| 1. |
- Kalenyuk, Aleksey A., et al.
(author)
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Unusual two-dimensional behavior of iron-based superconductors with low anisotropy
- 2017
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In: Physical Review B. - 2469-9950 .- 2469-9969. ; 96:13
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Journal article (peer-reviewed)abstract
- We study angular-dependent magnetoresistance in iron-based superconductors Ba1−xNaxFe2As2 and FeTe1−xSex. Both superconductors have relatively small anisotropies γ∼2 and exhibit a three-dimensional (3D) behavior at low temperatures. However, we observe that they start to exhibit a profound two-dimensional behavior at elevated temperatures and in applied magnetic field parallel to the surface. We conclude that the unexpected two-dimensional (2D) behavior of the studied low-anisotropic superconductors is not related to layeredness of the materials, but is caused by appearance of surface superconductivity when magnetic field exceeds the upper critical field Hc2(T) for destruction of bulk superconductivity. We argue that the corresponding 3D-2D bulk-to-surface dimensional transition can be used for accurate determination of the upper critical field.
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| 2. |
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| 3. |
- Galin, Mikhail A., et al.
(author)
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Synchronization of Large Josephson-Junction Arrays by Traveling Electromagnetic Waves
- 2018
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In: Physical Review Applied. - 2331-7019. ; 9:5
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Journal article (peer-reviewed)abstract
- Mutual synchronization of many Josephson junctions is required for superradiant enhancement of the emission power. However, the larger the junction array is, the more difficult is the synchronization, especially when the array size becomes much larger than the emitted wavelength. Here, we study experimentally Josephson emission from such larger-than-the-wavelength Nb/NbSi/Nb junction arrays. For one of the arrays we observe a clear superradiant enhancement of emission above a threshold number of active junctions. The arrays exhibit strong geometrical resonances, seen as steps in current-voltage characteristics. However, radiation patterns of the arrays have forward-backward asymmetry, which is inconsistent with the solely geometrical resonance (standing-wave) mechanism of synchronization. We argue that the asymmetry provides evidence for an alternative mechanism of synchronization mediated by unidirectional traveling-wave propagation along the array (such as a surface plasmon). In this case, emission occurs predominantly in the direction of propagation of the traveling wave. Our conclusions are supported by numerical modeling of Josephson traveling-wave antenna. We argue that such a nonresonant mechanism of synchronization opens a possibility for phase locking of very large arrays of oscillators.
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| 4. |
- Kalenyuk, Aleksey A., et al.
(author)
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Phase-Sensitive Evidence for the Sign-Reversal s± Symmetry of the Order Parameter in an Iron-Pnictide Superconductor Using Nb/Ba1−xNaxFe2As2 Josephson Junctions
- 2018
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 120:6
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Journal article (peer-reviewed)abstract
- Josephson current provides a phase-sensitive tool for probing the pairing symmetry. Here we present an experimental study of high-quality Josephson junctions between a conventional s-wave superconductor Nb and a multiband iron-pnictide Ba1−xNaxFe2As2. Junctions exhibit a large enough critical current density to preclude the d-wave symmetry of the order parameter in the pnictide. However, the IcRn product is very small ≃3μV, which is not consistent with the sign-preserving s++ symmetry either. We argue that the small IcRn value, along with its unusual temperature dependence, provides evidence for the sign-reversal s± symmetry of the order parameter in Ba1−xNaxFe2As2. We conclude that it is the phase sensitivity of our junctions that leads to an almost complete (below a subpercent) cancellation of supercurrents from sign-reversal bands in the pnictide.
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| 5. |
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| 6. |
- Dremov, Viacheslav V., et al.
(author)
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Local Josephson vortex generation and manipulation with a Magnetic Force Microscope
- 2019
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 10
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Journal article (peer-reviewed)abstract
- Josephson vortices play an essential role in superconducting quantum electronics devices. Often seen as purely conceptual topological objects, 2π-phase singularities, their observation and manipulation are challenging. Here we show that in Superconductor—Normal metal—Superconductor lateral junctions Josephson vortices have a peculiar magnetic fingerprint that we reveal in Magnetic Force Microscopy (MFM) experiments. Based on this discovery, we demonstrate the possibility of the Josephson vortex generation and manipulation by the magnetic tip of a MFM, thus paving a way for the remote inspection and control of individual nano-components of superconducting quantum circuits.
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| 7. |
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| 8. |
- Fedorov, A. S., et al.
(author)
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Charge-transfer plasmons with narrow conductive molecular bridges : A quantum-classical theory
- 2019
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In: Journal of Chemical Physics. - : AMER INST PHYSICS. - 0021-9606 .- 1089-7690. ; 151:24
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Journal article (peer-reviewed)abstract
- We analyze a new type of plasmon system arising from small metal nanoparticles linked by narrow conductive molecular bridges. In contrast to the well-known charge-transfer plasmons, the bridge in these systems consists only of a narrow conductive molecule or polymer in which the electrons move in a ballistic mode, showing quantum effects. The plasmonic system is studied by an original hybrid quantum-classical model accounting for the quantum effects, with the main parameters obtained from first-principles density functional theory simulations. We have derived a general analytical expression for the modified frequency of the plasmons and have shown that its frequency lies in the near-infrared (IR) region and strongly depends on the conductivity of the molecule, on the nanoparticle-molecule interface, and on the size of the system. As illustrated, we explored the plasmons in a system consisting of two small gold nanoparticles linked by a conjugated polyacetylene molecule terminated by sulfur atoms. It is argued that applications of this novel type of plasmon may have wide ramifications in the areas of chemical sensing and IR deep tissue imaging. Published under license by AIP Publishing.
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| 9. |
- Boris, Andrey A., et al.
(author)
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Quantization of the superconducting energy gap in an intense microwave field
- 2015
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 92:17
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Journal article (peer-reviewed)abstract
- We study experimentally photon-assisted tunneling in Nb/AlOx/Nb Josephson junctions. We perform a quantitative calibration of the microwave field inside the junction. This allows direct verification of the quantum efficiency of microwave photon detection, which corresponds to tunneling of one electron per one absorbed microwave photon. We observe that voltages of photon-assisted tunneling steps vary both with the microwave power and the tunneling current. However, this variation is not monotonous but staircaselike. The phenomenon is caused by mutual locking of positive and negative step series. A similar locking is observed with Shapiro steps. As a result, the superconducting gap assumes quantized values equal to multiples of the quarter of the photon energy. The quantization is a manifestation of nonequilibrium tuning (suppression or enhancement) of superconductivity by the microwave field.
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| 10. |
- Borodianskyi, Evgenii A., et al.
(author)
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Josephson emission with frequency span 1–11 THz from small Bi2Sr2CaCu2O8+δ mesa structures
- 2017
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 8
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Journal article (peer-reviewed)abstract
- Mesa structures made of Bi2Sr2CaCu2O8+δ high-temperature superconductor represent stacks of atomic scale intrinsic Josephson junctions. They can be used for generation of high-frequency electromagnetic waves. Here we analyze Josephson emission from small-but-high mesas (with a small area, but containing many stacked junctions). We have found strong evidence for tunable terahertz emission with a good efficacy in a record high-frequency span 1–11 THz, approaching the theoretical upper limit for this superconductor. Emission maxima correspond to in-phase cavity modes in the mesas, indicating coherent superradiant nature of the emission. We conclude that terahertz emission requires a threshold number of junctions N ~ 100. The threshold behavior is not present in the classical description of stacked Josephson junctions and suggests importance of laser-like cascade amplification of the photon number in the cavity.
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