H.M. Duan

Single crystal studies of the pairing mechanism in Tl2Ba2CuO6 superconductors

Abstract Reasonably large crystals (several mm size) of Tl 2 Ba 2 CuO 6 (referred as Tl-2201) were grown by a self-flux technique. T c s were altered by annealing the crystals in different atmospheres and at different temperatures. As the oxygen content increased, the T c decreased from 90 K to 0 K. The anisotropic resistivities ϱ ab (along ab-plane) and ϱ c (along c-axis) were measured on these crystals by Montgomery method. ϱ c was found to be 2–3 orders of magnitude greater than ϱ ab . The anisotropy ratio (ϱ c /ϱ ab ) increases with T c . For higher T c crystals, the superconducting and the normal state properties of ϱ ab can be explained by the nested Fermi surface behavior with a T-dependence of the in-plane resistivities. For lower T c crystals, the normal Fermi liquid picture emerges from the T 2 -dependence of the in-plane resistivities. Unlike YBa 2 Cu 3 O 7 , Bi-2212, and other copper oxide superconductors, ϱ c for Tl-2201 crystals is metal-like. The paraconductivity data shows that Tl-2201 is a two-dimensional superconductor. The c-axis Seebeck coefficients are observed to be p-type for all crystals in the whole temperature range studied whereas some of the corresponding ab-plane Seebeck coefficients change sign from n-type to p-type and some are n-type down to T c . The thermopower behavior is still theoretically unclear.

Thermal conductivity of single crystal Tl2Ba2CuO6 (Tl-2201) in applied Magnetic fields

Abstract We report the dependence of the thermal conductivity κ(H, T) of single crystal Tl-2201 samples on both magnetic field and temperature. The samples have a superconducting phase transition temperature T c ∼86K. We develop a two component method to analyze the magnetic field dependence of thermal conductivity. Considering the temperature dependence of the two components, we provide the first experimental separation of the quasiparticle and phonon contribution to the thermal conductivity in high temperature superconductors.

Direct evidence of spontaneous creation of flux lines in Tl2Ba2Cu1O6 superconductors

Abstract We report magnetization studies on single crystals of Tl 2 Ba 2 Cu 1 O 6 superconductors. For magnetic field parallel to the c -axis configuration, low-field magnetization measurements show H c1 =0 at T s =86.5 K while the mean-field transition temperature T c0 =88.8 K. The transition width in zero-field-cooled magnetization increases linearly with field, but with a finite intercept at H =0, consistent with the H c1 =0 measurements. The experimental results strongly suggest the Tl-compound is Josephson coupled between superconducting layers, and our results provide direct evidence of the spontaneous excitations of flux lines in this system.

Single crystal growth of Tl-based superconductors

Abstract Large single crystals of Tl 2 Ba 2 CuO 6 and Tl 2 Ba 2 CaCu 2 O 8 superconductors were grown by a two-step self-flux process. In the first step, a mixture of Tl 2 O 3 , BaO (and CaO for 2212 crystals) and CuO was heated at 940°C for about ten minutes to form bulk superconductors. In the second step, the bulk compound was re-ground, re-pelletized and placed into an Al 2 O 3 crucible which was then sealed. The sealed crucible was heated up to above melting point of the bulk superconductor. The crystals were grown as the temperature cooled down slowly. Crystals are plate-like with in-plane crystal size as large as 12 mmx8 mm and the thickness range from 20 μm to 0.8 mm. X-ray diffraction experiments, SEM and other chemical analysis results have shown the crystals to be of high quality and chemically uniform.

THERMOPOWER OF TL2BA2CACU2O8 SINGLE-CRYSTALS

The anisotropic thermopower of Tl2Ba2CaCu2O8 single crystals has been measured. The room temperature value of Sab is about 14 μV/K. With the decrease of temperature Sab increases linearly in a speed of (0.06 ÷ 0.065) μV/K2 down to about 160 K below which Sab flattens off and drops to zero at Tc. Sc at room temperature is around 30 μV/K and only weakly depends on temperature. The superconducting transition determined by thermopower measurements shows that the transition along the c-axis is always lower than that in the (a, b)-plane.

Asymmetry between flux penetration and flux expulsion in Tl-2212 superconductors

Abstractdc magnetic hysteresis as well as flux penetration and flux expulsion were investigated in Tl2−yBa2CaCu2O8−x polycrystals and monocrystals. All measurements were performed at 35 K and in the 0–5 T field range. Hysteresis measurements revealed an irreversibility field of about 2 T. Existing models predict identical field-cooled (fc) and zero-field-cooled (zfc) magnetizations and vanishing time dependence above this field. Although the identical fc and zfc magnetizations are in fact observed, the time dependence vanishes only for flux penetration after zero-field cooling; a remanence is preserved after field cooling and decays with a finite relaxation rate. Activation energies calculated on the basis of the thermal activation model display a pronounced field dependence, and arelower for flux penetration than for flux expulsion in high fields (H≥3 T) for all orientations. This behavior of extreme layered superconductors contradicts classical theoretical models and questions the original definition of the irreversibility line as well. All of our results are consistent with the recent theory of lock-in transition, and can be well interpreted by using those principles.

Magnetic penetration depth in high-Tc superconducting Tl2Ca1Ba2Cu2O8−δ single crystals

The temperature dependence of Cu-O plane magnetic penetration depth,λab(T), for T1-2122 single crystals has been determined by measuring the low-field d.c. magnetization using a SQUID susceptometer. Almost full (97%) diamagnetism at low temperature was reached by carefully minimizing the background field, which is an essential condition for this method.λab(0) was obtained uniquely by a self-consistent condition to be 182 nm. Results of our measurements show a deviation from the single-band BCSs-wave prediction. The exponential behavior in very limited temperature regions may be reconciled with the BCS-like prediction. However, ourλ(T) data follow the theoretical prediction very accurately for ap-wave polar state pairing.

Anisotropic resistivities of Tl2Ba2CuO6+d single crystals with different oxygen concentrations

We report for the first time single crystal anisotropic resistivity measurements of the single-copper-oxide layer material Tl2Ba2CuO6+d. At room temperature, the in-plane resistivities were on the order of 5X 10^(-4)Ω-cm and decrease with temperature according to a simple power law. C-direction resistivities were hvo orders of magnitude larger, also decrease with temperature.

Forward and reverse flux creep in single crystal Tl2Ca2BaCu2Oχ superconductors

Abstract We report the observation of novel flux motion in single-crystal superconducting Tl 2 Ca 2 BaCu 2 O χ . Application of a field H ⩽1 kG following zero-field cooling results in typical logarithmic flux creep, with a peak in δM/δln t near 18 K. Above 50 K, different flux dynamics are observed: an exponential decrease in the diamagnetic moment that gives way near 70 K to the appearance of reverse creep, i.e., increasingly negative magnetization. This latter effect begins after a temperature- and field-dependent wait time. While conventional theory explains inward flux creep, we must invoke flux-line roughening, drive by pinning randomness, to give rise to reverse creep.

Field dependent thermal conductivity of Tl2Ba2CuO6: evidence for Andreev scattering of quasiparticles

Abstract We have studied the thermal conductivity of Tl 2 Ba 2 CuO 6 (Tl-2201) crystals as functions of temperature and magnetic field. A thermal conductivity peak is observed near 0.6 T c which is greatly reduced on application of a magnetic field along the c axis. We argue that the suppression of the peak at relatively low fields (≤ 8 T ⪡ H c2 ) permits us to separate electronic and phononic contributions. Fields applied in plane are less effective, but give the same reduction when applied either parallel or perpendicular to the temperature gradient. A model based on Andreev reflection by vortex screening currents is shown to be consistent with these results.