2,714 research outputs found
Microscopic Approach to Magnetism and Superconductivity of -Electron Systems with Filled Skutterudite Structure
In order to gain a deep insight into -electron properties of filled
skutterudite compounds from a microscopic viewpoint, we investigate the
multiorbital Anderson model including Coulomb interactions, spin-orbit
coupling, and crystalline electric field effect. For each case of
=113, where is the number of electrons per rare-earth ion, the
model is analyzed by using the numerical renormalization group (NRG) method to
evaluate magnetic susceptibility and entropy of electron. In order to make
further step to construct a simplified model which can be treated even in a
periodic system, we also analyze the Anderson model constructed based on the
- coupling scheme by using the NRG method. Then, we construct an orbital
degenerate Hubbard model based on the - coupling scheme to investigate
the mechanism of superconductivity of filled skutterudites. In the 2-site
model, we carefully evaluate the superconducting pair susceptibility for the
case of =2 and find that the susceptibility for off-site Cooper pair is
clearly enhanced only in a transition region in which the singlet and triplet
ground states are interchanged.Comment: 14 pages, 11 figures, Typeset with jpsj2.cl
Analytical and numerical study of hardcore bosons in two dimensions
We study various properties of bosons in two dimensions interacting only via
onsite hardcore repulsion. In particular, we use the lattice spin-wave
approximation to calculate the ground state energy, the density, the condensate
density and the superfluid density in terms of the chemical potential. We also
calculate the excitation spectrum, . In addition, we performed
high precision numerical simulations using the stochastic series expansion
algorithm. We find that the spin-wave results describe extremely well the
numerical results over the {\it whole} density range . We
also compare the lattice spin-wave results with continuum results obtained by
summing the ladder diagrams at low density. We find that for
there is good agreement, and that the difference between the two methods
vanishes as for . This offers the possibility of obtaining
precise continuum results by taking the continuum limit of the spin-wave
results for all densities. Finaly, we studied numerically the finite
temperature phase transition for the entire density range and compared with low
density predictions.Comment: 10 pages, 8 figures include
Multifractals of Normalized First Passage Time in Sierpinski Gasket
The multifractal behavior of the normalized first passage time is
investigated on the two dimensional Sierpinski gasket with both absorbing and
reflecting barriers. The normalized first passage time for Sinai model and the
logistic model to arrive at the absorbing barrier after starting from an
arbitrary site, especially obtained by the calculation via the Monte Carlo
simulation, is discussed numerically. The generalized dimension and the
spectrum are also estimated from the distribution of the normalized first
passage time, and compared with the results on the finitely square lattice.Comment: 10 pages, Latex, with 3 figures and 1 table. to be published in J.
Phys. Soc. Jpn. Vol.67(1998
Study of Low Energy Spin Rotons in the Fractional Quantum Hall Effect
Motivated by the discovery of extremely low energy collective modes in the
fractional quantum Hall effect (Kang, Pinczuk {\em et al.}), with energies
below the Zeeman energy, we study theoretically the spin reversed excitations
for fractional quantum Hall states at and 3/7 and find qualitatively
different behavior than for . We find that a low-energy,
charge-neutral "spin roton," associated with spin reversed excitations that
involve a change in the composite-fermion Landau level index, has energy in
reasonable agreement with experiment.Comment: Postscript figures included. Accepted in Phys. Rev. B (Rapid
Communication
Conductance through Quantum Dots Studied by Finite Temperature DMRG
With the Finite temperature Density Matrix Renormalization Group method
(FT-DMRG), we depeloped a method to calculate thermo-dynamical quantities and
the conductance of a quantum dot system. Conductance is written by the local
density of states on the dot. The density of states is calculated with the
numerical analytic continuation from the thermal Green's function which is
obtained directly from the FT-DMRG. Typical Kondo behaviors in the quantum dot
system are observed conveniently by comparing the conductance with the magnetic
and charge susceptibilities: Coulomb oscillation peaks and the unitarity limit.
We discuss advantage of this method compared with others.Comment: 14 pages, 13 fiure
The numerical renormalization group method for quantum impurity systems
In the beginning of the 1970's, Wilson developed the concept of a fully
non-perturbative renormalization group transformation. Applied to the Kondo
problem, this numerical renormalization group method (NRG) gave for the first
time the full crossover from the high-temperature phase of a free spin to the
low-temperature phase of a completely screened spin. The NRG has been later
generalized to a variety of quantum impurity problems. The purpose of this
review is to give a brief introduction to the NRG method including some
guidelines of how to calculate physical quantities, and to survey the
development of the NRG method and its various applications over the last 30
years. These applications include variants of the original Kondo problem such
as the non-Fermi liquid behavior in the two-channel Kondo model, dissipative
quantum systems such as the spin-boson model, and lattice systems in the
framework of the dynamical mean field theory.Comment: 55 pages, 27 figures, submitted to Rev. Mod. Phy
Criterion for weak spin-orbit coupling in heavy-fermion superconductivity: A numerical renormalization-group study
A criterion for effective irrelevancy of the spin-orbit coupling in the
heavy-fermion superconductivity is discussed on the basis of the impurity
Anderson model with two sets of Kramers doublets. Using Wilson's numerical
renormalization-group method, we demonstrate a formation of the quasiparticle
as well as the renormalization of the rotational symmetry-breaking interaction
in the lower Kramers doublet (quasispin) space. A comparison with the quasispin
conserving interaction exhibits the effective irrelevancy of the
symmetry-breaking interaction for the splitting of two doublets Delta larger
than the characteristic energy of the local spin fluctuation T_K. The formula
for the ratio of two interactions is also determined.Comment: 4 pages, 4 figures (2 color figures
A Composite Fermion Hofstader Problem: Partially Polarized Density Wave States in the 2/5 FQHE
It is well-known that the 2/5 state is unpolarized at zero Zeeman energy,
while it is fully polarized at large Zeeman energies. A novel state with
charge/spin density wave order for Composite Fermions is proposed to exist at
intermediate values of the Zeeman coupling for 2/5. This state has half the
maximum possible polarization, and can be extended to other incompressible
fractions. A Hartree-Fock calculation based on the new approach for all
fractional quantum Hall states developed by R.Shankar and the author is used to
demonstrate the stability of this state to single-particle excitations, and
compute gaps. We compare our results with a very recent experiment which shows
direct evidence for the existence of such a state, and also with more indirect
evidence from past experiments.Comment: One reference added, minor clarifying change
Domain Formation in v=2/3 Fractional Quantum Hall Systems
We study the domain formation in the v=2/3 fractional quantum Hall systems
basing on the density matrix renormalization group (DMRG) analysis. The
ground-state energy and the pair correlation functions are calculated for
various spin polarizations. The results confirm the domain formation in
partially spin polarized states, but the presence of the domain wall increases
the energy of partially spin polarized states and the ground state is either
spin unpolarized state or fully spin polarized state depending on the Zeeman
energy. We expect coupling with external degrees of freedom such as nuclear
spins is important to reduce the energy of partially spin polarized state.Comment: 7 pages, submitted to J. Phys. Soc. Jp
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