96 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
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.
Janus-faced influence of the Hund's rule coupling in strongly correlated materials
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
N\'eel temperatures in Tc-based perovskites are naturally explained.Comment: 4 pages + supplementary materia
Rationalizing doping and electronic correlations in LaFeAs
We compute the electronic properties of the normal state of uncollapsed
LaFeAs, taking into account local dynamical correlations by means of
slave-spin mean-field+density-functional theory. Assuming the same local
interaction strength used to model the whole electron- and hole-doped
BaFeAs family, our calculations reproduce the experimental Sommerfeld
specific heat coefficient, which is twice the value predicted by uncorrelated
band theory. We find that LaFeAs has a reduced bare bandwidth and this
solves the apparent paradox of its sizeable correlations despite its nominal
valence d, which would imply extreme overdoping and uncorrelated
behaviour in BaFeAs. Our results yield a consistent picture of the
whole 122 family and point at the importance of the correlation strength,
rather than sheer doping, in the interpretation of the phase diagram of
iron-based superconductorsComment: 5 pages, 4 figure
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
A continuous-time solver for quantum impurity models
We present a new continuous time solver for quantum impurity models such as
those relevant to dynamical mean field theory. It is based on a stochastic
sampling of a perturbation expansion in the impurity-bath hybridization
parameter. Comparisons to quantum Monte Carlo and exact diagonalization
calculations confirm the accuracy of the new approach, which allows very
efficient simulations even at low temperatures and for strong interactions. As
examples of the power of the method we present results for the temperature
dependence of the kinetic energy and the free energy, enabling an accurate
location of the temperature-driven metal-insulator transition.Comment: Published versio
Orbital-Selective Mott transition out of band degeneracy lifting
We outline a general mechanism for Orbital-selective Mott transition (OSMT),
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.Comment: Published versio
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