Mihkel's MSc

Local structure and dynamics in spin-Peierls compound TiPO₄ : a ³¹P NMR study

Quasi one dimensional conductors and spin chains are popular subjects of study both in theoretical and experimental physics. Because of sophisticated magnetic field – temperature phase diagrams and unique quantum effects taking place they regained the attention of many researchers. One of those phenomenons is spin-Peierls (SP) effect in low dimensional magnetic structures. In spin-Peierls state the ground state of magnetic structure isn’t ordered ferromagnetic or antiferromagnetic, but diamagnetic singlet state, which is usually accompanied by dimerization of linear magnetic chain.

The first compounds where SP effect was found were TTF-CuBDT ane TTF-AuBDT, where in linear antiferromagnetically interacting S = 1/2 chains the transition into singlet state were observed on temperatures T_{SP_Cu} = 12K, T_{SP_Au} = 2K, respectively. The first anorganic SP compound was discovered in 1993, when it was found, that in magnetic fields H > 12.5T the diamagnetic dimerized SP state chain in CuGeO₃ transformed in temperature T_SP = 14.7K into incommensurate SP state. In titanium compounds the SP transition takes place in remarkably higher temperatures than 14.7K – in TiOCl the transition occurs in T_SP = 92K/65K, in TiOBr T_SP = 48K/27K, in TiPO₄ T_SP = 112K/73K. It is important to notice, that the SP transition in all the mentioned Ti compounds takes place in two steps – reducing the temperature causes firstly the transition from paramagnetic state into incommensurate SP phase, which in lower temperatures is followed by transition into commensurate SP state.

In the master’s thesis it was investigated the structural changes of TiPO₄ when going from paramagnetic phase into dimerized spin-Peierls state, using nuclear magnetic resonance techniques on phosphorus isotope ³¹P. Earlier Glaum et al. X-ray structural analysis studies didn’t see any noticeable changes in TiPO₄ structure, which gave the motivation to determine the chemical shift tensor of ³¹P and the orientation of its main axes with respect to crystal lattice. Knowing the tensors and their orientations enables to investigate the structural changes on a lot more precise level.

A byproduct of master’s thesis was the approximate energetic gap width in spin excitation spectrum, which was found from the temperature dependence of ³¹P spin-lattice relaxation.