Gapless quantum spin liquid, stripe and antiferromagnetic phases in frustrated Hubbard models in two dimensions

TL;DR

This paper investigates the phase diagram of the frustrated t-t' Hubbard model on a square lattice, revealing antiferromagnetic, stripe, and quantum spin liquid phases near the Mott transition using path-integral renormalization group methods.

Contribution

It provides the first numerical evidence of a nonmagnetic insulating quantum spin liquid phase in a 2D Hubbard model near the Mott transition, highlighting its gapless excitations and ground state degeneracy.

Findings

Identification of antiferromagnetic and stripe phases depending on t'

Discovery of a quantum spin liquid phase with gapless excitations

Observation of a longer-period antiferromagnetic insulator at intermediate t'/t

Abstract

Unique features of non-magnetic insulator phase are shown, and phase diagram for t-t' Hubbard model on square lattice is presented. Using the path-integral renormalization group method, we find antiferromagnetic phase for small next-nearest neighbor transfer t' and stripe (or collinear) phase for large t' in the Mott insulating region of strong on site interaction U. For intermediate t'/t~ 0.7 at large U/t>7, we find longer-period antiferromagnetic insulator phase with 2*4 structure. In the Mott insulating region, we also find quantum spin liquid (in other words, non-magnetic insulator) phase near the Mott transition to paramagnetic metals. Correlated electrons often crystallize to the Mott insulator usually with some magnetic orders, whereas the "quantum spin liquid" has been a long-sought issue. We report numerical evidences that a nonmagnetic insulating (NMI) phase gets stabilized near the Mott transition with remarkable properties: The 2D Mott insulators on geometrically frustrated lattices contain a phase with gapless spin excitations and degeneracy of the ground state in the whole Brillouin zone of the total momentum. It has an interpretation for an unexplored type of a quantum liquid. The present concept is useful in analyzing a variety of experimental results in frustrated magnets including organic BEDT-TTF compounds and 3He atoms adsorbed on graphite.