Little-Parks Oscillations Research Articles

Electron transport in mesoscopic structures

Recent advances in electron beam lithography have allowed to prepare high-quality metal structures with submicrometer dimensions. At low temperatures the size of these structures can become smaller than the coherence length over which the electrons maintain their phase memory. Therefore, a mesoscopic regime can be reached, where the quantum interference between conduction electron waves becomes important. The interference effects can be conveniently tuned by applying a perpendicular magnetic field. Using a lift-off technique we have prepared various submicrometer Al and Au structures. In the superconducting Al loops the Ginzburg-Landau coherence length, which diverges near T c, corresponds to the relevant characteristic length scale. Due to their mesoscopic sample size the Al loops behave as superconducting quantum interference devices (SQUIDs), even when no artificial Josephson junctions are present in the branches of the loop. The mesoscopic confinement of the superconducting wave function causes in addition anomalous Little-Parks oscillations of T c when a magnetic field is applied. We have been able to further reduce the linewidth of the mesoscopic sample by using the tip of a scanning tunneling microscope (STM) for local exposure of a very thin Langmuir-Blodgett layer used as an electron sensitive resist.

Newly Observed Phase Coherent Electron Transport Properties in the Mesoscopic Loop Structure of Aluminum Wire

We have identified two new features related to the coherent transport in the mesoscopic loop structure of aluminum wire, including the autocorrelation of the conductance fluctuations beyond Bc and fine structure in the low-field magnetoresistance curve in the superconducting transition regime, which, to the best of our knowledge, have not been reported in the literature. Since the electrons in Al have a phase coherence length larger than 1 μm at or below T = 3 K, which is comparable to the dimensions of the structure, the wave nature of the electronic transport has been clearly observed: the universal conductance fluctuations, the Aharonov-Bohm oscillations, and the Altshuler-Aronov-Spivak oscillations. Due to the transition of Al to a superconducting state at T = 1.3 K, the coherent phenomena of Cooper pairs, i.e., the Little-Parks oscillations, have also been observed.

Little-Parks and Aharonov-Bohm oscillations in fractional Hall regime: Manifestation of Chern-Simons gauge flux.

We show that the recently developed Chern-Simons gauge theory for fractional quantum Hall effect leads naturally to a striking prediction that an applied gate voltage coupled to an isolated region in a two-dimensional electron gas can induce Little-Parks oscillations of the longitudinal conductance in the quantum Hall devices at odd denominator filling fractions. For even denominator filling fractions, the Chern-Simons fermions are in a Fermi liquid state, and the conductance fluctuations are similar to the Aharonov-Bohm conductance oscillations of a mesoscopic metal.

The critical field of a hollow type-II superconducting cylinder

An exact calculation is given of the upper critical field of a hollow isotropic type-II superconducting cylinder. This field is the analogue of the surface-sheath critical field Hc3. As the temperature decreases below Tc the critical field corresponds to increasing values of the azimuthal quantum number mod m mod , i.e. the fluxoid number. The results are compared with those for a flat surface, a film and a solid cylinder and with the approximate result for a thin-walled cylinder. For the last case, it is shown bow the Little-Parks oscillations are modified as the cylinder wall thickness increases. The effects of normal-metal cladding on the inner and outer surfaces are calculated.

Field enhanced superconductivity in mesoscopic loops

Abstract The superconducting phase boundary TC(H) in a mesoscopic loop reveals the presence of ordinary Little-Parks oscillations as well as an anomalous field enhancement of the superconducting transition temperature Tc for low flux quanta. The amplitude of the Tc enhancement depends on the current through the loop imposed by the external current source. For high currents, exceeding a critical value, a crossover to a TC(H) boundary without any Little-Parks oscillations is observed.

Anomalous Little-Parks oscillations in mesoscopic loops.

Anomalous resistance oscillations have been observed in meso- scopic superconducting Al loops. As for the classical Little-Parks effect, the oscillation period is governed by the superconducting fluxoid quantization. The oscillation amplitude shows, however, an unusual magnetic-field and current dependence for low flux quanta, which is completely different from the Little-Parks oscillations.

Anomalous Little-Parks oscillations and field enhanced superconductivity in mesoscopic loops

Quantum interference phenomena have been studied in superconducting Al loops with dimensions smaller than the temperature dependent coherence length ξ(T). Besides the classical Little-Parks (LP) effect, anomalous resistance oscillations have been observed whose amplitude shows an unusual magnetic field and current dependence at low flux quanta. The magnetic field (H) - critical temperature (Tc) phase boundary has been measured for different transport currents jt. A clear field enhancement of Tc is observed at low current densities. The amplitude of the field enhancement of Tc is strongly influenced by the transport current through the loop. Higher jt values induce a transition from the Tc(H) boundary with LP oscillations to a new regime without oscillations. This transition is governed by the interplay between the characteristic superconducting length scale ξ(T), the loop circumference 2πr and the de Broglie wavelength λD ∝ (1/jt), tuned by the transport current.

Little-Parks oscillations of Tc in patterned microstructures of the oxide superconductor YBa2Cu3O7: Experimental limits on fractional-statistics-particle theories.

We report observation of Little-Parks oscillations in arrays with a 4-\ensuremath{\mu}${\mathrm{m}}^{2}$ plaquette area, fabricated from c^-axis-oriented ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ films. Near the mean-field transition temperature the magnitude of the oscillations implies a zero-temperature coherence length \ensuremath{\xi}(0)=15\ifmmode\pm\else\textpm\fi{}2 A\r{}. For temperatures more than 1 K below the mean-field transition, the magnitude of the oscillations increases dramatically, apparently signaling a crossover to the dynamics of a Josephson-coupled network. Precise experimental limits can be placed on the value of the flux quantum, severely restricting any anomalous value which may be proposed on the basis of fractional-statistics-particle theories.

Negative magnetoresistance in small superconducting loops and wires.

We report an experimental attempt to measure resistively the predicted periodic reentrant behavior of small superconducting loops in external magnetic fields. The superconductivity in the contact probes was deliberately weakened in order to minimize their influence on the loops. As expected, the loops exhibit Little-Parks oscillations near the top of the resistive transition. At lower temperatures, instead of the reentrant behavior, the samples show an unexpected negative magnetoresistance. We interpret this result as being due to a nonequilibrium situation existing between normal and superconducting regions in the sample.