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Register a new userSnAs-based layered superconductor NaSn2As2
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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.
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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.
Superconducting transition temperature (Tc) reported in SnAs-based layered compound NaSn$_2$As$_2$ varies from 1.2 to 1.6 K, implying that its superconductivity is critically sensitive to non-stoichiometry. Here, we demonstrate that Na-doping on the Sn site (Na$_{1+x}$Sn$_{2-x}$As$_2$) is effective in enhancing superconductivity, leading to Tc = 2.1 K for x = 0.4. First-principles calculation indicates that such a doping, or Na$_{rm Sn}$ antisite defects, is energetically favored over other cation vacancies. Our results pave the way for increasing Tc of layered tin pnictide superconductors.
Layered superconductors have provided some interesting fields in condensed matter physics owing to the low dimensionality of their electronic states. For example, the high-Tc (high transition temperature) cuprates and the Fe-based superconductors possess a layered crystal structure composed of a stacking of spacer (blocking) layers and conduction (superconducting) layers, CuO2 planes or Fe-Anion layers. The spacer layers provide carriers to the conduction layers and induce exotic superconductivity. Recently, we have reported superconductivity in the novel BiS2-based layered compound Bi4O4S3. It was found that superconductivity of Bi4O4S3 originates from the BiS2 layers. The crystal structure is composed of a stacking of BiS2 superconducting layers and the spacer layers, which resembles those of high-Tc cuprate and the Fe-based superconductors. Here we report a discovery of a new type of BiS2-based layered superconductor LaO1-xFxBiS2, with a Tc as high as 10.6 K.
We have employed a new route to synthesize single phase F-doped LaOFeAs compound and confirmed the superconductivity above 20 K in this Fe-based system. We show that the new superconductor has a rather high upper critical field of about 54 T. A clear signature of superconducting gap opening below T$_c$ was observed in the far-infrared reflectance spectra, with 2$Delta/textit{k}T_capprox$3.5-4.2. Furthermore, we show that the new superconductor has electron-type conducting carrier with a rather low carrier density.
The recent discovery of superconductivity in NaSn$_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.
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