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Register a new userMagnetic Moment Collapse-Driven Mott Transition in MnO
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The metal-insulator transition in correlated electron systems, where electron states transform from itinerant to localized, has been one of the central themes of condensed matter physics for more than half a century. The persistence of this question has been a consequence both of the intricacy of the fundamental issues and the growing recognition of the complexities that arise in real materials, even when strong repulsive interactions play the primary role. The initial concept of Mott was based on the relative importance of kinetic hopping (measured by the bandwidth) and on-site repulsion of electrons. Real materials, however, have many additional degrees of freedom that, as is recently attracting note, give rise to a rich variety of scenarios for a ``Mott transition. Here we report results for the classic correlated insulator MnO which reproduce a simultaneous moment collapse, volume collapse, and metallization transition near the observed pressure, and identify the mechanism as collapse of the magnetic moment due to increase of crystal field splitting, rather than to variation in the bandwidth.
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We present a new approach based on the static density functional theory (DFT) to describe paramagentic MnO, which is a representative paramagnetic Mott insulator. We appended the spin noncollinearity and the canonical ensemble to the magnetic sampling method (MSM), which is one of the supercell approaches based on disordered local moment model. The combination of the noncollinear MSM (NCMSM) with DFT$+U$ represents a highly favorable computational method called NCMSM$+U$ to accurately determine the paramagnetic properties of MnO with moderate numerical cost. The effects of electron correlations and spin noncollinearity on the properties of MnO were also investigated. We revealed that the spin noncollinearity plays an important role in determining the detailed electronic profile and precise energetics of paramagnetic MnO. Our results illustrate that the NCMSM$+U$ approach may be used as an alternative to the $textit{ab initio}$ framework of dynamic mean field theory based on DFT in the simulation of the high-temperature properties of Mott insulators.
The electronic structure, magnetic moment, and volume collapse of MnO under pressure are obtained from four different correlated band theory methods; local density approximation + Hubbard U (LDA+U), pseudopotential self-interaction correction (pseudo-SIC), the hybrid functional (combined local exchange plus Hartree-Fock exchange), and the local spin density SIC (SIC-LSD) method. Each method treats correlation among the five Mn 3d orbitals (per spin), including their hybridization with three O $2p$ orbitals in the valence bands and their changes with pressure. The focus is on comparison of the methods for rocksalt MnO (neglecting the observed transition to the NiAs structure in the 90-100 GPa range). Each method predicts a first-order volume collapse, but with variation in the predicted volume and critical pressure. Accompanying the volume collapse is a moment collapse, which for all methods is from high-spin to low-spin (5/2 to 1/2), not to nonmagnetic as the simplest scenario would have. The specific manner in which the transition occurs varies considerably among the methods: pseudo-SIC and SIC-LSD give insulator-to-metal, while LDA+U gives insulator-to-insulator and the hybrid method gives an insulator-to-semimetal transition. Projected densities of states above and below the transition are presented for each of the methods and used to analyze the character of each transition. In some cases the rhombohedral symmetry of the antiferromagnetically ordered phase clearly influences the character of the transition.
The vicinity of a Mott insulating phase has constantly been a fertile ground for finding exotic quantum states, most notably the high Tc cuprates and colossal magnetoresistance manganites. The layered transition metal dichalcogenide 1T-TaS2 represents another intriguing example, in which the Mott insulator phase is intimately entangled with a series of complex charge-density-wave (CDW) orders. More interestingly, it has been recently found that 1T-TaS2 undergoes a Mott-insulator-to-superconductor transition induced by high pressure, charge doping, or isovalent substitution. The nature of the Mott insulator phase and transition mechanism to the conducting state is still under heated debate. Here, by combining scanning tunneling microscopy (STM) measurements and first-principles calculations, we investigate the atomic scale electronic structure of 1T-TaS2 Mott insulator and its evolution to the metallic state upon isovalent substitution of S with Se. We identify two distinct types of orbital textures - one localized and the other extended - and demonstrates that the interplay between them is the key factor that determines the electronic structure. Especially, we show that the continuous evolution of the charge gap visualized by STM is due to the immersion of the localized-orbital-induced Hubbard bands into the extended-orbital-spanned Fermi sea, featuring a unique evolution from a Mott gap to a charge-transfer gap. This new mechanism of orbital-driven Mottness collapse revealed here suggests an interesting route for creating novel electronic state and designing future electronic devices.
We compute the electronic structure, spin and charge state of Fe ions, and structural phase stability of paramagnetic CaFeO$_3$ under pressure using a fully self-consistent in charge density DFT+dynamical mean-field theory method. We show that at ambient pressure CaFeO$_3$ is a negative charge-transfer insulator characterized by strong localization of the Fe $3d$ electrons. It crystallizes in the monoclinic $P2_1/n$ crystal structure with a cooperative breathing mode distortion of the lattice. While the Fe $3d$ Wannier occupations and local moments are consistent with robust charge disproportionation of Fe ions in the insulating $P2_1/n$ phase, the physical charge density difference around the structurally distinct Fe A and Fe B ions with the ``contracted and ``expanded oxygen octahedra, respectively, is rather weak, $sim$0.04. This implies the importance of the Fe $3d$ and O $2p$ negative charge transfer and supports the formation of a bond-disproportionated state characterized by the Fe A $3d^{5-delta}underline{L}^{2-delta}$ and Fe B $3d^5$ valence configurations with $delta ll 1$, in agreement with strong hybridization between the Fe $3d$ and O $2p$ states. Upon compression above $sim$41 GPa CaFeO$_3$ undergoes the insulator-to-metal phase transition (IMT) which is accompanied by a structural transformation into the orthorhombic $Pbnm$ phase. The phase transition is accompanied by suppression of the cooperative breathing mode distortion of the lattice and, hence, results in the melting of bond disproportionation of the Fe ions. Our analysis suggests that the IMT transition is associated with orbital-dependent delocalization of the Fe $3d$ electrons and leads to a remarkable collapse of the local magnetic moments. Our results imply the crucial importance of the interplay of electronic correlations and structural effects to explain the properties of CaFeO$_3$.
Switching voltage of first-order metal-insulator transition (MIT) in VO_2, an inhomogeneous strongly correlated system, is changed by irradiating an infrared light with wavelength, 1.5 micrometer, and applying the electric field (photo-induced switching). This was predicted in the hole-driven MIT theory in which hole doping of a low concentration below 0.01% into conduction band (Fermi surface) induces the abrupt MIT as correlation effect. The switching is explained by the Mott transition not the Peierls transition.
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