Superconductivity and quantum transport in thin crystals of TaSe₂ tuned by ionic gating

Superconductivity and quantum transport in two-dimensional (2D) crystals of transition metal dichalcogenides have been a focus of materials and condensed matter physics research in recent years. These 2D crystals have been found to exhibit novel properties not found in the bulk. Bulk 2H-TaSe₂ is a metal featuring superconductivity at very low temperatures (T_c = 0.15 K), but few-layer TaSe₂ was found to show enhanced T_c. On the other hand, the charge density wave (CDW) did not show similar behavior. We studied superconductivity and quantum transport properties of thin crystal of 2H-TaSe₂ obtained by mechanical exfoliation with its electronic properties tuned by ionic gating using a polymer electrolyte. We found that the superconducting transition temperature increases monotonically with decreasing thickness. The transition temperature of CDW was found to decrease only slightly as the crystal gets thinner, consistent with the expectation that superconductivity and CDW compete. We also found that the magnetoresistance (MR) of thin crystals of TaSe₂ showed a crossover from positive to negative MR at low magnetic fields as the gate voltage was ramped up. The change in the sign of MR is argued to be a result of competition between weak antilocalization and weak localization.