Superconductivity in a misfit layered (SnS)1.15(TaS2) compound

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Cite This: Chem. Mater. 2018, 30, 1373−1378

Superconductivity in a Misfit Layered (SnS)1.15(TaS2) Compound Raman Sankar,*,†,‡,○ G. Peramaiyan,†,○ I. Panneer Muthuselvam,†,‡,§ Cheng-Yen Wen,∥ Xiaofeng Xu,⊥,# and F. C. Chou*,‡,∇,¶ †

Institute of Physics, Academia Sinica, Taipei 10617, Taiwan Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan § Department of Materials Science, Central University of Tamil Nadu, Neelakudi, Thiruvarur 610005, Tamil Nadu, India ∥ Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan ⊥ Advanced Functional Materials Lab and Department of Physics, Changshu Institute of Technology, Changshu 215500, China # Department of Physics, Hangzhou Normal University, Hangzhou 310036, China ∇ National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan ¶ Taiwan Consortium of Emergent Crystalline Materials, Ministry of Science and Technology, Taipei 10622, Taiwan ‡

ABSTRACT: We report the single crystal growth and superconducting properties of a misfit layered (SnS)1.15(TaS2) compound. The transport, magnetic, and thermodynamic properties revealed the superconducting transition with an onset temperature of Tc ∼ 3.01 K. The high resolution transmission electron microscopy (HRTEM) image clearly shows the misfit stacking of SnS and TaS2 layers. Based on the Werthamer−Helfand−Hohenberg (WHH) formula and Ginzburg−Landau theory, the upper critical fields are Hc2(0) = 0.64 ± 0.06 T and 0.22 ± 0.02 T with coherence lengths of ξ = 22.67 and 38.68 nm for field applied perpendicular (H⊥) and parallel (H//) to the plane, respectively. On the basis of the specific heat measurement data analysis of derived parameters including Sommerfeld coefficient γ = 5.831 ± 0.012 mJ mol−1 K−2, Debye temperature ΘD = 151 K, specific heat jump ΔCe/γTc = 0.812, and electron−phonon coupling constant λel−ph ∼ 0.724, all indicate the weak-coupling nature for (SnS)1.15(TaS2) as a misfit layered superconductor. Resistivity measurements show that Tc increases from temperature 3.01 to 3.85 K at 1.95 GPa, and linear dependence of Tc as a function of pressure (P) is observed up to 1.583 GPa.

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INTRODUCTION

degree of layer mismatch, and is related to the tolerance factors, which can be estimated from the corresponding ionic radii.13 The 2D structural flexibility under temperature leads to three different phases, including trigonal 1T-TaS2,14 hexagonal 2HTaS2,15 and rhombohedral 3R-TaS2.16 Among the numerous forms of transition metal dichalcogenides, TaS2 has attracted more attention due the observed charge density wave (CDW) accompanied by chiral charge order and superconducting transition in 2H-TaS2.17,18 Due to its structural flexibility, it enables organic and inorganic chemical intercalation to reduce the CDW and raise the superconducting transition temperatures. The hexagonal 2H-TaS2 phase shows superconductivity

The misfit layered compounds attract special attention due to their incommensurate layered structural features that are linked to their unique physical properties.1 The transition metal dichalcogenides having incommensurate intercalated layers sandwiched in between have been reported as showing superconductivity. The general formula of this class of material is represented as (MX)1+δ(TX2)m with m = 1, 2, 3, where MX represents the monochalcogenides layer of M = Sn, Pb, Bi, Sb, and rare earth elements and TX2 represents the transition metal dichalcogenides (TMDC) layer including T = transition metal and X = S, Se, and Te. It is noted that m denotes the number of TMDC layers stacked before each MX layer is intercalated. In particular, the value of δ (0.08 ≤ δ ≤ 0.28) represents the mismatch index.2−12 The noninteger value of δ indicates the © 2018 American Chemical Society

Received: December 8, 2017 Revised: January 20, 2018 Published: January 22, 2018 1373

DOI: 10.1021/acs.chemmater.7b04998 Chem. Mater. 2018, 30, 1373−1378

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Chemistry of Materials at 0.8 K with CDW onset near ∼78 K,17 and the Tc raises from 0.8 to 5.5 K via various organic and inorganic intercalations.19 The trigonal 1T-TaS2 shows only a CDW phase transition at 136 K, and superconductivity occurs under the influence of high pressure.20 Recently, photoemission microspectrocopy provided experimental evidence for the stabilization of misfit compounds through metal cross substitution (partial transition metal substitution from the TX2 (T = Ti, Nb) layer into the MX (M = Pb, Sn) layer and vice versa), and ab initio electronicstructure calculations predicted that the nonstoichiometric (substitution of Pb by Ta in (PbS)1.14(TaS2)) plays a significant role in stabilization of the misfit layer compounds.21,22 Superconducting properties of (SnS)1.15TaS2 misfit layered compound with Tc ∼ 2.9 and 2.85 K have been reported based on studies using powder samples before.6,23,24 Here, we report the detailed procedure for the growth of large size single crystals of (SnS)1.15TaS2. The misfit character has been confirmed with clear pictures obtained with the high resolution transmission electron microscopy (HRTEM). Thorough characterization of a (SnS)1.15TaS2 single crystal sample showing superconductivity onset of Tc ∼ 3.01 K is provided by electric transport, magnetic susceptibility, and specific heat measurements. We found that the superconducting transition temperature (Tc) increases linearly up to 3.77 K under pressure of about 1.583 GPa and then start to saturate when the applied pressure reaches 1.95 GPa.

Figure 1. (a) Crystal structure of (SnS)1.15(TaS2) viewed along the baxis with incommensurability shown in the SnS layer. Ta, Sn, and S atoms are shown in red, blue, and green colors, respectively. (b) Photograph of the as-grown (SnS)1.15(TaS2) single crystal grown from the chemical vapor transport method. (c) X-ray diffraction patterns of (SnS)1.15(TaS2) (upper panel) and 2H-TaS2 (lower panel) crystals, showing preferred orientation of (00l) planes.

is similar to that of a typical 2H-TaS2.26 Figure 2a,b shows the HRTEM image of misfit (SnS)1.15(TaS2), which confirms the regular stacking of SnS and TaS2 layers, which is similar to those reported misfit compounds of (PbS)1.13TaS2 and (SbS)1.16TaS2.27 Resistivity measurements were performed on the ab-plane of (SnS)1.15(TaS2) crystal by employing the four probe method. The resistivity change as a function of temperature is shown in Figure 3a for both in-plane and cross-plane directions, respectively. For the cross-plane direction, the variation of resistivity above ∼50 K is linear to show a metallic behavior with residual resistivity ratio (RRR) about 11, which is comparable to the single crystalline misfit layered compounds reported in the literature, including (BiSe)l.l0(NbSe2) (RRR = 4), (BiS) 1.11 (NbS 2 ) (RRR = 7), and polycrystalline (SnSe)1.18(TiSe2)2 (RRR = 10).28,29 The expanded view of the resistivity vs temperature plot (inset of Figure 3a) shows the onset of superconducting transition temperature Tc ∼ 3.01 K under ambient pressure. The onset of Tc ∼ 3.01 K for (SnS)1.15(TaS2) is higher than that of the pristine 2H-TaS2 (Tc = 1.90 K),30 and the misfit compounds of (SnS) 1.1 (NbS 2 ) (T c = 2.75 K) 6 and (PbS)1.14(NbS2) (Tc = 2.72 K).23 The Hall resistivity of (SnS)1.15(TaS2) is found to be positive with linear dependence of magnetic field up to 9 T at 4 K, as shown in Figure 3b. The inset shows the temperature dependence of the Hall coefficient, which does not show carrier sign change from 300 K down to 4 K. From the field dependence of Hall resistivity, carrier density is calculated to be nh ∼ 4.9 × 1021 cm−3 at 4 K. Figure 3c shows the influence of pressure on the temperature dependent resistivity of (SnS)1.15(TaS2). For the hydrostatic pressure dependent resistivity measurements, samples were loaded into a commercial piston-type pressure cell. The actual pressure of the sample was determined by measuring the superconducting transition temperatures of Pb, and the pressure is applied on the (00l) plane. Daphne 7373 oil was applied as the pressure transmission media. The same contacts were used throughout

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RESULTS AND DISCUSSION We have grown (SnS)1.15(TaS2) single crystals by the chemical vapor transport method. The high quality fine powders of Sn (99.99%), Ta (99.99%), and S (99.99%) in the misfit stoichiometric ratios were used to synthesize the title compound. The mixtures were thoroughly ground and sealed in an evacuated quartz tube and heated at 900 °C for several hours. The prereacted product was loaded into vacuum sealed quartz tubes together with iodine as the transporting agent in an appropriate quantity of ∼1 mg/cm3. A quartz ampule containing source mixtures was placed in a two zone furnace, where the temperatures for the source powders and growth regions were set at 800 and 950 °C separated by about 40 cm, respectively. After a growth period of 200 h, good quality single crystals of (SnS)1.15(TaS2) were seen at the end of the ampules kept at 800 °C. Figure 1b shows the as-grown (SnS)1.15(TaS2) crystal. Powder X-ray diffraction (PWXRD) study was carried out on the as-grown crystal plates of the layered compounds (SnS)1.15(TaS2) and 2H-TaS2, as shown in Figure 1c. The XRD pattern of (SnS)1.15(TaS2) is compared with 2H-TaS2, which confirmed the misfit phase. The PWXRD pattern recorded (00l) planes of preferred orientation for (SnS)1.15(TaS2) indicates that c = 23.7697 Å, which is in accordance with the orthorhombic phase of TaS2 subsystem having a = 5.7406 Å, b = 3.3082 Å, and c = 23.7697 Å and SnS having lattice parameters of a = 5.749 Å, b = 5.737 Å, and c = 11.8755 Å, respectively.25 Since the unit cell parameters for the two subsystems are mutually incommensurate; i.e., the layer misfit must induce rippling plane on one of the subsystems relative to the other. Figure 1a depicts the crystal structure of misfit layered compound (SnS)1.15(TaS2), which consists regular stacking of SnS layers of distorted NaCl-type structure and the 2H-TaS2 layers. It is clearly seen that the Ta sitting in the trigonal prismatic center of six S atoms in TiS6 coordination 1374

DOI: 10.1021/acs.chemmater.7b04998 Chem. Mater. 2018, 30, 1373−1378

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Figure 2. HRTEM images of (SnS)1.15(TaS2) compound: (a) the plane-view along the [001] direction and (b) the cross-sectional view revealing the regular stacking of SnS and TaS2 layers. Red, blue, and green colors denote Ta, Sn, and S atoms, respectively.

Figure 3. (a) Electrical resistivity for the ab-plane of (SnS)1.15(TaS2) crystal as a function of temperature for the in-plane and cross-plane directions. The inset shows the expanded view of superconducting transition of zero resistance near 3.01 K. (b) Field dependence of Hall resistivity measured at 4 K. The inset shows the temperature dependence of Hall coefficient (RH). (c) Temperature-dependent resistivity of (SnS)1.15(TaS2) under pressure. (d) Evolution of Tc under pressure.

sharp superconducting transition and linear pressure dependence of Tc below 1.583 GPa in a (SnS)1.15(TaS2) compound needs further investigation. The photoemission spectral studies of (SnS)1.15(TaS2) have revealed that the Fermi energy of (SnS)1.15(TaS2) and α-SnS (a p-type semiconductor)33 are at the same level, and the obtained emission spectrum of (SnS)1.15(TaS2) is proposed to be the superposition of the individual spectra of α-SnS and TaS2; hence, no charge transfer from the SnS layer to TaS2 layer.34,35 In addition, the 2H-TaS2 phase showed negative Hall coefficient below 56 K,36 whereas 1T-TaS2 showed positive Hall coefficient below 200 K.37 In general, the misfit

the measurements under different pressures such that the geometric errors in the contact size were identical for different runs. Upon increasing the pressure from ambient value taken to be 0 to 1.95 GPa, Tc increases linearly up to 1.583 GPa and starts to saturate at 1.95 GPa as shown in Figure 3d. An initial slope of 0.5 K/GPa is found below 1.583 GPa, which then slows down to 0.21 K/GPa, between 1.583 and 1.95 GPa. The similar behavior of Tc under the influence of pressure is observed in a layered 2H-TaS2 compound,31 and in the (PbSe)1.16(TiSe2)2 misfit superconductor, the superconducting transition temperature (Tc) is initially suppressed and then slightly increased with the increase of the applied pressure.32 A 1375

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Figure 4. (a, b) Temperature dependent resistivity measured in various magnetic for the cross-plane (H∥c I⊥c) and in-plane (H∥c I∥c) directions. (c, d) The corresponding temperature dependence of upper critical fields Hc2 taken from the Tc onset. Insets show the WHH fit yielding a zero temperature limit of the upper critical field Hc2(0).

Figure 5. (a) Zero-field cooled (ZFC) and field cooled (FC) magnetization as a function of temperature at 50 Oe. (b) Temperature dependence of AC susceptibilities measured with rf field of 1 Oe in frequencies of 1−500 Hz. (c) Specific heat curves of C/T vs T2 measured in magnetic fields of H = 0 and 1 T. Sommerfeld and phononic coefficients were obtained from the linear fit of C/T = γ + βT2 for Cp under 1 T. (d) Plot of (C − Cn)/T vs T determines a ΔCe/γTc value of 0.812, where Cn is the phonon contribution to specific heat.

were found.1,4,38 In the case of (SnS)1.17(NbS2), a p-type metallic conduction was found, which has been proposed due to less charge transfer from Sn2+.38 The field-dependent Hall resistivity of (SnS)1.15(TaS2) suggests that the introduction of the SnS layer would not donate electron (e−) into the TaS2 layer, but the hole doping may be coming from a much less e−

compounds could be regarded as the combination of the two subsystems. Based on the rigid band model, it is revealed that the electron transfer occurs from MX to TX2 layer leading to negative Hall coefficient, which has been verified with many monolayer types of di- and trivalent M cations; large charge transfer in trivalent M and less charge transfer in divalent M 1376

DOI: 10.1021/acs.chemmater.7b04998 Chem. Mater. 2018, 30, 1373−1378

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Chemistry of Materials transfer as in the case of (SnS)1.17(NbS2) system. Detailed firstprinciples calculation is required to probe the possible reason for the stability of the (SnS)1.15(TaS2) compound. The temperature dependent upper critical magnetic fields Hc2 for the cross-plane and in-plane directions are obtained from the resistivity measurement in applied magnetic field between 0 and 2 T, as shown in Figure 4a,b. The transition widths are broadened, and the onsets of Tc are reduced with increasing field, as shown in Figure 4c,d. The zero temperature limit of the upper critical field Hc2(0) for the Tc is calculated from the Werthamer−Helfand−Hohenberg (WHH) formula dHc 2 dT

( )

Hc 2(0) = −0.693Tc

superconductor.41 The specific heat jump from the electronic contribution (Ce) is obtained by subtracting the normal state specific heat i.e phonon contribution Cn from the total. Figure 5d shows the plot of C−Cn/T vs T, which yields ΔCe/γTc ∼ 0.812 at the onset of superconducting transition to be lower than the Bardeen−Cooper−Schrieffer (BCS) theoretical value of 1.43 but is comparable to those of the misfit superconductors of (SnSe)1.18(TiSe2)2 (0.88)29 and (Pb0.6 Sn0.4 Se)1.16(TiSe2)2 (1.38).42 In summary, large size plate-like single crystals of misfit layered compound (SnS)1.15(TaS2) were synthesized and grown by the chemical vapor transport method. The misfit phase of (SnS)1.15(TaS2) was confirmed by the X-ray diffraction and HRTEM analyses. Electric transport and magnetic property measurements confirmed the superconducting transition with the onset of a critical temperature of ∼3.01 K. The upper critical field Hc2(0) is calculated to be 0.64 ± 0.06 T and 0.22 ± 0.02 T for the cross-plane and in-plane directions, respectively. The coherence lengths ξ estimated following the Ginzburg− Landau theory are 22.67 and 38.68 nm for the cross-plane and in-plane orientations, respectively. The specific heat jump of ΔCe/γTc = 0.812 confirms the bulk superconductivity with a Sommerfeld coefficient γ = 5.831 ± 0.012 mJ mol−1 K−2. The electron−phonon coupling constant λel−ph ∼ 0.724 suggests that the (SnS)1.15(TaS2) misfit layer compound is a weakcoupling BCS superconductor.

.39 By fitting the WHH equation T = Tc

dHc2/dT = −Hc2(0)/0.693Tc, as shown in the inset of Figures 4c,d, Hc2(0) is estimated to be 0.64 ± 0.06 T and 0.22 ± 0.02 T for the cross-plane and in-plane directions, respectively. Using the estimated upper critical field Hc2(0) values, the coherence length ξ is calculated to be 22.67 and 38.68 nm for the crossplane and in-plane directions, respectively, following the Ginzburg−Landau theory Hc2 = Φ0/2πξ2, where Φ0 is the flux quantum. The coherence length of (SnS)1.15(TaS2) is found to be very close to that of the Sn-based misfit superconductor (SnS)1.17(NbS2).38 Figure 5a shows the dc magnetization as a function of temperature M(T) measured at 50 Oe, in zero field (ZFC) and field cooled (FC) cycles for both H∥c and H⊥c orientations. A strong FC diamagnetic signal below ∼3 K confirms the Meissner effect with onset of Tc ∼ 3.01 K. The significant reduction of FC value below Tc is due to the flux pinning. The AC susceptibility measurement was carried out in various frequencies (1, 10, 100, 500 Hz) using a rf field of 1 Oe parallel to the c-axis. The real (χ′) and imaginary (χ″) parts of the AC susceptibilities confirmed the superconducting transition has Tc ∼ 3.01 K, as shown in Figure 5b. Specific heat measurement has also been used to probe the superconducting phase transition. Figure 5c shows the specific heat measurement results plotted in Cp/T vs T2 under the applied fields of 0 and 1 T. The anomaly observed in the specific heat curve confirms the superconducting phase transition of Tc ∼ 3.01 K, but the anomaly vanishes under magnetic field of 1 T, which indicates the Hc2 is at least 1T and in agreement with the Hc2 estimated using resistivity data as shown in Figure 4. Since the measured specific heat contains electronic and lattice contributions, the Sommerfeld coefficient (γ) and the phononic coefficient (β) can be obtained from the linear fit of Cp/T = γ + βT2, yielding γ = 5.831 ± 0.012 mJ mol−1 K−2 and β = 2.79 ± 0.02 mJ mol−1 K−4. The Sommerfeld coefficient of (SnS)1.15(TaS2) is lower than that of 2H-TaS2 (γ = 8.5 ± 0.10 mJ mol−1 K−2) and its intercalated compounds (7.4−9.5 mJ mol−1 K−2).19 The Debye temperature ΘD is then calculated from the relation β = (12π4nR)/(5ΘD3) to be ∼151 K, where n = 5 is a number of atoms per unit cell without considering the misfit index and R is the ideal gas constant. The electron−phonon coupling constant λel‑ph is calculated to be ∼0.724 with the Mcmillan formula,40

AUTHOR INFORMATION

Corresponding Authors

*Raman Sankar. Phone: +886-02-3366 3826. Fax: +886-023366 3843. E-mail: [email protected]. *F. C. Chou. Phone: +886-02-3366 3826. Fax: +886-02-3366 3843. E-mail: [email protected]. ORCID

I. Panneer Muthuselvam: 0000-0002-7763-5915 Cheng-Yen Wen: 0000-0002-9788-4329 Author Contributions ○

R.S. and G.P. contributed equally to this work. R.S, and F.C.C. designed the work and grew the crystals. R.S. G.P.and I.P.M.carried out magnetic, specific heat and transport measurements . C. Y. W performed HRTEM. X. X performed pressure dependence of resistivity. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.S. and F.C.C. acknowledge the support provided by the Academia Sinica research program on Nanoscience and Nanotechnology under Project Number NM004. F.C.C. acknowledges support from the Ministry of Science and Technology in Taiwan under Project Number MOST-1022119-M-002-004. We thank the Nanoscience and Technology thematic research program of Academia Sinica, Taiwan. I.P.M. thanks Department of Science and Technology in India for the support of INSPIRE faculty Award No. DST/INSPIRE/04/ 2016/002275.

( ) − 1.04 = 1.04 + ln( )(1 − 0.62μ*) μ* ln

λel − ph

■

1.45Tc ΘD

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1.45Tc ΘD

where μ* is the Coulomb pseudopotential set to be 0.15. The λel−ph value suggests that (SnS)1.15(TaS2) is a weak coupled

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Chemistry of Materials

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DOI: 10.1021/acs.chemmater.7b04998 Chem. Mater. 2018, 30, 1373−1378