V. Müller

Observation of a lambda-shaped ultrasonic attenuation peak in superconducting UPt3

Abstract We report measurements of the temperature dependence of the ultrasonic attenuation in superconducting UPt3. In the vicinity of the superconducting transition temperature a pronounced longitudinal attenuation peak is observed. Such a peak does not appear for transverse ultrasound. Our experimental results can be explained by a “correlation hump” of the density-density correlation function.

Evidence for ferroelectric phase transitions and excitonic high-Tc superconductivity in YBa2Cu3O7−δ from ultrasonic measurements

Abstract We report on pronounced anomalies of the absorption and dispersion of the sound-induced rf magnetic field in superconducting YBa 2 Cu 3 O 7−δ ( δ K T K and for a near coincidence of the superconducting transition and a ferroelectric phase transition at about 90 K.

NQR investigations of the high-Tc superconductor Ba2YCU3O7

Abstract NQR measurements of 63,65Cu in both the normal and superconducting state are reported for the Cu1 and Cu2 sites in Ba2YCu3O7. An anomalous behaviour in the vicinity and below Tc is observed: after a steep decrease at Tc the quadrupole frequency increases with decreasing temperature.

Magnetic penetration depth in oriented YBaCuO powder samples

Abstract For oriented YBa2Cu3O7-δ powder samples we report for what we believe to be the first time on measurements of the temperature dependence of the magnetic penetration depth and of the lower critical magnetic field Bc1(T). The results are found to agree with theoretical expressions derived for weak-coupled BCS-type superconductors in the clean local limit. From the lower critical field values Bc1∣(0) = (340 ± 50)G and Bc1⊥(0) = (113 ± 30)G the magnitude of the anisotropic penetration depth has been determined yielding λ ∣ (0) = (1400 ± 200) A and λ ⊥ (0) = (6100 ± 1000) A .

Coexistence of high-Tc superconductivity and ferro- or antiferroelectricity in YBa2Cu3O7-δ

Abstract We report on pronounced anomalies of the absorption and dispersion of the sound-induced rf magnetic field in YBa2Cu3O7-δ. The experimental findings are highly indicative of a near-coincidence of the superconducting transition and an antiferrodistortive transition into either an antiferroelectric or improper ferroelectric state. To our knowledge the experimental technique presented here, so far is the only method which offers the possibility to show experimentally that both superconductivity and ferro-/antiferroelectricity may coexist. Furthermore, it is demonstrated that in YBa2Cu3O7-δ microtwinning plays an important role for the dielectric phase transition to become manifest. The possible relationship between our findings and high-Tc superconductivity is also discussed.

NMR investigations on oriented grains of superconducting YBa2Cu3O7

Oriented powdered samples of YBa2Cu3O7 were prepared by exposing the mechanically vibrated powder at 4.5 K to a magnetic field of 1.2 T. NMR experiments on 63,65Cu were performed and the results are compared with those of non-oriented samples. The data confirm the assignment of νQ1 ≅ 22 MHz to the Cu(1) and νQ2 ≅ 31.5 MHz to the Cu(2) sites. By means of the less common low-field NMR technique (νLarmor < νQuadrupole) the asymmetry parameter at the Cu(2) sites was determined to be η < 0.01, and at 100 K the Knight shift in the (a,b) plane was found to be K⊥ = (0.35 ± 0.1)% which is about a factor 2 smaller than the values reported in literature so far. Using published NMR data for 63Cu in Cu2O and in antiferromagnetic YBa2Cu3O6, our results are indicative for fluctuations of the Cu(1) ions between the ionic states Cu+ and Cu3+, and of the Cu(2) ions between Cu+ and Cu2+.

Ultrasonic hysteresis effects and the formation of twin domains in high-Tc superconductors

Abstract We report on ultrasonic experiments on YBa2Cu3O7. The formation of twins as a precursor of a first order phase transition is discussed.

Sound Velocity and Ultrasonic Absorption in High-Tc-Superconductors

We report on strong hysteresis effects in the temperature dependence of the longitudinal ultrasonic attenuation and group-velocity in YBa2Cu3O7.

Possible relationship between Cu-NQR frequencies and Cu valence states in CuO2-planes for high-T c superconductors

We report on63Cu-NQR frequency νNQR measurements on various high-Tc superconductors. An empirical relationship betweenTc and νNQR is presented. We attribute this finding to variations in the valence state of the in-plane Cu ions which, due to the unusual electric polarizability of the O2− ions, depends sensitively on the Cu−O distance.

Possible origin of high-Tc superconductivity in (1-2-3) oxides

Abstract Based on recent NMR and NQR experiments performed on YBa2Cu3O7 an illustrative model for the formation of hole pairs in RBa2Cu3O7 compounds (R= rare earth) is presented.