C.-J. Chien

Phase Dependent Thermopower in Andreev Interferometers

We report measurements of the thermopower S of mesoscopic Andreev interferometers, which are hybrid loops with one arm fabricated from a superconductor (Al), and one arm from a normal metal (Au). S depends on the phase of electrons in the interferometer, oscillating as a function of magnetic flux with a period of one flux quantum (= h/2e). The magnitude of S increases as the temperature T is lowered, reaching a maximum around T = 0.14 K, and decreases at lower temperatures. The symmetry of S oscillations with respect to magnetic flux depends on the topology of the sample.

Reentrance effect in normal-metal/superconducting hybrid loops

We have measured the transport properties of two mesoscopic hybrid loops composed of a normal-metal arm and a superconducting arm. The samples differed in the transmittance of the normal/superconducting interfaces. While the low transmittance sample showed monotonic behavior in the low temperature resistance, magnetoresistance and differential resistance, the high transmittance sample showed reentrant behavior in all three measurements. This reentrant behavior is due to coherent Andreev reflection at the normal/superconducting interfaces. We compare the reentrance effect for the three different measurements and discuss the results based on the theory of quasiclassical Greens functions.

Extrinsic origins of electrical transport anomalies near the superconducting transition in mesoscopic aluminum lines

We investigate whether inadvertent noise currents may induce the previously reported resistive peak and negative differential resistance (dV/dI) anomalies near the superconducting transition in quasi-one-dimensional aluminum lines. Although our samples show no intrinsic anomalies, low frequency current deliberately applied to the sample in addition to the ordinary measuring current induces the anomalies. In our work the resistive peak results from simple mixing, but the negative dV/dI feature arises from a more complex effect.

Resistance anomalies in mesoscopic superconducting Al structures

We show that the recently reported anomalous resistance peaks near the superconducting transition of quasi-one-dimensional mesoscopic Al structures can be induced by applying either radio-frequency radiation or a sufficiently high dc current. The nonmonotonic R(T) curves can be linked to changes in the voltage-current characteristics below the superconducting transition. The observed dependence on the position of the voltage probes is consistent with recent theoretical calculations which interpret the anomalies in terms of the charge imbalance near phase-slip centers created at specific spots on the sample.

Thermopower of Mesoscopic Normal-metal/Superconductor Heterostructures

Measurements of micron-size doubly connected normal-Metal/superconductor (NS) heterostructures show athermoelectric response which oscillates as a function ofmagnetic field, with a fundamental period corresponding to oneflux quantum h/2e through the area of the loop. Theoscillations of the thermopower are either symmetric orantisymmetric with respect to magnetic field, depending on thetopology of the sample. The temperature dependence of theamplitude of the oscillations shows a minimum at a temperatureof 0.14 K, although this nonmonotonic behavior doesnot appear to be related to the reentrance effect recentlyobserved in the electrical transport properties of NSstructures.

New phenomena in nanostructured materials

In recent years, a level of nanoscale fabrication has been achieved where dimensions and energy scales are such that quantum effects are dominant and can even be controlled in a variety of condensed matter systems. In this paper, several examples are given to illustrate the influence of nanostructuring on the physical properties of superconducting and normal magnetic systems. For superconductors we will discuss quantum interference effects in mesoscopic single loops and multiloop structures and the confinement of flux lines by a lattice of artificial submicron antidots. For magnetic layers we will analyze the size dependence of the Kondo and spin-glass effects.