Lattice dynamics and electronic excitations in a large family of lacunar spinels with a breathing pyrochlore lattice structure

Abstract

Reproducing the electronic structure of AM${}_4$X${}_8$ lacunar spinels with a breathing pyrochlore lattice is a great theoretical challenge due to the interplay of various factors. The character of the M${}_4$X${}_4$ cluster orbitals is critically influenced by the Jahn-Teller instability, the spin-orbit interaction, and also by the magnetic state of the clusters. Consequently, to reproduce the narrow-gap semiconducting nature of these moderately correlated materials requires advanced approaches, since the strength of the inter-cluster hopping is strongly affected by the character of the cluster orbitals. In order to provide a solid experimental basis for theoretical studies, we performed broadband optical spectroscopy on a large set of lacunar spinels, with systematically changing ions at the A and M sites as well as the ligand (A=Ga, Ge, Al; M=V, Mo, Nb, Ta; X=S, Se). Our study covers the range of phonon excitations and also electronic transitions near the gap edge. In the phonon excitation spectrum a limited subset of the symmetry allowed modes is observed in the cubic state, with a few additional modes emerging upon the symmetry-lowering structural transition. All the infrared active modes are assigned to vibrations of the ligands and ions at the A sites, with no obvious contribution from the M-site ions. Concerning the electronic states, we found that all compounds are narrow-gap semiconductors ($E_{\mathrm{g}}=130-350$meV) already in their room-temperature cubic state and their structural transitions induce weak, if any, changes in the band gap. The gap value is decreased when substituting S with Se and also when replacing $3d$ ions by $4d$ or $5d$ ions at the M sites.

11 pages, 7 figures

References in corpus (10)

• Electric-Field-Induced Resistive Switching in a Family of Mott Insulators : towards Non-Volatile Mott-RRAM Memories
• Resistive switching induced by electronic avalanche breakdown in GaTa${}_4$Se${}_{8-x}$Te${}_x$ narrow gap Mott Insulators
• Skyrmion robustness in non-centrosymmetric magnets with axial symmetry: The role of anisotropy and tilted magnetic fields
• First-Order Insulator-to-Metal Mott Transition in the Paramagnetic 3D System GaTa4Se8
• Characteristics of ferroelectric-ferroelastic domains in N{é}el-type skyrmion host GaV${}_4$S${}_8$
• Possible emergence of a skyrmion phase in ferroelectric GaMo${}_4$S${}_8$
• Mapping Skyrmion Stability in Uniaxial Lacunar Spinel Magnets from First-Principles
• Structural, magnetic, electric, dielectric, and thermodynamic properties of multiferroic GeV4S8
• Localised Wannier orbital basis for the Mott insulators GaV4S8 and GaTa4Se8
• Charge density functional plus $U$ calculation of lacunar spinel GaM${}_4$Se${}_8$ (M = Nb, Mo, Ta, and W)

Cited by in corpus (9)

• Macroscopic Manifestation of Domain-wall Magnetism and Magnetoelectric Effect in a Néel-type Skyrmion Host
• Cooperative cluster-Jahn-Teller effect as a possible route to antiferroelectricity
• Magnetic avalanche of non-oxide conductive domain walls
• Quasimolecular $J_{\mathrm{tet}}$=3/2 moments in the cluster Mott insulator GaTa${}_4$Se${}_8$
• Crystal and Electronic Structure of GaTa${}_4$Se${}_8$ From First-Principle Calculations
• Bond Ordering and Molecular Spin-Orbital Fluctuations in the Cluster Mott Insulator GaTa${}_4$Se${}_8$
• Spin dynamics in the skyrmion-host lacunar spinel GaV4S8
• Optical, dielectric, and magnetoelectric properties of ferroelectric and antiferroelectric lacunar spinels
• Analytic description of spin waves in dipolar/octupolar pyrochlore magnets