J. Phys. Soc. Jpn. 86, 123701 (2017) [4 Pages]
LETTERS

SnAs-Based Layered Superconductor NaSn2As2

Jun Sung Kim
JPSJ News Comments 14,  13 (2017).

+ Affiliations
Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan

Superconducting behavior with exotic characteristics is often observed in materials with a layered two-dimensional crystal structure. Low dimensionality affects the electronic structure of these materials, potentially leading to high transition temperatures (Tc) and/or unconventional pairing mechanisms. In this letter, we report on superconductivity in layered tin arsenide NaSn2As2. The crystal structure consists of Sn2As2 bilayers, bound by the van der Waals forces and 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 suggest that layered SnAs is a basic structure, offering another universality class in the family of layered superconductors. The results provide a new platform for the studies of physics and chemistry of low-dimensional superconductors with lone pair electrons.

©2017 The Author(s)
This article is published by the Physical Society of Japan under the terms of the Creative Commons Attribution 4.0 License. Any further distribution of this work must maintain attribution to the author(s) and the title of the article, journal citation, and DOI.

Crystals with layered structure have been attractive for exploration of high-transition-temperature (\(T_{\text{c}}\)) superconductivity, and for discussions of the mechanisms of unconventional superconductivity, as exemplified by the work on cuprates1) and Fe-based superconductors.2) The discovery of a basic structure, which plays the role of a superconducting layer, such as the CuO2 plane and the Fe2An2 (\(\text{An}=\text{P}\), As, S, Se, Te) layer, has opened new vistas in the fields of physics and chemistry of low-dimensional superconductors, because many structural analogs could be designed by varying the structure or the alignment of the spacer layers and/or superconducting layers. Since 2012, our group has been developing BiCh2-based (\(\text{Ch}=\text{S}\), Se) layered superconductors.35) Although early studies have suggested conventional pairing mechanisms for BiCh2-based compounds,610) recent studies, including first-principles calculations,11) angle-resolved photoemission spectroscopy studies,12) and the isotope effect studies,13) have indicated unconventional pairing mechanisms in BiCh2-based superconductors. One candidate mechanism is charge fluctuation-mediated, which was proposed based on the neutron studies of the in-plane local structure of RE(O,F)BiS2,1416) where RE represents a rare earth element. The locally distorted in-plane (Bi–Ch plane) structure should originate from the structural instability owing to Bi3+ lone pair electrons. Studies of chemical pressure effects in RE(O,F)BiS2 suggested that these lone-pair effects are likely related to the emergence of superconductivity.17) In addition, the presence of lone pairs induces the van-der-Waals (vdW) gap between BiCh planes; hence, the crystals of BiCh2-based superconductors can be readily exfoliated, which is useful for investigating electronic properties using surface sensitive probes,12,18) and for developing other functionalities.

In this letter, we present a novel layered NaSn2As2 superconductor containing Sn2+ ions with lone-pair electrons. NaSn2As2 crystallizes in a trigonal \(R\bar{3}m\) unit cell characterized by SnAs bilayers separated by six coordinated Na+ cations, as schematically shown in the inset of Fig. 1.19,20) SnAs bilayers are bound by the vdW forces with an approximately 3.3-Å-wide gap between the Sn atoms of adjacent layers. Assuming a +2 oxidation state for Sn, and a −3 oxidation state for As,21) a conducting SnAs layer is negatively charged as [Sn2As2]2−. Thus, NaSn2As2 is not an electron-balanced compound, as was recently reported for Li\(_{1-x}\)Sn\(_{2+x}\)As2,22) which contrasts with isostructural electron-balanced compounds SrSn2As223) and EuSn2As2.24) Owing to the vdW gap between adjacent SnAs layers, these compounds can be readily exfoliated using both mechanical and liquid-phase methods.20,24) Density functional theory calculations suggest that the Fermi surface in these compounds dominantly consists of hybridization between Sn and As orbitals.20,24) Because there exist various structurally analogous compounds with conducting SnPn layers (\(\text{Pn}=\text{P}\), As, Sb), such as ASn2Pn2 (\(\text{A}=\text{Li}\), Na, K, Sr, Eu1921)), ASnPn,23,26,27) and Sn4Pn3,28,29) the discovery of superconductivity in NaSn2As2 is likely to open new vistas in the field of physics and chemistry of layered superconductors.


Figure 1. (Color online) PXRD pattern of NaSn2As2. Vertical tick marks represent the Bragg diffraction angles of NaSn2As2. The asterisk denotes the diffraction peak owing to the elemental Sn. The inset schematically shows the crystal structure of NaSn2As2 (\(R\bar{3}m\) space group). The black solid line represents the unit cell in the hexagonal setting. The crystal structure was depicted using VESTA.32)

Single crystals of NaSn2As2 were synthesized by melting raw elements. A surface oxide layer of Na (Sigma-Aldrich, 99.9%) was mechanically cleaved before experiments. Sn (99.99%) and As (99.9999%) were purchased from Kojundo Chemical Laboratory and used without further purification. The raw elements were handled in an Ar-filled glovebox with a gas-purifier system. The mixture at a stoichiometric ratio of Na, Sn, and As was sealed in an evacuated quartz tube, heated at 750 °C for 20 h, and cooled to 350 °C in 10 h. Energy dispersive X-ray spectroscopy revealed that the chemical composition of the obtained products was \(\text{Na}:\text{Sn}:\text{As}= 1:1.89(8):2.12(7)\). The phase purity of the sample was examined using a powder X-ray diffractometer (PXRD; Rigaku miniflex 600), using the powders that were prepared by grinding the obtained crystals. Lattice parameters were calculated using the Rietveld refinement method, using RIETAN-FP.30) Single-crystal X-ray diffractometry (XRD; Rigaku XtaLAB) for a selected small single crystal with dimensions of \(0.06\times 0.1\times 0.07\) mm3 was performed and the results were analyzed using the program SHELX-97.31) Temperature (T) dependence of electrical resistivity (ρ) of a sample flake with dimensions of \(3\times 1\times 0.04\) mm3 was measured using the four-terminal method with a physical property measurement system (PPMS; Quantum Design) equipped with a 3He-probe system. A magnetic field was applied along the c-axis direction of the crystal. The dependence of the specific heat (C) on T was measured using the relaxation method with PPMS.

Figure 1 shows the PXRD pattern of the obtained sample. Almost all of the sample's diffraction peaks were assigned to those of the NaSn2As2 phase, indicating that the obtained sample was nearly single-phase. However, tiny diffraction peaks attributable to the elemental Sn were also observed, as indicated by the asterisk in Fig. 1. Lattice parameters calculated using the Rietveld refinement method of the PXRD pattern were \(a = 4.00409(10)\) Å and \(c = 27.5944(5)\) Å. The crystal's structural parameters, obtained from single-crystal XRD, are summarized in Table I. In general, these structural parameters were in a good agreement with those that were reported previously.19,20) We cannot exclude the possibility of some (of the order of several percent) Na deficiency in the present structural refinement. However, we note that refinements by partially substituting the Sn site by Na did not improve the structural refinement, unlike in the case of isostructural Li\(_{1-x}\)Sn\(_{2+x}\)As2.22)

Data table
Table I. Structural parameters of NaSn2As2 at room temperature determined by single-crystal XRD measurements. \(R_{1}\) and \(wR_{2}\) are the reliability factors and B is the equivalent isotropic atomic displacement parameter. Standard deviations are given in parentheses. Site occupancy of each site is unity.

Figures 2(a) and 2(b) show the ρT plots for NaSn2As2. Metallic behavior of the sample's electrical resistivity was observed at temperatures above 10 K. A sharp drop in ρ was observed at 1.3 K, accompanied by zero resistivity at temperatures under 1.1 K, which indicates the superconductivity transition. Note that a steep drop, except for this range of temperatures was not observed [Fig. 2(b)], although a small amount of elemental Sn (\(T_{\text{c}} = 3.7\) K) was detected using PXRD. The transition temperature of NaSn2As2 shifted toward lower temperatures with increasing the intensity of the applied magnetic field, as shown in Fig. 2(c). The temperatures \(T_{\text{c}}^{90\%}\) and \(T_{\text{c}}^{\text{zero}}\), obtained from the temperature dependences of electrical resistivity for the applied magnetic field up to 3000 Oe, are shown in Fig. 2(d). Here, \(T_{\text{c}}^{90\%}\) is defined as the temperature at which ρ is at 90% of the value at 3 K (normal states), as indicated by a dashed line in Fig. 3(c). The dependence of the upper critical field (\(H_{\text{c2}}\)) on temperature is still almost linear at \(T\approx 0.5\) K. From the Werthamer–Helfand–Hohemberg (WHH) model,33) the value of \(H_{\text{c2}}\) at 0 K was estimated to be \(H_{\text{c2}}(0)\sim 2000\) Oe. From \(H_{\text{c2}}\), the coherence length was estimated to be ∼41 nm using the equation of \(\xi^{2} =\Phi_{0}/2\pi\mu_{0}H_{\text{c2}}\). Note that a kink was observed in ρT data for magnetic field intensities of 1500 and 2000 Oe, at ∼0.7 K [Fig. 2(c)]. This likely reflects highly anisotropic superconducting states owing to the sample's layered structure; \(H_{\text{c2}}\) is believed to be higher when the magnetic field is applied parallel to the ab-plane. Although the magnetic field was applied along the c-axis of the crystal, some domain areas may be tilted with respect to the magnetic field. Another possibility is local inhomogeneity on the nanoscale, which has been revealed for Li\(_{1-x}\)Sn\(_{2+x}\)As2 by combinational analysis using nuclear magnetic resonance, transmission electron microscopy, and neutron diffraction.22) The ρT measurements for the magnetic field oriented in parallel to the ab plane were not performed in the present study, owing to the limitations of the measurement apparatus.


Figure 2. (Color online) (a) T dependence of electrical resistivity (ρ) of NaSn2As2. (b) ρT data for temperatures under 4 K. (c) ρT data for applied magnetic field (H) up to 3000 Oe. The magnetic field was applied along the c-axis. The dashed line represents 90% of ρ at 3 K. (d) Magnetic field-temperature phase diagram of NaSn2As2. The dashed lines represent the least-squares fits to data.


Figure 3. (Color online) (a) T dependence of specific heat (C) divided by T for NaSn2As2 at magnetic fields (H) of 0 and 30000 Oe. (b) \(C/T\)\(T^{2}\) plot at 30000 Oe. The red solid line is the least-squares fit. (c) \(C_{\text{el}}\)T plot at 0 Oe. The black solid line is used to estimate the specific heat jump (\(\Delta C_{\text{el}}\)) at \(T_{\text{c}}\).

Figure 3(a) shows \(C/T\) as a function of T, for the applied magnetic fields of 0 and 30000 Oe. A steep jump in \(C/T\) is observed at \(T_{\text{c}} = 1.3\) K, which is consistent with the superconductivity transition seen in the ρT data. Figure 3(b) shows the \(C/T\)\(T^{2}\) plot measured at 30000 Oe. By performing least-squares fitting to \(C/T =\gamma +\beta T^{2}\), where γ and β represent the electronic specific heat parameter and the phonon contribution parameter, respectively, we obtained those parameters as \(\gamma = 3.97\) mJ·mol−1·K−2 and \(\beta = 1.120\) mJ·mol−1·K−2. The Debye temperature \(\Theta_{\text{D}} = (12\pi^{4}rN_{\text{A}}k_{\text{B}}/5\beta)^{1/3}\) was estimated to be 205 K, where \(r = 5\) is the number of atoms per formula unit, \(N_{\text{A}}\) is the Avogadro constant, and \(k_{\text{B}}\) is the Boltzmann constant, respectively. The electronic contribution to specific heat, \(C_{\text{el}}\), was determined by subtracting the contribution of phonons, which can be described as \(\beta T^{3}\) for \(T\ll \Theta_{\text{D}}\), from total C. Figure 3(c) shows the \(C_{\text{el}}/T\)T plot. The specific heat jump at \(T_{\text{c}}\) (\(\Delta C_{\text{el}}\)) was 7.82 mJ·mol−1·K−2. From the obtained parameters, \(\Delta C_{\text{el}}/\gamma T_{\text{c}}\) was calculated as 1.50, which was slightly larger than the value expected from the weak-coupling Bardeen–Cooper–Schrieffer (BCS) approximation (\(\Delta C_{\text{el}}/\gamma T_{\text{c}} = 1.43\)). The slightly larger superconducting gap may result from the strong-coupling nature of superconductivity, but it seems reasonable as a fully gapped superconductor. The electron–phonon coupling constant (λ) can be determined from Macmillan's theory,34) which gives \begin{equation*} \lambda = \frac{1.04 + \mu^{*}\ln (\Theta_{\text{D}}/1.45T_{\text{c}})}{(1 - 0.62\mu^{*})\ln (\Theta_{\text{D}}/1.45T_{\text{c}}) - 1.04}, \end{equation*} where \(\mu^{*}\) is defined as the Coulomb pseudopotential. Taking \(\mu^{*} = 0.13\) gives \(\lambda = 0.44\), which is consistent with the weak or intermediate coupling strength of the BCS superconductivity. As a result, the bulk nature of superconductivity at \(T_{\text{c}} = 1.3\) K in NaSn2As2 has been confirmed from electrical resistivity and specific heat measurements.

We briefly discuss the difference between the crystal structures of NaSn2As2 and Li\(_{1-x}\)Sn\(_{2+x}\)As2 by comparing the observations for layered FeAs-based compounds LiFeAs35) and NaFeAs.36) In the LiFeAs phase, a Li deficiency is present and the obtained sample (Li\(_{1-x}\)FeAs) exhibits superconductivity without any magnetic ordering,35) which indicates that the Li deficiency results in carrier doping and suppression of antiferromagnetic ordering, which is typical of the parent phase of FeAs-based superconductors. In contrast, NaFeAs is nearly stoichiometric and exhibits antiferromagnetic ordering, and Co-doped NaFe\(_{2-x}\)CoxAs2 becomes superconducting.36) The different physical properties may result from the different ionic radii: 92 and 118 pm for Li+ and Na+, respectively, when the coordination number is 8. Based on these differences between alkaline site deficiencies of Li- and Na-containing FeAs systems, we speculate that our NaSn2As2 sample is almost stoichiometric, similar to NaFeAs, and is almost free of site deficiencies or solutions with Sn, while a Li/Sn solution was reported for Li\(_{1-x}\)Sn\(_{2+x}\)As2.22)

In this letter, we described and characterized a novel layered NaSn2As2 superconductor. Because there exist various structural analogs of SnPn layers, superconductivity of SnPn layers may in general emerge when carriers are doped with these SnAs-based layered compounds. Furthermore, a three-dimensional Dirac semimetal state and a non-trivial topological phase were predicted in these compounds.37,38) Our present discovery opens new vistas for the fields of physics and chemistry of low-dimensional superconductors and related notable phenomena.

Acknowledgement

This work was partly supported by Grants-in-Aid for Scientific Research (Nos. 15H05886, 15H05884, 16H04493, 17K19058, 16K17944, and 15H03693), JST-PRESTO (No. JPMJPR16R1), and JST-CREST (No. JPMJCR16Q6), Japan.


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