192 research outputs found
Alternating commensurate-incommensurate structures in the magnetic phase diagram of CsNiF3
The magnetic phase diagram of the quasi one-dimensional spinchain system
CsNiF below the N\'eel temperature is determined. For magnetic fields
perpendicular to the spin chains incommensurate phases are predicted. From
linear spin-wave theory we obtain the instability line of the paramagnetic
phase as a function of the strength and the direction of the field. The system
undergoes a transition to a commensurate or an incommensurate phase depending
on the direction of the magnetic field. In the commensurate phase the
characterizing wave vector is locked to values describing a two-sublattice
structure, whereas in the incommensurate phase the wave vector changes
continuously between the corresponding two-sublattice wave vectors.Comment: 11 pages, LaTeX, 5 figures, sent to PRB Rapid Communicatio
Spontaneous symmetry breaking and the limit
We point out a basic ambiguity in the limit of the connected
propagator in a spontaneously broken phase. This may represent an indication
that the conventional singlet Higgs boson, rather than being a purely massive
field, might have a gap-less branch. This would dominate the energy spectrum
for and give rise to a very weak, long-range force. The
natural interpretation is in terms of density fluctuations of the `Higgs
condensate': in the region of very long wavelengths, infinitely larger than the
Fermi scale, it cannot be treated as a purely classical c-number field.Comment: 17 pages, LaTex, small changes and some comments adde
Order of Two-Dimensional Isotropic Dipolar Antiferromagnets
The question of the existence of order in two-dimensional isotropic dipolar
Heisenberg antiferromagnets is studied. It is shown that the dipolar
interaction leads to a gap in the spin-wave energy and a nonvanishing order
parameter. The resulting finite N\'eel-temperature is calculated for a square
lattice by means of linear spin-wave theory.Comment: 10 pages, REVTEX, 1 figure available upon request, TUM-CP-93-0
Superdiffusivity of the 1D lattice Kardar-Parisi-Zhang equation
The continuum Kardar-Parisi-Zhang equation in one dimension is lattice
discretized in such a way that the drift part is divergence free. This allows
to determine explicitly the stationary measures. We map the lattice KPZ
equation to a bosonic field theory which has a cubic anti-hermitian
nonlinearity. Thereby it is established that the stationary two-point function
spreads superdiffusively.Comment: 21 page
Kinetic Limit for Wave Propagation in a Random Medium
We study crystal dynamics in the harmonic approximation. The atomic masses
are weakly disordered, in the sense that their deviation from uniformity is of
order epsilon^(1/2). The dispersion relation is assumed to be a Morse function
and to suppress crossed recollisions. We then prove that in the limit epsilon
to 0 the disorder averaged Wigner function on the kinetic scale, time and space
of order epsilon^(-1), is governed by a linear Boltzmann equation.Comment: 71 pages, 3 figure
Ac Stark Effects and Harmonic Generation in Periodic Potentials
The ac Stark effect can shift initially nonresonant minibands in
semiconductor superlattices into multiphoton resonances. This effect can result
in strongly enhanced generation of a particular desired harmonic of the driving
laser frequency, at isolated values of the amplitude.Comment: RevTeX, 10 pages (4 figures available on request), Preprint
UCSBTH-93-2
Dynamical Kohn Anomaly in Surface Acoustic Wave Response in Quantum Hall Systems Near
The dynamical analog of the Kohn Anomaly image of the Fermi Surface is
demonstrated for the response functions to the surface acoustic waves in
Quantum Hall Systems near . Kinks appear in the velocity shift
and attenuation coefficient . The effect is considerably
enhanced under periodic modulation and should be observable.Comment: 5 pages, 2 figures, the published versio
Kirchhoff's Loop Law and the maximum entropy production principle
In contrast to the standard derivation of Kirchhoff's loop law, which invokes
electric potential, we show, for the linear planar electric network in a
stationary state at the fixed temperature,that loop law can be derived from the
maximum entropy production principle. This means that the currents in network
branches are distributed in such a way as to achieve the state of maximum
entropy production.Comment: revtex4, 5 pages, 2 figure
Theoretical search for Chevrel phase based thermoelectric materials
We investigate the thermoelectric properties of some semiconducting Chevrel
phases. Band structure calculations are used to compute thermopowers and to
estimate of the effects of alloying and disorder on carrier mobility. Alloying
on the Mo site with transition metals like Re, Ru or Tc to reach a
semiconducting composition causes large changes in the electronic structure at
the Fermi level. Such alloys are expected to have low carrier mobilities.
Filling with transition metals was also found to be incompatible with high
thermoelectric performance based on the calculated electronic structures.
Filling with Zn, Cu, and especially with Li was found to be favorable. The
calculated electronic structures of these filled Chevrel phases are consistent
with low scattering of carriers by defects associated with the filling. We
expect good mobility and high thermopower in materials with the composition
close to (Li,Cu)MoSe, particularly when Li-rich, and recommend this
system for experimental investigation.Comment: 4 two-column pages, 4 embedded ps figure
Decoherence in circuits of small Josephson junctions
We discuss dephasing by the dissipative electromagnetic environment and by
measurement in circuits consisting of small Josephson junctions. We present
quantitative estimates and determine in which case the circuit might qualify as
a quantum bit. Specifically, we analyse a three junction Cooper pair pump and
propose a measurement to determine the decoherence time .Comment: 4 pages, 4 figure
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