12,075 research outputs found
Quantum Monte Carlo study for multiorbital systems with preserved spin and orbital rotational symmetries
We propose to combine the Trotter decomposition and a series expansion of the
partition function for Hund's exchange coupling in a quantum Monte Carlo (QMC)
algorithm for multiorbital systems that preserves spin and orbital rotational
symmetries. This enables us to treat the Hund's (spin-flip and pair-hopping)
terms, which is difficult in the conventional QMC method. To demonstrate this,
we first apply the algorithm to study ferromagnetism in the two-orbital Hubbard
model within the dynamical mean-field theory (DMFT). The result reveals that
the preservation of the SU(2) symmetry in Hund's exchange is important, where
the Curie temperature is grossly overestimated when the symmetry is degraded,
as is often done, to Ising (Z). We then calculate the spectral
functions of SrRuO by a three-band DMFT calculation with tight-binding
parameters taken from the local density approximation with proper rotational
symmetry.Comment: 9 pages, 9 figures. Typos corrected, some comments and references
adde
Phase mixing of shear Alfvén waves as a new mechanism for electron acceleration in collisionless, kinetic plasmas
Particle-in-cell (kinetic) simulations of shear Alfv´en wave (AW) interaction with one-dimensional, across the uniform-magnetic field, density inhomogeneity (phase mixing) in collisionless plasma were performed for the first time. As a result, a new electron acceleration mechanism is discovered. Progressive distortion of the AW front, due to the differences in local Alfv´en speed, generates electrostatic fields nearly parallel to the magnetic field, which accelerate electrons via Landau damping. Surprisingly, the amplitude decay law in the inhomogeneous regions, in the kinetic regime, is the same as in the MHD approximation described by Heyvaerts and Priest (1983 Astron. Astrophys. 117 220)
Field-induced long-range order in the S=1 antiferromagnetic chain
The quasi-one dimensional S=1 antiferromagnet in magnetic field H is
investigated with the exact diagonalization of finite chains and the mean field
approximation for the interchain interaction. In the presence of the single-ion
anisotropy D, the full phase diagram in the plane is presented for H
\parallel D and H \perp D. The shape of the field-induced long-range ordered
phase is revealed to be quite different between the two cases, as observed in
the recent experiment of NDMAP. The estimated ratio of the interchain and
intrachain couplings of NDMAP (J'/J ~ 10^{-3}) is consistent with the neutron
scattering measurement.Comment: 4 pages, Revtex, with 6 eps figure
Metamagnetism of antiferromagnetic XXZ quantum spin chains
The magnetization process of the one-dimensional antiferromagnetic Heisenberg
model with the Ising-like anisotropic exchange interaction is studied by the
exact diagonalization technique. It results in the evidence of the first-order
spin flop transition with a finite magnetization jump in the N\'eel ordered
phase for . It implies that the S=1/2 chain is an exceptional case
where the metamagnetic transition becomes second-order due to large quantum
fluctuations.Comment: 4 pages, Revtex, with 6 eps figure
Critical exponent in the magnetization curve of quantum spin chains
The ground state magnetization curve around the critical magnetic field
of quantum spin chains with the spin gap is investigated. We propose a size
scaling method to estimate the critical exponent defined as from finite cluster calculation. The applications of the
method to the S=1 antiferromagnetic chain and S=1/2 bond alternating chain lead
to a common conclusion . The same result is derived for both edges
of the magnetization plateau of the S=3/2 antiferromagnetic chain with the
single ion anisotropy.Comment: 4 pages, Revtex, with 4 eps figure
How Do Nonlinear Voids Affect Light Propagation ?
Propagation of light in a clumpy universe is examined. As an inhomogeneous
matter distribution, we take a spherical void surrounded by a dust shell where
the ``lost mass'' in the void is compensated by the shell. We study how the
angular-diameter distance behaves when such a structure exists. The
angular-diameter distance is calculated by integrating the Raychaudhuri
equation including the shear. An explicit expression for the junction condition
for the massive thin shell is calculated. We apply these results to a dust
shell embedded in a Friedmann universe and determine how the distance-redshift
relation is modified compared with that in the purely Friedmann universe. We
also study the distribution of distances in a universe filled with voids. We
show that the void-filled universe gives a larger distance than the FRW
universe by at if the size of the void is of the
Horizon radius.Comment: To appear in Prog. Theor. Phys. 10
Field induced transition of the S=1 antiferromagnetic chain with anisotropy
The ground state magnetization process of the S=1 antiferromagnetic chain
with the easy-axis single-ion anisotropy described by negative is
investigated. It is numerically found that a phase transition between two
different gapless phases occurs at an intermediate magnetic field between the
starting and saturation points of the magnetization for . The
transition is similar to the spin flopping, but it is second-order and not
accompanied with any significant anomalous behaviors in the magnetization
curve. We also present the phase diagrams in the m-D and H-D planes which
reveal a possible re-entrant transition.Comment: 6 pages, Revtex, with 6 eps figures, to appear in Phys. Rev. B (Sep.
1
Static Gravitational Global Monopoles
Static solutions in spherical symmetry are found for gravitating global
monopoles. Regular solutions lacking a horizon are found for , where is the scale of symmetry breaking. Apparently
regular solutions with a horizon are found for 1/\sqrt{8\pi} \le \eta \alt
\sqrt{3/8\pi}. Though they have a horizon, they are not Schwarzschild. The
solution for is argued to have a horizon at infinity.
The failure to find static solutions for
is consistent with findings that topological inflation begins at .Comment: 4 pages, 6 figure
Numerical Renormalization Group Study of non-Fermi-liquid State on Dilute Uranium Systems
We investigate the non-Fermi-liquid (NFL) behavior of the impurity Anderson
model (IAM) with non-Kramers doublet ground state of the f configuration
under the tetragonal crystalline electric field (CEF). The low energy spectrum
is explained by a combination of the NFL and the local-Fermi-liquid parts which
are independent with each other. The NFL part of the spectrum has the same form
to that of two-channel-Kondo model (TCKM). We have a parameter range that the
IAM shows the divergence of the magnetic susceptibility together with
the positive magneto resistance. We point out a possibility that the anomalous
properties of UThRuSi including the decreasing resistivity
with decreasing temperature can be explained by the NFL scenario of the TCKM
type. We also investigate an effect of the lowering of the crystal symmetry. It
breaks the NFL behavior at around the temperature, , where
is the orthorhombic CEF splitting. The NFL behavior is still expected above the
temperature, .Comment: 25 pages, 12 figure
- …