M. Yu. Mikhailov

Superconducting single-photon detector made of MoSi film

We fabricated and characterized nanowire superconducting single-photon detectors made of 4 nm thick amorphous Mo x Si1−x films. At 1.7 K the best devices exhibit a detection efficiency (DE) up to 18% at 1.2 wavelength of unpolarized light, a characteristic response time of about 6 ns and timing jitter of 120 ps. The DE was studied in wavelength range from 650 nm to 2500 nm. At wavelengths below 1200 nm these detectors reach their maximum DE limited by photon absorption in the thin MoSi film.

Interfacial superconductivity in bilayer and multilayer IV–VI semiconductor heterostructures

A comprehensive investigation and comparison of the superconducting properties of bilayer and multilayer epitaxial heterostructures of IV–VI semiconductors exhibiting superconductivity at critical temperatures Tc⩽6.5K is carried out. The superconductivity of these systems is due to inversion of the bands in the narrow-gap semiconductors on account of the nonuniform stresses created by the grids of misfit dislocations arising at the interfaces during the epitaxial growth. It is found that Tc and the character of the superconducting transition of bilayer PbTe∕PbS heterostructures depend on the thickness d of the semiconductor layers and are directly related to the quality of the grids of misfit dislocations at the interfaces (the number and type of structural defects in the grids). Substantial differences in the behavior of bilayer sandwiches and superlattices are found. The minimum thickness d at which superconductivity appears is several times larger for bilayer than for multilayer systems. The upper crit...

Commensurate vortex lattices in thin vanadium films and in V/Si superconducting superlattices

The features of the Shubnikov phase in thin films of type-II superconductors are investigated in the case when the magnetic field is parallel to the surface of the film. Measurements of the nonmonotonic dependence of the critical current Ic on the magnetic field H∥ reveal commensurate vortex lattices with different numbers of vortex chains in the film. It is proved experimentally that in homogeneous films the commensurability effect between the vortex lattice parameter and the thickness of the film can be observed only for the ideal state of the film boundaries, admitting the formation of an infinite lattice of the vortices and their images. Worsening of the film surface smoothness and of the plane-parallel precision of the two surfaces of the film leads to vanishing of the oscillations of Ic and to a sharp decrease of the critical current. The lock-in transition due to the influence of the surface barrier is observed in the films for the first time. It is found that a thin-film layered sample exhibits an...

Mo/Si superlattices: phase transitions between the commensurate states in the vortex ensemble

The avalanche-type jumps of the resistivity in the mixed state have been observed on the artificial superconducting multilayers Mo/Si with weak Josephson interlayer coupling when an applied magnetic field was oriented along the layer planes. These jumps are periodic with an inverse magnetic field. The simple model is proposed which explains the jump patterns on different samples as the manifestation of the phase transitions between various commensurate states of the vortex lattice. The values of the jump fields and the distances between these fields are quantitatively consistent with the model considered. It is assumed that the set of the jumps corresponds to the sequence of the upper critical fields for different commensurate lattices in the strongly layered superconductor. These phase transitions belong to the class of the first order transitions in distinction to the corresponding transition in homogeneous type-II superconductors. The resistive curves are hysteretic and reveal considerable supercooling.

Mo/Si and W/Si multilayers: commensurated vortex lattice structures and superconductivity reentrance

Abstract We report on the unusual behavior of Mo/Si and W/Si multilayers (MLs) in parallel and slightly inclined to the ML planes magnetic fields. Field dependencies of resistivity are nonmonotonic and display minima or zero resistance regions (ZRR) in magnetic fields different from zero. We show that such a behavior is connected with strong intrinsic pinning and vortex lattice (VL) commensurability with underlying layered structure. The locations of ZRR correspond to stable VL configurations.

Commensurability effect and lock-in transition in Mo/Si superconducting superlattices

We report the first observation of the lock-in transition in artificial superconducting superlattices, which takes place in tilted magnetic fields. The measurements were carried out on the Mo/Si layered system. The temperature dependence of the critical angle for the trapping of the vortices in the orientation parallel to the layer planes is determined by the previously known resistive method and by a new method based on the effect of commensurability between the intervortex distance and the superlattice wavelength. The temperature dependences of the critical angle obtained by the two methods practically coincide. The experimental results are consistent with the theoretical predictions of Feinberg and Villard.

Photon counting statistics of superconducting single-photon detectors made of a three-layer WSi film

Superconducting nanowire single-photon detectors (SNSPD) are used in quantum optics when record-breaking time resolution, high speed, and exceptionally low levels of dark counts (false readings) are required. Their detection efficiency is limited, however, by the absorption coefficient of the ultrathin superconducting film for the detected radiation. One possible way of increasing the detector absorption without limiting its broadband response is to make a detector in the form of several vertically stacked layers and connect them in parallel. For the first time we have studied single-photon detection in a multilayer structure consisting of three superconducting layers of amorphous tungsten silicide (WSi) separated by thin layers of amorphous silicon. Two operating modes of the detector are illustrated: an avalanche regime and an arm-trigger regime. A shift in these modes occurs at currents of ∼0.5–0.6 times the critical current of the detector.Superconducting nanowire single-photon detectors (SNSPD) are used in quantum optics when record-breaking time resolution, high speed, and exceptionally low levels of dark counts (false readings) are required. Their detection efficiency is limited, however, by the absorption coefficient of the ultrathin superconducting film for the detected radiation. One possible way of increasing the detector absorption without limiting its broadband response is to make a detector in the form of several vertically stacked layers and connect them in parallel. For the first time we have studied single-photon detection in a multilayer structure consisting of three superconducting layers of amorphous tungsten silicide (WSi) separated by thin layers of amorphous silicon. Two operating modes of the detector are illustrated: an avalanche regime and an arm-trigger regime. A shift in these modes occurs at currents of ∼0.5–0.6 times the critical current of the detector.

Commensurate vortex lattices and oscillation effects in superconducting Mo/Si and W/Si multilayers

We report experimental results of a vortex lattice structure investigation in artificial superconducting Mo/Si and W/Si superlattices. Resistance R and critical current Ic measurements in parallel magnetic fields are performed as well as measurements in tilted magnetic fields. At temperatures where the condition of strong layering is satisfied the dependences Ic(H∥) and R(H∥) exhibit oscillatory behavior. It is shown that the appearance of oscillations and of reentrant behavior (vanishing of the resistivity in definite ranges of H∥) are due to the strong intrinsic pinning and to the effect of commensurability between the vortex lattice period and multilayer wavelength. The locations of Ic(H∥) and R(H∥) extrema correspond to the stable states of a commensurate vortex lattice. Our experimental data are in good quantitative agreement with the Ivlev, Kopnin, and Pokrovsky (IKP) theory. It is shown that the values of the commensurability fields depend exclusively on the superlattice period s and anisotropy coe...

Interlayer interaction in Mo/Si superlattices in a normal and superconducting states

Abstract The electronic interlayer interaction in superconducting Mo/Si multilayers was determined by measurements of the upper critical magnetic fields in the superconducting state, and of the magnetoconductivity in the normal state in magnetic fields parallel and perpendicular to the layer planes. These measurements allow the determination of the anisotropy parameter, γ , and of the effective thickness, L eff , which characterize the interlayer interaction between the Mo layers across the Si layer. For Mo/Si series with a constant Si layer thickness, the L eff oscillates as a function of the Mo layer thickness in correlation with the oscillations of the anisotropy parameter γ . The Josephson nature of the interlayer coupling in the superconducting state is confirmed by measurements of the magnetoconductivity on a sample set with a constant Mo layer thickness and variable Si layer thickness. The procedure of evaluating the effective metal layer thickness L eff of the multilayers proposed in this work serves as a new quantitative method to determine the interlayer interaction in the layered systems.

Interlayer interaction in Mo/Si multilayers

Abstract Interlayer interaction in Mo/Si superconducting multilayers was determined by two methods involving the measurements of the critical magnetic fields and magnetoresistivity (MR) in a normal state at both parallel and perpendicular H orientations. Data on MR allow one to determine the effective thickness of the metal layer L ∗ which characterizes interlayer interaction. On the Mo/Si series with the constant Si layer thickness LSi the oscillations of L ∗ are observed which correlate closely with ones of anisotropy parameter γ. The tunnel (Josephson) nature of the interlayer coupling is confirmed by the data obtained on multilayer series with variable LSi. On these samples the correlation between γ and L ∗ behavior is also found.