431 research outputs found

### The importance of thermal disorder and electronic occupation for the T-dependence of the optical conductivity in FeSi and MnSi

The spectral weight (SW) for optical transitions in FeSi and MnSi are
calculated as function of temperature by means of LMTO-LDA band calculations.
The main effects, caused by structural disorder and electronic Fermi-Dirac
distribution, act oppositely on the T-dependence of the SW, while the variation
of the magnetic moment in MnSi has only a minor effect. The calculations agree
with the experimental findings of an increasing SW in FeSi and a decreasing SW
in MnSi as function of T. The results can be understood from the change of the
bandstructure with disorder.Comment: (5 pages, 4 figures

### Electronic, dynamical, and thermal properties of ultra-incompressible superhard rhenium diboride: A combined first-principles and neutron scattering study

Rhenium diboride is a recently recognized ultra-incompressible superhard
material. Here we report the electronic (e), phonon (p), e-p coupling and
thermal properties of ReB$_2$ from first-principles density-functional theory
(DFT) calculations and neutron scattering measurements. Our calculated elastic
constants ($c_{11}$ = 641 GPa, $c_{12}$ = 159 GPa, $c_{13}$ = 128 GPa, $c_{33}$
= 1037 GPa, and $c_{44}$ = 271 GPa), bulk modulus ($B$ $\approx$ 350 GPa) and
hardness ($H$ $\approx$ 46 GPa) are in good agreement with the reported
experimental data. The calculated phonon density of states (DOS) agrees very
well with our neutron vibrational spectroscopy result. Electronic and phonon
analysis indicates that the strong covalent B-B and Re-B bonding is the main
reason for the super incompressibility and hardness of ReB$_2$. The thermal
expansion coefficients, calculated within the quasi-harmonic approximation and
measured by neutron powder diffraction, are found to be nearly isotropic in $a$
and $c$ directions and only slightly larger than that of diamond in terms of
magnitude. The excellent agreement found between calculations and experimental
measurements indicate that first-principles calculations capture the main
interactions in this class of superhard materials, and thus can be used to
search, predict, and design new materials with desired properties.Comment: submitted to pr

### Anharmonic stabilization of the high-pressure simple cubic phase of calcium

The phonon spectrum of the high-pressure simple cubic phase of calcium, in
the harmonic approx- imation, shows imaginary branches that make it
mechanically unstable. In this letter, the phonon spectrum is recalculated
using density-functional theory (DFT) ab initio methods fully including
anharmonic effects up to fourth order at 50 GPa. Considering that perturbation
theory cannot be employed with imaginary harmonic frequencies, a variational
procedure based on the Gibbs- Bogoliubov inequality is used to estimate the
renormalized phonon frequencies. The results show that strong quantum
anharmonic effects make the imaginary phonons become positive even at zero
temperature so that the simple cubic phase becomes mechanically stable, as
experiments suggest. Moreover, our calculations find a superconducting Tc in
agreement with experiments and predict an anomalous behavior of the specific
heat.Comment: 5 pages, 3 figure

### Ab initio investigation of Elliott-Yafet electron-phonon mechanism in laser-induced ultrafast demagnetization

The spin-flip (SF) Eliashberg function is calculated from first-principles
for ferromagnetic Ni to accurately establish the contribution of Elliott-Yafet
electron-phonon SF scattering to Ni's femtosecond laser-driven demagnetization.
This is used to compute the SF probability and demagnetization rate for
laser-created thermalized as well as non-equilibrium electron distributions.
Increased SF probabilities are found for thermalized electrons, but the induced
demagnetization rate is extremely small. A larger demagnetization rate is
obtained for {non-equilibrium} electron distributions, but its contribution is
too small to account for femtosecond demagnetization.Comment: 5 pages, 3 figures, to appear in PR

### Small Fermi energy, zero point fluctuations and nonadiabaticity in MgB$_2$

Small Fermi energy effects are induced in MgB$_2$ by the low hole doping in
the $\sigma$ bands which are characterized by a Fermi energy $E_{\rm F}^\sigma
\sim 0.5$ eV. We show that, due to the particularly strong deformation
potential relative to the $E_{2g}$ phonon mode, lattice fluctuations are
reflected in strong fluctuations in the electronic band structure. Quantum
fluctuations associated to the zero-point lattice motion are responsible for an
uncertainty of the Fermi energy of the order of the Fermi energy itself,
leading to the breakdown of the adiabatic principle underlying the
Born-Oppenheimer approximation in MgB$_2$ even if $\omega_{\rm ph}/E_{\rm F}
\sim 0.1-0.2$, where $\omega_{\rm ph}$ are the characteristic phonon
frequencies. This amounts to a new nonadiabatic regime, which could be relevant
to other unconventional superconductors.Comment: to appear on Physical Review

### Electron-Phonon Interactions for Optical Phonon Modes in Few-Layer Graphene

We present a first-principles study of the electron-phonon (e-ph)
interactions and their contributions to the linewidths for the optical phonon
modes at $\Gamma$ and K in one to three-layer graphene. It is found that due to
the interlayer coupling and the stacking geometry, the high-frequency optical
phonon modes in few-layer graphene couple with different valence and conduction
bands, giving rise to different e-ph interaction strengths for these modes.
Some of the multilayer optical modes derived from the $\Gamma$-$E_{2g}$ mode of
monolayer graphene exhibit slightly higher frequencies and much reduced
linewidths. In addition, the linewidths of K-$A'_1$ related modes in
multilayers depend on the stacking pattern and decrease with increasing layer
numbers.Comment: 6 pages,5 figures, submitted to PR

### Electron-phonon coupling and its evidence in the photoemission spectra of lead

We present a detailed study on the influence of strong electron-phonon
coupling to the photoemission spectra of lead. Representing the strong-coupling
regime of superconductivity, the spectra of lead show characteristic features
that demonstrate the correspondence of physical properties in the normal and
the superconducting state, as predicted by the Eliashberg theory. These
features appear on an energy scale of a few meV and are accessible for
photoemission only by using modern spectrometers with high resolution in energy
and angle.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Let

### Unraveling the acoustic electron-phonon interaction in graphene

Using a first-principles approach we calculate the acoustic electron-phonon
couplings in graphene for the transverse (TA) and longitudinal (LA) acoustic
phonons. Analytic forms of the coupling matrix elements valid in the
long-wavelength limit are found to give an almost quantitative description of
the first-principles based matrix elements even at shorter wavelengths. Using
the analytic forms of the coupling matrix elements, we study the acoustic
phonon-limited carrier mobility for temperatures 0-200 K and high carrier
densities of 10^{12}-10^{13} cm^{-2}. We find that the intrinsic effective
acoustic deformation potential of graphene is \Xi_eff = 6.8 eV and that the
temperature dependence of the mobility \mu ~ T^{-\alpha} increases beyond an
\alpha = 4 dependence even in the absence of screening when the full coupling
matrix elements are considered. The large disagreement between our calculated
deformation potential and those extracted from experimental measurements (18-29
eV) indicates that additional or modified acoustic phonon-scattering mechanisms
are at play in experimental situations.Comment: 7 pages, 3 figure

### The electron-phonon coupling strength at metal surfaces directly determined from the Helium atom scattering Debye-Waller factor

A new quantum-theoretical derivation of the elastic and inelastic scattering
probability of He atoms from a metal surface, where the energy and momentum
exchange with the phonon gas can only occur through the mediation of the
surface free-electron density, shows that the Debye-Waller exponent is directly
proportional to the electron-phonon mass coupling constant $\lambda$. The
comparison between the values of $\lambda$ extracted from existing data on the
Debye-Waller factor for various metal surfaces and the $\lambda$ values known
from literature indicates a substantial agreement, which opens the possibility
of directly extracting the electron-phonon coupling strength in quasi-2D
conducting systems from the temperature or incident energy dependence of the
elastic Helium atom scattering intensities.Comment: 14 pages, 2 figures, 1 tabl

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