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Register a new userChiral superconductivity in the alternate stacking compound 4Hb-TaS$_2$
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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.
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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.
The tetragonal Mo$_5$PB$_2$ compound was recently reported to show superconductivity with a critical temperature up to 9.2 K. In search of evidence for multiple superconducting gaps in Mo$_5$PB$_2$, comprehensive measurements, including magnetic susceptibility, electrical resistivity, heat capacity, and muon-spin rotation and relaxation ($mu$SR) measurements were carried out. Data from both low-temperature superfluid density and electronic specific heat suggest a nodeless superconducting ground state in Mo$_5$PB$_2$. Two superconducting energy gaps $Delta_0$ = 1.02 meV (25%) and 1.49 meV (75%) are required to describe the low-$T$ electronic specific-heat data. The multigap features are clearly evidenced by the field dependence of the electronic specific-heat coefficient and the Gaussian relaxation rate in the superconducting state (i.e., superfluid density), as well as by the temperature dependence of the upper critical field. By combining our extensive experimental results with numerical band-structure calculations, we provide compelling evidence of multigap superconductivity in Mo$_5$PB$_2$.
We investigated the superconducting gap structure of SrNi$_2$P$_{2}$ ($T_c$=1.4 K) via low-temperature magneto-thermal conductivity $kappa(T,H)$ measurements. Zero field thermal conductivity $kappa$ decreases exponentially $kappa propto$ exp($-aT_c/T$) with $a$=1.5, in accord with the BCS theory, and rolls over to a phonon-like $kappapropto T^3$ behavior at low temperature, similar to a number of conventional s-wave superconductors. In addition, we observed a concave field dependence of the residual linear term $kappa_0(H)/T$. These facts strongly rule out the presence of nodes in the superconducting energy gap of SrNi$_2$P$_{2}$. Together with a fully gapped Fermi surface in the superconducting state of BaNi$_2$As$_{2}$ ($T_c$=0.6-0.7 K), demonstrated in our recent work, these results lead us to postulate that fully gapped superconductivity is a universal feature of Ni-based pnictide superconductors.
The pressure effects on the antiferromagentic orders in iron-based ladder compounds CsFe$_2$Se$_3$ and BaFe$_2$S$_3$ have been studied using neutron diffraction. With identical crystal structure and similar magnetic structures, the two compounds exhibit highly contrasting magnetic behaviors under moderate external pressures. In CsFe$_2$Se$_3$ the ladders are brought much closer to each other by pressure, but the stripe-type magnetic order shows no observable change. In contrast, the stripe order in BaFe$_2$S$_3$, undergoes a quantum phase transition where an abrupt increase of N$acute{e}$el temperature by more than 50$%$ occurs at about 1 GPa, accompanied by a jump in the ordered moment. With its spin structure unchanged, BaFe$_2$S$_3$ enters an enhanced magnetic phase that bears the characteristics of an orbital selective Mott phase, which is the true neighbor of superconductivity emerging at higher pressures.
We report the synthesis, magnetic susceptibility and crystal structure analysis for NbB2+x (x = 0.0 to 1.0) samples. The study facilitates in finding a correlation among the lattice parameters, chemical composition and the superconducting transition temperature Tc. Rietveld analysis is done on the X- ray diffraction patterns of all synthesized samples to determine the lattice parameters. The a parameter decreases slightly and has a random variation with increasing x, while c parameter increases from 3.26 for pure NbB2 to 3.32 for x=0.4 i.e. NbB2.4. With higher Boron content (x>0.4) the c parameter decreases slightly. The stretching of lattice in c direction induces superconductivity in the non- stoichiometric niobium boride. Pure NbB2 is non-superconductor while the other NbB2+x (x>0.0) samples show diamagnetic signal in the temperature range 8.9-11K. Magnetization measurements (M-H) at a fixed temperature of 5K are also carried out in both increasing and decreasing directions of field. The estimated lower and upper critical fields (Hc1 & Hc2) as viewed from M-H plots are around 590 and 2000Oe respectively for NbB2.6 samples. In our case, superconductivity is achieved in NbB2 by varying the Nb/B ratios, rather than changing the processing conditions as reported by others.
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