Katsuyoshi Hamasaki

Microstructure and superconductivity in epitaxial MgO/NbN multilayers

The aim of this investigation is to achieve high transition temperature, low resistivity films under conditions suitable for use in all-NbN Josephson junction fabrication. For this purpose, the heteroepitaxy of NbN on MgO films is investigated. For substrate temperature of 200°C or thereabouts, polycrystalline MgO films of (200) orientation were formed on (100) silicon, (102) α-Al2O3, and fused silica. Based on XTEM, HEED and X-ray data, heteroepitaxial growth ((100)NbN//(100)MgO) was obtained in NbN films and NbN/MgO/NbN trilayers deposited on (200) oriented MgO films. The epitaxial NbN films of 10 nm thickness had high transition temperatures of about 14 K, and relatively low residual resistivities of less than 200 µΩcm. Using the GL relationships, we calculated the Ginzburg-Landau parameters, λ(0) and ξ, of our NbN films in the dirty limit. The calculated ξ(4.2 K) and λ(0) are ~4.5 nm and ~300 nm, respectively.

High frequency properties of all-NbN nanobridges with gap structure in I-V curves

All-NbN nanobridges with gap structure in I-V curves have been reproducibly constructed using RIE and lift-off techniques. The nanobridges had a width of 2 μm, and a thickness of c R N products of ∼3 mV, and low excess current. Small-area dc SQUIDs were made using the nanobridges, and analyses of the response to magnetic flux were performed. The current-phase relationship of the nano-bridges was found to be close to sinusoidal. The maximum LC resonant voltage was about 1.2 mV, corresponding to a frequency of 580 GHz. The IF peak was obtained up to the bias voltage of about 4 mV in 101 GHz Josephson mixing.

Critical currents of superconducting PbMo 6 S 8 tapes

An electro-plating and diffusion method has been developed for the production of superconducting ternary lead molybdenum sulfide (PbMo 6 S 8 ) tapes. These tapes were prepared by plating a Mo tape with lead, and with subsequent heat treatment in sulfur (S) vapour from molybdenum sulfide (MoS 2 ). Samples were examined with SEM, XMA, and X-ray diffraction Optimal reaction-temperature and Pb-plating conditions gave a critical temperature of about 14 K, and a resistive transition width of less than 1 K. The value of J c was relatively large, and reached 1.8\times10^{4} A/cm2in a 14 T magnetic field at 4.2 K. The relatively large values of J c in strong magnetic field suggest the need for a more detailed analysis of the preparation conditions and the pinning mechanism in this compound.

Critical currents in the Chevrel‐phase lead molybdenum sulfide thin films

Critical currents in thin films of Chevrel‐phase lead molybdenum sulfide have been measured as a function of applied magnetic field up to 8.5 T. The magnetic field was perpendicular or parallel to the film plane, and a 5‐μV/cm criterion was used to define the critical current density Jc. The maximum value of critical current density was above 106 A/cm2 at zero field and 5×104 A/cm2 at 8 T. These values are greater than the best reported so far.

Self-planarizing process for the fabrication of Bi2Sr2CaCu2Ox stacks

We developed a new fabrication process for stacked intrinsic Josephson junctions using Bi2Sr2CaCu2Ox (Bi-2212) single crystals. For the fabrication of self-planarized stacks, the Bi-2212 around the stack was changed into an insulator by dipping it in a solution of dilute hydrochloric acid. For the solution concentration 0.5%, however, the planarization was spoiled. The current-voltage characteristic of the stacks showed distinct resistive branches with large hysteresis at 77K. The number of intrinsic junctions in the stacks linearly decreased with decreasing the concentration of the solution in the range from 0.05% to 0.2 %. The good controllability of the number of junctions in the self-planarized stacks may be useful for electronic device applications.

Sectional structures and electrical properties of ultrathin NbN/MgO bilayers on Si(100)

NbN/MgO bilayers deposited on Si(100) substrates by rf‐sputtering were studied using cross‐sectional transmission electron microscopy (XTEM). The XTEM results reveal that the 10‐nm‐thick MgO layer forms two layers of different structures, i.e., an amorphous layer (∼4 nm) and a highly (100) oriented polycrystalline layer (∼6 nm). We also found that the structure of MgO underlayers exhibits a strong effect on Tc of on‐deposited NbN layers.

Effect of N2 partial pressure on the structure of MgO thin films deposited by radio frequency-magnetron sputtering with single-crystal MgO target

We report the effect of N2 partial pressure on the structure of MgO thin films deposited on Si(100) substrates by radio frequency-magnetron sputtering in Ar+N2 mixture. The films were evaluated using x-ray diffractmeter. As increasing N2 partial pressure, the intensity of (200)MgO reflection increased and saturated above partial pressure of 4.4×10−1 Pa, which corresponds to N2 flow rate of 3 sccm. Effect of N2 gas on the structure of MgO thin films were investigated by plasma emission spectroscopy. The increase of the optical emission intensity of NH spectrum results in the increase of intensity of (200)MgO reflection. We found the strong relationship between the emission intensity of NH spectrum and intensity of (200)MgO reflection.

Fabrication of thin film Nb‐(fine Nb wires)‐NbN weak links for superconducting quantum interference device applications

Thin film Nb‐(fine Nb wires)‐NbN dc‐superconducting quantum interference devices (SQUIDs) with well‐balanced critical current have been reproducibly fabricated by applying an electric field simultaneously to two insulating Nb/MgO/NbN edge‐sandwiches (each junction area=0.2 μm2) in a superconducting loop, in liquid helium. Although the critical current of a dc‐SQUID is not yet controlled sufficiently, this fabrication technique is well suited to use in short weak link dc‐SQUIDs, thanks to the reproducibility of dc‐SQUIDs with well‐balanced critical current, the simplicity of fabrication processes, and the large device resistance.

1/f noise in high current density NbN/AlN/NbN tunnel junctions

Low frequency noise characteristics of epitaxial NbN/AlN/NbN tunnel junctions have been investigated. For all of our junctions with different current densities, the voltage noise power spectrum S/sub v/(f) showed a frequency dependence of 1/f type. The magnitude of the S/sub v/(f) exhibited two distinct types of dependency on the current density and AlN barrier thickness. We consider that this may result from a difference in the crystal structures of the AlN barrier by consulting with the barrier thickness dependence of tunnel barrier heights. We also estimated the 1/f noise parameter /spl eta/ using the Rogers and Buhrmans empirical theory for the S/sub v/(f), and investigated the relationship between the /spl eta/ and the current density J/sub c/ and the tunnel barrier thickness d/sub AlN/. The tunnel barrier was characterized by investigating the /spl eta/-d/sub AlN/ relation. It was found that the /spl eta/-J/sub c/ and /spl eta/-d/sub AlN/ relation in our high current density junctions, i.e, in the epitaxial tunnel junctions, are different from nonepitaxial tunnel junctions.

Coherent behavior of 2N‐Josephson junction closed loop

Voltage‐current characteristics of a superconducting closed loop which consists of a parallel combination of two linear series arrays of N‐Josephson junctions are theoretically and experimentally investigated for N=3. The results show that the rf‐induced constant voltage steps in v‐i characteristics in the presence of the rf radiation of frequency F appear at voltages corresponding to nNFΦ0 with an integer n. We find that the junctions show a coherent behavior even though the closed loop does not consist of identical junctions. The voltage standard application of this circuit seems feasible.