Mitsumasa Suzuki

Two-axis goniometer for use in the mixing chamber of a dilution refrigerator

Abstract We have constructed a goniometer to rotate a sample around two independent axes in the mixing chamber of a dilution refrigerator. Two small computer-controlled stepper motors wound with superconducting wire rotate the sample around the axes with an angular range of 360 °. The goniometer was designed to investigate the angular dependence of the magnetic de Haas-van Alphen effect of heavy electron materials in a magnetic field of up to 14.5 T.

Preparation and properties of c-axis-oriented NdBa2Cu3O7−δ films by dc sputtering

Abstract NdBa 2 Cu 3 O 7− δ (NdBCO) is known to have the highest T c , among the rare earth REBa 2 Cu 3 O 7− δ (RBCO) family, and to show higher irreversibility fields than YBa 2 Cu 3 O 7− δ (YBCO). Recently, excellent superconducting NdBCO films have been synthesized by a laser ablation technique. In this work, the transition temperature, phase and growth morphology of NdBCO films deposited on (100) SrTiO 3 (STO) and (100) MgO substrates by dc sputtering have been investigated. The transition temperature T c is closely related with the c -axis lattice constant and shows a maximum around 1.173 nm. The c -axis lattice constant of the films strongly depends on oxygen pressure P o . High- T c NdBCO films are obtained around P o =350 Pa. The spiral growth morphology of the films is affected by deposition rate and takes place at low deposition rate. The best film has shown a zero resistance transition temperature of 92.6 K.

Critical Current Density in Superconducting Nb3Ge and NbN Films at Fields up to 30 T

The superconducting critical current densities Jc of Nb3Ge and NbN films prepared by sputtering have been studied in fields up to 30 T for the perpendicular field direction H⊥ to the film surface and up to 23 T for the parallel field direction H//. The Nb3Ge film exhibits Jc exceeding 1 ×105(1 ×104) A/cm2 in fields below 21(30) T for H⊥ and below 23 T for H//. The Jc of the NbN film for H⊥ is higher than 1 ×105 A/cm2 in fields below 21 T, while that for H// is much reduced. Their pinning properties and upper critical fields Hc2 are presented.

Superconducting Tc's of Sputter-Deposited Nb–Ge Films and Tc Nonuniformity within Films

The superconducting transition temperature Tc and nonuniformity of Nb–Ge films prepared on sapphire substraies from compound targets by dc sputtering have been studied. The relationships between the transition temperature, the electrical resistivity ratio and the film thickness have been studied using targets with different compositions, and the conditions for preparing high-Tc. Nb–Ge films below ~2000 A in thickness have been determined. The Tc values for Nb–Ge films are appreciably improved in films thinner than ~1000 A with a Ge-richer target (Nb/Ge=2.8) and onset Tcs greater than 20 K are obtained in films with thicknesses of ~500 A. The nonuniformity of Tc in the films has been measured. The high Tc arises from the film surface layer and the Tc variation along the film thickness is greatly affected by changes in the substrate temperature during deposition.

Effects of surface treatment of single-crystal MgO substrates on the growth and properties of YBa/sub 2/Cu3O/sub 7-y/ films

The preferred orientation and the inplane alignment of YBa/sub 2/Cu/sub 3/O/sub 7-y/ (YBCO) thin films deposited on MgO [100] substrates by on-axis DC magnetron sputtering have been investigated. The deposition temperature highly influences the preferred orientation of films. Films were oriented with the a-axis perpendicular to the substrates for low temperature depositions and with the c-axis for high temperature depositions. In the c-axis oriented films, there exist YBCO grains with the a(or b)-axis parallel to MgO [100] and 4-5/spl deg/ -rotated ones. 45/spl deg/ -rotated grains exclusively grow under the conditions of higher temperatures and on the substrates treated by acid-etching. The volume ratio between both types of YBCO grains is strongly connected with critical current density, even in the c-axis oriented films.<<ETX>>

High Field Properties of Superconducting Nb3Ge Films Prepared by Chemical Vapor Deposition

The superconducting critical current densities Jc of Nb3Ge films prepared by chemical vapor deposition have been investigated in magnetic fields parallel and perpendicular to the film plane up to above 20 T. The Jc and its field dependence are strongly influenced by any change in the microstructure of films deposited at different deposition temperatures Td. Films deposited at a low Td of 800°C exhibit a high Jc below 20 T and show a slight difference between parallel and perpendicular field dependences of Jc. As Td is increased to 900°C, the Jc of films is appreciably lowered over the entire field range and the difference between two field dependences of Jc is remarkably enlarged. The upper critical field Hc2 of these films has been obtained by extrapolating the Kramer plot of Jc1/2H1/4 versus H to J1/2cH1/4=0, and were estimated to range from 28 to 31 T at 4.2 K.

Upper critical fields of superconducting Nb3Ge films

The upper critical field Hc2 (4.2 K) of superconducting Nb3Ge films prepared by sputtering and chemical vapor deposition (CVD) has been measured in a pulsed field. The Hc2 of these films is generally related to their superconducting critical temperature Tc. Sputtered films exhibit a linear increase of Hc2 from 25.4 to 33.3 T for a Tc range 18.6–21.3 K. Although the data of CVD‐prepared films are scattered due to nonuniformity in the films, a similar dependence of Hc2 on Tc has been found.

Characteristics of High-Tc Nb3Ge Films Prepared by Chemical Vapor Deposition

The properties of Nb–Ge films prepared by chemical vapor deposition have been studied in order to clarify the characteristics of high-TcNb3Ge. The effects of the deposition temperature and film composition on the transition temperature, residual resistance ratio, residual resistivity, A15 lattice parameter and growth of the A15 and non-A15 phases are described. The characteristics of the high-Tc phase are discussed in terms of correlations between the transition temperature, A15 lattice parameter and film composition. For films with transition temperatures around 20 K, the A15 lattice parameter is ~5.140 A, and the film composition (Nb/Ge ratio) falls in the range 2.5–3.0. Micrographs of some films, taken by scanning electron microscopy, reveal that the growth morphology of films changes markedly as the deposition temperature varies from 750 to 900°C.

Superconducting Critical Current Densities of Sputter-Deposited Nb3Ge Films

Critical current densities Jc of sputter-deposited Nb3Ge films with thicknesses of less than 1 µm have been measured in applied fields either parallel or normal to the film plane. Current densities in the 105 A/cm2 range are obtained in a parallel field of 15 T. A film with the highest Jc consists of regular fine grains of ~1000 A in columnar diameter. Jcs of thicker films where A-15 grains with different sizes are observed are relatively low.

Anisotropic resistivity of in-plane-aligned La2-xSrxCuO4(100) films on LaSrGaO4(100) substrates

We prepared in-plane-aligned La2-xSrxCuO4(100) films using LaSrGaO4(100) substrates by KrF laser ablation. The scan of X-ray diffraction and the large anisotropy of resistivity indicate a high degree of in-plane epitaxy. The T c of the in-plane-aligned La2-xSrxCuO4 (x=0.15) was 21.6 K. The critical temperatures measured along the c-axis were usually higher than those measured along the a-axis. We also observed voltage peaks at just above T c, corresponding to the abrupt decrease of the anisotropy of resistivity at T c.