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Register a new userA simple metal-insulator criterion for the doped Mott-Hubbard materials
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Added by
Vladimir Gavrichkov A.
Publication date
2014
fields
Physics
and research's language is
English
Authors
V. A. Gavrichkov
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We derived a simple metal-insulator criterion in analytical form for the doped Mott-Hubbard materials. Its readings closely related to the orbital and spin nature of the ground states of the unit cell. The available criterion readings (metal or insulator) in the paramagnetic phase points to the possibility of the insulator state of doped materials with the forbidden first removal electron states. According to its physical meaning the result is similar to Wilsons criterion in the itinerant electron systems. An application of the criterion to the high-Tc cuprates discussed.
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The temperature ($T$) dependent metal-insulator transition (MIT) in VO$_2$ is investigated using bulk sensitive hard x-ray ($sim$ 8 keV) valence band, core level, and V 2$p-3d$ resonant photoemission spectroscopy (PES). The valence band and core level spectra are compared with full-multiplet cluster model calculations including a coherent screening channel. Across the MIT, V 3$d$ spectral weight transfer from the coherent ($d^1underbar{it {C}}$ final) states at Fermi level to the incoherent ($d^{0}+d^1underbar{it {L}}$ final) states, corresponding to the lower Hubbard band, lead to gap-formation. The spectral shape changes in V 1$s$ and V 2$p$ core levels as well as the valence band are nicely reproduced from a cluster model calculations, providing electronic structure parameters. Resonant-PES finds that the $d^1underbar{it{L}}$ states resonate across the V 2$p-3d$ threshold in addition to the $d^{0}$ and $d^1underbar{it {C}}$ states. The results support a Mott-Hubbard transition picture for the first order MIT in VO$_2$.
The spectral weight evolution of the low-dimensional Mott insulator TiOCl upon alkali-metal dosing has been studied by photoelectron spectroscopy. We observe a spectral weight transfer between the lower Hubbard band and an additional peak upon electron-doping, in line with quantitative expectations in the atomic limit for changing the number of singly and doubly occupied sites. This observation is an unconditional hallmark of correlated bands and has not been reported before. In contrast, the absence of a metallic quasiparticle peak can be traced back to a simple one-particle effect.
We point out the generic competition between the Hunds coupling and the spin-orbit coupling in correlated materials, and this competition leads to an electronic dilemma between the Hunds metal and the relativistic insulators. Hunds metals refer to the fate of the would-be insulators where the Hunds coupling suppresses the correlation and drives the systems into correlated metals. Relativistic Mott insulators refer to the fate of the would-be metals where the relativistic spin-orbit coupling enhances the correlation and drives the systems into Mott insulators. These contradictory trends are naturally present in many correlated materials. We study the competition between Hunds coupling and spin-orbit coupling in correlated materials and explore the interplay and the balance from these two contradictory trends. The system can become a spin-orbit-coupled Hunds metal or a Hunds assisted relativistic Mott insulator. Our observation could find a broad application and relevance to many correlated materials with multiple orbitals.
Detailed understanding of the role of single dopant atoms in host materials has been crucial for the continuing miniaturization in the semiconductor industry as local charging and trapping of electrons can completely change the behaviour of a device. Similarly, as dopants can turn a Mott insulator into a high temperature superconductor, their electronic behaviour at the atomic scale is of much interest. Due to limited time resolution of conventional scanning tunnelling microscopes, most atomic scale studies in these systems focussed on the time averaged effect of dopants on the electronic structure. Here, by using atomic scale shot-noise measurements in the doped Mott insulator Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$, we visualize sub-nanometer sized objects where remarkable dynamics leads to an enhancement of the tunnelling current noise by at least an order of magnitude. From the position, current and energy dependence we argue that these defects are oxygen dopant atoms that were unaccounted for in previous scanning probe studies, whose local environment leads to charge dynamics that strongly affect the tunnelling mechanism. The unconventional behaviour of these dopants opens up the possibility to dynamically control doping at the atomic scale, enabling the direct visualization of the effect of local charging on e.g. high T$_{text{c}}$ superconductivity.
Despite many efforts to rationalize the strongly correlated electronic ground states in doped Mott insulators, the nature of the doping induced insulator to metal transition is still a subject under intensive investigation. Here we probe the nanoscale electronic structure of the Mott insulator Sr$_2$IrO$_{4-delta}$ with low-temperature scanning tunneling microscopy and find enhanced local density of states (LDOS) inside the Mott gap at the location of individual apical oxygen site defects. We visualize paths of enhanced conductance arising from the overlapping of defect states which induces finite LDOS at the Fermi level. By combining these findings with the typical spatial extension of isolated defects of about 2~nm, we show that the insulator to metal transition in Sr$_2$IrO$_{4-delta}$ is of percolative nature.
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