| 1. |
- Kapran, Olena M., 1993-, et al.
(author)
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Observation of the dominant spin-triplet supercurrent in Josephson spin valves with strong Ni ferromagnets
- 2020
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In: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 2:1
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Journal article (peer-reviewed)abstract
- We study experimentally nanoscale Josephson junctions and Josephson spin valves containing strongly ferromagnetic Ni interlayers. We observe that in contrast to junctions, spin valves with the same geometry exhibit anomalous Ic(H) patterns with two peaks separated by a dip. We develop several techniques for in situ characterization of micromagnetic states in our nanodevices, including magnetoresistance, absolute Josephson fluxometry, and first-order-reversal-curves analysis. They reveal a clear correlation of the dip in supercurrent with the antiparallel state of a spin valve and the peaks with two noncollinear magnetic states, thus providing evidence for generation of spin-triplet superconductivity. A quantitative analysis, based on micromagnetic simulations, brings us to the conclusion that the triplet current in our Ni-based spin valves is approximately three times larger than the conventional spin-singlet supercurrent.
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| 2. |
- Cattaneo, Roger, et al.
(author)
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Observation of collective excitation of surface plasmon resonances in large Josephson junction arrays
- 2022
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In: Beilstein Journal of Nanotechnology. - : Beilstein Institut. - 2190-4286. ; 13, s. 1578-1588
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Journal article (peer-reviewed)abstract
- Josephson junctions can be used as sources of microwave radiation. However, synchronization of many junctions is required for achieving a coherent amplification of the emitted power. In this work we present an experimental study of large arrays containing up to one thousand Nb/NbxSi1−x/Nb junctions. The arrays exhibit profound cavity mode resonances, corresponding to the formation of standing waves at the electrode/substrate interface. We observe that resonant steps in the current–voltage characteristics appear above some threshold number of junctions, Nth ≈ 100, and then progressively enhance in amplitude with further increment of the number of junctions in the resistive oscillating state. We use an external detector to measure the emission of electromagnetic waves. The emission power correlates with the step amplitude. Our results indicate that the emission is facilitated by the cavity modes in the electrodes. The modes are collectively excited by active junctions. In turn, the standing wave imprints its order on the array, facilitating mutual phase-locking of junctions. This provides an indirect coupling mechanism, allowing for the synchronization of junctions, which do not directly interact with each other. Our results demonstrate that electrodes can effectively work as a common external resonator, facilitating long-range phase-locking of large junction arrays with sizes larger than the emitted wavelength.
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| 3. |
- Galin, Mikhail A., et al.
(author)
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Coherent amplification of radiation from two phase-locked Josephson junction arrays
- 2022
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In: Beilstein Journal of Nanotechnology. - : Beilstein Institut. - 2190-4286. ; 13, s. 1445-1457
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Journal article (peer-reviewed)abstract
- We analyze experimentally and theoretically mutual phase locking and electromagnetic interaction between two linear arrays with a large number of Josephson junctions. Arrays with different separation, either on the same chip or on two separate substrates are studied. We observe a large coherent gain, up to a factor of three, of emitted power from two simultaneously biased arrays, compared to the sum of powers from two individually biased arrays. The phenomenon is attributed to the phase locking of junctions in different arrays via a common electromagnetic field. Remarkably, the gain can exceed the factor of two expected for a simple constructive interference of two oscillators. The larger gain is explained by an additional consequence of mutual interaction between two large arrays. Mutual phase locking of large arrays does not only result in constructive interference outside the arrays, but also improved synchronization of junctions inside each array. Our conclusion is supported by numerical modelling.
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| 4. |
- Golod, Taras, 1981-, et al.
(author)
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Demonstration of a superconducting diode-with-memory, operational at zero magnetic field with switchable nonreciprocity
- 2022
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 13:1
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Journal article (peer-reviewed)abstract
- Superconducting diodes, operational at zero magnetic field, can be used in supercomputers. Here, the authors demonstrate prototypes of diodes-with-memory, based on Nb Josephson junctions, with a large and switchable nonreciprocity at zero field. Diode is one of the basic electronic components. It has a nonreciprocal current response, associated with a broken space/time reversal symmetry. Here we demonstrate prototypes of superconducting diodes operational at zero magnetic field. They are based on conventional niobium planar Josephson junctions, in which space/time symmetry is broken by a combination of self-field effect from nonuniform bias and stray fields from a trapped Abrikosov vortex. We demonstrate that nonreciprocity of critical current in such diodes can reach an order of magnitude and rectification efficiency can exceed 70%. Furthermore, we can easily change the diode polarity and switch nonreciprocity on/off by changing the bias configuration and by trapping/removing of a vortex. This facilitates a memory functionality. We argue that such a diode-with-memory can be used for a future generation of in-memory superconducting computers.
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| 5. |
- Golod, Taras, 1981-, et al.
(author)
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Word and bit line operation of a 1 x 1 μm2 superconducting vortex-based memory
- 2023
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In: Nature Communications. - 2041-1723. ; 14:1
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Journal article (peer-reviewed)abstract
- The lack of dense random access memory is one of the main bottlenecks for the creation of a digital superconducting computer. In this work we study experimentally vortex-based superconducting memory cells. Three main results are obtained. First, we test scalability and demonstrate that the cells can be straightforwardly miniaturized to submicron sizes. Second, we emphasize the importance of conscious geometrical engineering. In the studied devices we introduce an asymmetric easy track for vortex motion and show that it enables a controllable manipulation of vortex states. Finally, we perform a detailed analysis of word and bit line operation of a 1 x 1 mu m(2) cell. High-endurance, non-volatile operation at zero magnetic field is reported. Remarkably, we observe that the combined word and bit line threshold current is significantly reduced compared to the bare word-line operation. This could greatly improve the selectivity of individual cell addressing in a multi-cell RAM. The achieved one square micron area is an important milestone and a significant step forward towards creation of a dense cryogenic memory.
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| 6. |
- Grebenchuk, S. Yu, et al.
(author)
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Nonlocal Long-Range Synchronization of Planar Josephson-Junction Arrays
- 2022
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In: Physical Review Applied. - 2331-7019. ; 17:6
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Journal article (peer-reviewed)abstract
- We study arrays of planar Nb Josephson junctions with contacts to intermediate electrodes, which allow measurements of individual junctions and, thus, provide an insight into intricate array dynamics. We observe strong indications for array phase locking, despite a significant interjunction separation. Several unusual phenomena are reported, such as a bistable critical current with reentrant superconductivity upon switching of nearby junctions; and “incorrect” Shapiro steps, occurring at mixing frequencies between the external rf radiation and the internal Josephson frequency in nearby junctions. Our results reveal a surprisingly strong and long-range interjunction interaction, which is attributed to nonlocality of planar-junction electrodynamics, caused by the long-range spreading of stray electromagnetic fields. The nonlocality greatly enhances the high-frequency interjunction coupling and enables large-scale synchronization. Therefore, we conclude that planar geometry is advantageous for the realization of coherent Josephson electronics.
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| 7. |
- Hovhannisyan, Razmik A., et al.
(author)
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Holographic reconstruction of magnetic field distribution in a Josephson junction from diffraction-like Ic(H) patterns
- 2022
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In: Physical Review B. - 2469-9950 .- 2469-9969. ; 105:21
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Journal article (peer-reviewed)abstract
- A general problem of magnetic sensors is a trade-off between spatial resolution and magnetic-field sensitivity. With decreasing sensor size its resolution is improved but the sensitivity is deteriorated. Obviation of such a trade-off requires development of super-resolution imaging technique not limited by sensor size. Here we present a proof of concept for a super-resolution method of magnetic imaging by a Josephson junction (JJ). It is based on a solution of an inverse problem—reconstruction of a local magnetic-field distribution within a junction from the dependence of the critical current on an external magnetic field, Ic(H). The method resembles the Fourier-transform holography, with the diffractionlike Ic(H) pattern serving as a hologram. A simple inverse problem solution, valid for an arbitrary symmetric case, is derived. We verify the method numerically and show that the accuracy of reconstruction does not depend on the junction size and is only limited by the field range of the Ic(H) pattern. Finally, the method is tested experimentally using planar Nb JJs. Super-resolution reconstruction of stray magnetic fields from an Abrikosov vortex, trapped in the junction electrodes, is demonstrated. Thus our method facilitates both high field sensitivity and high spatial resolution, obviating the trade-off problem of magnetic sensors. We conclude that the holographic magnetic imaging by a planar JJ can be used in scanning probe microscopy
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| 8. |
- Hovhannisyan, Razmik A., et al.
(author)
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Superresolution magnetic imaging by a Josephson junction via holographic reconstruction of I c ( H ) modulation
- 2023
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In: Physical Review Applied. - 2331-7019. ; 20:6
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Journal article (peer-reviewed)abstract
- This work provides a proof -of -concept for superresolution magnetic imaging using a single Josephson junction. The technique resembles digital holography: magnetic patterns are obtained via an inverseproblem solution from diffractionlike modulation of the junction's critical current, I c (H) . We demonstrate numerical reconstruction of complex two-dimensional patterns, verify the technique experimentally using Nb-based planar junctions, and fabricate an operational sensor on a cantilever. Our results show that Josephson holography allows for both high spatial resolution (approximately 20 nm) and high field sensitivity (approximately 10 - 11 T R root Hz), thus resolving the trade-off problem between resolution and sensitivity in magnetic scanning probe imaging.
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| 9. |
- Jacobs, Thorsten, et al.
(author)
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Sequence of Quantum Phase Transitions in Bi2Sr2CaCu2O8+δ Cuprates Revealed by In Situ Electrical Doping of One and the Same Sample
- 2016
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 116:6
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Journal article (peer-reviewed)abstract
- Our recently discovered electrical doping technique allows a broad-range variation of carrier concentration without changing the chemical composition. We show that it is possible to induce superconductivity in a nondoped insulating sample and to tune it reversibly all the way to an overdoped metallic state. This way, we can investigate the whole doping diagram of one and the same sample. Our study reveals two distinct critical points. The one at the overdoped side is associated with the onset of the pseudogap and with the metal-to-insulator transition in the c-axis transport. The other at optimal doping is associated with the appearance of a dressed electron energy. Our study confirms the existence of multiple phase transitions under the superconducting dome in cuprates.
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| 10. |
- 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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