Strong electronic correlations are often associated with the proximity of a Mott insulating state. In recent years however, it has become increasingly clear that the Hund's rule coupling (intra-atomic exchange) is responsible for strong correlations in multi-orbital metallic materials which are not close to a Mott insulator. Hund's coupling has two effects: it influences the energetics of the Mott gap and strongly suppresses the coherence scale for the formation of a Fermi-liquid. A global picture has emerged recently, which emphasizes the importance of the average occupancy of the shell as a control parameter. The most dramatic effects occur away from half-filling or single occupancy. The theoretical understanding and physical properties of these 'Hund's metals' are reviewed, together with the relevance of this concept to transition-metal oxides of the 3d, and especially 4d series (such as ruthenates), as well as to the iron-based superconductors (iron pnictides and chalcogenides).

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...

We calculate the electronic structure of Sr(2)RuO(4), treating correlations within dynamical mean-field theory. The approach successfully reproduces several experimental results and explains the key properties of this material: the anisotropic mass renormalization of quasiparticles and the crossover into an incoherent regime above a low temperature scale. While the orbital differentiation originates from the proximity of the van Hove singularity, strong correlations are caused by the Hund's coupling. The generality of this mechanism for other correlated materials is pointed out.

We investigate transport in strongly correlated metals. Within dynamical mean-field theory, we calculate the resistivity, thermopower, optical conductivity and thermodynamic properties of a hole-doped Mott insulator. Two well-separated temperature scales are identified: T(FL) below which Landau Fermi liquid behavior applies, and T(MIR) above which the resistivity exceeds the Mott-Ioffe-Regel value and bad-metal behavior is found. We show that quasiparticle excitations remain well defined above T(FL) and dominate transport throughout the intermediate regime T(FL)

International audienceWe present the TRIQS/DFTTools package, an application based on the TRIQS library that connects this toolbox to realistic materials calculations based on density functional theory (DFT). In particular, TRIQS/DFTTools together with TRIQS allows an efficient implementation of DFT plus dynamical mean-field theory (DMFT) calculations. It supplies tools and methods to construct Wannier functions and to perform the DMFT self-consistency cycle in this basis set. Post-processing tools, such as band-structure plotting or the calculation of transport properties are also implemented. The package comes with a fully charge self-consistent interface to the Wien2k band structure code, as well as a generic interface that allows to use TRIQS/DFTTools together with a large variety of DFT codes. It is distributed under the GNU General Public License (GPLv3)

The existence of a quantum spin liquid (QSL) in which quantum fluctuations of spins are sufficiently strong to preclude spin ordering down to zero temperature was originally proposed theoretically more than 40 years ago, but its experimental realisation turned out to be very elusive. Here we report on an almost ideal spin liquid state that appears to be realized by atomic-cluster spins on the triangular lattice of a charge-density wave (CDW) state of 1T-TaS 2 . In this system, the charge excitations have a well-defined gap of ∼ 0.3 eV, while nuclear magnetic quadrupole resonance and muon spin relaxation experiments reveal that the spins show gapless quantum spin liquid dynamics and no long range magnetic order down to 70 mK. Canonical T 2 power-law temperature dependence of the spin relaxation dynamics characteristic of a QSL is observed from 200 K to T f = 55 K. Below this temperature we observe a new gapless state with reduced density of spin excitations and high degree of local disorder signifying new quantum spin order emerging from the QSL.arXiv:1704.06450v1 [cond-mat.str-el]

We investigate the interplay of spin-orbit coupling (SOC) and electronic correlations in Sr_{2}RuO_{4} using dynamical mean-field theory. We find that SOC does not affect the correlation-induced renormalizations, which validates Hund's metal picture of ruthenates even in the presence of the sizable SOC relevant to these materials. Nonetheless, SOC is found to change significantly the electronic structure at k points where a degeneracy applies in its absence. We explain why these two observations are consistent with one another and calculate the effects of SOC on the correlated electronic structure. The magnitude of these effects is found to depend on the energy of the quasiparticle state under consideration, leading us to introduce the notion of an energy-dependent quasiparticle spin-orbit coupling λ^{*}(ω). This notion is generally applicable to all materials in which both the spin-orbit coupling and electronic correlations are sizable.

Using analytical arguments and the numerical renormalization group method we investigate the spin-thermopower of a quantum dot in a magnetic field. In the particle-hole symmetric situation the temperature difference applied across the dot drives a pure spin current without accompanying charge current. For temperatures and fields at or above the Kondo temperature, but of the same order of magnitude, the spin-Seebeck coefficient is large, of the order of kB/|e|. Via a mapping, we relate the spin-Seebeck coefficient to the charge-Seebeck coefficient of a negative-U quantum dot where the corresponding result was recently reported by Andergassen et al. in Phys. Rev. B 84, 241107 (2011). For several regimes we provide simplified analytical expressions. In the Kondo regime, the dependence of the spin-Seebeck coefficient on the temperature and the magnetic field is explained in terms of the shift of the Kondo resonance due to the field and its broadening with the temperature and the field. We also consider the influence of breaking the particle-hole symmetry and show that a pure spin current can still be realized provided a suitable electric voltage is applied across the dot. Then, except for large asymmetries, the behavior of the spin-Seebeck coefficient remains similar to that found in the particle-hole symmetric point.