463 research outputs found
First and second order transition of frustrated Heisenberg spin systems
Starting from the hypothesis of a second order transition we have studied
modifications of the original Heisenberg antiferromagnet on a stacked
triangular lattice (STA-model) by the Monte Carlo technique. The change is a
local constraint restricting the spins at the corners of selected triangles to
add up to zero without stopping them from moving freely (STAR-model). We have
studied also the closely related dihedral and trihedral models which can be
classified as Stiefel models. We have found indications of a first order
transition for all three modified models instead of a universal critical
behavior. This is in accordance with the renormalization group investigations
but disagrees with the Monte Carlo simulations of the original STA-model
favoring a new universality class. For the corresponding x-y antiferromagnet
studied before, the second order nature of the transition could also not be
confirmed.Comment: 31 pages, 13 figures, to be published in Euro. J. Phys.
A Spin-1/2 Model for CsCuCl_3 in an External Magnetic Field
CsCuCl_3 is a ferromagnetically stacked triangular spin-1/2 antiferromagnet.
We discuss models for its zero-temperature magnetization process. The models
range from three antiferromagnetically coupled ferromagnetic chains to the full
three-dimensional situation. The situation with spin-1/2 is treated by
expansions around the Ising limit and exact diagonalization. Further,
weak-coupling perturbation theory is used mainly for three coupled chains which
are also investigated numerically using the density-matrix renormalization
group technique. We find that already the three-chain model gives rise to the
plateau-like feature at one third of the saturation magnetization which is
observed in magnetization experiments on CsCuCl_3 for a magnetic field
perpendicular to the crystal axis. For a magnetic field parallel to the crystal
axis, a jump is observed in the experimental magnetization curve in the region
of again about one third of the saturation magnetization. In contrast to
earlier spinwave computations, we do not find any evidence for such a jump with
the model in the appropriate parameter region.Comment: 13 pages LaTeX2e with EPJ macro package (included), 8 (e)ps figures
included using psfig.sty; this is the final version to appear in Eur. Phys. J
B; a few further explanations and one reference adde
Application of Microcanonical Temperature to the Spin Crossover of Fe-co Compounds
Using the Rugh's microcanonical approach to temperature we study the classical model of three dimensional spin-crossover of Fe-Co compounds. These compounds are characterized by magnetic ions that can be in a high-spin or low-spin state. We consider the case of diamagnetic low-spin state. The values of the magnetization average, and fraction of high-spin/low-spin are studied over a wide range of values for the system size, temperature, magnetic field, energy difference, nearest neighbor coupling and exchange interaction. We also address the metastability according to the relative values of interaction parameters and the phase diagram of the model.
Keywords: phase transition, dynamical temperature, spin crossove
Quantum shock waves in the Heisenberg XY model
We show the existence of quantum states of the Heisenberg XY chain which
closely follow the motion of the corresponding semi-classical ones, and whose
evolution resemble the propagation of a shock wave in a fluid. These states are
exact solutions of the Schroedinger equation of the XY model and their
classical counterpart are simply domain walls or soliton-like solutions.Comment: 15 pages,6 figure
Dynamical Effective Medium Theory for Quantum Spins and Multipoles
A dynamical effective medium theory is presented for quantum spins and higher
multipoles such as quadrupole moments. The theory is a generalization of the
spherical model approximation for the Ising model, and is accurate up to
O(1/z_n) where z_n is the number of interacting neighbors. The polarization
function is optimized under the condition that it be diagonal in site indices.
With use of auxiliary fields and path integrals, the theory is flexibly applied
to quantum spins and higher multipoles with many interacting neighbors. A
Kondo-type screening of each spin is proposed for systems with extreme quantum
fluctuations but without conduction electrons.Comment: 16 pages, 3 Postscript figure
Oblique triangular antiferromagnetic phase in CsCuCoCl
The spin-1/2 stacked triangular antiferromagnet CsCuCoCl with
undergoes two phase transitions at zero field. The
low-temperature phase is produced by the small amount of Co doping. In
order to investigate the magnetic structures of the two ordered phases, the
neutron elastic scattering experiments have been carried out for the sample
with . It is found that the intermediate phase is identical to
the ordered phase of CsCuCl, and that the low-temperature phase is an
oblique triangular antiferromagnetic phase in which the spins form a triangular
structure in a plane tilted from the basal plane. The tilting angle which is
42 at K decreases with increasing temperature, and becomes
zero at K. An off-diagonal exchange term is proposed as the
origin of the oblique phase.Comment: 6 pages, 7 figure
Resonance in One--Dimensional Fermi--Edge Singularity
The problem of the Fermi--edge singularity in a one--dimensional
Tomonaga--Luttinger liquid is reconsidered. The backward scattering of the
conduction band electrons on the impurity--like hole in the valence band is
analyzed by mapping the problem onto a Coulomb gas theory. For the case when
the electron--electron interaction is repulsive the obtained exponent of the
one--dimensional Fermi--edge singularity appears to be different from the
exponent found in the previous studies. It is shown that the infrared physics
of the Fermi--edge singularity in the presence of backward scattering and
electron--electron repulsion resembles the physics of the Kondo problem.Comment: 38 pages and 1 figure, to be published in PR
TUNNELING SPECTROSCOPY OF QUANTUM CHARGE FLUCTUATIONS IN THE COULOMB BLOCKADE
We present a theory of Coulomb blockade oscillations in tunneling through a
pair of quantum dots connected by a tunable tunneling junction. The positions
and amplitudes of peaks in the linear conductance are directly related,
respectively, to the ground state energy and to the dynamics of charge
fluctuations. We study analytically both strong and weak interdot tunneling. As
the tunneling decreases, the period of the peaks doubles, as observed
experimentally. In the strong tunneling limit, we predict a striking power law
temperature dependence of the peak amplitudes.Comment: 4 pages, revtex3.0, 1 figure uuencode
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