Superconductor Tl Research Articles

Vortex States of Tl2Ba2CuO6+δStudied via205Tl NMR at 2 Tesla

We report a $^{205}$Tl NMR study of vortex state for an aligned polycrystalline sample of an overdoped high-$T_c$ superconductor Tl$_2$Ba$_2$CuO$_{6+\delta}$ ($T_{c}\sim$85 K) with magnetic field 2 T along the c axis. We observed an imperfect vortex lattice, so-called Bragg glass at $T$=5 K, coexistence of vortex solid with liquid between 10 and 60 K, and vortex melting between 65 and 85 K. No evidence for local antiferromagnetic ordering at vortex cores was found for our sample.

205Tl NMR study of vortex state of Tl 2Ba 2CuO 6+δ

We report a 205Tl NMR study of vortex state for an oriented polycrystalline sample of an overdoped high- T c superconductor Tl 2Ba 2CuO 6+δ ( T c∼85 K) with magnetic field 2 T along the c-axis. An obvious Redfield pattern of the 205Tl NMR spectrum for our sample at T=5 K indicates the vortex lattice state without local antiferromagnetic order at vortex cores. We emphasize that nonexponential nuclear spin-lattice relaxation curves indicate an imperfect vortex lattice or Bragg glass phase.

Effect of B2O3 addition on the formation and properties of Tl-2212 and Tl-2223 superconductors

We have studied the effect of B2O3 addition on the formation and properties of the Tl-based superconductors. Polycrystalline Tl-2212 and Tl-2223 samples were synthesized by adding B2O3 to the chemicals at the first and at the second stage of two stages sintering processes (3+1 h) at 860 °C. X-ray powder diffraction patterns, resistance versus temperature behaviours and the micrographs of the B2O3 free and B2O3 added samples have been analysed. The compositions and the critical temperatures of the samples indicate that about 1.0 weight percent B2O3 added at the first stage of sintering stabilize the Tl-2212 phase and at the second stage promote the formation of the Tl-2223 phase. For both of the phases, the a-lattice param-eter remains the same, but the c-parameter slightly decreases with B2O3 additions. Few small peaks due to the impurity phases appear in the diffraction patterns but none due to B2O3 or any other phase involving B atoms. We suggest that, with their small size B atoms may partially substitute Cu atoms and disperse at interstitial positions and grain boundaries. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Heat transport in a strongly overdoped cuprate: Fermi liquid and a pure d-wave BCS superconductor.

The transport of heat and charge in the overdoped cuprate superconductor Tl(2)Ba2CuO(6+delta) was measured down to low temperature. In the normal state, obtained by applying a magnetic field greater than the upper critical field, the Wiedemann-Franz law is verified to hold perfectly. In the superconducting state, a large residual linear term is observed in the thermal conductivity, in quantitative agreement with BCS theory for a d-wave superconductor. This is compelling evidence that the electrons in overdoped cuprates form a Fermi liquid, with no indication of spin-charge separation.

Lattice Dynamics and Normal Coordinate Analysis of HTSC Tl2CaBa2Cu2O8

The lattice dynamics of the high-temperature superconductor Tl 2 CaBa 2 Cu 2 O 8 has been investigated on the basis of the three-body-force shell model (TSM). The various interactions between ions are treated in a general way without making them numerically equal. The phonon frequency at the zone centre of Brillouin zone are presented and the vibrational assignments are discussed. Further, a normal coordinate calculation has also been employed to study the vibrational analysis of this compound. The normal coordinate analysis of the superconductor Tl 2 CaBa 2 Cu 2 O 8 has been calculated by using Wilson's F-G matrix method which is useful for the confirmation of our present investigation. The vibrational frequencies and the potential energy distribution (PED) of the optically active phonon modes are also presented.

AC susceptibility of high Tc superconductor Tl 0.8Pb 0.2Bi 0.2Sr 1.6Ba 0.4Ca 2Cu 3O x using a critical state model

The Anderson–Kim model is employed to calculate the temperature and AC magnetic field dependence of the complex susceptibility, χ= χ ′+i χ ′′, of high T c superconductor Tl 0.8Pb 0.2Bi 0.2Sr 1.6Ba 0.4Ca 2Cu 3O x . Intergranular Josephson vortices sweep in and out of the weak-link network, while intragranular Abrikosov vortices move in and out of the superconducting grains, both causing bulk pinning hysteresis loss. The calculated χ ′ and χ ′′ are consistent with experimental data very well by considering the proper volume fraction of superconducting grains and intergranular region. Using field dependence of critical current density obtained from fitting parameter, the property of intergranular region is discussed and the intergranular lower critical field H c1J is determined.

Mössbauer Spectroscopy and Preparation of Hg-1223, Tl-1223 and Tl-1212 Superconductors

Introduction of the Mossbauer nuclei 57Fe into Hg-1223 phase and that of 57Fe and 151Eu into Tl-1212 and Tl-1223 superconductors were investigated. Samples of high phase purity were obtained. Scanning electron microscopy and optical microscopy in normal and polarized light were employed to study the microstructure of the specimens. Energy dispersive X-ray analysis showed that 57Fe in the Hg-based samples and 57Fe as well as 151Eu in the Tl-based compounds, entered superconducting phases. Incorporation of Eu3+ into the superconducting phase favored the formation of the Tl-1212 phase. Mossbauer spectroscopy showed that Eu3+ entered the Ca-site. Two doublets found in the 57Fe Mossbauer spectra in both the Hg-1223 and the Tl-1223 phase referred to two different micro-environments of Fe3+. The assignment of the 57Fe Mossbauer spectra was made under the assumption that Fe favored the 1223 phase. Fe3+ may replace Cu in both the square pyramidal, five-fold oxygen coordinated Cu sites between the Ca and Ba-(Sr)-O layers, and in the square planar, four-fold oxygen coordinated Cu sites the Ca layers in the superconducting phases in both the Hg- and the Tl-based materials. From the relative areas of the two doublets, we concluded that the Fe3+ preferred the square planar Cu site.

The rotational magnetic process and effects of -irradiation on vortex flux pinning in Tl-2223 at low temperatures

Magnetization vector measurements were carried out in a polycrystalline sample of Tl-2223 superconductor in fixed fields at low temperatures. Demonstrating the broad distribution in the vortex flux pinning, the vortex flux density B was seen to consist of two component: the rotational component BR turns rigidly with sample rotation and the frictional component BF stays at fixed angle F relative to the applied field H. The pinning torques per vortex were calculated as a function of H at constant temperature. The -irradiated sample shows an increase in pinning torque of ratio seven to one at temperatures 4.2 K and 25 K compared to an unirradiated sample; this is due to defects induced by the gamma irradiation.

Influence of the Tl- and Hg-content on magnetic and transport properties of the Pb, Sr-doped Tl-1223 and Hg-1223 superconductors

The effects of the thallium and mercury content x on the as-sintered and post annealed samples of M x Pb 0.4Sr 1.6Ba 0.4Ca 2Cu 3O 8+ δ {M: Tl (0.32≤ x≤0.74) or Hg (0.18≤ x≤0.68)} have been studied by magnetization and transport measurements. For Tl-1223 we have found the optimum Tl doping level to be x=0.53 regarding the grain properties, the content of superconducting phase, the first penetration field H pl wl, the transport ( J c tr), magnetic intergrain ( J c M) and intragrain ( J c g) critical current densities. The critical temperature T c of the as-sintered Tl-1223 sample decreased with increasing Tl content. Post-annealing in oxygen improved the T c for Tl contents of x≥0.53 and had generally positive effects on the critical current densities. The intergrain properties of the Hg-1223 samples were much worse than those of the Tl-based superconductors.

Superconducting and microstructural properties of Fe-doped Tl-1223 cuprates

The Fe-doping effects on superconductivity, transport and microstructure are investigated by means of Hall coefficient measurements, thermogravimetric analysis, Mössbauer spectroscopy and the calculation of the point-charge-effective-valence model (PCEV model) in Fe-doped Tl 0.5Pb 0.5(Sr 0.8Ba 0.2) 2Ca 2(Cu 1− x Fe x ) 3O y (Tl-1223) superconductors. In the doping level of Fe ( x=0–0.05), both zero-resistance temperature T co and carrier concentration n H decrease linearly with Fe dopants increasing. The striking linear behavior of T co and n H suggests the potential effects of charge localization on the transport properties and also shows that Fe dopants occupy different Cu sites in a stable manner with Fe content increasing. The thermogravimetric analysis shows the Fe 3+ ions substituting for Cu 2+ ions can bring excess oxygen atoms into lattice and form some Fe–O defect clusters. Mössbauer measurements are performed to investigate the microstructure of Fe–O defects. In addition, the atomic position of Fe dopant and the electric field gradient (EFG) on Fe sites are investigated under the PCEV model. We found that the signs of the EFG on Fe sites are positive. According to quadrupole splittings deduced from Mössbauer spectroscopies and results of model calculation, it is further confirmed that there is a small shift of Fe ion relative to the original center of the CuO 2 network, and in some different kinds of Fe–O defect clusters, Fe dopants can lead to excess oxygen atoms entering lattice. These results provide a further support for the interpretation about the displacement of Fe dopant and the assignment of Fe site. Our discussion suggests that the microstructural distortion and the excess oxygen defect induced by Fe doping encourage in the localization of cruising carrier on CuO 2 planes, which is one of the reasons to explain the decrease of T c and carrier concentration.

Effect of fluorine on the phase formation and superconducting properties of Tl-1223 superconductors

Fluorination of (Tl 0.5, Pb 0.5)Sr 1.6Ba 0.4Ca 2Cu 3O y high-temperature superconductors has been performed through partial replacement of CuO by CuF 2 in the starting materials. Seven samples with nominal fluorine content x atom per formula unit (0≤ x≤3.6) were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), resistivity and magnetization M( H) measurements. Fluorine addition has raised the onset transition temperature to 128 K, and increased the critical current density to 300%, the value of the fluorine free sample. XRD, SEM and energy dispersive X-ray (EDX) results showed that the formation of the Tl-1223 phase was enhanced as fluorine was introduced, and the formation of large grains was observed. EDX spot analysis revealed the presence of fluorine as a species of the structure of the Tl-1223 grains. The c-axis of Tl-1223 decreased with increasing x, indicating that fluorine had replaced oxygen in the structure. The behavior of the normal state resistivity with temperature indicated that fluorine addition changed this material from overdoped to optimally doped, and upon further fluorine addition, changes it to slightly underdoped state. That provided an additional evidence that fluorine addition changes the density of charge carrier concentration in the Cu–O planes and improves the superconducting properties of this material.

Microstructure and properties of textured bulk phase Tl(Bi)-1223 superconductors

Bi-doped Tl-1223 superconductors of the composition Tl n− x Bi x Ba 0.2Sr 1.8Ca 2Cu 3O y ( n=0.96−1.12 and x=0.25, 0.3, 0.35, 0.4) and Tl n−0.25 Bi 0.25Ba 0.4Sr 1.6Ca 2Cu 3O y ( n=0.97−1.04) were prepared from oxidic Ba–Sr–Ca–Cu precursor material. Upon calcination of the metal nitrates, the respective amounts of Tl 2O 3 and Bi 2O 3 were added to the oxidic powders by milling. The samples were sintered in silver foil. The specimens consisted predominantly of the Tl-1223 phase with small amounts of Tl-1212 and secondary phases. T c(0)-values ranged between 114 K and 119 K with transition widths around 1 K. Transport critical current densities up to 1.8 kA cm −2 at 77 K were measured for the as-sintered samples. Texturing was obtained by grinding, repressing and resintering of the superconducting material followed by annealing at 750°C for 50 h in flowing oxygen. The transport critical current densities could be improved up to 4.8 kA cm −2 by the thermo-mechanical treatment. Microstructural studies yielded information on the alignment of the superconducting grains, on secondary phases and the porosities.

Doped Tl-1223 Superconductors

Doped Tl-1223 superconductors of the composition TlaBibPbcBanSr2−nCa2Cu3Oy (a = 0.50 − 0.76, b = 0, 0.16 and 0.3, c = 0.5, 0.24 and 0, n = 0.15 − 0.4) were prepared from oxidic Ba-Sr-Ca-Cu precursor material. Upon calcination of the tartrate gels or the nitrates, the respective amounts of Tl2O3, PbO and Bi2O3 were added to the oxidic powders by milling. The samples were sintered in silver foil. The specimens consisted predominantely of the Tl-1223 phase with small amounts of Tl-1212 and secondary phases. Tc(0)-values ranged between 115 K and 119 K with transition widths around 1 K. Transport critical current densities up to 1.4 kA cm−2 at 77 K were measured for the assintered samples. Texturing was obtained by repressing and resintering of the superconducting material followed by annealing at 750°C for 50 h in flowing oxygen. The transport critical current densities could be improved up to 7 kA cm−2 by the thermomechanical treatment. Microstructural studies yielded information on the alignment of the superconducting grains, on secondary phases and the porosities.

Subgap structures in the current-voltage characteristics of the intrinsic Josephson effect due to phonons

A modified RSJ model for the coupling of intrinsic Josephson oscillations and c-axis phonons in the high- T c superconductors Tl 2Ba 2Ca 2Cu 3O 10+δ and Bi 2Sr 2CaCu 2O 8+δ is developed. This provides a very good explanation for recently reported subgap structures in the I-V characteristics of the c-axis transport. It turns out that the voltages of these structures coincide with the eigenfrequencies of longitudinal optical phonons, providing a new measurement technique for this quantity. The significantly enhanced microwave emission at the subgap structures in both the GHz and THz region is discussed.

Anomalous behaviors in high temperature superconductors: Lattice effect and d-electron delocalization effect of the normal state on the Bi, Tl, and Y-system superconductors

The dynamic studies of the hyperfine interactions and the lattice vibrational behavior are performed by means of temperature-dependent Mossbauer spectroscopy on the Bi-, Tl-, and Y-based superconductors, respectively. The temperature-dependent Mossbauer recoil-free factor f show that, above, but relatively close to, the transition temperature, there is an anomalous lattice effect which is related to the softening of phonon vibrational modes for Bi-2223, Tl-1223, and Y-123 superconductors. Moreover, for such three systems, the anomalous decrease of the Mossbauer isomer shift above Tc reveal the delocalization effect of d-electrons. Since the lattice effect is accompanied by d-electron delocalization effect at the almost same temperature region, it is believed that the structural instability involved in phonon softening is strongly interrelated to the change of the electronic states, and the phase separation behavior.

Coupling between Phonons and Intrinsic Josephson Oscillations in Cuprate Superconductors

The recently reported subgap structures observed in the current-voltage characteristic of intrinsic Josephson junctions in the high-T_c superconductors Tl_2Ba_2Ca_2Cu_3O_{10+\delta} and Bi_2Sr_2CaCu_2O_{8+\delta} are explained by the coupling between c-axis phonons and Josephson oscillations. A model is developed where c-axis lattice vibrations between adjacent superconducting multilayers are excited by the Josephson oscillations in a resistive junction. The voltages of the lowest structures correspond well to the frequencies of longitudinal c-axis phonons with large oscillator strength in the two materials, providing a new measurement technique for this quantity.

Preparation and Characterization of Tl-2223 Superconductor Coating Using the Electrophoretic Deposition Method

Tl2Ba1.6Sr0.4Ca2Cu3O10+y(Tl-2223) superconductor coatings on Ag substrates were prepared using the electrophoretic deposition method. The Tl-2223 single phase was obtained by applying a high Tl composition of superconducting powder at the proper sintering conditions in order to prevent evaporation of Tl during the sintering process. Nominal Tl compositions of the superconducting powders were 2.00-2.50, which were stoichiometric and excess compositions. Electrophoretically deposited coatings using these superconducting powders were buried in the superconducting powder and sintered at 815°C for 6h in air. For the case of the nominal Ti compositions of 2.20-2.30, superconducting powders were used for the electrophoretic deposition, the Tl compositions of sintered superconductors were 1.80-1.87, the critical temperature (Tc, zero) was 110-112K, and these samples were almost single phase. On the other hand, Ag2O powder was electrophoretically deposited onto the Tl-2223 electrophoretic deposition coating. After heat treatment at 315°C for 2h, the Tl-2223 pre-sintered superconductor coating was coated with silver. It was then sintered at 815°C for 6h. Although they were not buried during the sintering process, the Tl compositions of the obtained samples were 1.81-1.83 with a Tc, zero of 111-113K. This silver coat acts to restrain Tl evaporation and protects the coating of the superconducting layer. Moreover, the critical current density (Jc) of silver coated superconductor coating was 67A/cm2 (77K, 0T) when a nominal Tl composition of 2.30 superconducting powder was used for the electrophoretic deposition.

Neutron diffraction and Mössbauer spectroscopy studies of the mixed valent T1-1212 ferrite (Tl 0.5Pb 0.5)Sr 3Fe 2O 8

The mixed valent 1212-ferrite (Tl 0.5Pb 0.5)Sr 3Fe 2O 8 has been characterized using powder neutron diffraction data collected at 1.5 and 293 K. The structure is confirmed to be close to that of the Tl-1212 superconductor (Tl 0.5Pb 0.5)Sr 2CaCu 2O 7 but with no deviation from a full occupancy of (Tl,Pb) and oxygen sites. 57Fe-Mössbauer spectroscopy data depend upon the temperature between 4.2 and 293 K. They have been explained on the basis of a mixture of trivalent and tetravalent iron states at 4.2 and 150 K. At temperature higher than 150 K the existence of intermediate iron states Fe n+ (3 < n < 4) due to electron transfer between Fe 3+ and Fe 4+ sites has to be taken into account to fit the data. EFG calculations based upon neutron diffraction results at 293 K confirm that local displacements of the apical oxygen O(2) can be considered to explain the appearance of the Mössbauer iron sites instead of the structure expected one.

Electronic Raman scattering of Tl-2201 and Tl-2223 and the symmetry of the energy gap

The high temperature superconductors Tl 2Ba 2CuO 6+δ (Tl-2201) and Tl 2Ba 2Cu 3O 10 (Tl-2223) with one and three CuO 2 planes were investigated by Raman scattering in order to see whether the electronic Raman scattering cross-section depends on the number of CuO 2 planes, i.e. sheets of the Fermi surface. We present measurements of polarized Raman spectra in different scattering geometries on two single crystals of Tl-2201 ( T c = 90 K, 80 K), and one single crystal of Tl-2223 ( T c = 118 K), for temperatures below and above T c. In B 1g-symmetry the peak in the electronic Raman scattering is found at 460 cm −1 ( T c = 90 K), or 430 cm −1 ( T c = 80 K) for Tl-2201 and at 610 cm −1 for Tl-2223. This corresponds to 2 Δ/ k B T c = 7.4 ± 0.4 for both compounds. The A 1g- and B 2g symmetry components show smaller values for the peak position and a decrease linear in ω of the scattering intensity towards lower frequencies. The B 1g-component shows a decrease cubic in ω of the scattering intensity towards lower frequencies. The spectral features of Tl-2201 and Tl-2223 are found to be independent of the number of CuO 2-planes, i.e. of specific details of the Fermi surface. These observations are similar to the measurements on over- and underdoped Bi 2Sr 2CaCu 2O 8+δ single crystals. They are consistent with a d x 2− y 2 -wave pairing mechanism, based on the theoretical model of Devereaux and Einzel.

NMR study of local hole distribution, spin fluctuation and superconductivity in Tl 2Ba 2Ca 2Cu 3O 10

63Cu, 17O and 205Tl NMR have been performed in the high- T c superconductor Tl 2Ba 2Ca 2Cu 3O 10 whose T c(max) is 127 K. The hole densities at Cu and oxygen sites in the CuO 2 plane have been extracted from the nuclear quadrupole frequency ν Q . The striking feature is that the Cu holes are significantly transferred to oxygen site due to strong hybridization between Cu and oxygen. From an analysis of T 1 and T 2 G , it has been found that the spectral weight of the spin fluctuation is transferred to higher energy compared to YBa 2Cu 3O 7, while the magnetic correlation length ξ does not differ much. Thus, it is suggested that the higher T c is due to higher characteristic energy of spin fluctuations, i.e. the superconductivity is spin fluctuation mediated. The superconducting properties are consistently explained by a d-wave superconductivity model with a finite density of states (DOS) at the Fermi level. We show that the disorder of the Ca/TlO layer caused by the partial inter-substitution of Tl and Ca is responsible for the potential scattering to produce such a DOS. It is found that if such a potential scattering were absent, T c would go up to 132 K which is quite close to the record T c realized in the Hg based compound.