Abstract
2H-TaSe2 has been one of unique transition metal dichalcogenides exhibiting several phase transitions due to a delicate balance among competing electronic ground states. An unusual metallic state at high-T is sequentially followed by an incommensurate charge density wave (ICDW) state at ≈122 K and a commensurate charge density wave (CCDW) state at ≈90 K, and superconductivity at TC ~ 0.14 K. Upon systematic intercalation of Pd ions into TaSe2, we find that CCDW order is destabilized more rapidly than ICDW to indicate a hidden quantum phase transition point at x ~ 0.09–0.10. Moreover, TC shows a dramatic enhancement up to 3.3 K at x = 0.08, ~24 times of TC in 2H-TaSe2, in proportional to the density of states N(EF). Investigations of upper critical fields Hc2 in single crystals reveal evidences of multiband superconductivity as temperature-dependent anisotropy factor γH = , quasi-linear increase of , and an upward, positive-curvature in near TC. Furthermore, analysis of temperature-dependent electronic specific heat corroborates the presence of multiple superconducting gaps. Based on above findings and electronic phase diagram vs x, we propose that the increase of N(EF) and effective electron-phonon coupling in the vicinity of CDW quantum phase transition should be a key to the large enhancement of TC in PdxTaSe2.
Highlights
The 2H-polymorph of tantalum diselenide (TaSe2) is yet another renowned CDW system, in which an unusual metallic state at high temperatures enters into an incommensurate-charge-density wave (ICDW) phase at TICDW ≈ 122 K, followed by a first order “lock-in” transition to a commensurate charge density-wave (CCDW) phase at TCCDW ≈ 90 K13–15
While the origin of the CDW instability in 2H-Transition metal dichalcogenides (TMDCs) has been a topic of debate for many years, most recent studies indicate that the CDW formation in 2H-TMDCs is driven by periodic lattice distortion induced by strong momentum-dependent electron-phonon coupling rather than by the Peierls-type mechanism[19,20,21,22,23]
We show that intercalation of Pd ion into 2H-TaSe2 in a broad doping ranges without altering the 2H structure enhances TC up to 3.3 K, the highest observable TC among the TaSe2 polytypes
Summary
We found that as the CCDW state is suppressed with increasing Pd content, the superconducting transition temperature increases continuously (e) Temperature dependence of the upper critical fields determined by the 50% of the normal state resistivity criterion. For better understanding of the superconducting states, we have measured the temperature-dependence and anisotropy of the upper critical fields, Hc2, of a PdxTaSe2 single crystal in a dilution refrigerator.
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