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

TL;DR

This study combines ab initio phonon calculations and experimental data to reveal how magnetic excitations in a molecular quantum spin liquid suppress phonon thermal transport, showing a power-law behavior linked to Dirac-like magnetic spectra.

Contribution

It provides the first detailed analysis of phonon and magnetic excitation interactions in a molecular quantum spin liquid using large-scale ab initio calculations and experimental thermal conductivity data.

Findings

Phonon velocity is unusually low (~700 m/s).

Thermal conductivity suppression follows a power-law behavior.

Magnetic excitations exhibit a Dirac-like spectrum.

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.