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Register a new userMott Transition of MnO under Pressure: Comparison of Correlated Band Theories
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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.
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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.
We study the electronic structure, magnetic state, and phase stability of paramagnetic BiNiO$_3$ near a pressure-induced Mott insulator-to-metal transition (MIT) by employing a combination of density functional and dynamical mean-field theory. We obtain that BiNiO$_3$ exhibits an anomalous negative-charge-transfer insulating state, characterized by charge disproportionation of the Bi $6s$ states, with Ni$^{2+}$ ions. Upon a compression of the lattice volume by $sim$4.8%, BiNiO$_3$ is found to make a Mott MIT, accompanied by the change of crystal structure from triclinic $Pbar{1}$ to orthorhombic $Pbnm$. The pressure-induced MIT is associated with the melting of charge disproportionation of the Bi ions, caused by a charge transfer between the Bi $6s$ and O $2p$ states. The Ni sites remain to be Ni$^{2+}$ across the MIT, which is incompatible with the valence-skipping Ni$^{2+}$/Ni$^{3+}$ model. Our results suggest that the pressure-induced change of the crystal structure drives the MIT in BiNiO$_3$.
We present a computational study of PbCoO$_3$ at ambient and elevated pressure. We employ the static and dynamic treatment of local correlation in form of density functional theory + $U$ (DFT+$U$) and + dynamical mean-field theory (DFT+DMFT). Our results capture the experimentally observed crystal structures and identify the unsaturated Pb $6s$ - O $2p$ bonds as the driving force beyond the complex physics of PbCoO$_3$. We provide a geometrical analysis of the structural distortions and discuss their implications, in particular, the internal doping, which triggers transition between phases with and without local moments and a site selective Mott transition in the low-pressure phase.
1T-TaS$_2$ undergoes successive phase transitions upon cooling and eventually enters an insulating state of mysterious origin. Some consider this state to be a band insulator with interlayer stacking order, yet others attribute it to Mott physics that support a quantum spin liquid state.Here, we determine the electronic and structural properties of 1T-TaS$_2$ using angle-resolved photoemission spectroscopy and X-Ray diffraction. At low temperatures, the 2$pi$/2c-periodic band dispersion, along with half-integer-indexed diffraction peaks along the c axis, unambiguously indicates that the ground state of 1T-TaS$_2$ is a band insulator with interlayer dimerization. Upon heating, however, the system undergoes a transition into a Mott insulating state, which only exists in a narrow temperature window. Our results refute the idea of searching for quantum magnetism in 1T-TaS$_2$ only at low temperatures, and highlight the competition between on-site Coulomb repulsion and interlayer hopping as a crucial aspect for understanding the materials electronic properties.
Calculations employing the local density approximation combined with static and dynamical mean-field theories (LDA+U and LDA+DMFT) indicate that the metal-insulator transition observed at 32 GPa in paramagnetic LaMnO3 at room temperature is not a Mott-Hubbard transition, but is caused by orbital splitting of the majority-spin eg bands. For LaMnO3 to be insulating at pressures below 32 GPa, both on-site Coulomb repulsion and Jahn-Teller distortion are needed.
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