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Register a new userRobust Gapless Superconductivity in 4Hb-TaS$_2$
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The superconducting TMD 4Hb-TaS$_2$ consists of alternating layers of H and T structures, which in their bulk form are metallic and Mott-insulating, respectively. Recently, this compound has been proposed as a candidate chiral superconductor, due to an observed enhancement of the muon spin relaxation at $T_c$. 4Hb-TaS$_2$ also exhibits a puzzling $T$-linear specific heat at low temperatures, which is unlikely to be caused by disorder. Elucidating the origin of this behavior is an essential step in discerning the true nature of the superconducting ground state. Here, we propose a simple model that attributes the $T$-linear specific heat to the emergence of a robust multi-band gapless superconducting state. We show that an extended regime of gapless superconductivity naturally appears when the pair-breaking scattering rate on distinct Fermi-surface pockets differs significantly, and the pairing interaction is predominantly intra-pocket. Using a tight-binding model derived from first-principle calculations, we show that the pair-breaking scattering rate promoted by slow magnetic fluctuations on the T layers, which arise from proximity to a Mott transition, can be significantly different in the various H-layer dominated Fermi pockets depending on their hybridization with T-layer states. Thus, our results suggest that the ground state of 4Hb-TaS$_2$ consists of Fermi pockets displaying gapless superconductivity, which are shunted by superconducting Fermi pockets that are nearly decoupled from the T-layers.
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Layered van der Waals (vdW) materials are emerging as one of the most versatile directions in the field of quantum condensed matter physics. They allow an unprecedented control of electronic properties via stacking of different types of two-dimensional (2D) materials. A fascinating frontier, largely unexplored, is the stacking of strongly-correlated phases of matter in vdW materials. Here, we study 4Hb-TaS$_2$, which naturally realizes an alternating stacking of a Mott insulator, recently reported as a gapless spin-liquid candidate(1T-TaS$_2$), and a 2D superconductor (1H-TaS$_2$). This raises the question of how these two components affect each other. We find a superconducting ground state with a transition temperature of 2.7K, which is significantly elevated compared to the 2H polytype (Tc=0.7K). Strikingly, the superconducting state exhibits signatures of time-reversal-symmetry breaking abruptly appearing at the superconducting transition, which can be naturally explained by a chiral superconducting state.
When approaching the atomically thin limit, defects and disorder play an increasingly important role in the properties of two-dimensional materials. Superconductivity is generally thought to be vulnerable to these effects, but here we demonstrate the contrary in the case of oxygenation of ultrathin tantalum disulfide (TaS$_2$). Our first-principles calculations show that incorporation of oxygen into the TaS$_2$ crystal lattice is energetically favourable and effectively heals sulfur vacancies typically present in these crystals, thus restoring the carrier density to the intrinsic value of TaS$_2$. Strikingly, this leads to a strong enhancement of the electron-phonon coupling, by up to 80% in the highly-oxygenated limit. Using transport measurements on fresh and aged (oxygenated) few-layer TaS$_2$, we found a marked increase of the superconducting critical temperature ($T_{mathrm{c}}$) upon aging, in agreement with our theory, while concurrent electron microscopy and electron-energy loss spectroscopy confirmed the presence of sulfur vacancies in freshly prepared TaS$_2$ and incorporation of oxygen into the crystal lattice with time. Our work thus reveals the mechanism by which certain atomic-scale defects can be beneficial to superconductivity and opens a new route to engineer $T_{mathrm{c}}$ in ultrathin materials.
1T-TaS$_2$ is a layered transition metal dichalgeonide with a very rich phase diagram. At T=180K it undergoes a metal to Mott insulator transition. Mott insulators usually display anti-ferromagnetic ordering in the insulating phase but 1T-TaS$_2$ was never shown to order magnetically. In this letter we show that 1T-TaS$_2$ has a large paramagnetic contribution to the magnetic susceptibility but it does not show any sign of magnetic ordering or freezing down to 20mK, as probed by $mu$SR, possibly indicating a quantum spin liquid ground state. Although 1T-TaS$_2$ exhibits a strong resistive behavior both in and out-of plane at low temperatures we find a linear term in the heat capacity suggesting the existence of a Fermi-surface, which has an anomalously strong magnetic field dependence.
Topological crystalline metals/semimetals (TCMs) have stimulated a great research interest, which broaden the classification of topological phases and provide a valuable platform to explore topological superconductivity. Here, we report the discovery of superconductivity and topological features in Pb-intercalated transition-metal dichalcogenide Pb$_{1/3}$TaS$_2$. Systematic measurements indicate that Pb$_{1/3}$TaS$_2$ is a quasi-two-dimensional (q-2D) type-II superconductor ({em T}$_c approx$ 2.8 K) with a significantly enhanced anisotropy of upper critical field ($gamma_{H_{c2}}$ = $H_{c2}^{ab}/H_{c2}^{c}$ $approx$ 17). In addition, first-principles calculations reveal that Pb$_{1/3}$TaS$_2$ hosts multiple topological Dirac fermions in the electronic band structure. We discover four groups of Dirac nodal lines on the $k_z = pi$ plane and two sets of Dirac points on the rotation/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 Pb$_{1/3}$TaS$_2$ is a promising arena to study the interplay between superconductivity and topological Dirac fermions.
To reveal the nature of elementary excitations in a quantum spin liquid (QSL), we measured low temperature thermal conductivity and specific heat of 1T-TaS$_2$, a QSL candidate material with frustrated triangular lattice of spin-1/2. The nonzero temperature linear specific heat coefficient $gamma$ and the finite residual linear term of the thermal conductivity in the zero temperature limit $kappa_0/T=kappa/T(Trightarrow 0)$ are clearly resolved. This demonstrates the presence of highly mobile gapless excitations, which is consistent with fractionalized spinon excitations that form a Fermi surface. Remarkably, an external magnetic field strongly suppresses $gamma$, whereas it enhances $kappa_0/T$. This unusual contrasting behavior in the field dependence of specific heat and thermal conductivity can be accounted for by the presence of two types of gapless excitations with itinerant and localized characters, as recently predicted theoretically (I. Kimchi et al., arXiv:1803.00013 (2018)). This unique feature of 1T-TaS$_2$ provides new insights into the influence of quenched disorder on the QSL.
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