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Electronic correlations and transport in multi-orbital systems
Horvat, Alen (Author), Mravlje, Jernej (Mentor) More about this mentor... This link opens in a new window

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Abstract
We investigate electronic correlations and transport in multi-orbital models with Hund’s rule coupling. Recent theoretical investigations of multi-orbital models pointed out the vital role of the Hund’s rule coupling, which induces strong correlations away from the Mott transition, suppresses the coherence scale and causes a two-stage Kondo screening. These properties were also observed experimentally in ruthenates and pnictides, which are hence called Hund’s metals. Impurity models play a central role in the dynamical mean-field theory, where we map the problem of the bulk to an impurity problem coupled to a self-determined bath. However, the multi-orbital impurity models are, in contrast to single-orbital impurity models, poorly understood. We investigate in detail the multi-orbital Anderson impurity model with Kanamori and Dworin-Narath interactions. First, we derive the corresponding Kondo models using the Schrieffer-Wolff transformation and apply the perturbative renormalization group theory to investigate their low-temperature properties. The Hund’s impurity undergoes a two-stage Kondo screening process, where the orbital degrees of freedom are screened at higher temperatures than the spin. Next, we solve the impurity problem with the numerical renormalization group impurity solver. We extend the solver to take advantage of the symmetry of the problem. We address the Kondo model and investigate the fixed points and their effects on impurity spectral functions. Last, we consider the spin-orbit coupling and show that the spin-orbit coupling affects the physics of the model only at temperatures above the orbital Kondo temperature. In the second part, we investigate the multi-orbital model on Bethe lattice within DMFT. We use the NRG impurity solver to calculate the spectral functions and transport properties, i.e., the temperature dependence of the Seebeck coefficient and resistivity. In the last part, we investigate the transport properties of the single-orbital Kondo-Hubbard model.

Language:English
Keywords:multi-orbital impurity models, Kondo effect, Hund’s metals, spin-orbit coupling, transport properties, numerical renormalization group
Work type:Doctoral dissertation (mb31)
Tipology:2.08 - Doctoral Dissertation
Organization:FMF - Faculty of Mathematics and Physics
Year:2019
COBISS.SI-ID:3336548 This link opens in a new window
Views:517
Downloads:223
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Secondary language

Language:Slovenian
Title:Elektronske korelacije in transport v več-orbitalnih sistemih
Abstract:
Preučujemo vpliv močnih korelacij na fizikalne lastnosti ter elektronski transport v več-orbitalnih sistemih z znatno Hundovo sklopitvijo. V to skupino snovi spadajo rutenati in pniktidi za katere so značilni nizka koherenčna skala, Kondovo senčenje spina ter tirne vrtilne količine pri različnih temperaturah ter močne korelacije tudi v območju faznega prostora, ki je daleč od Mottovega faznega prehoda. Takšne snovi imenujemo Hundove kovine. Pomembnost Hundove sklopitve v več-orbitalnih sistemih je bila izpostavljena tudi z uporabo teorije dinamičnega povprečnega polja (DMFT), ki je potrdila eksperimentalna dognanja. V teoriji dinamičnega povprečnega polja igrajo osrednjo vlogo magnetne nečistoče, saj mrežni model snovi preslikamo na model magnetne nečistoče, sklopljene na samo-usklajeno kopel. Fizika več-orbitalnih nečistoč je, za razliko od eno-orbitalnih nečistoč, slabo raziskana. V prvem delu podrobno obravnavamo več-orbitalni Andersonov model magnetne nečistoče. Nizkoenergijske lastnosti modela študiramo s perturbativno teorijo renormalizacijske grupe. Najprej s Shcrieffer-Wolffovo transformacijo izpeljemo Kondov model ter predstavimo fizikalne lastnosti modela. Nato Andersonov problem rešimo z uporabo numerične teorije renormalizacijske grupe (NRG), ki jo prilagodimo simetriji problema. Numerično rešimo tudi Kondov model, preiščemo fiksne točke ter študiramo njihov vpliv na fiziko nečistoče. Nato obravnavamo vpliv sklopitve spin-tir in razkrijemo, da ta začne igrati pomembno vlogo pri energijah, ki ustrezajo Kondovi temperaturi problema v odsotnosti sklopitve spin-tir. V drugem delu z uporabo DMFT obravnavamo vpliv korelacij na spektralne in transportne lastnosti več-orbitalnih sistemov. Temperaturno odvisnost upornosti ter Seebeckovega koeficienta izračunamo z metodo NRG. Kot drugi primer uporabe metod obravnavamo še Kondo-Hubbardov model.

Keywords:več-orbitalni modeli kvantnih nečistoč, Kondov pojav, Hundove kovine, sklopitev spin-tir, transportne lastnosti, numerična renormalizacijska grupa

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