B. Bucher

Strong pressure dependence of Tc of the new 80 K phase YBa2Cu4O8+x

Abstract The pressure dependence of the superconducting transition temperature Tc of the new structure YBa2Cu4O8 was measured on bulk material and single crystals as well. The differential dTc/dp was determined to be 0.55 K/kbar. Such a large change of Tc with increasing pressure was never observed before in compounds with a Tc of about 80 K. The results are discussed on the basis of Cryots model and on polaronic based superconductivity.

Phase diagrams of YBa2Cu4O8 and YBa2Cu3.5O7.5 in the pressure range 1 bar≤PO2≤3000 bar

Abstract The P-T-x phase diagram of the pseudobinary system (Y-Ba-Cu-O)-O 2 has been further investigated in the oxygen pressure range between 1 and 3000 bar. The stability ranges of the phases YBa 2 Cu 4 O 8 (124), YBa 2 Cu 3.5 O 7.5− x (123.5) and YBa 2 Cu 3 O 7− x (123) have been determined. Long duration experiments showed that the 123 phase is not stable at least down to 7 bar≤ P ≤20 bar oxygen and 900°C. It is not clear whether at lower pressures and temperatures the 123 phase is thermodynamically stable or metastable due to low reaction rates. In the presence of excess CuO, the 124 is the stable phase. The melting of 124 pellets at P O 2 =2800 bar shows that even at this pressure the 124 compound melts incongruently. Using the phase diagram data we could change the T c of 123.5 from 16 to 70 K by varying systematically the nonstoichiometry. Due to a narrow homogeneity range the T c of 124 remained constant but is different for powder pellets (81 K) and for crystals (70 K), probably due to the influence of the flux. Single crystals of both 124 and 123.5 with dimensions up to 4 mm were grown from the flux under high oxygen pressure.

The nonstoichiometry of the high-Tc superconductor Y2Ba4Cu7O15±x (14 K⩽Tc⩽68 K)

Abstract In this paper we present a study of the thermodynamic, structural and physical properties of Y2Ba4Cu7O15±x, the new high-Tc superconductor we have discovered recently. The nonstoichiometry of Y2Ba4Cu7O15±x has been varied between -0.70 T c , 1 2 =26.5 K ) indicating that possibly a coupling over long distance (11.5 A) takes place.

Anisotropic behavior in untwinned YBa2Cu4O8: Optical, magnetic and transport measurements at high pressure

Abstract We have investigated the anisotropic properties of untwinned, single crystal YBa 2 Cu 4 O 8 in the directions of the a - and b -axes. Along the b -direction, one has in addition to the plane a doubled CuO chain. Resistivity measurements (0–15 kbar) have revealed an anisotropy of a factor three. The transition temperature of one sample was different with respect to the a - and b -direction. Polarized microspot reflectivity spectra have shown a shift of the plasma edges of about 1 eV. The determination of H c2 ( T ) with H ext perpendicular and parallel to the plane also has given a high anisotropy. In the context of magnetic and elastic bound bipolarons which can suffer a Bose condensation some experimental facts can be understood.

PRESSURE DEPENDENCE OF Tc AND ANISOTROPIC FEATURES IN THE FAMILY Y2Ba4Cu6+nO14+n (n = 0,1,2)

Abstract The family Y 2 Ba 4 Cu 6 + n O 14 + n (n = 0,1,2) shows a wide range of the transition temperature T c . The pressure dependences of T c (dT c /dp) differ by one order of magnitude. We have shown that: 1. a) YBa 2 Cu 3 O 7 (123) has a differential dT c /dp from 0.03 K/kbar to 0.43 K/kbar, depending on the oxygen content. 2. b) YBa 2 Cu 3.5 O 7.5 (247) has a dT c /dp of 0.5 K/kbar with a leveling off of T c (p) at high pressure. 3. c) YBa 2 Cu 4 O 8 (124) has up to 18 kbar a dT c /dp of 0.57 K/kbar without a diminished pressure effect. Further, we report on the change of T c when Y is partially replaced by Sc or In. We have also investigated the 90 K compound (Y,Ba,Ca) 3 Cu 4 O 8 looking for a still high pressure dependence. We discuss the connection of dT c /dp with the strong shift of the apical oxygen towards the CuO plane at applied pressure, as detected by neutron diffraction. The n = 1 and n = 2 members have the great advantage, not to be twinned. So it was possible for the first time to resolve the anisotropy of the a- and b direction. We have measured the polarized reflectance R(ω) with respect to the a- and b direction of single crystals from 0.001 to 10 eV. The resistivity in a- and b direction ( ρ a and ρ b ) at ambient and high pressure reveals an anisotropy of a factor three and a deviation from the linear temperature behavior of ρ (T).

Structural and physical properties of the single and double chain YBaCuO phases 124 and 123.5

Abstract A review is given of the up to now experimentally determined structural and physical properties of the double chain compounds YBa2Cu4O8, in pure (Tc=806K) and Ca-substituted form (Tc=990 K), as well as Y2Ba4Cu7O15±x (Tc=19−14 K). These compounds belong together with 123 to the Y2Ba4Cu6+nO14+n family of a HBC superconductors. The structure of the first compound (124) is very similar to that of 123 but contains only double chains. These are thermodynamically stable up to 850°C and stoichiometric, therefore, both O÷T phase transition and a-b twinning are supressed. The structure of the second compound (123.5 or 247) is also derived from that of 123, but it contains alternating single and double chains which make the properties more complex than those of 124. All three up to now known HTc-compounds of the YBaCuO system are classical examples of structure sensitive superconductors, so that in depth structural characterization is necessary for the understanding of the physical properties. Most sensitiv structural feature is the apex of the pyramid (Cu2O1). Structural investigation show that its contraction can be correlated with the increase of Tc. Some properties of these compounds are not yet completely understood. Two examples are discussed here : a) The increase of Tc of 123.5 up to 94K, and b) the site of incorporation of Ca, in Ca-124, whose increase of Tc does not seem to be due to the substitution of Y by Ca, but of Ba by Ca. Evidence from various methods is presented. Last but not least, evidence is presented in this review, indicating that also other YBaCuO compounds, some with much higher Tc, may exist.

Optical study of the electronic phase transition of strongly correlated YbInCu 4

Infrared, visible, and near-UV reflectivity measurements are used to obtain conductivity as a function of temperature and frequency in YbInCu{sub 4}, which exhibits an isostructural phase-transition into a mixed-valent phase below T{sub v}{approx_equal}42 K. In addition to a gradual loss of spectral weight with decreasing temperature extending up to 1.5 eV, a sharp resonance appears at 0.25 eV in the mixed-valent phase. This feature can be described in terms of excitations into the Kondo (Abrikosov-Suhl) resonance, and, like the sudden reduction of resistivity, provides a direct reflection of the onset of coherence in this strongly correlated electron system. (c) 2000 The American Physical Society.

Physical properties of the homologous series Y2Ba4Cu6+nO14+n (n=0,1,2) at high pressure

Abstract We have measured resistivity and magnetic susceptibility of Y2Ba4Cu6+nO14+n (n=0,1,2) on bulk material and single crystals as well. We have found a pressure dependence of dTc/dp within the range of 0.05 to 0.55 K/kbar depending on the symmetry.

Coherence effects above Tc in YBa2Cu4O8

Abstract High temperature superconductors (HTSC) consist of CuO 2 layers which are separated by charge reservoirs (CuO-chains). They show strongly anisotropic physical parameters. Lower doped samples reveal metallic conductivity inside the layers but insulating temperature dependence for the out-of-plane direction. The resistivities of highly doped samples are metallic for all three directions. Evidence is presented that the temperature dependence for the resistivity perpendicular to the CuO 2 layers is related to the charge reservoirs in between the planes. The metallic out-of-plane resistivity might be traced back to a metallic conduction channel due to the charge reservoirs.