Conduttività e paradosso di Klein nel grafene

Lo scopo di questo lavoro è quello di analizzare le proprietà elettriche del grafene. Nel primo capitolo si introducono la struttura cristallina del materiale e, tramite il modello di approssimazione di tight binding, la sua struttura elettronica. Poi, sviluppando la relazione di dispersione intorno ai punti speciali della prima zona di Brillouin, viene evidenziata la particolare presenza dei coni di Dirac. Nel secondo capitolo si studia la conduttività del grafene partendo dal modello classico e arrivando alla conduttività minima, dimostrata tramite la formula di Landauer. Nel terzo capitolo viene evidenziato qual è il significato della corrispondenza tra i fermioni senza massa e gli elettroni nel grafene, dimostrando infine il paradosso di Klein

Spin-orbital Jahn-Teller bipolarons

Polarons and spin-orbit (SO) coupling are distinct quantum effects that play a critical role in charge transport and spin-orbitronics. Polarons originate from strong electron-phonon interaction and are ubiquitous in polarizable materials featuring electron localization, in particular $\mathrm{3d}$ transition metal oxides (TMOs). On the other hand, the relativistic coupling between the spin and orbital angular momentum is notable in lattices with heavy atoms and develops in $\mathrm{5d}$ TMOs, where electrons are spatially delocalized. Here we combine ab initio calculations and magnetic measurements to show that these two seemingly mutually exclusive interactions are entangled in the electron-doped SO-coupled Mott insulator $\mathrm{Ba_2Na_{1-x}Ca_xOsO_6}$ ($0< x < 1$), unveiling the formation of spin-orbital bipolarons. Polaron charge trapping, favoured by the Jahn-Teller lattice activity, converts the Os $\mathrm{5d^1}$ spin-orbital $\mathrm{J_{eff}=3/2}$ levels, characteristic of the parent compound $\mathrm{Ba_2NaOsO_6}$ (BNOO), into a bipolaron $\mathrm{5d^2}$ $\mathrm{J_{eff}=2}$ manifold, leading to the coexistence of different J-effective states in a single-phase material. The gradual increase of bipolarons with increasing doping creates robust in-gap states that prevents the transition to a metal phase even at ultrahigh doping, thus preserving the Mott gap across the entire doping range from $\mathrm{d^1}$ BNOO to $\mathrm{d^2}$ $\mathrm{Ba_2CaOsO_6}$ (BCOO)

Nuclear magnetic resonance study of the electron doped Dirac-Mott Insulator double perovskite Ba2Na(1-x)CaxOsO6

Osmium-based materials exhibit unconventional magnetism due to the interplay between spin orbit coupling and strong electronic correlations. In this thesis we investigate for the first time the effect of charge doping on the Dirac Mott insulator double perovskites Ba2Na(1-x)CaxOsO6 by using Nuclear Magnetic Resonance technique. We study in details the evolution of the magnetic phase diagram as a function of doping and applied magnetic field. The system rapidly evolves from a canted to a collinear antiferromagnetic ground state with a monotonic increase of the magnetic transition temperature. Furthermore, the system remains insulating in the whole range of doping, despite the injection of extra electrons. This indicates that the Dirac-Mott insulating state of this material is unusually robust. In particular, the results show an unexpected thermally activated charge dynamics which suggests the presence of polarons dominating the high temperature excitations of this Dirac-Mott insulator