
(or excitation) of modes. The current numerical results are preliminary, but
suggest that the PPI and MRI both significantly damp acoustic oscillations in
tori. | 0503305v1 |
2005-03-17 | The first observed stellar X-ray flare oscillation: Constraints on the flare loop length and the magnetic field | We present the first X-ray observation of an oscillation during a stellar
flare. The flare occurred on the active M-type dwarf AT Mic and was observed
with XMM-Newton. The soft X-ray light curve (0.2-12 keV) is investigated with
wavelet analysis. The flare's extended, flat peak shows clear evidence for a
damped oscillation with a period of around 750 s, an exponential damping time
of around 2000 s, and an initial, relative peak-to-peak amplitude of around
15%. We suggest that the oscillation is a standing magneto-acoustic wave tied
to the flare loop, and find that the most likely interpretation is a
longitudinal, slow-mode wave, with a resulting loop length of (2.5 +- 0.2) e10
cm. The local magnetic field strength is found to be (105 +- 50) G. These
values are consistent with (oscillation-independent) flare cooling time models
and pressure balance scaling laws. Such a flare oscillation provides an
excellent opportunity to obtain coronal properties like the size of a flare
loop or the local magnetic field strength for the otherwise
spatially-unresolved star. | 0503384v1 |
2005-04-25 | Emission-line spectroscopy of Damped Lyman Alpha Systems: The case of SBS 1543+593/HS 1543+5921 | We report HST/STIS spectroscopy and Gemini/GMOS-N imaging of the Damped Lyman
Alpha (DLA) system toward HS 1543+5921 caused by the host star-forming galaxy
(SFG) SBS 1543+593. The Gemini image shows new morphological details of this
well resolved DLA galaxy. In combination with previous optical spectra, the new
UV spectra enable us to compare for the first time, ionized and neutral
gas-phase alpha-element abundances derived from emission- and absorption-line
spectroscopy, in a bona fide DLA galaxy. The abundances we determine using
emission-line diagnostics agree with those from absorption-line diagnostics. We
present our results on a metallicity versus redshift diagram that combines
local HII regions and SFGs with high-redshift DLAs, and discuss implications
for the chemical evolution of galaxies. | 0504556v1 |
2005-04-27 | Self-consistent radiative effect on relativistic electromagnetic particle acceleration | We study the radiation damping effect on the relativistic acceleration of
electron-positron plasmas with two-and-half-dimensional particle-in-cell (PIC)
simulation. Particles are accelerated by Poynting flux via the diamagnetic
relativistic pulse accelerator (DRPA), and decelerated by the self-consistently
solved radiation damping force. With $\Omega_{ce}/\omega_{pe}\geq 10$, the
Lorentz factor of the highest energy particles reaches gamma>100, and the
acceleration still continues. The emitted radiation is peaked within few
degrees from the direction of Poynting flux and strongly linearly polarized,
which may be detectable in gamma-ray burst(GRB) observations. We also show that
the DRPA is insensitive to the initial supporting currents. | 0504612v2 |
2005-07-13 | UHE neutrino damping in a thermal gas of relic neutrinos | We present a calculation of the damping of an ultra-energetic (UHE) cosmic
neutrino travelling through the thermal gas of relic neutrinos, using the
formalism of finite-temperature field theory. From the self-energy diagram due
to Z exchange, we obtain the annihilation cross section for an UHE neutrino
interacting with an antineutrino from the background. This method allows us to
derive the full expressions for the UHE neutrino transmission probability,
taking into account the momentum of relic neutrinos. We compare our results
with the approximations in use in the literature. We discuss the effect of
thermal motion on the shape of the absorption dips for different UHE neutrino
fluxes as well as in the context of relic neutrino clustering. We find that for
ratios of the neutrino mass to the relic background temperature $10^2$ or
smaller, the thermal broadening of the absorption lines could significantly
affect the determination of the neutrino mass and of the characteristics of the
population of UHE sources. | 0507333v2 |
2005-07-25 | Post-inflation increase of the cosmological tensor-to-scalar perturbation ratio | We investigate the possibility that the amplitude of scalar density
perturbations may be damped after inflation. This would imply that CMB
anisotropies do not uniquely fix the amplitude of the perturbations generated
during inflation and that the present tensor-to-scalar ratio might be larger
than produced in inflation, increasing the prospects of detection of primordial
gravitational radiation. It turns out, however, that the damping of density
perturbations is hard to achieve. | 0507573v3 |
2005-09-08 | Possible Detection of Lyman-alpha Fluorescence from a Damped Lyman Alpha system at Redshift z=2.8 | We have detected Lyman-alpha emission from a damped Lyman-alpha system (DLA)
that lies near the bright quasar HS1549+1919. The DLA has the same redshift as
HS1549+1919 and was discovered in the spectrum of a faint QSO that lies 49"
away (380 proper kpc). The emission line's luminosity, double-peaked profile,
and small spatial separation from the DLA suggest that it may be fluorescent
Lyman-alpha emission from gas that is absorbing the nearby QSO's radiation. If
this is the case, our observations show that the DLA has a size of at least
1.5" and that the QSO's luminosity one million years ago was similar to its
luminosity today. A survey for similar systems within 1' of bright QSOs would
put interesting limits on the mean quasar lifetime. | 0509229v1 |
2005-11-17 | Measurement of the Spatial Cross-Correlation Function of Damped Lyman Alpha Systems and Lyman Break Galaxies | We present the first spectroscopic measurement of the spatial
cross-correlation function between damped Lyman alpha systems (DLAs) and Lyman
break galaxies (LBGs). We obtained deep u'BVRI images of nine QSO fields with
11 known z ~ 3 DLAs and spectroscopically confirmed 211 R < 25.5
photometrically selected z > 2 LBGs. We find strong evidence for an overdensity
of LBGs near DLAs versus random, the results of which are similar to that of
LBGs near other LBGs. A maximum likelihood cross-correlation analysis found the
best fit correlation length value of r_0 = 2.9^(+1.4)_(-1.5) h^(-1)Mpc using a
fixed value of gamma = 1.6. The implications of the DLA-LBG clustering
amplitude on the average dark matter halo mass of DLAs are discussed. | 0511509v1 |
2006-01-29 | Relative abundance pattern along the profile of high redshift Damped Lyman-alpha systems | We investigated abundance ratios along the profiles of six high-redshift
Damped Lyman-alpha systems, three of them associated with H2 absorption, and
derived optical depths in each velocity pixel. The variations of the pixel
abundance ratios were found to be remarkably small and usually smaller than a
factor of two within a profile. This result holds even when considering
independent sub-clumps in the same system. The depletion factor is
significantly enhanced only in those components where H2 is detected. There is
a strong correlation between [Fe/S] and [Si/S] abundances ratios, showing that
the abundance ratio patterns are definitely related to the presence of dust.
The depletion pattern is usually close to the one seen in the warm halo gas of
our Galaxy. | 0601664v1 |
2006-02-14 | VLT/UVES constraints on the carbon isotope ratio 12C/13C at z=1.15 toward the quasar HE 0515-4414 | We analyzed the CI lines associated with the damped Ly-alpha system observed
at zabs = 1.15 in the spectrum of HE 0515-4414 to derive the 12C/13C ratio. The
spectrum was obtained by means of the UV-Visual Echelle Spectrograph (UVES) at
the ESO Very Large Telescope (VLT). The obtained lower limit 12C/13C > 80
(2sigma C.L.) shows for the first time that the abundance of 13C in the
extragalactic intervening clouds is very low. This rules out a significant
contribution from intermediate-mass stars to the chemical evolution of matter
sampled by this line of sight. The estimated low amount of 13C is in agreement
with low abundances of nitrogen observed in damped Ly-alpha systems - the
element produced in the same nuclear cycles and from about the same stars as
13C. | 0602303v1 |
2006-06-08 | The detectability of HI 21-cm absorption in damped Lyman-alpha systems | In this paper we investigate the possible reasons why HI 21-cm absorption in
damped Lyman-alpha systems (DLAs) has only been detected at low redshift... We
suggest that the lack of 21-cm absorption detections at high redshift arises
from the fact that these DLAs are at similar angular diameter distances to the
background quasars (i.e. the distance ratios are always close to unity): Above
z~1.6 the covering factor becomes largely independent of the DLA--QSO distance,
making the high redshift absorbers much less effective at covering the
background continuum emission. At low redshift, small distance ratios are
strongly favoured by the 21-cm detections, whereas large ratios are favoured by
the non-detections. This mix of distance ratios gives the observed mix of
detections and non-detections at z<1.6.In addition to the predominance of large
distance ratios and non-detections at high redshift, this strongly suggests
that the observed distribution of 21-cm absorption in DLAs is dominated by
geometric effects. | 0606180v1 |
2006-07-15 | Damping and excitation variations of the solar acoustic modes using LOWL observations | We have used observations made with the helioseismic instrument LOWL
collected over $\sim$ 6 years to carry out an independent study of the
variations of the p-mode damping and excitation rates with solar activity. We
observe significant variations in the mode height, mode width and mode velocity
power over a wide range of angular degree values. Their sensitivities to solar
activity show clear evidence of frequency dependence, the modes in the
frequency range from 2700 and 3300 $\mu$Hz showing the largest variations and
exhibiting a maximum change centered around 3100 $\mu$Hz. As for the mode
energy supply rate, it is consistent, at the level of precision of the
observations, with a zero change along the solar cycle and over the range of
studied frequencies. Moreover, the variations with solar activity of each of
these parameters are observed to be more or less $\ell$-independent over the
range of studied angular degrees. Our results provide the first in-depth
confirmation of the findings obtained from GONG measurements for intermediate
angular degrees. | 0607346v1 |
2006-09-01 | A line profile analysis of the pulsating red giant star epsilon Ophiuchi (G9.5III) | So far, solar-like oscillations have been studied using radial velocity
and/or light curve variations, which reveal frequencies of the oscillation
modes. Line-profile variations, however, are also a valuable diagnostic to
characterise radial and non-radial oscillations, including frequencies,
amplitudes, the spherical mode wavenumbers (l,m) and the stellar inclination
angle. Here we present a line profile analysis of epsilon Ophiuchi, which is a
pulsating red giant. The main differences compared to previous line profile
analyses done for heat-driven oscillations are the small amplitudes and the
predicted short damping and re-excitation times in red giants.
Two line diagnostics have been tested to see whether these are sensitive to
the small line profile variations present in red giants. In addition, line
profiles have been simulated with short damping and re-excitation times and are
compared with the observations. This comparison reveals that non-radial modes
are detected in the observed line profile variations of epsilon Ophiuchi. This
is rather surprising, as theoretical predictions favours the occurrence of
radial modes. | 0609043v1 |
2006-10-11 | Determining the neutron star equation of state using the narrow-band gravitational wave detector Schenberg | We briefly review the properties of quasi-normal modes of neutron stars and
black holes. We analyse the consequences of a possible detection of such modes
via the gravitational waves associated with them, especially addressing our
study to the Brazilian spherical antenna, on which a possible detection would
occur at 3.0-3.4 kHz. A question related to any putative gravitational wave
detection concerns the source that produces it. We argue that, since the
characteristic damping times for the gravitational waves of neutron stars and
black holes are different, a detection can distinguish between them, and also
distinguish the neutron stars oscillating modes. Moreover, since the source can
be identified by its characteristic damping time, we are able to extract
information about the neutron star or black hole. This information would lead,
for example, to a strong constraint in the nuclear matter equation of state,
namely the compression modulus should be K=220 MeV. | 0610335v1 |
2006-12-14 | Three dimensional numerical simulations of acoustic wave field in the upper convection zone of the Sun | Results of numerical 3D simulations of propagation of acoustic waves inside
the Sun are presented. A linear 3D code which utilizes realistic OPAL equation
of state was developed by authors. Modified convectively stable standard solar
model with smoothly joined chromosphere was used as a background model. High
order dispersion relation preserving numerical scheme was used to calculate
spatial derivatives. The top non-reflecting boundary condition established in
the chromosphere absorbs waves with frequencies greater than the acoustic
cut-off frequency which pass to the chromosphere, simulating a realistic
situation. The acoustic power spectra obtained from the wave field generated by
sources randomly distributed below the photosphere are in good agreement with
observations. The influence of the height of the top boundary on results of
simulation was studied. It was shown that the energy leakage through the
acoustic potential barrier damps all modes uniformly and does not change the
shape of the acoustic spectrum. So the height of the top boundary can be used
for controlling a damping rate without distortion of the acoustic spectrum. The
developed simulations provide an important tool for testing local
helioseismology. | 0612364v1 |
2006-12-15 | Damp Mergers: Recent Gaseous Mergers without Significant Globular Cluster Formation? | Here we test the idea that new globular clusters (GCs) are formed in the same
gaseous ("wet") mergers or interactions that give rise to the young stellar
populations seen in the central regions of many early-type galaxies. We compare
mean GC colors with the age of the central galaxy starburst. The red GC
subpopulation reveals remarkably constant mean colors independent of galaxy
age. A scenario in which the red GC subpopulation is a combination of old and
new GCs (formed in the same event as the central galaxy starburst) can not be
ruled out; although this would require an age-metallicity relation for the
newly formed GCs that is steeper than the Galactic relation. However, the data
are also well described by a scenario in which most red GCs are old, and few,
if any, are formed in recent gaseous mergers. This is consistent with the old
ages inferred from some spectroscopic studies of GCs in external systems. The
event that induced the central galaxy starburst may have therefore involved
insufficient gas mass for significant GC formation. We term such gas-poor
events "damp" mergers. | 0612415v1 |
2006-12-21 | Accretion of Terrestrial Planets from Oligarchs in a Turbulent Disk | We have investigated the final accretion stage of terrestrial planets from
Mars-mass protoplanets that formed through oligarchic growth in a disk
comparable to the minimum mass solar nebula (MMSN), through N-body simulation
including random torques exerted by disk turbulence due to
Magneto-Rotational-Instability. For the torques, we used the semi-analytical
formula developed by Laughlin et al.(2004). The damping of orbital
eccentricities (in all runs) and type-I migration (in some runs) due to the
tidal interactions with disk gas are also included. We found that the orbital
eccentricities pumped up by the turbulent torques and associated random walks
in semimajor axes tend to delay isolation of planets, resulting in more
coagulation of planets than in the case without turbulence. The eccentricities
are still damped after planets become isolated. As a result, the number of
final planets decreases with increase in strength of the turbulence, while
Earth-mass planets with small eccentricities are still formed. In the case of
relatively strong turbulence, the number of final planets are 4-5 at 0.5-2AU,
which is consistent with Solar system, for relatively wide range of disk
surface density (~10^{-4}-10^{-2} times MMSN). | 0612619v1 |
2007-01-18 | Models of the Collisional Damping Scenario for Ice Giant Planets and Kuiper Belt Formatio | Chiang et al. 2006, hereafter C06 have recently proposed that the observed
structure of the Kuiper belt could be the result of a dynamical instability of
a system of ~5 primordial ice giant planets in the outer Solar System.
According to this scenario, before the instability occurred, these giants were
growing in a highly collisionally damped environment according to the arguments
in Goldreich et al. (2004a,b, hereafter G04). Here we test this hypothesis with
a series of numerical simulations using a new code designed to incorporate the
dynamical effects of collisions. We find that we cannot reproduce the observed
Solar System. In particular, G04 and C06 argue that during the instability, all
but two of the ice giants would be ejected from the Solar System by Jupiter and
Saturn, leaving Uranus and Neptune behind. We find that ejections are actually
rare and that instead the systems spread outward. This always leads to a
configuration with too many planets that are too far from the Sun. Thus, we
conclude that both G04's scheme for the formation of Uranus and Neptune and
C06's Kuiper belt formation scenario are not viable in their current forms. | 0701544v1 |
2007-02-05 | Ion Charge States in the Fast Solar Wind: New Data Analysis and Theoretical Refinements | We present a further investigation into the increased ionization observed in
element charge states in the fast solar wind compared to its coronal hole
source regions. Once ions begin to be perpendicularly heated by ion cyclotron
waves and execute large gyro-orbits, density gradients in the flow can excite
lower hybrid waves that then damp by heating electrons in the parallel
direction. We give further analysis of charge state data from polar coronal
holes at solar minimum and maximum, and also from equatorial coronal holes. We
also consider further the damping of lower hybrid waves by ions and the effect
of non-Maxwellian electron distribution functions on the degree of increased
ionization, both of which appear to be negligible for the solar wind case
considered here. We also suggest that the density gradients required to heat
electrons sufficiently to further ionize the solar wind can plausibly result
from the turbulent cascade of MHD waves. | 0702131v1 |
1995-10-11 | Multiple Transitions to Chaos in a Damped Parametrically Forced Pendulum | We study bifurcations associated with stability of the lowest stationary
point (SP) of a damped parametrically forced pendulum by varying $\omega_0$
(the natural frequency of the pendulum) and $A$ (the amplitude of the external
driving force). As $A$ is increased, the SP will restabilize after its
instability, destabilize again, and so {\it ad infinitum} for any given
$\omega_0$. Its destabilizations (restabilizations) occur via alternating
supercritical (subcritical) period-doubling bifurcations (PDB's) and pitchfork
bifurcations, except the first destabilization at which a supercritical or
subcritical bifurcation takes place depending on the value of $\omega_0$. For
each case of the supercritical destabilizations, an infinite sequence of PDB's
follows and leads to chaos. Consequently, an infinite series of period-doubling
transitions to chaos appears with increasing $A$. The critical behaviors at the
transition points are also discussed. | 9510003v1 |
1996-03-04 | Period Doublings in Coupled Parametrically Forced Damped Pendulums | We study period doublings in $N$ $(N=2,3,4, \dots)$ coupled parametrically
forced damped pendulums by varying $A$ (the amplitude of the external driving
force) and $c$ (the strength of coupling). With increasing $A$, the stationary
point undergoes multiple period-doubling transitions to chaos. We first
investigate the two-coupled case with $N=2$. For each period-doubling
transition to chaos, the critical set consists of an infinity of critical line
segments and the zero-coupling critical point lying on the line $A=A^*_i$ in
the $A-c$ plane, where $A^*_i$ is the $i$th transition point for the uncoupled
case. We find three kinds of critical behaviors, depending on the position on
the critical set. They are the same as those for the coupled one-dimensional
maps. Finally, the results of the $N=2$ case are extended to many-coupled cases
with $N \geq 3$, in which the critical behaviors depend on the range of
coupling. | 9603002v1 |
1996-12-05 | Fractal Basins of Attraction Associated with a Damped Newton's Method | An intriguing and unexpected result for students learning numerical analysis
is that Newton's method, applied to the simple polynomial z^3 - 1 = 0 in the
complex plane, leads to intricately interwoven basins of attraction of the
roots. As an example of an interesting open question that may help to stimulate
student interest in numerical analysis, we investigate the question of whether
a damping method, which is designed to increase the likelihood of convergence
for Newton's method, modifies the fractal structure of the basin boundaries.
The overlap of the frontiers of numerical analysis and nonlinear dynamics
provides many other problems that can help to make numerical analysis courses
interesting. | 9612010v1 |
1999-01-28 | Intrinsically localized chaos in discrete nonlinear extended systems | The phenomenon of intrinsic localization in discrete nonlinear extended
systems, i.e. the (generic) existence of discrete breathers, is shown to be not
restricted to periodic solutions but it also extends to more complex (chaotic)
dynamical behaviour. We illustrate this with two different forced and damped
systems exhibiting this type of solutions: In an anisotropic Josephson junction
ladder, we obtain intrinsically localized chaotic solutions by following
periodic rotobreather solutions through a cascade of period-doubling
bifurcations. In an array of forced and damped van der Pol oscillators, they
are obtained by numerical continuation (path-following) methods from the
uncoupled limit, where its existence is trivially ascertained, following the
ideas of the anticontinuum limit. | 9901030v1 |
1995-03-27 | Dynamics of a Strongly Damped Two-Level System: Beyond the DBGA | Dynamics of a dissipative two-level system is studied using quantum
relaxation theory. This calculation for the first time goes beyond the commonly
used dilute bounce gas approximation (DBGA), even for strong damping. The new
results obtained here deviate from the DBGA results at low temperatures,
however, the DBGA form is recovered at high temperatures. The results in the
parameter regime $ 1/2<\alpha <1$, where the model has connection with the
Kondo Hamiltonian, are of particular significance. In this regime, the spin
shows a cross-over to a slower exponential relaxation at intermediate times,
which is roughly half the relaxation rate at short times, as also observed in
Quantum Monte-Carlo simulation of the model. The asymptotic behavior of the
spin in the Kondo regime is in agreement with the exact conformal field theory
results for the Kondo model. A connection of the dissipative dynamics of the
two-level system with the quantum Zeno effect is also presented. | 9503133v1 |
1995-05-23 | The phase-dependent linear conductance of a superconducting quantum point contact | The exact expression for the phase-dependent linear conductance of a weakly
damped superconducting quantum point contact is obtained. The calculation is
performed by summing up the complete perturbative series in the coupling
between the electrodes. The failure of any finite order perturbative expansion
in the limit of small voltage and small quasi-particle damping is analyzed in
detail. In the low transmission regime this nonperturbative calculation yields
a result which is at variance with standard tunnel theory. Our result predicts
the correct sign of the quasi-particle pair interference term and exhibits an
unusual phase-dependence at low temperatures in qualitative agreement with the
available experimental data. | 9505102v1 |
1996-02-27 | Quasiparticle properties of a coupled quantum wire electron-phonon system | We study leading-order many-body effects of longitudinal optical (LO) phonons
on electronic properties of one-dimensional quantum wire systems. We calculate
the quasiparticle properties of a weakly polar one dimensional electron gas in
the presence of both electron-phonon and electron-electron interactions. The
leading-order dynamical screening approximation (GW approximation) is used to
obtain the electron self-energy, the quasiparticle spectral function, and the
quasiparticle damping rate in our calculation by treating electrons and phonons
on an equal footing. Our theory includes effects (within the random phase
approximation) of Fermi statistics, Landau damping, plasmon-phonon mode
coupling, phonon renormalization, dynamical screening, and impurity scattering.
In general, electron-electron and electron-phonon many-body renormalization
effects are found to be nonmultiplicative and nonadditive in our theoretical
results for quasiparticle properties. | 9602143v1 |
1996-02-29 | Dynamical response of a one dimensional quantum wire electron system | We provide a self-contained theoretical analysis of the dynamical response of
a one dimensional electron system, as confined in a semiconductor quantum wire,
within the random phase approximation. We carry out a detailed comparison with
the corresponding two and three dimensional situations, and discuss the
peculiarities arising in the one dimensional linear response from the
non-existence of low energy single-particle excitations and from the linear
nature of the long wavelength plasmon mode. We provide a critical discussion of
the analytic properties of the complex dielectric function in the complex
frequency plane. We investigate the zeros of the complex dielectric function,
and calculate the plasmon dispersion, damping, and plasmon spectral weight in
one dimension. We consider finite temperature and impurity scattering effects
on one dimensional plasmon dispersion and damping. | 9602157v1 |
1996-08-24 | New Universality Class at the Superconductor--Insulator Transition | We study dynamic properties of thin films near the superconductor - insulator
transition. We formulate the problem in a phase representation. The key new
feature of our model is the assumption of a {\it local} ohmic dissipative
mechanism. Coarse graining leads to a Ginzburg-Landau description, with
non-ohmic dynamics for the order parameter. For strong enough damping a new
universality class is observed. It is characterized by a {\it non-universal}
d.c. conductivity, and a damping dependent dynamical critical exponent. The
formulation also provides a description of the magnetic field-tuned transition.
Several microscopic mechanisms are proposed as the origin of the dissipation. | 9608115v1 |
1996-10-07 | Supersymmetric Fokker-Planck strict isospectrality | I report a study of the nonstationary one-dimensional Fokker-Planck solutions
by means of the strictly isospectral method of supesymmetric quantum mechanics.
The main conclusion is that this technique can lead to a space-dependent
(modulational) damping of the spatial part of the nonstationary Fokker-Planck
solutions, which I call strictly isospectral damping. At the same time, using
an additive decomposition of the nonstationary solutions suggested by the
strictly isospectral procedure and by an argument of Englefield [J. Stat. Phys.
52, 369 (1988)], they can be normalized and thus turned into physical
solutions, i.e., Fokker-Planck probability densities. There might be
applications to many physical processes during their transient period | 9610049v2 |
1997-04-03 | Quasiparticle Many-Body Dynamics of Highly Correlated Electronic Systems | The self-consistent theory of the correlation effects in Highly Correlated
Systems(HCS) is presented. The novel Irreducible Green's Functions(IGF) method
is discused in detail for the Hubbard model and random Hubbard model. The
interpolative solution for the quasiparticle spectrum, which is valid for both
the atomic and band limit is obtained. The (IGF) method permits to calculate
the quasiparticle spectra of many-particle systems with the complicated spectra
and strong interaction in a very natural and compact way. The inelastic
scattering corrections leads to the damping of the quasiparticles and are the
main topic of the present consideration. The calculation of the damping has
been done in a self-consistent way for both limits. For the random Hubbard
model the weak coupling case has been considered and the self-energy operator
has been calculated using the combination of the IGF method and Coherent
Potential Approximation (CPA). The other applications of the method to s-f
model, Anderson model, Heisenberg antiferromagnet, electron-phonon interaction
models are discussed briefly. | 9704028v1 |
1997-12-17 | Detecting flux creep in superconducting YBCO thin films via damping of the oscillations of a levitating permanent magnet | The damping of the oscillations of a small permanent magnet (spherical shape,
radius 0.1 mm) levitating between two parallel epitaxial YBCO films is measured
as a function of oscillation amplitude and temperature. At small amplitudes the
dissipation is found to be orders of magnitude lower than in bulk YBCO,
Q-factors exceeding one million at low temperatures. With increasing amplitude
the dissipation becomes exponentially large, exceeding the bulk values at large
drives. We describe our results by calculating the ac shielding currents
flowing through trapped flux whose motion gives rise to electric fields. We
find dissipation to originate from different mechanisms of flux dynamics. | 9712199v1 |
1998-02-06 | Spin polaron damping in the spin-fermion model for cuprate superconductors | A self-consistent, spin rotational invariant Green's function procedure has
been developed to calculate the spectral function of carrier excitations in the
spin-fermion model for the CuO2 plane. We start from the mean field description
of a spin polaron in the Mori-Zwanzig projection method. In order to determine
the spin polaron lifetime in the self-consistent Born approximation, the
self-energy is expressed by an irreducible Green's function. Both, spin polaron
and bare hole spectral functions are calculated. The numerical results show a
well pronounced quasiparticle peak near the bottom of the dispersion at
(pi/2,pi/2), the absence of the quasiparticle at the Gamma-point, a rather
large damping away from the minimum and an asymmetry of the spectral function
with respect to the antiferromagnetic Brillouin zone. These findings are in
qualitative agreement with photoemission data for undoped cuprates. The direct
oxygen-oxygen hopping is responsible for a more isotropic minimum at
(pi/2,pi/2). | 9802074v1 |
1998-02-24 | Resonant steps and spatiotemporal dynamics in the damped dc-driven Frenkel-Kontorova chain | Kink dynamics of the damped Frenkel-Kontorova (discrete sine-Gordon) chain
driven by a constant external force are investigated. Resonant steplike
transitions of the average velocity occur due to the competitions between the
moving kinks and their radiated phasonlike modes. A mean-field consideration is
introduced to give a precise prediction of the resonant steps. Slip-stick
motion and spatiotemporal dynamics on those resonant steps are discussed. Our
results can be applied to studies of the fluxon dynamics of 1D
Josephson-junction arrays and ladders, dislocations, tribology and other
fields. | 9802251v1 |
1998-05-09 | The resonance peak in cuprate superconductors | We pursue the consequences of a theory in which the resonance peak observed
in inelastic neutron scattering (INS) experiments on underdoped and optimally
doped YBa$_2$Cu$_3$O$_{6+x}$ compounds arises from a spin-wave excitation. We
find that it is heavily damped, and thus almost not observable, in the normal
state, but becomes visible in the superconducting state due to the drastic
decrease in spin damping. We show that a spin-fermion model correctly describes
the temperature dependence of the peak position for YBa$_2$Cu$_3$O$_7$, as well
as the doping dependence of the peak position and of the integrated intensity.
We explain why no resonance peak has been observed in La$_{2-x}$Sr$_x$CuO$_4$,
and make several predictions concerning resonance peaks in other cuprate
superconductors. | 9805107v1 |
1998-05-22 | On the driven Frenkel-Kontorova model: I. Uniform sliding states and dynamical domains of different particle densities | The dynamical behavior of a harmonic chain in a spatially periodic potential
(Frenkel-Kontorova model, discrete sine-Gordon equation) under the influence of
an external force and a velocity proportional damping is investigated. We do
this at zero temperature for long chains in a regime where inertia and damping
as well as the nearest-neighbor interaction and the potential are of the same
order. There are two types of regular sliding states: Uniform sliding states,
which are periodic solutions where all particles perform the same motion
shifted in time, and nonuniform sliding states, which are quasi-periodic
solutions where the system forms patterns of domains of different uniform
sliding states. We discuss the properties of this kind of pattern formation and
derive equations of motion for the slowly varying average particle density and
velocity. To observe these dynamical domains we suggest experiments with a
discrete ring of at least fifty Josephson junctions. | 9805287v1 |
1999-01-29 | Acoustic radiation controls friction: Evidence from a spring-block experiment | Brittle failures of materials and earthquakes generate acoustic/seismic waves
which lead to radiation damping feedbacks that should be introduced in the
dynamical equations of crack motion. We present direct experimental evidence of
the importance of this feedback on the acoustic noise spectrum of
well-controlled spring-block sliding experiments performed on a variety of
smooth surfaces. The full noise spectrum is quantitatively explained by a
simple noisy harmonic oscillator equation with a radiation damping force
proportional to the derivative of the acceleration, added to a standard viscous
term. | 9901350v2 |
1999-03-02 | d_{x^2-y^2}-Wave Pairing Fluctuations and Pseudo Spin Gap in Two-Dimensional Electron Systems | Pseudogap phenomena of high-T_c cuprates are examined. In terms of AFM
(antiferromagnetic) and dSC (d_{x^2-y^2}-wave superconducting) auxiliary fields
introduced to integrate out the fermions, the effective action for 2D electron
systems with AFM and dSC fluctuations is considered. By the self-consistent
renormalization (SCR), the NMR relaxation rate T_1^{-1}, the spin correlation
length \xi_\sigma and the pairing correlation length \xi_d are calculated. From
this calculation, a mechanism of the pseudogap formation emerges as the region
of dominant d-wave short-range order (SRO) over AFM-SRO. When damping for the
AFM fluctuation strongly depends on the dSC correlation length through the
formation of precursor singlets around (\pi,0) and (0,\pi) points in the
momentum space, the pseudogap appears in a region of the normal state
characterized by decreasing 1/T_1T and increasing AFM correlation length with
decrease in temperature. This reproduces a characteristic feature of the
pseudogap phenomena in many underdoped cuprates. When the damping becomes
insensitive to the dSC correlation length, the pseudogap region shrinks as in
the overdoped cuprates. | 9903030v2 |
1999-03-11 | Thermally activated escape rates of uniaxial spin systems with transverse field | Classical escape rates of uniaxial spin systems are characterized by a
prefactor differing from and much smaller than that of the particle problem,
since the maximum of the spin energy is attained everywhere on the line of
constant latitude: theta=const, 0 =< phi =< 2*pi. If a transverse field is
applied, a saddle point of the energy is formed, and high, moderate, and low
damping regimes (similar to those for particles) appear. Here we present the
first analytical and numerical study of crossovers between the uniaxial and
other regimes for spin systems. It is shown that there is one HD-Uniaxial
crossover, whereas at low damping the uniaxial and LD regimes are separated by
two crossovers. | 9903192v2 |
1999-04-19 | Numerical analysis of the dissipative two-state system with the density-matrix Hilbert-space-reduction algorithm | Ground state of the dissipative two-state system is investigated by means of
the Lanczos diagonalization method. We adopted the Hilbert-space-reduction
scheme proposed by Zhang, Jeckelmann and White so as to reduce the overwhelming
reservoir Hilbert space to being tractable in computers. Both the
implementation of the algorithm and the precision applied for the present
system are reported in detail. We evaluate the dynamical susceptibility
(resolvent) with the continued-fraction-expansion formula. Through analysing
the resolvent over a frequency range, whose range is often called `interesting'
frequency, we obtain the damping rate and the oscillation frequency. Our
results agree with those of a recent quantum Monte-Carlo study, which concludes
that the critical dissipation from oscillatory to over-damped behavior
decreases as the tunneling amplitude is strengthened. | 9904260v1 |
1999-05-02 | Finite Temperature Time-Dependent Effective Theory For The Goldstone Field In A BCS-Type Superfluid | We extend to finite temperature the time-dependent effective theory for the
Goldstone field (the phase of the pair field) $ \theta $ which is appropriate
for a superfluid containing one species of fermions with s-wave interactions,
described by the BCS Lagrangian. We show that, when Landau damping is
neglected, the effective theory can be written as a local time-dependent
non-linear Schr\"{o}dinger Lagrangian (TDNLSL) which preserves the Galilean
invariance of the zero temperature effective theory and is identified with the
superfluid component. We then calculate the relevant Landau terms which are
non-local and which destroy the Galilean invariance. We show that the retarded
$\theta$-propagator (in momentum space) can be well represented by two poles in
the lower-half frequency plane, describing damping with a predicted
temperature, frequency and momentum dependence. It is argued that the real
parts of the Landau terms can be approximately interpreted as contributing to
the normal fluid component. | 9905008v2 |
1999-06-29 | Simulation of I-V Hysteresis Branches in An Intrinsic Stack of Josephson Junctions in High $T_c$ Superconductors | I-V characteristics of the high T$_c$ superconductor
Bi$_2$Sr$_2$Ca$_1$C$_2$O$_8$ shows a strong hysteresis, producing many
branches. The origin of hysteresis jumps is studied by use of the model of
multi-layered Josephson junctions proposed by one of the authors (T. K.). The
charging effect at superconducting layers produces a coupling between the next
nearest neighbor phase-differences, which determines the structure of
hysteresis branches. It will be shown that a solution of phase motions is
understood as a combination of rotating and oscillating phase-differences, and
that, at points of hysteresis jumps, there occurs a change in the number of
rotating phase-differences. Effects of dissipation are analyzed. The
dissipation in insulating layers works to damp the phase motion itself, while
the dissipation in superconducting layers works to damp relative motions of
phase-differences. Their effects to hysteresis jumps are discussed. | 9906422v1 |
1999-07-05 | The interplay between flattening and damping of single particle spectra in strongly correlated Fermi systems | The self-consistent theory of the fermion condensation, a specific phase
transition which results in a rearrangement of the single particle degrees of
freedom in strongly correlated Fermi systems is developed. Beyond the phase
transition point, the single particle spectra are shown to be flat. The
interplay between the flattening and the damping of the single particle spectra
at $T\to 0$ is investigated. The width $\gamma(\epsilon)$ of the single
particle states is found to grow up linearly with $\epsilon$ over a wide range
of energy as in a marginal Fermi liquid. Our results gain insight into the
success of the phenomenological theory of the normal states of high-temperature
superconductors by Varma et al. | 9907061v1 |
1999-10-19 | Zener transitions between dissipative Bloch bands. II: Current Response at Finite Temperature | We extend, to include the effects of finite temperature, our earlier study of
the interband dynamics of electrons with Markoffian dephasing under the
influence of uniform static electric fields. We use a simple two-band
tight-binding model and study the electric current response as a function of
field strength and the model parameters. In addition to the Esaki-Tsu peak,
near where the Bloch frequency equals the damping rate, we find current peaks
near the Zener resonances, at equally spaced values of the inverse electric
field. These become more prominenent and numerous with increasing bandwidth (in
units of the temperature, with other parameters fixed). As expected, they
broaden with increasing damping (dephasing). | 9910290v1 |
1999-11-02 | Shifts and widths of collective excitations in trapped Bose gases by the dielectric formalism | We present predictions for the temperature dependent shifts and damping
rates. They are obtained by applying the dielectric formalism to a simple model
of a trapped Bose gas. Within the framework of the model we use lowest order
perturbation theory to determine the first order correction to the results of
Hartree-Fock-Bogoliubov-Popov theory for the complex collective excitation
frequencies, and present numerical results for the temperature dependence of
the damping rates and the frequency shifts. Good agreement with the
experimental values measured at JILA are found for the m=2 mode, while we find
disagreements in the shifts for m=0. The latter point to the necessity of a
non-perturbative treatment for an explanation of the temperature-dependence of
the m=0 shifts. | 9911018v1 |
2000-06-13 | Characteristic features of anharmonic effects in the lattice dynamics of fcc metals | The dispersion in the entire Brillouin zone and the temperature dependence
(right up to the melting temperature) of the anharmonic frequency shift and
phonon damping in a number of fcc metals is investigated on the basis of
microscopic calculations. It is found that the anharmonic effects depend
sharply on the wave vector in the directions $\Gamma$-X, X-W, and $\Gamma$-L
and, in contrast to bcc metals, the magnitude of the effects is not due to the
softness of the initial phonon spectrum. It is shown that the relative
frequency shifts and the phonon damping near melting do not exceed 10-20%. The
relative role of various anharmonic processes is examined, and the relation
between the results obtained and existing experimental data is discussed. | 0006205v1 |
2000-06-20 | Collisional damping of the collective oscillations of a trapped Fermi gas | We consider a Fermi gas confined by a harmonic trapping potential and we
highlight the role of the Fermi-Dirac statistics by studying frequency and
damping of collective oscillations of quadrupole type in the framework of the
quantum Boltzmann equation, in which statistical corrections are taken into
account in the collisional integral. We are able to describe the crossover from
the collisionless regime to the hydrodynamic one by introducing a
temperature-dependent relaxation time $\tau_Q$. We show that, in the degenerate
regime, the relaxation rate $1/\tau_Q$ exhibits a temperature dependence
different from the collision rate $\gamma$. We finally compare the collisional
properties of the Fermi gas with the ones of the Bose gas for temperatures
above the Bose-Einstein condensation. | 0006305v2 |
2000-06-21 | Resonances in the dynamics of $φ^4$ kinks perturbed by ac forces | We study the dynamics of $\phi^4$ kinks perturbed by an ac force, both with
and without damping. We address this issue by using a collective coordinate
theory, which allows us to reduce the problem to the dynamics of the kink
center and width. We carry out a careful analysis of the corresponding ordinary
differential equations, of Mathieu type in the undamped case, finding and
characterizing the resonant frequencies and the regions of existence of
resonant solutions. We verify the accuracy of our predictions by numerical
simulation of the full partial differential equation, showing that the
collective coordinate prediction is very accurate. Numerical simulations for
the damped case establish that the strongest resonance is the one at half the
frequency of the internal mode of the kink. In the conclusion we discuss on the
possible relevance of our results for other systems, especially the sine-Gordon
equation. We also obtain additional results regarding the equivalence between
different collective coordinate methods applied to this problem. | 0006313v1 |
2000-07-11 | Quantum phase transitions in d-wave superconductors | Motivated by the strong, low temperature damping of nodal quasiparticles
observed in some cuprate superconductors, we study quantum phase transitions in
d_{x^2-y^2} superconductors with a spin-singlet, zero momentum, fermion
bilinear order parameter. We present a complete, group-theoretic classification
of such transitions into 7 distinct cases (including cases with nematic order)
and analyze fluctuations by the renormalization group. We find that only 2, the
transitions to d_{x^2-y^2}+is and d_{x^2-y^2} + i d_{xy} pairing, possess
stable fixed points with universal damping of nodal quasiparticles; the latter
leaves the gapped quasiparticles along (1,0), (0,1) essentially undamped. | 0007170v3 |
2000-07-12 | Frequencies and Damping rates of a 2D Deformed Trapped Bose gas above the Critical Temperature | We derive the equation of motion for the velocity fluctuations of a 2D
deformed trapped Bose gas above the critical temperature in the hydrodynamical
regime. From this equation, we calculate the eigenfrequencies for a few
low-lying excitation modes. Using the method of averages, we derive a
dispersion relation in a deformed trap that interpolates between the
collisionless and hydrodynamic regimes. We make use of this dispersion relation
to calculate the frequencies and the damping rates for monopole and quadrupole
mode in both the regimes. We also discuss the time evolution of the wave packet
width of a Bose gas in a time dependent as well as time independent trap. | 0007210v2 |
2000-09-01 | The Broad Brillouin Doublet and CP of Ktao_3 : Second Sound vs. Two-Phonon Difference Scattering | Low-T Brillouin spectra of the incipient ferroelectric KTaO3 exhibit a broad
central peak (CP), and additional Brillouin doublets (BD), that can both be
related to phonon-density fluctuations. On the basis of new high-resolution
neutron data obtained of low-lying phonon branches, we analysed the
phonon-kinetics mechanisms that are possibly the origin of these unusual
features. Firstly, transverse acoustic (TA) phonons whose normal damping is
faster than the BD frequency can produce hydrodynamic second sound. Secondly,
two-phonon difference scattering from low damping thermal transverse phonons
contribute to the spectra with either a sharp or a broader doublet, depending
on the phonon group velocity and anisotropy of dispersion surfaces. The
position of the observed sharp doublet is consistent with both mechanisms, but
a comparison of the computed and experimental anisotropies favours the second
process. | 0009011v1 |
2000-12-29 | Finite Temperature Time-Dependent Effective Theory for the Phase Field in two-dimensional d-wave Neutral Superconductor | We derive finite temperature time-dependent effective actions for the phase
of the pairing field, which are appropriate for a 2D electron system with both
non-retarded d- and s-wave attraction. As for s-wave pairing the d-wave
effective action contains terms with Landau damping, but their structure
appears to be different from the s-wave case due to the fact that the Landau
damping is determined by the quasiparticle group velocity v_{g}, which for
d-wave pairing does not have the same direction as the non-interacting Fermi
velocity v_{F}. We show that for d-wave pairing the Landau term has a linear
low temperature dependence and in contrast to the s-wave case are important for
all finite temperatures. A possible experimental observation of the phase
excitations is discussed. | 0012511v4 |
2001-01-23 | Ginzburg-Landau theory for the time-dependent phase field in a two-dimensional d-wave superconductor | We derive a finite temperature time-dependent effective theory for the phase
$\theta$ of the pairing field, which is appropriate for a 2D conducting
electron system with non-retarded d-wave attraction. As for s-wave pairing the
effective action contains terms with Landau damping, but their structure
appears to be different from the s-wave case due to the fact that the Landau
damping is determined by the quasiparticle group velocity $v_g$, which for the
d-wave pairing does not have the same direction as the non-interacting Fermi
velocity $v_F$. We show that for the d-wave pairing the Landau terms have a
linear low temperature dependence and in contrast to the s-wave case are
important for all finite temperatures. | 0101353v1 |
2001-06-05 | Damping and frequency shift in the oscillations of two colliding Bose-Einstein condensates | We have investigated the center-of-mass oscillations of a Rb87 Bose-Einstein
condensate in an elongated magneto-static trap. We start from a trapped
condensate and we transfer part of the atoms to another trapped level, by
applying a radio-frequency pulse. The new condensate is produced far from its
equilibrium position in the magnetic potential, and periodically collides with
the parent condensate. We discuss how both the damping and the frequency shift
of the oscillations are affected by the mutual interaction between the two
condensates, in a wide range of trapping frequencies. The experimental data are
compared with the prediction of a mean-field model. | 0106072v1 |
2001-07-12 | Ferromagnetism in the Hubbard model | We investigate the possibility and stability of bandferromagnetism in the
single-band Hubbard model. This model poses a highly non-trivial many-body
problem the general solution of which has not been found up to now.
Approximations are still unavoidable. Starting from a simple two-pole ansatz
for the spectral density our approach is systematically improved by focusing on
the influence of quasiparticle damping and the correct weak-and strong coupling
behaviour. The compatibility of the different aproximative steps with decisive
moment sum rules is analysed and the importance of a spin-dependent band shift
mediated by higher correlation functions is worked out. Results are presented
in terms of temperature- and band occupation-dependent quasiparticle densities
of states and band structures as well as spontaneous magnetisations,
susceptibilities and Curie temperatures for varying electron densities and
coupling strengths. Comparison is made to numerically essentially exact Quantum
Monte Carlo calculations recently done by other authors using dynamical mean
field theory for infinite-dimensional lattices. The main conclusion will be
that the Hubbard model provides a qualitatively correct description of
bandferromagnetism if quasiparticle damping and selfconsistent spin-dependent
bandshifts are properly taken into account. | 0107255v1 |
2001-09-10 | Finite temperature theory of the scissors mode in a Bose gas using the moment method | We use a generalized Gross-Pitaevskii equation for the condensate and a
semi-classical kinetic equation for the noncondensate atoms to discuss the
scissors mode in a trapped Bose-condensed gas at finite temperatures. Both
equations include the effect of $C_{12}$ collisions between the condensate and
noncondensate atoms. We solve the coupled moment equations describing
oscillations of the quadrupole moments of the condensate and noncondensate
components to find the collective mode frequencies and collisional damping
rates as a function of temperature. Our calculations extend those of
Gu\'ery-Odelin and Stringari at T=0 and in the normal phase. They complement
the numerical results of Jackson and Zaremba, although Landau damping is left
out of our approach. Our results are also used to calculate the quadrupole
response function, which is related to the moment of inertia. It is shown
explicitly that the moment of inertia of a trapped Bose gas at finite
temperatures involves a sum of an irrotational component from the condensate
and a rotational component from the thermal cloud atoms. | 0109149v1 |
2002-01-24 | Dynamic light scattering from colloidal fractal monolayers | We address experimentally the problem of how the structure of a surface
monolayer determines the visco-elasticity of the interface. Optical microscopy
and surface quasi--elastic light scattering have been used to characterize
aggregation of CaCO$_3$ particles at the air--water interface. The structures
formed by cluster-cluster aggregation are two dimensional fractals which grow
to eventually form a percolating network. This process is measured through
image analysis. On the same system we measure the dynamics of interfacial
thermal fluctuations (surface ripplons), and we discuss how the relaxation
process is affected by the growing clusters. We show that the structures start
damping the ripplons strongly when the two length scales are comparable. No
macroscopic surface pressure is measured and this is in contrast to lipid,
surfactant or polymer monolayers at concentrations corresponding to surface
coverage. This observation and the difficulty in fitting the ripplon spectrum
with traditional models suggest that a different physical mechanism might be
responsible for the observed damping of ripplons in this system. | 0201438v1 |
2002-08-08 | Excitation and damping of collective modes of a Bose-Einstein condensate in a one-dimensional lattice | The mode structure of a Bose-Einstein condensate non-adiabatically loaded
into a one-dimensional optical lattice is studied by analyzing the visibility
of the interference pattern as well as the radial profile of the condensate
after a time-of-flight. A simple model is proposed that predicts the short-time
decrease of the visibility as a function of the condensate parameters. In the
radial direction, heavily damped oscillations are observed, as well as an
increase in the condensate temperature. These findings are interpreted as a
re-thermalization due to dissipation of the initial condensate excitations into
high-lying modes. | 0208162v1 |
2002-08-12 | Spectral Properties of the Generalized Spin-Fermion Models | In order to account for competition and interplay of localized and itinerant
magnetic behaviour in correlated many body systems with complex spectra the
various types of spin-fermion models have been considered in the context of the
Irreducible Green's Functions (IGF) approach. Examples are generalized d-f
model and Kondo-Heisenberg model. The calculations of the quasiparticle
excitation spectra with damping for these models has been performed in the
framework of the equation- of-motion method for two-time temperature Green's
Functions within a non-perturbative approach. A unified scheme for the
construction of Generalized Mean Fields (elastic scattering corrections) and
self-energy (inelastic scattering) in terms of the Dyson equation has been
generalized in order to include the presence of the two interacting subsystems
of localized spins and itinerant electrons. A general procedure is given to
obtain the quasiparticle damping in a self-consistent way. This approach gives
the complete and compact description of quasiparticles and show the flexibility
and richness of the generalized spin-fermion model concept. | 0208227v1 |
2002-08-13 | Itinerant Antiferromagnetism of Correlated Lattice Fermions | The problem of finding of the ferromagnetic and antiferromagnetic "symmetry
broken" solutions of the correlated lattice fermion models beyond the
mean-field approximation has been investigated. The calculation of the
quasiparticle excitation spectra with damping for the single- and multi-orbital
Hubbard model has been performed in the framework of the equation- of-motion
method for two-time temperature Green's Functions within a non-perturbative
approach. A unified scheme for the construction of Generalized Mean Fields
(elastic scattering corrections) and self-energy (inelastic scattering) in
terms of the Dyson equation has been generalized in order to include the
presence of the "source fields". The damping of quasiparticles, which reflects
the interaction of the single-particle and collective degrees of freedom has
been calculated. The "symmetry broken" dynamical solutions of the Hubbard
model, which correspond to various types of itinerant antiferromagnetism has
been discussed. This approach complements previous studies and clarifies the
nature of the concepts of itinerant antiferromagnetism and "spin-aligning
field" of correlated lattice fermions. | 0208242v1 |
2003-01-21 | Zero temperature damping of Bose-Einstein condensate oscillations by vortex-antivortex pair creation | We investigate vortex-antivortex pair creation in a supersonically expanding
and contracting quasi-2D Bose-Einstein condensate at zero temperature. For
sufficiently large amplitude condensate oscillations, pair production provides
the leading dissipation mechanism. The condensate oscillations decay in a
nonexponential fashion, and the dissipation rate depends strongly on the
oscillation amplitude. These features allow to distinguish the decay due to
pair creation from other possible damping mechanisms. Experimental observation
of the predicted oscillation behavior of the superfluid gas provides a direct
confirmation of the hydrodynamical analogy of quantum electrodynamics and
quantum vortex dynamics in two spatial dimensions. | 0301397v3 |
2003-02-10 | Damped orbital excitations in the titanates | A possible mechanism for the removal of the orbital degeneracy in RTiO3
(where R=La, Y, ...) is considered. The calculation is based on the
Kugel-Khomskii Hamiltonian for electrons residing in the t2g orbitals of the Ti
ions, and uses a self-consistent pe rturbation expansion in the interaction
between the orbital and the spin degrees of freedom. The latter are assumed to
be ordered in a Neel state, brought about by delicate interactions that are not
included in the Kugel-Khomskii Hamiltonian. Within our model calculations, each
of the t2g bands is found to acquire a finite, temperature-dependent
dispersion, that lifts the orbital degeneracy. The orbital excitations are
found to be heavily damped over a rather wide band. Consequently, they do not
participate as a separate branch of excitations in the low-temperature
thermodynamics.e | 0302182v1 |
2003-03-20 | Energies and damping rates of elementary excitations in spin-1 Bose-Einstein condensed gases | Finite temperature Green's function technique is used to calculate the
energies and damping rates of elementary excitations of the homogeneous,
dilute, spin-1 Bose gases below the Bose-Einstein condensation temperature both
in the density and spin channels. For this purpose the self-consistent
dynamical Hartree-Fock model is formulated, which takes into account the direct
and exchange processes on equal footing by summing up certain classes of
Feynman diagrams. The model is shown to fulfil the Goldstone theorem and to
exhibit the hybridization of one-particle and collective excitations correctly.
The results are applied to the gases of ^{23}Na and ^{87}Rb atoms. | 0303424v3 |
2003-04-17 | Non-Fermi liquid behavior from two-dimensional antiferromagnetic fluctuations: a renormalization-group and large-N analysis | We analyze the Hertz-Moriya-Millis theory of an antiferromagnetic quantum
critical point, in the marginal case of two dimensions (d=2,z=2). Up to
next-to-leading order in the number of components (N) of the field, we find
that logarithmic corrections do not lead to an enhancement of the Landau
damping. This is in agreement with a renormalization-group analysis, for
arbitrary N. Hence, the logarithmic effects are unable to account for the
behavior reportedly observed in inelastic neutron scattering experiments on
CeCu_{6-x}Au_x. We also examine the extended dynamical mean-field treatment
(local approximation) of this theory, and find that only subdominant
corrections to the Landau damping are obtained within this approximation, in
contrast to recent claims. | 0304415v1 |
2003-05-21 | The path-coalescence transition and its applications | We analyse the motion of a system of particles subjected a random force
fluctuating in both space and time, and experiencing viscous damping. When the
damping exceeds a certain threshold, the system undergoes a phase transition:
the particle trajectories coalesce. We analyse this transition by mapping it to
a Kramers problem which we solve exactly. In the limit of weak random force we
characterise the dynamics by computing the rate at which caustics are crossed,
and the statistics of the particle density in the coalescing phase. Last but
not least we describe possible realisations of the effect, ranging from
trajectories of raindrops on glass surfaces to animal migration patterns. | 0305491v2 |
2003-05-21 | Magnetoresistive response of a high mobility 2DES under electromagnetic wave excitation | Oscillations of the resistance observed under electromagnetic wave excitation
in the high mobility GaAs/AlGaAs 2DES are examined as a function of the
radiation frequency and the power, utilizing an empirical lineshape based on
exponentially damped sinusoids. The fit-analysis indicates the resistance
oscillation frequency, F, increases with the radiation frequency, n, at the
rate dF/dn = 2.37 mTesla/GHz; the damping parameter, a, is approximately
independent of n at constant power; and the amplitude, A, of the oscillations
grows slowly with the incident power, at a constant temperature and frequency.
The lineshape appears to provide a good description of the data. | 0305507v2 |
2003-06-16 | Infrared Spectroscopy of Quantum Crossbars | Infrared (IR) spectroscopy can be used as an important and effective tool for
probing periodic networks of quantum wires or nanotubes (quantum crossbars,
QCB) at finite frequencies far from the Luttinger liquid fixed point. Plasmon
excitations in QCB may be involved in resonance diffraction of incident
electromagnetic waves and in optical absorption in the IR part of the spectrum.
Direct absorption of external electric field in QCB strongly depends on the
direction of the wave vector ${\bf q}.$ This results in two types of $1D\to 2D$
dimensional crossover with varying angle of an incident wave or its frequency.
In the case of QCB interacting with semiconductor substrate, capacitive contact
between them does not destroy the Luttinger liquid character of the long wave
QCB excitations. However, the dielectric losses on a substrate surface are
significantly changed due to appearance of additional Landau damping. The
latter is initiated by diffraction processes on QCB superlattice and manifests
itself as strong but narrow absorption peaks lying below the damping region of
an isolated substrate.Submi | 0306409v1 |
2003-06-19 | Superradiant light scattering from a moving Bose-Einstein condensate | We investigate the interaction of a moving BEC with a far detuned laser beam.
Superradiant Rayleigh scattering arises from the spontaneous formation of a
matter-wave grating due to the interference of two wavepackets with different
momenta. The system is described by the CARL-BEC model which is a
generalization of the Gross-Pitaevskii model to include the self-consistent
evolution of the scattered field. The experiment gives evidence of a damping of
the matter-wave grating which depends on the initial velocity of the
condensate. We describe this damping in terms of a phase-diffusion decoherence
process, in good agreement with the experimental results. | 0306500v2 |
2003-07-25 | Finite temperature excitations of a trapped Bose-Fermi mixture | We present a detailed study of the low-lying collective excitations of a
spherically trapped Bose-Fermi mixture at finite temperature in the
collisionless regime. The excitation frequencies of the condensate are
calculated self-consistently using the static Hartree-Fock-Bogoliubov theory
within the Popov approximation. The frequency shifts and damping rates due to
the coupled dynamics of the condensate, noncondensate, and degenerate Fermi gas
are also taken into account by means of the random phase approximation and
linear response theory. In our treatment, the dipole excitation remains close
to the bare trapping frequency for all temperatures considered, and thus is
consistent with the generalized Kohn theorem. We discuss in some detail the
behavior of monopole and quadrupole excitations as a function of the Bose-Fermi
coupling. At nonzero temperatures we find that, as the mixture moves towards
spatial separation with increasing Bose-Fermi coupling, the damping rate of the
monopole (quadrupole) excitation increases (decreases). This provides us a
useful signature to identify the phase transition of spatial separation. | 0307638v1 |
2003-09-18 | Memory-function approach to the normal-state optical properties of the Bechgaard salt (TMTSF)_2PF_6 | The gauge invariant, two-component optical conductivity model, with a
correlation gap structure related to the umklapp scattering processes, is
applied to the quasi-one-dimensional electronic systems and compared to the
recent measurements on the Bechgaard salt (TMTSF)_2PF_6. The optical response
of both the insulating and metallic state is found for the half-filled
conduction band, depending on the ratio between the correlation energy scale 2
\Delta^0_2 and the transfer integral in the direction perpendicular to the
conducting chains, t_{{\rm b}'}. The estimated value 2 \Delta^0_2/t_{{\rm b}'}
agrees reasonably well with the previous experimental and theoretical
conclusions. Parallel to the chains the thermally activated conduction
electrons in the insulating state are found to exhibit an universal behaviour,
accounting for the observed single-particle optical conductivity of the ordered
ground state of charge-density-wave systems. The band parameters and the
related damping energies suitable to the normal metallic state of (TMTSF)_2PF_6
are estimated from the measured spectra. Not only the spectral weights but also
the damping energies clearly indicate an opening of the correlation gap in the
charge excitation spectrum | 0309419v1 |
2003-10-03 | Effects of electrostatic fields and Casimir force on cantilever vibrations | The effect of an external bias voltage and fluctuating electromagnetic fields
on both the fundamental frequency and damping of cantilever vibrations is
considered. An external voltage induces surface charges causing
cantilever-sample electrostatic attraction. A similar effect arises from
charged defects in dielectrics that cause spatial fluctuations of electrostatic
fields. The cantilever motion results in charge displacements giving rise to
Joule losses and damping. It is shown that the dissipation increases with
decreasing conductivity and thickness of the substrate, a result that is
potentially useful for sample diagnostics. Fluctuating electromagnetic fields
between the two surfaces also induce attractive (Casimir) forces. It is shown
that the shift in the cantilever fundamental frequency due to the Casimir force
is close to the shift observed in recent experiments of Stipe et al. Both the
electrostatic and Casimir forces have a strong effect on the cantilever
eigenfrequencies, and both effects depend on the geometry of the cantilever
tip. We consider cylindrical, spherical, and ellipsoidal tips moving parallel
to a flat sample surface. The dependence of the cantilever effective mass and
vibrational frequencies on the geometry of the tip is studied both numerically
and analytically. | 0310081v1 |
2003-10-07 | Precessional switching of thin nanomagnets: analytical study | We study analytically the precessional switching of the magnetization of a
thin macrospin. We analyze its response when subjected to an external field
along its in-plane hard axis. We derive the exact trajectories of the
magnetization. The switching versus non switching behavior is delimited by a
bifurcation trajectory, for applied fields equal to half of the effective
anisotropy field. A magnetization going through this bifurcation trajectory
passes exactly along the hard axis and exhibits a vanishing characteristic
frequency at that unstable point, which makes the trajectory noise sensitive.
Attempting to approach the related minimal cost in applied field makes the
magnetization final state unpredictable. We add finite damping in the model as
a perturbative, energy dissipation factor. For a large applied field, the
system switches several times back and forth. Several trajectories can be gone
through before the system has dissipated enough energy to converge to one
attracting equilibrium state. For some moderate fields, the system switches
only once by a relaxation dominated precessional switching. We show that the
associated switching field increases linearly with the damping parameter. The
slope scales with the square root of the effective anisotropy. Our simple
concluding expressions are useful to assess the potential application of
precessional switching in magnetic random access memories. | 0310147v1 |
2003-11-27 | Temperature dependent Bogoliubov approximation in the classical fields approach to weakly interacting Bose gas | A classical fields approximation to the finite temperature microcanonical
thermodynamics of weakly interacting Bose gas is applied to the idealized case
of atoms confined in a box with periodic boundary conditions. We analyze in
some detail the microcanonical temperature in the model. We also analyze the
spectral properties of classical amplitudes of the plane waves -- the
eigenmodes of the time averaged one--particle density matrix. Looking at the
zero momentum component -- the order parameter of the condensate, we obtain the
nonperturbative results for the chemical potential. Analogous analysis of the
other modes yields nonperturbative temperature dependent Bogoliubov frequencies
and their damping rates. Damping rates are linear functions of momenta in the
phonon range and show more complex behavior for the particle sector. Where
available, we make comparison with the analytic estimates of these quantities. | 0311622v1 |
2003-12-29 | Plasmon attenuation and optical conductivity of a two-dimensional electron gas | In a ballistic two-dimensional electron gas, the Landau damping does not lead
to plasmon attenuation in a broad interval of wave vectors q << k_F. Similarly,
it does not contribute to the optical conductivity \sigma (\omega, q) in a wide
domain of its arguments, E_F > \omega > qv_F, where E_F, k_F and v_F are,
respectively, the Fermi energy, wavevector and velocity of the electrons. We
identify processes that result in the plasmon attenuation in the absence of
Landau damping. These processes are: the excitation of two electron-hole pairs,
phonon-assisted excitation of one pair, and a direct plasmon-phonon conversion.
We evaluate the corresponding contributions to the plasmon linewidth and to the
optical conductivity. | 0312684v3 |
2004-03-05 | Mode-coupling theory and molecular dynamics simulation for heat conduction in a chain with transverse motions | We study heat conduction in a one-dimensional chain of particles with
longitudinal as well as transverse motions. The particles are connected by
two-dimensional harmonic springs together with bending angle interactions. The
problem is analyzed by mode-coupling theory and compared with molecular
dynamics. We find very good, quantitative agreement for the damping of modes
between a full mode-coupling theory and molecular dynamics result, and a
simplified mode-coupling theory gives qualitative description of the damping.
The theories predict generically that thermal conductance diverges as N^{1/3}
as the size N increases for systems terminated with heat baths at the ends. The
N^{2/5} dependence is also observed in molecular dynamics which we attribute to
crossover effect. | 0403162v1 |
2004-03-21 | Evidence for Superfluidity in a Resonantly Interacting Fermi Gas | We observe collective oscillations of a trapped, degenerate Fermi gas of
$^6$Li atoms at a magnetic field just above a Feshbach resonance, where the
two-body physics does not support a bound state. The gas exhibits a radial
breathing mode at a frequency of 2837(05) Hz, in excellent agreement with the
frequency of $\nu_H\equiv\sqrt{10\nu_x\nu_y/3}=2830(20)$ Hz predicted for a
{\em hydrodynamic} Fermi gas with unitarity limited interactions. The measured
damping times and frequencies are inconsistent with predictions for both the
collisionless mean field regime and for collisional hydrodynamics. These
observations provide the first evidence for superfluid hydrodynamics in a
resonantly interacting Fermi gas. | 0403540v2 |
2004-03-22 | Parametric Driving of Dark Solitons in Atomic Bose-Einstein Condensates | A dark soliton oscillating in an elongated harmonically-confined atomic
Bose-Einstein condensate continuously exchanges energy with the sound field.
Periodic optical `paddles' are employed to controllably enhance the sound
density and transfer energy to the soliton, analogous to parametric driving. In
the absence of damping, the amplitude of the soliton oscillations can be
dramatically reduced, whereas with damping, a driven soliton equilibrates as a
stable dark soliton with lower energy, thereby extending the soliton lifetime
up to the lifetime of the condensate. | 0403566v2 |
2004-04-09 | Network-Induced Oscillatory Behavior in Material Flow Networks | Network theory is rapidly changing our understanding of complex systems, but
the relevance of topological features for the dynamic behavior of metabolic
networks, food webs, production systems, information networks, or cascade
failures of power grids remains to be explored. Based on a simple model of
supply networks, we offer an interpretation of instabilities and oscillations
observed in biological, ecological, economic, and engineering systems. We find
that most supply networks display damped oscillations, even when their units -
and linear chains of these units - behave in a non-oscillatory way. Moreover,
networks of damped oscillators tend to produce growing oscillations. This
surprising behavior offers, for example, a new interpretation of business
cycles and of oscillating or pulsating processes. The network structure of
material flows itself turns out to be a source of instability, and cyclical
variations are an inherent feature of decentralized adjustments. | 0404226v1 |
2004-04-28 | Dynamics of the Electro-Reflective Response of TaS3 | We have observed a large (~1%) change in infrared reflectance of the
charge-density-wave (CDW) conductor, orthorhombic TaS3, when its CDW is
depinned. The change is concentrated near one current contact. Assuming that
the change in reflectance is proportional to the degree of CDW polarization, we
have studied the dynamics of CDW repolarization through position dependent
measurements of the variation of the electro-reflectance with the frequency of
square wave voltages applied to the sample, and have found that the response
could be characterized as a damped harmonic oscillator with a distribution of
relaxation (i.e. damping) times. The average relaxation time, which increases
away from the contacts, varies with applied voltage as t0 ~ 1/V^p with p ~ 3/2,
but the distribution of times broadens as the voltage approaches the depinning
threshold. Very low resonant frequencies (~ 1 kHz) indicate a surprisingly
large amount of inertia, which is observable in the time dependence of the
change in reflectance as a polarity dependent delay of ~ 100 microsec. | 0404690v1 |
2004-05-06 | Damping of the de Haas-van Alphen oscillations in the superconducting state of MgB_2 | The de Haas-van Alphen (dHvA) signal arising from orbits on the $\pi$ Fermi
surface sheet of the two-gap superconductor MgB$_2$ has been observed in the
vortex state below $H_{c2}$. An extra attenuation of the dHvA signal, beyond
those effects described in the conventional Lifshitz-Kosevich expression, is
seen due to the opening of the superconducting gap. Our data show that the
$\pi$ band gap is still present up to $H_{c2}$. The data are compared to
current theories of dHvA oscillations in the superconducting state which allow
us to extract estimates for the evolution of the $\pi$ band gap with magnetic
field. Contrary to results for other materials, we find that the most recent
theories dramatically underestimate the damping in MgB$_2$. | 0405120v1 |
2004-05-26 | Magnetic droplets in a metal close to a ferromagnetic quantum critical point | Using analytical and path integral Monte Carlo methods, we study the
susceptibility $\chi_{dc}(T)$ of a spin-S impurity with XY rotational symmetry
embedded in a metal. Close to a ferromagnetic quantum critical point, the
impurity polarizes conduction electrons in its vicinity and forms a large
magnetic droplet with moment M>>S. At not too low temperatures, the strongly
damping paramagnon modes of the conduction electrons suppress large quantum
fluctuations (or spin flips) of this droplet. We show that the susceptibility
follows the law $\chi_{dc}(T)=(M^{2}/T)[1-(\pi g)^{-1}\ln(gE_{0}/T)]$, where
the parameter g>>1 describes the strong damping by conduction electrons, and
E_0 is the bandwidth of paramagnon modes. At exponentially low temperatures T
<< T_{*} ~ E_{0}\exp(-\pi g/2) we show that spin flips cannot be ignored. In
this regime we find that $\chi_{dc}(T) \approx \chi_{dc}(0)
[1-(2/3)(T/T_{*})^2]$, where $\chi_{dc}(0)\sim M^{2}/T_{*}$ is finite and
exponentially large in g. We also discuss these effects in the context of the
multi-channel Kondo impurity model. | 0405618v2 |
2004-06-29 | Pairing gaps in atomic gases at the BCS-BEC crossover | Strong evidence for pairing and superfluidity has recently been found in
atomic Fermi gases at the BCS-BEC crossover both in collective modes and RF
excitation energies. It is argued that the scale for the effective pairing gaps
measured in RF experiments is set by the lowest quasiparticle in-gap excitation
energies. These are calculated at the BCS-BEC crossover from semiclassical
solutions to the Bogoliubov-deGennes equations. The strong damping of the
radial breathing mode observed in the BCS limit occur when the lowest
quasiparticle excitation energies coincide with the radial frequency, which
indicates that a coupling between them take place. | 0406714v3 |
2004-08-26 | Decoherence of Rabi oscillations in a single quantum dot | We develop a realistic model of Rabi oscillations in a quantum-dot
photodiode. Based in a multi-exciton density matrix formulation we show that
for short pulses the two-level models fails and higher levels should be taken
into account. This affects some of the experimental conclusions, such as the
inferred efficiency of the state rotation (population inversion) and the
deduced value of the dipole interaction. We also show that the damping observed
cannot be explained using \emph{constant} rates with fixed pulse duration. We
demonstrate that the damping observed is in fact induced by an off-resonant
excitation to or from the continuum of wetting layer states. Our model
describes the nonlinear decoherence behavior observed in recent experiments. | 0408570v2 |
2004-08-28 | Breakdown of Hydrodynamics in the Radial Breathing Mode of a Strongly-Interacting Fermi Gas | We measure the magnetic field dependence of the frequency and damping time
for the radial breathing mode of an optically trapped, Fermi gas of $^6$Li
atoms near a Feshbach resonance. The measurements address the apparent
discrepancy between the results of Kinast et al., [Phys. Rev. Lett. {\bf 92},
150402 (2004)] and those of Bartenstein et al., [Phys. Rev. Lett. {\bf 92},
203201 (2004)]. Over the range of magnetic field from 770 G to 910 G, the
measurements confirm the results of Kinast et al. Close to resonance, the
measured frequencies are in excellent agreement with predictions for a unitary
hydrodynamic gas. At a field of 925 G, the measured frequency begins to
decrease below predictions. For fields near 1080 G, we observe a breakdown of
hydrodynamic behavior, which is manifested by a sharp increase in frequency and
damping rate. The observed breakdown is in qualitative agreement with the sharp
transition observed by Bartenstein et al., at 910 G. | 0408634v2 |
2004-10-01 | Magnetic Properties of Cuprate Perovskites | The magnetic susceptibility of underdoped yttrium and lanthanum cuprates is
interpreted based on the self-consistent solution of the t-J model of a Cu-O
plane. The calculations reproduce correctly the frequency dependencies of the
susceptibility in YBa2Cu3O_{7-y} and La_{2-x}Sr_xCuO4 attributing their
dissimilarity to the difference in the damping of spin excitations. In
YBa2Cu3O_{7-y} these excitations are well defined at the antiferromagnetic wave
vector Q=(\pi,\pi) even in the normal state which manifests itself in a
pronounced maximum -- the resonance peak -- in the susceptibility. In
La_{2-x}Sr_xCuO4 the spin excitations are overdamped which leads to a broad
low-frequency feature in the susceptibility. The low-frequency
incommensurability in the magnetic response is attributed to a dip in the
magnon damping at Q. The calculated concentration and temperature dependencies
of the incommensurability parameter conform with experimental observations.
Generally the incommensurate magnetic response is not accompanied with an
inhomogeneity of the carrier density. | 0410009v1 |
2004-11-12 | Microwave induced resistance oscillations on a high-mobility 2DEG: absorption/reflection and temperature damping experiments | In this work we address experimentally a number of unresolved issues related
to microwave induced resistance oscillations (MIRO) and the zero-resistance
states observed recently on very high-mobility 2D electron gases in GaAs/AlGaAs
heterostructures. In particular, we examine electrodynamic effects via
reflection/absorption experiments and study the exact waveform of MIRO and
their damping due to temperature. It is shown that electrodynamic effects due
to metallic-like reflection and plasmons are important producing a wide
cyclotron resonance line and a number of oscillations which do not coincide
with the MIRO. To describe the MIRO waveform a simple model was employed
involving radiation-induced scattering with displacement. A very good
correlation was found between the temperature dependencies of the quantum
lifetime from MIRO and the transport scattering time from the electron
mobility. The results are compared with measurements of Shubnikov-de Haas
oscillations down to 30 mK on the same sample. | 0411338v1 |
2004-11-17 | Path integral derivation of Bloch-Redfield equations for a qubit weakly coupled to a heat bath: Application to nonadiabatic transitions | Quantum information processing has greatly increased interest in the
phenomenon of environmentally-induced decoherence. The spin boson model is
widely used to study the interaction between a spin-modelling a quantum
particle moving in a double well potential-and its environment-modelled by a
heat bath of harmonic oscillators. This paper extends a previous analysis of
the static spin boson study to the driven spin boson case, with the derivation
of an exact integro-differential equation for the time evolution of the
propagator of the reduced spin density matrix. This is the first main result.
By specializing to weak damping we then obtain the next result, a set of
Bloch-Redfield equations for the equilibrium fixed spin initial condition.
Finally we show that these equations can be used to solve the classic
dissipative Landau-Zener problem and illustrate these solutions for the weak
damping case. The effect of dissipation is seen to be minimised as the speed of
passage is increased, implying that qubits need to be switched as fast as
possible. | 0411443v1 |
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