1,221 research outputs found
Cooper pair islanding model of insulating nanohoneycomb films
We first review evidence for the Cooper pair insulator (CPI) phase in amorphous nanohoneycomb (NHC) films. We then extend our analysis of superconducting islands induced by film thickness variations in NHC films to examine the evolution of island sizes through the magnetic field-driven SIT. Finally, using the islanding picture, we present a plausible model for the appearance and behavior of the CPI phase in amorphous NHC films
Theory of High-Tc Superconducting Cuprates Based on Experimental Evidence
A model of superconductivity in layered high-temperature superconducting
cuprates is proposed, based on the extended saddle point singularities in the
electron spectrum, weak screening of the Coulomb interaction and
phonon-mediated interaction between electrons plus a small short -range
repulsion of Hund's, or spin-fluctuation, origin. This permits to explain the
large values of , features of the isotope effect on oxygen and copper, the
existence of two types of the order parameter, the peak in the inelastic
neutron scattering, the positive curvature of the upper critical field, as
function of temperature etc.Comment: RevTeX 3.x, 11 Postscript figures (included); send comments to
[email protected]
Fate of the Bose insulator in the limit of strong localization and low Cooper-pair density in ultrathin films
A Bose insulator composed of a low density of strongly localized Cooper pairs develops at the two-dimensional superconductor to insulator transition (SIT) in a number of thin film systems. Investigations of ultrathin amorphous PbBi films far from the SIT described here provide evidence that the Bose insulator gives way to a second insulating phase with decreasing film thickness. At a critical film thickness dc the magnetoresistance changes sign from positive, as expected for boson transport, to negative, as expected for fermion transport, signs of local Cooper-pair phase coherence effects on transport vanish, and the transport activation energy exhibits a kink. Below dc pairing fluctuation effects remain visible in the high-temperature transport while the activation energy continues to rise. These features show that Cooper pairing persists and suggest that the localized unpaired electron states involved in transport are interspersed among regions of strongly localized Cooper pairs in this strongly localized, low Cooper-pair density phase
Collapse of the Cooper pair phase coherence length at a superconductor to insulator transition
We present investigations of the superconductor to insulator transition (SIT)
of uniform a-Bi films using a technique sensitive to Cooper pair phase
coherence. The films are perforated with a nanohoneycomb array of holes to form
a multiply connected geometry and subjected to a perpendicular magnetic field.
Film magnetoresistances on the superconducting side of the SIT oscillate with a
period dictated by the superconducting flux quantum and the areal hole density.
The oscillations disappear close to the SIT critical point to leave a
monotonically rising magnetoresistance that persists in the insulating phase.
These observations indicate that the Cooper pair phase coherence length, which
is infinite in the superconducting phase, collapses to a value less than the
interhole spacing at this SIT. This behavior is inconsistent with the gradual
reduction of the phase coherence length expected for a bosonic, phase
fluctuation driven SIT. This result starkly contrasts with previous
observations of oscillations persisting in the insulating phase of other films
implying that there must be at least two distinct classes of disorder tuned
SITs
Disorder influences the quantum critical transport at a superconductor-to-insulator transition
We isolated flux disorder effects on the transport at the critical point of the quantum magnetic field tuned superconductor-to-insulator transition (BSIT). The experiments employed films patterned into geometrically disordered hexagonal arrays. Spatial variations in the flux per unit cell, which grow in a perpendicular magnetic field, constitute flux disorder. The growth of flux disorder with magnetic field limited the number of BSITs exhibited by a single film due to flux matching effects. The critical metallic resistance at successive BSITs grew with flux disorder contrary to predictions of its universality. These results open the door for controlled studies of disorder effects on the universality class of an ubiquitous quantum phase transition
Fluctuation Effects in High Sheet Resistance Superconducting Films
As the normal state sheet resistance, , of a thin film superconductor
increases, its superconducting properties degrade. For
superconductivity disappears and a transition to a nonsuperconducting state
occurs. We present electron tunneling and transport measurements on ultrathin,
homogeneously disordered superconducting films in the vicinity of this
transition. The data provide strong evidence that fluctuations in the amplitude
of the superconducting order parameter dominate the tunneling density of states
and the resistive transitions in this regime. We briefly discuss possible
sources of these amplitude fluctuation effects. We also describe how the data
suggest a novel picture of the superconductor to nonsuperconductor transition
in homogeneous 2D systems.Comment: 11 pages, 5 figure
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