Janus-Faced Influence of Hund’s Rule Coupling in Strongly Correlated Materials

Abstract

We show that in multi-band metals the correlations are strongly affected by the Hund's rule coupling, which depending on the filling promotes metallic, insulating or bad-metallic behavior. The quasiparticle coherence and the proximity to a Mott insulator are influenced distinctly and, away from single-and half-filling, in opposite ways. A strongly correlated bad-metal far from a Mott phase is found there. We propose a concise classification of 3d and 4d transition-metal oxides within which the ubiquitous occurrence of strong correlations in Ru-and Cr-based oxides, as well as the recently measured high Néel temperatures in Tc-based perovskites are naturally explained.Hund's rules determine the ground-state of an isolated atom by accounting for the dependence of the Coulomb repulsion between electrons on their relative spin and orbital configurations. In insulating solids, their role is to select the relevant atomic multiplets, which are then coupled by inter-site magnetic interactions. In contrast, the effects of the Hund's rule coupling in metallic compounds are less understood. The difficulty lies in dealing with the localized (atomic) and itinerant characters of electrons on equal footing, a key issue for materials with strong electron correlations [1]. Despite increasing awareness of the physical relevance of the Hund's rule coupling for such materials [2][3][4][5][6][7][8], a global view is still lacking.In this article, we fill this gap and provide a classification with respect to the number of electrons filling the active orbitals. We show that, aside from the case of a singly-occupied shell (where metallicity is favored [4,8]), or a half-filled shell (where it promotes Mott insulating behaviour [9,10]) the Hund's rule coupling induces conflicting tendencies and thus causes strong correlations far from the insulating state ('bad-metal' behavior). This picture explains the observed physical properties of a number of transition-metal oxides and allows for predictions on novel ones, such as Technetium compounds.In order to describe all these possibilities and illustrate our key-point in a simple context, we consider a model of three identical bands with semicircular density-of-states of half-bandwidth D filled by N electrons per site. This is relevant, for example, to transition-metal oxides with cubic symmetry and a partially filled t 2g shell well separated from the empty e g shell. The standard interaction Hamiltonian [1] can be written as:whereN denotes total charge, S spin and T the angular momentum operators. U is the intra-orbital interaction and J is the Hund's rule coupling. The Hund's rule coupling favors, in decreasing order: configurations with parallel spins in different orbitals, with parallel spins in the same orbital, and with opposite spins in the same orbital, maximizing S and then T . We solve the model using dynamical mean-field theory (DMFT) [11] which maps a correlated electron system onto a quantum-impurity problem: an effective atom coupled to a self-consistent environment. This approa...

ARPES view of orbitally resolved quasiparticle lifetimes in iron pnictides

“…One expects E coh ZW and γ ∼1/ZW [42]. In iron pnictides, the situation is of course more complicated because of the interaction between the different bands, especially the coherence depends on the Hund's couplings J between them [44,45]. Nevertheless, it seems important to keep in mind that ZW can be very small in iron pnictides, and, in fact, significantly smaller for d xz /d yz than d xy .…”

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“…For the Cr site, we ignore SOC and find the atomic excitation gap to obtain U 1 = U 1 + 2J H1 [33]. The effective repulsion is enhanced by Hund's coupling, well known [34,35] for half-filled orbitals. On the Os site, we include λ 2 and find U 2 = U 2 + 2J H2 − 3ξ 2 /2 [33], so that SOC reduces the atomic charge gap.…”

Mott transition, spin-orbit effects, and magnetism in Ca2RuO4

“…1) is made of planes of corner-sharing RuO 6 octahedra. It belongs to the Ca 2−x Sr x RuO 4 family, well known for its exotic electronic properties, including, among others, spin-triplet superconductivity [1], Hund's coupling physics [2,3], heavy-fermion [4] and spin-glass behavior [5], as well as a series of structural, magnetic, and electronic phase transitions [6,7]. Ca 2 RuO 4 itself exhibits a peculiar paramagnetic metal-insulator transition [6] (MIT) at T MIT = 360 K, basically concurrent with the change from L-Pbca (long c axis) to S-Pbca (short c axis) structure [8,9] at T S = 356 K. Similar transitions have been reported when Ca is partially replaced by Sr (x 0.2) or under pressure [10,11].…”