Spin Singlet Formation from S = 1/2 Tetrahedral Clusters

TL;DR

This study demonstrates the formation of a spin singlet state with a significant excitation gap in a cluster Mott insulator, driven by structural instability and intercluster interactions in Nb$_4$ tetrahedral clusters.

Contribution

It reveals the mechanism of spin singlet formation from S=1/2 tetrahedral clusters in a Mott insulator, highlighting the role of Jahn-Teller instability and intercluster coupling.

Findings

Spin singlet state with ~200 K gap in GaNb$_4$S$_8$

Structural instability triggers singlet formation at 32 K

Formation of Nb$_8$ octamer suggested by lattice symmetry

Abstract

Muon spin relaxation ($\mu$SR) and nuclear magnetic resonance (NMR) experiments revealed that the spin singlet state with an excitation gap of $\sim$200 K is realized from $S = 1/2$ Nb$_4$ tetrahedral clusters in a cluster Mott insulator GaNb$_4$S$_8$. The intercluster cooperative phenomenon to the singlet state at $T_\mathrm{S} = 32$ K is triggered by intracluster Jahn-Teller type structural instability developed from $\sim$$3T_\mathrm{S}$. Referring to the lattice symmetry, the formation of Nb$_8$ octamer (Nb$_4$--Nb$_4$ bond) is suggested.