Jhy-Jiun Chang

Gap enhancement in superconducting thin films due to microwave irradiation

We investigated the gap enhancement and the change of the quasiparticle density in superconductors driven in a uniformly steady-state manner by microwave irradiation. Effects associated with the frequency and strength of the driving microwave field as well as the thermal coupling of the superconductor and the temperature bath have been considered. We found that in addition to the energy redistribution of quasiparticles by the microwaves, the irradiated film could have lower quasiparticle density than the nonirradiated one, leading to an additional mechanism for enhancement of the gap. In the case of tight thermal coupling of the film and the bath, the gap enhancement due to the reduction of the quasiparticle density can be larger than that due to the redistribution of the quasiparticles.

An experimental investigation of YBa2Cu3O7 films at millimeter-wave frequencies

We have measured the millimeter-wave (100 GHz) surface resistance of high-quality laserdeposited YBa2Cu3O7 films on SrTiO3 and LaA103 substrates. Due to finite film thickness, radiation losses are important in the normal state and in the superconducting state nearTc. These effects are calculated andRs characteristic of the ohmics losses in the film are extracted from the data. The surface resistanceRs drops rapidly atTc, and a detailed comparison with calculations which include finite mean free path effects suggests a gap which exceeds the weak coupling BCS limit.

Interaction between microwaves and a single vortex in a long Josephson tunnel junction

The effects of microwave radiation on the vortex modes of Josephson tunnel junctions have been examined. Current steps are induced on the resistive branches at voltages V=(n/m)(hν/2e) where n and m are integers and ν is the frequency of the incident radiation. The system we have studied is a well‐defined one in which vortices undergoing periodic motion interact with the periodic perturbation of the microwave radiation. Vertical steps are induced when resonance occurs.

Energy gap suppression and instability in superconducting tin films under strong quasiparticle injection

Double tunnel junctions have been used to investigate the properties of superconducting Sn films under quasiparticle injection. At high injection level, the gap is discontinuously suppressed to zero from a finite value. This instability for ambient Ta>Tλ, the λ temperature of liquid helium, results from the film current exceeding its critical value. Below the instability the general behavior of the gap suppression is shown to be consistent with the modified heating model.

Nonlocal response to a focused laser beam in one-dimensional Josephson tunnel junctions

A perturbation method is used to calculate the change of the total zero‐voltage current ΔIc(y0) flowing through a one‐dimensional Josephson tunnel junction due to a focused laser beam irradiated at y0. Nonlocal response of the phase difference function φ as well as the local response of the  amplitude J to the laser beam are included. In the junction configuration we considered, it is found that the nonlocal effect can, depending upon the external magnetic field, make the spatial dependences of ΔIc(y0) and the current distribution different.

Simulations of fine structures on the zero field steps of Josephson tunnel junctions

Fine structures on the zero field steps of long Josephson tunnel junctions are simulated for junctions with the bias current injected into the junction at the edges. These structures are due to the coupling between self‐generated plasma oscillations and the traveling fluxon. The plasma oscillations are generated by the interaction of the bias current with the fluxon at the junction edges. On the first zero field step, the voltages of successive fine structures are given by Vn=ℏ/2e(2ωp/n), where n is an even integer.

Observation of step structures on the resistive branches of long Josephson tunnel junctions

Abstract We have observed well defined current steps superimposed on the resistive branches associated with non-resonant vortex propagation in a long Sn-oxide-Sn junction. Experimental data suggest that these step structures are due to the interaction between the traveling vortex and the cavity modes whose frequencies are given by νn = nνc, where νc is the frequency of the fundamental cavity mode and n is an integer. The voltage-frequency relation for those steps are found to be 2eV = (n/m)hνc, where m is an integer greater than n. This is different from that of the Fiske steps and the zero field steps of a short junction for which the fraction n/m is replaced by an integer equal to or greater than unity.

Calculation of the effects of a focused laser beam on the dynamic Josephson tunneling states

Perturbative calculations of the voltage response of constant‐current biased Josephson tunnel junctions to a focused laser beam have been performed. The dependence of the voltage modification as a function of the laser beam position was calculated. The results were found to be qualitatively different for junctions biased on the first zero‐field current step but with either vortex‐mode or cavity‐mode excitations. Therefore, the recently developed scanning focused laser beam technique operated in this mode can be a simple and effective method for probing the nature of the dynamic Josephson tunneling state.

Energy spectra of phonons emitted from a nonequilibrium superconducting thin film

Results of a theoretical investigation of the energy spectra of phonons generated by a superconducting thin film under IR and optical pumping, phonon injection via heat pumping, and electron injections through SIS tunnel junctions are reported. The coupled quasiparticle and phonon kinetic equations and a modified BCS gap equation are used as the basis of our study. The use of the thin films as phonon generators or phonon convertors to produce narrow band phonons of energy in the meV range and the mechanisms that limit the bandwidth are discussed.

Charge transfer in electrostatically coupled chains: Effects of the nearest neighbor Coulomb correlations

The effects of the nearest neighbor Coulomb correlations on the character of the charge transfer between donor and acceptor molecular chains are investigated. We use a modified Hubbard model for the individual donor and acceptor chains. In this model the intramolecular Coulomb interactions are taken to be infinitely large, and the transfer integrals are considered to be infinitely small. All the intermolecular interactions except those between the nearest neighbors along the chains are treated within the mean field approximation. The free energy of this model can be calculated for an arbitrary strength of the nearest neighbor interaction and results are given for the temperature dependences of the charge transfer and the zero field paramagnetic spin susceptibility.