arXiv subjects
Publications and source records attributed to L. M. Hernandez.
A parallel field negative magnetoresistance has been found in quench-condensed ultrathin films of amorphous bismuth in the immediate vicinity of the thickness-tuned superconductor-insulator transition. The effect appears to be a signature of quantum fluctuations of the order parameter associated with the quantum critical point.
Measurements of resistance vs. temperature have been carried out on a sequence of quench-condensed ultrathin films of amorphous bismuth (a-Bi). The resistance below about 0.1K was found to be temperature independent in a range of films with thicknesses spanning the superconductor-to-insulator transition that would be inferred by analyzing data obtained above 0.14K. Film magnetoresistance was temperature dependent in the same temperature range over which the resistance was temperature independent. This implies that the low temperature metallic regime is intrinsic, and not a consequence of failure to cool the electrons.
Dramatic glass-like behavior involving nonexponential relaxation of in-plane electrical conduction, has been induced in quench-condensed ultrathin amorphous Bi films by the application of a parallel magnetic field. The effect is found over a narrow range of film thicknesses well on the insulating side of the thickness-tuned superconductor-insulator transition. The simplest explanation of this field-induced glass-like behavior is a Pauli principle blocking of hopping transport between singly occupied states when electrons are polarized by the magnetic field. A second explanation involving weak coupling of superconducting clusters by intercluster interactions of random sign is also discussed.
A Kelvinox 400 dilution refrigerator with the ability to load samples onto the mixing chamber from the bottom of the cryostat has been combined with an ultrahigh-vacuum (UHV) deposition chamber equipped with molecular beam sources. The liquid helium cooled sample transfer mechanism is used in a manner that allows films to be grown on substrates which are kept at temperatures of order 8K with chamber pressures in the 10^-9 to 10^-10 Torr range. This system facilitates the growth of quench-condensed ultrathin films which must always be kept below ~ 12K in a UHV environment during and after growth. Measurements can be made on the films down to millikelvin temperatures and in magnetic fields up to 15 T.
Nonlinear I-V characteristics have been observed in insulating quench-condensed films which are locally superconducting. We suggest an interpretation in terms of the enhancement of conduction by the depinning of a Cooper pair charge density wave, Cooper pair crystal, or Cooper pair glass that may characterize the insulating regime of locally superconducting films. We propose that this is a more likely description than the Coulomb blockade or charge-anticharge unbinding phenomena.