Bad metal and negative compressibility transitions in a two-band Hubbard model

TL;DR

This paper investigates negative electronic compressibility and bad metal states in a two-band Hubbard model near integer filling, revealing a feedback mechanism that could enable tunable capacitance in heterostructures.

Contribution

It introduces a slave boson approach with Ising-like pseudospins to analyze charge instabilities and negative compressibility in strongly correlated systems near a Mott transition.

Findings

Negative compressibility states can be sustained in heterostructures.

Capacitance can be tuned via small electronic density variations.

Transitions between different capacitance states are possible through electric pulses.

Abstract

We analyze the paramagnetic state of a two-band Hubbard model with finite Hund's coupling close to integer filling at $n=2$ in two spacial dimensions. Previously, a Mott metal-insulator transition was established at $n=2$ with a coexistence region of a metallic and a bad metal state in the vicinity of that integer filling. The coexistence region ends at a critical point beyond which a charge instability persists. Here we investigate the transition into negative electronic compressibility states for an extended filling range close to $n=2$ within a slave boson setup. We analyze the separate contributions from the (fermionic) quasiparticles and the (bosonic) multiparticle incoherent background and find that the total compressibility depends on a subtle interplay between the quasiparticle excitations and collective fields. Implementing a Blume-Emery-Griffiths model approach for the slave bosons, which mimics the bosonic fields by Ising-like pseudospins, we suggest a feedback mechanism between these fields and the fermionic degrees of freedom. We argue that the negative compressibility can be sustained for heterostructures of such strongly correlated planes and results in a large capacitance of these structures. The strong density dependence of these capacitances allows to tune them through small electronic density variations. Moreover, by resistive switching from a Mott insulating state to a metallic state through short electric pulses, transitions between fairly different capacitances are within reach.