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Register a new userDoping a correlated band insulator: A new route to half metallic behaviour
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We demonstrate in a simple model the surprising result that turning on an on-site Coulomb interaction U in a doped band insulator leads to the formation of a half-metallic state. In the undoped system, we show that increasing U leads to a first order transition between a paramagnetic, band insulator and an antiferomagnetic Mott insulator at a finite value U_{AF}. Upon doping, the system exhibits half metallic ferrimagnetism over a wide range of doping and interaction strengths on either side of U_{AF}. Our results, based on dynamical mean field theory, suggest a novel route to half-metallic behavior and provide motivation for experiments on new materials for spintronics.
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We demonstrate, using dynamical mean-field theory with the hybridization expansion continuous time quantum montecarlo impurity solver, a rich phase diagram with {em correlation driven metallic and half-metallic phases} in a simple model of a correlated band insulator, namely, the half-filled ionic Hubbard model (IHM) with first {em and} second neighbor hopping ($t$ and $t$), an on-site repulsion $U$, and a staggered potential $Delta$. Without $t$ the IHM has a direct transition from a paramagnetic band insulator (BI) to an antiferromagnetic Mott insulator (AFI) phase as $U$ increases. For weak to intermediate correlations, $t$ frustrates the AF order, leading to a paramagnetic metal (PM) phase, a ferrimagnetic metal (FM) phase and an anti-ferromagnetic half-metal (AFHM) phase in which electrons with one spin orientation, say up-spin, have gapless excitations while the down-spin electrons are gapped. For $t$ less than a threshold $ t_1$, there is a direct, first-order, BI to AFI transition as $U$ increases, as for $t=0$; for $t_4< t < Delta/2$, the BI to AFI transition occurs via an intervening PM phase. For $t > Delta/2$, there is no BI phase, and the system has a PM to AFI transition as $U$ increases. In an intermediate-range $t_2 < t < t_3$, as $U$ increases the system undergoes four transitions, in the sequence BI $rightarrow$ PM $rightarrow$ FM $rightarrow$ AFHM $rightarrow$ AFI; the FM phase is absent in the ranges of $t$ on either side, implying three transitions. The BI-PM, FM-AFHM and AFHM-AFI transitions, and a part of the PM-FM transition are continuous, while the rest of the transitions are first order in nature. The PM, FM and the AFHM phases have, respectively, spin symmetric, partially polarized and fully polarized electron [hole] pockets around the ($pmpi/2$, $pmpi/2$) [($pm pi, 0$), ($0. pm pi$)] points in the Brillouin zone.
We present a novel route for attaining unconventional superconductivity (SC) in a strongly correlated system without doping. In a simple model of a correlated band insulator (BI) at half-filling we demonstrate, based on a generalization of the projected wavefunctions method, that SC emerges when e-e interactions and the bare band-gap are both much larger than the kinetic energy, provided the system has sufficient frustration against the magnetic order. As the interactions are tuned, SC appears sandwiched between the correlated BI followed by a paramagnetic metal on one side, and a ferrimagnetic metal, antiferromagnetic (AF) half-metal, and AF Mott insulator phases on the other side.
We introduce the notion of the strongly correlated band insulator (SCI), where the lowest energy excitations are collective modes (excitons) rather than the single particles. We construct controllable 1/N expansion for SCI to describe their observables properties. A remarkable example of the SCI is bilayer graphene which is shown to be tunable between the SCI and usual weak coupling regime.
The resistance of a conventional insulator diverges as temperature approaches zero. The peculiar low temperature resistivity saturation in the 4f Kondo insulator (KI) SmB6 has spurred proposals of a correlation-driven topological Kondo insulator (TKI) with exotic ground states. However, the scarcity of model TKI material families leaves difficulties in disentangling key ingredients from irrelevant details. Here we use angle-resolved photoemission spectroscopy (ARPES) to study FeSb2, a correlated d-electron KI candidate that also exhibits a low temperature resistivity saturation. On the (010) surface, we find a rich assemblage of metallic states with two-dimensional dispersion. Measurements of the bulk band structure reveal band renormalization, a large temperature-dependent band shift, and flat spectral features along certain high symmetry directions, providing spectroscopic evidence for strong correlations. Our observations suggest that exotic insulating states resembling those in SmB6 and YbB12 may also exist in systems with d instead of f electrons.
NaV2O4 crystals were grown under high pressure using a NaCl flux, and the crystals were characterized with X-ray diffraction, electrical resistivity, heat capacity, and magnetization. The structure of NaV2O4 consists of double chains of edge-sharing VO6 octahedra. The resistivity is highly anisotropic, with the resistivity perpendicular to the chains more than 20 times greater than that parallel to the chains. Magnetically, the intrachain interactions are ferromagnetic and the interchain interactions are antiferromagnetic; 3D antiferromagnetic order is established at 140 K. First principles electronic structure calculations indicate that the chains are half metallic. Interestingly, the case of NaV2O4 seems to be a quasi-1D analogue of what was found for half-metallic materials.
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