Superconductivity enhancement in Ta1+x Se2 with a randomly stacked structure

A series of Ta1+x Se2 (0.00 ≤ x ≤ 0.12) samples was prepared and characterized by x-ray powder diffraction, resistance, magnetization and specific heat measurements. Based on the fitting of the diffraction patterns, we found that Ta1+x Se2 has two randomly stacked structures with distinct stacking units: 2Hb and 3R slabs when x ≤ 0.06 and single TaSe2 layers with Ta atoms in trigonal prismatic coordination with Se when x > 0.06. The superconducting critical temperatures (T c) of the Ta1+x Se2 samples can be significantly enhanced from 0.14 K for pure TaSe2 to above 2 K in the Ta-rich samples with both structures. The maximum T c of 3.2 K was obtained in Ta1.06Se2. The remarkable enhancement of T c in Ta1+x Se2 originates from the enhancement of N(E F) and λ.