Makoto Maki

Vortex phase diagram as a function of oxygen deficiency in untwinned YBa{sub 2}Cu{sub 3}O{sub y}

Magnetization and resistivity measurements were performed in un-twinned YBa2Cu3Oy(YBCO) single crystals with different oxygen content. We find that the vortex phase diagram is a strong function of the oxygen deficiency; (1) For fully oxidized YBCO (y ≃ 7, Tc ≃ 87.5 K), the first-order vortex lattice melting line Tm(H) is observed up to 30 T. (2) For slightly overdoped YBCO (Tc≃ 92 K), Tm(H) and the vortex glass transition line Tg(H) terminate at the multicritical point and the field-driven disordering transition line separates the vortex solid phase into the Bragg glass and the vortex glass phases. (3) For slightly underdoped YBCO (Tc≃ 92.3 K), Tg(H) terminates well below the critical point of Tm(H), indicating the existence of the vortex slush regime by the introduced oxygen deficiency.

Identification of Atomic Layers of YBa2Cu3O7-δ by Scanning Tunneling Microscopy and Spectroscopy

Low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) measurements have been performed on YBa 2 Cu 3 O 7-δ single crystals cleaved below 20 K in order to obtain clean surfaces...

Melting transition of vortex matter in YBa2Cu3Oy with various oxygen contents

Abstract The effect of oxygen content on the vortex phase diagram of untwinned YBa2Cu3Oy (YBCO) single crystals has been studied by magnetization and resistivity measurements in high magnetic fields up to 30 T. The cantilever magnetometry with a capacitance torque meter has been developed for precise magnetization measurement in high magnetic field. As for fully oxidized and optimally doped YBCO, the magnetic properties near the first-order vortex lattice melting transition line are discussed in comparison with the vortex phase diagram obtained from the resistivity measurement.

Dielectric Anisotropy in the Charge-Density-Wave State of K0:3MoO3

Low-frequency dielectric response of the charge-density-wave (CDW) state has been studied in three orthogonal directions of K 0.3 MoO 3 . Broad relaxation at radio frequencies is observed both in the CDW-chain axis and in the inter-chain directions, although the dielectric strength is much smaller and the inductive response at dc biases above the threshold electric field is broader in the latter case. These results suggest that the phason excitations survive in the inter-chain direction in spite of the commensurability-pinning effects.

Vortex phase transition and oxygen vacancy in YBa2Cu3Oy single crystals

The atomic images of YBa2Cu3Oy (YBCO, y≃6.94) single crystals are measured by low-temperature scanning tunneling microscopy (LT-STM). We find that the oxygen vacancies form clusters along the b-axis in the CuO chain layer. Near the clusters, the strong atomic distortion is observed along the a-axis. The vortex phase diagram is very sensitive to the amount of oxygen vacancies in the region between optimal and overdoped regions. For optimally doped untwinned YBCO (y≃6.92), we find a first-order transition TL(H) in the vortex liquid above the terminal point Hmcp (≃7 T) of both the vortex glass line Tg (H) and the field-driven disordering transition line H*(T). The obtained small entropy change (∼0.02 kB/vortex/layer) and the critical endpoint Hcep (≃11 T) of TL(H) indicate that the vortex liquid undergoes the vortex slush regime before the solidification into the vortex glass phase. Below Hmcp, the vortex liquid phase shows the first-order melting transition into the Bragg glass phase.

Out-of-Plane Dielectric Behavior of Insulating Bi2Sr2RECu2O8+δ (RE = Dy, Y, and Er)

This report describes the low-frequency dielectric response of lightly doped Bi 2 Sr 2 RE Cu 2 O 8+δ ( RE : rare earth) with various oxygen contents. The out-of-plane dielectric constant e c ′ exhibits an anomalous temperature dependence that is independent of RE elements, suggesting that the insulating Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) systems have a two-dimensional dielectric character at low temperatures. In disordered solids such as RE -substituted Bi2212 compounds, the charge carrier displays a localized character and the forced movement causes a dc current flow. Furthermore, the oxygen dopants act as trapped sites and strongly affect the out-of-plane dielectric properties.

Scanning tunneling spectroscopy studies on vortices in YBa2Cu3Oy single crystals

Abstract Low-temperature scanning tunneling spectroscopy (STS) has been performed on YBa 2 Cu 3 O y (YBCO) in the magnetic field μ 0 H =1 T. Clear vortex images are obtained on the surface chemically etched with 1% Br-ethanol. We measured on the twinned crystals and directly observed the effect of twin boundary on the vortex arrangement. Vortices close to the twin boundary align themselves parallel to it, suggesting that vortices are effectively trapped by the twin boundary. The vortices very close to the twin boundary form almost square lattice. Such tendency decreases with increasing distance and the vortices form a hexagonal lattice in the region far from the twin boundary. However, the orientation of hexagonal lattice is not consistent with the previous reports. Possible origins of such disagreement are described in this paper. We also show spectra observed in the scan region and discuss their features.

Low-temperature STM/STS of high-Tc superconductors

Abstract We have carried out low-temperature scanning tunneling microscopy and spectroscopy studies on high- T c superconductors, Bi 2 Sr 2 CaCu 2 O 8+ δ and YBa 2 Cu 3 O 7− δ single crystals. We present an experimental method and STM/STS results of these materials. Samples were cleaved below 20 K in order to obtain clean surfaces. For YBa 2 Cu 3 O 7− δ , we observe an atomic image of BaO and charge density waves image on the CuO chain layer. Differential tunneling conductance spectrum of YBa 2 Cu 3 O 7− δ has a clear superconducting energy gap along 100 nm path.

Thermal Conductivity of La2-x-yNdySrxCuO4

We have measured the thermal conductivity κ of La 2- x - y R y Sr x CuO 4 ( R =Nd, Sm, Eu, Gd) and found that it shows the enhancement below the transition temperature T d2 from the LTO1 (low-temperature orthorhombic-1) to LTT (low-temperature tetragonal) (or LTO2 (low temperature orthorhombic-2)) phase and the magnitude of the enhancement strongly depends on x and y . We propose that the characteristic enhancement of κ below T d2 reflects the nature of the structure below T d2 . From the results that in La 1.875- y Nd y Sr 0.125 CuO 4 , the LTT (or LTO2) phase appears at y >1/8, we propose the pair substitution of Sr 2+ and Nd 3+ ions where the loss of the strain energy is smaller and the local LTT (or LTO2) distortion is more largely enhanced than in the case of the independent substitution. We show that various characteristic properties associated with the LTT (or LTO2) transition could be explained naturally from the stand point that the local lattice distortion induced by the substituted ion plays an...

Suppression of the superconducting transition temperature Tc around x~0.115 in La2-xSrxCuO4.

Detailed studies of the superconducting diamagnetism of La{sub 2{minus}{ital x}{minus}{ital y}}Nd{sub {ital y}}Sr{sub {ital x}}CuO{sub 4} show that the characteristic concentration {ital x}{sub dip} at which the largest {ital T}{sub {ital c}} suppression is observed changes from 0.115 to 0.125 on Nd doping around {ital y}{approximately}0.12, above which the structural transition to the low-temperature tetragonal (LTT) phase is observed. This indicates that also in La{sub 2{minus}{ital x}}Sr{sub {ital x}}CuO{sub 4}, the {ital x}{sub dip} value is, in principle, also 1/8 as in La{sub 2{minus}{ital x}}Ba{sub {ital x}}CuO{sub 4}. We propose that the largest {ital T}{sub {ital c}} suppression around {ital x}{approximately}0.115 in La{sub 2{minus}{ital x}}Sr{sub {ital x}}CuO{sub 4} is the result of the existence of two competitive factors. One is the local distortion expanding the LTT region around the Sr{sup 2+} ion, which is largest at {ital x}=1/8 and acts to suppress {ital T}{sub {ital c}}. The other is the antiferromagnetic spin fluctuation energy, which might increase with {ital x} and acts to increase {ital T}{sub {ital c}}. On Nd doping, the former factor becomes larger around {ital x}=0.125 than around {ital x}=0.115 and the {ital x}{sub dip} value moves to 0.125. {copyright} {ital 1996 The American Physical Society.}