A. G. Sivakov

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.

Low-temperature scanning laser microscopy of individual filaments extracted from (Bi, Pb)2Sr2Ca2Cu3O10+x tapes

The method of low-temperature scanning laser microscopy is applied to visualize the resistive state in individual superconducting filaments extracted from (Bi, Pb)2Sr2Ca2Cu3O10+x/Ag tapes. This technique is capable of imaging the distributions of the critical currents over a sample. Using the nonbolometric response, a spatial resolution of about 1 μm is demonstrated for 10-μm-thick filaments. Some of the resistively visualized grain boundaries between crystallites show Josephson behavior.

Spatially resolved characterization of superconducting films and cryoelectronic devices by means of low temperature scanning laser microscope

Abstract The work describes the low temperature scanning laser microscopy technique used for spatially resolved characterization of superconducting films and film-based cryoelectronic circuits in the temperature range from 2 to 300 K. The determination of superconducting parameters for separate elements of a high T c Josephson junctions array and imaging of the resistive transition in a high T c superconducting polycrystalline film are demonstrated. The spatial evolution of the resistive state of a Sn thin film strip associated with the phase slip lines formation is visualized.

Spatially resolved measurements of critical parameters in superconducting filaments by laser scanning technique

We present an experimental study of local transport properties of superconducting filaments by means of low temperature scanning laser microscopy (LTSLM). A novel theoretical model that treats the laser-beam-induced voltage response is proposed and applied to analyze the measured data. The method of LTSLM allows to visualize the local inhomogeneities in individual filaments extracted from (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10+x//Ag tapes. Using the proposed theoretical analysis we extract the spatial distributions of the critical currents and critical temperatures over the sample. Tile data obtained by LTLSM show good agreement with standard non-local transport measurements. Using autocorrelation analysis of the voltage response, the characteristic length of the critical current variation is retrieved for filaments that belong to different parts of the tape.

Laser scanning visualization of evolution of vortex instability in current-carrying superconducting strips

The low temperature scanning laser microscopy is used to study spatial characteristics of current-induced resistive states in wide superconducting Sn films. The irregularities of edge potential barrier and the current distribution in a film cross section are visualized. The results are in good qualitative agreement with Aslamazov-Lempitsky theory.

Visualization of resistive states of superconducting films

Abstract A method is described for visualization of various resistive states in two-dimensional superconductors when scanning their surface with a laser beam. Experiments were made on Sn films to obtain a two-dimensional picture of a separate phase slip line, and a temperature field was plotted of a superconducting film under inhomogeneous heat transfer. The sample IV curve with nodal points of resistive states under study is given together with the block diagram of the experimental set-up.

Observation of stochastic resonance in percolative Josephson media

Measurements of the electrical response of granular Sn-Ge thin films below the superconducting transition temperature are reported. The addition of an external noise to the magnetic field applied to the sample is found to increase the sample voltage response to a small externally applied ac signal. The gain coefficient for this signal and the signal-to-noise ratio display clear maxima at particular noise levels. We interpret these observations as a stochastic resonance in the percolative Josephson media which occurs close to the percolation threshold.

A low temperature system with a pulse UV laser for scribing HTSC films and single crystals

Abstract A simple laser scribing equipment used to pattern high-Tc films and single crystals within a micron accuracy is described. A focused beam of a UV pulsed laser serves as a cutting tool. A low temperature attachment enables all operations to be performed at sample temperatures ranging from 65 to 300 K. Thus it provides a means to fit the critical current of any element in a cryoelectronic circuit to a desirable value during its operational conditions.

Microstructural and transport properties of superconducting FeTe0.65Se0.35 crystals

The issue concerning the nature and the role of microstructural inhomogeneities in iron chalcogenide superconducting crystals of FeTe0.65Se0.35 and their correlation with transport properties of this system was addressed. The presented data demonstrate that chemical disorder originating from the kinetics of the crystal growth process significantly influences the superconducting properties of an Fe–Te–Se system. Transport measurements of the transition temperature and critical current density performed for microscopic bridges allow us to deduce the local properties of a superconductor with microstructural inhomogeneities, and significant differences were noted. The variances observed in the local properties were explained as a consequence of weak superconducting links existing in the studied crystals. The results confirm that the inhomogeneous spatial distribution of ions and small hexagonal symmetry nanoscale regions with nanoscale phase separation also seem to enhance the superconductivity in this system with respect to the values of the critical current density. Magnetic measurements performed in order to determine, in an alternative way, the values of the critical current density, as well as to find the relaxation rate and to check the scaling of the pinning force, confirm the conclusions drawn from the transport measurements.

Averaged effective pinning potential in YBCO single crystals near Tc

The dynamics of magnetic flux trapped in low dc magnetic fields (of the order of the Earths field) was for the first time studied experimentally in single-crystal YBCO samples with unidirectional twin boundaries in the temperature range near Tc (0.8 < T/Tc < 0.99). Strong pinning in the system of unidirectional planar defects was demonstrated, and a significant deviation from monotonous behavior was established for the averaged effective pinning potential Up(T) for the trapped flux of low density. In order to compare different methods of Jc determination, the field dependences of the magnetization loop width M(H), which are related to the effective pinning and Jc, were obtained, and resistive measurements on microbridges made from the same single crystals were carried out.