Excited States beyond Mott Gap in Half-Filled-Band Hubbard Model

TL;DR

This paper investigates excited states beyond the Mott gap in a two-dimensional Hubbard model, revealing conditions under which paramagnetic and superconducting states become conductive and highlighting the stability of antiferromagnetic states.

Contribution

It introduces a variational Monte Carlo approach to study excited states in the Hubbard model, identifying the stability of antiferromagnetic states and the conditions for conduction in paramagnetic and superconducting states.

Findings

Antiferromagnetic states are most stable for low doublon density.

Paramagnetic and d-wave superconducting states become conductive above a certain doublon threshold.

s-wave superconducting states are not stabilized in the studied parameter range.

Abstract

In connection with recent experiments on excitation in which Mott insulators change to conductors, we study the properties of excited states beyond the Mott gap as quasi-stationary states for a two-dimensional Hubbard (t-t'-U) model at half filling. A variational Monte Carlo method is used with trial wave functions for paramagnetic or normal (PM), superconducting with dx2-y2-wave (d-SC), isotropic s-wave, and extended s-wave symmetries, and antiferromagnetic (AF) states. The excited states are generated by imposing a minimum number of doubly occupied sites (doublons) D_L on the lowest-energy states. For U>W (W: band width), d_L=D_L/Ns (Ns: number of sites) corresponds to the excitation intensity. It is found that the AF state is the most stable among the states we treated for d_L<0.14 and insulating. The PM and d-SC states become conductive over a threshold d_Lc, and the conduction is caused by unbound doublons and holons (empty sites). The PM state arises for d_L>0.14, but the d-SC state is always hidden by the AF state. The s-wave-type superconducting states are not stabilized for any parameter set.