171 research outputs found
Low temperature structural phase transition and incommensurate lattice modulation in the spin gap compound BaCuSi2O6
Results of high resolution x-ray diffraction experiments are presented for
single crystals of the spin gap compound BaCuSiO in the temperature
range from 16 to 300 K. The data show clear evidence of a transition from the
room temperature tetragonal phase into an incommensurately modulated
orthorhombic structure below 100 K. This lattice modulation is
characterized by a resolution limited wave vector {\bf
q}=(0,0.13,0) and its 2 and 3 harmonics. The phase
transition is first order and exhibits considerable hysteresis. This
observation implies that the spin Hamiltonian representing the system is more
complex than originally thought.Comment: 4 pages, 4 figure
Influence of an external magnetic field on the decoherence of a central spin coupled to an antiferromagnetic environment
Using the spin wave approximation, we study the decoherence dynamics of a
central spin coupled to an antiferromagnetic environment under the application
of an external global magnetic field. The external magnetic field affects the
decoherence process through its effect on the antiferromagnetic environment. It
is shown explicitly that the decoherence factor which displays a Gaussian decay
with time depends on the strength of the external magnetic field and the
crystal anisotropy field in the antiferromagnetic environment. When the values
of the external magnetic field is increased to the critical field point at
which the spin-flop transition (a first-order quantum phase transition) happens
in the antiferromagnetic environment, the decoherence of the central spin
reaches its highest point. This result is consistent with several recent
quantum phase transition witness studies. The influences of the environmental
temperature on the decoherence behavior of the central spin are also
investigated.Comment: 29 preprint pages, 4 figures, to appear in New Journal of Physic
From Kondo Effect to Fermi Liquid
The Kondo effect has been playing an important role in strongly correlated
electon systems. The important point is that the magnetic impurity in metals is
a typical example of the Fermi liquid. In the system the local spin is
conserved in the ground state and continuity with respect to Coulomb repulsion
is satisfied. This nature is satisfied also in the periodic systems as far
as the systems remain as the Fermi liquid. This property of the Fermi liquid is
essential to understand the cuprate high-Tc superconductors (HTSC). On the
basis of the Fermi liquid theory we develop the transport theory such as the
resistivity and the Hall coefficient in strongly correlated electron systems,
such as HTSC, organic metals and heavy Fermion systems. The significant role of
the vertex corrections for total charge- and heat-currents on the transport
phenomena is explained. By taking the effect of the current vertex corrections
into account, various typical non-Fermi-liquid-like transport phenomena in
systems with strong magnetic and/or superconducting flucutations are explained
within the Fermi liquid theory.Comment: 14 pages, an article for the special edition of JPSJ "Kondo Effect --
40 Years after the Discovery
Magnetization process of the spin-1/2 XXZ models on square and cubic lattices
The magnetization process of the spin-1/2 antiferromagnetic XXZ model with
Ising-like anisotropy in the ground state is investigated. We show numerically
that the Ising-like XXZ models on square and cubic lattices show a first-order
phase transition at some critical magnetic field. We estimate the value of the
critical field and the magnetization jump on the basis of the Maxwell
construction. The magnetization jump in the Ising-limit is investigated by
means of perturbation theory. Based on our numerical results, we briefly
discuss the phase diagram of the extended Bose-Hubbard model in the hard-core
limit.Comment: 13 pages, RevTex, 7 PostScript figures, to appear in Phys.Rev.
Entanglement Mean Field Theory and the Curie-Weiss Law
The mean field theory, in its different hues, form one of the most useful
tools for calculating the single-body physical properties of a many-body
system. It provides important information, like critical exponents, of the
systems that do not yield to an exact analytical treatment. Here we propose an
entanglement mean field theory (EMFT) to obtain the behavior of the two-body
physical properties of such systems. We apply this theory to predict the phases
in paradigmatic strongly correlated systems, viz. the transverse anisotropic
XY, the transverse XX, and the Heisenberg models. We find the critical
exponents of different physical quantities in the EMFT limit, and in the case
of the Heisenberg model, we obtain the Curie-Weiss law for correlations. While
the exemplary models have all been chosen to be quantum ones, classical
many-body models also render themselves to such a treatment, at the level of
correlations.Comment: 5 pages, 4 figure
A polarized neutron-scattering study of the Cooper-pair moment in Sr2RuO4
We report a study of the magnetization density in the mixed state of the
unconventional superconductor S2RuO4. On entering the superconducting state we
find no change in the magnitude or distribution of the induced moment for a
magnetic field of 1 Tesla applied within the RuO2 planes. Our results are
consistent with a spin-triplet Cooper pairing with spins lying in the basal
plane. This is in contrast with similar experiments performed on conventional
and high-Tc superconductors.Comment: Submitted to Physical Review Letter
Spin-Glass State in
Magnetic susceptibility, magnetization, specific heat and positive muon spin
relaxation (\musr) measurements have been used to characterize the magnetic
ground-state of the spinel compound . We observe a spin-glass
transition of the S=1/2 spins below characterized
by a cusp in the susceptibility curve which suppressed when a magnetic field is
applied. We show that the magnetization of depends on the
magnetic histo Well below , the muon signal resembles the dynamical
Kubo-Toyabe expression reflecting that the spin freezing process in results Gaussian distribution of the magnetic moments. By means of
Monte-Carlo simulati we obtain the relevant exchange integrals between the spins in this compound.Comment: 6 pages, 16 figure
Spin-Atomic Vibration Interaction and Spin-Flip Hamiltonian of a Single Atomic Spin in a Crystal Field
We derive the spin-atomic vibration interaction and the
spin-flip Hamiltonian of a single atomic spin in a crystal field.
We here apply the perturbation theory to a model with the spin-orbit
interaction and the kinetic and potential energies of electrons. The model also
takes into account the difference in vibration displacement between an
effective nucleus and electrons, \Delta {{\boldmath r}}. Examining the
coefficients of and , we first show that
appears for \Delta {{\boldmath r}}0, while is present
independently of \Delta {{\boldmath r}}. As an application, we next obtain
and of an Fe ion in a crystal field of tetragonal
symmetry. It is found that the magnitudes of the coefficients of
can be larger than those of the conventional spin-phonon interaction depending
on vibration frequency. In addition, transition probabilities per unit time due
to and are investigated for the Fe ion with an
anisotropy energy of , where is an anisotropy constant and
is the component of a spin operator.Comment: 55 pages, 17 figures, to be published in J. Phys. Soc. Jpn. 79 (2010)
No. 11, typos correcte
Low Temperature Properties of Anisotropic Superconductors with Kondo Impurities
We present a self-consistent theory of superconductors in the presence of
Kondo impurities, using large- slave-boson methods to treat the impurity
dynamics. The technique is tested on the s-wave case and shown to give good
results compared to other methods for . We calculate low temperature
thermodynamic and transport properties for various superconducting states,
including isotropic s-wave and representative anisotropic model states with
line and point nodes on the Fermi surface.Comment: 21 pages, RevTeX 3.0, 12 figures available upon request, UF preprin
Homogenized dynamics of stochastic partial differential equations with dynamical boundary conditions
A microscopic heterogeneous system under random influence is considered. The
randomness enters the system at physical boundary of small scale obstacles as
well as at the interior of the physical medium. This system is modeled by a
stochastic partial differential equation defined on a domain perforated with
small holes (obstacles or heterogeneities), together with random dynamical
boundary conditions on the boundaries of these small holes.
A homogenized macroscopic model for this microscopic heterogeneous stochastic
system is derived. This homogenized effective model is a new stochastic partial
differential equation defined on a unified domain without small holes, with
static boundary condition only. In fact, the random dynamical boundary
conditions are homogenized out, but the impact of random forces on the small
holes' boundaries is quantified as an extra stochastic term in the homogenized
stochastic partial differential equation. Moreover, the validity of the
homogenized model is justified by showing that the solutions of the microscopic
model converge to those of the effective macroscopic model in probability
distribution, as the size of small holes diminishes to zero.Comment: Communications in Mathematical Physics, to appear, 200
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