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Register a new userA ferroelectric-like structural transition in a metal
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Metals cannot exhibit ferroelectricity because static internal electric fields are screened by conduction electrons, but in 1965, Anderson and Blount predicted the possibility of a ferroelectric metal, in which a ferroelectric-like structural transition occurs in the metallic state. Up to now, no clear example of such a material has been identified. Here we report on a centrosymmetric (R-3c) to non-centrosymmetric (R3c) transition in metallic LiOsO3 that is structurally equivalent to the ferroelectric transition of LiNbO3. The transition involves a continuous shift in the mean position of Li+ ions on cooling below 140K. Its discovery realizes the scenario described by Anderson and Blount, and establishes a new class of materials whose properties may differ from those of normal metals.
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LiOsO3 is one of the first materials identified in a recent literature as a polar metal, a class of materials that are simultaneously noncentrosymmetric and metallic. In this work, the linear and nonlinear optical susceptibility of LiOsO3 is studied by means of ellipsometry and optical second harmonic generation (SHG). Strong optical birefringence is observed using spectroscopic ellipsometry. The nonlinear optical susceptibility extracted from SHG polarimetry reveals that the tensor components are of the same magnitude as in isostructural insulator LiNbO3, except the component along the polar axis d33, which is suppressed by an order of magnitude. Temperature-dependent SHG measurements in combination with Raman spectroscopy indicate a continuous order-disorder type polar phase transition at 140 K. Linear and nonlinear optical microscopy techniques reveal 109 deg/71 deg ferroelastic domain walls, like in other trigonal ferroelectrics. No 180 deg polar domain walls are observed to emerge across the phase transition.
Synchrotron X-ray total scattering studies of structural changes in rutile VO2 at the metal-insulator transition temperature of 340 K reveal that monoclinic and tetragonal phases of VO2 coexist in equilibrium, as expected for a first-order phase transition. No evidence for any distinct intermediate phase is seen. Unbiased local structure studies of the changes in V--V distances through the phase transition, using reverse Monte Carlo methods, support the idea of phase coexistence and point to the high degree of correlation in the dimerized low-temperature structure. No evidence for short range V--V correlations that would be suggestive of local dimers is found in the metallic phase.
A pressure-induced simultaneous metal-insulator transition (MIT) and structural-phase transformation in lithium hydride with about 1% volume collapse has been predicted by means of the local density approximation (LDA) in conjunction with an all-electron GW approximation method. The LDA wrongly predicts that the MIT occurs before the structural phase transition. As a byproduct, it is shown that only the use of the generalized-gradient approximation together with the zero-point vibration produces an equilibrium lattice parameter, bulk modulus, and an equation of state that are in excellent agreement with experimental results.
Rutile ($R$) phase VO$_2$ is a quintessential example of a strongly correlated bad-metal, which undergoes a metal-insulator transition (MIT) concomitant with a structural transition to a V-V dimerized monoclinic phase below T$_{MIT} sim 340K$. It has been experimentally shown that one can control this transition by doping VO$_2$. In particular, doping with oxygen vacancies ($V_O$) has been shown to completely suppress this MIT {em without} any structural transition. We explain this suppression by elucidating the influence of oxygen-vacancies on the electronic-structure of the metallic $R$ phase VO$_2$, explicitly treating strong electron-electron correlations using dynamical mean-field theory (DMFT) as well as diffusion Monte Carlo (DMC) flavor of quantum Monte Carlo (QMC) techniques. We show that $V_O$s tend to change the V-3$d$ filling away from its nominal half-filled value, with the $e_{g}^{pi}$ orbitals competing with the otherwise dominant $a_{1g}$ orbital. Loss of this near orbital polarization of the $a_{1g}$ orbital is associated with a weakening of electron correlations, especially along the V-V dimerization direction. This removes a charge-density wave (CDW) instability along this direction above a critical doping concentration, which further suppresses the metal-insulator transition. Our study also suggests that the MIT is predominantly driven by a correlation-induced CDW instability along the V-V dimerization direction.
The flat band has attracted a lot of attention because it gives rise to many exotic phases, as recently demonstrated in magic angle twisted bilayer graphene. Here, based on first-principles calculations, we identify a metal-insulator transition in boron triangular Kagome lattice with a spin-polarized flat band at 2/3-filling. This phase transition is accompanied by the formation of a Wigner crystal, which is driven by Fermi surface nesting effect and thereby strong electron-phonon interactions, keeping ferromagnetism. Our calculation results suggest that boron triangular Kagome lattices with partially filled flat bands may open a new playground for many exotic quantum phases in two-dimensional systems, such as Winger crystallization and fractional quantum Hall states.
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