Abstract
Lacunar spinels with a chemical formula of form a populous family of narrow‐gap semiconductors, which offer a fertile ground to explore correlation and quantum phenomena, including transition between Mott and spin‐orbit insulator states, ferro/antiferroelectricity driven by cluster Jahn‐Teller effect, and magnetoelectric response of magnetic skyrmions with polar dressing. The electronic and magnetic properties of lacunar spinels are determined to a large extent by their molecular‐crystal‐like structure. The interplay of electronic correlations with spin‐orbit and vibronic couplings leads to a complex electronic structure already on the single‐cluster level, which—together with weaker intercluster interactions—gives rise to a plethora of unconventional correlated states. This review primarily focuses on recent progresses in the field of optical, dielectric, and magnetoelectric properties on lacunar spinels. After introducing the main structural aspects, lattice dynamics and electronic structure of these compounds are discussed on the basis of optical spectroscopy measurements. Dielectric and polarization studies reveal the main characteristics of their low‐temperature ferro‐ or antiferroelectric phases as well as orbital fluctuations in their high‐temperature cubic state. Strong couplings between spin, lattice, and orbital degrees of freedom are manifested in singlet formation upon magnetostructural transitions, the emergence of various multiferroic phases, and exotic domain‐wall functionalities.
Highlights
Cubic spinels with the chemical formula AM2X4 are unarguably one of the largest compound families, comprising a plethora of transition metal (M ) oxides and chalcogenides [1, 2, 3]
Lacunar spinels, being a populous sub-class of spinels, have been attracting increasing attention, since they offer a fertile ground for intriguing correlated and quantum states, such as a topological superconductor state [4], spinorbit entangled molecular Jeff=3/2 states [5, 6], Neel-type skyrmions with polar dressing [7], molecular cluster orbital-driven ferroelectricity [7, 8, 9, 10, 11, 12] and antiferroelectricity [13], multiple multiferroic phases [7, 9, 14, 15], and magnetic states confined to polar domain walls [14]
The term lacunar refers to an important structural motif, namely the lack of every second A-site ion with respect to the normal spinel structure, as described by the chemical formula A ◻ M4X8 or AM4X8
Summary
Cubic spinels with the chemical formula AM2X4 are unarguably one of the largest compound families, comprising a plethora of transition metal (M ) oxides and chalcogenides [1, 2, 3]. The tendency to form cluster orbitals is widely accepted for the V- and Mo-based compounds, like GaV4S(e) and GaMo4S(e)8 [21, 22, 26], where the uppermost partially occupied molecular orbital is a triply degenerate t2 level, hosting a single unpaired electron and hole, respectively (see Figure 1d) This orbital degeneracy is lifted by a cooperative Jahn-Teller distortion at TJT, which reduces the symmetry to polar rhombohedral (R3m). In contrast to all aforementioned lacunar spinels, the highest molecular orbital level of the M4 cluster in this compound is occupied by two unpaired electrons with total spin S = 1 This leads to a structural change to the polar orthorhombic space group Imm upon TJT and an elongation and shortening of two opposite V-V bonds of the V4 tetrahedra, respectively. The present review summarizes the progress achieved along these lines and discusses open questions and future directions
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