Shi-Ping Zhou

Nonequilibrium photoresponse of YBa2Cu3O7−x granular films to 8 mm microwave radiation

Nonequilibrium photoresponse behavior has been investigated for YBa2Cu3O7−x (YBCO) granular films to 8 mm microwave radiation under various bias currents and magnetic fields. The measurements reveal that the nonequilibrium photoresponse mode occurs only in the tail region of the resistance transition curve R(T) from the normal to the superconducting state, where transportation behavior of the granular superconducting film is found to be characterized by the Kosterlitz–Thouless (KT) phase transition model. Based on the KT model, the photoresponse mechanism has been interpreted in terms of the depinning process of the unbinding vortices, which are generated from the decoupling process of the vortex–antivortex pairs by current, and are held at the intrinsic pinning sites of the granular high-Tc superconducting films at low temperature. Under the co-action of the bias current and the incident microwave photons, these unbinding vortices will be driven out of the pinning center, creating viscous motion in the J...

The self-sustained current oscillation and the dynamics in superlattices under the action of electric and magnetic fields

Self-sustained time-dependent current oscillations have been found in weakly coupled GaAs∕AlAs superlattices when the sequential resonant tunneling between adjacent quantum wells is the main electron transport mechanism. The oscillation regime was tunable by varying the doping densities and applied dc voltages. Based on the discrete sequential tunneling model, we theoretically studied the magnetic field dependence of the oscillation. The magnetic field B seems to be favorable for the formation of the static electric-field domains and to depress the current oscillation. Thus, the oscillation regime will be narrowed as the magnetic field strength increases. Driven by a transverse external microwave excitation, the system shows interesting nonlinear behaviors like quasiperiodicity, frequency locking, and periodicity.

Microwave detection using Y–Ba–Cu–O granular film bridge device

Abstract High-Tc superconducting microwave video detection is investigated based on the Y–Ba–Cu–O granular film bridge device. By optimizing the device parameters, a detectivity of about 104 V/W is achievable, and the noise equivalent power at T=70 K is about 5.2×10−14 W/Hz1/2. To identify the response time of the device, the response voltage is measured with incident radiation chopped at 500 kHz, and a rise time of less than 3.9×10 −8 s is observed. These results compare favorably with that of high-Tc superconducting bolometers for millimeter wave bands. On the other hand, the dynamic resistance RN of the device is found to increase as the device is illuminated by incident irradiation. This effect cannot be interpreted in terms of the Josephson dissipation mechanism, and a discussion about this is presented.

The s+idx2−y2 pairing symmetry in high temperature superconductors

Abstract The pairing symmetry in high temperature superconductors YBa2Cu3O7−δ is studied. In the framework of Ginzburg–Landau model, the coexistence of s- and d-wave states has been discussed from the group theory analysis point of view. The s+idx2−y2 pairing symmetry argument can provide us a plausible account of the experimental results of the Josephson tunneling along in YBa2Cu3O7−δ/YBa2Cu3O7−δ junctions and the c-axis tunneling between YBCO/Pb junctions as well.

Optically tuned superconductive-dielectric resonators with whispering-gallery modes

An analytical method is presented for calculating the resonant frequency andQ-factor of a superconducting dielectric disk resonator operating in millimeter-wave regime with whispering-gallery mode. Resonant frequency shift due to the optical generation of quasi-particles in superconducting film is investigated as a function of photon flux. An optically tunable resonant frequency of about 500 MHz is estimated, and good agreement is found between numerical results and experimental ones.

Optical detector using granular YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// weak link-type junctions

Experimental photoresponse study of granular YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) weak link is reported. The Josephson critical current I/sub c/ is suppressed with light illumination of the junction, produced mainly from optical generation of quasiparticles. At lower temperature (T=64.5 K), the hysteresis in I-V characteristic of the weak link appears, which may be found useful for optical detection purpose. This was supported from preliminary experiments.

Numerical Simulations of the Fluxon Dynamics of a Two-Dimensional Josephson Junction System

A direct perturbation method applied to a long Josephson junction molded by one-dimensional sine-Gordon equation is presented and the modulation of perturbations on fluxon velocity and stable state is studied. Simulations of a two-dimensional Josephson junction with dc bias current in an external magnetic field provide an I–V characteristic curve for the system and determine the various dynamic behaviors, including the periodic, quasiperiodic, and chaotic motions. Physical explanations for the behaviors are presented. The numerical results are in good agreement with the energetic analysis.

Oblique Vortex Lattice Implies Unconventional Pairing Symmetry in High Temperature Superconductors

AbstractThe symmetry of order parameters of YBa2Cu3O7 − δ high temperature superconductor was studied with the Ginzburg–Landau theory. The vortex lattice of a YBa2Cu3O7 superconductor is oblique at a temperature well below the transition temperature Tc, where the mixed s–

Intermittent superconductivity in three-dimensional mesoscopic rings

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