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In order to obtain experimental evidence for the influence of flux focusing effect on the characteristics of planar grain boundary (GB) Josephson junctions, we measured the current–voltage characteristics and magnetic-field-dependent current–voltage steps of integrated YBa2Cu3O7- δ bicrystal GB Junctions with different junction widths. Under the self-magnetic field, the critical current of the GB junction is proportional to the square root of the junction width, which is different from the traditional linear relationship between the critical current and the junction width. When a vertical magnetic field is applied, the flux flow current–voltage step could be observed, and the step voltage increases monotonously with an increase in the junction width. When a magnetic field rotating in the bc plane is applied, the voltage value of the flux flow step decreases slowly as the magnetic field inclined angle increases when the field inclined angle is relatively large. Based on these results, we demonstrate the presence of the magnetic flux focusing effect on YBa2Cu3O7- δ GB Josephson junctions. The size of flux focusing in the GB is closely associated with the width of the junctions and the orientation of the magnetic field.

By measuring the dependence of transverse (Hall) and longitudinal magnetoresistance on temperature, the vortex easy-flow channel effect in an YBCO microbridge with an inclined GB was studied when the magnetic field was rotated in the ac plane of the film. In addition, the magnetic field angular dependence of critical current density in an YBCO microbridge with a GB perpendicular to bridge was measured when the magnetic field was rotated in the ab plane of the film. These measurement results demonstrate that there still can be vortex components moving inside and along the plane of the GB even though the vortices diagonally cross the GB.

We fabricate a microscale electromechanical system, in which a suspended superconducting membrane, treated as a mechanical oscillator, capacitively couples to a superconducting microwave resonator. As the microwave driving power increases, nonmonotonic dependence of the resonance frequency of the mechanical oscillator on the driving power has been observed. We also demonstrate the optical switching of the resonance frequency of the mechanical oscillator. Theoretical models for qualitative understanding of our experimental observations are presented. Our experiment may pave the way for the application of a mechanical oscillator with its resonance frequency controlled by the electromagnetic and/or optical fields, such as a microwave-optical interface and a controllable element in a superqubit-mechanical oscillator hybrid system., 8 pages,4 figures

By adjusting the frequency of the cavity, we perform a microwave reflection measurement and directly probe the coherence and interference effects in a phase-sensitive Josephson parametric amplifier. We demonstrate the shift in the peak and the dip in the reflection spectra of the amplifier, which operates in the phase-sensitive mode. The behavior of the shift can be precisely controlled by tuning the pump power, the frequency of the cavity, and the relative phase between the incident signal and pump field. Theoretical simulations are in good agreement with the experimental results. These results provide an alternative way of controlling the parametric process by adjusting the frequency of the cavity.

By applying a coupling microwave field gated by random-telegraph-noise (RTN), we report the experimental demonstration of dynamically modulated Autler-Townes effect (ATE) in an artificial atom---a superconducting transmon qubit---and obtain the distinguished characteristics when a system is affected by RTN. By further applying another coupling microwave filed on the transmon, we use an intense bichromatic field to couple the energy levels. Depending on the detuning of the couplers, remarkable interference patterns emerge in the spectrum, which come from the dressed states induced by the modulated ATE. Numerical calculations agree well with the experimental data.

We investigated the current–voltage characteristics and the critical current density (J c) of a YBa2Cu3O7−δ grain boundary (GB) long Josephson junction with respect to magnetic field (H) under different application directions. When H is rotated within the bc-plane of the film, a small peak of J c is observed when H is parallel to the c-axis, which denotes that there are some residual c-axis dislocations in the GB. When H is rotated within the ac-plane, some vortex-flow steps can be observed in the current–voltage curve at a high inclination angle of H. The vortex-flow step becomes smaller and wider with the decreasing angle between H and the c-axis. These results are a clear demonstration of vortex splitting at the GB plane in YBa2Cu3O7−δ GB long Josephson junctions.