34,982 research outputs found
Collective atomic recoil motion in short-pulse multi-matter-optical wave mixing
An analytical perturbation theory of short-pulse, matter-wave superradiant
scatterings is presented. We show that Bragg resonant enhancement is
incapacitated and both positive and negative order scatterings contribute
equally. We further show that propagation gain is small and scattering events
primarily occur at the end of the condensate where the generated field has
maximum strength, thereby explaining the apparent ``asymmetry" in the scattered
components with respect to the condensate center. In addition, the generated
field travels near the speed of light in a vacuum, resulting in significant
spontaneous emission when the one-photon detuning is not sufficiently large.
Finally, we show that when the excitation rate increases, the generated-field
front-edge-steepening and peak forward-shifting effects are due to depletion of
the ground state matter wave.Comment: This manuscript was submitted for publication in Nov., 200
Single-cluster dynamics for the random-cluster model
We formulate a single-cluster Monte Carlo algorithm for the simulation of the
random-cluster model. This algorithm is a generalization of the Wolff
single-cluster method for the -state Potts model to non-integer values
. Its results for static quantities are in a satisfactory agreement with
those of the existing Swendsen-Wang-Chayes-Machta (SWCM) algorithm, which
involves a full cluster decomposition of random-cluster configurations. We
explore the critical dynamics of this algorithm for several two-dimensional
Potts and random-cluster models. For integer , the single-cluster algorithm
can be reduced to the Wolff algorithm, for which case we find that the
autocorrelation functions decay almost purely exponentially, with dynamic
exponents , and for , and
4 respectively. For non-integer , the dynamical behavior of the
single-cluster algorithm appears to be very dissimilar to that of the SWCM
algorithm. For large critical systems, the autocorrelation function displays a
range of power-law behavior as a function of time. The dynamic exponents are
relatively large. We provide an explanation for this peculiar dynamic behavior.Comment: 7 figures, 4 table
Computing the Least-core and Nucleolus for Threshold Cardinality Matching Games
Cooperative games provide a framework for fair and stable profit allocation
in multi-agent systems. \emph{Core}, \emph{least-core} and \emph{nucleolus} are
such solution concepts that characterize stability of cooperation. In this
paper, we study the algorithmic issues on the least-core and nucleolus of
threshold cardinality matching games (TCMG). A TCMG is defined on a graph
and a threshold , in which the player set is and the profit of
a coalition is 1 if the size of a maximum matching in
meets or exceeds , and 0 otherwise. We first show that for a TCMG, the
problems of computing least-core value, finding and verifying least-core payoff
are all polynomial time solvable. We also provide a general characterization of
the least core for a large class of TCMG. Next, based on Gallai-Edmonds
Decomposition in matching theory, we give a concise formulation of the
nucleolus for a typical case of TCMG which the threshold equals . When
the threshold is relevant to the input size, we prove that the nucleolus
can be obtained in polynomial time in bipartite graphs and graphs with a
perfect matching
Probing the Melting of a Two-dimensional Quantum Wigner Crystal via its Screening Efficiency
One of the most fundamental and yet elusive collective phases of an
interacting electron system is the quantum Wigner crystal (WC), an ordered
array of electrons expected to form when the electrons' Coulomb repulsion
energy eclipses their kinetic (Fermi) energy. In low-disorder, two-dimensional
(2D) electron systems, the quantum WC is known to be favored at very low
temperatures () and small Landau level filling factors (), near the
termination of the fractional quantum Hall states. This WC phase exhibits an
insulating behavior, reflecting its pinning by the small but finite disorder
potential. An experimental determination of a vs phase diagram for
the melting of the WC, however, has proved to be challenging. Here we use
capacitance measurements to probe the 2D WC through its effective screening as
a function of and . We find that, as expected, the screening
efficiency of the pinned WC is very poor at very low and improves at higher
once the WC melts. Surprisingly, however, rather than monotonically
changing with increasing , the screening efficiency shows a well-defined
maximum at a which is close to the previously-reported melting temperature
of the WC. Our experimental results suggest a new method to map out a vs
phase diagram of the magnetic-field-induced WC precisely.Comment: The formal version is published on Phys. Rev. Lett. 122, 116601
(2019
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