RESUMO
The defect-rich morphology of YBa2Cu3O7-δ (YBCO) thin films leads to a glass-like arrangement of Abrikosov vortices which causes the resistance to disappear in vanishing current densities. This vortex glass consists of entangled vortex lines and is identified by a characteristic scaling of the voltage-current isotherms. Randomly distributed columnar defects stratify the vortex lines and lead to a Bose glass. Here, we report on the observation of an ordered Bose glass in a YBCO thin film with a hexagonal array of columnar defects with 30 nm spacings. The periodic pinning landscape was engineered by a focused beam of 30 keV He+ ions in a helium-ion microscope.
RESUMO
Low-temperature scanning electron microscopy (LTSEM) has been used to image the supercurrent distribution in ramp-type Josephson junctions between Nb and either the electron-doped cuprate Nd_{2-x}Ce_{x}CuO_{4-y} or the hole-doped cuprate YBa_{2}Cu_{3}O_{7}. For zigzag-shaped devices in the short junction limit the critical current is strongly suppressed at zero applied magnetic field. The LTSEM images show that this is due to the Josephson current counterflow in neighboring 0 and pi facets, which is induced by the d_{x;{2}-y;{2}} order parameter in the cuprates. Thus, LTSEM provides imaging of the sign change of the superconducting order parameter, which can also be applied to other types of Josephson junctions.
RESUMO
Josephson junctions with a phase drop of pi in the ground state allow us to create vortices of supercurrent carrying only half of the magnetic flux quantum Phi0 approximately 2.07 x 10(-15) Wb. Such semifluxons have twofold degenerate ground states denoted upward arrow (with flux +Phi0/2 and supercurrent circulating clockwise) and downward arrow (with flux -Phi0/2 and supercurrent circulating counterclockwise). We investigate a molecule consisting of two coupled semifluxons in a 0-pi-0 long Josephson junction. The fluxes (polarities) of semifluxons are measured by two on-chip SQUIDs. By varying the dc bias current applied to the 0-pi-0 junction, we demonstrate controllable manipulation and switching between two states, upward arrow downward arrow and downward arrow upward arrow, of a semifluxon molecule. These results provide a major step towards employing semifluxons as bits or qubits for classical and quantum digital electronics.
RESUMO
A novel dynamical effect is presented: systematic drift of a topological soliton in ac-driven weakly damped systems with periodic boundary conditions. The effect is demonstrated in detail for a long annular Josephson junction. Unlike earlier considered cases of the ac-driven motion of fluxons (kinks), in the present case the long junction is spatially uniform. Numerical simulations reveal that progressive motion of the fluxon commences if the amplitude of the ac drive exceeds a threshold value. The direction of the motion is randomly selected by initial conditions, and a strong hysteresis is observed. An analytical approach to the problem is based on consideration of the interaction between plasma waves emitted by the fluxon under the action of the ac drive and the fluxon itself, after the waves complete round trip in the annular junction. The analysis predicts instability of the zero-average-velocity state of the fluxon interacting with its own radiation tails, provided that the drive's amplitude exceeds an explicitly found threshold. The result is valid if the phase shift phi of the radiation wave, gained after the round trip, is such that sin phi<0, the threshold amplitude strongly depending on phi. A very similar dependence is found in the simulations, testifying to the relevance of the analytical consideration.
