Superconducting & mesoscopic structures

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Non-equilibrium effects and mechanisms of the resistive state of bulk and thin film superconductors
and superconducting single photon detectors
A. G. Sivakov (Group Leader), A. S. Pokhila, M. Yu. Mikhailov, A. E. Kolinko, S. A. Kruhlov

Low temperature scanning laser microscopy of superconductors
We develop the Low Temperature Scanning Laser Microscopy as a spatially resolved method of testing superconducting parameters in bulk superconductors, thin films superconductors and superconducting devices. We have used low temperature scanning laser microscopy to visualize the phase-slip lines in wide superconducting films.
Fig.1 Generation of PSLs in a uniform 30 µm wide Sn strip induced by the bias current increasing gradually from (a) to d). Images (a), (b), and (c) are taken at the 1-st, 2-nd and 3-d steps of IV-curve. Image (d) is obtained in a high current egion.

Laser scanning microscopy of superconductive parameters of multi-band superconductors

Current-voltage characteristics of iron-containing chalcogenide FeTe films and MgB2 were investigated at different temperatures and the LTSLM images of resistive state were obtained. No difference between behavior of multiband and traditional superconductors in the resistive state was obtained.

Fig.2 The voltage-ampere characteristic of the sample (a), its initial part (b) and the images of the LSM-responses which have been obtained at different currents through the sample (c).
Fig.3 The family of the IVCs MgB2 film at different temperatures.
Fig.4 SLM images of PSL in MgB2 film.

LTSM investigations of HTC thin films
Vortex motion in polycrystalline HTC thin films was investigated. Local critical currents suppress due to disorientation of the grains and local defects was obtained.
Fig.5 Half-tone dc voltage response map of polycrystalline YBCO film with percolative current flow through a maze of weak links between grains at < Tc. The arrow denotes current flow direction.
Fig.6 250x250 µm2 2-D LSM dc voltage image of the resistive state in a YBCO strip fragment induced by transport current I=140mA at temperature T=88.7K: (a) an artificial linear defect (crack) is present near the bottom edge, (b) the second laser beam generates additional inhomogeneity. The strip width is 200µm, it is indicated by arrows and outlined by dashed lines.
Oscillations of critical superconducting current in thin doubly-connected Sn films in an external perpendicular magnetic field
We provided experimental and theoretical studies of critical current oscillations in thin doubly-connected Sn films in an external perpendicular magnetic field. The experiments were performed on samples that consisted of two wide electrodes joined together by two narrow channels. The length of the channels l satisfied the condition l >> ξ (ξ is the Ginzburg–Landau coherence length). At temperatures close to the critical temperature Tc, the dependence of the critical current Ic on average external magnetic flux Φe has the form of a piecewise linear function, periodic with respect to the flux quantum Φ0. The amplitude of the Ic oscillation at a given temperature is proportional to the factor ξ/l. Moreover, the dependence Ic = Ice) is found to be multivalued, hence indicating the presence of metastable states. Based on the Ginzburg–Landau approximation, a theory was constructed that explains the above features of the oscillation phenomenon taking a perfectly symmetric system as an example. Further, the experiments displayed the effects related to the critical currents imbalance between the superconducting channels, i.e., shift of the maxima of the dependence Ic = Ice) accompanied by an asymmetry with respect to the transport current direction.
Superconducting nanowire single photon detectors (SNSPDs)
SNSPDs are highly efficient single photon detectors of visible and near infrared radiation based on superconducting nanowires patterned out of ultrathin films. The current choice of superconducting materials suitable for SNSPD development is limited. Prior to nanostructuring, the as-deposited ultrathin films should provide a continuous surface down to few nanometers thickness. Of all materials for use as SNSPDs studied so far, the only commercially available SNSPD detectors are made of crystalline NbN, NbTiN and amorphous WSi ultrathin films. Particularly important is the possibility for uniform deposition across large areas in order to achieve uniform templates for later detector manufacture.
We are involved in the development of new materials for SNSPDs. We have studied superconducting properties of ultrathin amorphous molybdenum silicon and tungsten silicon films. For the first time, we developed an efficient superconducting single photon detector made of amorphous MoSi film. The critical temperature of the studied amorphous films can be easily tuned by thickness or stoichiometry. We deposit our ultrathin amorphous films by magnetron co-sputtering from two targets onto rotating substrate. We were able to demonstrate 18 % of detection efficiency for 1200 nm wavelength at 1.8K, a response time 6 ns and timing jitter of 120 ps for MoSi-based SNSPDs [, , arxiv; , ].
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