Zhao Shi-Ping

Electron tunneling spectroscopy of Bi2Ca1Sr2Cu2Oy superconductor

Abstract We investigated the electron tunneling spectroscopy of single crystal Bi2Ca1Sr2Cu2Oy superconductor of Tc=84 K, using a point contact system. A well-formed gap structure has been observed at liquid helium temperature. In the analysis of our data, we found that an excellent fit to the experimental dI dV curve could be achieved by assuming the density of states of the form N(E)=Re [ E E 2 −Δ 2 ], where Δ is complex and independent of energy. A parallel conductance due to the shunt resistance other than NS tunneling mechanism has been considered and the least square calculation resulted in a reduced energy gap of 2Δ = 5.0 . Discussions are presented.

Fabrication of high-quality submicron Nb/Al-AlOx/Nb tunnel junctions

Nb/Al–AlOx/Nb tunnel junctions are often used in the studies of macroscopic quantum phenomena and superconducting qubit applications of the Josephson devices. In this work, we describe a convenient and reliable process using electron beam lithography for the fabrication of high-quality, submicron-sized Nb/Al–AlOx/Nb Josephson junctions. The technique follows the well-known selective Nb etching process and produces high-quality junctions with Vm = 100 mV at 2.3 K for the typical critical current density of 2.2 kA/cm2, which can be adjusted by controlling the oxygen pressure and oxidation time during the formation of the tunnelling barrier. We present the results of the temperature dependence of the sub-gap current and in-plane magnetic-field dependence of the critical current, and compare them with the theoretical predictions.

Femtosecond Optical Detection of Quasiparticle Dynamics in Single-Crystal Bi2Sr2CaCu2O8+δ

Quasiparticle dynamics of an optimally doped Bi2Sr2CaCu2O8+δ single crystal is investigated by the femtosecond pump-probe technique. Temperature dependences of amplitude of the photoinduced differential reflectivity and the relaxation time show the evidence of strong phonon bottleneck. The experimental results are analysed by the Rothwarf–Taylor model.

Fabrication of Intrinsic Josephson Junctions on Bi2Sr2CaCu2O8+? Single Crystals with Reproducible Surface Junctions

Intrinsic Josephson junctions on Bi2Sr2CaCu2O8+δ single crystals are successfully fabricated using photolithography and Ar ion milling. Here we discuss the properties of the surface CuO2/metal bilayers and surface junctions prepared with different crystal cleavage conditions and metal film deposition techniques. We show that, by cleaving the crystal at liquid nitrogen temperature in vacuum, the contents of the (interstitial) oxygen in the Bi-O layers become stable upon cleavage, leading to a CuO2/metal bilayer and to surface junctions with reproducible properties. These results can be useful for practical device fabrications as well as for the studies of the contact properties between high Tc superconductors and normal metals.

Analysis of Step Etching on SrTiO3 Substrates for the Step-Edge YBCO Josephson Junctions

A way to determine some important etching parameters in the step fabrication for high Tc step-edge Josephson junctions is described based on an analysis of the dynamics of the etching process. The optimum thickness of the etch mask is defined with negligible recession of the mask edge during the etch. Under this condition, the equilibrium angle of the steps etched on the SrTiO3 (STO) substrate with an Nb mask has been calculated to be about 76°. With optimized mask thickness, its sharp sidewall and straight edge, high-quality steps on STO substrates with step height from 200-300 nm and step angle above 70° are made. Josephson junctions and dc-SQUIDs with high reproducibility and less parameter scatter are obtained on the step substrates. PACS: 74. 50. +r, 74. 76. Bz, 85. 25. Dq

Fabrication of rhenium Josephson junctions

Rhenium is a superconductor with a relatively weak tendency to oxidize, which is advantageous in superconducting quantum circuit and qubit applications. In this work, Re/Al—AlOx/Re Josephson tunnel junctions were fabricated using a selective film-etching process similar to that developed in Nb trilayer technology. The Re films had a superconducting transition temperature of 4.8 K and a transition width of 0.2 K. The junctions were found to be highly reproducible using the fabrication process and their characteristics had good quality with a low leakage current and showed a superconducting gap of 0.55 meV.

Nb/Al-AlOx/Nb junctions with controllable critical current density for qubit application

Nb/Al-AlOx/Nb tunnel junctions with controllable critical current density JC are fabricated using the standard selective Nb etching process. Tunnel barriers are formed in different oxygen exposure conditions (oxygen pressure P and oxidation time t), giving rise to JC ranging from 100 A/cm2 to above 2000 A/cm2. JC shows a familiar linear dependence on P × t in logarithmic scales. We calculate the energy levels of the phase- and flux-type qubits using the achievable junction parameters and show that the fabricated Nb/Al-AlOx/Nb tunnel junctions can be used conveniently for quantum computation applications in the future.

Research on high-T-c SQUID based non-destructive evaluation

A non-destructive evaluation system based on high-Tc dc-SQUID (superconducting quantum interference device) incorporating a gradient field excitation has been built. By using this system a 1mm-diameter hole at a depth of 2mm inside an aluminium plate at room temperature can be easily detected and imaged in an unshielded environment. The relation between the spatial resolution, or the smallest detectable flaw size and experimental parameters is briefly analysed in terms of a simple metal ring model. The result shows that the spatial resolution depends strongly on the sensor-sample separation as well as on some other parameters, such as signal-to-noise ratio of excitation, excitation frequency and material conductivity.A non-destructive evaluation system based on high-Tc dc-SQUID (superconducting quantum interference device) incorporating a gradient field excitation has been built. By using this system a 1mm-diameter hole at a depth of 2mm inside an aluminium plate at room temperature can be easily detected and imaged in an uushielded environment. The relation between the spatial resolution, or the smallest detectable flaw size, and experimental parameters is briefly analysed in terms of a simple metal ring model. The result shows that the spatial resolution depends strongly on the sensor-sample separation as well as on some other parameters, such as signal-to-noise ratio of excitation, excitation frequency and material conductivity.

Retrieval of original signals for superconducting quantum interference device operating in flux locked mode

We discuss a simple relation between the input and output signals of a superconducting quantum interference device magnetometer operating in flux locked mode in a cosine curve approximation. According to this relation, an original fast input signal can be easily retrieved from its distorted output response. This technique can be used in some areas such as sensitive and fast detection of magnetic or metallic grains in medicine and food security checking.

Phase escape of current-biased Josephson junctions

Switching current distributions of an Nb/Al–AlOx/Nb Josephson junction are measured in a temperature range from 25 mK to 800 mK. We analyse the phase escape properties by using the theory of Larkin and Ovchinnikov (LO) which takes discrete energy levels into account. Our results show that the phase escape can be well described by the LO approach for temperatures near and below the crossover from thermal activation to macroscopic quantum tunneling. These results are helpful for further study of macroscopic quantum phenomena in Josephson junctions where discrete energy levels need to be considered.