Akinobu Irie

Observation of Josephson Junctionlike Behavior in Single-Crystal (Bi, Pb)2Sr2CaCu2Oy

The dependences of critical current on the temperature and magnetic field and the current-voltage characteristics in response to incident microwave radiation are measured in the direction of the c-axis of the 80 K superconducting phase (Bi, Pb)2Sr2CaCu2Oy bulk single crystal. Josephson junctionlike behavior is successfully observed, and it shows that the single crystal itself is composed of Josephson junctions along the c-direction and that its electrical transport in this direction is limited by intrinsic tunnel barriers existing in the single crystal.

Fiske and flux-flow modes of the intrinsic Josephson junctions in Bi2Sr2CaCu2Oy mesas

We report the observation of the highest velocity mode of Josephson vortex motion in mesas consisting of 5–15 intrinsic Josephson junctions with an area of 37×30 μm2 on Bi2Sr2CaCu2Oy single crystals. By applying the dc magnetic field parallel to CuO2 planes in the mesas, resonant steps and flux flow branches have been observed on the current–voltage characteristics along the c-axis of the mesas. The characteristic velocities of the intrinsic Josephson junctions are estimated to be (1.5–1.8)×106 m/s, which corresponds to the highest velocity mode, from the voltages of Fiske steps and the maximum voltages of the flux-flow branches.

Microwave response of intrinsic Josephson junctions in BSCCO single crystals

We have studied the microwave response of an array of intrinsic Josephson junctions in the c-axis direction in large Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 7/ (BSCCO) single crystals indicating multiple resistive branches on the I-V characteristics. Under the 8-10 GHz microwave irradiation, they showed three types of responses depending on its power P. For relatively low P, the Shapiro steps were observed, whose voltage separations were two order magnitude larger than that calculated from /spl Delta/V=hf/2e, indicating that about one hundred intrinsic junctions in the crystal are phase-locked. For high P, two types of RF-induced steps were observed. In the case of resonance mode, the I-V curve showed the pronounced constant voltage step together with subharmonic ones. On the other hand, for off-resonance mode the broad step was observed. The voltages of these steps increased lineally with p/sup 1/2/. These results suggest that the fluxon motions in intrinsic Josephson junctions interact with external RF field.<<ETX>>

YBCO DC SQUID of MOCVD Thin Film Bridge

Using YBa2Cu3O7-x (YBCO) thin films (0.5~1.0 µm in thickness) with c axis orientation, we fabricated dc superconducting quantum interference devices (SQUIDs). YBCO films were deposited on MgO(100) single-crystal substrates by MOCVD. These films exhibit zero resistance above 80 K and critical current density of 104 to 105 A/cm2 at 77 K. The SQUIDs with microbridge are fabricated by photolithography and chemical etching techniques. The maximum voltage modulation is about 80 µV for the YBCO dc SQUID with an inductance of 70 pH, and intrinsic energy sensitivity is 740 h (h: Plancks constant) at 4.2 K. Furthermore, the SQUID was operated in flux-locked loop. The minimum flux noise in the flux-locked SQUID is 1.5×10-5 \varPhi0/√Hz in the white noise region at 4.2 K. The experiments suggest that the current-phase relationship of a micrometer-long YBCO bridge may be sinusoidal.

A high-performance micro-GRIN-chip spot-size converter formed with focused ion beam

A high-performance spot-size converter composed of a micrograded-index slab chip with a hemi-cylindrical endface for conversion between single-mode fibers (SMFs) and fine-core waveguides has been fabricated using the focused ion beam etching technique. The microchip has the index difference as much as 1.8 in the index-gradient direction and the input hemi-cylindrical endface of curvature radius as small as 6.3 /spl mu/m. The measured full width at the 1/e/sup 2/ maximum of the resulting focused spot intensity distribution is 1.8/spl times/2.5 /spl mu/m/sup 2/ when the input end is excited by an SMF of 10.8-/spl mu/m-spot diameter at the wavelength of 1.55 /spl mu/m.

Transport properties of small-sized intrinsic Josephson junctions in Bi2Sr2CaCu2Oy

Abstract Transport properties of small-sized intrinsic Josephson junctions in Bi 2 Sr 2 CaCu 2 O y have been systematically investigated with respect to both junction size and temperature. Using electron-beam lithography and Ar ion milling, we have successfully fabricated mesas with lateral dimensions ranging from sub-μm 2 to several tens μm 2 on the same crystal. Typical multiple resistive branches with hysteresis and well defined gap structures were observed in the I – V characteristics of the mesas at 4.2 K. In addition, we observed the suppression of the critical current densities as lateral dimensions of the mesa decrease down to less than 1 μm 2 . We interpret the decrease in J c of submicron junctions in terms of the contribution of the Coulomb blockade effect.

Microwave phase locking steps in intrinsic Josephson junctions of Bi2Sr2CaCu2Oy single crystals

Abstract The microwave-induced phase-locking phenomena in the intrinsic Josephson junctions in Bi 2 Sr 2 CaCu 2 O y single crystal mesas have been investigated experimentally. For the mesas with the dimensions much larger than the Josephson penetration depth, obvious microwave-induced steps composed of fundamental and subharmonic steps have been successfully observed in the current-voltage characteristics. The voltages of both steps increase proportionally to the microwave power, and the amplitudes of the subharmonic steps show periodical change with the microwave frequency. These dynamical properties can be interpreted to be caused by the microwave-induced phase-locking fluxon motion in the intrinsic Josephson junctions.

Devil's staircases and continued fractions in Josephson junctions

The detailed numerical simulations of the IV-characteristics of Josephson junction under external electromagnetic radiation show devils staircases within different bias current intervals. We have found that the observed steps form very precisely continued fractions. Increasing of the amplitude of radiation shifts the devils staircases to higher Shapiro steps. The algorithm of appearing and detection of the subharmonics with increasing radiation amplitude is proposed. We demonstrate that subharmonic steps registered in the famous experiments by A. H. Dayem and J. J. Wiegand [Phys. Rev 155, 419 (1967)] and J. Clarke [Phys. Rev. B 4, 2963 (1971)] also form continued fractions.

Josephson Current in Microbridges of YBa2Cu3O7-δ Thin Films Prepared by CVD

Single crystallike microbridges are fabricated from YBa2Cu3O7-δ thin films made by the CVD method. The thicknesses of the bridges range from 0.2 to 1 µm, and the width and length are both about 1 µm. The critical current densities of the bridges range from 104 to 105 A/cm2 at 77 K in zero field. The bridges show clear ac and dc Josephson effects. The single bridges look like typical Josephson junctions, and all transport current in the bridges behaves like Josephson current. However, the temperature dependences of critical current density show Jc∝(1-T/Tc)3/2., the bridges behave like one-dimensional long superconductors.

Structured chaos in a devil's staircase of the Josephson junction

The phase dynamics of Josephson junctions (JJs) under external electromagnetic radiation is studied through numerical simulations. Current-voltage characteristics, Lyapunov exponents, and Poincaré sections are analyzed in detail. It is found that the subharmonic Shapiro steps at certain parameters are separated by structured chaotic windows. By performing a linear regression on the linear part of the data, a fractal dimension of D = 0.868 is obtained, with an uncertainty of ±0.012. The chaotic regions exhibit scaling similarity, and it is shown that the devils staircase of the system can form a backbone that unifies and explains the highly correlated and structured chaotic behavior. These features suggest a system possessing multiple complete devils staircases. The onset of chaos for subharmonic steps occurs through the Feigenbaum period doubling scenario. Universality in the sequence of periodic windows is also demonstrated. Finally, the influence of the radiation and JJ parameters on the structured chaos is investigated, and it is concluded that the structured chaos is a stable formation over a wide range of parameter values.