7,781 research outputs found
Universal conductance in quantum wires in the presence of Umklapp scattering
The effects of Umklapp scattering on the zero-temperature conductance in
one-dimensional quantum wires are reexamined by taking into account both the
screening of external potential and the non-uniform chemical potential shift
due to electron-electron interaction. It is shown that in the case away from
half-filling the conductance is given by the universal value, , even in
the presence of Umklapp scattering, owing to these renormalization effects of
external potential. The conclusion is in accordance with the recent claim
obtained for the system with non-interacting leads being attached to a quantum
wire.Comment: 5 pages, to be published in Euro. Phys. J.
Chain breaks and the susceptibility of Sr_2Cu_{1-x}Pd_xO_{3+\delta} and other doped quasi one-dimensional antiferromagnets
We study the magnetic susceptibility of one-dimensional S=1/2
antiferromagnets containing non-magnetic impurities which cut the chain into
finite segments. For the susceptibility of long anisotropic Heisenberg
chain-segments with open boundaries we derive a parameter-free result at low
temperatures using field theory methods and the Bethe Ansatz. The analytical
result is verified by comparing with Quantum-Monte-Carlo calculations. We then
show that the partitioning of the chain into finite segments can explain the
Curie-like contribution observed in recent experiments on
Sr_2Cu_{1-x}Pd_xO_{3+\delta}. Possible additional paramagnetic impurities seem
to play only a minor role.Comment: 4 pages, 3 figures, final versio
Magnetohydrodynamic Simulations of A Rotating Massive Star Collapsing to A Black Hole
We perform two-dimensional, axisymmetric, magnetohydrodynamic simulations of
the collapse of a rotating star of 40 Msun and in the light of the collapsar
model of gamma-ray burst. Considering two distributions of angular momentum, up
to \sim 10^{17} cm^2/s, and the uniform vertical magnetic field, we investigate
the formation of an accretion disk around a black hole and the jet production
near the hole. After material reaches to the black hole with the high angular
momentum, the disk is formed inside a surface of weak shock. The disk becomes
in a quasi-steady state for stars whose magnetic field is less than 10^{10} G
before the collapse. We find that the jet can be driven by the magnetic fields
even if the central core does not rotate as rapidly as previously assumed and
outer layers of the star has sufficiently high angular momentum. The magnetic
fields are chiefly amplified inside the disk due to the compression and the
wrapping of the field. The fields inside the disk propagate to the polar region
along the inner boundary near the black hole through the Alfv{\'e}n wave, and
eventually drive the jet. The quasi-steady disk is not an advection-dominated
disk but a neutrino cooling-dominated one. Mass accretion rates in the disks
are greater than 0.01 Msun/sec with large fluctuations. The disk is transparent
for neutrinos. The dense part of the disk, which locates near the hole, emits
neutrino efficiently at a constant rate of < 8 \times 10^{51} erg/s. The
neutrino luminosity is much smaller than those from supernovae after the
neutrino burst.Comment: 42 pages, accepted for publication in the Astrophysical Journal. A
paper with higher-resolution figures available at
http://www.ec.knct.ac.jp/~fujimoto/collapsar/mhd-color.pd
Drude Weight at Finite Temperatures for Some Non-Integrable Quantum Systems in One Dimension
Using conformal perturbation theory, we show that for some classes of the
one-dimensional quantum liquids that possess the Luttinger liquid fixed point
in the low energy limit, the Drude weight at finite temperatures is
non-vanishing, even when the system is {\it non-integrable} and the total
current is not conserved. We also obtain the asymptotically exact
low-temperature formula of the Drude weight for Heisenberg XXZ spin chains,
which agrees quite well with recent numerical data.Comment: Title is a little changed. References are updated. To appear in
Physical Review Letter
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