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Search Results: 1 - 10 of 418191 matches for " S. V. Dubonos "
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Observation of dc voltage on segments of an inhomogeneous superconducting loop
S. V. Dubonos,V. I. Kuznetsov,A. V. Nikulov
Physics , 2001,
Abstract: In order to verify a possibility of a dc voltage predicted on segments of an inhomogeneous superconducting loop the Little-Parks oscillations are investigated on symmetrical and asymmetric Al loops. The amplitude of the voltage oscillations $\Delta V$ measured on segments of symmetrical loop increases with the measuring current $I_{m}$ and $\Delta V = 0$ at $I_{m}= 0$ in accordance with the classical Little-Parks experiment. Whereas the $\Delta V$ measured on segments of asymmetric loop has a maximum value at $I_{m}= 0$. The observation of the dc voltage at $I_{m}= 0$ means that one of the loop segments is a dc power source and others is a load. The dc power can be induced by both thermal fluctuation and a external electric noise.
Quantum voltage oscillations observed on segments of an inhomogeneous superconducting loop
S. V. Dubonos,V. I. Kuznetsov,A. V. Nikulov
Physics , 2003,
Abstract: A theoretical prediction published in Phys.Rev. B 64, 012505 (2001) is corroborated experimentally. This corroboration of the theoretical result published in Physical Review B was rejected for publication in the same journal. Such inconsistency is connected with a contradiction to the second law of thermodynamics. According to the theory Phys.Rev. B 64, 012505 (2001) the energy of a noise can be transformed to a dc power in superconducting loop and therefore a dc voltage oscillating with magnetic field can be observed on segments of an inhomogeneous loop in the temperature region close to the superconducting transition. Such quantum voltage oscillations observed in the present work can be induced both by an external electrical noise and by internal equilibrium noise (thermal fluctuations). Violation of the second law takes place in the second case. First of all the theoretical result published by Phys.Rev.B is challenge to the second law but about this contradiction was openly written only in the first version (see physics/0105059) of the experimental paper rejected by Referees of Phys.Rev.B. The transformation of an external electrical noise to the dc voltage was confirmed recently in the paper published in JETP Letters 77, 371 (2003) (see also cond-mat/0303538).
Segment of an inhomogeneous mesoscopic loop as a dc power source
S. V. Dubonos,V. I. Kuznetsov,A. V. Nikulov
Physics , 2011,
Abstract: A dc voltage changed periodically with magnetic field is observed on segments of asymmetric aluminum loop without any external dc current at temperatures corresponded to superconducting transition. According to this experimental result a segment of the loop is a dc power source. A possibility of a persistent voltage on segments of an inhomogeneous normal metal mesoscopic loop follows from this result.
Rectified dc voltage versus magnetic field in a superconducting asymmetric figure-of-eight-shaped microstructure
V. I. Kuznetsov,A. A. Firsov,S. V. Dubonos
Physics , 2007, DOI: 10.1103/PhysRevB.77.094521
Abstract: We have measured periodic oscillations of rectified dc voltage versus magnetic field V_{dc}(B) in a superconducting aluminum thin-film circular-asymmetric figure-of-eight microstructure threaded by a magnetic flux and biased with a sinusoidal alternating current (without a dc component) near the critical temperature. The Fourier spectra of these V_{dc}(B) functions contain fundamental frequencies representing periodic responses of the larger and smaller asymmetric circular loops, composing the microstructure, to the magnetic field. The higher harmonics of the obtained fundamental frequencies result from the non-sinusoidal character of loop circulating currents. The presence of the difference and summation frequencies in these spectra points to the interaction between the quantum states of both loops. Magnitudes of the loop responses to the bias ac and magnetic field vary with temperature and the bias current amplitude, both in absolute values and with respect to each other. The strongest loop response appears when the average resistive state of the loop corresponds to the midpoint of the superconducting-normal phase transition.
Quantum oscillations of rectified dc voltage as a function of magnetic field in an "almost" symmetric superconducting ring
V. I. Kuznetsov,A. A. Firsov,S. V. Dubonos
Physics , 2007, DOI: 10.3103/S1062873807080084
Abstract: Periodic quantum oscillations of a rectified dc voltage Vdc(B) vs the perpendicular magnetic field B were measured near the critical temperature Tc in a single superconducting aluminum almost symmetric ring (without specially created circular asymmetry) biased by alternating current with a zero dc component. With varying bias current and temperature, these Vdc(B) oscillations behave like the Vdc(B) oscillations observed in a circular-asymmetric ring but are of smaller amplitude. The Fourier spectra of the Vdc(B) functions exhibit a fundamental frequency, corresponding to the ring area, and its higher harmonics. Unexpectedly, satellite frequencies depending on the structure geometry and external parameters were found next to the fundamental frequency and around its higher harmonics.
Destruction of the phase coherence by the magnetic field in the fluctuation region of thin superconducting film
A. V. Nikulov,S. V. Dubonos,Y. I. Koval
Physics , 2002, DOI: 10.1007/BF02435484
Abstract: It was shown as long ago as 1998 cond-mat/9811051 that the vortex lattice melting theories are science fiction because the Abrikosov state is not the vortex lattice with crystalline long-range order. Nevertheless this false conception is used up to now in many works including the lecture notes cond-mat/0111052 providing a pedagogical introduction. This false conception has resulted from visual but incorrect ideas. In spite of outward appearances only long-range order - long-range phase coherence exists in the Abrikosov state. The phase transition interpreted erroneously as the vortex lattice melting is in reality a disappearance of long-range phase coherence. The investigation of the nonlocal resistivity made in this work is experimental evidence that a state without phase coherence, differed qualitatively from mythical vortex liquid, is observed above this transition.
Fine Structure in Magnetization of Individual Fluxoid States
A. K. Geim,S. V. Dubonos,J. J. Palacios
Physics , 2000, DOI: 10.1103/PhysRevLett.85.1528
Abstract: Each time a vortex enters or exits a small superconductor, a different fluxoid state develops. We have observed splitting and sharp kinks on magnetization curves of such individual states. The features are the manifestation of first and second order transitions, respectively, and reveal the existence of distinct vortex phases within a superconducting state with a fixed number of fluxoids. We show that the kinks indicate the merger of individual vortices into a single giant vortex while the splitting is attributed to transitions between different arrays of the same number of vortices.
Contradiction between the results of observations of resistance and critical current quantum oscillations in asymmetric superconducting rings
V. L. Gurtovoi,S. V. Dubonos,S. V. Karpii,A. V. Nikulov,V. A. Tulin
Physics , 2010,
Abstract: Magnetic field dependences of critical current, resistance, and rectified voltage of asymmetric (half circles of different widths) and symmetrical (half circles of equal widths) aluminum rings were measured in the temperature region close to the superconducting transition. All these dependences are periodic magnetic field functions with periods corresponding to the flux quantum in the ring. The periodic dependences of critical current measured in opposite directions were found to be close to each other for symmetrical rings and shifted with respect to each other by half the flux quantum in asymmetric rings with ratios between half circle widths of from 1.25 to 2. This shift of the dependences by a quarter of the flux quantum as the ring becomes asymmetric makes critical current anisotropic, which explains the rectification effect observed for asymmetric rings. Shifts of the extrema of the periodic dependences of critical current by a quarter of the flux quantum contradict directly to the results obtained by measuring asymmetric ring resistance oscillations, whose extrema are, as for symmetrical rings, observed at magnetic fluxes equal to an integer and a half of flux quanta.
Proximity Effect and Spontaneous Vortex Phase in Planar SF-Structures
V. V. Ryazanov,V. A. Oboznov,A. S. Prokofiev,S. V. Dubonos
Physics , 2003, DOI: 10.1134/1.1561979
Abstract: The proximity effect in SF structures was examined. It is shown that, due to the oscillations of the induced superconducting order parameter in a ferromagnet, the critical temperature of an SF-bilayer becomes minimal when the thickness of the ferromagnetic layer is close to a quarter of the period of spatial oscillations. It is found that the spontaneous vortex state arisen in the superconductor due to the proximity of the magnetic domain structure of a ferromagnet brings about noticeable magnetoresistive effects.
Observation of the Little-Parks Oscillations in a System of Asymmetric Superconducting Rings
A. A. Burlakov,V. L. Gurtovoi,S. V. Dubonos,A. V. Nikulov,V. A. Tulin
Physics , 2008, DOI: 10.1134/S0021364007200052
Abstract: Little-Parks oscillations are observed in a system of 110 series-connected aluminum rings 2000 nm in diameter with the use of measuring currents from 10 nA to 1000 nA. The measurements show that the amplitude and character of the oscillations are independent of the relation between the measuring current and the amplitude of the persistent current. By using asymmetric rings, it is demonstrated that the persistent current has clockwise or contra-clockwise direction. This means that the total current in one of the semi-rings may be directed against the electric field at measurement of the Little-Parks oscillations. The measurements at zero and low measuring current have revealed that the persistent current, like the conventional circulating current, causes a potential difference on the semi-rings with different cross sections in spite of the absence of the Faraday's voltage.
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