Low-temperature thermal conductivity measurements were performed on single crystal of PbTaSe$_2$, a noncentrosymmetric superconductor with topological bulk nodal lines in the electronic band structure. It is found that the residual linear term $\\kappa_0/T$ is negligible in zero magnetic field. Furthermore, the field dependence of $\\kappa_0/T$ exhibits a clear \"$S$\"-shape curve. These results suggest that PbTaSe$_2$ has multiple nodeless superconducting gaps. Therefore, the spin-triplet state with gap nodes does not play an important role in this noncentrosymmetric superconductor with strong spin-orbital coupling. The fully gapped superconducting state also meets the requirement of a topological superconductor, if PbTaSe$_2$ is indeed the case.

Multiple nodeless superconducting gaps in noncentrosymmetric superconductor PbTaSe$_{2}$ with topological bulk nodal lines

The combination of nontrivial topology and charge density wave (CDW) has been proposed as a powerful resource for realizing novel quantum phenomena such as axion electrodynamics and the anomalous Hall effect. Hence, topological materials with CDW states attract great interest, yet they are still very rare, particularly in the 2D limit. Here, it is predicted that monolayer NbTe2 in its high‐symmetry 1T phase stabilized by anharmonicity at room temperature exhibits nontrivial topology sensitive to electronic interactions: it changes from a quantum spin Hall (QSH) state to a quantum anomalous Hall (QAH) state when the Hubbard potential U exceeds a critical value. At low temperature, a 4 × 4 CDW order emerges and coexists with the nontrivial topology. Meanwhile, the critical U increases because CDW reduces density of states at Fermi level. More interestingly, in contrast to the high‐symmetry structure that actually is a topologically nontrivial metal, the CDW structure shows an insulating nontrivial gap either in the QSH or the QAH phase, indicating CDW is an effective means to modulate the topological state for developing new functions and devices. These discoveries establish NbTe2 as a promising candidate to explore exotic quantum states at the confluence of nontrivial topology, electronic correlation, and CDW.

Realization of Coexisting Charge Density Wave and Quantum Spin/Anomalous Hall State in Monolayer NbTe2

/pdf/wannsymm-a-symmetry-analysis-code-for-wannier-orbitals-1rksyan4.pdf

WannSymm: A symmetry analysis code for Wannier orbitals

Topological crystalline metals and semimetals (TCMs) have stimulated great research interest, which broadens the classification of topological phases and provides a valuable platform to explore topological superconductivity. Here, we report the discovery of superconductivity and topological features in Pb-intercalated transition-metal dichalcogenide ${\mathrm{Pb}}_{1/3}\mathrm{Ta}{\mathrm{S}}_{2}$. Systematic measurements indicate that ${\mathrm{Pb}}_{1/3}\mathrm{Ta}{\mathrm{S}}_{2}$ is a quasi-two-dimensional (q-2D) type-II superconductor ($T{}_{c}\ensuremath{\approx}2.8$ K) with a significantly enhanced anisotropy of upper critical field (${\ensuremath{\gamma}}_{{H}_{c2}}={H}_{c2}^{ab}/{H}_{c2}^{c}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}17$). In addition, first-principles calculations reveal that ${\mathrm{Pb}}_{1/3}\mathrm{Ta}{\mathrm{S}}_{2}$ hosts multiple topological Dirac fermions in the electronic band structure. We discover four groups of Dirac nodal lines on the ${k}_{z}=\ensuremath{\pi}$ plane and two sets of Dirac points on the rotation or screw axes, which are protected by crystalline symmetries and robust against spin-orbit coupling (SOC). Dirac-cone-like surface states emerge on the (001) surface because of band inversion. Our work shows that the TCM candidate ${\mathrm{Pb}}_{1/3}\mathrm{Ta}{\mathrm{S}}_{2}$ is a promising arena to study the interplay between superconductivity and topological Dirac fermions.

Anisotropic superconductivity in the topological crystalline metal Pb 1 / 3 TaS 2 with multiple Dirac fermions

Herein, first-principles calculations were employed to study the electronic, topological, and superconducting properties of InTaX2(X = S, Se). InTaX2exhibits nodal lines in the absence of spin-orbit coupling (SOC); on SOC inclusion, the nodal lines form Weyl rings with the Weyl points classified as a type-II Weyl semimetal (WSM) with tilted cones. Using Green functions method calculations, surface states distinguishable from the bulk states, and Fermi arcs surface states were visualized on the (001) easily cleavable indium terminated surface of both materials. The electron-phonon calculations using the Allen-Dynes relations predict InTaSe2and InTaS2to be superconducting around 2.38 K and 3.25 K. The prediction of these exotic properties in InTaX2(X = S, Se) makes them suitable for experimental validation of topological superconductivity in type-II WSMs.

Superconductivity in quasi-2D InTaX2(X = S, Se) type-II Weyl semimetals.

Topological materials and topological phases have recently become a hot topic in condensed matter physics. In this work, we report a topological nodal-line semimetal InxTaSe2, in the presence of both charge density wave (CDW) and superconductivity. In the x = 0.58 samples, the 2 * /3 commensurate CDW (CCDW) and the 2 * 2 CCDW are observed below 116 K and 77 K, respectively. Consistent with theoretical calculations, the spin-orbital coupling gives rise to two two-fold-degenerate nodal rings (Weyl rings) connected by drumhead surface states, confirmed by angle-resolved photoemission spectroscopy. Our results suggest that the 2 * 2 CCDW ordering gaps out one Weyl ring in accordance with the CDW band folding, while the other Weyl ring remains gapless with intact surface states. In addition, superconductivity emerges at 0.91 K, with the upper critical field deviating from the s-wave behavior at low temperature, implying possibly unconventional superconductivity. Therefore, InxTaSe2 represents an interesting material system to study the interplay between CDW, nontrivial band topology and superconductivity.

Anisotropic gapping of topological Weyl rings in the charge-density-wave superconductor InxTaSe2

Topological materials and topological phases have recently become a hot topic in condensed matter physics. In this work, we report an In-intercalated transition-metal dichalcogenide InxTaSe2 (named 112 system), a topological nodal-line semimetal in the presence of both charge density wave (CDW) and superconductivity. In the x = 0.58 sample, the 2×3 commensurate CDW (CCDW) and the 2×2 CCDW are observed below 116 and 77 K, respectively. Consistent with theoretical calculations, the spin–orbital coupling gives rise to two twofold-degenerate nodal rings (Weyl rings) connected by drumhead surface states, confirmed by angle-resolved photoemission spectroscopy. Our results suggest that the 2×2 CCDW ordering gaps out one Weyl ring in accordance with the CDW band folding, while the other Weyl ring remains gapless with intact surface states. In addition, superconductivity emerges at 0.91 K, with the upper critical field deviating from the s-wave behavior at low temperature, implying possibly unconventional superconductivity. Therefore, we think this type of the 112 system may possess abundant physical states and offer a platform to investigate the interplay between CDW, nontrivial band topology and superconductivity.

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Anisotropic gapping of topological Weyl rings in the charge-density-wave superconductor InxTaSe2

Anisotropic gapping of topological Weyl rings in the charge-density-wave superconductor InxTaSe2