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Of all publications in the section: 20
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Article
Golikova T. E., Wolf M. J., Beckmann D. et al. Superconductor Science and Technology. 2021. Vol. 34. No. 9.

A nonmonotonic dependence of the critical Josephson supercurrent on the injection current through a normal metal/ferromagnet weak link from a single domain ferromagnetic strip has been observed experimentally in nanofabricated planar crosslike S-N/F-S Josephson structures. This behavior is explained by 0–π and π–0 transitions, which can be caused by the suppression and Zeeman splitting of the induced superconductivity due to interaction between N and F layers, and the injection of spin-polarized current into the weak link. A model considering both effects has been developed. It shows the qualitative agreement between the experimental results and the theoretical model in terms of spectral supercurrent-carrying density of states of S-N/F-S structures and the spin-dependent double-step nonequilibrium quasiparticle distribution.

Added: Jun 7, 2021
Article
Golikova T., Wolf M., Beckmann D. et al. Superconductor Science and Technology. 2021. Vol. 34.

A nonmonotonic dependence of the critical Josephson supercurrent on the injection current through a normal metal/ferromagnet weak link from a single domain ferromagnetic strip has been observed experimentally in nanofabricated planar crosslike S-N/F-S Josephson structures. This behavior is explained by 0–π and π–0 transitions, which can be caused by the suppression and Zeeman splitting of the induced superconductivity due to interaction between N and F layers, and the injection of spin-polarized current into the weak link. A model considering both effects has been developed. It shows the qualitative agreement between the experimental results and the theoretical model in terms of spectral supercurrent-carrying density of states of S-N/F-S structures and the spin-dependent double-step nonequilibrium quasiparticle distribution.

Added: Oct 15, 2021
Article
Arutyunov K., Lehtinen J. S. Superconductor Science and Technology. 2013. Vol. 26. No. 3. P. 039503-039503.

On the second page, after equation (1), the statement ‘. . . where RN is the normal state resistance, RQ D h=2e D 6:45 k is the superconducting quantum resistance, . . . ’ contains a misprint. The correct expression is RQ D h=.2e/2 D 6:45 k.

Added: Mar 24, 2014
Article
Shcherbakova A., Fedorov K., Shulga K. et al. Superconductor Science and Technology. 2015. Vol. 28. P. 025009-25013.

We describe fabrication and testing of composite flux qubits combining Nb- and Al-based superconducting circuit technology. This hybrid approach to making qubits allows for employing π-phase shifters fabricated using well-established Nb-based technology of superconductor–ferromagnet–superconductor Josephson junctions. The important feature here is to obtain high interface transparency between Nb and Al layers without degrading sub-micron shadow mask. We achieve this by in situ Ar etching using e-beam gun. Shadow-evaporated Al/AlOx/Al Josephson junctions with Nb bias pads show the expected current–voltage characteristics with reproducible critical currents. Using this technique, we fabricated composite Nb/Al flux qubits with Nb/CuNi/Nb π-shifters and measured their magnetic field response. The observed offset between the field responses of the qubits with and without π-junction is attributed to the π phase shift. The reported approach can be used for implementing a variety of hybrid Nb/Al superconducting quantum circuits.

Added: Oct 23, 2016
Article
Ovchenkov Y., Chareev D., Kulbachinskii V. et al. Superconductor Science and Technology. 2017. Vol. 30. No. 3. P. 035017.

The field and temperature dependencies of the resistivity and Hall effect are measured for FeSeS (x = 0.04, 0.09, and 0.19) single crystals. Sample FeSeS does not show a transition to an orthorhombic phase and at low temperatures exhibits transport properties, which are very different from those of orthorhombic samples. The behavior of FeSeS is well described by the simple two-band model with comparable values of the hole and electron mobilities. The characteristics of the low-temperature transport properties of the orthorhombic Fe(SeS) samples are largely determined by the presence of a small number of highly mobile carriers, which may originate from the local regions of the Fermi surface, presumably, nearby the Van Hove singularity points. Our results, for the first time, demonstrate a strong evolution of a tiny band of highly mobile electrons at a tetragonal to orthorhombic quantum phase transition. The behavior of this band can be the reason for the diverging nematic susceptibility, determined from elastoresistivity, which is considered one of the most intriguing phenomena in the physics of iron-based superconductors.

Added: Apr 8, 2017
Article
Goltsman G., Antipov S., Trifonov A. et al. Superconductor Science and Technology. 2019. Vol. 32. No. 7. P. 1-7.

We report on the signal-to-noise and gain bandwidth of a niobium nitride (NbN) hot-electron bolometer (HEB) mixer at 2 THz fabricated on a sapphire substrate with a GaN buffer layer. Two mixers with different DC properties and geometrical dimensions were studied and they demonstrated very close bandwidth performance. The signal-to-noise bandwidth is increased to 8 GHz in comparison to the previous results, obtained without a buffer-layer. The data were taken in a quasi-optical system with the use of the signal-to-noise method, which is close to the signal levels used in actual astrophysical observations. We find an increase of the gain bandwidth to 5 GHz. The results indicate that prior results obtained on a substrate of crystalline GaN can also be obtained on a conventional sapphire substrate with a few micron MOCVD-deposited GaN buffer-layer.

Added: Oct 30, 2019
Article
Korneeva Y., Манова Н. Н., Симонов Н. О. et al. Superconductor Science and Technology. 2021. Vol. 34. No. 8.

We report our study of detection efficiency (DE) saturation in wavelength range 400–1550 nm for the NbN superconducting microstrip single-photon detectors (SMSPDs) featuring the strip width up to 3 µm. We observe an expected decrease of the DE saturation plateau with the increase of photon wavelength and decrease of film sheet resistance. At 1.7 K temperature DE saturation can be clearly observed at 1550 nm wavelength in strip with the width up to 2 µm when sheet resistance of the film is above 630 Ω sq−1. In such strips the length of the saturation plateau almost does not depend on the strip width. We used these films to make meander-shaped detectors with the light sensitive area from 20 × 20 µm2 to a circle 50 µm in diameter. In the latter case, the detector with the strip width of 0.49 µm demonstrates saturation of DE up to 1064 nm wavelength. Although DE at 1310 and 1550 nm is not saturated, it is as high as 60%. The response time is limited by the kinetic inductance and equals to 20 ns (by 1/e decay), timing jitter is 44 ps. When coupled to multi-mode fibre large-area meanders demonstrate significantly higher dark count rate which we attribute to thermal background photons, thus advanced filtering technique would be required for practical applications.

Added: Sep 20, 2021
Article
Korneeva Y., Золотов Ф. И., Korneev A. Superconductor Science and Technology. 2021. P. 1-10.

We report our study of detection efficiency (DE) saturation in wavelength range 400 - 1550 nm for the NbN Superconducting Microstrip Single-Photon Detectors (SMSPD) featuring the strip width up to 3 μm. We observe an expected decrease of the $DE$ saturation plateau with the increase of photon wavelength and decrease of film sheet resistance. At 1.7 K temperature DE saturation can be clearly observed at 1550 nm wavelength in strip with the width up to 2 μm when sheet resistance of the film is above 630Ω/sq. In such strips the length of the saturation plateau almost does not depend on the strip width. We used these films to make meander-shaped detectors with the light sensitive area from 20×20μm2 to a circle 50 μm in diameter. In the latter case, the detector with the strip width of 0.49 μm demonstrates saturation of DE up to 1064 nm wavelength. Although DE at 1310 and 1550 nm is not saturated, it is as high as 60%. The response time is limited by the kinetic inductance and equals to 20 ns(by 1/e decay), timing jitter is 44 ps. When coupled to multi-mode fibre large-area meanders demonstrate significantly higher dark count rate which we attribute to thermal background photons, thus advanced filtering technique would be required for practical applications.

Added: Jun 9, 2021
Article
K. I. Kugel, Bianconi A., Poccia N. et al. Superconductor Science and Technology. 2015. Vol. 28. No. 2. P. 024005-1-024005-8.
The arrested nanoscale phase separation in a two-band Hubbard model for strongly correlated charge carriers is shown to occur in a particular range in the vicinity of the topological Lifshitz transition, where the Fermi energy crosses the bottom of the narrow band and a new sheet of the Fermi surface related to the charge carriers of the second band comes into play. We determine the phase separation diagram of this two-band Hubbard model as a function of two variables, the charge carrier density and the energy shift between the chemical potential and the bottom of the second band. In this phase diagram, we first determine a line of quantum critical points for the Lifshitz transition and find criteria for the electronic phase separation resulting in an inhomogeneous charge distribution. Finally, we identify the critical point in the presence of a variable long-range Coulomb interaction where the scale invariance of the coexisting phases with different charge densities appears. We argue that this point is relevant for the regime of scale invariance of the nanoscale phase separation in cuprates like it was first observed in La2CuO4.1.
Added: Mar 12, 2016
Article
Bezuglyi E. V., Vasenko A. S., Bratus E. N. et al. Superconductor Science and Technology. 2007. Vol. 20. No. 6. P. 529-541.
We formulate a theoretical framework to describe multiparticle current transport in planar superconducting tunnel junctions with diffusive electrodes. The approach is based on the direct solving of quasiclassical Keldysh - Green function equations for nonequilibrium superconductors, and consists of a combination of circuit theory analysis and improved perturbation expansion. The theory predicts a much greater scaling parameter for the subharmonic gap structure of the tunnel current in diffusive junctions compared to the one in ballistic junctions and mesoscopic constrictions with the same barrier transparency.
Added: Nov 29, 2017
Article
Smirnov K., Divochiy A., Vakhtomin Y. et al. Superconductor Science and Technology. 2018. Vol. 31. No. 3. P. 1-7.

The possibility of creating NbN superconducting single-photon detectors with saturated dependence of quantum efficiency (QE) versus normalized bias current was investigated. It was shown that the saturation increases for the detectors based on finer films with a lower value of R s300/R s20. The decreasing of R s300/R s20 was related to the increasing influence of quantum corrections to conductivity of superconductors and, in turn, to the decrease of the electron diffusion coefficient. The best samples have a constant value of system QE 94% at I b /I c ~ 0.8 and wavelength 1310 nm.

Added: Mar 14, 2018
Article
Bezuglyi E. V., Vasenko A. S., Bratus’ E. N. Superconductor Science and Technology. 2017. Vol. 30. No. 025011. P. 1-8.

A specific property of a planar tunnel junction with thin-film diffusive plates and long enough leads, typical for most of practical situations, is essential enhancement of its transmission coefficient compared to the bare transparency of the tunnel barrier [1,2]. In voltage-biased junctions, this creates favorable conditions for strong nonequilibrium of quasiparticles in the junction plates and leads, produced by multiparticle tunneling. We study theoretically the interplay between the nonequilibrium and relaxation processes in such junctions and found that nonequilibrium in the leads noticeably modifies the current-voltage characteristic at eV>2Δ , especially the excess current, whereas strong diffusive relaxation restores the result of the classical tunnel model. At eV≤2Δ , diffusive relaxation decreases the peaks of the multiparticle currents. Inelastic relaxation in the junction plates essentially suppresses the n -particle currents (n>2 ) by the factor n for odd and n/2 for even n . The results may be important for the problem of decoherence in Josephson-junction based superconducting qubits.

Added: Nov 22, 2016
Article
Zorro M., Saraiva T., Silva C. Superconductor Science and Technology. 2014. Vol. 27. No. 055002. P. 1-12.

We investigate theoretically the nucleation of superconductivity in a thin film pierced by ferromagnetic dots. We demonstrate that localized superconductivity near the ferromagnet/superconductor interface is sustained above the second critical field Hc2 in spite of the deleterious proximity effect of the ferromagnet. This phase is characterized by Little–Parks oscillations similar to those found in perforated films, where surface superconductivity is stabilized by the enhanced properties of the superconductor-vacuum boundary. Here, localization is provided by field compensation induced by the ferromagnet stray field near the ferromagnetic dot. We also show that for an array of such ferromagnetic dots the localized phase around each dot actually percolates through the entire sample, rendering bulk superconductivity at fields considerably higher than Hc2.

Added: Feb 3, 2020
Article
Shurakov A., Lobanov Y., Goltsman G. Superconductor Science and Technology. 2015. Vol. 29. No. 2. P. 023001.

The discovery of hot-electron phenomena in a thin superconducting film in the last century was followed by numerous experimental studies of its appearance in different materials aiming for a better understanding of the phenomena and consequent implementation of terahertz detection systems for practical applications. In contrast to the competitors such as superconductor-insulator-superconductor tunnel junctions and Schottky diodes, the hot electron bolometer (HEB) did not demonstrate any frequency limitation of the detection mechanism. The latter, in conjunction with a decent performance, rapidly made the HEB mixer the most attractive candidate for heterodyne observations at frequencies above 1 THz. The successful operation of practical instruments (the Heinrich Hertz Telescope, the Receiver Lab Telescope, APEX, SOFIA, Hershel) ensures the importance of the HEB technology despite the lack of rigorous theoretical routine for predicting the performance. In this review, we provide a summary of experimental and theoretical studies devoted to understanding the HEB physics, and an overview of various fabrication routes and materials.

Added: Feb 19, 2016
Article
Korneeva Y., Mikhailov M., Pershin Y. et al. Superconductor Science and Technology. 2014. Vol. 27. No. 9. P. 095012.

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.

Added: Mar 31, 2015
Article
Goltsman G., Zhang W., Miao W. et al. Superconductor Science and Technology. 2014. Vol. 27. No. 8. P. 085013.

In this paper we study the temperature dependence of the receiver noise temperature and IF noise bandwidth of superconducting hot electron bolometer (HEB) mixers. Three superconducting NbN HEB devices of different transition temperatures (Tc) are measured at 0.85 THz and 1.4 THz at different bath temperatures (Tbath) between 4 K and 9 K. Measurement results demonstrate that the receiver noise temperature of superconducting NbN HEB devices is nearly constant for Tbath/Tc, less than 0.8, which is consistent with the simulation based on a distributed hot-spot model. In addition, the IF noise bandwidth appears independent of Tbath/ Tc, indicating the dominance of phonon cooling in the investigated HEB devices.

Added: Feb 24, 2016
Article
Yagovtsev V., Pugach N., Eschrig M. Superconductor Science and Technology. 2021. Vol. 34. P. 1-10.

The magnetization in a superconductor induced due to the inverse proximity effect is  investigated in hybrid bilayers containing a superconductor and a ferromagnetic  insulator or a strongly spin-polarized ferromagnetic metal. The study is performed  within a quasiclassical Green function framework, wherein Usadel equations are solved  with boundary conditions appropriate for strongly spin-polarized ferromagnetic  materials. A comparison with recent experimental data is presented. The singlet to triplet  conversion of the superconducting correlations as a result of the proximity effect with a  ferromagnet is studied.

Added: Dec 7, 2020
Article
Arutyunov K., Lehtinen J. S. Superconductor Science and Technology. 2012. Vol. 25. P. 1240071-1240075.

In a number of recent experiments it has been demonstrated that in ultra-narrow superconducting channels quantum fluctuations of the order parameter, alternatively called quantum phase slips, are responsible for the finite resistance well below the critical temperature. Acceptable agreement between those experiments and the models describing quantum fluctuations in quasi-one-dimensional superconductors has been established. However, the very concept of phase slip is justified when these fluctuations are relatively rare events, meaning that the effective resistance of the system should be much smaller than the normal state equivalent. In this paper we study the limit of the strong quantum fluctuations where the existing models are not applicable. In the particular case of ultra-thin titanium nanowires, it is demonstrated that below the expected critical temperature the resistance does not demonstrate any trend towards the conventional for a superconductor zero-resistivity state even at negligibly small measuring currents. The application of a small magnetic field leads to an unusual negative magnetoresistance, which becomes more pronounced at lower temperatures. The origin of the negative magnetoresistance effect is not clear.

Added: Mar 24, 2014
Article
Ozhegov R., Gorshkov K., Goltsman G. et al. Superconductor Science and Technology. 2011. Vol. 24. No. 3. P. 1-4.

We present the results of stability testing of a terahertz radiometer based on a superconducting receiver with a SIS tunnel junction as the mixer and a flux-flow oscillator as the local oscillator. In the continuum mode, the receiver with a noise temperature of 95 K at 510 GHz measured over the intermediate frequency (IF) passband of 4-8 GHz offered a noise equivalent temperature difference of 10 ± 1 mK at an integration time of 1 s. We offer a method to significantly increase the integration time without the use of complex measurement equipment. The receiver observed a strong signal over a final detection bandwidth of 4 GHz and offered an Allan time of 5 s.

Added: Oct 12, 2017
Article
Galin M., Klushin A., Kurin V. et al. Superconductor Science and Technology. 2015. Vol. 28. No. 5. P. 055002.

Various applications in the field of terahertz technology are in urgent need of compact, wide-tunable solid-state continuous wave radiation sources with a moderate power. However, satisfactory solutions for the THz frequency range are scarce yet. Here we report on coherent radiation from a large planar array of Josephson junctions (JJs) in the frequency range between 0.1 and 0.3 THz. The external resonator providing the synchronization of JJ array is identified as a straight fragment of a single-strip-line containing the junctions themselves. We demonstrate a prototype of the quasioptical heterodyne receiver with the JJ array as a local oscillator and a hot-electron bolometer mixer.

Added: Oct 15, 2015