Reproducing dynamical excitations observed in resonant inelastic X-ray scattering through Bethe-Salpeter calculations
le vendredi 25 mai 2018 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Resonant inelastic X-ray scattering (RIXS) is a relatively new technique for probing low energy excitations in materials. In addition to traditional techniques, such as angle resolved photoemission, it has become an important, high precision characterization tool of strongly correlated electron materials. To calculate RIXS, and related core and valence level spectra, we solve the Bethe-Salpeter equation (BSE) based on a self-energy corrected density functional theory electronic structure. I outline our implementation of the BSE and use SrVO3 for demonstration. Non fluorescence features in RIXS arise from a dynamic response of the system to the intermediate state perturbation. Since the Bethe-Salpeter equation is typically reduced to the static limit in practice, these dynamic excitations are generally not reproduced. To include interactions beyond the static BSE I introduce the cumulant expansion. Spectral functions derived from a GW self-energy are typically inadequate when the dressed Green’s function is built via the Dyson equation. With the same GW self-energy, a superior Green’s function and spectral function, implicitly including vertex corrections, is obtained through the cumulant expansion. I consider application of cumulant spectral functions to photoemission, photoabsorption, and X-ray scattering. Lastly, vibronic coupling has important impacts on these spectra. I show how to calculation the phonon contribution to photoemission, absorption and scattering with a vibronic cumulant.

Vladimir Dobrosavljevic (Florida State University)

Quantum Critical Behavior at the Mott Point: the Status Quo
le vendredi 1er juin 2018 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : According to early ideas of Mott and Anderson, the interaction-driven metal-insulator transition – the Mott transition – remains a sharp T=0 phase transition even in absence of any spin or charge ordering. Should this phase transition be regarded as a quantum critical point? This basic question has long remained controversial, although it bears direct relevance to many puzzling phenomena in strongly correlated electronic systems. This talk will provide a brief overview of several new theoretical ideas and methods, developed in the last 25 years, that shed new light on this important problem. Especially useful information has very recently been obtained from the study of a class of “maximally frustrated” Hubbard models, which can be solved using non-perturbative methods of Dynamical Mean-Field Theory (DMFT). This theory identified the relevant quantum critical region associated with the Mott metal-insulator transition and found remarkable scaling behavior of transport properties, characteristic of quantum criticality. Precisely this kind of behavior was in very recent experiments on organic Mott systems [1,2]. The detailed mapping of this family of experimental systems to the theoretical phase diagram, identifying the Quantum Widom Line as the relevant organizing principle, has been provided by very recent optical conductivity studies [3]. Further relevance of these ideas for other systems will also be discussed, including the long-standing chicken-and-egg problem of the Mott-Pierls transition in VO2 [4].
[1] Quantum criticality of Mott transition in organic materials, Tetsuya Furukawa, K. Miyagawa, H. Taniguchi, R. Kato & K. Kanoda, Nature Physics 11, 221–224 (2015); See also: http://condensedconcepts.blogspot.ae/2015/03/quantum-criticality-near-mott.html
[3] Quantum spin liquids unveil the genuine Mott state, A. Pustogow, M. Bories, A. Löhle, R. Rösslhuber, E. Zhukova, B. Gorshunov, S. Tomić, J.A. Schlueter, R. Hübner, T. Hiramatsu, Y. Yoshida, G. Saito, R. Kato, T.-H. Lee, V. Dobrosavljević, S. Fratini, M. Dressel, arXiv:1710.07241.
[4] Resolving the VO2 controversy: Mott mechanism dominates the insulator-to-metal transition, O. Nájera, M. Civelli, V. Dobrosavljević, and M. J. Rozenberg, Phys. Rev. B 95, 035113 (2017); Phys. Rev. B 97, 045108 (2018).