Specific heat of the 90-K superconductor Tl2Ba2CuO6 ("2201") prepared in high pressure Ar or He gas (original) (raw)
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A new elaboration process of the superconducting Tl2Ba2CuO6 phase with Tc=90K
We have synthesized high quality ceramic T12Ba2CulO6+ ~ (2201) samples using the high pressure, high temperature route. At high oxygen content, the structure is orthorhombic and the samples are metallic but non-superconducting. Upon lowering the oxygen content, the symmetry changes from orthorhombic to tetragonal or pseudo-tetragonal. In the latter phase, the maximum superconducting critical temperature reaches 92 K Optical micrographs show large 2201 grains and some traces of impurity phases like T12Ba2Os, Ba2Cu30 x and CuO. X-ray diffraction shows only the 2201 phase. Plasma emission spectroscopy indicates that the global sample stoichiometry is TI:Ba:Cu=2:2:I. This analysis proves that the high pressure route effectively prevents the thallium evaporation. HREM investigations exclude the possibility of cation vacancies. Microprobe analyses (EDAX) show no variation of the cation stoichiometry between the 2201 grains. X-ray diffraction on a superconducting single crystal yields a refined composition Tll.94Ba2Cul.060 6.
Superconductivity at 133K in Tl2Ba2Ca2Cu3O10 under high pressure
1993
The pressure dependence of the superconducting transition temperature T, of TltBa&a&usO,o+, (n-2223) has been measured under pressures up to 13 GPa ( 130 kbar ). At 4.2 GPa a maximum is observed with a T, of 133 K. This is the highest T, yet observed for any high-T, superconductor with the exception of the recently discovered mercury-based compound Hg-Ba,Ca,CusOs+, (Hg-1223). Because of some structural differences between Tl-2223 and Hg-1223, a slightly higher maximum Z', is predicted for Hg-1223. Both T. and its pressure dependence are discussed on the basis of a simple charge-transfer model.
Physica C: Superconductivity, 1995
The anomalous sharp peak of the thermoelectric power, S(T), that we have recently observed just above the superconducting transition in some non-fully oxygenated YtBa2CU3OT_ n polycrystalline samples, with T~ ~ 91 K, and also the rounded S(T) behavior around T¢ of the fully oxygenated samples (with negative S(T) values), are now analyzed in this paper on the grounds of different existing approaches for the influence of thermal fluctuations on S(T) near the Superconducting transition. This analysis confirms our previous proposal that these sharp S(T) peaks near T c, that were also observed by other groups, are not intrinsic and that these anomalies are probably due to the presence in the samples of small oxygen content inhomogeneities (less than 4% of the average sample oxygenation) at long length scales, which lead to inhomogeneities in the critical temperature and in the S(T) sign. Although the uncertainties on the S(T) background may introduce some ambiguity, our present analysis also confirms our earlier proposal that the intrinsic critical behavior of S(T) near T c in Y1Ba2Cu307_n compounds is mainly driven by that of the electrical conductivity, i.e., we found that the thermoelectric coefficient, L(T), of copper-oxide superconductors does not present a strong critical behavior above the superconducting transition, in agreement with earlier theoretical predictions of Maki. Also, our previous analysis of the influence on S(T) of uniformly and non-uniformly distributed oxygen content inhomogeneities is now extended to new possible experimental situations. These results show, in particular, that the strong decrease of the anomalous S(T) peak amplitude due to the presence of a magnetic field, an effect observed by various groups and that has remained unexplained until now, may be easily understood in terms of oxygen content inhomogeneities. By taking into account the dependence of the effective thermopower on the relative strength of these inhomogeneities, it is possible to explain the change of sign of S(T) observed by various authors in the normal region in some polycrystalline samples. Such a dependence also provides a simple and plausible explanation of the existing controversial experimental results concerning the possible opposite sign of S in the a and b directions of untwinned crystals. The S(T) peak will also be briefly compared with the electrical resistivity peak that we have observed previously in other YIBa2Cu307_ ~ crystals. * Corresponding author. 0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All fights reserved SSDI 0921-4534(95)00509-9
Physical review. B, Condensed matter, 1992
We observe significant frequency shifts of oxygen vibrations in (Tl,Cd)2(Ba, La)2CuOs+s upon doping; the frequency shifts result from charge transfer from the intermediate Tl02 layers to the Cu02 planes. These observations are interpreted by comparison with data on (La, Ba)2CiiOs+s, which is isostructural with (Tl,Cd)2(Ba, La)2CiiOs+s, except that it contains no intermediate T102 filling layers. The data suggest that the twice higher T, in the Tl compound results from a superconducting pairing interaction between carriers in the Cu02 planes aided by dynamic charge transfer along the c axis, with the polarizable structure between the conducting layers playing an important role.
Physical Review B, 1988
Using a powder diffractometer with a continuous flow arrangement, the lattice parameters of a sample of YBa2Cu307 s (with b estimated to be between 0.10 and 0.15) were determined from 300 K down to 80 K with a temperature regulation of 0.1 K. The diffractogram showed a single phase with the orthorhombic structure. Measurements from 100-80 K were performed at close temperature intervals and showed an anomalous change in c in a narrow temperature range around T,. This anomaly correlates with the anomaly in the nuclear quadrupole resonance frequency, positron annihilation lifetimes, and sound velocity observed by some workers. A smaller decrease in c was observed around 240 K as the sample was cooled, which again correlates with anomalies observed in sound velocity and attenuation and differential scanning calorimetry. The present observations differ from the recently published results of Horn et al. Since the discovery of superconductivity around 90 K by Wu et al. ' in a multiphase sample of Y-Ba-Cu-O, the superconducting phase has been identified as a distorted oxygen-deficient perovskite with the composition YBa2Cu307-s. This has an orthorhombic structure. The lattice parameters have been measured by several work-ers~s in diff'erent temperature ranges using both x-ray and neutron diffraction. However, the parameter 8 has not been specifically indicated in some of these reports though the samples were superconducting around 90 K. Anomalous behavior has been observed in the sound velocity through the transition. 9 '3 A sharp anomaly in the
Physical Review B, 1990
A detailed study of infrared properties (reflectivity, conductivity, and dielectric response), emphasizing reproducible results from fully oxygenated YBa2Cu307 crystals (T, =93 K) and films, is presented. The extrapolated values of 0. &(co) at low frequency are roughly consistent with the measured temperature-dependent dc resistivity. Although not well understood, this infrared conductivity can be interpreted in terms of a frequency-dependent scattering rate of-kT+Ae, with a lowfrequency mass enhancement of roughly 2 to 4 associated with a carrier-spin related interaction. Infrared measurements polarized along the c axis suggest a conductivity anisotropy of roughly 40:1 near T, in the normal state. In the superconducting state an energy scale of 2h, = 3kT, is suggested by c-axis polarized measurements, while a much larger characteristic energy of 2h, b-8kT, is evident in the (a-b)-plane conductivity. From the area missing from the conductivity up to this very 0 large gap, a reasonable estimate (=1700 A) for the (a-b)-plane penetration depth is obtained. Evidence for non-BCS temperature dependence, strong pair breaking scattering, and possible fluctuation effects is discussed. A comparison to infrared data from Bi2Sr,CaCu208 "shows a similarly large energy scale, 2b, b-8kT"' for the cubic Bap 6Ko 4Bi03 superconductor a more conventional energy scale, 2b =4kT, is observed. The unusually large energy scale obtained from the (a-b)-plane measurements of the layered cuprates lies far beyond the range of previously studied superconducting energy gaps (25 =3 to SkT,).
Magnetic properties of the Tl2Ba2CuO6+x 90K superconductor
Journal of Alloys and Compounds, 195 (1993) 607-610, 1993
We have measured magnetic properties of high quality TI2Ba2CuO6+x 90 K tetragonal superconducting and orthorhombic metallic non-superconducting ceramics. The AC susceptibility shows distinct intra- and intergrain transitions down to H=0.004 Oe rms. The field cooling measurement at 20 Oe shows a sharp transition at 90 K and 50% of full flux exclusion at 5 K. At T>150 K, i.e. above the region where superconducting fluctuations contribute, the normal-state susceptibility is temperature independent and diamagnetic with a value of -5x10-8 emu/g. The intergrain critical current densities determined at B=1 T, using Bean's critical state model, are very low, 6000 A/cm2 at 5 K and below 1 A/cm2 at 40 K.
Oxygen deficiency dependence of transition temperature in (Sm, Er) Ba2Cu3O7−δ superconductors
Bulletin of Materials Science, 1997
We have investigated the effects of oxygen deficiency (6) on the transition temperature (To) of (Sin, Er)Ba2CU3OT_ ~ superconductors by incorporating the effects of the two dimensional (2I)) acoustic phonons and plasmons in the framework of strong coupling theory. The proposed approach for yttrium cuprates properly takes care of the double CuO 2 plane in a unit cell and has been found earlier to be successful in describing the pairing mechanism as well as the variation of T c with 6 in YBa2CU3OT_ 6 system. The coupling strength (2), the screening parameter (/~*) and the two dimensional acoustic phonon (plasmon) energy hco (oJ +) as a function of oxygen deficiency is worked out. Finally, the transition temperature is evaluated and is found to be consistent with the earlier experimental data on yttrium euprates. Thus, coupled phonon-plasmon mechanism is adequate to understand the nature of pairing mechanism and oxygen deficiency dependence of transition temperature in 90 K (Sm, Er) Ba 2 Cu 3 0 7-6 superconductors.