Transition from metallic to tunneling-type conductance in metal-metal and normal-metal--superconductor point contacts
- Department of Physics, Indian Institute of Science, Bangalore 560012 (India)
We have investigated the microshort--to--tunneling crossover in normal-normal ([ital N]-[ital N]) and normal-metal--superconductor ([ital N]-[ital S]) point-contact junctions at 4.2 K as the junction conductance ([ital G][sub 0]) is varied. The microshort (or metallic'') point contact with the dynamic conductance [ital G] ([ital V]) having a negative derivative with respect to bias [ital V] (i.e., [partial derivative][ital G]/[partial derivative][ital V][lt]0) changes over to a tunneling-type junction with [partial derivative][ital G]/[partial derivative][ital V][gt]0 when [ital G][sub 0][congruent]3--5 mS. We show that this is the region where the contact radius [alpha] is a few times [ital k][sub [ital F]][sup [minus]1] ([ital k][sub [ital F]] is the Fermi wave vector) and the crossover in the sign of [partial derivative][ital G]/[partial derivative][ital V] occurs due to electron confinement in a length scale comparable to [ital k][sub [ital F]][sup [minus]1]. The effect of confinement is to make the electrons crossing the constriction evanescent in nature as the junction size is reduced progressively and the energy for lateral confinement becomes comparable to [ital E][sub [ital F]]. We argue that in this extreme ballistic regime, the classical Sharvin approach breaks down as quantum effects due to electron confinement takes over. This happens much before single-atom'' contact is reached which signals the onset of vacuum tunneling. As a further test of the electron confinement effects in point contacts, we investigated clean [ital N]-[ital S] microshorts showing near-ideal Andreev reflection. We find that in [ital N]-[ital S] junctions, the Andreev reflection (which is a manifestation of superconductivity) gradually vanishes when the contact radius [alpha][congruent]0.1[xi] ([xi] is the coherence length) and the energy cost of electron confinement is larger than the superconducting energy gap [Delta].
- OSTI ID:
- 7155451
- Journal Information:
- Physical Review, B: Condensed Matter; (United States), Vol. 46:22; ISSN 0163-1829
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
SUPERCONDUCTIVITY AND SUPERFLUIDITY
SUPERCONDUCTING JUNCTIONS
ELECTRIC CONDUCTIVITY
TUNNEL EFFECT
COHERENCE LENGTH
CONFINEMENT
ENERGY GAP
TEMPERATURE DEPENDENCE
TEMPERATURE RANGE 0000-0013 K
TRANSPORT THEORY
DIMENSIONS
ELECTRICAL PROPERTIES
JUNCTIONS
LENGTH
PHYSICAL PROPERTIES
TEMPERATURE RANGE
665411* - Basic Superconductivity Studies- (1992-)
665412 - Superconducting Devices- (1992-)