875 research outputs found
The microscopic model for the magnetic subsystem in HoNi2B2C
We demonstrate that the system of localized magnetic moments in
HoNiBC can be described by the 4-positional clock model. This model, at
a proper choice of the coupling constants, yields several metamagnetic phases
in magnetic field at zero temperature in full agreement with the experimental
phase diagram. The model incorporates couplings between not nearest neighbors
in the direction perpendicular to the ferromagnetic planes. The same model
leads to a c-modulated magnetic phase near the Curie temperature. The
theoretical value of the modulation wave-vector agrees surprisingly well with
that observed by the neutron diffraction experiment without new adjustable
parameters.Comment: 4 pages, 3 Postscript figures, RevTeX, uses EPSF.st
Correlations in noisy Landau-Zener transitions
We analyze the influence of classical Gaussian noise on Landau-Zener
transitions during a two-level crossing in a time-dependent regular external
field. Transition probabilities and coherence factors become random due to the
noise. We calculate their two-time correlation functions, which describe the
response of this two-level system to a weak external pulse signal. The spectrum
and intensity of the magnetic response are derived. Although fluctuations are
of the same order of magnitude as averages, the results is obtained in an
analytic form.Comment: 12 pages LaTex with 6 EPS figure
Thermal melting of density waves on the square lattice
We present the theory of the effect of thermal fluctuations on commensurate
"p x p" density wave ordering on the square lattice (p >= 3, integer). For the
case in which this order is lost by a second order transition, we argue that
the adjacent state is generically an incommensurate striped state, with
commensurate p-periodic long range order along one direction, and
incommensurate quasi-long-range order along the orthogonal direction. We also
present the routes by which the fully disordered high temperature state can be
reached. For p=4, and at special commensurate densities, the "4 x 4"
commensurate state can melt directly into the disordered state via a self-dual
critical point with non-universal exponents.Comment: 12 pages, 5 figure
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