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Register a new userField-induced multiple metal-insulator crossovers of correlated Dirac electrons of perovskite CaIrO$_3$
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The interplay between electron correlation and topology of relativistic electrons may lead to a new stage of the research on quantum materials and emergent functions. The emergence of various collective electronic orderings/liquids, which are tunable by external stimuli, is a remarkable feature of correlated electron systems, but has rarely been realized in the topological semimetals with high-mobility relativistic electrons. Here, we report that the correlated Dirac electrons with the Mott criticality in perovskite CaIrO$_3$ show unconventional field-induced successive metal-insulator-metal crossovers in the quantum limit accompanying a giant magnetoresistance (MR) with MR ratio of 3,500 % (18 T and 1.4 K). The analysis shows that the insulating state originates from the collective electronic ordering such as charge/spin density wave promoted by electron correlation, whereas it turns into the quasi-one-dimensional metal at higher fields due to the field-induced reduction of chemical potential, highlighting the highly field-sensitive character of correlated Dirac electrons.
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Metal-insulator transition (MIT) is one of the most conspicuous phenomena in correlated electron systems. However such transition has rarely been induced by an external magnetic field as the field scale is normally too small compared with the charge gap. In this paper we present the observation of a magnetic-field-driven MIT in a magnetic semiconductor $beta $-EuP$_3$. Concomitantly, we found a colossal magnetoresistance (CMR) in an extreme way: the resistance drops billionfold at 2 kelvins in a magnetic field less than 3 teslas. We ascribe this striking MIT as a field-driven transition from an antiferromagnetic and paramagnetic insulator to a spin-polarized topological semimetal, in which the spin configuration of $mathrm{Eu^{2+}}$ cations and spin-orbital coupling (SOC) play a crucial role. As a phosphorene-bearing compound whose electrical properties can be controlled by the application of field, $beta $-EuP$_3$ may serve as a tantalizing material in the basic research and even future electronics.
Dynamical mean-field theory (DMFT) has been employed in conjunction with density functional theory (DFT+DMFT) to investigate the metal-insulator transition (MIT) of strongly correlated $3d$ electrons due to quantum confinement. We shed new light on the microscopic mechanism of the MIT and previously reported anomalous subband mass enhancement, both of which arise as a direct consequence of the quantization of V $xz(yz)$ states in the SrVO$_3$ layers. We therefore show that quantum confinement can sensitively tune the strength of electron correlations, leading the way to applying such approaches in other correlated materials.
We consider the dimer Hubbard model within Dynamical Mean Field Theory to study the interplay and competition between Mott and Peierls physics. We describe the various metal-insulator transition lines of the phase diagram and the break down of the different solutions that occur along them. We focus on the specific issue of the debated Mott-Peierls insulator crossover and describe the systematic evolution of the electronic structure across the phase diagram. We found that at low intra-dimer hopping the emerging local magnetic moments can unbind above a characteristic singlet temperature $T^*$. Upon increasing the inter-dimer hopping subtle changes occur in the electronic structure. Notably, we find Hubbard bands of a mix character with coherent and incoherent excitations. We argue that this state is relevant for VO$_2$ and its signatures may be observed in spectroscopic studies, and possibly through pump-probe experiments.
The mechanisms that drive metal-to-insulator transitions (MIT) in correlated solids are not fully understood. For example, the perovskite (PV) SrCoO3 is a FM metal while the oxygen-deficient (n-doped) brownmillerite (BM) SrCoO2.5 is an anti-ferromagnetic (AFM) insulator. Given the magnetic and structural transitions that accompany the MIT, the driver for such a MIT transition is unclear. We also observe that the perovskite metals LaNiO3, SrFeO3, and SrCoO3 also undergo MIT when n-doped via high-to-low valence compositional changes. Also, pressurizing the insulating BM SrCoO2.5 phase, drives a gap closing. Using DFT and correlated diffusion Monte Carlo approaches we demonstrate that the ABO3 perovskites most prone to MIT are self hole-doped materials, reminiscent of a negative charge-transfer system. Upon n-doping away from the negative-charge transfer metallic phase, an underlying charge-lattice (or e-phonon) coupling drives the system to a bond-disproportionated gapped state, thereby achieving ligand hole passivation at certain sites only, leading to charge-disproportionated states. The size of the gap opened is correlated with the size of the hole-filling at these ligand sites. This suggests that the interactions driving the gap opening to realize a MIT even in correlated metals is the charge-transfer energy, but it couples with the underlying phonons to enable the transition to the insulating phase. Other orderings (magnetic, charge, etc.) driven by weaker interactions are secondary and may assist gap openings at small dopings, but its the charge-transfer energy that predominantly determines the bandgap, with a negative energy preferring the metallic phase. This n-doping can be achieved by modulations in stoichiometry or composition or pressure. Hence, controlling the amount of the ligand-hole is key in controlling MIT. We compare our predictions to experiments where possible.
We discuss Mott insulating and metallic phases of a model with $e_g$ orbital degeneracy to understand physics of Mn perovskite compounds. Quantum Monte Carlo and Lanczos diagonalization results are discussed in this model. To reproduce experimental results on charge gap and Jahn-Teller distortions, we show that a synergy between the strong correlation effects and the Jahn-Teller coupling is important. The incoherent charge dynamics and strong charge fluctuations are characteristic of the metallic phase accompanied with critical enhancement of short-ranged orbital correlation near the insulator.
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