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.
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