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Register a new userSuppression of axionic charge density wave and onset of superconductivity in the chiral Weyl semimetal Ta$_2$Se$_8$I
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A Weyl semimetal with strong electron-phonon interaction can show axionic coupling in its insulator state at low temperatures, owing to the formation of a charge density wave (CDW). Such a CDW emerges in the linear chain compound Weyl semimetal Ta$_2$Se$_8$I below 263 K, resulting in the appearance of the dynamical condensed-matter axion quasiparticle. In this study, we demonstrate that the interchain coupling in Ta$_2$Se$_8$I can be varied to suppress the CDW formation with pressure, while retaining the Weyl semimetal phase at high temperatures. Above 17 GPa, the Weyl semimetal phase does not survive and we induce superconductivity, due to the amorphization of the iodine sub-lattice. Structurally, the one-dimensional Ta-Se-chains remain intact and provide a superconducting channel in one dimension. We highlight that our results show a near-complete suppression of the gap induced by the axionic charge-density wave at pressures inaccessible to previous studies. Including this CDW phase, our experiments and theoretical predictions and analysis reveal the complete topological phase diagram of Ta$_2$Se$_8$I and its relationship to the nearby superconducting state. The results demonstrate Ta$_2$Se$_8$I to be a distinctively versatile platform for exploring correlated topological states.
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Charge density wave (CDW), the periodic modulation of the electronic charge density, will open a gap on the Fermi surface that commonly leads to decreased or vanishing conductivity. On the other hand superconductivity, a commonly believed competing order, features a Fermi surface gap that results in infinite conductivity. Here we report that superconductivity emerges upon Se doping in CDW conductor ZrTe$_{3}$ when the long range CDW order is gradually suppressed. Superconducting critical temperature $T_c(x)$ in ZrTe$_{3-x}$Se$_x$ (${0leq}xleq0.1$) increases up to 4 K plateau for $0.04$$leq$$x$$leq$$0.07$. Further increase in Se content results in diminishing $T_{c}$ and filametary superconductivity. The CDW modes from Raman spectra are observed in $x$ = 0.04 and 0.1 crystals, where signature of ZrTe$_{3}$ CDW order in resistivity vanishes. The electronic-scattering for high $T_{c}$ crystals is dominated by local CDW fluctuations at high temperures, the resistivity is linear up to highest measured $T=300K$ and contributes to substantial in-plane anisotropy.
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.
The temperature dependence of the phonon spectrum in the superconducting transition metal dichalcogenide 2H-NbS$_2$ is measured by diffuse and inelastic x-ray scattering. A deep, wide and strongly temperature dependent softening, of the two lowest energy longitudinal phonons bands, appears along the $mathrm{Gamma M}$ symmetry line in reciprocal space. In sharp contrast to the iso-electronic compounds 2H-NbSe$_2$, the soft phonons energies are finite, even at very low temperature, and no charge density wave instability occurs, in disagreement with harmonic ab-initio calculations. We show that 2H-NbS$_2$ is at the verge of the charge density wave transition and its occurrence is only suppressed by the large anharmonic effects. Moreover, the anharmonicity and the electron phonon coupling both show a strong in-plane anisotropy.
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$_2$.
Layered transition metal dichalcogenide WTe$_2$ has recently attracted significant attention due to the discovery of an extremely large magnetoresistance, a predicted type-II Weyl semimetallic state, and the pressure-induced superconducting state. By a careful measurement of the superconducting upper critical fields as a function of the magnetic field angle at a pressure as high as 98.5 kbar, we provide the first detailed examination of the dimensionality of the superconducting condensate in WTe$_2$. Despite the layered crystal structure, the upper critical field exhibits a negligible field anisotropy. The angular dependence of the upper critical field can be satisfactorily described by the anisotropic mass model from 2.2 K ($T/T_csim0.67$) to 0.03 K ($T/T_csim0.01$), with a practically identical anisotropy factor $gammasim1.7$. The temperature dependence of the upper critical field, determined for both $Hperp ab$ and $Hparallel ab$, can be understood by a conventional orbital depairing mechanism. Comparison of the upper critical fields along the two orthogonal field directions results in the same value of $gammasim1.7$, leading to a temperature independent anisotropy factor from near $T_c$ to $<0.01T_c$. Our findings thus identify WTe$_2$ as a nearly isotropic superconductor, with an anisotropy factor among one of the lowest known in superconducting transition metal dichalcogenides.
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