The magnetically driven metal-insulator transition (MIT) was predicted by Slater in the fifties. Here a long-range antiferromagnetic (AF) order can open up a gap at the Brillouin electronic band boundary regardless of the Coulomb repulsion magnitude. However, while many low-dimensional organic conductors display evidence for an AF driven MIT, in three-dimensional (3D) systems the Slater MIT still remains elusive. We employ terahertz and infrared spectroscopy to investigate the MIT in the NaOsO3 3D antiferromagnet. From the optical conductivity analysis we find evidence for a continuous opening of the energy gap, whose temperature dependence can be well described in terms of a second order phase transition. The comparison between the experimental Drude spectral weight and the one calculated through Local Density Approximation (LDA) shows that electronic correlations play a limited role in the MIT. All the experimental evidence demonstrates that NaOsO3 is the first known 3D Slater insulator.
LiOsO3 has been recently identified as the first unambiguous "ferroelectric metal", experimentally realizing a prediction from 1965 by Anderson and Blount. In this work, we investigate the metallic state in LiOsO3 by means of infrared spectroscopy supplemented by Density Functional Theory and Dynamical Mean Field Theory calculations. Our measurements and theoretical calculations clearly show that LiOsO3 is a very bad metal with a small quasiparticle weight, close to a Mott-Hubbard localization transition. The agreement between experiments and theory allows us to ascribe all the relevant features in the optical conductivity to strong electron-electron correlations within the t2g manifold of the osmium atoms. Introduction -Ferroelectric materials display a spontaneous polarization due to an inversion symmetry breaking induced by the macroscopic ordering of local dipole moments. Multiferroic materials are defined by coexistent and coupled magnetic and ferroelectric order. It is a natural expectation that ferroelectric (and consequently multiferroic) ordering can only happen in insulators in order to avoid the metallic charge carriers to screen out the ferroelectric polarization. The existence of "ferroelectric metals" challenging this expectation has been hypothesized in 1965 by Anderson and Blount [1], who have shown that ferroelectricity can occur in a metal as long as the electrons at the Fermi level are decoupled from the ferroelectric distortions. Under these circumstances, the ferroelectric ordering can take place through a second-order transition. In 2013 the first unambiguous realization of this proposal has been reported in LiOsO 3 , where a second-order transition leads to a ferroelectric ionic structure below 140 K while the material remains conducting [2]. This behavior is accompanied by a large residual resistivity which exceeds by two orders of magnitude that of prototypical metals as gold, and by a CurieWeiss like behavior of the magnetic susceptibility in the ordered phase which suggests the presence of almost localized magnetic moments, characteristic precursors of Mott localization [2].
Vanadium sesquioxide V2O3 is considered a textbook example of Mott-Hubbard physics. In this paper we present an extended optical study of its whole temperature/doping phase diagram as obtained by doping the pure material with M=Cr or Ti atoms (V1−xMx)2O3. We reveal that its thermodynamically stable metallic and insulating phases, although macroscopically equivalent, show very different low-energy electrodynamics. The Cr and Ti doping drastically change both the antiferromagnetic gap and the paramagnetic metallic properties. A slight chromium content induces a mesoscopic electronic phase separation, while the pure compound is characterized by short-lived quasiparticles at high temperature. This study thus provides a new comprehensive scenario of the Mott-Hubbard physics in the prototype compound V2O3.
Using angle resolved photoemission spectroscopy (ARPES) we report the first band dispersions and distinct features of the bulk Fermi surface (FS) in the paramagnetic metallic phase of the prototypical metal-insulator transition material V2O3. Along the c-axis we observe both an electron pocket and a triangular hole-like FS topology, showing that both V 3d a1g and e π g states contribute to the FS. These results challenge the existing correlation-enhanced crystal field splitting theoretical explanation for the transition mechanism and pave the way for the solution of this mystery.PACS numbers: 79.60.-i,71.27.+a,71.30.+h Since its seminal report in 1969 [1][2][3], the metalinsulator transition (MIT) in the alloy system (V 1−x Cr x ) 2 O 3 has stood as a mystery for many decades. For x=0 and with decreasing temperature (T ) there is a transition from a paramagnetic metal (PM) to an antiferromagnetic insulator (AFI). With increasing x the AFI phase persists but the PM phase gives way to a paramagnetic insulator (PI) along an (x, T ) line terminating at higher T in a critical point. The initial identification of the latter transition as the long sought experimental example of the Mott MIT [4] inherent in the one-band Hubbard model was quickly challenged [5] on the grounds that the complexity of the actual multi-orbital electronic structure must be essential for the transition. This complexity consists of four V 3+ions per rhombohedral unit cell with each ion having two 3d electrons to distribute in the two lowest energy trigonal crystal field split 3d states, an orbital singlet a 1g and an orbital doublet e π g . Scenarios for reducing this complexity back to a one-band model [6] were eventually abandoned after X-ray absorption spectroscopy (XAS) showed that the V 3+ ions are in a Hund's rule S=1 state and that both the e π g and a 1g states are always occupied, albeit with different occupation ratios e/a(=e π g :a 1g ) in the three phases [7].The advent of dynamic mean field theory (DMFT) [8] combined [9] with band structure from density functional theory (DFT), supported by bulk sensitive angle integrated photoemission spectra for (V 1−x Cr x ) 2 O 3 [10, 11], gave the first real hope that the mystery could be solved within a realistic multi-orbital calculation. Indeed a series of DFT+ DMFT studies in the first decade of this century [12][13][14] gradually coalesced around a narrative in which the MIT is enabled by a strong many-body enhancement of the trigonal crystal field splitting and thus the orbital polarization of the quasi-particle (QP) bands based on the e π g and a 1g states. In 2007 the claim in Ref. [14] "to have demystified the nature of the metalinsulator transition in V 2 O 3 " seemed well justified by the consensus. The study suggested that in the PM phase the Fermi surface (FS) is formed entirely from an a 1g QP band, while all the e π g QP bands lie entirely below the Fermi energy, E F , a scenario consistent with electron counting only by virtue of QP weights sufficiently reduced from...
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