Limits of sympathetic cooling of fermions: The role of the heat capacity of the coolant

The sympathetic cooling of an initially degenerate Fermi gas by either an ideal Bose gas below $T_c$ or an ideal Boltzmann gas is investigated. It is shown that the efficiency of cooling by a Bose gas below $T_c$ is by no means reduced when its heat capacity becomes much less than that of the Fermi gas, where efficiency is measured by the decrease in the temperature of the Fermi gas per number of particles evaporated from the coolant. This contradicts the intuitive idea that an efficient coolant must have a large heat capacity. In contrast, for a Boltzmann gas a minimal value of the ratio of the heat capacities is indeed necessary to achieve T=0 and all of the particles must be evaporated.Comment: 5 pages, 3 figure

Evidence for A Two-dimensional Quantum Wigner Solid in Zero Magnetic Field

We report the first experimental observation of a characteristic nonlinear threshold behavior from dc dynamical response as an evidence for a Wigner crystallization in high-purity GaAs 2D hole systems in zero magnetic field. The system under increasing current drive exhibits voltage oscillations with negative differential resistance. They confirm the coexistence of a moving crystal along with striped edge states as observed for electrons on helium surfaces. However, the threshold is well below the typical classical levels due to a different pinning and depinning mechanism that is possibly related to a quantum process

Multiwinner Voting with Fairness Constraints

Multiwinner voting rules are used to select a small representative subset of candidates or items from a larger set given the preferences of voters. However, if candidates have sensitive attributes such as gender or ethnicity (when selecting a committee), or specified types such as political leaning (when selecting a subset of news items), an algorithm that chooses a subset by optimizing a multiwinner voting rule may be unbalanced in its selection -- it may under or over represent a particular gender or political orientation in the examples above. We introduce an algorithmic framework for multiwinner voting problems when there is an additional requirement that the selected subset should be "fair" with respect to a given set of attributes. Our framework provides the flexibility to (1) specify fairness with respect to multiple, non-disjoint attributes (e.g., ethnicity and gender) and (2) specify a score function. We study the computational complexity of this constrained multiwinner voting problem for monotone and submodular score functions and present several approximation algorithms and matching hardness of approximation results for various attribute group structure and types of score functions. We also present simulations that suggest that adding fairness constraints may not affect the scores significantly when compared to the unconstrained case.Comment: The conference version of this paper appears in IJCAI-ECAI 201

Calibrating dipolar interaction in an atomic condensate

We revisit the topic of a dipolar condensate with the recently derived more rigorous pseudo-potential for dipole-dipole interaction [A. Derevianko, Phys. Rev. A {\bf 67}, 033607 (2003)]. Based on the highly successful variational technique, we find that all dipolar effects estimated before (using the bare dipole-dipole interaction) become significantly larger, i.e. are amplified by the new velocity-dependent pseudo-potential, especially in the limit of large or small trap aspect ratios. This result points to a promising prospect for detecting dipolar effects inside an atomic condensate.Comment: 5 figures, to be publishe

Magneto-controlled nonlinear optical materials

We exploit theoretically a magneto-controlled nonlinear optical material which contains ferromagnetic nanoparticles with a non-magnetic metallic nonlinear shell in a host fluid. Such an optical material can have anisotropic linear and nonlinear optical properties and a giant enhancement of nonlinearity, as well as an attractive figure of merit.Comment: 11 pages, 2 figures. To be published in Appl. Phys. Let

Higher Order Corrections to Density and Temperature of Fermions from Quantum Fluctuations

A novel method to determine the density and temperature of a system based on quantum Fermionic fluctuations is generalized to the limit where the reached temperature T is large compared to the Fermi energy {\epsilon}f . Quadrupole and particle multiplicity fluctuations relations are derived in terms of T . The relevant Fermi integrals are numerically solved for any values of T and compared to the analytical approximations. The classical limit is obtained, as expected, in the limit of large temperatures and small densities. We propose simple analytical formulas which reproduce the numerical results, valid for all values of T . The entropy can also be easily derived from quantum fluctuations and give important insight for the behavior of the system near a phase transition. A comparison of the quantum entropy to the entropy derived from the ratio of the number of deuterons to neutrons gives a very good agreement especially when the density of the system is very low

Sigma_c Dbar and Lambda_c Dbar states in a chiral quark model

The S-wave Sigma_c Dbar and Lambda_c Dbar states with isospin I=1/2 and spin S=1/2 are dynamically investigated within the framework of a chiral constituent quark model by solving a resonating group method (RGM) equation. The results show that the interaction between Sigma_c and Dbar is attractive, which consequently results in a Sigma_c Dbar bound state with the binding energy of about 5-42 MeV, unlike the case of Lambda_c Dbar state, which has a repulsive interaction and thus is unbound. The channel coupling effect of Sigma_c Dbar and Lambda_c Dbar is found to be negligible due to the fact that the gap between the Sigma_c Dbar and Lambda_c Dbar thresholds is relatively large and the Sigma_c Dbar and Lambda_c Dbar transition interaction is weak.Comment: 7 pages,2 figures. arXiv admin note: text overlap with arXiv:nucl-th/0606056 by other author