Aviad Frydman

Tunable superconducting nanoinductors

We characterize inductors fabricated from ultra-thin, approximately 100 nm wide strips of niobium (Nb) and niobium nitride (NbN). These nanowires have a large kinetic inductance in the superconducting state. The kinetic inductance scales linearly with the nanowire length, with a typical value of 1 nH µm(-1) for NbN and 44 pH µm(-1) for Nb at a temperature of 2.5 K. We measure the temperature and current dependence of the kinetic inductance and compare our results to theoretical predictions. We also simulate the self-resonant frequencies of these nanowires in a compact meander geometry. These nanowire inductive elements have applications in a variety of microwave frequency superconducting circuits.

Reset dynamics and latching in niobium superconducting nanowire single-photon detectors

We study the reset dynamics of niobium(Nb)superconductingnanowire single-photon detectors (SNSPDs) using experimental measurements and numerical simulations. The numerical simulations of the detection dynamics agree well with experimental measurements, using independently determined parameters in the simulations. We find that if the photon-induced hotspot cools too slowly, the device will latch into a dc resistive state. To avoid latching, the time for the hotspot to cool must be short compared to the inductive time constant that governs the resetting of the current in the device after hotspot formation. From simulations of the energy relaxation process, we find that the hotspot cooling time is determined primarily by the temperature-dependent electron-phonon inelastic time. Latching prevents reset and precludes subsequent photon detection. Fast resetting to the superconducting state is, therefore, essential, and we demonstrate experimentally how this is achieved. We compare our results to studies of reset and latching in niobium nitride SNSPDs.

Niobium Superconducting Nanowire Single-Photon Detectors

We investigate the performance of superconducting nanowire photon detectors fabricated from ultra-thin Nb. A direct comparison is made between these detectors and similar nanowire detectors fabricated from NbN. We find that Nb detectors are significantly more susceptible than NbN to thermal instability (latching) at high bias. We show that the devices can be stabilized by reducing the input resistance of the readout. Nb detectors optimized in this way are shown to have approximately 2/3 the reset time of similar large-active-area NbN detectors of the same geometry, with approximately 6% detection efficiency for single photons at 470 nm.

The Higgs mode in disordered superconductors close to a quantum phase transition

The Higgs mechanism is best known for generating mass for subatomic particles. Less well-known is that the idea originated in the study of superconductivity, and can be tested in the laboratory.

Universal transport in two-dimensional granular superconductors

The transport properties of quench condensed granular superconductors (Pb, Sn, and Pb-Ag films) are presented and analyzed. These systems exhibit transitions from insulating to superconducting behavior as a function of intergrain spacing. Superconductivity is characterized by broad transitions in which the resistance drops exponentially with reducing temperature. The slope of the log R versus T curves turns out to be universally dependent on the normal state film resistance for all measured granular systems. It does not depend on the material, critical temperature, geometry, or experimental setup. We discuss possible physical scenarios to explain these findings.

Percolation model for the superconductor-insulator transition in granular films

We study the temperature dependence of the superconductor-insulator transition in granular superconductors. Empirically, these systems are characterized by very broad resistance tails, which depend exponentially on the temperature, and the normal state resistance. We model these systems by two-dimensional random resistor percolation networks in which the resistance between two grains is governed either by Josephson junction coupling (Cooper pairs tunneling) or by quasiparticle tunneling. Our numerical simulations as well as an effective medium evaluation explain the experimental results over a wide range of temperatures and resistances. Using effective medium approximation we find an analytical expression for the effective resistance of the system and the value of the critical resistance separating conducting from insulating branches.

Magnetic coding in systems of nanomagnetic particles

Nanomagnetic systems show exotic memory effects in the dc magnetization as a function of temperature. We present magnetization measurements on systems of nanomagnetic particles that show that such systems store the memory of either decrease or increase of magnetic field enabling a magnetic “coding” of “0”s and “1”s. Application of a field larger than a critical field, H* erases the memory effects. We show that this behavior can be explained by a wide distribution of grain sizes having different blocking temperatures. The effect can be used as a tool for measuring spacial and temporal magnetization changes of a magnetic surface.

Resistance distribution in the hopping percolation model

We study the distribution function P (rho) of the effective resistance rho in two- and three-dimensional random resistor networks of linear size L in the hopping percolation model. In this model each bond has a conductivity taken from an exponential form sigma proportional to exp (-kappar) , where kappa is a measure of disorder and r is a random number, 0< or = r < or =1 . We find that in both the usual strong-disorder regime L/ kappa(nu) >1 (not sensitive to removal of any single bond) and the extreme-disorder regime L/ kappa(nu) <1 (very sensitive to such a removal) the distribution depends only on L/kappa(nu) and can be well approximated by a log-normal function with dispersion b kappa(nu) /L , where b is a coefficient which depends on the type of lattice, and nu is the correlation critical exponent.

The superconductor insulator transition in systems of ultrasmall grains

Abstract We present transport measurements on quench condensed granular Pb films in which the grains are about 40–80 A in diameter. These films show a crossover from an insulator to a superconductor behavior as the nominal thickness of the layer is increased. This transition is different in nature than those seen in quench condensed systems reported in the past where the films were either uniform or granular with grain diameters on the order of 200 A. We discuss possible physical mechanisms for these transitions.

Spreading of a mercury droplet on thin gold films

The spreading of a small mercury droplet (150μm) on thin gold films is studied, using an optical microscope enhanced with a differential interference contrast system. The growing interfaces are analyzed in order to determine the roughness (α) and growth (β) exponents. For gold film thickness of 1500A we find that α=0.88±0.03 and β=0.76±0.03, while for gold thickness of 3000A, α=0.96±0.04 and β=1.00±0.04. Both sets of exponents satisfy the scaling relation α+α/β=2. In both systems the roughness exponent α crosses over to a value close to 0.5 in the final stages of the experiment and for relatively long length scales (order of a few microns).