Changkang Chen

Phase diagram and single crystal growth of (La, Sr)2CuO4 from CuO solution

The system (La,Sr)2CuO4-CuO has been investigated by differential thermal analysis (DTA), thermogravimetric analysis (TGA) and slow-cooling flux growth experiments. The phase diagram is given and was used in the crystal growth experiments. The crystal products have been characterised by EPMA and by X-ray powder pattern and single crystal techniques. Large single crystals have been grown from CuO as flux by spontaneous nucleation and top seeding.

The flux growth of crystals of (La,Sr)2CuO4 and (La,Sr)CuO3

Abstract The phase diagram of the system (La,Sr) 2 CuO 4 -CuO was investigated by differential thermal analysis (DTA), by thermogravimetric analysis (TGA) and by top-seeded as well as slow-cooled experiments. (La,Sr) 2 CuO 4 melts incongruently at 1350 °C. Crystals of (La,Sr) 2 CuO 4 up to (8x4x2) mm 3 were grown on seeds attached to a wire, with the aid of TGA. Single crystals (6.5 x 5x0.2)mm 3 in size were grown by slowly cooling the batches. The Sr content in these crystals was nearly one third of that in the melt. Crystals of (La,Sr)CuO 3 are obtained under very slow cooling conditions and single crystal La 2 CuO 4 (9.5 x 5 x 2)mm 3 in size has been grown from the system La 2 CuO 4 - CuO.

Raman investigation of the orthorhombic-tetragonal phase transition in La2−xSrxCuO4 superconductors

Abstract We present polarized Raman spectra from oriented single crystals of La 2 CuO 4 and superconducting La 1.85 Sr 0.15 CuO 4 that encompass the temperature range above and below the orthorhombic-to-tetragonal phase transition. This transition has a striking effect upon the Raman spectra. As approached from below (the orthorhombic phase), the α zz spectrum from La 2 CuO 4 shows a strong line at 104 cm −1 that broadens and softens and a weak line at 271 cm −1 that gradually disappears, both lines being unobservable in the tetragonal phase. The soft phonon is not seen in the orthorhombic phase of the Sr-doped sample, but the weak 271 cm −1 line is seen. The oxygen E g mode of the tetragonal phase is observed to split into B 2g and B 3g components in the orthorhombic phase.

Relationship between crystal growth, structure and spectral characteristics of NdAl3(BO3)4 with order-disorder structure

By means of spontaneous nucleation or seeded growth, the laser crystal NdAl3(BO3)4 (NAB) has been grown from the flux system K2SO4-MoO3-H3BO3. X-ray diffraction analysis confirmed that this crystal had the R32 space group but with disorder in some degree, giving the crystal a higher laser threshold and absorption. Kinetic analysis on the experiments of seed dissolution allowed the activity energy of diffusion and the parameters of diffusion rate at high temperature to be calculated. The maximum stable growth rate is estimated to be far less than normal. We conclude that it would be advantageous to modify the flux system to one with smaller complex ions or clusters.

Femtosecond dynamics of BSCCO-2212

We present measurements of the dynamics of the electronic excitations of BSCCO-2212 measured with 150fs resolution using ultrafast time-resolved spectroscopy. The dynamics is clearly different in the normal, pseudogap and superconducting states, and from that measured in YBa2Cu3O7−δ.

Crystal growth and characterization of superconducting lead cuprates

Abstract High quality single crystals of Pb 2 Sr 2 (R/Ca)Cu 3 O 8 + y ( R = Nd , Sm , Gd , Dy , Er , Tm , Yb , Lu and Y ), the 2213 phase of the lead cuprate family, have been grown by PbO/NaCl flux and seed crystal methods. The role of NaCl and the pseudobinary phase diagrams of the crystal growth system are investigated. A correlation between crystal quality and superconducting properties of Pb 2 Sr 2 (R/Ca)Cu 3 O 8 + y is observed. The 1212 lead cuprate phase has also been prepared in single crystal form by a semi-ceramic method. The oxygen partial pressure seems to be the key to the determination of the product compound in the crystal growth process; the 2213 lead cuprate phase can be prepared under low oxygen partial pressure while the 1212 phase requires an oxidizing atmosphere.

Evaporation kinetics of a halide flux system for the growth of Bi-Sr-Ca-Cu-O superconducting crystals

Abstract Two phases in the Bi-Sr-Ca-Cu-O system have been prepared as single crystal platelets from alkali halide fluxes. One is superconducting at 80 K, with the formula BiSr 0.8 Ca 0.6 CuO x . The other, which apparently forms more readily, has the approximate formula Bi 2.1 Sr 1.4 Ca 0.3 CuO x . The flux growth process involves both growth by slow cooling and solvent evaporation which is important not only for control of nucleation and crystal growth but also for the formation of various crystalline phases. The evaporation from the solution of Bi-Sr-Ca-Cu-O in the flux of mixed alkali halides was measured by the thermogravimetric method at different temperatures, holding times and cooling rates. A first order kinetic model is proposed and the experimental data are analysed. At constant temperature, the relative amount of solvent evaporation is directly proportional to the evaporating time while the relationship between the evaporating rate constant and the temperature follows the Arrhenius formula, from which it is possible to obtain an activation energy of evaporation. Under cooling at constant rate, on the other hand, the evaporating behaviour is dependent on the temperature function of evaporation f(x) , which has also been calculated from the kinetic parameters.

Characterisation and transport measurements on single crystals in the BiSrCuO system

Abstract A series of single crystals of Bi 2+x Sr 2−x′ CuO 6±y have been prepared and characterized by means of high precision electron probe microanalysis and by transport measurements. The superconducting transition temperatures of our samples, for which 0.1 ≤ x∼x′ ≤ 0.4, are found to be very sensitive to x and y but insensitive to pressure.

Relationship between growth, structure, and superconductivity of single crystal YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}

We study the crystal growth of YBa2Cu3O7-δwith additives, using ac susceptibility, X-ray diffraction and STM. One specific additive, BaF2, dramatically improves both the crystal growth and the superconducting properties. A cluster growth model is proposed to describe the function of the additive. F−reduces the Al contamination of Cu-O chains, and the orthorhombicity increases. On a nanometer scale, the roughness of the chains decreases markedly. Superconductivity is further enhanced using non-contaminating crucibles. Large single crystals with TC= 93.7 K and ΔTC= 0.1K were grown in Y2O3or BaZrO3crucibles using the BaF2additive.

Femtosecond dynamics of BSCCO-2212 in the weak and strong excitation limits☆

Abstract We present measurements of the ultrafast optical response of BSCCO-2212 in the weak perturbation limit (photoexcitation density ≤ 10μJcm−2), where we focus attention on the dynamics of electronic correlations associated with the pseudogap, and also in the strong perturbation limit (≥ 1JJcm−2), where the superconducting condensate is destroyed.