Superfluid Motion of Light

Superfluidity, the ability of a fluid to move without dissipation, is one of the most spectacular manifestations of the quantum nature of matter. We explore here the possibility of superfluid motion of light. Controlling the speed of a light packet with respect to a defect, we demonstrate the presence of superfluidity and, above a critical velocity, its breakdown through the onset of a dissipative phase. We describe a possible experimental realization based on the transverse motion through an array of waveguides. These results open new perspectives in transport optimization.Comment: 4 pages, 3 figure

Failed theories of superconductivity

Almost half a century passed between the discovery of superconductivity by Kamerlingh Onnes and the theoretical explanation of the phenomenon by Bardeen, Cooper and Schrieffer. During the intervening years the brightest minds in theoretical physics tried and failed to develop a microscopic understanding of the effect. A summary of some of those unsuccessful attempts to understand superconductivity not only demonstrates the extraordinary achievement made by formulating the BCS theory, but also illustrates that mistakes are a natural and healthy part of the scientific discourse, and that inapplicable, even incorrect theories can turn out to be interesting and inspiring.Comment: 14 pages, 3 figures (typos fixed), to appear in: Bardeen Cooper and Schrieffer: 50 YEARS, edited by Leon N Cooper and Dmitri Feldma

Children’s sensitivity to speaker accuracy and explanatory competence with biological concepts

This thesis investigated children’s selective trust in contexts that extend beyond a direct comparison of a distinctly accurate labeller with a distinctly inaccurate labeller in the domain of artifacts. First, it was examined whether children’s evaluations of informant trustworthiness is similar across the biological domain and the domain of artifacts. Second, it was investigated how children interpret novel labellers when compared to accurate and inaccurate labellers. Finally, children were presented with informants who provided functional or surface information for body parts to determine whether they prefer learning from informants who provided functional explanations. Across five experiments, children aged between 3 and 8 years of age (N = 379) were tested. The main findings were as follows: (a) 4- and 5-year-olds knew more about external body parts than internal organs; (b) 5-year-olds began to appreciate that speakers offering novel information were more trustworthy than those offering inaccurate information; (c) 4- to 8-year-olds had difficulty with distinguishing between informants who provided either functional explanations or obvious descriptions for highly unfamiliar organs; (d) however, when presented with informants who provided either functional or obvious information for highly familiar body parts, 8-year-olds (and to some extent, 5-year-olds) showed better recall of which informant provided a particular type of explanation, but they did not consider either informant to be a more trustworthy source. These findings indicate that children demonstrate selective trust in the biological domain, as well as in contexts that go beyond comparing accurate and inaccurate labellers. It is apparent that children are balanced in their evaluations of informants who provide new information, as well as those provide information that varies in explanatory depth. However, they are yet to fully consider functional explanations to be superior to superficial descriptions

Local Information Processing in Adults with High Functioning Autism and Asperger Syndrome: The Usefulness of Neuropsychological Tests and Self-Reports

Local information processing in 42 adults with high functioning autism, 41 adults with Asperger syndrome and 41 neurotypical adults was examined. Contrary to our expectations, the disorder groups did not outperform the neurotypical group in the neuropsychological measures of local information processing. In line with our hypotheses, the self-reports did show higher levels of local information processing and a stronger tendency to use systemizing strategies in the two disorder groups. Absent and weak correlations were found between the self-reports and the two neuropsychological tasks in the three groups. The neuropsychological tests and the self-reports seem to measure different underlying constructs. The self-reports were most predictive of the presence of an autism spectrum diagnosis

A branch-point approximant for the equation of state of hard spheres

Using the first seven known virial coefficients and forcing it to possess two branch-point singularities, a new equation of state for the hard-sphere fluid is proposed. This equation of state predicts accurate values of the higher virial coefficients, a radius of convergence smaller than the close-packing value, and it is as accurate as the rescaled virial expansion and better than the Pad\'e [3/3] equations of state. Consequences regarding the convergence properties of the virial series and the use of similar equations of state for hard-core fluids in $d$ dimensions are also pointed out.Comment: 6 pages, 4 tables, 3 figures; v2: enlarged version, extension to other dimensionalities; v3: typos in references correcte

Quantum Statistics of Interacting Dimer Spin Systems

The compound TlCuCl3 represents a model system of dimerized quantum spins with strong interdimer interactions. We investigate the triplet dispersion as a function of temperature by inelastic neutron scattering experiments on single crystals. By comparison with a number of theoretical approaches we demonstrate that the description of Troyer, Tsunetsugu, and Wuertz [Phys. Rev. B 50, 13515 (1994)] provides an appropriate quantum statistical model for dimer spin systems at finite temperatures, where many-body correlations become particularly important.Comment: 4 pages, 4 figures, to appear in Physical Review Letter

Many-body effects in nuclear structure

We calculate, for the first time, the state-dependent pairing gap of a finite nucleus (120Sn) diagonalizing the bare nucleon-nucleon potential (Argonne v14) in a Hartree-Fock basis (with effective k-mass m_k eqult to 0.7 m), within the framework of the BCS approximation including scattering states up to 800 MeV above the Fermi energy to achieve convergence. The resulting gap accounts for about half of the experimental gap. We find that a consistent description of the low-energy nuclear spectrum requires, aside from the bare nucleon-nucleon interaction, not only the dressing of single-particle motion through the coupling to the nuclear surface, to give the right density of levels close to the Fermi energy (and thus an effective mass m* approximately equal to m), but also the renormalization of collective vibrational modes through vertex and self-energy processes, processes which are also found to play an essential role in the pairing channel, leading to a long range, state dependent component of the pairing interaction. The combined effect of the bare nucleon-nucleon potential and of the induced pairing interaction arising from the exchange of low-lying surface vibrations between nucleons moving in time reversal states close to the Fermi energy accounts for the experimental gap.Comment: 5 pages, 4 figures; author list correcte

Bayesian network conflict detection for normative monitoring of black-box systems

Bayesian networks are interpretable probabilistic models that can be constructed from both data and domain knowledge. They are applied in various domains and for different tasks, including that of anomaly detection, for which an easy to compute measure of data conflict exists. In this paper we consider the use of Bayesian networks to monitor input-output pairs of a black-box AI system, to establish whether the output is acceptable in the current context in which the AI system operates. A Bayesian network-based prescriptive, or normative, model is assumed that includes context variables relevant for deciding what is or is not acceptable. We analyse and adjust the conflict measure to make it applicable to our new type of monitoring setting

BCS ansatz, Bogoliubov approach to superconductivity and Richardson-Gaudin exact wave function

The Bogoliubov approach to superconductivity provides a strong mathematical support to the wave function ansatz proposed by Bardeen, Cooper and Schrieffer (BCS). Indeed, this ansatz --- with all pairs condensed into the same state --- corresponds to the ground state of the Bogoliubov Hamiltonian. Yet, this Hamiltonian only is part of the BCS Hamiltonian. As a result, the BCS ansatz definitely differs from the BCS Hamiltonian ground state. This can be directly shown either through a perturbative approach starting from the Bogoliubov Hamiltonian, or better by analytically solving the BCS Schr\"{o}dinger equation along Richardson-Gaudin exact procedure. Still, the BCS ansatz leads not only to the correct extensive part of the ground state energy for an arbitrary number of pairs in the energy layer where the potential acts --- as recently obtained by solving Richardson-Gaudin equations analytically --- but also to a few other physical quantities such as the electron distribution, as here shown. The present work also considers arbitrary filling of the potential layer and evidences the existence of a super dilute and a super dense regime of pairs, with a gap \emph{different} from the usual gap. These regimes constitute the lower and upper limits of density-induced BEC-BCS cross-over in Cooper pair systems.Comment: 15 pages, no figure

The moderating effect of cognitive abilities on the association between sensory processing and emotional and behavioural problems and social participation in autistic individuals

Education and Child Studie