Magnetic correlations in the high-temperature superconductor – Niels Bohr Institute - University of Copenhagen

Niels Bohr Institute > Calendar > 2009 > Magnetic correlations ...

Magnetic correlations in the high-temperature superconductor

Magnetic correlations in the high-temperature superconductor La1.88Sr0.12CuO4

Superconductivity provides one of the most fascinating and intriguing problems of condensed
matter physics. One of the first high-temperature superconductor was the cuprate La2-xSrxCuO4 (LSCO), which was discovered more than 20 years ago. The phase of the material changes with Sr-doping, x: At zero doping the material is a antiferromagnetically ordered Mott insulator, whereas for dopings above x=0.05 the material becomes superconducting.

In the doping range x=0.05 to x=0.13 LSCO shows magnetic order at incommensurate positions, which are shifted slightly from the antiferromagnetic ordering position. Low-energy magnetic fluctuations are also present at the incommensurate positions. Since the discovery of co-existent magnetic order and fluctuations and superconductivity it has been a leading question whether the phases compete or if the magnetic ordering somehow acts as an agent for the superconducting pairing mechanism.

Different aspects of the magnetic correlations have been the subject of this thesis work. The sample, La1.88Sr0.12CuO4, has a doping value for which the magnetic order is well developed, and the superconducting transition temperature is slightly suppressed (Tc =27 K). The temperature- and field dependence of the low-energy magnetic fluctuations has been investigated. A partial suppression of the scattering intensity is observed for low energy transfers and a possible explanation of this feature is presented. Furthermore a study of the magnetic correlations between the superconducting CuO2 planes has shown that there is only a very weak coupling between the CuO2 planes. Questions adressed are the origin of the incommensurate magnetic order and fluctuations and the effect of an applied magnetic field. Different scenarios are discussed with the outset in the experimental data.