Yoshimi Nakayama

Compositional dependence of transport anisotropy in large (La, Sr)2CuO4 single crystals and second peak in magnetization curves

Abstract Large single crystals of (La 1 − x Sr x ) 2 CuO 4 , up to 5 mm along the crystal c -axis, have been grown over a composition range of x = 0.03 to 0.15, covering the whole superconducting region in this system. A complete set of electrical resistivity data along both the c -axis ( ϱ c ) and ab -plane ( ϱ ab ) have been obtained via the DC four-probe method for the first time. As x was varied, both resistivities changed monotonically from semiconductive to metallic in character and the critical temperature, T c defined by the mid point of the resistive transition curves, passed through a maximum value of 38.8 K at x = 0.072. The anisotropy ratio ϱ r / g 9 ab at 50 K decreased drastically with increasing x , from nearly 4000 to 160, while it was appoximately 350 at x = 0.072> Magnetization hysteresis curves ( H || c , ± 9 T) revealed the presence of an additional pinning mechanism in the over-doped region ( x ⩾ 0.072) giving rise to a second peak at moderately high fields. This was attributed to enhanced flux pinning by oxygen vacancies nominally present in this composition range.

In situ growth of Bi-Sr-Ca-Cu-O thin films by molecular beam epitaxy technique with pure ozone

Thin films of Bi-Sr-Ca-Cu-O have been grown in situ on SrTiO3 (001) substrates using a molecular beam epitaxy technique with a pure ozone source. This allows us to grow highly oriented films at substrate temperature of 700°C without heat treatment in oxygen. One of the films showed a superconducting transition at 10.0 K.

Epitaxial Growth of Bi-Sr-Ca-Cu-O Thin Films by Molecular Beam Epitaxy Technique with Shutter Control

Thin films of Bi2(Sr, Ca)3Cu2Oy have been prepared in situ by a molecular beam epitaxy technique in an ozone atmosphere of 4×10-4 Pa. The molecular beams were shutter controlled and films were deposited layer by layer. In situ reflection high energy electron diffraction (RHEED) showed the formation of the perovskite-related structure even at the initial deposition (one unit cell). The resistance of one of the films became zero at 30 K without heat treatment in oxygen.

Anisotropic magnetization, flux pinning and critical current density of La-Sr-Cu-O single crystal

Magnetization curves of large single crystals of (La1-xSrx)2CuO4 (x=0.065 and 0.07) were measured at temperatures from 4.2 to 30K under magnetic fields of up to 9T. The crystals utilized had long dimensions in either the c-axis or the a-axis direction along which the magnetic field was applied, in order to minimize the demagnetization effect. The analysis of the hysteresis data revealed strong anisotropy in flux pinning and critical current density, Jcab(H//c), Jcab (H//ab) and Jcc (H//ab). It was found that the field and temperature dependence was most pronounced for Jcab(H//c) and discussion will be made for this particular case in terms of the conventional scaling law.

Physical Properties of an 80 K-Superconductor: Bi-Sr-Ca-Cu-O Ceramics

The magnetization curve, tunneling spectra, and normal-state susceptibility were measured in well-characterized single-phase 80 K superconductor, Bi-Sr-Ca-Cu-O ceramic samples. The obtained result is very similar to that of other copper-oxide superconductors.

Superconducting Films of YBa2Cu3Ox and Bi-Sr-Ca-Cu-O Fabricated by Electron-Beam Deposition with a Single Source

Superconducting thin films of YBa2Cu3Ox and Bi-Sr-Ca-Cu-O were made by electron-beam deposition with a sintered pellet as an evaporation source. With a c-axis orientation perpendicular to the substrates, YBa2Cu3Ox grows on MgO(100) and Bi-Sr-Ca-Cu-O grows on SrTiO3(100), and the critical temperatures for zero resistivity were 78 and 72 K, respectively.

Nonlinear Current Dependence of Potential Barrier for Flux Creep in Superconducting La-Sr-Cu-O Single Crystal

The potential barrier for flux bundles, U, was estimated from the observed relaxation of the magnetization, M, with time, t, in a superconducting La-Sr-Cu-O single crystal. It is shown that U depends on the current density, J, and the temperature, T, as U(J, T)=U0[(J0/J)α-1]. The present result is different from the relationship of U\proptln (J0/J) observed in Y-Ba-Cu-O films, and is similar to the one predicted by the collective theory. It is also pointed out that the observed increase of the effective pinning potential, Uc*=kBT(-∂ln M/∂ln t)-1, with increasing T does not result from the T dependence of U0, but is only an apparent anomaly originating from the above nonlinear U-J characteristics.

Molecular beam epitaxy growth of BiSrCaCuO thin films

Abstract Thin films of BiSrCaCuO have been grown in situ by a molecular beam epitaxy technique. The molecular beams were shutter controlled and films were deposited layer by layer in an ozone atmosphere of 4×10 −4 Pa. In-situ reflection high-energy electron diffraction (RHEED) showed the formation of the perovskite-related structure at the initial deposition(one unit cell).

In-Situ Growth of Sequentially Deposited Bi-Sr-Ca-Cu-O Thin Films by Molecular Beam Epitaxy Technique

Thin Films of Bi2Sr2Can-1CunOy(n=1~5) were prepared in situ by molecular beam epitaxy technique. All films were deposited sequentially by shutter control using pure ozone gas. Films of n=2, 3, 4 and 5 exhibit superconductivity below 65, 40, 75 and 30 K without post annealing process. The resistivity measurements in magnetic fields were performed for the film of n=4. The upper critical field and GL coherence length are estimated.

Preparation and Physical Properties of Bi-Sr-Ca-Cu-O Films Grown by Molecular Beam Epitaxy Using Pure Ozone

High-T. superconductor, Bi-Sr-Ca-Cu-O, has attracted much attention, since it has high transition temperature and the variable CuO2 layer numbers per unit cell. It has also larger anisotropy than the typical high-T c , material YBa2Cu3O7 and has a very short coherence length, especially, parallel to c axis. Thus the fluctuation due to the 2-dimensional nature is expected to be observed more clearly in this material than in others and in fact it is observed in several physical properties. In view of this situation it is very important to investigate physical properties of this material, in particular single crystal as well as high-quality single-crystal thin film of this material.