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

The Political Economy of Reform in Brazil’s Civil Servant Pension Scheme

Brazil, pension, political economy, social security

An Unsupervised Method for Estimating the Global Horizontal Irradiance from Photovoltaic Power Measurements

In this paper, we present a method to determine the global horizontal irradiance (GHI) from the power measurements of one or more PV systems, located in the same neighborhood. The method is completely unsupervised and is based on a physical model of a PV plant. The precise assessment of solar irradiance is pivotal for the forecast of the electric power generated by photovoltaic (PV) plants. However, on-ground measurements are expensive and are generally not performed for small and medium-sized PV plants. Satellite-based services represent a valid alternative to on site measurements, but their space-time resolution is limited. Results from two case studies located in Switzerland are presented. The performance of the proposed method at assessing GHI is compared with that of free and commercial satellite services. Our results show that the presented method is generally better than satellite-based services, especially at high temporal resolutions

Hund-enhanced electronic compressibility in FeSe and its correlation with Tc_c

We compute the compressibility of the conduction electrons in both bulk orthorhombic FeSe and monolayer FeSe on SrTiO$_3$ 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

Chemosynthetic gas exchanger Final report, 8 Jun. 1964 - 14 Mar. 1967

Chemosynthetic bioregenerative system for long-term space exploration that employs functional coupling of water with biosynthesis by hydrogen bacteri