1,976 research outputs found
Thermoelectric transport in strained Si and Si/Ge heterostructures
The anisotropic thermoelectric transport properties of bulk silicon strained
in [111]-direction were studied by detailed first-principles calculations
focussing on a possible enhancement of the power factor. Electron as well as
hole doping were examined in a broad doping and temperature range. At low
temperature and low doping an enhancement of the power factor was obtained for
compressive and tensile strain in the electron-doped case and for compressive
strain in the hole-doped case. For the thermoelectrically more important high
temperature and high doping regime a slight enhancement of the power factor was
only found under small compressive strain with the power factor overall being
robust against applied strain. To extend our findings the anisotropic
thermoelectric transport of an [111]-oriented Si/Ge superlattice was
investigated. Here, the cross-plane power factor under hole-doping was
drastically suppressed due to quantum-well effects, while under electron-doping
an enhanced power factor was found. With that, we state a figure of merit of
ZT and ZT at T=\unit[300]{K} and T=\unit[900]{K} for the
electron-doped [111]-oriented Si/Ge superlattice. All results are discussed in
terms of band structure features
Strong influence of the complex bandstructure on the tunneling electroresistance: A combined model and ab-initio study
The tunneling electroresistance (TER) for ferroelectric tunnel junctions
(FTJs) with BaTiO_{3} (BTO) and PbTiO}_{3} (PTO) barriers is calculated by
combining the microscopic electronic structure of the barrier material with a
macroscopic model for the electrostatic potential which is caused by the
ferroelectric polarization. The TER ratio is investigated in dependence on the
intrinsic polarization, the chemical potential, and the screening properties of
the electrodes. A change of sign in the TER ratio is obtained for both barrier
materials in dependence on the chemical potential. The inverse imaginary Fermi
velocity describes the microscopic origin of this effect; it qualitatively
reflects the variation and the sign reversal of the TER. The quantity of the
imaginary Fermi velocity allows to obtain detailed information on the transport
properties of FTJs by analyzing the complex bandstructure of the barrier
material.Comment: quality of figures reduce
On calculating the Berry curvature of Bloch electrons using the KKR method
We propose and implement a particularly effective method for calculating the
Berry curvature arising from adiabatic evolution of Bloch states in wave vector
k space. The method exploits a unique feature of the Korringa-Kohn-Rostoker
(KKR) approach to solve the Schr\"odinger or Dirac equations. Namely, it is
based on the observation that in the KKR method k enters the calculation via
the structure constants which depend only on the geometry of the lattice but
not the crystal potential. For both the Abelian and non-Abelian Berry curvature
we derive an analytic formula whose evaluation does not require any numerical
differentiation with respect to k. We present explicit calculations for Al, Cu,
Au, and Pt bulk crystals.Comment: 13 pages, 5 figure
Influence of strain on anisotropic thermoelectric transport of BiTe and SbTe
On the basis of detailed first-principles calculations and semi-classical
Boltzmann transport, the anisotropic thermoelectric transport properties of
BiTe and SbTe under strain were investigated. It was found that
due to compensation effects of the strain dependent thermopower and electrical
conductivity, the related powerfactor will decrease under applied in-plane
strain for BiTe_2_3_2_3$ suggests, that already in the single crystalline system
strong anisotropic scattering effects should play a role
On the Correlation between the Magnetic Activity Levels, the Metallicities and the Radii of Low-Mass Stars
The recent burst in the number of radii measurements of very low-mass stars
from eclipsing binaries and interferometry of single stars has opened more
questions about what can be causing the discrepancy between the observed radii
and the ones predicted by the models. The two main explanations being proposed
are a correlation between the radius of the stars and their activity levels or
their metallicities. This paper presents a study of such correlations using all
the data published to date. The study also investigates correlations between
the radii deviation from the models and the masses of the stars. There is no
clear correlation between activity level and radii for the single stars in the
sample. Those single stars are slow rotators with typical velocities v_rot sini
< 3.0 km s^-1. A clear correlation however exists in the case of the faster
rotating members of binaries. This result is based on the of X-ray emission
levels of the stars. There also appears to be an increase in the deviation of
the radii of single stars from the models as a function of metallicity, as
previously indicated by Berger et al. (2006). The stars in binaries do not seem
to follow the same trend. Finally, the Baraffe et al. (1998) models reproduce
well the radius observations below 0.30-0.35Msun, where the stars become fully
convective, although this result is preliminary since almost all the sample
stars in that mass range are slow rotators and metallicities have not been
measured for most of them. The results in this paper indicate that stellar
activity and metallicity play an important role on the determination of the
radius of very low-mass stars, at least above 0.35Msun.Comment: 22 pages, 4 figures. Accepted for publication on Ap
Simulations of core convection in rotating A-type stars: Differential rotation and overshooting
We present the results of 3--D simulations of core convection within A-type
stars of 2 solar masses, at a range of rotation rates. We consider the inner
30% by radius of such stars, thereby encompassing the convective core and some
of the surrounding radiative envelope. We utilize our anelastic spherical
harmonic (ASH) code, which solves the compressible Navier-Stokes equations in
the anelastic approximation, to examine highly nonlinear flows that can span
multiple scale heights. The cores of these stars are found to rotate
differentially, with central cylindrical regions of strikingly slow rotation
achieved in our simulations of stars whose convective Rossby number (R_{oc}) is
less than unity. Such differential rotation results from the redistribution of
angular momentum by the nonlinear convection that strongly senses the overall
rotation of the star. Penetrative convective motions extend into the overlying
radiative zone, yielding a prolate shape (aligned with the rotation axis) to
the central region in which nearly adiabatic stratification is achieved. This
is further surrounded by a region of overshooting motions, the extent of which
is greater at the equator than at the poles, yielding an overall spherical
shape to the domain experiencing at least some convective mixing. We assess the
overshooting achieved as the stability of the radiative exterior is varied, and
the weak circulations that result in that exterior. The convective plumes serve
to excite gravity waves in the radiative envelope, ranging from localized
ripples of many scales to some remarkable global resonances.Comment: 48 pages, 16 figures, some color. Accepted to Astrophys. J. Color
figures compressed with appreciable loss of quality; a PDF of the paper with
better figures is available at
http://lcd-www.colorado.edu/~brownim/core_convectsep24.pd
On internal wave breaking and tidal dissipation near the centre of a solar-type star
We study the fate of internal gravity waves, which are excited by tidal
forcing by a short-period planet at the interface of convection and radiation
zones, approaching the centre of a solar-type star. We study at what amplitude
these wave are subject to instabilities. These instabilities lead to wave
breaking whenever the amplitude exceeds a critical value. Below this value, the
wave reflects perfectly from the centre of the star. Wave breaking results in
spinning up the central regions of the star, and the formation of a critical
layer, which acts as an absorbing barrier for ingoing waves. As these waves are
absorbed, the star is spun up from the inside out. This results in an important
amplitude dependence of the tidal quality factor Q'. If the tidal forcing
amplitude exceeds the value required for wave breaking, efficient dissipation
results over a continuous range of tidal frequencies, leading to Q' \approx
10^5 (P/1day)^(8/3), for the current Sun. This varies by less than a factor of
5 throughout the range of G and K type main sequence stars, for a given orbit.
We predict fewer giant planets with orbital periods of less than about 2 days
around such stars, if they cause breaking at the centre, due to the efficiency
of this process. This mechanism would, however, be ineffective in stars with a
convective core, such as WASP-18, WASP-12 and OGLE-TR-56, perhaps partly
explaining the survival of their close planetary companions.Comment: 22 pages, 10 figures, accepted in MNRAS, abstract shortened (!
Electrostrictive counter-force on fluid microdroplet in short laser pulse
When a micrometer-sized fluid droplet is illuminated by a laser pulse, there
is a fundamental distinction between two cases. If the pulse is short in
comparison with the transit time for sound across the droplet, the disruptive
optical Abraham-Minkowski radiation force is countered by electrostriction and
the net stress is compressive. In contrast, if the pulse is long on this scale,
electrostriction is cancelled by elastic pressure and the surviving term of the
electromagnetic force, the Abraham-Minkowski force, is disruptive and deforms
the droplet. Ultrashort laser pulses are routinely used in modern experiments,
and impressive progress has moreover been made on laser manipulation of liquid
surfaces in recent times, making a theory for combining the two pertinent. We
analyze the electrostrictive contribution analytically and numerically for a
spherical droplet.Comment: 3 pages, 3 figures, accepted for publication in Optics Letter
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