Orbital and magnetic excitations in the spin-chain compound TiPO4

Condensed Matter Seminars

Speaker: Jochen Geck
Date & Time: Thursday March 2, 2017 - 2:00 PM
Location: Hennings 318
Local Contact: George Sawatzky
Intended Audience: Graduate

We present high-resolution resonant inelastic x-ray scattering data as well many-body quantum chemistry calculations for the spin-chain compound TiPO₄. This combined approach allows identifying the d-level electron configuration of Ti in this material, which is found to be at odds with canonical local ligand-field theory. Specifically, we find the ground state to be composed of an admixture of z² and zx orbitals, highly unusual for six-fold coordinated d-metal sites. Our studies also reveal that this orbital state does not change significantly with temperature, excluding earlier proposals of an orbital reordering. Furthermore, the theoretical results indicate that the unexpected ground state orbital is related to the S=0 singlet bonds: to obtain the orbital ground state right, the quantum magnetism has to be included. We also observe dispersing magnetic excitations and resolve a spin gap of about 52 meV at low temperatures. This is a surprisingly large value compared to the observed Ti-Ti exchange of about 45 meV.  We interpret the spin dynamics in terms of bound spinon pairs (triplets) in a strongly dimerized spin chain.  In accordance with the above, our theoretical calculations reveal a dramatic dependence of the Ti-Ti exchange on the corresponding bond geometry.