V. A. Tulin

Little-Parks Effect in a System of Asymmetric Superconducting Rings

Little-Parks oscillations are observed in a system of 110 series-connected aluminum rings 2 μm in diameter with the use of measuring currents from 10 nA to 1 μA. The measurements show that the amplitude and character of 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 a direction. This means that, in the Little-Parks experiment, the total current in one of the half-rings may be directed against the electric field.Little-Parks oscillations are observed in a system of 110 series-connected aluminum rings 2 μm in diameter with the use of measuring currents from 10 nA to 1 μA. 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.

Contradiction between the Results of Observations of Resistance and Critical Current Quantum Oscillations in Asymmetric Superconducting Rings

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 close to the super-conducting transition were measured. 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 effect of alternating current rectification observed for asymmetric rings. Shifts of the extrema of the periodic dependences of critical current by a quarter of the flux quantum directly contradict 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.

Dependence of the magnitude and direction of the persistent current on the magnetic flux in superconducting rings

The obtained periodic magnetic-field dependences Ic+(Φ/Φ0) and Ic−(Φ/Φ0) of the critical current measured in opposite directions on asymmetric superconducting aluminum rings has made it possible to explain previously observed quantum oscillations of dc voltage as a result of alternating current rectification. It was found that a higher rectification efficiency of both single rings and ring systems is caused by hysteresis of the current-voltage characteristics. The asymmetry of current-voltage characteristics providing the rectification effect is due to the relative shifts of the magnetic dependences Ic−(Φ/Φ0) = Ic+(Φ/Φ0 + Δϕ) of the critical current measured in opposite directions. This shift means that the position of Ic+(Φ/Φ0) and Ic−(Φ/Φ0) minima does not correspond to n + 0.5 magnetic flux Φ quanta, which is in direct contradiction to measured Little-Parks resistance oscillations. Despite this contradiction, the amplitude Ic, an(Φ/Φ0) = Ic+(Φ/Φ0) − Ic−(Φ/Φ0) of critical current anisotropy oscillations and its variations with temperature correspond to the expected amplitude of persistent current oscillations and its variations with temperature.

Possibility of persistent voltage observation in a system of asymmetric superconducting rings

Abstract The possibility of observing persistent voltage in superconducting rings of different arm widths is experimentally investigated. It was previously found that switching of the arms between superconducting and normal states by an AC current induces DC voltage oscillation in the magnetic field with a period corresponding to the flux quantum inside the ring. We used systems with a large number of asymmetric rings connected in series to investigate the possibility of observing this quantum phenomenon near the superconducting transition, where thermal fluctuations lead to switching of ring segments without an external influence and the persistent current is much smaller than in the superconducting state.

Weak dissipation does not result in the disappearance of the persistent current

Recent experiments confirm a prediction made by I.O. Kulik forty years ago to the effect that energy dissipation does not suppress the equilibrium circular current observed in the normal state of superconducting rings and normal metal rings. Conflicting interpretations of the persistent current as a Brownian motion or a dissipationless current are compared from the standpoint of observations of this phenomenon when an electric potential difference is present. Distinctions between quantum phenomena at the atomic and mesoscopic levels are emphasized. It is pointed out that quantum oscillations in the magnetic field of the potential difference observed in asymmetric rings with a persistent current can be verified experimentally under thermodynamic equilibrium.

Superconducting quantum interference device without Josephson junctions

A new type of a superconducting quantum interference device (SQUID) based on a single superconducting loop without Josephson junctions and with asymmetric contacts has been proposed. This SQUID offers advantages in simplicity of fabrication and a steeper dependence of measured quantities on the magnetic flux. To confirm the possibility of making this type of SQUID, the magnetic field dependence of the critical current in an aluminum ring with asymmetric contacts has been experimentally investigated.

Synchronization of high-frequency vibrations of slipping phase centers in a tin whisker under microwave radiation

Current-voltage characteristics of a system with a variable number of slipping phase centers resulting from phase separation in a tin whisker under external microwave field with a frequency Ω/2π≅35–45 GHz have been studied experimentally. Emergence and disappearance of steps with zero slope in a whisker’s current-voltage characteristic at Um/n=(m/n)UΩ, where m and n are integers and UΩ is determined by Josephson’s formula ℏΩ=2eUΩ, have been investigated. Microwave field generated by slipping phase centers is nonharmonic, and the system of slipping phase centers permits synchronization of internal oscillations at a microwave frequency by an external field with a frequency which is the n-th harmonic of internal oscillations. The estimated microwave power generated by a whisker is 10−8 W. Stimulation of superconductivity in a current-carrying whisker has been detected.

Multiple superconducting ring ratchets for ultrasensitive detection of non-equilibrium noises

Magnetic quantum periodicity in the dc voltage is observed when asymmetric rings are switched between superconducting and normal state by a noise or ac current. This quantum effect is used for the detection of a non-equilibrium noise using a system of 667 asymmetric aluminum rings of 1 μm diameter connected in series. Any noise down to the equilibrium one can be detected with large number of asymmetric rings. The equilibrium noise can induce the dc voltage close to the superconducting transition. In our sample we observe the dc voltage oscillation only below the superconducting transition.

High-frequency absorption of the dynamic mixed state in the surface superconductivity region

We analyze the absorption of a high-frequency electromagnetic field in the type II superconductor Pb0.8In0.2 in magnetic fields Hc2 < H < Hc3. The absorption component proportional to the rate of variation of the external magnetic field is detected. We assume that this absorption component is associated with the dynamic mixed state of the superconducting shell containing 2D magnetic flux vortices (Kulik vortices). The motion of these vortices under the action of the critical current ensures the required difference between the external and internal magnetic inductions of the superconducting shell upon a change in the external magnetic field. This model correctly describes the observed behavior of absorption of rf electromagnetic radiation.

Multiple current states of two phase-coupled superconducting rings

The states of two phase-coupled superconducting rings have been investigated. Multiple current states have been revealed in the dependence of the critical current on the magnetic field. The performed calculations of the critical currents and energy states in a magnetic field have made it possible to interpret the experiment as the measurement of energy states into which the system comes with different probabilities because of the equilibrium and nonequilibrium noises upon the transition from the resistive state to the superconducting state.