Fluctuating pseudospin dimers in cluster Mott insulator*

The Lacunar spinel, GaTa₄Se₈, is a unique Mott insulator that hosts molecular spin-orbit J_eff-3/2 states as correlation units. The material undergoes a concurrent magnetic and structural transition at 50 K and forms a singlet ground state below the transition temperature. The magnetic ground states are entangled to the lattice distortion, and a preceding phonon mode was observed by neutron scattering measurements. However, there are no signs of the transition in the local distortion from x-ray and neutron pair distribution function measurements indicating the fluctuating dimer phase in GaTa₄Se₈. In this talk, I will discuss a series of x-ray and neutron pair distribution function measurements combining the phonon analysis to uncover the fluctuating pseudospin dimer phase in this representative cluster Mott insulator.