Franziska Hammerath visits Zagreb

In the week November 13-19, 2011, Franziska Hammerath from the Leibniz Institute for Solid State Research (IFW), Dresden is visiting PDFS in Zagreb. During the visit she will get acquainted with the new Laboratory for ssNMR and the other research groups for solid state physics in Zagreb. On Tuesday she will give a seminar "Unusual NMR Line Broadening and Spin Gap in the Zigzag S=1/2 Spin Chain Compound Sr_1-xCa_xCuO₂"

Abstract of the seminar:

"We present ⁶³Cu Nuclear Magnetic Resonance (NMR) measurements on undoped  SrCuO₂ and Ca-doped Sr_0.9Ca_0.1CuO₂ single crystals. The crystal structure contains one-dimensional CuO2 double chains that are magnetically decoupled due to frustration. The system orders magnetically only below 2K. Nevertheless, the Cu NMR spectra broaden already at temperatures below 100K and show an anomalous two peak structure at low temperatures, which are still far above the magnetic ordering temperature. For the Ca-doped sample, this broadening is reduced. The reason for this unusual broadening is not known, but a similar broadening has been reported for the single chain compound Sr₂CuO₃.
The Cu NMR spin-lattice relaxation rate, T₁^-1 , is temperature independent for SrCuO_2, as it is expected for a one-dimensional S=1/2 Heisenberg spin chain. Doping with nonmagnetic, isovalent Ca takes place on the Sr sites outside the spin chains, and should not aect the magnetic properties of the compound. It is therefore very surprising that we do observe a decrease of T₁^-1 in Ca-doped Sr_0.9Ca_0.1CuO₂ for temperatures below 90K that clearly evidences the opening of a gap in the spin excitation spectrum. Density Matrix Renormalization Group (DMRG) calculations of the J₁-J₂ Heisenberg model are presented to discuss the origin of this spin gap.

[1] F. Hammerath et al., Phys. Rev. Lett. 107, 017203 (2011)."