B. Liang

Resonant Spin Excitation in an Overdoped High Temperature Superconductor

An inelastic neutron scattering study of overdoped Bi(2)Sr(2)CaCu(2)O(8+delta) ( T(c) = 83 K) has revealed a resonant spin excitation in the superconducting state. The mode energy is E(res) = 38.0 meV, significantly lower than in optimally doped Bi(2)Sr(2)CaCu(2)O(8+delta) ( T(c) = 91 K, E(res) = 42.4 meV). This observation, which indicates a constant ratio E(res)/k(B)T(c) approximately 5.4, helps resolve a long-standing controversy about the origin of the resonant spin excitation in high temperature superconductors.

Critical point and the nature of the pseudogap of single-layered copper-oxide Bi2Sr2-xLaxCuO6+δ superconductors

We apply strong magnetic fields of H=28.5 to 43 T to suppress superconductivity (SC) in the cuprates Bi2Sr2-xLaxCuO6+delta (x=0.65, 0.40, 0.25, 0.15, and 0), and investigate the low temperature (T) normal state by 63Cu nuclear spin-lattice relaxation rate (1/T1) measurements. We find that the pseudogap (PG) phase persists deep inside the overdoped region but terminates at x approximately 0.05, which corresponds to the hole doping concentration of approximately 0.21. Beyond this critical point, the normal state is a Fermi liquid that persists as the ground state when superconductivity is removed by the magnetic field. A comparison of the superconducting state with the H-induced normal state in the x=0.40 (Tc=32 K) sample indicates that there remains substantial part of the Fermi surface even in the fully developed PG state, which suggests that the PG and SC are coexisting matters.

Superconducting properties of ultrathin Bi2Sr2CaCu2O8+x single crystals

We use Ar-ion milling to thin Bi2Sr2CaCu2O8+x (Bi2212) single crystals down to a few nanometers or one to two (CuO2)2 layers. In decreasing the thickness, the superconducting transition temperature gradually decreases to zero and the in-plane resistivity increases to large values indicating the existence of a superconductor-insulator transition in ultrathin Bi2212 single crystals

Single intrinsic Josephson junction with double-sided fabrication technique

We make stacks of intrinsic Josephson junctions (IJJs) embedded in the bulk of very thin (d⩽100nm) Bi2Sr2CaCu2O8+x single crystals. By precisely controlling the etching depth during the double-sided fabrication process, the stacks can be reproducibly tailor-made to be of any microscopic height (0−9nm<d), i.e., enclosing a specified number of IJJ (0–6), including the important case of a single junction. We discuss reproducible gaplike features in the current-voltage characteristics of the samples at high bias.

Investigation on crystal growth of BiSrCuO system under high oxygen pressure using TSFZ method

Abstract Crystal growth was investigated in the system BiSrCuO using the travelling solvent floating zone (TSFZ) method. Crystal of formula Bi 2+x Sr 2−x CuO 6+δ was obtained under oxygen pressure between 200–700 kPa. Platelet with an a - b surface of several cm 2 and along the c -axis > 1 mm was cleaved mechanically. A tetragonal phase was determined from the crystal grown under 700 kPa. Solubility of strontium was in the range of 0.09 x x c ∼5–10 K, was found for the crystals. The electric properties of the crystals were affected by the amount of oxygen as well as Sr content.

Intrinsic Josephson junctions fabricated in a well-controlled way

With etching rates of argon ion milling greatly reduced, the conventional fabrication process of mesa-structured intrinsic Josephson junctions is improved by precisely controlling and measuring the parameters of the fabrication, which include the thickness of metal layers and the time and etching rates of argon ion milling. We have obtained intrinsic Josephson junctions of mesa structure, in which the number of junctions ranges from 3 to 12 and is controllable with the error within ±1. The thickness of the Bi2Sr2CaCu2O8+x layer damaged by the etching process is about 2.7 nm, which also contributes to the junction number in intrinsic Josephson junctions. The critical current of each junction in the intrinsic Josephson junctions is consistent except for a surface junction, which shows a nonlinear resistance in the current–voltage (I–V) curves. With the fabrication of two top electrodes on the mesa and four-probe measurement, the surface junction is eliminated and the standard supercurrent is registered in the I–V curves.

Magnetic properties of Sm2Co17−xGax(x=0–7) compounds

The study of the effect of Ga substitution on the magnetic properties of arc-melted Sm2Co17−xGax (x=0–7) compounds has been carried out by means of x-ray diffraction (XRD) and magnetic measurements. XRD patterns show that all samples of Sm2Co17−xGax (x=0–7) crystallize in the rhombohedral Th2Zn17-type structure. The Ga substitution for Co leads to an approximately linear decrease in the Curie temperature and a rapid decrease in the saturation magnetic moment which is faster than that in the case of magnetic dilution. Spin-reorientation transition is observed for Sm2Co11Ga6 compound at 32 K. In order to determine the room-temperature easy magnetization direction (EMD), XRD measurements are performed on magnetically aligned samples with x⩽5. The result shows that the EMD of these compounds corresponds to the c axis. The anisotropy constants K1 and K2 of Sm2Co17−xGax compounds were derived by fitting the hard-direction magnetization recoil curves measured on magnetically aligned powder samples as well as by ...

Thickness dependence of the superconducting properties of ultra-thin Bi2Sr2CaCu2O8+x single crystals

With a cleaving technique and Ar-ion etching, we make ultra-thin bridges from Bi2Sr2CaCu2O8+x single crystals. Superconductivity can be observed in the bridges only a few unit cells thick. A superconductor–insulator transition occurs when the thickness is lower than 4.5 nm. The two-dimensional nature of the superconducting transition is examined in these ultra-thin samples. We see deviations from the usual Kosterlitz–Thouless-type behaviour and discuss a possible explanation.

Josephson-vortex flow resistance in Bi2Sr2Ca2Cu3Oy single crystals and its possible application in the manipulation of spin and charge textures in diluted magnetic semiconductors

In this work, the flow of the Josephson vortices (JVs) has been studied for the highly anisotropic Bi2Sr2Ca2Cu3Oy (Bi2223) single crystals. A giant flow of JVs or giant positive magnetoresistance (MR) of over 500%–2000% was obtained in fields of 0.1–5T and remained almost constant over a wide temperature range from 110 down to 4K, in contrast to superconducting vortices (SVs), which only produced MR in the vicinity of Tc. The flow of the JVs is expected to be much faster than that of SVs. It is proposed that the Josephson vortices could be used to manipulate the spin and charge in magnetic semiconductors in the same way as SVs [M. Berciu, T. G. Rappoport, and B. Janko, Nature (London) 435, 71 (2005)]. Hybrid systems consisting of layered superconductors with Josephson junctions and magnetic semiconductors will be discussed.

Sr effects on anomalous Pr ordering in PrBa2Cu3O7 system

In order to make clear the Ba-site Sr substitution effect, we measured the PrBa2−xSrxCu3−yAlyO7 single crystals, and PrBa2−xSrxCu2.7Mo0.3O7 polycrystalline samples by magnetic susceptibility. It is found that, Sr doping causes a change in the crystal field interaction at the Pr site and therefore a change in the magnetic anisotropy for crystals. Sr increases the TN(Pr) slightly first in the range of 0⩽x⩽0.2, then decreases slightly in 0.2<x<1, and decreases abruptly after x⩾1. No TN(Pr) was observed for x⩾1.5. The results are discussed based on the variation of hybridization between Pr ions and O 2pπ of the CuO2 planes.