Spectroscopic Studies on the Metal–Insulator Transition Mechanism in Correlated Materials

Abstract

The metal-insulator transition (MIT) in correlated materials is a novel phenomenon that accompanies a large change in resistivity, often many orders of magnitude. It is important in its own right but its switching behavior in resistivity can be useful for device applications. From the material physics point of view, the starting point of the research on the MIT should be to understand the microscopic mechanism. Here, an overview of recent efforts to unravel the microscopic mechanisms for various types of MITs in correlated materials is provided. Research has focused on transition metal oxides (TMOs), but transition metal chalcogenides have also been studied. Along the way, a new class of MIT materials is discovered, the so-called relativistic Mott insulators in 5d TMOs. Distortions in the MO (M = transition metal) octahedron are found to have a large and peculiar effect on the band structure in an orbital dependent way, possibly paving a way to the orbital selective Mott transition. In the final section, the character of the materials suitable for applications is summarized, followed by a brief discussion of some of the efforts to control MITs in correlated materials, including a dynamical approach using light.

Observation of Kondo hybridization with an orbital-selective Mott phase in 4d Ca2-xSrxRuO4

“…2a-d. As x decreases from x = 0.5 to 0.2, gradual suppression of the spectral weight is observed near the Fermi level generating a soft gap 19 , as shown in Fig. S2b (Supplementary Information 2).…”

Magnetically induced metal-insulator transition in Pb2CaOsO6

“…In NaOsO 3 , the MIT occurs over a temperature range of some 400 K, and such a continuous transition has been argued to be a signature of a spin-driven Lifshitz transition rather than a Slater transition [9]. The electronic configuration of Os in NaOsO 3 is 5d 3 corresponding to half-filled t 2g states, whereas it is 5d 2 in Pb 2 CaOsO 6 . Hence, the effect of spin-orbit coupling is expected to be larger in Pb 2 CaOsO 6 , and as a result the mechanism of the MIT may be different from that in NaOsO 3 .…”

“…Metal-to-insulator transitions (MIT) can occur through a variety of mechanisms [1,2]. Perhaps the best known is that of the Mott insulator in which strong Coulomb interactions between electrons open up a gap at the Fermi energy [3].…”

“…The extended 5d states in these systems are relatively weakly interacting as required for the Slater mechanism, but the band splitting caused by strong spin-orbit coupling can bring about Mott-like behavior instead. Indeed, later work indicated that mechanisms other than the Slater mechanism may be behind the MIT in the above-mentioned 5d oxides [2,9,10].…”

Correlated Magnetic Weyl Semimetal State in Strained Pr2Ir2O7

“…DOI: 10.1002/adma.202008528 matter research, [1] and have drawn intensive investigations independently. Electronic correlations (U) have been well studied in the last several decades [2][3][4] and play a crucial role in generating various exotic quantum pheno mena, such as metal-insulator transition, [2] colossal magnetoresistance, [3] and unconventional superconductivity. [4] Recently, topological nontrivial systems, including Weyl [5][6][7][8] (or Dirac) [9] and nodal-line semimetals, [10,11] have been developed swiftly, since the discovery of a topological insulator (TI) with strong spin-orbit coupling (λ).…”