Effect of superradiance on transport of diffusing photons in cold atomic gases

We show that in atomic gases cooperative effects like superradiance and subradiance lead to a potential between two atoms that decays like $1/r$. In the case of superradiance, this potential is attractive for close enough atoms and can be interpreted as a coherent mesoscopic effect. The contribution of superradiant pairs to multiple scattering properties of a dilute gas, such as photon elastic mean free path and group velocity, is significantly different from that of independent atoms. We discuss the conditions under which these effects may be observed and compare our results to recent experiments on photon transport in cold atomic gases.Comment: 4 pages and 1 figur

An electronic Mach-Zehnder interferometer in the Fractional Quantum Hall effect

We compute the interference pattern of a Mach-Zehnder interferometer operating in the fractional quantum Hall effect. Our theoretical proposal is inspired by a remarkable experiment on edge states in the Integer Quantum Hall effect (IQHE). The Luttinger liquid model is solved via two independent methods: refermionization at nu=1/2 and the Bethe Ansatz solution available for Laughlin fractions. The current differs strongly from that of single electrons in the strong backscattering regime. The Fano factor is periodic in the flux, and it exhibits a sharp transition from sub-Poissonian (charge e/2) to Poissonian (charge e) in the neighborhood of destructive interferences

Vortex nucleation through edge states in finite Bose-Einstein condensates

We study the vortex nucleation in a finite Bose-Einstein condensate. Using a set of non-local and chiral boundary conditions to solve the Schr$\ddot{o}$dinger equation of non-interacting bosons in a rotating trap, we obtain a quantitative expression for the characteristic angular velocity for vortex nucleation in a condensate which is found to be 35% of the transverse harmonic trapping frequency.Comment: 24 pages, 8 figures. Both figures and the text have been revise

Coherent Backscattering of Ultracold Atoms

We report on the direct observation of coherent backscattering (CBS) of ultracold atoms, in a quasi-two-dimensional configuration. Launching atoms with a well-defined momentum in a laser speckle disordered potential, we follow the progressive build up of the momentum scattering pattern, consisting of a ring associated with multiple elastic scattering, and the CBS peak in the backward direction. Monitoring the depletion of the initial momentum component and the formation of the angular ring profile allows us to determine microscopic transport quantities. The time resolved evolution of the CBS peak is studied and is found a fair agreement with predictions, at long times as well as at short times. The observation of CBS can be considered a direct signature of coherence in quantum transport of particles in disordered media. It is responsible for the so called weak localization phenomenon, which is the precursor of Anderson localization.Comment: 5 pages, 4 figure

Energy levels and their correlations in quasicrystals

Quasicrystals can be considered, from the point of view of their electronic properties, as being intermediate between metals and insulators. For example, experiments show that quasicrystalline alloys such as AlCuFe or AlPdMn have conductivities far smaller than those of the metals that these alloys are composed from. Wave functions in a quasicrystal are typically intermediate in character between the extended states of a crystal and the exponentially localized states in the insulating phase, and this is also reflected in the energy spectrum and the density of states. In the theoretical studies we consider in this review, the quasicrystals are described by a pure hopping tight binding model on simple tilings. We focus on spectral properties, which we compare with those of other complex systems, in particular, the Anderson model of a disordered metal.Comment: 15 pages including 19 figures. Review article, submitted to Phil. Ma

Dissipation-driven superconductor-insulator transition in linear arrays of Josephson junctions capacitively coupled to metallic films

We study the low-temperature properties of linear Josephson-junction arrays capacitively coupled to a proximate two-dimensional diffusive metal. Using bosonization techniques, we derive an effective model for the array and obtain its critical properties and phases at T = 0 using a renormalization group analysis and a variational approach. While static screening effects given by the presence of the metal can be absorbed in a renormalization of the parameters of the array, backscattering originated in the dynamically screened Coulomb interaction produces a non-trivial stabilization of the insulating groundstate and can drive a superconductor-insulator transition. We study the consequences for the transport properties in the low-temperature regime. In particular, we calculate the resisitivity as a function of the temperature and the parameters of the array, and obtain clear signatures of a superconductor-insulator transition that could be observed in experiments.Comment: 10 pages, 5 figures, submitted to Physical Review

Athletic trainers\u27 or athletic training students\u27 perceptions of sexual harassment by student-athletes in the collegiate setting

Sexual harassment is an issue that occurs in healthcare professions and on college campuses nationwide. Athletic trainers employed at the collegiate setting are healthcare professionals who work in close conjunction with student-athletes, which may predispose a risk of a sexual harassment occurrence. A study was conducted to investigate the sexual harassment of athletic trainers and athletic training students by student athletes at the collegiate setting. A Qualtrics survey, containing closed- and open-ended questions, was sent out to 297 CAATE-accredited athletic training program directors; the athletic training staff at these institutions were also sent the survey as well. The program directors were asked to send the survey to the students currently enrolled in the program. Quantitative data were analyzed using Qualtrics and the qualitative data were analyzed using themes and coding. The study revealed that athletic trainers and athletic training students were sexually harassed by student-athletes on at least one occasion. 40% of the respondents stated being sexually harassed by a student-athlete, and 59% observed sexual harassment of a member of the athletic training staff on at least one instance

Dissipative phase-fluctuations in superconducting wires capacitively coupled to diffusive metals

We study the screening of the Coulomb interaction in a quasi one-dimensional superconductor given by the presence of either a one- or a two-dimensional non-interacting electron gas. To that end, we derive an effective low-energy phase-only action, which amounts to treating the Coulomb and superconducting correlations in the random-phase approximation. We concentrate on the study of dissipation effects in the superconductor, induced by the effect of Coulomb coupling to the diffusive density-modes in the metal, and study its consequences on the static and dynamic conductivity. Our results point towards the importance of the dimensionality of the screening metal in the behavior of the superconducting plasma mode of the wire at low energies. In absence of topological defects, and when the screening is given by a one-dimensional electron gas, the superconducting plasma mode is completely damped in the limit $q\to 0$, and consequently superconductivity is lost in the wire. In contrast, we recover a Drude-response in the conductivity when the screening is provided by a two-dimensional electron gas.Comment: 16 pages, 8 figures, 1 table, 2 appendice