610 research outputs found
Band structure analysis of the conduction-band mass anisotropy in 6H and 4H SiC
The band structures of 6H and 4H SiC calculated by means of the FP-LMTO
method are used to determine the effective mass tensors for their
conduction-band minima. The results are shown to be consistent with recent
optically detected cyclotron resonance measurements and predict an unusual band
filling dependence for 6H-SiC.Comment: 5 pages including 4 postscript figures incorporated with epsfig figs.
available as part 2: sicfig.uu self-extracting file to appear in Phys. Rev.
B: Aug. 15 (Rapid Communications
Heisenberg exchange in magnetic monoxides
The superexchange intertacion in transition-metal oxides, proposed initially
by Anderson in 1950, is treated using contemporary tight-binding theory and
existing parameters. We find also a direct exchange for nearest-neighbor metal
ions, larger by a factor of order five than the superexchange. This direct
exchange arises from Vddm coupling, rather than overlap of atomic charge
densities, a small overlap exchange contribution which we also estimate. For
FeO and CoO there is also an important negative contribution, related to Stoner
ferromagnetism, from the partially filled minority-spin band which broadens
when ionic spins are aligned. The corresponding J1 and J2 parameters are
calculated for MnO, FeO, CoO, and NiO. They give good accounts of the Neel and
the Curie-Weiss temperatures, show appropriate trends, and give a reasonable
account of their volume dependences. For MnO the predicted value for the
magnetic susceptibility at the Neel temperature and the crystal distortion
arising from the antiferromagnetic transition were reasonably well given.
Application to CuO2 planes in the cuprates gives J=1220oK, compared to an
experimental 1500oK, and for LiCrO2 gives J1=4 50oK compared to an experimental
230oK.Comment: 21 pages, 1 figure, submitted to Phys. Rev. B 1/19/07. Realized
J=4V^2/U applies generally, as opposed to J=2V^2/U from one-electron theory
(1/28 revision
Small, Dense Quark Stars from Perturbative QCD
As a model for nonideal behavior in the equation of state of QCD at high
density, we consider cold quark matter in perturbation theory. To second order
in the strong coupling constant, , the results depend sensitively on
the choice of the renormalization mass scale. Certain choices of this scale
correspond to a strongly first order chiral transition, and generate quark
stars with maximum masses and radii approximately half that of ordinary neutron
stars. At the center of these stars, quarks are essentially massless.Comment: ReVTeX, 5 pages, 3 figure
Band Calculations for Ce Compounds with AuCu-type Crystal Structure on the basis of Dynamical Mean Field Theory I. CePd and CeRh
Band calculations for Ce compounds with the AuCu-type crystal structure
were carried out on the basis of dynamical mean field theory (DMFT). The
auxiliary impurity problem was solved by a method named NCAvc
(noncrossing approximation including the state as a vertex correction).
The calculations take into account the crystal-field splitting, the spin-orbit
interaction, and the correct exchange process of the virtual excitation. These are necessary features in the
quantitative band theory for Ce compounds and in the calculation of their
excitation spectra. The results of applying the calculation to CePd and
CeRh are presented as the first in a series of papers. The experimental
results of the photoemission spectrum (PES), the inverse PES, the
angle-resolved PES, and the magnetic excitation spectra were reasonably
reproduced by the first-principles DMFT band calculation. At low temperatures,
the Fermi surface (FS) structure of CePd is similar to that of the band
obtained by the local density approximation. It gradually changes into a form
that is similar to the FS of LaPd as the temperature increases, since the
band shifts to the high-energy side and the lifetime broadening becomes
large.}Comment: 12 pasges, 13 figure
Dynamics of the rotational degrees of freedom in a supercooled liquid of diatomic molecules
Using molecular dynamics computer simulations, we investigate the dynamics of
the rotational degrees of freedom in a supercooled system composed of rigid,
diatomic molecules. The interaction between the molecules is given by the sum
of interaction-site potentials of the Lennard-Jones type. In agreement with
mode-coupling theory (MCT), we find that the relaxation times of the
orientational time correlation functions C_1^(s), C_2^(s) and C_1 show at low
temperatures a power-law with the same critical temperature T_c, and which is
also identical to the critical temperature for the translational degrees of
freedom. In contrast to MCT we find, however, that for these correlators the
time-temperature superposition principle does not hold well and that also the
critical exponent gamma depends on the correlator. We also study the
temperature dependence of the rotational diffusion constant D_r and demonstrate
that at high temperatures D_r is proportional to the translational diffusion
constant D and that when the system starts to become supercooled the former
shows an Arrhenius behavior whereas the latter exhibits a power-law dependence.
We discuss the origin for the difference in the temperature dependence of D (or
the relaxation times of C_l^(s) and D_r. Finally we present results which show
that at low temperatures 180 degree flips of the molecule are an important
component of the relaxation dynamics for the orientational degrees of freedom.Comment: 17 pages of RevTex, 12 figure
Second harmonic generation in SiC polytypes
LMTO calculations are presented for the frequency dependent second harmonic
generation (SHG) in the polytypes 2H, 4H, 6H, 15R and 3C of SiC. All
independent tensor components are calculated. The spectral features and the
ratios of the 333 to 311 tensorial components are studied as a function of the
degree of hexagonality. The relationship to the linear optical response and the
underlying band structure are investigated. SHG is suggested to be a sensitive
tool for investigating the near band edge interband excitations.Comment: 12 pages, 10 figure
Molecular Dynamics Simulations
A tutorial introduction to the technique of Molecular Dynamics (MD) is given,
and some characteristic examples of applications are described. The purpose and
scope of these simulations and the relation to other simulation methods is
discussed, and the basic MD algorithms are described. The sampling of intensive
variables (temperature T, pressure p) in runs carried out in the microcanonical
(NVE) ensemble (N= particle number, V = volume, E = energy) is discussed, as
well as the realization of other ensembles (e.g. the NVT ensemble). For a
typical application example, molten SiO2, the estimation of various transport
coefficients (self-diffusion constants, viscosity, thermal conductivity) is
discussed. As an example of Non-Equilibrium Molecular Dynamics (NEMD), a study
of a glass-forming polymer melt under shear is mentioned.Comment: 38 pages, 11 figures, to appear in J. Phys.: Condens. Matte
Quasiparticle dynamics in ferromagnetic compounds of the Co-Fe and Ni-Fe systems
We report a theoretical study of the quasiparticle lifetime and the
quasiparticle mean free path caused by inelastic electron-electron scattering
in ferromagnetic compounds of the Co-Fe and Ni-Fe systems. The study is based
on spin-polarized calculations, which are performed within the
approximation for equiatomic and Co- and Ni-rich compounds, as well as for
their constituents. We mainly focus on the spin asymmetry of the quasiparticle
properties, which leads to the spin-filtering effect experimentally observed in
spin-dependent transport of hot electrons and holes in the systems under study.
By comparing with available experimental data on the attenuation length, we
estimate the contribution of the inelastic mean free path to the latter.Comment: 10 pages, 10 figure
CoRoT photometry and high-resolution spectroscopy of the interacting eclipsing binary AU Mon
Analyses of very accurate CoRoT space photometry, past Johnson V
photoelectric photometry and high-resolution \'echelle spectra led to the
determination of improved and consistent fundamental stellar properties of both
components of AU Mon. We derived new, accurate ephemerides for both the orbital
motion (with a period of 11.113d) and the long-term, overall brightness
variation (with a period of 416.9d) of this strongly interacting Be + G
semi-detached binary. It is shown that this long-term variation must be due to
attenuation of the total light by some variable circumbinary material. We
derived the binary mass ratio = 0.17\p0.03 based on the
assumption that the G-type secondary fills its Roche lobe and rotates
synchronously. Using this value of the mass ratio as well as the radial
velocities of the G-star, we obtained a consistent light curve model and
improved estimates of the stellar masses, radii, luminosities and effective
temperatures. We demonstrate that the observed lines of the B-type primary may
not be of photospheric origin. We also discover rapid and periodic light
changes visible in the high-quality residual CoRoT light curves. AU Mon is put
into perspective by a comparison with known binaries exhibiting long-term
cyclic light changes.Comment: Accepted for publication in MNRA
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