Quantum spin liquid states in the two dimensional kagome antiferromagnets, ZnxCu4-x(OD)6Cl2

TL;DR

This study provides experimental evidence for quantum spin liquid states in frustrated kagome antiferromagnets, showing how doping and lattice distortion influence the emergence of valence bond solid and spin liquid phases.

Contribution

It demonstrates the existence of quantum paramagnetic states, including valence bond solid and spin liquid phases, in the kagome lattice of ZnxCu4-x(OD)6Cl2, highlighting the effects of doping and lattice distortion.

Findings

Observation of dispersionless excitation mode below 20 K in Cu4(OD)6Cl2

Suppression of valence bond solid state with Zn doping

Featureless spin fluctuations in undistorted kagome planes at high doping

Abstract

A three-dimensional system of interacting spins typically develops static long-range order when it is cooled. If the spins are quantum (S = 1/2), however, novel quantum paramagnetic states may appear. The most highly sought state among them is the resonating valence bond (RVB) state in which every pair of neighboring quantum spins form entangled spin singlets (valence bonds) and the singlets are quantum mechanically resonating amongst all the possible highly degenerate pairing states. Here we provide experimental evidence for such quantum paramagnetic states existing in frustrated antiferromagnets, ZnxCu4-x(OD)6Cl2, where the S = 1/2 magnetic Cu2+ moments form layers of a two-dimensional kagome lattice. We find that in Cu4(OD)6Cl2, where distorted kagome planes are weakly coupled to each other, a dispersionless excitation mode appears in the magnetic excitation spectrum below ~ 20 K, whose characteristics resemble those of quantum spin singlets in a solid state, known as a valence bond solid (VBS), that breaks translational symmetry. Doping nonmagnetic Zn2+ ions reduces the distortion of the kagome lattice, and weakens the interplane coupling but also dilutes the magnetic occupancy of the kagome lattice. The VBS state is suppressed and for ZnCu3(OD)6Cl2 where the kagome planes are undistorted and 90% occupied by the Cu2+ ions, the low energy spin fluctuations in the spin liquid phase become featureless.