Power-Law Suppression of Phonon Thermal Transport by Magnetic Excitations in a Molecular Quantum Spin Liquid

Abstract

We present large-scale ab initio phonon calculations for the molecular quantum spin liquid X[Pd(dmit)2]2. An unusually low average phonon velocity ( 700 {m/s}) and optical modes below 10 cm^{-1} confine the Debye T^{3} regime to T < 2 K. As the transfer-integral anisotropy approaches the maximally frustrated regime (t'/t \to 1), the lattice stiffens, ruling out lattice softening as the origin of the spin-liquid state. By quantifying the additional suppression of the thermal conductivity from experimental data, we observe a power-law behavior consistent with two-dimensional magnetic excitations with a nodal, approximately linear (Dirac-like) spectrum.