Fermi Velocity Renormalization Research Articles

Oxygen-controlled transition from tunnel-like to a weak-link behavior in YBa 2Cu 3O 7−δ/YBa 2CoCu 2O y/YBa 2Cu 3O 7−δ wedge-edge junctions

Differential conductance measurements in YBa 2Cu 3O 7−δ/YBa 2CoCu 2O y /YBa 2Cu 3O 7−δ (YBCO/YBCoCO/YBCO) wedge-edge junctions are reported on. The properties of the junctions change dramatically with the oxygen concentration in the films. Junctions prepared in an oxygen-deficient atmosphere show differential conductance which increases with the bias voltage. This behavior is typical of high- T c tunnel junctions. In contrast, junctions prepared with fully oxygenated films show weak-link behavior where the differential conductance decreases with the bias voltage. In the tunnel-like type junctions a series of peaks is observed in the differential conductance versus voltage. We identify these peaks as due to Tomasch oscillations in the cover electrode along the c direction, yielding Δ c = 2.1 mV and a renormalized Fermi velocity v′ F c = 2.4 × 10 7 cm/s.

Scaling theory of interacting light and heavy fermions in one dimension.

We consider a one-dimensional model of interacting electrons. The Fermi surface crosses two bands, with light and heavy masses, respectively. Both intraband and interband interactions are assumed. The earlier renormalization-group treatment of the model by Varma and Zawadowski is extended to check the validity of the scaling assumption and to calculate the next corrections to the scaling equations. These corrections indicate a Fermi-velocity renormalization. Depending on the initial parameters, two kinds of behavior can appear. Either the interband charge fluctuations are negligible and then the dominant instabilities are determined by the interband exchange and the intraband backscattering with the two velocities remaining different, or the interband charge fluctuations become dominant and the velocities scale to a common value. The phase diagram of the model is also determined.

Tomasch effect and superconducting proximity

Tunneling conductance measurements (dI/dV)(V) on thick and clean super-conducting films (S 1 ) backed by normal metals (M 2 ) show geometrical resonance effects, which have been associated with the variation δΔ(x) of the pair potential near the interface. We analyze both theoretically and experimentally the three factors involved in the resonance effect on S 1 /M 1 sandwiches where one of the films is superconducting. The first factor, period, has already received considerable attention and is connected with the ratioV /d 1 of the renormalized Fermi velocity and the thicknessd 1 of the film S 1 . We show that generally subharmonic effects, related to multiple interference effects, are expected. This result is demonstrated experimentally on Pb/M 2 sandwiches. The phase depends on the sign of δΔ(x), the detailed shape of its variation, and on the nature of the tunneling process. Unfortunately the interplay among these parameters is difficult to analyze, although directive tunneling is found to fit better with experiments than the generally assumed diffuse tunneling. The amplitude of the resonance should be proportional to the change of δΔ(x). The ambiguity in the experimental results is pointed out and an experiment that shows clearly the role of the superconducting proximity mechanisms is described where the proximity on several simultaneously prepared Pb/Al films is controlled by deposition of various third upper layers. The theoretical discussion is an extension of the Ishii matrix formulation, itself based on an application of the Andreev scattering process.