2,332 research outputs found
Hund's metals, explained
A possible practical definition for a Hund's metal is given, as a metallic
phase - arising consistently in realistic simulations and experiments in
Fe-based superconductors and other materials - with three features: large
electron masses, high-spin local configurations dominating the paramagnetic
fluctuations and orbital-selective correlations. These features are triggered
by, and increase with the proximity to, a Hund's coupling-favored Mott
insulator that is realized for half-filled conduction bands. A clear crossover
line is found where these three features get enhanced, departing from the Mott
transition at half filling and extending in the interaction/doping plane,
between a normal (at weak interaction and large doping) and a Hund's metal (at
strong interaction and small doping). This phenomenology is found identically
in models with featureless bands, highlighting the generality of this physics
and its robustness by respect to the details of the material band structures.
Some analytical arguments are also given to gain insight into these defining
features. Finally the attention is brought on the recent theoretical finding of
enhanced/diverging electronic compressibility near the Hund's metal crossover,
pointing to enhanced quasiparticle interactions that can cause or boost
superconductivity or other instabilities.Comment: Lecture prepared for the Autumn School on Correlated Electrons, 25-29
September 2017, Juelich. To appear on: E. Pavarini, E. Koch, R. Scalettar,
and R. Martin (eds.) The Physics of Correlated Insulators, Metals, and
Superconductors Modeling and Simulation Vol. 7 Forschungszentrum Juelich,
2017, ISBN 978-3-95806-224-5 http://www.cond- mat.de/events/correl1
Hund-enhanced electronic compressibility in FeSe and its correlation with T
We compute the compressibility of the conduction electrons in both bulk
orthorhombic FeSe and monolayer FeSe on SrTiO substrate, including
dynamical electronic correlations within slave-spin mean-field +
density-functional theory. Results show a zone of enhancement of the electronic
compressibility crossing the interaction-doping phase diagram of these
compounds in accord with previous simulations on iron pnictides and in general
with the phenomenology of Hund's metals. Interestingly at ambient pressure FeSe
is found slightly away from the zone with enhanced compressibility but moved
right into it with hydrostatic pressure, while in monolayer FeSe the stronger
enhancement region is realized on the electron-doped side. These findings
correlate positively with the enhancement of superconductivity seen in
experiments, and support the possibility that Hund's induced many-body
correlations boost superconductive pairing when the system is at the frontier
of the normal- to Hund's-metal crossover.Comment: 6 pages, 2 figure
Non-Fermi Liquid Behavior and Double-Exchange Physics in Orbital-Selective Mott Systems
We study a multi-band Hubbard model in its orbital selective Mott phase, in
which localized electrons in a narrow band coexist with itinerant electrons in
a wide band. The low-energy physics of this phase is shown to be closely
related to that of a generalized double-exchange model. The high-temperature
disordered phase thus differs from a Fermi liquid, and displays a finite
scattering rate of the conduction electrons at the Fermi level, which depends
continuously on the spin anisotropy.Comment: 5 pages, minor typos correcte
Slave spin cluster mean field theory away from half-filling: Application to the Hubbard and the extended Hubbard Model
A new slave-spin representation of fermion operators has recently been
proposed for the half-filled Hubbard model. We show that with the addition of a
gauge variable, the formalism can be extended to finite doping. The resulting
spin problem can be solved using the cluster mean-field approximation. This
approximation takes short-range correlations into account by exact
diagonalization on the cluster, whereas long-range correlations beyond the size
of clusters are treated at the mean-field level. In the limit where the cluster
has only one site and the interaction strength is infinite, this approach
reduces to the Gutzwiller approximation. There are some qualitative differences
when the size of the cluster is finite. We first compute the critical for
the Mott transition as a function of a frustrating second-neighbor interaction
on lattices relevant for various correlated systems, namely the cobaltites, the
layered organic superconductors and the high-temperature superconductors. For
the triangular lattice, we also study the extended Hubbard model with
nearest-neighbor repulsion. In additionto a uniform metallic state, we find a
charge density wave in a broad doping regime,
including commensurate ones. We find that in the large limit, intersite
Coulomb repulsion strongly suppresses the single-particle weight of the
metallic state.Comment: 10 pages, 11 figures, submitted to PR
Role of oxygen-oxygen hopping in the three-band copper-oxide model: quasiparticle weight, metal insulator and magnetic phase boundaries, gap values and optical conductivity
We investigate the effect of oxygen-oxygen hopping on the three-band
copper-oxide model relevant to high- cuprates, finding that the physics is
changed only slightly as the oxygen-oxygen hopping is varied. The location of
the metal-insulator phase boundary in the plane of interaction strength and
charge transfer energy shifts by eV or less along the charge transfer
axis, the quasiparticle weight has approximately the same magnitude and doping
dependence and the qualitative characteristics of the electron-doped and
hole-doped sides of the phase diagram do not change. The results confirm the
identification of LaCuO as a material with intermediate correlation
strength. However, the magnetic phase boundary as well as higher-energy
features of the optical spectrum are found to depend on the magnitude of the
oxygen-oxygen hopping. We compare our results to previously published one-band
and three-band model calculations.Comment: 13.5 pages, 16 figure
Antiferromagnetism and the gap of a Mott insulator: Results from analytic continuation of the self-energy
Direct analytic continuation of the self energy is used to determine the
effect of antiferromagnetic ordering on the spectral function and optical
conductivity of a Mott insulator. Comparison of several methods shows that the
most robust estimation of the gap value is obtained by use of the real part of
the continued self energy in the quasiparticle equation within the single-site
dynamical mean field theory of the two dimensional square lattice Hubbard
model, where for U slightly greater than the Mott critical value,
antiferromagnetism increases the gap by about 80%.Comment: 8 pages, 9 figures. An error in normalization of optical conductivity
(Fig. 9) corrected. to appear in Phys. Rev.
Supersolidity, entropy and frustration
We study the properties of t-t'-V model of hard-core bosons on the triangular
lattice that can be realized in optical lattices. By mapping to the spin-1/2
XXZ model in a field, we determine the phase diagram of the t-V model where the
supersolid characterized by the ordering pattern (x,x,-2x') ("ferrimagnetic" or
SS A) is a ground state for chemical potential \mu >3V. By turning on either
temperature or t' at half-filling \mu =3V, we find a first order transition
from SS A to the elusive supersolid characterized by the (x,-x,0) ordering
pattern ("antiferromagnetic" or SS C). In addition, we find a large region
where a superfluid phase becomes a solid upon raising temperature at fixed
chemical potential. This is an analog of the Pomeranchuk effect driven by the
large entropic effects associated with geometric frustration on the triangular
lattice.Comment: 4 pages, igures, LaTe
Charge Disproportionation, Mixed Valence, and Janus Effect in Multiorbital Systems: A Tale of Two Insulators
Multiorbital Hubbard models host strongly correlated "Hund's metals" even for interactions much stronger than the bandwidth. We characterize this interaction-resilient metal as a mixed-valence state. In particular, it can be pictured as a bridge between two strongly correlated insulators: a high-spin Mott insulator and a charge-disproportionated insulator which is stabilized by a very large Hund's coupling. This picture is confirmed comparing models with negative and positive Hund's coupling for different fillings. Our results provide a characterization of the Hund's metal state and connect its presence with charge disproportionation, which has indeed been observed in chromates and proposed to play a role in iron-based superconductors
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