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Register a new userInterplay of charge density wave and multiband superconductivity in 2$H$-Pd$_x$TaSe$_2$
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2$H$-TaSe$_2$ 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 $approx$ 122 K and a commensurate charge density wave (CCDW) state at $approx$ 90 K, and superconductivity at $T_{rm{C}}sim$0.14 K. Upon systematic intercalation of Pd ions into TaSe$_2$, we find that CCDW order is destabilized more rapidly than ICDW to indicate a hidden quantum phase transition point at $x$$sim$0.09-0.10. Moreover, $T_{rm{C}}$ shows a dramatic enhancement up to 3.3 K at $x$ = 0.08, $sim$24 times of $T_{rm{C}}$ in 2$H$-TaSe$_2$, in proportional to the density of states $N(E_F)$. Investigations of upper critical fields $H_{c2}$ in single crystals reveal evidences of multiband superconductivity as temperature-dependent anisotropy factor $gamma_H$ = $H_{c2}^{ab}$/$H_{c2}^{c}$, quasi-linear increase of $H_{c2}^{c}(T)$, and an upward, positive-curvature in $H_{c2}^{ab}(T)$ near $T_{rm{C}}$. 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 $N(E_F)$ and effective electron-phonon coupling in the vicinity of CDW quantum phase transition should be a key to the large enhancement of $T_{rm{C}}$ in Pd$_x$TaSe$_2$.
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We carried out a comprehensive study of the electronic, magnetic, and thermodynamic properties of Ni-doped ZrTe$_2$. High quality Ni$_{0.04}$ZrTe$_{1.89}$ single crystals show a possible coexistence of charge density waves (CDW, T$_{CDW}approx287$,K) with superconductivity (T$_capprox 4.1$,K), which we report here for the first time. The temperature dependence of the lower (H$_{c_1}$) and upper (H$_{c_2}$) critical magnetic fields both deviate significantly from the behaviors expected in conventional single-gap s-wave superconductors. However, the behaviors of the normalized superfluid density $rho_s(T)$ and H$_{c_2}(T)$ can be described well using a two-gap model for the Fermi surface, in a manner consistent with conventional multiband superconductivity. Electrical resistivity and specific heat measurements show clear anomalies centered near 287,K suggestive of CDW phase transition. Additionally, electronic-structure calculations support the coexistence of electron-phonon multiband superconductivity and CDW order due to the compensated disconnected nature of the electron- and hole-pockets at the Fermi surface. Our calculations also suggest that ZrTe$_2$ is a non-trivial topological type-II Dirac semimetal. These findings highlight that Ni-doped ZrTe2 is uniquely important for probing the coexistence of superconducting and CDW ground states in an electronic system with non-trivial topology.
Recently it was found that selenium doping can suppress the charge-density-wave (CDW) order and induce bulk superconductivity in ZrTe$_3$. The observed superconducting dome suggests the existence of a CDW quantum critical point (QCP) in ZrTe$_{3-x}$Se$_x$ near $x approx$ 0.04. To elucidate its superconducting state near the CDW QCP, we measure the thermal conductivity of two ZrTe$_{3-x}$Se$_x$ single crystals ($x$ = 0.044 and 0.051) down to 80 mK. For both samples, the residual linear term $kappa_0/T$ at zero field is negligible, which is a clear evidence for nodeless superconducting gap. Furthermore, the field dependence of $kappa_0/T$ manifests multigap behavior. These results demonstrate multiple nodeless superconducting gaps in ZrTe$_{3-x}$Se$_x$, which indicates conventional superconductivity despite of the existence of a CDW QCP.
By measuring the temperature dependence of the resistance, we investigated the effect of Cu doping on superconductivity (SC) in Cu-doped TaSe$_3$ in which the charge density wave (CDW) transition is induced by Cu doping. We observed an emergence of a region where the SC transition temperature ($T_mathrm{C}$) decreased in samples with higher Cu concentrations and found that the region tended to expand with increasing Cu concentration. In addition, the temperature dependence of the upper critical field ($H_mathrm{C2}$) of Cu-doped TaSe$_3$ was found to differ from that of pure TaSe$_3$. Based on these experimental results and the fact that the SC of TaSe$_3$ is filamentary, we conclude that SC is suppressed locally by Cu doping and competes with the CDW in Cu-doped TaSe$_3$. The resistance anomaly due to the CDW transition was extremely small and the size of the anomaly was enhanced with increasing Cu concentration but the temperature at which the anomaly appeared hardly changed. This result of the anomaly and the local suppression of SC imply that the induced CDWs are short-range order in the vicinity of Cu atoms. We also discuss the effect of the pinning of CDWs on the relationship between SC and short-range order CDWs.
Transition-metal dichalcogenides (TMDs) $MX_2$ ($M$ = Ti, Nb, Ta; $X$ = S, Se, Te) exhibit a rich set of charge density wave (CDW) orders, which usually coexist and/or compete with superconductivity. The mechanisms of CDWs and superconductivity in TMDs are still under debate. Here we perform an investigation on a typical TMD system, 1emph{T}-TaSe$_{2-x}$Te$_x$ ($0 leq x leq 2$). Doping-induced disordered distribution of Se/Te suppresses CDWs in 1emph{T}-TaSe$_2$. A domelike superconducting phase with the maximum $T_textrm{c}^{textrm{onset}}$ of 2.5 K was observed near CDWs. The superconducting volume is very small inside the CDW phase and becomes very large instantly when the CDW phase is fully suppressed. The observations can be understood based on the strong emph{textbf{q}}-dependent electron-phonon coupling-induced periodic-lattice-distortion (PLD) mechanism of CDWs. The volume variation of superconductivity implies the emergence of domain walls in the suppressing process of CDWs. Our concluded scenario makes a fundamental understanding about CDWs and related superconductivity in TMDs.
HoTe$_{3}$, a member of the rare-earth tritelluride ($R$Te$_{3}$) family, and its Pd-intercalated compounds, Pd$_x$HoTe$_{3}$, where superconductivity (SC) sets in as the charge-density wave (CDW) transition is suppressed by the intercalation of a small amount of Pd, are investigated using angle-resolved photoemission spectroscopy (ARPES) and electrical resistivity. Two incommensurate CDWs with perpendicular nesting vectors are observed in HoTe$_{3}$ at low temperatures. With a slight Pd intercalation ($x$ = 0.01), the large CDW gap decreases and the small one increases. The momentum dependence of the gaps along the inner Fermi surface (FS) evolves from orthorhombicity to near tetragonality, manifesting the competition between two CDW orders. At $x$ = 0.02, both CDW gaps decreases with the emergence of SC. Further increasing the content of Pd for $x$ = 0.04 will completely suppress the CDW instabilities and give rise to the maximal SC order. The evolution of the electronic structures and electron-phonon couplings (EPCs) of the multiple CDWs upon Pd intercalation are carefully scrutinized. We discuss the interplay between multiple CDW orders, and the competition between CDW and SC in detail.
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