Lecture Markus Grüninger
|11 februari 2005||FWN-Building 5113.0201, Nijenborgh 4, 9747 AG, Groningen|
|Speaker:||Dr. Markus Grüninger|
|Affiliation:||RWTH Aachen, Germany|
|Title:||The quest for collective orbital excitations and a novel approach to the magnetic excitations of the high Tc cuprates|
|Date:||Fri Feb 11, 2005|
|Host:||P.H.M. van Loosdrecht|
|Telephone:||+31 50 363 8149 / 4974 (secr.)|
In correlated electron systems, spin and orbital degrees of freedom are intimately connected. While many interesting properties have been discovered in, e.g., low-dimensional quantum spin systems, novel quantum phenomena in systems with orbital degeneracy are still under debate. The central issue is whether the orbital degeneracy is lifted by the exchange interactions between the orbitals or by the coupling to the lattice, since orbital fluctuations are suppressed in the latter case. A study of the orbital excitation spectra may help to discriminate between the two mechanisms.
TiOCl has been discussed as the second inorganic spin-Peierls compound, and the importance of orbital fluctuations has been suggested. We have observed a large splitting of the t2g orbitals by means of optical conductivity measurements, and conclude that orbital fluctuations do not play an important role in this compound. In YTiO3, we have observed the orbital excitations in the optical conductivity and in Raman scattering.
The polarization dependence and the resonance behavior observed in the Raman data suggest a collective nature of the orbital excitations. Finally, we will address recent progress regarding the magnetic excitation spectra of the high Tc cuprate superconductors. Here, a quantitative description of neutron scattering data has been achieved on the basis of coupled spin ladders.
|Laatst gewijzigd:||22 oktober 2012 14:31|