Presence Of Superconductivity Research Articles

Theory of Andreev resonances in quantum dots

We present a comprehensive theory of the electrical conductance G of phase-coherent, multi-channel, resonant structures in the presence of superconductivity. When voltages of the order of the level spacing are applied, particle-hole symmetry is broken and our results differ significantly from earlier descriptions. After deriving generalizations of the well-known Breit-Wigner formula, valid in the presence of superconductivity, results for resonant transport in three classes of structure are obtained. First, for a superconducting dot (SDOT) connected to normal contacts (N), we examine the change in conductance as the magnitude of the superconducting order parameter increases from zero. The change is typically negative, except near a normal-state resonance, where large positive changes can occur. Secondly, for a structure comprising a normal (N) contact, a normal dot (NDOT) and a superconducting (S) contact, we predict that finite-voltage, differential conductance resonances are strongly suppressed by the switching on of superconductivity in the S contact. In the weak-coupling limit, resonances which survive have a double-peaked line-shape. Thirdly, analytic results are presented for superconductivity-enhanced, quasi-particle interferometers (SEQUINs), which demonstrate that resonant SEQUINs can provide galvanometric magnetic flux detectors, with a sensitivity in excess of the flux quantum.

Magnetic and transport properties of 214 oxides: Dependence on doping

We consider how doping can be described in terms of the charge-transfer insulator concept. We discuss and compare a few models for the band structure for the doped charges. This has led us to the conclusion that the band structure stability problem is one of the main issues in any correspondence between results for thet-J model and, say, the three-band model for the slightly doped layered oxides. The stability criterion is formulated and its implications discussed. Provided a phenomenological conduction band is chosen to satisfy the criterion of stability, a detailed picture of how dopants influence the spin wave spectrum atT=0 is presented. The basic physics for the destruction of the antiferromagnetic (AF) long-range order is rather model-independent: the long-range order (atT=0) disappears due to the Cerenkov effect when the Fermi velocity first exceeds the spin wave velocity. We then discuss the overall spectrum of spin excitations and see that the spin wave attenuation for x<x c,T= 0 due to Landau damping appears in the range of magnon momentak(x)=2m * s±α√x. We also argue that in the presence of superconductivity, the Cerenkov effect is eliminated due to the gap in the spectrum. This may restore the role of the AF fluctuations as the main source of dissipation at the lowest temperatures. A brief discussion of how interaction with magnons may affect the hole spectrum concludes the paper.

A simple contactless method for the study of superconductivity and critical shielding currents

A technique for detecting superconductivity in small samples of high temperature superconductor is described. The sample is placed within the inductor of a Colpitts oscillator. The precise quantity of feedback is finely adjusted to maintain oscillation at a set amplitude. The presence of superconductivity is detected through a small but sudden change in frequency and an increase in the gain required to maintain oscillations. The value of critical shielding current may also be estimated in these experiments. The various modes of application of the technique are illustrated by typical experimental results.

High pressure-high temperature synthesis of distorted perovskite-type Cu-based oxides

Abstract We have synthesized the rhombohedrally distorted perovskite phase of LaCuO3 by reacting mixtures of La2O3 and CuO in an oxygen rich atmosphere at 1500°C and 6.5 GPa. We find this phase to be metastable; at 410°C and ambient pressure, it undergoes an irreversible transition to a tetragonal structure. By selective replacement of some or all of the La or Cu with one or more of the following elements: Ba, Ca, Cr, Ni, Pb, Sc, Sr, Ti, Y, Zn, and Zr, over 150 different alloys have been formed. Magnetic susceptibility measurements have failed to reveal the presence of superconductivity in any of these new polymorphs.

Elusive superconductivity in polycrystalline samples of layered lanthanum nickelates

La 2- x NiO 4, La 2- x Sr x NiO 4 and related layered nickelates have been investigated for possible presence of superconductivity. While there is clear onset of diamagnetism around 20 K in many of these nickelates, we do not, however, find any anomaly in the electrical resistivity, magnetoresistance or thermopower around 20 K. High energy spectroscopic studies show Ni to be in the 2+ oxidation state accompanied by a substantial proportion of oxygen holes.

Synthesis and magnetic properties of the Bi-Sr-Ca-Cu oxide 80- and 110-K superconductors.

We report on the observation of both the 80- and 110-K superconducting transitions in a nominally $\mathrm{BiSrCa}{\mathrm{Cu}}_{2}{\mathrm{O}}_{x}$ material produced by a new chemical synthesis procedure as well as by solidstate reaction. We also discuss a method for the confirmation of the presence of superconductivity when only a small amount is present.

Superconductivity in layered nickel oxides

Likely presence of superconductivity in layered nickelates of K2NiF4 structure is pointed out.

Fabrication of Y-Ba-Cu-O mixed phase superconductors

Mixed phase superconductors have been fabricated from Y2O3, BaCO3, and Cu2O which show an onset temperature of 96 K. The presence of superconductivity is found to depend on cooling rate, the presence of oxygen, and temperature.

Influence of superconductivity on magnetic exchange interactions

Using the formulation due to Fedro and Robinson of magnetic exchange in the presence of superconductivity, the mean field magnetic interaction energy has been calculated for rare earth (RE) ions in the RERh4B4 lattice. It is shown that increasing the strength of the superconducting interaction relative to the magnetic interaction results in a decrease of ferromagnetic coupling, and in some cases may drive ferromagnetic systems into antiferromagnetism.

Effects of Fermi Surface Nesting on Magnetic Instability in Magnetic Superconductors

Possible magnetic instability in magnetic superconductors is studied by calculating the wave number dependent susceptibility χ( q ) in the presence of superconductivity. Effects of Fermi surface nesting on χ( q ) are examined for three models: Free electron gas model and tightly binding cosine band model of one-dimension, and Liu's model for imperfect nesting. All model calculations reveal that stable magnetic ordering which coexists with superconductivity is antiferromagnetic one. Possibility of an incommensurate spin density wave state is discussed.

Superconductivity and charge-density waves

A theory of two competing order parameters---superconductivity and charge-density waves---as applied to layered compounds is presented. Both effects are caused by the phonon-mediated attractive interaction between itinerant electrons in narrow-band materials. The interplay between superconductivity and charge-density waves is analyzed. The main results of our model calculation are: (i) all metallic charge-density systems should be superconducting at low temperatures: (ii) the presence of superconductivity tends to reduce the charge-density wave and vice versa; (iii) under some conditions it is possible to have a charge-density-wave---induced semiconductor which, at low temperatures, makes a transition to a superconducting state. This last case is examined in detail.

Four critical fields in superconducting indium lead alloys

Data on some polycrystalline indium-lead alloys that confirm the presence of superconductivity in fields greater than the upper critical field and show the temperature dependence and concentration dependence of the four critical fields at solute concentrations up to 6% Pb are presented. (L.B.S.)