Bond ordering and molecular spin-orbital fluctuations in the cluster Mott insulator GaTa4Se8

Resonating holes vs molecular spin-orbit coupled states in group-5 lacunar spinels

“…This result suggests a physical picture for the nonmagnetic states found in the Nb- and Ta-based lacunar spinels: as the M 4 -cluster electrons are prone to stronger fluctuations with larger spatial spread, inter-cluster couplings are able to create “valence bond” spin singlets, as concluded from experiment 27 – 29 . The peculiar pseudospin structure must play a role in the superconductivity mechanism under pressure, which is speculated to be unconventional owing to closeness of magnetic states and spin fluctuations.…”

Cluster Rearrangement by Chiral Charge Order in Lacunar Spinel GaNb4Se8

“…26−29 In contrast, GaM 4 X 8 with M = Nb, Ta exhibit different types of distortion. The lattice of GaTa 4 Se 8 changes to tetragonal below T M = 50 K. 20,25 GaNb 4 S 8 and GaNb 4 Se 8 have orthorhombic distortion in the low-temperature phase below T M = 32 and 33 K, respectively. 20,32 These low-temperature phases are characterized by nonmagnetic (spin-singlet) nature confirmed by nuclear magnetic resonance.…”

“…Furthermore, in a magnetic field, magnetic orders including a skyrmion lattice emerge at lower temperatures in the rhombohedral phase. − In contrast, GaM 4 X 8 with M = Nb, Ta exhibit different types of distortion. The lattice of GaTa 4 Se 8 changes to tetragonal below T M = 50 K. , GaNb 4 S 8 and GaNb 4 Se 8 have orthorhombic distortion in the low-temperature phase below T M = 32 and 33 K, respectively. , These low-temperature phases are characterized by nonmagnetic (spin-singlet) nature confirmed by nuclear magnetic resonance. , However, GaNb 4 Se 8 undergoes another phase transition at T C = 50 K . Powder X-ray diffraction (XRD) experiments suggest the cubic space group P 2 1 3 in the intermediate-temperature phase between T M and T C …”

“…Lacunar spinel compounds GaM 4 X 8 (M = V, Nb, Mo, Ta, and X = S, Se) are Mott insulator systems that exhibit various electronic and magnetic properties, such as pressure-induced superconductivity, − negative colossal magnetoresistance, spin-singlet state, , spin–orbit entangled molecular J eff state, − and magnetic skyrmions. − Since the formal valence of M is 3.25+, the average number of d electrons are 7/4 and 11/4 for group V (M = V, Nb, Ta) and group VI (M = Mo) transition metals, respectively. Since only half of the tetrahedral sites are occupied by Ga atoms, the pyrochlore network of M atoms takes on a breathing-type structure, resulting in M 4 tetrahedral clusters.…”