615 research outputs found
Thermal transport of the XXZ chain in a magnetic field
We study the heat conduction of the spin-1/2 XXZ chain in finite magnetic
fields where magnetothermal effects arise. Due to the integrability of this
model, all transport coefficients diverge, signaled by finite Drude weights.
Using exact diagonalization and mean-field theory, we analyze the temperature
and field dependence of the thermal Drude weight for various exchange
anisotropies under the condition of zero magnetization-current flow. First, we
find a strong magnetic field dependence of the Drude weight, including a
suppression of its magnitude with increasing field strength and a non-monotonic
field-dependence of the peak position. Second, for small exchange anisotropies
and magnetic fields in the massless as well as in the fully polarized regime
the mean-field approach is in excellent agreement with the exact
diagonalization data. Third, at the field-induced quantum critical line between
the para- and ferromagnetic region we propose a universal low-temperature
behavior of the thermal Drude weight.Comment: 9 pages REVTeX4 including 5 figures, revised version, refs. added,
typos correcte
Bound states in weakly disordered spin ladders
We study the appearance of bound states in the spin gap of spin-1/2 ladders
induced by weak bond disorder. Starting from the strong-coupling limit, i.e.,
the limit of weakly coupled dimers, we perform a projection on the
single-triplet subspace and derive the position of bound states for the single
impurity problem of one modified coupling as well as for small impurity
clusters. The case of a finite concentration of impurities is treated with the
coherent-potential approximation in the strong-coupling limit and compared with
numerical results. Furthermore, we analyze the details in the structure of the
density of states and relate their origin to the influence of impurity
clusters.Comment: 2 pages, 1 figure. Proceedings of SCES'04, to appear in Physica
Magnetic heat conductivity in : linear temperature dependence
We present experimental results for the thermal conductivity of the
pseudo 2-leg ladder material . The strong buckling of the ladder
rungs renders this material a good approximation to a Heisenberg-chain.
Despite a strong suppression of the thermal conductivity of this material in
all crystal directions due to inherent disorder, we find a dominant magnetic
contribution along the chain direction.
is \textit{linear} in temperature, resembling the
low-temperature limit of the thermal Drude weight of the
Heisenberg chain. The comparison of and
yields a magnetic mean free path of \AA, in good agreement with magnetic measurements.Comment: appears in PR
Bond-impurity induced bound states in disordered spin-1/2 ladders
We discuss the effect of weak bond-disorder in two-leg spin ladders on the
dispersion relation of the elementary triplet excitations with a particular
focus on the appearance of bound states in the spin gap. Both the cases of
modified exchange couplings on the rungs and the legs of the ladder are
analyzed. Based on a projection on the single-triplet subspace, the
single-impurity and small cluster problems are treated analytically in the
strong-coupling limit. Numerically, we study the problem of a single impurity
in a spin ladder by exact diagonalization to obtain the low-lying excitations.
At finite concentrations and to leading order in the inter-rung coupling, we
compare the spectra obtained from numerical diagonalization of large systems
within the single-triplet subspace with the results of diagrammatic techniques,
namely low-concentration and coherent-potential approximations. The
contribution of small impurity clusters to the density of states is also
discussed.Comment: 9 pages REVTeX4 including 7 figures, final version; Fig. 5 modifie
Constraints on narrow exotic states from K+p and K0_Lp scattering data
We consider the effect of exotic S=+1 resonances Theta+ and Theta++ on K+p
elastic scattering data (total cross section) and the process K0_Lp-->K0_Sp.
Data near the observed Theta+(1540) are examined for evidence of additional
states. The width limit for a Theta++ state is reconsidered and shown to be
much less than 1 MeV.Comment: 4 pages, 3 eps figures; minor corrections, one fig adde
Optical Properties of MFe_4P_12 filled skutterudites
Infrared reflectance spectroscopy measurements were made on four members of
the MFe_4P_12 family of filled skutterudites, with M=La, Th, Ce and U. In
progressing from M=La to U the system undergoes a metal-insulator transition.
It is shown that, although the filling atom induces such dramatic changes in
the transport properties of the system, it has only a small effect on lattice
dynamics. We discuss this property of the compounds in the context of their
possible thermoelectric applications.Comment: Manuscript in ReVTeX format, 7 figures in PostScirpt forma
Low thermal conductivity of the layered oxide (Na,Ca)Co_2O_4: Another example of a phonon glass and an electron crystal
The thermal conductivity of polycrystalline samples of (Na,Ca)Co_2O_4 is
found to be unusually low, 20 mW/cmK at 280 K. On the assumption of the
Wiedemann-Franz law, the lattice thermal conductivity is estimated to be 18
mW/cmK at 280 K, and it does not change appreciably with the substitution of Ca
for Na. A quantitative analysis has revealed that the phonon mean free path is
comparable with the lattice parameters, where the point-defect scattering plays
an important role. Electronically the same samples show a metallic conduction
down to 4.2 K, which strongly suggests that NaCo_2O_4 exhibits a glass-like
poor thermal conduction together with a metal-like good electrical conduction.
The present study further suggests that a strongly correlated system with
layered structure can act as a material of a phonon glass and an electron
crystal.Comment: 5 pages 3 figures, to be published in Phys. Rev.
Improved ventricular function during inhalation of PGI(2) aerosol partly relies on enhanced myocardial contractility
Inhaled prostacyclin (PGI(2)) aerosol induces selective pulmonary vasodilation. Further, it improves right ventricular ( RV) function, which may largely rely on pulmonary vasodilation, but also on enhanced myocardial contractility. We investigated the effects of the inhaled PGI(2) analogs epoprostenol (EPO) and iloprost (ILO) on RV function and myocardial contractility in 9 anesthetized pigs receiving aerosolized EPO (25 and 50 ng center dot kg(-1) center dot min(-1)) and, consecutively, ILO (60 ng center dot kg(-1) center dot min(-1)) for 20 min each. We measured pulmonary artery pressure ( PAP), RV ejection fraction (RVEF) and RV end-diastolic-volume (RV-EDV), and left ventricular end-systolic pressure-volume-relation (end-systolic elastance, E-es). EPO and ILO reduced PAP, increased RVEF and reduced RVEDV. E-es was enhanced during all doses tested, which reached statistical significance during EPO25ng and ILO, but not during EPO50ng. PGI(2) aerosol enhances myocardial contractility in healthy pigs, contributing to improve RV function. Copyright (C) 2005 S. Karger AG, Basel
Can we continue research in splenectomized dogs? Mycoplasma haemocanis: Old problem - New insight
We report the appearance of a Mycoplasma haemocanis infection in laboratory dogs, which has been reported previously, yet, never before in Europe. Outbreak of the disease was triggered by a splenectomy intended to prepare the dogs for a hemorrhagic shock study. The clinical course of the dogs was dramatic including anorexia and hemolytic anemia. Treatment included allogeneic transfusion, prednisone, and oxytetracycline. Systematic follow-up (n=12, blood smears, antibody testing and specific polymerase chain reaction) gives clear evidence that persistent eradication of M. haemocanis is unlikely. We, therefore, had to abandon the intended shock study. In the absence of effective surveillance and screening for M. haemocanis, the question arises whether it is prudent to continue shock research in splenectomized dogs. Copyright (C) 2004 S. Karger AG, Basel
Thermal Conductivity of Spin-1/2 Chains
We study the low-temperature transport properties of clean one-dimensional
spin-1/2 chains coupled to phonons. Due to the presence of approximate
conservation laws, the heat current decays very slowly giving rise to an
exponentially large heat conductivity, . As a result of an
interplay of Umklapp scattering and spinon-phonon coupling, the characteristic
energy scale turns out to be of order , where is
the Debye energy, rather than the magnetic exchange interaction -- in
agreement with recent measurements in SrCuO compounds. A large magnetic field
strongly affects the heat transport by two distinct mechanisms. First, it
induces a LINEAR spinon--phonon coupling, which alters the nature of the fixed point: the elementary excitations of the system are COMPOSITE
SPINON-PHONON objects. Second, the change of the magnetization and the
corresponding change of the wave vector of the spinons strongly affects the way
in which various Umklapp processes can relax the heat current, leading to a
characteristic fractal--like spiky behavior of when plotted as a
function of magnetization at fixed T.Comment: 16 pages, RevTex4, 2 figures included; revised refs. and some useful
comments on experimental relevance. On July 12 2005, added an appendix
correcting an error in the form of the phonon propagator. The main result is
unchange
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