Weak Ferromagnetic Exchange and Anomalous Specific Heat in ZnCu3(OH)6Cl2

TL;DR

This paper investigates the low-energy spin excitations in the kagome antiferromagnet ZnCu3(OH)6Cl2, proposing a spinon Fermi surface state stabilized by ferromagnetic interactions, supported by variational calculations.

Contribution

It introduces a variational wavefunction approach demonstrating that ferromagnetic next-nearest-neighbor interactions favor a spinon Fermi surface in the kagome lattice.

Findings

A Gutzwiller-projected wavefunction reproduces known dimerization patterns.

Small ferromagnetic interactions stabilize a spinon Fermi surface.

The proposed state explains low-energy excitations observed experimentally.

Abstract

Experimental evidence for a plethora of low energy spin excitations in the spin-1/2 kagome antiferromagnet ZnCu3(OH)6Cl2 may be understandable in terms of an extended Fermi surface of spinons coupled to a U(1) gauge field. We carry out variational calculations to examine the possibility that such a state may be energetically viable. A Gutzwiller-projected wavefunction reproduces the dimerization of a kagome strip found previously by DMRG. Application to the full kagome lattice shows that the inclusion of a small ferromagnetic next-nearest-neighbor interaction favors a ground state with a spinon Fermi surface.