Spin singlet formation in MgTi$_2$O$_4$: evidence of a helical dimerization pattern

TL;DR

MgTi$_2$O$_4$ undergoes a metal-insulator transition characterized by spin singlet formation and a helical dimerization pattern, confirmed through diffraction and band structure calculations, revealing orbital order and insulating behavior.

Contribution

This study provides the first detailed structural evidence of helical dimerization and spin singlet formation in MgTi$_2$O$_4$, combining experimental diffraction data with theoretical calculations.

Findings

Formation of Ti-Ti dimers with short bonds forming helices

Transition to a band insulator with orbital order below 260 K

Magnetic susceptibility reduction indicating singlet state

Abstract

The transition metal spinel MgTi$_2$O$_4$ undergoes a metal-insulator transition on cooling below $T_{M-I} = 260$ K. A sharp reduction of the magnetic susceptibility below $T_{M-I}$ suggests the onset of a magnetic singlet state. Using high-resolution synchrotron and neutron powder diffraction, we have solved the low-temperature crystal structure of MgTi$_2$O$_4$, which is found to contain dimers with short Ti-Ti distances (the locations of the spin singlets) alternating with long bonds to form helices. Band structure calculations based on hybrid exchange density functional theory show that, at low temperatures, MgTi$_2$O$_4$ is an orbitally ordered band insulator.