2,929 research outputs found
Numerical Calculation of Convection with Reduced Speed of Sound Technique
Context. The anelastic approximation is often adopted in numerical
calculation with low Mach number, such as stellar internal convection. This
approximation requires frequent global communication, because of an elliptic
partial differential equation. Frequent global communication is negative factor
for the parallel computing with a large number of CPUs.
Aims. The main purpose of this paper is to test the validity of a method that
artificially reduces the speed of sound for the compressible fluid equations in
the context of stellar internal convection. The reduction of speed of sound
allows for larger time steps in spite of low Mach number, while the numerical
scheme remains fully explicit and the mathematical system is hyperbolic and
thus does not require frequent global communication.
Methods. Two and three dimensional compressible hydrodynamic equations are
solved numerically. Some statistical quantities of solutions computed with
different effective Mach numbers (due to reduction of speed of sound) are
compared to test the validity of our approach.
Results. Numerical simulations with artificially reduced speed of sound are a
valid approach as long as the effective Mach number (based on the reduced speed
of sound) remains less than 0.7.Comment: 16 pages, 10 figures, accepted to A&
Quadrupole Susceptibility of Gd-Based Filled Skutterudite Compounds
It is shown that quadrupole susceptibility can be detected in Gd compounds
contrary to our textbook knowledge that Gd ion induces pure spin moment
due to the Hund's rules in an coupling scheme. The ground-state multiplet
of Gd is always characterized by =7/2, where denotes total
angular momentum, but in a - coupling scheme, one electron in =7/2
octet carries quadrupole moment, while other six electrons fully occupy =5/2
sextet, where denotes one-electron total angular momentum. For realistic
values of Coulomb interaction and spin-orbit coupling, the ground-state
wavefunction is found to contain significant amount of the - coupling
component. From the evaluation of quadrupole susceptibility in a simple
mean-field approximation, we point out a possibility to detect the softening of
elastic constant in Gd-based filled skutterudites.Comment: 8 pages, 4 figure
Modulation Doping of a Mott Quantum Well by a Proximate Polar Discontinuity
We present evidence for hole injection into LaAlO3/LaVO3/LaAlO3 quantum wells
near a polar surface of LaAlO3 (001). As the surface is brought in proximity to
the LaVO3 layer, an exponential drop in resistance and a decreasing positive
Seebeck coefficient is observed below a characteristic coupling length of 10-15
unit cells. We attribute this behavior to a crossover from an atomic
reconstruction of the AlO2-terminated LaAlO3 surface to an electronic
reconstruction of the vanadium valence. These results suggest a general
approach to tunable hole-doping in oxide thin film heterostructures.Comment: 16 pages, 7 figure
Kondo Effect in an Electron System with Dynamical Jahn-Teller Impurity
We investigate how Kondo phenomenon occurs in the Anderson model dynamically
coupled with local Jahn-Teller phonons. It is found that the total angular
moment composed of electron pseudo-spin and phonon angular moments is screened
by conduction electrons. Namely, phonon degrees of freedom essentially
contribute to the formation of singlet ground state. A characteristic
temperature of the Kondo effect due to dynamical Jahn-Teller phonons is
explained by an effective - Hamiltonian with anisotropic exchange
interaction obtained from the Jahn-Teller-Anderson model in a non-adiabatic
region.Comment: 5 pages, 3 figure
Multipole as -Electron Spin-Charge Density in Filled Skutterudites
It is shown that -electron multipole is naturally defined as spin-charge
one-electron density operator in the second-quantized form with the use of
tensor operator on the analogy of multipole expansion of electromagnetic
potential from charge distribution in electromagnetism. Due to this definition
of multipole, it is possible to determine multipole state from a microscopic
viewpoint on the basis of the standard linear response theory for multipole
susceptibility. In order to discuss multipole properties of filled
skutterudites, we analyze a seven-orbital impurity Anderson model by employing
a numerical renormalization group method. We show our results on possible
multipole states of filled skutterudite compounds.Comment: To appear in the Proceedings of International Conference on "New
Quantum Phenomena in Skutterudite and Related Systems" (September 2007, Kobe,
Japan
Effective Crystalline Electric Field Potential in a j-j Coupling Scheme
We propose an effective model on the basis of a - coupling scheme to
describe local -electron states for realistic values of Coulomb interaction
and spin-orbit coupling , for future development of microscopic
theory of magnetism and superconductivity in -electron systems, where
is the number of local electrons. The effective model is systematically
constructed by including the effect of a crystalline electric field (CEF)
potential in the perturbation expansion in terms of . In this paper,
we collect all the terms up to the first order of . Solving the
effective model, we show the results of the CEF states for each case of
=25 with symmetry in comparison with those of the Stevens
Hamiltonian for the weak CEF. In particular, we carefully discuss the CEF
energy levels in an intermediate coupling region with in the order
of 0.1 corresponding to actual -electron materials between the and
- coupling schemes. Note that the relevant energy scale of is the
Hund's rule interaction. It is found that the CEF energy levels in the
intermediate coupling region can be quantitatively reproduced by our modified
- coupling scheme, when we correctly take into account the corrections in
the order of in addition to the CEF terms and Coulomb interactions
which remain in the limit of =. As an application of the
modified - coupling scheme, we discuss the CEF energy levels of filled
skutterudites with symmetry.Comment: 12 pages, 7 figures. Typeset with jpsj2.cl
Multipole correlations in low-dimensional f-electron systems
By using a density matrix renormalization group method, we investigate the
ground-state properties of a one-dimensional three-orbital Hubbard model on the
basis of a j-j coupling scheme. For , where is a parameter
to control cubic crystalline electric field effect, one orbital is itinerant,
while other two are localized. Due to the competition between itinerant and
localized natures, we obtain orbital ordering pattern which is sensitive to
, leading to a characteristic change of quadrupole state
into an incommensurate structure. At , all the three orbitals are
degenerate, but we observe a peak at in quadrupole
correlation, indicating a ferro-orbital state, and the peak at in
dipole correlation, suggesting an antiferromagnetic state. We
also discuss the effect of octupole on magnetic anisotropy.Comment: 4 pages, 3 figures, Proceedings of ASR-WYP-2005 (September 27-29,
2005, Tokai
Multipole State of Heavy Lanthanide Filled Skutterudites
We discuss multipole properties of filled skutterudites containing heavy
lanthanide Ln from a microscopic viewpoint on the basis of a seven-orbital
Anderson model. For Ln=Gd, in contrast to naive expectation, quadrupole moments
remain in addition to main dipole ones. For Ln=Ho, we find an exotic state
governed by octupole moment. For Ln=Tb and Tm, no significant multipole moments
appear at low temperatures, while for Ln=Dy, Er, and Yb, dipole and
higher-order multipoles are dominant. We briefly discuss possible relevance of
these multipole states with actual materials.Comment: 5 pages, 3 figure
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