22,895 research outputs found
Thermal expansion of the magnetically ordering intermetallics RTMg (R = Eu, Gd and T = Ag, Au)
We report measurements of the thermal expansion for two Eu- and two
Gd-based intermetallics which exhibit ferro- or antiferromagnetic phase
transitions. These materials show sharp positive (EuAgMg and GdAuMg) and
negative (EuAuMg and GdAgMg) peaks in the temperature dependence of the thermal
expansion coefficient which become smeared and/or displaced in an
external magnetic field. Together with specific heat data we determine the
initial pressure dependences of the transition temperatures at ambient pressure
using the Ehrenfest or Clausius-Clapeyron relation. We find large pressure
dependences indicating strong spin-phonon coupling, in particular for GdAgMg
and EuAuMg where a quantum phase transition might be reached at moderate
pressures of a few GPa.Comment: 6 pages, 3 figure
Chaotic Quantum Decay in Driven Biased Optical Lattices
Quantum decay in an ac driven biased periodic potential modeling cold atoms
in optical lattices is studied for a symmetry broken driving. For the case of
fully chaotic classical dynamics the classical exponential decay is quantum
mechanically suppressed for a driving frequency \omega in resonance with the
Bloch frequency \omega_B, q\omega=r\omega_B with integers q and r.
Asymptotically an algebraic decay ~t^{-\gamma} is observed. For r=1 the
exponent \gamma agrees with as predicted by non-Hermitian random matrix
theory for q decay channels. The time dependence of the survival probability
can be well described by random matrix theory. The frequency dependence of the
survival probability shows pronounced resonance peaks with sub-Fourier
character.Comment: 7 pages, 5 figure
Nonlinear magnetoplasmons in strongly coupled Yukawa plasmas
The existence of plasma oscillations at multiples of the magnetoplasmon
frequency in a strongly coupled two-dimensional magnetized Yukawa plasma is
reported, based on extensive molecular dynamics simulations. These modes are
the analogues of Bernstein modes which are renormalized by strong interparticle
correlations. Their properties are theoretically explained by a dielectric
function incorporating the combined effect of a magnetic field, strong
correlations and finite temperature
Directed polymers in random media under confining force
The scaling behavior of a directed polymer in a two-dimensional (2D) random
potential under confining force is investigated. The energy of a polymer with
configuration is given by H\big(\{y(x)\}\big) = \sum_{x=1}^N \exyx
+ \epsilon \Wa^\alpha, where is an uncorrelated random potential
and \Wa is the width of the polymer. Using an energy argument, it is
conjectured that the radius of gyration and the energy fluctuation
of the polymer of length in the ground state increase as
and respectively with and for . A
novel algorithm of finding the exact ground state, with the effective time
complexity of \cO(N^3), is introduced and used to confirm the conjecture
numerically.Comment: 9 pages, 7 figure
Nonequilibrium stabilization of charge states in double quantum dots
We analyze the decoherence of charge states in double quantum dots due to
cotunneling. The system is treated using the Bloch-Redfield generalized master
equation for the Schrieffer-Wolff transformed Hamiltonian. We show that the
decoherence, characterized through a relaxation and a dephasing time
, can be controlled through the external voltage and that the
optimum point, where these times are maximum, is not necessarily in
equilibrium. We outline the mechanism of this nonequilibrium-induced
enhancement of lifetime and coherence. We discuss the relevance of our results
for recent charge qubit experiments.Comment: 5 pages, 5 figure
The spin state transition in LaCoO; revising a revision
Using soft x-ray absorption spectroscopy and magnetic circular dichroism at
the Co- edge we reveal that the spin state transition in LaCoO
can be well described by a low-spin ground state and a triply-degenerate
high-spin first excited state. From the temperature dependence of the spectral
lineshapes we find that LaCoO at finite temperatures is an inhomogeneous
mixed-spin-state system. Crucial is that the magnetic circular dichroism signal
in the paramagnetic state carries a large orbital momentum. This directly shows
that the currently accepted low-/intermediate-spin picture is at variance.
Parameters derived from these spectroscopies fully explain existing magnetic
susceptibility, electron spin resonance and inelastic neutron data
Few-Particle Effects in Semiconductor Quantum Dots: Observation of Multi-Charged-Excitons
We investigate experimentally and theoretically few-particle effects in the
optical spectra of single quantum dots (QDs). Photo-depletion of the QD
together with the slow hopping transport of impurity-bound electrons back to
the QD are employed to efficiently control the number of electrons present in
the QD. By investigating structurally identical QDs, we show that the spectral
evolutions observed can be attributed to intrinsic, multi-particle-related
effects, as opposed to extrinsic QD-impurity environment-related interactions.
From our theoretical calculations we identify the distinct transitions
related to excitons and excitons charged with up to five additional electrons,
as well as neutral and charged biexcitons.Comment: 4 pages, 4 figures, revtex. Accepted for publication in Physical
Review Letter
On Which Length Scales Can Temperature Exist in Quantum Systems?
We consider a regular chain of elementary quantum systems with nearest
neighbor interactions and assume that the total system is in a canonical state
with temperature . We analyze under what condition the state factors into a
product of canonical density matrices with respect to groups of subsystems
each, and when these groups have the same temperature . While in classical
mechanics the validity of this procedure only depends on the size of the groups
, in quantum mechanics the minimum group size also depends
on the temperature ! As examples, we apply our analysis to different types
of Heisenberg spin chains.Comment: To appear in: Proceedings of the SPQS conference, J. Phys. Soc. Jpn.
74 (2005) Supp
- …