1,749 research outputs found
High-Temperature Dynamics of Spin Glasses
We develop a systematic expansion method of physical quantities for the SK
model and the finite-dimensional model of spin glasses in
non-equilibrium states. The dynamical probability distribution function is
derived from the master equation using a high temperature expansion. We
calculate the expectation values of physical quantities from the dynamical
probability distribution function. The theoretical curves show satisfactory
agreement with Monte Carlo simulation results in the appropriate temperature
and time regions. A comparison is made with the results of a dynamics theory by
Coolen, Laughton and Sherrington.Comment: 24 pages, figures available on request, LaTeX, uses jpsj.sty, to be
published in J. Phys. Soc. Jpn. 66 No. 7 (1997
New Josephson Plasma Modes in Underdoped YBa2Cu3O6.6 Induced by Parallel Magnetic Field
The c-axis reflectivity spectrum of underdoped YBa2Cu3O6.6 (YBCO) is measured
below Tc=59K in parallel magnetic fields H//CuO2 up to 7T. Upon application of
a parallel field, a new peak appears at finite frequency in the optical
conductivity at the expense of suppression of c-axis condensate weight. We
conclude that the dramatic change originates from different Josephson coupling
strengths between bilayers with and without Josephson vortices. We find that
the 400cm^-1 broad conductivity peak in YBCO gains the spectral weight under
parallel magnetic field; this indicates that the condensate weight at \omega =0
is distributed to the intra-bilayer mode as well as to the new optical
Josephson mode.Comment: 4 pages, 3 figure
Quantum Speedup by Quantum Annealing
We study the glued-trees problem of Childs et. al. in the adiabatic model of
quantum computing and provide an annealing schedule to solve an oracular
problem exponentially faster than classically possible. The Hamiltonians
involved in the quantum annealing do not suffer from the so-called sign
problem. Unlike the typical scenario, our schedule is efficient even though the
minimum energy gap of the Hamiltonians is exponentially small in the problem
size. We discuss generalizations based on initial-state randomization to avoid
some slowdowns in adiabatic quantum computing due to small gaps.Comment: 7 page
Faster annealing schedules for quantum annealing
New annealing schedules for quantum annealing are proposed based on the
adiabatic theorem. These schedules exhibit faster decrease of the excitation
probability than a linear schedule. To derive this conclusion, the asymptotic
form of the excitation probability for quantum annealing is explicitly obtained
in the limit of long annealing time. Its first-order term, which is inversely
proportional to the square of the annealing time, is shown to be determined
only by the information at the initial and final times. Our annealing schedules
make it possible to drop this term, thus leading to a higher order (smaller)
excitation probability. We verify these results by solving numerically the
time-dependent Schrodinger equation for small size systemsComment: 10 pages, 5 figures, minor correction
Influence of Topological Edge States on the Properties of Al/Bi2Se3/Al Hybrid Josephson Devices
In superconductor-topological insulator-superconductor hybrid junctions, the
barrier edge states are expected to be protected against backscattering, to
generate unconventional proximity effects, and, possibly, to signal the
presence of Majorana fermions. The standards of proximity modes for these types
of structures have to be settled for a neat identification of possible new
entities. Through a systematic and complete set of measurements of the
Josephson properties we find evidence of ballistic transport in coplanar
Al-Bi2Se3-Al junctions that we attribute to a coherent transport through the
topological edge state. The shunting effect of the bulk only influences the
normal transport. This behavior, which can be considered to some extent
universal, is fairly independent of the specific features of superconducting
electrodes. A comparative study of Shubnikov - de Haas oscillations and
Scanning Tunneling Spectroscopy gave an experimental signature compatible with
a two dimensional electron transport channel with a Dirac dispersion relation.
A reduction of the size of the Bi2Se3 flakes to the nanoscale is an unavoidable
step to drive Josephson junctions in the proper regime to detect possible
distinctive features of Majorana fermions.Comment: 11 pages, 14 figure
Application of the quantum spin glass theory to image restoration
Quantum fluctuation is introduced into the Markov random fields (MRF's) model
for image restoration in the context of Bayesian approach. We investigate the
dependence of the quantum fluctuation on the quality of BW image restoration by
making use of statistical mechanics. We find that the maximum posterior
marginal (MPM) estimate based on the quantum fluctuation gives a fine
restoration in comparison with the maximum a posterior (MAP) estimate or the
thermal fluctuation based MPM estimate.Comment: 19 pages, 9 figures, 1 table, RevTe
Z2-vortex order of frustrated Heisenberg antiferromagnets in two dimensions
We discuss the recent experimental data on various frustrated
quasi-two-dimensional Heisenberg antiferromagnets from the viewpoint of the
Z2-vortex order, which include S=3/2 triangular-lattice antiferromagnet NaCrO2,
S=1 triangular-lattice antiferromagnet NiGa2S4, S=1/2 organic
triangular-lattice antiferromagnets \kappa-(BEDT-TTF)2Cu2(CN)3 and
EtMe3Sb[Pd(dmit)2]2, and S=1/2 kagome-lattice antiferromagnet volborthite
Cu3V2O7(OH)22H2O, etc.Comment: Proceedings of the International Conference on Frustration in
Condensed Matter (ICFCM
Statistical Transfer Matrix Study of the Multileg Ising Ladders and Tubes
Finite temperature properties of symmetric multileg Ising ladders and
tubes are investigated using the statistical transfer matrix method. The
temperature dependences of the specific heat and entropy are calculated. In the
case of tubes, it is found that the ground state entropy shows an even-odd
oscillation with respect to the number of legs. The same type of oscillation is
also found in the ground state energy. On the contrary, these oscillations do
not take place in ladders. From the temperature-dependence of the specific
heat, it is found that the lowest excitation energy is 4J for even-leg ladders
while it is 2J otherwise, The physical origin of these behaviors is discussed
based on the structure of excitations.Comment: 6 pages, 9 figure
Evidence for unconventional superconducting fluctuations in heavy-fermion compound CeNi2Ge2
We present evidence for unconventional superconducting fluctuations in a
heavy-fermion compound CeNiGe. The temperature dependence of the
Ge nuclear-spin-lattice-relaxation rate indicates the
development of magnetic correlations and the formation of a Fermi-liquid state
at temperatures lower than K, where is constant. The
resistance and measured on an as-grown sample decrease below K and K, respectively; these
are indicative of the onset of superconductivity. However, after annealing the
sample to improve its quality, these superconducting signatures disappear.
These results are consistent with the emergence of unconventional
superconducting fluctuations in close proximity to a quantum critical point
from the superconducting to the normal phase in CeNiGe.Comment: 4pages,5figures,to appear in J. Phys. Soc. Jp
Residual Energies after Slow Quantum Annealing
Features of the residual energy after the quantum annealing are investigated.
The quantum annealing method exploits quantum fluctuations to search the ground
state of classical disordered Hamiltonian. If the quantum fluctuation is
reduced sufficiently slowly and linearly by the time, the residual energy after
the quantum annealing falls as the inverse square of the annealing time. We
show this feature of the residual energy by numerical calculations for
small-sized systems and derive it on the basis of the quantum adiabatic
theorem.Comment: 4 pages, 2 figure
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