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Register a new userNodeless superconductivity in the SnAs-based van der Waals type superconductor NaSn2As2
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We grew the single crystals of the SnAs-based van der Waals (vdW)-type superconductor NaSn$_2$As$_2$ and systematically measured its resistivity, specific heat, and ultralow-temperature thermal conductivity. The superconducting transition temperature $T_c$ = 1.60 K of our single crystal is 0.3 K higher than that previously reported. A weak but intrinsic anomaly situated at 193 K is observed in both resistivity and specific heat, which likely arises from a charge-density-wave (CDW) instability. Ultralow-temperature thermal conductivity measurements reveal a fully-gapped superconducting state with a negligible residual linear term in zero magnetic field, and the field dependence of $kappa_0 / T$ further suggests NaSn$_2$As$_2$ is an $s$-wave superconductor.
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Superconductivity with exotic properties has often been discovered in materials with a layered (two-dimensional) crystal structure. The low dimensionality affects the electronic structure of materials, which could realize a high transition temperature (Tc) and/or unconventional pairing mechanisms. Here, we report the superconductivity in a layered tin arsenide NaSn2As2. The crystal structure consists of (Sn2As2)2- bilayers, which is bound by van-der-Waals forces, separated by Na+ ions. Measurements of electrical resistivity and specific heat confirm the bulk nature of superconductivity of NaSn2As2 with Tc of 1.3 K. Our results propose that the SnAs layers will be a basic structure providing another universality class of a layered superconducting family, and it provides a new platform for the physics and chemistry of low-dimensional superconductors with lone pair electrons.
We report a comprehensive study of the noncentrosymmetric superconductor Mo$_3$P. Its bulk superconductivity, with $T_c = 5.5$ K, was characterized via electrical resistivity, magnetization, and heat-capacity measurements, while its microscopic electronic properties were investigated by means of muon-spin rotation/relaxation ($mu$SR) and nuclear magnetic resonance (NMR) techniques. In the normal state, NMR relaxation data indicate an almost ideal metallic behavior, confirmed by band-structure calculations, which suggest a relatively high electron density of states, dominated by the Mo $4d$-orbitals. The low-temperature superfluid density, determined via transverse-field $mu$SR and electronic specific heat, suggest a fully-gapped superconducting state in Mo$_3$P, with $Delta_0= 0.83$ meV, the same as the BCS gap value in the weak-coupling case, and a zero-temperature magnetic penetration depth $lambda_0 = 126$ nm. The absence of spontaneous magnetic fields below the onset of superconductivity, as determined from zero-field $mu$SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of Mo$_3$P and, hence, spin-singlet pairing.
The recent discovery of superconductivity in NaSn$_2$As$_2$ with a van der Waals layered structure raises immediate questions on its pairing mechanism and underlying electronic structure. Here, we present measurements of the temperature-dependent magnetic penetration depth $lambda(T)$ in single crystals of NaSn$_2$As$_2$ down to $sim40$ mK. We find a very long penetration depth $lambda (0) = 960$ nm, which is strongly enhanced from the estimate of first-principles calculations. This enhancement comes from a short mean free path $ell approx 1.7$ nm, indicating atomic scale disorder possibly associated with the valence-skipping states of Sn. The temperature dependence of superfluid density is fully consistent with the conventional fully gapped s-wave state in the dirty limit. These results suggest that NaSn$_2$As$_2$ is an ideal material to study quantum phase fluctuations in strongly disordered superconductors with its controllable dimensionality.
Structural and superconducting transitions of layered van der Waals (vdW) hydrogenated germanene (GeH) were observed under high-pressure compression and decompression processes. GeH possesses a superconducting transition at critical temperature (Tc) of 5.41 K at 8.39 GPa. A crystalline to amorphous transition occurs at 16.80 GPa while superconductivity remains. An abnormally increased Tc up to 6.1 K has been observed in the decompression process while the GeH remained amorphous. Thorough in-situ high-pressure synchrotron X-ray diffraction and in-situ high-pressure Raman spectroscopy with the density functional theory simulations suggest that the superconductivity of GeH should be attributed to the increased density of states at the Fermi level as well as the enhanced electron-phonon coupling effect under high pressure. The decompression-driven superconductivity enhancement arises from pressure-induced phonon softening related to an in-plane Ge-Ge phonon mode. As an amorphous metal hydride superconductor, GeH provides a platform to study amorphous hydride superconductivity in layered vdW materials.
The designer approach has become a new paradigm in accessing novel quantum phases of matter. Moreover, the realization of exotic states such as topological insulators, superconductors and quantum spin liquids often poses challenging or even contradictory demands for any single material. For example, it is presently unclear if topological superconductivity, which has been suggested as a key ingredient for topological quantum computing, exists at all in any naturally occurring material . This problem can be circumvented by using designer heterostructures combining different materials, where the desired physics emerges from the engineered interactions between the different components. Here, we employ the designer approach to demonstrate two major breakthroughs - the fabrication of van der Waals (vdW) heterostructures combining 2D ferromagnetism with superconductivity and the observation of 2D topological superconductivity. We use molecular-beam epitaxy (MBE) to grow two-dimensional islands of ferromagnetic chromium tribromide (CrBr$_3$) on superconducting niobium diselenide (NbSe$_2$) and show the signatures of one-dimensional Majorana edge modes using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). The fabricated two-dimensional vdW heterostructure provides a high-quality controllable platform that can be integrated in device structures harnessing topological superconductivity. Finally, layered heterostructures can be readily accessed by a large variety of external stimuli potentially allowing external control of 2D topological superconductivity through electrical, mechanical, chemical, or optical means.
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