25 research outputs found
Hot Electron Effects in the 2D Superconductor-Insulator Transition
The parallel magnetic field tuned two-dimensional superconductor-insulator
transition has been investigated in ultrathin films of amorphous Bi. The
resistance is found to be independent of temperature on both sides of the
transition below approximately 120 mK. Several observations suggest that this
regime is not intrinsically "metallic" but results from the failure of the
films' electrons to cool. The onset of this temperature-independent regime can
be moved to higher temperatures by either increasing the measuring current or
the level of electromagnetic noise. Temperature scaling is successful above 120
mK. Electric field scaling can be mapped onto temperature scaling by relating
the electric fields to elevated electron temperatures. These results cast doubt
on the existence of an intrinsic metallic regime and on the independent
determination of the correlation length and dynamical critical exponents
obtained by combining the results of electric field and temperature scaling.Comment: 4 pages, 4 figure
Evidence of Spatially Inhomogeous Pairing on the Insulating Side of a Disorder-Tuned Superconductor-Insulator Transition
Measurements of transport properties of amorphous insulating indium oxide
thin films have been interpreted as evidence of the presence of superconducting
islands on the insulating side of a disorder-tuned superconductor-insulator
transition. Although the films are not granular, the behavior is similar to
that observed in granular films. The results support theoretical models in
which the destruction of superconductivity by disorder produces spatially
inhomogenous pairing with a spectral gap.Comment: Revised title and content/argument. Totals: 4 pages, 3 figure
Electrostatic Tuning of the Superconductor-Insulator Transition in Two Dimensions
Superconductivity has been induced in insulating ultra-thin films of
amorphous bismuth using the electric field effect. The screening of
electron-electron interaction was found to increase with electron concentration
in a manner correlated with the tendency towards superconductivity. This does
not preclude an increase in the density of states being important in the
development of superconductivity. The superconductor-insulator transition
appears to belong to the universality class of the three dimensional XY model.Comment: Four pages, three figures. Revised slightly to reflect referees'
comment
Electrostatic- and Parallel Magnetic Field- Tuned Two Dimensional Superconductor-Insulator Transitions
The 2D superconductor-insulator transition in disordered ultrathin amorphous
bismuth films has been tuned both by electrostatic electron doping using the
electric field effect and by the application of parallel magnetic fields.
Electrostatic doping was carried out in both zero and nonzero magnetic fields,
and magnetic tuning was conducted at multiple strengths of electrostatically
induced superconductivity. The transitions were analyzed using finite size
scaling with critical exponent products nu*z = 0.65-0.7. The parallel critical
magnetic field increased with electron transfer as (dn_c-dn)^0.33, where dn is
the electron transfer and dn_c is its critical value, and the critical
resistance decreased linearly with dn. However at lower temperatures, in the
insulating regime, the resistance became larger than expected from
extrapolation of its temperature dependence at higher temperatures, and scaling
failed. These observations imply that although the electrostatic- and parallel
magnetic field- tuned superconductor-insulator transitions would appear to
belong to the same universality class and to be delineated by a robust phase
boundary that can be crossed either by tuning electron density or magnetic
field, in the case of the field-tuned transition at the lowest temperatures,
some different type of physical behavior turns on in the insulating regime.Comment: About 11 pages, with 14 figures. To be submitted to Phys Rev
Localization and Superconductivity in Doped Semiconductors
Motivated by the discovery of superconductivity in boron-doped (B-doped)
diamond, we investigate the localization and superconductivity in heavily doped
semiconductors. The competition between Anderson localization and s-wave
superconductivity is investigated from the microscopic point of view. The
effect of microscopic inhomogeneity and the thermal fluctuation in
superconductivity are taken into account using the self-consistent 1-loop-order
theory with respect to superconducting fluctuation. The crossover from
superconductivity in the host band to that in the impurity band is described on
the basis of the disordered three-dimensional attractive Hubbard model for
binary alloys. We show that superconductor-insulator transition (SIT)
accompanies the crossover. We point out an enhancement of Cooper pairing in the
crossover regime. Further localization of the electron wave function gives rise
to incoherent Cooper pairs and the pseudogap above T_c. A global phase diagram
is drawn for host band superconductivity, impurity band superconductivity,
Anderson localization, Fermi liquid state, and pseudogap state. A theoretical
interpretation is proposed for superconductivity in the doped diamond, SiC, and
Si.Comment: Final version for publication. To appear in J. Phys. Soc. Jpn. (2009)
No.
Superconductor-insulator transition in thin metallic films induced by interface-roughness scattering
Local switching of two-dimensional superconductivity using the ferroelectric field effect
Correlated oxides display a variety of extraordinary physical properties including high-temperature superconductivity1 and colossal magnetoresistance2. In these materials, strong electronic correlations often lead to competing ground states that are sensitive to many parameters—in particular the doping level—so that complex phase diagrams are observed. A flexible way to explore the role of doping is to tune the electron or hole concentration with electric fields, as is done in standard semiconductor field effect transistors3. Here we demonstrate a model oxide system based on high-quality heterostructures in which the ferroelectric field effect approach can be studied. We use a single-crystal film of the perovskite superconductor Nb-doped SrTiO3 as the superconducting channel and ferroelectric Pb(Zr,Ti)O3 as the gate oxide. Atomic force microscopy is used to locally reverse the ferroelectric polarization, thus inducing large resistivity and carrier modulations, resulting in a clear shift in the superconducting critical temperature. Field-induced switching from the normal state to the (zero resistance) superconducting state was achieved at a well-defined temperature. This unique system could lead to a field of research in which devices are realized by locally defining in the same material superconducting and normal regions with 'perfect' interfaces, the interface being purely electronic. Using this approach, one could potentially design one-dimensional superconducting wires, superconducting rings and junctions, superconducting quantum interference devices (SQUIDs) or arrays of pinning centres