Gapless excitations in the ground state of $1T\text{−}{\mathrm{TaS}}_2$

$1T\text{−}{\mathrm{TaS}}_2$ is a layered transition-metal dichalcogenide with a very rich phase diagram. At $T=180$ K it undergoes a metal to Mott insulator transition. Mott insulators usually display antiferromagnetic ordering in the insulating phase but $1T\text{−}{\mathrm{TaS}}_2$ was never shown to order magnetically. In this paper we show that $1T\text{−}{\mathrm{TaS}}_2$ has a large paramagnetic contribution to the magnetic susceptibility but it does not show any sign of magnetic ordering or freezing down to 20 mK, as probed by muon spin relaxation, possibly indicating a quantum spin liquid ground state. Although $1T\text{−}{\mathrm{TaS}}_2$ exhibits a strong resistive behavior both in and out of plane at low temperatures we find a linear term in the heat capacity suggesting the existence of a Fermi surface, which has an anomalously strong magnetic field dependence.

Figure 1

(a) Normalized resistance vs temperature. The black (blue) curve is in-plane (out-of-plane) normalized resistance, all measured in four-contact configuration. (b) Magnetic molar susceptibility vs temperature, obtained from SQUID magnetization measurements at constant magnetic fields of 1 and 3 T. The red line is the fit to Curie-Weiss form at the temperature range of 2 to 175 K.

Figure 2

$\mu \mathrm{SR}$ results. (a) Averaged muon polarization at 50-G transverse field at several temperatures, below and above ${\theta }_{\text{CW}}$. The circles are measurement results, while the solid lines are the fit results. (b) Temperature dependence of the muon spin damping rate, $\lambda$. $\lambda$ remains constant down to 20 mK (TF measurements) and 70 mK (ZF measurements), implying the absence of frozen moments. The ZF asymmetry was fitted using $P\left(t\right)=A\text{exp}(-\lambda t)$ and the TF asymmetry was fitted using $P\left(t\right)=A\text{exp}(-\lambda t)\text{cos}(\gamma Bt+\varphi )$. The solid lines are the average $\lambda$ from the ZF and TF measurements, respectively. (c) Longitudinal field $\mu \mathrm{SR}$ asymmetry, measured at 10 K and at three different magnetic fields. The symbols are measurement results while the solid lines are the fit results using $P\left(t\right)=A\text{exp}(-\lambda t)$. There is a complete decoupling of the ${\mu }^{+}$ spins from nuclear dipolar fields already at 50 G, suggesting all static moments are much smaller than 50 G.

Figure 3

Heat capacity measurements of $1T\text{−}{\mathrm{TaS}}_2$ at eight magnetic fields, plotted as $C_P/T$ as a function of $T^2$. The inset displays larger temperature range, which exhibits a linear relation according to the Debye form $C_P\propto T^3$. The main panel focuses on the low-temperature regime where the deviation from the Debye behavior is noticeable. The solid lines are fits to Eq. (3) modeling the data using both a Schottky term and linear contribution.

Figure 4

Field dependence of the heat-capacity linear term. $\gamma \left(H\right)$ decreases almost monotonically with the applied magnetic field.