Intrinsic new properties of a quantum spin liquid

TL;DR

This study investigates the intrinsic properties of a quantum spin liquid in copper-based two-dimensional triangular-lattice compounds, revealing scale-invariant fluctuations and missing magnetic entropy indicative of highly entangled quantum states.

Contribution

It provides the first detailed thermodynamic and muon spin relaxation measurements on these specific spin liquids, uncovering intrinsic quantum fluctuations and entropy behavior.

Findings

Half of the magnetic entropy is missing down to very low temperatures.

Low-temperature specific heat and relaxation rates are temperature-independent, then logarithmically decrease.

Properties are intrinsic, not due to impurities, indicating massive quantum fluctuations.

Abstract

Quantum fluctuations are expected to lead to highly entangled spin-liquid states in certain two-dimensional spin-1/2 compounds. We have synthesized and measured thermodynamic properties and muon spin relaxation rates in the copper-based two-dimensional triangular-lattice spin liquids Lu$_3$Cu$_2$Sb$_3$O$_{14}$ and Lu$_3$CuZnSb$_3$O$_{14}$. The former is the least disordered of this kind discovered to date. Magnetic entropy generation at high temperatures has been ruled out after carefully correcting for the lattice specific heat. Surprisingly, roughly half of the magnetic entropy is missing down to temperatures of O(10$^{-3}$) the exchange energy, independent of magnetic field up to $g\mu_B H \gtrsim k_B\Theta_W$, where $\Theta_W$ is the Weiss temperature. The magnetic specific heat divided by temperature $C_M(T)/T$ and muon spin relaxation rate $\lambda(T)$ are both temperature-independent at low temperatures, followed by logarithmic decreases with increasing temperature. This behavior can be simply characterized by scale-invariant time-dependent fluctuations with a single parameter. Since no cooperative effects due to impurities are observed, the measured properties are intrinsic. They are evidence that in Lu$_3$Cu$_2$Sb$_3$O$_{14}$ massive quantum fluctuations lead to either a gigantic specific heat peak from singlet excitations at very low temperatures or, perhaps less likely, an extensively degenerate possibly topological singlet ground state.