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We argue that photon counts in a superconducting nanowire single-photon detector (SNSPD) are caused by the transition from a current-biased metastable superconducting state to the normal state. Such a transition is triggered by vortices crossing the thin and narrow superconducting strip from one edge to another due to the Lorentz force. Detector counts in SNSPDs may be caused by three processes: (a) a single incident photon with sufficient energy to break enough Cooper pairs to create a normal-state belt across the entire width of the strip (direct photon count), (b) thermally induced single-vortex crossing in the absence of photons (dark count), which at high-bias currents releases the energy sufficient to trigger the transition to the normal state in a belt across the whole width of the strip, and (c) a single incident photon of insufficient energy to create a normal-state belt but initiating a subsequent single-vortex crossing, which provides the rest of the energy needed to create the normal-state belt (vortex-assisted single-photon count). We derive the current dependence of the rate of vortex-assisted photon counts. The resulting photon count rate has a plateau at high currents close to the critical current and drops as a power-law with high exponent at lower currents. While the magnetic field perpendicular to the film plane does not affect the formation of hot spots by photons, it causes the rate of vortex crossings (with or without photons) to increase. We show that by applying a magnetic field one may characterize the energy barrier for vortex crossings and identify the origin of dark counts and vortex-assisted photon counts.

A vortex crossing a thin-film superconducting strip from one edge to the other, perpendicular to the bias current, is the dominant mechanism of dissipation for films of thickness d on the order of the coherence length ξ and of width w much narrower than the Pearl length Λ ≫ w ≫ ξ. At high bias currents, I * < I < Ic, the heat released by the crossing of a single vortex suffices to create a belt-like normal-state region across the strip, resulting in a detectable voltage pulse. Here Ic is the critical current at which the energy barrier vanishes for a single vortex crossing. The belt forms along the vortex path and causes a transition of the entire strip into the normal state. We estimate I * to be roughly Ic/3. Further, we argue that such "hot" vortex crossings are the origin of dark counts in photon detectors, which operate in the regime of metastable superconductivity at currents between I * and Ic. We estimate the rate of vortex crossings and compare it with recent experimental data for dark counts. For currents below I * , i.e., in the stable superconducting but resistive regime, we estimate the amplitude and duration of voltage pulses induced by a single vortex crossing.

We present NMR data in the normal and superconducting states of CeCoIn5 for fields close to Hc2(0)= 11.8 T in the ab plane. Recent experiments identified a first-order transition from the normal to superconducting state for H > 10.5 T, and a new thermodynamic phase below 290 mK within the superconducting state. We find that the Knight shifts of the In(1), In(2) and the Co are discontinuous across the first-order transition and the magnetic linewidths increase dramatically. The broadening differs for the three sites, unlike the expectation for an Abrikosov vortex lattice, and suggests the presence of static spin moments in the vortex cores. In the low-temperature and highfield phase the broad NMR lineshapes suggest ordered local moments, rather than a long wavelength quasiparticle spin density modulation expected for an FFLO phase.PACS numbers: 71.27.+a, 74.70.Tx, 75.20.Hr One of the most intriguing properties observed in Kondo lattice systems is the emergence of unconventional superconductivity near a quantum critical point (QCP). By varying some external parameter such as field or pressure, an antiferromagnetic ground state can be tuned such that the transition temperature goes to zero at the QCP. As the tuning parameter increases past the QCP, conventional Fermi-liquid behavior is recovered below a characteristic temperature T FL [1]. Superconductivity often emerges as the ground state of the system for sufficiently low temperatures in the vicinity of the QCP [2]. The heavy-fermion superconductor CeCoIn 5 exhibits many properties typical of a Kondo lattice system at a QCP. In particular, T FL appears to vanish at the superconducting critical field H c2 (T = 0) for fields along the c axis, suggesting the presence of a field-tuned QCP [3,4]. This interpretation has remained contentious because the ordered state associated with the QCP is superconductivity rather than antiferromagnetism. One explanation is that an antiferromagnetic (AFM) phase is hidden within the superconducting phase diagram, which is the genitor of both the QCP and non-Fermi liquid behavior in the vicinity of H c2 (0). However, when the superconductivity is suppressed with Sn doping, the QCP tracks H c2 (0), and no magnetic state emerges in the phase diagram, whereas pressure separates the QCP [5].In fact, there is a field-induced state, which we will refer to as the B phase, in the H − T phase diagram of CeCoIn 5 that exists just below H c2 (0). The order parameter of the B phase could be either (1) a different symmetry of the superconducting order parameter, (2) a fieldinduced magnetic phase, or (3) a Fulde-Ferrell-LarkinOvchinnikov (FFLO) superconducting phase [6,7,8,9]. The normal to superconducting transition in this system has a critical point at (H, T ) ∼ (10.5T, 0.75K), separating a second to first order transition, and the B phase exists below a temperature T 0 (H) ∼ 290 mK and is bounded by T c (H). NMR experiments suggest the presence of excess quasiparticles associated with nodes in the superconducting FFLO wavefunction [10,11,1...

We present the first femtosecond studies of electron-phonon (e-ph) thermalization in heavy-fermion compounds. The e-ph thermalization time tau(ep) increases below the Kondo temperature by more than 2 orders of magnitude as T=0 K is approached. Analysis using the two-temperature model and numerical simulations based on Boltzmann's equations suggest that this anomalous slowing down of the e-ph thermalization derives from the large electronic specific heat and the suppression of scattering between heavy electrons and phonons.

The mean-square relative displacements (MSRD) of atomic pair motions in crystals are studied as a function of pair distance and temperature using the atomic pair distribution function (PDF). The effects of the lattice vibrations on the PDF peak widths are modelled using both a multi-parameter Born von-Karman (BvK) force model and a single-parameter Debye model. These results are compared to experimentally determined PDFs. We find that the near-neighbor atomic motions are strongly correlated, and that the extent of this correlation depends both on the interatomic interactions and crystal structure. These results suggest that proper account of the lattice vibrational effects on the PDF peak width is important in extracting information on static disorder in a disordered system such as an alloy. Good agreement is obtained between the BvK model calculations of PDF peak widths and the experimentally determined peak widths. The Debye model successfully explains the average, though not detailed, natures of the MSRD of atomic pair motion with just one parameter. Also the temperature dependence of the Debye model largely agrees with the BvK model predictions. Therefore, the Debye model provides a simple description of the effects of lattice vibrations on the PDF peak widths.

A decrease in the rotational period observed in torsional oscillator measurements was recently taken as a possible indication of a putative supersolid state of helium. We reexamine this interpretation and note that the decrease in the rotation period is also consistent with a solidification of a small liquidlike component into a lowtemperature glass. Such a solidification may occur by a low-temperature quench of topological defects (e.g., grain boundaries or dislocations) which we examined in an earlier work. The low-temperature glass can account for not only a monotonic decrease in the rotation period as the temperature is lowered but also explains the peak in the dissipation occurring near the transition point. Unlike the non-classical rotational inertia scenario, which depends on the supersolid fraction, the dependence of the rotational period on external parameters, e.g., the oscillator velocity, provides an alternate interpretation of the oscillator experiments.

We study the c-axis transport of stacked, intrinsic junctions in Bi 2 Sr 2 CaCu 2 O 81d single crystals, fabricated by the double-sided ion beam processing technique from single crystal whiskers. Measurements of the I-V characteristics of these samples allow us to obtain the temperature and voltage dependence of the quasiparticle c-axis conductivity in the superconducting state, the Josephson critical current, and the superconducting gap. We show that the BCS d-wave model in the clean limit for resonant impurity scattering, with a significant contribution from coherent interlayer tunneling, describes satisfactorily the low temperature and low energy c-axis transport of both quasiparticles and Cooper pairs. [S0031-9007 (99)09468-5] PACS numbers: 74.25.Fy, 74.50. + r, 74.72.HsThe observation of the pseudogap in the underdoped cuprate superconductors YBa 2 Cu 3 O 72d , La 22x Sr x CuO 41d , and Bi 2 Sr 2 CaCu 2 O 81d (Bi-2212) is indicative of the breakdown of the Fermi-liquid theory above T c in these systems [1]. On the other hand, the superconducting state is usually discussed in the BCS d-wave pairing model, which is based on the Fermi-liquid picture. Such an approach may be limited because (i) the properties of the normal state determine the mechanism of superconductivity, and (ii) the ratio 2D 0 ͞T c is well above the BCS ratio for d-wave pairing and is strongly doping dependent. Specifically, the BCS approach may fail in describing the properties of the superconducting state that are directly related to the quasiparticles, while the electrodynamics, based on supercurrents (macroscopic quantum phenomena), is insensitive to the pairing mechanism.The interlayer currents of both quasiparticles and Cooper pairs may be studied in highly anisotropic Bi-2212 crystals with Josephson interlayer coupling by measuring the I-V characteristic of the c-axis current. Such measurements provide information on the voltage and temperature dependence of the quasiparticle c-axis current, the energy gap, and the Josephson interlayer current. These data allow us to check the validity of the BCS d-wave model and determine the degree of the coherence of the interlayer tunneling. The question of coherence in both the normal and superconducting state is the focus of numerous studies (see, for example, [2-5]). Recently, Tanabe et al. [3] and Schlenga et al. [4] measured the quasiparticle c-axis transport in the superconducting state of Bi-2212 crystals and concluded that their data support the d-wave pairing scenario. However, their results for the quasiparticle current are insufficient to determine the nature of the interlayer transport and the effect of intralayer scattering on this transport.Our measurements of I-V characteristics have been performed on stacked, intrinsic mesa junctions, fabricated from high quality single crystal Bi-2212 whiskers by double-sided focused ion beam (FIB) processing [6]. For the fabrication, we used the conventional FIB machine of Seiko Instruments Corp., SMI 9800 (SP) with Ga 1 -ion beam. The details of...