Enhanced low-temperature entropy and flat-band ferromagnetism in the t-J model on the sawtooth lattice

TL;DR

This paper investigates the t-J model on the sawtooth lattice, revealing flat-band ferromagnetism, high zero-temperature entropy, and unique thermodynamic properties due to localized excitations in a frustrated lattice system.

Contribution

It demonstrates the emergence of flat-band ferromagnetism and entropy in the t-J model on a frustrated sawtooth lattice, highlighting features similar to the Hubbard model.

Findings

Flat lowest single-fermion band leads to localized excitations.

Exact many-electron states can be constructed from localized states.

Presence of finite zero-temperature entropy and ferromagnetic ground state.

Abstract

Using the example of the sawtooth chain, we argue that the t-J model shares important features with the Hubbard model on highly frustrated lattices. The lowest single-fermion band is completely flat (for a specific choice of the hopping parameters $t_{i,j}$ in the case of the sawtooth chain), giving rise to single-particle excitations which can be localized in real space. These localized excitations do not interact for sufficient spatial separations such that exact many-electron states can also be constructed. Furthermore, all these excitations acquire zero energy for a suitable choice of the chemical potential $\mu$. This leads to: (i) a jump in the particle density at zero temperature, (ii) a finite zero-temperature entropy, (iii) a ferromagnetic ground state with a charge gap when the flat band is fully occupied and (iv) unusually large temperature variations when $\mu$ is varied adiabatically at finite temperature.