Chiral Unitary Quantum Phase Transition in 2H-Fe$_x$TaSe$_2$

Abstract: We have observed a metal-insulator transition of a quasi-two dimensional electronic system in transition metal dichalcogenide $2H$-TaSe$_2$ caused by doping iron. The sheet resistance of $2H$-Fe$_x$TaSe$_2$ ($0 \leq x \leq 0.120$) single crystals rises about $10^6$ times with the increasing of $x$ at the lowest temperature. We investigated the temperature dependence of the resistance and found a metal-insulator transition with a critical sheet resistance $11.7 \pm 5.4$ k$\rm{\Omega}$. The critical exponent of the localization length $\nu$ is estimated $0.31 \pm 0.18$. The values of the critical sheet resistance and $\nu$ are accordant to those of the \textit{chiral unitary class} (less than $h/1.49e^2=17.3$ k$\rm{\Omega}$ and $0.35 \pm 0.03$, respectively). We suggest that $2H$-Fe$_x$TaSe$_2$ is classified as the chiral unitary class, not as standard unitary class.