Continuous collapse of antiferromagnetic order toward a quantum critical point in a single-component molecular material

Abstract

We uncover the pressure variation of electronic states of the single-component molecular material $\mathrm{Pd}{(\mathrm{tmdt})}_{2}$ that hosts the antiferromagnetic (AFM) Mott insulator at ambient pressure, using $^{13}\mathrm{C}$ nuclear magnetic resonance (NMR) spectroscopy. Under pressures up to 8.5 kbar, the NMR spectral broadening and the peak structure in the temperature dependence of the spin-lattice relaxation rate $1\text{/}{T}_{1}$ are observed and indicate that the AFM transition persists in this pressure range. However, the Neel temperature and the AFM magnetic moment are continuously decreased toward a critical pressure of $\ensuremath{\sim}9$ kbar. At 10 kbar, these AFM features in the spectra and $1\text{/}{T}_{1}$ are suppressed, and instead $1\text{/}{T}_{1}T$ exhibits a critical increase toward zero temperature, complying with the self-consistent renormalization (SCR) theory in two-dimensional AFM itinerant magnets [$1\text{/}{T}_{1}T$ $\ensuremath{\sim}$ (${T+\ensuremath{\theta})}^{\ensuremath{-}1}$], which also explains the systematic change of $1\text{/}{T}_{1}T$ at higher pressures. The present results indicate that $\mathrm{Pd}{(\mathrm{tmdt})}_{2}$ shows a pressure-induced quantum phase transition from the AFM Mott insulator to the paramagnetic metal at the quantum critical point of $\ensuremath{\sim}9$ kbar.