arXiv subjects
Publications and source records attributed to S. V. Dubonos.
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.
Magnetic dependencies of the critical current of superconducting, aluminium rings, with homogeneous section and with small narrow spot, are measured. Only permitted state at each magnetic flux value is observed in the conventional ring because of strong discreteness of its spectrum. But two states are observed because of the small narrow spot.
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.
The obtained periodic magnetic-field dependencies of the critical current, measured in opposite directions on asymmetric superconducting aluminum rings, allow to explain observed earlier quantum oscillations of a dc voltage as a result of alternating current rectification. It is found, that the high efficiency of the rectification of both individual rings and ring systems is connected to a hysteresis of the current-voltage characteristics. The asymmetry of the current-voltage characteristics providing the rectification effect is due to the relative shifts of the magnetic dependencies of the critical current measured in opposite directions. This shift means that position of the critical current minimums does not correspond to n + 0.5 magnetic flux quantum which is in the direct contradiction with measurement results of the Little-Parks resistance oscillations. Despite of this contradiction the amplitude of the critical current anisotropy oscillations and its variations with temperature correspond to expected amplitude of the persistent current oscillations and to its variations with temperature.
We analyze theoretically and experimentally vortex configurations in mesoscopic superconducting squares. Our theoretical approach is based on the analytical solution of the London equation using Green's-function method. The potential-energy landscape found for each vortex configuration is then used in Langevin-type molecular-dynamics simulations to obtain stable vortex configurations. Metastable states and transitions between them and the ground state are analyzed. We present our results of the first direct visualization of vortex patterns in micrometer-sized Nb squares, using the Bitter decoration technique. We show that the filling rules for vortices in squares with increasing applied magnetic field can be formulated, although in a different manner than in disks, in terms of formation of vortex "shells".
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.
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.
The interaction between the quantum states of two aluminum superconducting rings forming an 8-shape circular-asymmetric microstructure was examined under a threading magnetic flux and bias by an alternating current without a dc component. Quantum oscillations of the rectified dc voltage Vdc(B) as a function of magnetic field were measured in the 8-shape microstructure at various bias ac currents and temperatures close to critical. Fourier and wavelet analyses of Vdc(B) functions revealed the presence of various combination frequencies in addition to two ring fundamental frequencies, which suggests the interaction in the structure. Deviation of the Vdc(B) function from oddness with respect to the magnetic field direction was found for the first time.
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.
In order to clear up a question on possibility of a dc voltage proportional to the persistent current in normal metal and semiconductor mesoscopic loops conjectural causes of this phenomenon observed in superconductor loops is investigated.
Multi-quanta, or giant, vortices (GVs) are known to appear in very small superconductors near the superconducting transition due to strong confinement of magnetic flux. Here we present evidence for a new, pinning-related, mechanism for the formation of GVs. Using Bitter decoration to visualise vortices in small Nb disks, we show that confinement in combination with strong disorder causes individual vortices to merge into clusters or even GVs well below Tc and Hc2, in contrast to well-defined shells of individual vortices found in the absence of pinning.
Multiple and single measurements of quantum states of mesoscopic superconducting loops are carried out in order to investigate a possibility of macroscopic quantum superposition and of creation of a superconductor quantum register. Asymmetric superconducting rings are used in order to be convinced that single measurement gives result corresponding to one of permitted states and multiple one gives an average value on these states. We have measured magnetic dependencies of resistance, rectified voltage and critical current on these rings. The observed quantum oscillations of the resistance and the rectified voltage, corresponding to multiple measurement of quantum states, give evidence of two permitted states at the magnetic flux inside the ring divisible by half of the flux quantum. But the observed quantum oscillations of the critical current, corresponding to single measurement, not only does not confirm these two quantum states, but are in a direct contradiction with the observed oscillations of the resistance. It is assumed that the observed contradiction between the results of measurements made on the same ring can testify to violation of the principle of realism on the mesoscopic level that is a necessary condition for an opportunity of creation of a quantum computer.
Mesoscopic loop is proposed in many works as possible solid-state quantum bit, i.e. two-state quantum system. The quantum oscillations of resistance and of rectified voltage observed on asymmetric superconducting loops give evidence of the two states at magnetic flux divisible by half of the flux quantum. But our measurements of quantum oscillations of the critical current of these loops have given results coming into irreconcilable contradictions with result of the observations of the quantum oscillations of resistance.
The Little-Parks oscillations of the resistance and the quantum oscillations of the rectified voltage observed for asymmetric superconducting Al rings give experimental evidence of two degenerate energy states at the magnetic flux $Φ_{0}$/2. The quantum oscillations of the critical current as a function of magnetic field have also been measured. On the one hand, these oscillations confirm that the quantum oscillations of the rectified voltage are a consequence of periodical dependence of the asymmetry of the current-voltage curves and, on the other hand, comparison of the oscillations with Little-Parks measurements results in contradiction.
The quantum oscillations of the dc voltage are induced on segments of asymmetric superconducting loops by an external ac current or noise. The dependencies of the amplitude of the quantum oscillations on amplitude of inducing ac current are measured at different temperatures below superconducting transition Tc on aluminum asymmetric loops and systems of the loops connected in series. The measured values of the maximum amplitude of the quantum oscillations, the amplitude of the ac current inducing this maximum dc voltage and the critical amplitude of the ac current decrease with temperature increase to Tc. The extrapolation of these measured dependencies to the region near superconducting transition allows to make a calibration of asymmetric superconducting loops as quantum detector of noise. The calibration restores an amplitude profile of the noise pulses from a measured temperature dependence of an amplitude of the quantum oscillations induced by this noise. It is found that rectification efficiency, determined as relation of the maximum amplitude of the quantum oscillations to the ac current amplitude inducing it, decreases near superconducting transition Tc. High efficiency of rectification observed below Tc is consequence of irreversibility of the current-voltage curves. Increase of the rectification efficiency is achieved in multiple series connected loop structures.
The quantum rectification effect observed on asymmetric superconducting loops is proposed to use as a basic of a noise detector with maximum sensitivity. The measurements show that a critical amplitude of noise or ac current decreases down to zero near superconducting transition, Tc, just as superconducting critical current. Therefore any how weak noise, right down to the equilibrium one, can induce the quantum oscillations of the dc voltage near Tc. The transformation of the power of random noise into the dc power observed on asymmetric superconducting loops gives very important advantage: even very weak noise can give enough high output power in a system with large number of loops.
The current-voltage curves and magnetic dependence of the critical current of asymmetric superconducting loops are measured. It was found that sign and value of the asymmetry of the current-voltage curves changes with value of magnetic field, periodically for single loop and system of identical loops. The obtained results allow to explain the quantum oscillation of the dc voltage, observed below superconducting transition in the previous works, as rectification of ac current or noise.
Electronic properties of materials are commonly described by quasiparticles that behave as non-relativistic electrons with a finite mass and obey the Schroedinger equation. Here we report a condensed matter system where electron transport is essentially governed by the Dirac equation and charge carriers mimic relativistic particles with zero mass and an effective "speed of light" c* ~10^6m/s. Our studies of graphene - a single atomic layer of carbon - have revealed a variety of unusual phenomena characteristic of two-dimensional (2D) Dirac fermions. In particular, we have observed that a) the integer quantum Hall effect in graphene is anomalous in that it occurs at half-integer filling factors; b) graphene's conductivity never falls below a minimum value corresponding to the conductance quantum e^2/h, even when carrier concentrations tend to zero; c) the cyclotron mass m of massless carriers with energy E in graphene is described by equation E =mc*^2; and d) Shubnikov-de Haas oscillations in graphene exhibit a phase shift of pi due to Berry's phase.