111 research outputs found
Quantum spin metal state on a decorated honeycomb lattice
We present a modification of exactly solvable spin-(1/2) Kitaev model on the
decorated honeycomb lattice, with a ground state of "spin metal" type. The
model is diagonalized in terms of Majorana fermions; the latter form a 2D
gapless state with a Fermi-circle those size depends on the ratio of exchange
couplings. Low-temperature heat capacity C(T) and dynamic spin susceptibility
\chi(\omega,T) are calculated in the case of small Fermi-circle. Whereas
C(T)\sim T at low temperatures as it is expected for a Fermi-liquid, spin
excitations are gapful and \chi(\omega,T) demonstrate unusual behaviour with a
power-law peak near the resonance frequency. The corresponding exponent as well
as the peak shape are calculated.Comment: 4 pages, 3 figure
Power-law spin correlations in a perturbed honeycomb spin model
We consider spin- model on the honeycomb lattice~\cite{Kitaev06}
in presence of a weak magnetic field . Such a perturbation
destroys exact integrability of the model in terms of gapless fermions and
\textit{static} fluxes. We show that it results in appearance of a
long-range tail in the irreducible dynamic spin correlation function: , where is
proportional to the density polarization function of fermions
Optimal fluctuation approach to a directed polymer in a random medium
A modification of the optimal fluctuation approach is applied to study the
tails of the free energy distribution function P(F) for an elastic string in
quenched disorder both in the regions of the universal behavior of P(F) and in
the regions of large fluctuations, where the behavior of P(F) is non-universal.
The difference between the two regimes is shown to consist in whether it is
necessary or not to take into account the renormalization of parameters by the
fluctuations of disorder in the vicinity of the optimal fluctuation.Comment: 4 pages, no figure
Current responses and voltage fluctuations in Josephson-junction systems
We consider arrays of Josephson junctions as well as single junctions in both
the classical and quantum-mechanical regimes, and examine the generalized
(frequency-dependent) resistance, which describes the dynamic responses of such
Josephson-junction systems to external currents. It is shown that the
generalized resistance and the power spectrum of voltage fluctuations are
related via the fluctuation-dissipation theorem. Implications of the obtained
relations are also discussed in various experimental situations.Comment: 4 pages, To appear in Europhys. Let
Vortex Entanglement and Broken Symmetry
Based on the London approximation, we investigate numerically the stability
of the elementary configurations of entanglement, the twisted-pair and the
twisted-triplet, in the vortex-lattice and -liquid phases. We find that, except
for the dilute limit, the twisted-pair is unstable and hence irrelevant in the
discussion of entanglement. In the lattice phase the twisted-triplet
constitutes a metastable, confined configuration of high energy. Loss of
lattice symmetry upon melting leads to deconfinement and the twisted-triplet
turns into a low-energy helical configuration.Comment: 4 pages, RevTex, 2 figures on reques
Identification of the vortex glass phase by harmonics of the AC magnetic susceptibility
We compared the AC magnetic susceptibility behaviour for the vortex glass
phase and for the creep phenomena with an inhomogeneous pinning potential. The
temperature dependence of the harmonics of the susceptibility have been
numerically simulated with these two models, and we studied them as a function
of the frequency, in terms of Cole-Cole plots. From our analysis we show that
it is possible to distinguish between the two different phases, because of
their clear differences in the Cole-Cole plots behaviour with the frequency.Comment: 8 pages, 2 figures to be published on "The Journal of Physics and
Chemistry of Solids
Phase Transitions in a Model Anisotropic High Tc Superconductor
We carry out simulations of the anisotropic uniformly frustrated 3D XY model,
as a model for vortex line fluctuations in high Tc superconductors. We compute
the phase diagram as a function of temperature and anisotropy, for a fixed
applied magnetic field. We find that superconducting coherence parallel to the
field persists into the vortex line liquid state, and that this transition lies
well below the "mean-field" cross-over from the vortex line liquid to the
normal state.Comment: 23 pages + 19 ps figure
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