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  • 1.
    Borysov, Stanislav S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). KTH Royal Institute of Technology, Sweden; Los Alamos National Laboratory, USA.
    Forchheimer, Daniel
    Haviland, David B.
    Dynamic calibration of higher eigenmode parameters of a cantilever in atomic force microscopy by using tip-surface interactions2014In: Beilstein Journal of Nanotechnology, ISSN 2190-4286, Vol. 5, p. 1899-1904Article in journal (Refereed)
    Abstract [en]

    We present a theoretical framework for the dynamic calibration of the higher eigenmode parameters (stiffness and optical lever inverse responsivity) of a cantilever. The method is based on the tip-surface force reconstruction technique and does not require any prior knowledge of the eigenmode shape or the particular form of the tip-surface interaction. The calibration method proposed requires a single-point force measurement by using a multimodal drive and its accuracy is independent of the unknown physical amplitude of a higher eigenmode.

  • 2.
    Cattaneo, Roger
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Galin, Mikhail A.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Observation of collective excitation of surface plasmon resonances in large Josephson junction arrays2022In: Beilstein Journal of Nanotechnology, ISSN 2190-4286, Vol. 13, p. 1578-1588Article in journal (Refereed)
    Abstract [en]

    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.

  • 3. Galin, Mikhail A.
    et al.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Shereshevsky, Ilya A.
    Vdovicheva, Nadezhda K.
    Kurin, Vladislav V.
    Coherent amplification of radiation from two phase-locked Josephson junction arrays2022In: Beilstein Journal of Nanotechnology, ISSN 2190-4286, Vol. 13, p. 1445-1457Article in journal (Refereed)
    Abstract [en]

    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.

  • 4.
    Kapran, Olena M.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Morari, Roman
    Golod, Taras
    Stockholm University, Faculty of Science, Department of Physics.
    Borodianskyi, Evgenii A.
    Stockholm University, Faculty of Science, Department of Physics.
    Boian, Vladimir
    Prepelita, Andrei
    Klenov, Nikolay
    Sidorenko, Anatoli S.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics. State University, Russia.
    In situ transport characterization of magnetic states in Nb/Co superconductor/ferromagnet heterostructures2021In: Beilstein Journal of Nanotechnology, ISSN 2190-4286, Vol. 12, p. 913-923Article in journal (Refereed)
    Abstract [en]

    Employment of the non-trivial proximity effect in superconductor/ferromagnet (S/F) heterostructures for the creation of novel superconducting devices requires accurate control of magnetic states in complex thin-film multilayers. In this work, we study experimentally in-plane transport properties of microstructured Nb/Co multilayers. We apply various transport characterization techniques, including magnetoresistance, Hall effect, and the first-order-reversal-curves (FORC) analysis. We demonstrate how FORC can be used for detailed in situ characterization of magnetic states. It reveals that upon reduction of the external field, the magnetization in ferromagnetic layers first rotates in a coherent scissor-like manner, then switches abruptly into the antiparallel state and after that splits into the polydomain state, which gradually turns into the opposite parallel state. The polydomain state is manifested by a profound enhancement of resistance caused by a flux-flow phenomenon, triggered by domain stray fields. The scissor state represents the noncollinear magnetic state in which the unconventional odd-frequency spin-triplet order parameter should appear. The non-hysteretic nature of this state allows for reversible tuning of the magnetic orientation. Thus, we identify the range of parameters and the procedure for in situ control of devices based on S/F heterostructures.

  • 5. Krasnov, Mikhail M.
    et al.
    Novikova, Natalia D.
    Cattaneo, Roger
    Stockholm University, Faculty of Science, Department of Physics.
    Kalenyuk, Alexey A.
    Stockholm University, Faculty of Science, Department of Physics. National Academy of Sciences of Ukraine, Ukraine; Kyiv Academic University, Ukraine.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics. Moscow Institute of Physics and Technology, Russia.
    Design aspects of Bi2Sr2CaCu2O8+δ THz sources: optimization of thermal and radiative properties2021In: Beilstein Journal of Nanotechnology, ISSN 2190-4286, Vol. 12, p. 1392-1403Article in journal (Refereed)
    Abstract [en]

    Impedance matching and heat management are important factors influencing the performance of terahertz sources. In this work we analyze thermal and radiative properties of such devices based on mesa structures of a layered high-temperature superconductor Bi2Sr2CaCu2O8+delta. Two types of devices are considered containing either a conventional large single crystal or a whisker. We perform numerical simulations for various geometrical configurations and parameters and make a comparison with experimental data for the two types of devices. It is demonstrated that the structure and the geometry of both the superconductor and the electrodes play important roles. In crystal-based devices an overlap between the crystal and the electrode leads to appearance of a large parasitic capacitance, which shunts terahertz emission and prevents impedance matching with open space. The overlap is avoided in whisker-based devices. Furthermore, the whisker and the electrodes form a turnstile (crossed-dipole) antenna facilitating good impedance matching. This leads to more than an order of magnitude enhancement of the radiation power efficiency in whisker-based, compared to crystal-based, devices. These results are in good agreement with presented experimental data.

  • 6.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    A distributed active patch antenna model of a Josephson oscillator2023In: Beilstein Journal of Nanotechnology, ISSN 2190-4286, Vol. 14, p. 151-164Article in journal (Refereed)
    Abstract [en]

    Optimization of Josephson oscillators requires a quantitative understanding of their microwave properties. A Josephson junction has a geometry similar to a microstrip patch antenna. However, it is biased by a dc current distributed over the whole area of the junction. The oscillating electric field is generated internally via the ac-Josephson effect. In this work, I present a distributed, active patch antenna model of a Josephson oscillator. It takes into account the internal Josephson electrodynamics and allows for the determination of the effective input resistance, which couples the Josephson current to cavity modes in the transmission line formed by the junction. The model provides full characterization of Josephson oscillators and explains the origin of the low radiative power efficiency. Finally, I discuss the design of an optimized Josephson patch oscillator capable of reaching high efficiency and radiation power for emission into free space.

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