N. Motohira

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.

Anisotropy of Bi2Sr2CaCu2O8+y revisited

Abstract The anisotropic superconductivity in Bi 2 Sr 2 CaCu 2 O 8+ y is investigated by precision measurements of resistivity and torque as a function of the magnetic field angle in the reversible regime. Scaling of the angular dependences of these quantities with an effective field, H (sin 2 θ+γ −2 cos 2 θ) 1 2 , θ being the field angle measured from the basal plane, suggests values of the anisotropy parameter γ≈200 or larger, which substantially exceeds previous estimates. In the small θ region, the effect of a layer gains importance.

In-plane and out-of-plane thermal conductivity of La2−æSræCuO4 single crystals

Abstract We report the first measurements of the in-plane and out-of-plane thermal conductivity ( K ab and K c ) for a series of La 2−ae Sr ae CuO 4 single crystals grown by the travelling solvent floating zone method. Corresponding to the superconducting-nonsuperconducting transition between ae =0.20 and 0.30, a quantitative change is observed in both K ab and K c , indicating an enhanced electronic contribution and / or a change in the phonon-electron scattering. We also observed a sharp increase of thermal conductivity just below T c in both directions.

Intercalation of Bi2Sr2Can−1CunOy with iodine and other halogens

Abstract Intercalation of iodine and bromine into polycrystalline (n = 1,2,3) and single crystal (n + 2) specimens of Bi 2 Sr 2 Ca n−1 Cu n O y has been examined. Intercalation of iodine is accompanied by an expansion of the c-axis of approximately 0.7nm in all 3 systems, while, for n·2, Br insertion leads to half this expansion. Susceptibility and resistivity measurements are discussed. An investigation of the Hall coefficient in an n=2 crystal suggests a modest charge transfer to the CuO 2 planes.

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.

Electron irradiation effect on the double hump magnetization loop in BSCCO crystals

Abstract Magnetic hysteresis loops of Bi2.2Sr1.8CaCu2O8+∂ crystals were measured using a local Hall probe magnetometer (LHMP). In particular we investigated the second hump of the magnetic hysteresis curve which appears suddenly at low temperature (below 35K) in our crystals. After moderate irradiation damage, produced by 2.5MeV electrons at low temperature, this hump was strongly enhanced and shifted to lower magnetic fields. This observation points to a change in flux creep dynamics as the origin of the second hump.

Temperature dependence of the lower critical field under the effect of surface barriers in Bi2Sr2CaCu2O8 crystal

Abstract The magnetization properties of Bi-2212 single crystals were measured by local Hall probe magnetometry (LHPM). Features characteristic of the Bean-Livingston surface barrier were observed. The lower critical field H c1 was determined by considering the effect of the surface barrier on the magnetization. The temperature variation of H c1 shows saturation at low temperature which is consistent with the prediction based on the BCS theory.

The first magnetic penetration field in BSCCO single crystals, temperature dependence and electron irradiation effect

Abstract We report measurements of the first magnetic penetration field, H p , of Bi 2.2 Sr 1.89 CaCu 2 O 8+ ϖ crystals by a novel local Hall probe technique. We demonstrate that H p is slightly enhanced above the thermodynamic lower critical field, H c1 , by the Bean-Livingston surface barrier, and that irreversible magnetization at temperatures between 50K and T c (87.8K) arises mainly from the presence of this barrier. We argue that the non-linear temperature variation of H p observed, results from a temperature-dependent efficiency of the Bean-Livingston barrier.

Enhancement on Jc of Bi2Sr2CaCu2Oy by electron irradiation

Abstract Effect of 28 MeV electron irradiation was investigated on the critical current density (J c ) of high-T c superconductor Bi 2 Sr 2 CaCu 2 O y single crystal grown by floating zone method. For the single crystal sample irradiated with a fluence between 3.4 × 10 17 cm −2 and 2.3 × 10 18 cm −2 , J c was enhanced at temperatures from 4.2 to 40 K below 8 T in magnetic field parallel to c-axis of the crystal. Enhancement ratio in J c by irradiation increased with increasing fluence below 40 K, while at 60 K it has a maximum value with a fluence of 1.2 × 10 18 cm −2 . The maximum value 80 was attained in enhancement ratio of J c at 40 K and 0.3 T.

Atomic resolution STM/STS on oxide superconductors down to 4.2 K

Atomic resolution STM/STS measurement on the cleaved basal plane and on the edge of it has been performed for the first time on the layered cuprate superconductors at low temperatures down to 4.2 K. The topographic image of the a-b plane taken at 4.2 K clearly showed the atomic corrugation structure along the b-axis. The STS observations both on the a-b plane and the cross sectional edge surface lead to the electronic scheme that the superconductivity appears only in CuO 2 layer, and that the BiO layer is essentially semiconducting even below T c .