2,191 research outputs found
Thermal conductivity of the thermoelectric layered cobalt oxides measured by the Harman method
In-plane thermal conductivity of the thermoelectric layered cobalt oxides has
been measured using the Harman method, in which thermal conductivity is
obtained from temperature gradient induced by applied current. We have found
that the charge reservoir block (the block other than the CoO block)
dominates the thermal conduction, where a nano-block integration concept is
effective for material design. We have further found that the thermal
conductivity shows a small but finite in-plane anisotropy between and
axes, which can be ascribed to the misfit structure.Comment: 4 pages, 4 figures, J. Appl. Phys. (scheduled on July 1, 2004
Frustrated electron liquids in the Hubbard model
The ground state of the Hubbard model is studied within the constrained
Hilbert space where no order parameter exists. The self-energy of electrons is
decomposed into the single-site and multisite self-energies. The calculation of
the single-site self-energy is mapped to a problem of self-consistently
determining and solving the Anderson model. When an electron reservoir is
explicitly considered, it is proved that the single-site self-energy is that of
a normal Fermi liquid even if the multisite self-energy is anomalous. Thus, the
ground state is a normal Fermi liquid in the supreme single-site approximation
(S^3A). In the strong-coupling regime, the Fermi liquid is stabilized by the
Kondo effect in the S^3A and is further stabilized by the Fock-type term of the
superexchange interaction or the resonating-valence-bond (RVB) mechanism beyond
the S^3A. The stabilized Fermi liquid is frustrated as much as an RVB spin
liquid in the Heisenberg model. It is a relevant unperturbed state that can be
used to study a normal or anomalous Fermi liquid and an ordered state in the
whole Hilbert space by Kondo lattice theory. Even if higher-order multisite
terms than the Fock-type term are considered, the ground state cannot be a Mott
insulator. It can be merely a gapless semiconductor even if the multisite
self-energy is so anomalous that it is divergent at the chemical potential. A
Mott insulator is only possible as a high temperature phase.Comment: 11 pages, no figur
Rashba spin splitting in biased semiconductor quantum wells
Rashba spin splitting (RSS) in biased semiconductor quantum wells is
investigated theoretically based on the eight-band envelope function model. We
find that at large wave vectors, RSS is both nonmonotonic and anisotropic as a
function of in-plane wave vector, in contrast to the widely used linear and
isotropic model. We derive an analytical expression for RSS, which can
correctly reproduce such nonmonotonic behavior at large wave vectors. We also
investigate numerically the dependence of RSS on the various band parameters
and find that RSS increases with decreasing band gap and subband index,
increasing valence band offset, external electric field, and well width. Our
analytical expression for RSS provides a satisfactory explanation to all these
features.Comment: 5 pages, 4 figures, author names corrected, submitted to Phys. Rev.
Fluctuation Effects on the Quadrupolar Ordering in Magnetic Field
Effects of magnetic field on the quadrupolar ordering are investigated with
inclusion of fluctuation of order parameters. For the simplest model with the
nearest-neighbor quadrupolar interaction, the transition temperature and the
specific heat are derived by the use of the recently proposed effective medium
theory. It is shown that magnetic field H has two competing effects on the
quadrupolar ordering; one is to encourage the ordering by suppressing the
fluctuation among different components of order parameters, and the other is to
block the ordering as in antiferromagnets. The former is found to be of order
H^2 and the latter of order H^4. Hence the fluctuation is suppressed for weak
fields, and the transition temperature increases with magnetic field. The
fluctuation effect is so strong that the entropy released at the quadrupolar
ordering is only about half of the full value ln 4 even without the Kondo
effect.Comment: 10 pages including 3 Postscript figure
Resonant X-Ray Scattering from the Quadrupolar Ordering Phase of CeB_6
We theoretically investigate the origin of the resonant x-ray scattering
(RXS) signal near the Ce absorption edge in the quadrupolar ordering
phase of CeB, considering the intersite interaction between the
states in the initial state. The anisotropic charge distribution of the
states modulates the states through the intra-atomic Coulomb interaction
and thereby generates a large RXS superlattice intensity. The temperature and
magnetic field dependence indicates that the induced dipolar and octupolar
orders have little influence on the RXS spectra, in good agreement with the
recent experiment.Comment: 4 pages, 4 figure
Theory and design of quantum cascade lasers in (111) n-type Si/SiGe
Although most work towards the realization of group IV quantum cascade lasers (QCLs) has focused on valence band transitions, there are many desirable properties associated with the conduction band. We show that the commonly cited shortcomings of n-type Si/SiGe heterostructures can be overcome by moving to the (111) growth direction. Specifically, a large band offset and low effective mass are achievable and subband degeneracy is preserved. We predict net gain up to lattice temperatures of 90 K in a bound-to-continuum QCL with a double-metal waveguide, and show that a Ge interdiffusion length of at least 8 ā« across interfaces is tolerable
Crystal Field, Magnetic Anisotropy and Excitations in Rare-Earth Hexaborides
We clarify the role of crystalline electric field (CEF) induced magnetic
anisotropy in the ground state and spin-wave spectrum of cubic rare-earth
materials with dominating isotropic magnetic exchange interactions. In
particular we study the hexaboride NdB_6 which is shown to exhibit strong
spin-quadrupolar coupling. The CEF scheme is analyzed and a non-collinear
magnetization response is found. The spin orientation in the
antiferromagnetically ordered ground-state is identified. Moreover, the spin
excitations are evaluated and in agreement with inelastic neutron scattering a
suppression of one of the two magnetic modes in the strong-coupling regime is
predicted.Comment: 4 pages, 1 eps-figur
Quadrupolar Order in Isotropic Heisenberg Models with Biquadratic Interaction
Through Quantum Monte Carlo simulation, we study the biquadratic-interaction
model with the SU(2) symmetry in two and three dimensions. The zero-temperature
phase diagrams for the two cases are identical and exhibit an intermediate
phase characterized by finite quadrupole moment, in agreement with mean-field
type arguments and the semi-classical theory. In three dimensions, we
demonstrate that the model in the quadrupolar regime has a phase transition at
a finite temperature. In contrast to predictions by mean-field theories, the
phase transition to the quadrupolar phase turns out to be of the second order.
We also examine the critical behavior in the two marginal cases with the SU(3)
symmetry.Comment: 4 pages 5 figure
Magnetic Field Dependence of the Paramagnetic to the High Temperature Magnetically Ordered Phase Transition in CeB6
We have measured the magnetic field dependence of the paramagnetic to high
temperature magnetically ordered phase transition TQ(H) in CeB6 from 2 to 30 T
using cantilever magnetometry. It is found that the phase separation
temperature continuously increases in field with an increasingly positive
slope. In addition, we find that measurements in strong magnetic field
gradients have no effect on the phase transition.Comment: 6 total page including 3 figures, submitted to Physical Review B
(also available at
http://publish.aps.org/eprint/gateway/eplist/aps1999dec08_006) v2: Corrected
typos, added 1 reference, minor content changes (deleted 1 sentence in
introduction, added 2 sentences in discussion to explain added reference),
fixed the "et al"s in the bibliograph
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