G. Sangiovanni

A microscopic view on the Mott transition in chromium-doped V 2 O 3

V(2)O(3) is the prototype system for the Mott transition, one of the most fundamental phenomena of electronic correlation. Temperature, doping or pressure induce a metal-to-insulator transition (MIT) between a paramagnetic metal (PM) and a paramagnetic insulator. This or related MITs have a high technological potential, among others, for intelligent windows and field effect transistors. However the spatial scale on which such transitions develop is not known in spite of their importance for research and applications. Here we unveil for the first time the MIT in Cr-doped V(2)O(3) with submicron lateral resolution: with decreasing temperature, microscopic domains become metallic and coexist with an insulating background. This explains why the associated PM phase is actually a poor metal. The phase separation can be associated with a thermodynamic instability near the transition. This instability is reduced by pressure, that promotes a genuine Mott transition to an eventually homogeneous metallic state.

Oxide heterostructures for efficient solar cells.

We propose an unexplored class of absorbing materials for high-efficiency solar cells: heterostructures of transition-metal oxides. In particular, LaVO(3) grown on SrTiO(3) has a direct band gap ∼1.1  eV in the optimal range as well as an internal potential gradient, which can greatly help to separate the photogenerated electron-hole pairs. Furthermore, oxide heterostructures afford the flexibility to combine LaVO(3) with other materials such as LaFeO(3) in order to achieve even higher efficiencies with band-gap graded solar cells. We use density-functional theory to demonstrate these features.

Dichotomy between Large Local and Small Ordered Magnetic Moments in Iron-Based Superconductors

We study a four-band model for iron-based superconductors within the local density approximation combined with dynamical mean-field theory (LDA+DMFT). This successfully reproduces the results of models which take As p degrees of freedom explicitly into account and has several physical advantages over the standard five d-band model. Our findings reveal that the new superconductors are more strongly correlated than their single-particle properties suggest. Two-particle correlation functions unveil the dichotomy between local and ordered magnetic moments in these systems, calling for further experiments to better resolve the short time scale spin dynamics.

Raman-scattering measurements and theory of the energy-momentum spectrum for underdoped Bi2Sr2CaCuO(8+δ) superconductors: evidence of an s-wave structure for the pseudogap.

S. Sakai , S. Blanc, M. Civelli, Y. Gallais, M. Cazayous , M.-A. Méasson, J. S. Wen, Z. J. Xu, G. D. Gu, G. Sangiovanni , Y. Motome, K. Held, A. Sacuto, A. Georges , and M. Imada Centre de Physique Théorique, École Polytechnique, CNRS, 91128 Palaiseau, France, Department of Applied Physics, University of Tokyo, Hongo, Tokyo 113-8656, Japan, JST-CREST, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan, Laboratoire Matériaux et Phénom̀ enes Quantiques (UMR 7162 CNRS), Université Paris Diderot-Paris 7, Bat. Condorcet, 75205 P aris Cedex 13, France, Laboratoire de Physique des Solides, Université Paris-Su d, CNRS, UMR 8502, F-91405 Orsay Cedex, France, Matter Physics and Materials Science, Brookhaven National Laboratory (BNL), Upton, NY 11973, USA, Institut für Theoretische Physik und Astrophysik, Univer sität Würzburg, Am Hubland, D-97074 Würzburg, Germany, Institute for Solid State Physics, Vienna University of Tec hnology, 1040 Vienna, Austria, Collège de France, 11 place Marcelin Berthelot, 75005 Pari s, France, DPMC, Université de Genève, 24 quai Ernest Ansermet, CH-1 211 Genève, Suisse (Dated: May 2, 2014)

Inequivalent routes across the Mott transition in V2O3 explored by X-ray absorption.

The changes in the electronic structure of V2O3 across the metal-insulator transition induced by temperature, doping, and pressure are identified using high resolution x-ray absorption spectroscopy at the V pre-K edge. Contrary to what has been taken for granted so far, the metallic phase reached under pressure is shown to differ from the one obtained by changing doping or temperature. Using a novel computational scheme, we relate this effect to the role and occupancy of the a{1g} orbitals. This finding unveils the inequivalence of different routes across the Mott transition in V2O3.

Photoemission kinks and phonons in cuprates.

Arising from: F. Giustino, M. L. Cohen & S. G. Louie 452, 975–978 (2008)10.1038/nature06874One of the possible mechanisms of high transition temperature (Tc) superconductivity is Cooper pairing with the help of bosons, which change the slope of the electronic dispersion as observed by photoemission. Giustino et al. calculated that in the high temperature superconductor La1.85Sr0.15CuO4 crystal lattice vibrations (phonons) should have a negligible effect on photoemission spectra and concluded that phonons do not have an important role. Here we show that the calculations used by Giustino et al. do not reproduce the huge influence of electron–phonon coupling on important phonons observed in experiments. Thus, we would similarly expect that these calculations do not explain the role of electron–phonon coupling for the electronic dispersion.

Phase Separation Close to the Density-Driven Mott Transition in the Hubbard-Holstein Model

The density-driven Mott transition is studied by means of dynamical mean-field theory in the Hubbard-Holstein model, where the Hubbard term leading to the Mott transition is supplemented by an electron-phonon (e-ph) term. We show that an intermediate e-ph coupling leads to a first-order transition at T=0, which is accompanied by a phase separation between a metal and an insulator. The compressibility in the metallic phase is substantially enhanced. At quite larger values of the coupling, a polaronic phase emerges coexisting with a nonpolaronic metal.

Relevance of hybridization and filling of 3d orbitals for the Kondo effect in transition metal phthalocyanines.

Magnetic organic molecules, such as 3d transition metal phthalocyanines (TMPc), exhibit properties which make them promising candidates for future applications in magnetic data storage or spin-based data processing. Due to their small size, however, TMPc molecules are prone to quantum effects. For example, the interaction of uncompensated molecular spins with conduction electrons of the substrate may lead to the formation of a many-body singlet state, which gives rise to the so-called Kondo effect. Although the Kondo effect of TMPc molecules has been the object of several investigations, a consistent picture to describe under which conditions a Kondo state is formed is still missing. Here, we study the Kondo properties of MnPc on Ag(001) by means of the low-temperature scanning tunneling spectroscopy (LT-STS) measurements. Differential conductance dI/dU spectra reveal a zero-bias peak that is localized on the Mn ion site. Ab initio calculations combined with a many-body treatment of the multiorbital interaction show that the local Hund coupling favors the high-spin configuration on the 3d shell of the central TM atom. Therefore, each orbital gets close to its individual half-filling creating the necessary condition for many of the 3d orbitals to contribute to the observed Kondo resonance. This, however, happens only for the 3dz(2) orbital, whose hybridization to the substrate is much stronger than for the other orbitals thanks to its shape and its orientation.

Conserved quantities of S U ( 2 ) -invariant interactions for correlated fermions and the advantages for quantum Monte Carlo simulations

In the context of realistic calculations for strongly correlated materials with

Electronics with Correlated Oxides: SrVO(3)/SrTiO(3) as a Mott Transistor.

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