Resistive upper critical field of single crystals of Tl2Ba2CuO6

We have measured the resistive upper critical field of overdoped single crystals of Tl₂Ba₂CuO₆ from the zero-field transition temperatureT _c (approximately 20 K) to temperatures as low as 12 mK, corresponding to less than 0.001T _c. In sharp contrast to the predictions of standard theories of superconductivity, the critical field is found to rise steeply with positive curvature as the temperature is reduced, and no sign of saturation is observed down to the lowest temperatures reached.     

Is Tl2Ba2CuO6 a superconductor-metal array?

From structural analysis we have shown in a previous investigation that the series TlBa₂Ca _n Cu _n+1O_2n+5 and Tl₂Ba₂Ca _n Cu _n+1O_2n+6 are composed of superconductor-semiconductor arrays. In this paper, we demonstrate that the unit cell of the Tl₂Ba₂Ca _n Cu _n+1O_2n+6 series can further be viewed as composed of a composite block of superconductor-metal-semiconductor, where TlBa₂CuO₅ is the fundamental superconductor, TlO the metallic unit, and CaCuO₂ the semiconductor block. It is indicated that theT _c of Tl(2201) can be deduced from that of Tl(1201) based on Kresin's model of composite systems.     

Vortex Dynamics in a Thin Film of Tl2Ba2CuO6

The vortex dynamics in a Tl ₂ Ba ₂ CuO ₆ (Tl-2201) thin film with a critical temperature T _c = 80.8 K has been studied by electrical resistivity measurements for magnetic fields 0B12 T. The vortex glass line, as determined from the disappearance of linear resistivity, was found to be well described by B _g (T) = B ₀ [(1–T/T _c )/(T/T _c )], with B ₀ 0.65 T and 1.9. The glass line of Tl-2201 is observed to be located below the one of Tl ₂ Ba ₂ CaCu ₂ O ₈₊ (Tl-2212) in a magnetic phase diagram based on a reduced temperature scale T/T _c , thus suggesting a higher anisotropy for Tl-2201.     

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

The temperature dependence of Cu-O plane magnetic penetration depth,λ _ab (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.λ _ab (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.     

Magnetic penetration depth of Tl2Ba2CuO6+δ :T c ∼σ0 in the overdoped region

We report transverse-field muon-spin-rotation experiments carried out on Tl₂Ba₂CuO_6+δ . This system spans the whole overdoped regime, andT _c is reduced by excess oxygen doping, which increases the normal-state carrier concentration. In the heavily overdoped regimeσ(0) is found to scale linearly with the superconducting critical temperatureT _c , similar to the behavior previously observed for other cuprates in the underdoped regime. However, for the overdoped region one has to explain the reduction ofσ ₀, thus the increase of the magnetic penetration depthλ, in spite of an increasing normal-state carrier concentration. We discuss some possible explanations for this behavior.     

Electronic specific heat of Tl2Ba2CuO6+δ from 2 K to 300 K for 0≤δ≤0.1

Using a high-precision differential technique with a resolution of 1∶10⁴, we have measured the heat capacity of Tl₂Ba₂CuO_6+δ over a temperature range 2–300 K for 0≤δ≤0.1. Anomalies atT _c are seen for all superconducting compositions measured, and the results are consistent with a temperature- andδ-independent normal-state electronic termγ _n～0.6 mJ/g-at. K². The samples with the largerT _c 's exhibit strong fluctuations in their specific heat, typical of a highly anisotropic 2D superconductor, but there is some evidence that these fluctuations become weaker atT _c falls-consistent with an increase in the coherence length on hole doping. At temperatures below 5 K an upturn in the data is observed which appears to increase in magnitude asT _c falls, correlating with the increase in the Curie term of published susceptibility data.     

Superconductivity in single crystals of the nonperovskite-like cuprate Ba2Cu3O6−x

Spontaneous crystallization from a solution in a melt at a pressure of (0.8–1)×10⁻³ Pa was used to obtain single crystals of a new nonperovskite-like superconducting phase Ba₂Cu₃O_6−x space group P _ccm, a=13.065, b=20.654, and c=11.431 Å). The superconducting properties of the crystals were investigated by modulated microwave absorption. The superconducting transition temperatures were 5 K (sample No. 1), 7 K and 13 K (sample No. 2). The superconductivity in the Ba₂Cu₃O_6−x crystals is attributed to the presence of CuO₂ chains of edge-sharing copper-oxygen squares. Nonperovskite-like Ba₂Cu₃O_6−x crystals are a new class of one-dimensional (1D) superconductors. Pis’ma Zh. Tekh. Fiz. 23, 27–34 (December 26, 1997)     

Enhancement ofT c (∼ 103 K) in calcium-free Tl2Ba2CuO x (2201) compound

EnhancedT _c in calcium-free Tl compounds of the series Tl _m Ba₂Ca _n−1Cu _n O _x (2201) has been reported. Three different starting compositions (2201, 1201 and 2202) were studied extensively with varying conditions of preparation. Under optimized conditions (sintering temperature 970°C and duration 3–10 min) the highestT _c(onset) ranges from 103 K to ∼ 115 K andT _c (zero) ∼ 95 K was found. XRD studies showed the transformation of all the three nominal compositions into 2201 phase with differentT _cs.     

Controlled growth of CsI〈Tl〉 single crystals

CsI〈Tl〉 crystals have been grown by the axial-heat-flux-close-to-the-phase-interface (AHP) method in a purpose-designed apparatus at an argon overpressure. The furnace and crystallizer of the apparatus have been designed to ensure considerable (5 to 100 K/cm) axial and low (within 1 K/cm) radial temperature gradients at the growth interface. The effects of the melt layer thickness, temperature gradient, and activator concentration in the melt on the crystal quality have been studied. The results demonstrate that reducing the AHP heater temperature for even a short time (by 2°C in 3 min) markedly raises the actual growth rate, from 2 to 15 mm/h, and leads to entrapment of bubbles of various diameters and TlI inclusions.     

Anomalous heating rate dependence of thermoluminescence in Tl2GaInS4 single crystals

Trap centers in Tl₂GaInS₄ single crystals have been investigated by thermoluminescence (TL) measurements in the temperature range of 10–110 K. Curve-fitting method was utilized to evaluate the activation energies (12 and 26 meV) of revealed two trap centers. Thermal cleaning method was applied to the spectra and two overlapping peaks were reduced to one single peak corresponding to deeper trap level. The characteristics of heating rate dependence and trap distribution of the separated peak have been studied using experimental techniques based on various heating rates and various illumination temperatures, respectively. Anomalous heating rate dependence was observed carrying out TL measurements with various heating rate between 0.2 and 1.2 K s^?1. This behavior was explained on the base of a semi-localized transition model. Moreover, increase of activation energy from 27 to 67 meV was revealed for the applied illumination temperature range of 26–36 K.     

Lower critical magnetic field of lithium containing Bi2Sr2CaCu2O8+δ single crystals

Lower critical magnetic field strength at 0 K, Hcl (T); T=0 K, has been evaluated on single-crystal Bi2.2Sr1.8Ca(Cu1-x, Lix)2Oy with 0≤x≤0.13 from d.c. susceptibility data. Significant change is noted between lithiuim-doped and undoped crystals: the value of Hcl (0) decreases with increasing lithium content when the magnetic field is applied to the direction parallel to the ab-plane, i.e. the cleaved crystalline plane, while the value remains constant when the direction of the field is perpendicular to the ab-plane. This could be attributed to the change in penetration depth along the c-axis, i.e. λc, with lithium incorporation. Analyses of diamagnetism versus temperature data were made, which exhibited no significant change in electron-transport mechanism with the variation of lithium content. This revised version was published online in November 2006 with corrections to the Cover Date.     

Growth and characterizations of Ba2Ti2Fe2As4O single crystals

Abstract

Single crystals of a new iron-based superconductor Ba₂Ti₂Fe₂As₄O have been grown successfully via a Ba₂As₃-flux method in a sealed evacuated quartz tube. Bulk superconductivity with T_c ～ 21.5 K was demonstrated in resistivity and magnetic susceptibility measurements after the as-grown crystals were annealed at 500 °C in vacuum for a week. X-ray diffraction patterns confirm that the annealed and the as-grown crystals possess the identical crystallographic structure of Ba₂Ti₂Fe₂As₄O. Energy-dispersive x-ray spectra indicate that partial Ti/Fe substitution exists in the [Fe₂As₂] layers and the annealing process redistributes the Ti within the Fe-plane. The ordered Fe-plane stabilized by annealing exhibits superconductivity with magnetic vortex pinned by Ti.     

Low-temperature structural studies of Tl2Ba2CaCu2O x

Low-temperature x-ray diffraction studies of superconducting Tl-2212 ceramics revealed anomalies in thec cell parameter andc-axis thermal expansion coefficient near the superconducting transition. Air and vacuum anneals in the range 400–550‡C demonstrate that these anomalies correlate with the superconducting properties of the material. The anomalous temperature variation of thermal expansion suggests that the superconducting transition is accompanied by significant changes in the phonon spectrum.     

Study of the piezoelectric properties of Ba2Ge2TiO8 glass-ceramic and single crystals

Glass-ceramics of composition Ba₂Ge₂TiO₈ with oriented crystallites were prepared by crystallizing the glasses in a temperature gradient. The degree of preferred orientation of the crystallites as a function of depth was studied. Piezoelectric resonance properties of both the glass-ceramics and the single crystals were studied. The piezoelectricd ₃₃ coefficient, the frequency constants and the electromechanical coupling coefficients of the glass-ceramic were comparable to the single-crystal values.     

The upper critical field of layered superconductors

The results of a theoretical study of the upper critical fields of layered superconducting compounds are presented. It is found that for sufficiently decoupled layers the parallel critical fieldsH _c2,‖ can be quite large compared to those expected with ordinary type II superconductors and essentially determined by the destruction of the superconductivity in the individual superconducting layers. It is found thatH _c2,‖ can be calculated from standard pair breaking theory in terms of a pair breaking parameter appropriate for layered compounds. The pair breaking parameter includes both orbital effects and the effects of Pauli paramagnetism and spin-orbit scattering.     

Trapping centers and their distribution in Tl2InGaSe4 single crystals by thermally stimulated luminescence

Thermoluminescence (TL) measurements in Tl₂InGaSe₄-layered single crystals have been carried out in the temperature range of 10–200 K at various heating rates (0.2–1.0 K s^?1) to get information about the characteristics of traps. Two TL overlapping glow peaks related to defect levels have been clearly observed. Thermal cleaning procedure was applied to the glow curves to separate overlapped peaks. Initial rise, peak shape, and heating rate methods were used to calculate the activation energies of the revealed traps. The energy values of 5 and 28 meV were evaluated for the peaks observed at low and high temperatures, respectively. Moreover, heating rate dependence and traps distribution analysis were also investigated on the curve obtained after thermal cleaning. The activation energies of the distributed trapping centers were found to be increasing from 29 to 151 meV with increasing the illumination temperature from 42 to 80 K.     

IR operation by third harmonic generation of Tl4PbTe3 and Tl4SnS3 single crystals

We have discovered substantial influence of the 3 μs CO₂ laser treatment on the output third harmonic generation (THG) for Tl₄SnS₃ and Tl₄PbTe₃ single crystals. The treatment was performed in the regime of the pump-probing delay. The optimal pump-probe delaying time was 230 ps for the Tl₄SnS₃ crystals and 250 ps for the Tl₄PbTe₃ single crystals. It is crucial that for the Tl₄PbTe₃ the effect is substantially smaller. The effect may be explained within a framework of conception of excitation of the effective phonon subsystem effectively contributing to the THG.     

Flux Dynamics in Tl2Ba2CaCu2O8 Thin Films

Investigations on flux dynamics of ring-shaped T1₂Ba₂CaCu₂O₈ superconducting thin films have been carried out by measuring the temperature dependent magnetization M(T) during field-cooling (FC) and zero-field-cooling (ZFC) processes. For a given magnetic field, from the magnetization behavior two distinct temperatures, T_kink and T_irr, can be defined. Below T_kink, a clear hysteretic behavior of M(T) is observed leading to a large irreversible signal ΔM = M_FC ? M_ZFC. Above T_kink, this irreversible signal, though being very small, is still non-zero until it eventually vanishes at a higher temperature T_irr. Above T_irr, both curves M_FC(T) and M_ZFC(T) merge together and become temperature independent. We attribute the first region ( T< T_kink) to a 3D vortex-glass phase, the second region (T_kink < T < T_irr) to a vortex line liquid state and the third region (T_irr < T < T_c) to a pancake liquid state.     

Preparation and properties of Tl2Ba2CaCu2O8 thin films

Anex situ process has been developed to produce thin superconducting Tl₂Ba₂CaCu₂O₈ films. The properties of films grown on different substrates using different annealing regimes were studied. Critical temperatures of 103–107 K were measured on films prepared in a broad range of annealing temperatures on SrTiO₃, LaAlO₃, and Y-ZrO₂ substrates. A critical current density,J _c, of 2×10⁶ A/cm² at 77 K was measured on LaAlO₃. Film morphology was studied by SEM, AFM, and STM.