Orbital-Selective Mott Transition out of Band Degeneracy Lifting

Abstract

We outline a general mechanism for orbital-selective Mott transition, the coexistence of both itinerant and localized conduction electrons, and show how it can take place in a wide range of realistic situations, even for bands of identical width and correlation, provided a crystal field splits the energy levels in manifolds with different degeneracies and the exchange coupling is large enough to reduce orbital fluctuations. The mechanism relies on the different kinetic energy in manifolds with different degeneracy. This phase has Curie-Weiss susceptibility and non-Fermi-liquid behavior, which disappear at a critical doping, all of which is reminiscent of the physics of the pnictides.

Photoemission studies of the near EFspectral weight shifts in FeSe1−xTexsuperconductor

“…This is a temperature induced crossover from a metallic state in which all the t 2g orbitals (xy, xz/yz) near the E F are occupied to a state in which occupancy of the xz/yz orbitals are depleted, signifying a Mott transition. This kind of spectral weight transfer which is a characteristic of Mottness [27], has earlier been identified as Orbital Selective Mott Transition (OSMT) [8]. Our study presents the first observation of a temperature induced crossover to a low temperature OSM phase in the family of Fe chalcogenides although such phenomena was earlier observed in A x Fe 2−y Se 2 (A = K, Rb) where a high temperature OSM phase was identified [28].…”

“…Furthermore, the electronic properties of these superconductors with their moderate electron correlations, are controlled mostly by the competing inter-and intra-orbital interactions near the E F region. It has been pointed out [8] that these interactions generate an Orbital Selective Mott Phase (OSMP) in which electrons in some orbitals are Mott localized while others remain itinerant. Earlier, such an OSMP was identified in Ca 2−x Sr x RuO 4 [9,10].…”

Slave Boson Theory of Orbital Differentiation with Crystal Field Effects: Application toUO2

“…The fact that the overall occupancy of the 5f levels deviates considerably from an integer value confirms the importance of covalency effects in UO 2 , which has been pointed out also in previous experimental and theoretical studies [46][47][48][49]. Note also that the Mott-localized Γ 8 degrees of freedom have occupancy close to integer, which is a factor that is known to promote localization [3]. Let us now address the question of what is the physical origin of the strong CFS orbital differentiation in UO 2 .…”

“…Let us now address the question of what is the physical origin of the strong CFS orbital differentiation in UO 2 . The first important observation is that the importance of the CFS splittings in UO 2 is not related with the U5f crystal fields (on-site energy splittings) [2][3][4], which are very small in this material (∼ 7 meV ). In fact, a direct calculation shows that neglecting the CFS contributions to the on-site energy splittings [17] does not affect sensibly any of the results considered above (data not shown).…”

“…We apply our theory to the archetypical nuclear fuel UO2, and show that the ground state of this system displays a pronounced orbital differention within the 5f manifold, with Mott localized Γ8 and extended Γ7 electrons. Orbital differentiation, where states with different orbital character exhibit different levels of correlation, is a pervasive phenomena in condensed matter systems [1][2][3][4], which gives rise to multiple functionalities in strongly correlated multiorbital systems. In all known Mott systems in nature only a fraction of electrons form localized magnetic moments, while the other electronic states are extended (but away from the Fermi level).…”