Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Superconductivity in Weyl semimetal candidate MoTe2

Superconductivity in Weyl semimetal candidate MoTe2 ARTICLE Received 10 Sep 2015 | Accepted 15 Feb 2016 | Published 14 Mar 2016 DOI: 10.1038/ncomms11038 OPEN Superconductivity in Weyl semimetal candidate MoTe 1 1 2 1 1 1 Yanpeng Qi , Pavel G. Naumov , Mazhar N. Ali , Catherine R. Rajamathi , Walter Schnelle , Oleg Barkalov , 3 1 1 1 1 1 1 Michael Hanfland , Shu-Chun Wu , Chandra Shekhar , Yan Sun , Vicky Su¨ , Marcus Schmidt , Ulrich Schwarz , 4 4 4 5 5 4 2 Eckhard Pippel , Peter Werner , Reinald Hillebrand , Tobias Fo¨rster , Erik Kampert , Stuart Parkin , R.J. Cava , 1 1,6 1 Claudia Felser , Binghai Yan & Sergey A. Medvedev Transition metal dichalcogenides have attracted research interest over the last few decades due to their interesting structural chemistry, unusual electronic properties, rich intercalation chemistry and wide spectrum of potential applications. Despite the fact that the majority of related research focuses on semiconducting transition-metal dichalcogenides (for example, MoS ), recently discovered unexpected properties of WTe are provoking strong interest 2 2 in semimetallic transition metal dichalcogenides featuring large magnetoresistance, pressure-driven superconductivity and Weyl semimetal states. We investigate the sister compound of WTe , MoTe , predicted to be a Weyl semimetal and a quantum spin Hall 2 2 insulator in bulk and monolayer form, respectively. We find that bulk MoTe exhibits superconductivity with a transition temperature of 0.10 K. Application of external pressure dramatically enhances the transition temperature up to maximum value of 8.2 K at 11.7 GPa. The observed dome-shaped superconductivity phase diagram provides insights into the interplay between superconductivity and topological physics. 1 2 Max Planck Institute for Chemical Physics of Solids, No¨thnitzer Strae 40, 01187 Dresden, Germany. Department of Chemistry, Princeton University, 3 4 Princeton, New Jersey 08544, USA. European Synchrotron Radiation Facility, BP 220, 38043 Grenoble, France. Max Planck Institute of Microstructure Physics, 06120 Halle, Germany. Dresden High Magnetic Field Laboratory (HLD-EMFL), Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany. Max Planck Institute for the Physics of Complex Systems, 01187 Dresden, Germany. Correspondence and requests for materials should be addressed to B.Y. (email: Yan@cpfs.mpg.de) or to S.A.M. (email: Sergiy.Medvediev@cpfs.mpg.de). NATURE COMMUNICATIONS | 7:11038 | DOI: 10.1038/ncomms11038 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11038 ransition metal dichalcogenides (TMDs) have attracted The discovery of superconductivity in WTe is apparently tremendous attention due to their rich physics and contradictory to previous theoretical predictions , which claim 1–11 Tpromising potential applications . TMDs share the that 2H TMDs may become superconducting at high P, but the same formula, MX , where M is a transition metal (for 1T phases will not. Thus the investigation of other TMDs for the example, Mo or W) and X is a chalcogenide atom (S, Se appearance of superconductivity under pressure is of big interest. and Te). These compounds typically crystallize in many Molybdenum ditelluride (MoTe ) is unique among the TMDs 0 0 structures, including 2H-, 1T-, 1T - and T -type lattices. The since it is the only material that can be grown in both 2H and 1T most common structure is the 2H phase, where M atoms are forms, allowing for direct examination of this theory. If super- trigonal-prismatically coordinated by the chalcogenide atoms. conductivity exists in 1T -MoTe , it may allow the topological These planes then stack on one other with van der Waals gaps edge states to also become superconducting because of the inbetween. In contrast, the 1T structure corresponds to proximity effect in a bulk superconductor. This would open up a octahedral coordination of M. The 1T phase is a monoclinic new platform for the study of topological superconductivity, lattice that can be interpreted as a distortion of the 1T phase by which has potential application in quantum computation . the formation of in-plane M–M bonds, resulting in a pseudo- Regarding the recently anticipated Weyl semimetal phase in MoTe hexagonal layer with zigzag metal chains. Finally, the T phase is (ref. 24), discovery of superconductivity may introduce a new 0 25–27 very similar to the 1T phase, but the layers stack in a direct pathway for the exploration of topological superconductivity fashion, resulting in a higher-symmetry orthorhombic structure. along with emergent space-time supersymmetry . Depending on the synthesis technique, the same composition of Here, we report on the transport properties of the 2H, 1T and T MX can crystallize in a variety of structures with very different polytypes of MoTe under various applied P.Wefind that 2 2 electronic properties. For example, MoTe exists in 2H, 1T and T -MoTe exhibits superconductivity with T ¼ 0.10 K, according 2 d 2 c 12–14 T structures , while WTe has commonly been observed in to electrical resistivity (r) measurements. Application of relatively d 2 the T structure . The 2H and 1T compounds are primarily low pressures below 1 GPa dramatically enhances the T ,and a d c semiconducting, whereas the 1T and T compounds are typically dome-shaped T –P phase diagram is observed with maximum d c semimetallic. T ¼ 8.2 K at 11.7 GPa; this is B80 times larger than the ambient Very recently, semimetallic TMDs have attracted considerable pressure value. In contrast, we do not observe any traces of attention because of the discovery of salient quantum phenom- superconductivity in the 2H phase, even when it becomes metallic ena. For instance, T  WTe has been found to exhibit an under P. We assume that the extreme sensitivity of the super- d 2 16,17 extremely large magnetoresistance , pressure (P)-driven conductivity to P is a consequence of the unique electronic structure. superconductivity (highest resistive transition temperature Thus, MoTe presents the opportunity to study the interaction of T E7 K at 16.8 GPa) (refs 18,19), and a large and linear Nernst topological physics and superconductivity in a bulk material. effect . Further, this material has been theorized to constitute the first example of a type-II Weyl semimetal . Moreover, the Results 1T -MX monolayer has been predicted to be a two-dimensional Structure and transport properties at ambient pressure. 6 0 topological insulator . Prior physical properties measurements, synthesized 1T -MoTe a b (000) (011) (022) (002) (004) [100] d c 1T’ T 1T' 302 304 306 308 310 312 Volume (Å ) Figure 1 | MoTe crystal structure. (a) HAADF-STEM image of 1T -MoTe along the [100] zone (scale bar, 0.5 nm). The red rectangle shows HAADF 2 2 simulated image, and the red and blue spheres in the yellow rectangle represent Te and Mo atoms, respectively. (b) Corresponding electron diffraction 0 0 images. (c)1T and T -MoTe crystal structures. (d) Energy-volume dependence for 1T and T phases from DFT calculations. d 2 d 2 NATURE COMMUNICATIONS | 7:11038 | DOI: 10.1038/ncomms11038 | www.nature.com/naturecommunications E –E (meV) d NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11038 ARTICLE samples were structurally characterized (Fig. 1) using single- crystal x-ray diffraction (SXRD) and high-angle annular dark-field scanning transmission electron microscopy (HAADF- MoTe STEM). The atomic arrangement of the 1T structure was Cooling Warming determined using high-resolution HAADF-STEM images and diffraction patterns, as shown in Fig. 1a,b and Supplementary Fig. 1a,b. The crystal structures of 1T and T -MoTe are sketched d 2 1.2 in Fig. 1c. At room temperature, the crystals exhibit the expected monoclinic 1T -MoTe structure, while the SXRD measurements 1.0 at 120 K indicate a transition into the orthorhombic T structure. The 1T -MoTe structure crystallizes in the P2 /m space group 2 1 with lattice parameters of a¼ 6.320 Å, b ¼ 3.469 Å, c¼ 13.86 Å 0.8 and b¼ 93.917; these results are consistent with the previously 200 220 240 260 280 Temperature (K) reported structure . The Raman spectra at ambient P contain two characteristic peaks (Supplementary Fig. 1c), which are due 0 50 100 150 200 250 300 to the A and B vibrational modes of the 1T -MoTe structure; g g 2 Temperature (K) this is also in agreement with a previous report . A full structural solution was obtained for the orthorhombic T phase at 0.4 120 K, the refined parameters are given in Supplementary Tables 1 and 2. Temperature dependence of electrical resistivity of MoTe 0.3 down to a minimum temperature of T ¼ 0.08 K at ambient min pressure is presented in Fig. 2. In contrast to the 2H phase, which displays semiconducting behaviour, 1T -MoTe is semimetallic in nature. At zero field, the room-temperature resistivity is r¼ 1.0 0.2 5  7 10 O m, which decreases to 2.8 10 O m at 0.25 K, yielding a residual resistance ratio (RRR) E36. At TE250 K an anomaly with thermal hysteresis (Fig. 2a, inset) is observed, 0.1 which is associated with the first-order structural phase transition 0 14,30 from the 1T to the T polytype . A range of magneto- transport properties has been measured at zero pressure on our 0.0 MoTe crystals (Supplementary Figs 2–4 and Supplementary 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Note 1). From Hall effect measurements, MoTe shows dominant Temperature (K) electron-type transport. Within a single-band model the electron 19  3 concentration n is estimated to 5 10 cm at 2 K and Figure 2 | Resistivity of 1T -MoTe at ambient pressure. e 2 20  3 8  10 cm at 300 K (Supplementary Fig. 2), which is close to (a) Temperature-dependent resistivity at near zero pressure. Inset: anomaly reported values . In addition, T -MoTe gradually becomes with hysteresis observed at B250 K. This hysteresis is associated with the d 2 superconducting below TB0.3 K (the onset of transition), while structural phase transition from 1T -MoTe to T -MoTe .(b) Resistivity 2 d 2 zero resistance is observed at T ¼ 0.10 K (Fig. 2b). Note that, detail from 0.08 to 1.2 K. Superconductivity is observed with onset at although potential superconductivity atB0.25 K in MoTe has E0.25 K and zero resistance at T ¼ 0.10 K. 2 c been briefly mentioned in the literature , no related data have been published. stabilizes the monoclinic 1T structure. Increase of P at room temperature results in enhancement of monoclinic distortion (increase of the monoclinic angle b). In an isothermal run at 1T –T structural transition under pressure. It is well known that high pressure can effectively modify lattice structures and the 135 K the reversible orthorhombic T to monoclinic 1T transition is observed at E0.8 GPa (E0.4 GPa) at pressure corresponding electronic states in a systematic fashion. Hence, we measured r(T) for the same 1T -MoTe single crystal at various increase (decrease) (Fig. 4c). Thus, application of P well below 1 GPa decreases the temperature of structural transition to below pressure values P (Fig. 3). Figure 3a shows the typical r(T) curves for P up to 34.9 GPa. For increasing P, the metallic characteristic 135 K. Furthermore, at PE1.5 GPa, the 1T structure remains becomes stronger and r decreases over the entire temperature stable down to at least 80 K. The quantitative discrepancy in the T values derived from structural and resistivity data is most likely range. At low pressures, resistance curves exhibit an anomaly at a temperature T , associated with the monoclinic 1T –orthorhom- due to nonhydrostatic pressure conditions in the resistivity measurements, and the thermal hysteresis since the resistivity bic T structural phase transition similarly to the ambient pres- sure data. With pressure increase, the resistivity anomaly becomes curves are recorded with increasing temperatures. The stability of MoTe in different phases can be explained less pronounced whereas the temperature of anomaly T is sig- s 2 nificantly shifted to lower T and disappears completely above using total energy calculations within density-functional theory (DFT). The optimized lattice constants are very close to 4 GPa. Thus, the application of P tends to stabilize the monoclinic phase. In addition, the Raman spectra recorded at room tem- experimental values for both phases, as shown in Supplementary Fig. 6 and Supplementary Table 3. After evaluating the total perature under different pressures (Fig. 4a) contain only two 0 29 characteristic peaks for the 1T -structure A and B modes . The energies of the two phases at ambient pressure, we found that the g g T phase exhibits slightly lower energy (0.5 meV per formula frequencies of both vibrational modes increase gradually with no discontinuities as P increases (Fig. 4b) indicating the absence of unit) than the 1T phase. This is consistent with the fact that the major structural phase transition in the whole studied pressure low- and high-T phases are T and 1T , respectively, without external pressure. As the 1T phase can be obtained by sliding range at room temperature. The SXRD data (Fig. 4c and Supplementary Fig. 5) also indicate that application of pressure between layers of the T phase, the former exhibits a slightly NATURE COMMUNICATIONS | 7:11038 | DOI: 10.1038/ncomms11038 | www.nature.com/naturecommunications 3 –6 Resistivity (10 Ω m) –6 Resistivity (10 Ω m) –6 Resistivity (10 Ω m) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11038 0.8 GPa MoTe 34.9 GPa 0 50 100 150 200 250 300 Temperature (K) bc 0.4 0.20 34.9 GPa 0.3 0.15 4.4 GPa 0.2 0.10 0.8 GPa 11.7 GPa 7.2 GPa 0.1 0.05 11.7 GPa 21.2 GPa 27.1 GPa 0.0 0.00 46 8 10 02468 10 Temperature (K) Temperature (K) de 3.0 T 0.3 11.7 GPa 1.1 GPa Fitting 0.2 0 T 1.6 T 0.4 T 0.1 0.8 T 0.0 0 02 4 6 8 10 02 46 8 Temperature (K) Temperature (K) Figure 3 | Transport properties of 1T -MoTe as a function of pressure. (a) Electrical resistivity as a function of temperature for pressures of 0.76 34.9 GPa. The anomaly associated with the structural transition is completely suppressed with increasing pressure. (b,c) Electrical resistivity as a function of temperature for pressures of 0.7 11.7 and 11.7 34.9 GPa, respectively. Clear electrical resistivity drops and zero-resistance behaviour are apparent. T increases under increasing pressure and a dome-shaped superconducting phase in pressure–temperature space is observed for the maximum superconducting transition temperature corresponding to T ¼ 8.2 K at 11.7 GPa. (d) Temperature dependence of resistivity under different magnetic fields of up to 3 T at 11.2 GPa. (e) Temperature dependence of MoTe upper critical field H . T is defined as temperature at which resistivity drops to 90% of its 2 c2 c 1 þ a residual value in normal state. The red curve is the best least squares fit of the equation H (T)¼ H *(1—T/T ) to the experimental data. c2 c2 c 4 NATURE COMMUNICATIONS | 7:11038 | DOI: 10.1038/ncomms11038 | www.nature.com/naturecommunications –6 –6 Resistivity (10 Ω m) Resistivity (10 Ω m) –6 Resistivity (10 Ω m) –6 Resistivity (10 Ω m) H (T) 0 c2 Temperature (K) NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11038 ARTICLE ab MoTe 0.8 GPa 3.5 GPa 010 20 30 Pressure (GPa) 8.8 GPa 14.8 GPa 295 K 21.2 GPa 135 K 135 K 0.0 0.5 1.0 1.5 2.0 150 200 250 300 350 –1 Pressure (GPa) Raman shift (cm ) 0 0 Figure 4 | High-pressure Raman spectroscopy and structural studies of 1T -MoTe . (a) Pressure-dependent Raman signals for 1T -MoTe at room 2 2 temperature. The Raman spectra contain two characteristic peaks due to the A and B vibrational modes of the 1T -MoTe structure. (b) Frequencies of A g g 2 g and B modes as function of pressure. The frequencies of both vibrational modes increase gradually and continuously as the pressure increases. (c) Pressure dependence of the monoclinic angle b obtained from SXRD studies. Isothermal compression at room temperature (red filled squares) shows increase of the monoclinic distortion with pressure, whereas reversible orthorhombic T –monoclinic 1T transition is observed in isothermal compression (filled blue circles)/decompression (open blue circles) run at 135 K. The values of Raman frequencies in b and monoclinic angle in c at each pressure are average values obtained from several Raman spectra (XRD patterns) collected from different areas across the sample. The error bars for Raman frequencies in b and monoclinic angle in c due to s.d. are smaller than the symbols size. smaller equilibrium volume than the latter, as also revealed from 300 25 the lattice parameters measured via SXRD. As illustrated by the T (resistivity) MoTe energy-volume profile in Fig. 1d, external pressure will stabilize T (XRD) T (run 1) the 1T phase with the smaller volume (and correspondingly T (run 2) higher density) by increasing the shift between neighbouring c T (decompression) layers. T (shielding effect) The dome-shaped superconductivity behaviour. Our pressure studies have revealed that the T is very sensitive to pressure. That is, T increases dramatically to 5 K at relatively low pressures 5 Superconductivity below 1 GPa, before beginning a slower increase to a maximum T of 8.2 K at 11.7 GPa (Figs 3b and 5). Beyond this pressure, T 0 0 decreases and no superconductivity with T 41.5 K is found at c 0 5 101520253035 P434.9 GPa (Fig. 3c). Remarkably, the drastic increase of T at Pressure (GPa) low pressures is associated with a sharp decrease of the 1T –T Figure 5 | MoTe electronic phase diagram. The black and green squares structural phase transition temperature T . Subsequently at higher represent the structural phase transition temperature T obtained from pressures, T still increases to its maximum value with increasing resistivity and single-crystal synchrotron x-ray diffraction data. The red, P but with significantly lower rate. Our findings demonstrate that blue and olive circles represent the T extracted from various electrical the strong enhancement of T at relatively low P is associated with resistance measurements, and the magenta triangles represent the T suppression of the 1T –T structural phase transition. All the determined from the magnetization measurements. The error bars deduced characteristic temperatures in the above experimental results are from resistivity measurements values of T (red, olive and blue solid circles) summarized in the T–P phase diagram in Fig. 5. A dome-shaped due to s.d. of resistivity values (Methods section) are smaller than the superconducting phase boundary is obtained for MoTe , with a symbols size. sharp slope towards the zero-P end of the diagram. The bulk character of the superconductivity is confirmed by observations of the magnetic shielding effect in the low pressure low pressure range (Fig. 5). Further, we conducted resistivity range and at 7.5 GPa (Supplementary Fig. 7). The onset measurements in the vicinity of T for various external magnetic temperatures of the diamagnetism are consistent with that of fields. As can be seen in Fig. 3d, the zero-resistance-point T the resistivity drop and confirm the drastic increase of T in the under P¼ 11.2 GPa is gradually suppressed with increasing field. NATURE COMMUNICATIONS | 7:11038 | DOI: 10.1038/ncomms11038 | www.nature.com/naturecommunications 5 Intensity (arb. u.) –1 Raman shift (cm ) Monoclinic angle  (degrees) Temperature (K) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11038 Deviating from the Werthamer–Helfand–Hohenberg theory Experimental details of high-pressure measurements. A non-magnetic diamond anvil cell was used for r measurements under P values of up to 40 GPa. based on the single-band model, the upper critical field, H (T), c2 A cubic BN/epoxy mixture was used for the insulating gaskets and Pt foil was of MoTe has a positive curvature close to T (H ¼ 0), as shown in 2 c employed in the electrical leads. The diameters of the flat working surface of the Fig. 3e. This is similar to the behaviours of both WTe (ref. 18) diamond anvil and the sample chamber were 500 and 200 mm, respectively. The and NbSe (ref. 32). The experimental H (T) data can be initial sample thickness was E40 mm. Electrical resistivity at zero magnetic field 2 c2 was measured using the dc current in van der Pauw technique in a customary described within the entire T/T range by the expression 1þ a cryogenic setup (lowest achievable temperature 1.5 K). The resistivity values were H (T)¼ H *(1—T/T ) (refs 18,33). The fitting parameter c2 c2 c defined as an average of five successive measurements at constant temperature. H * ¼ 4.0 T can be considered as the upper limit for the upper c2 Resistivity measurements in magnetic field were performed on PPMS. Pressure was critical field H (0), which yields a Ginzburg–Landau coherence measured using the ruby scale by measuring the luminescence from small chips c2 of ruby placed in contact with the sample. length x (0) of B9 nm. The corresponding data obtained at GL Magnetization was measured on MoTe (m¼ 3.1 mg) in a pressure cell P¼ 1.1 GPa is also shown in Fig. 3e. It is also worth noting that (m¼ 170 mg) for Pr0.7 GPa and TZ0.5 K (Quantum Design Magnetic Property our estimated value of H (0) is well below the Pauli-Clogston c2 Measurement System (MPMS), iQuantum He insert). Shielding (after zero-field limit. cooling) and Meiner effect curves (in field-cooling) were recorded. We repeated the high-pressure experiments using different The high-P Raman spectra were recorded using a customary micro-Raman spectrometer with a HeNe laser as the excitation source and a single-grating crystal flakes. Similar superconducting behaviour with almost spectrograph with 1 cm resolution. Raman scattering was calibrated using Ne identical T was observed. For comparison with 1T -MoTe ,we c 2  1 lines with an uncertainty of 1cm . also measured r(T) for the 2H-MoTe single crystal at various High-pressure diffraction experiments have been performed at ID09A pressure values. We found a pressure-induced metallization at synchrotron beamline using monochromatic x-ray beam (E ¼ 30 keV, l ¼ 0.413 Å) 2 37 focused to 15 10mm on the sample . We used a membrane-driven high- 15 GPa (Supplementary Fig. 8), which is consistent with previous pressure cell equipped with Boehler-Almax seats and diamond anvil design, theoretical predictions . However, in contrast, we did not detect allowing an opening cone of 64. The culet size was 600 mm and the sample was any signature of superconductivity in the 2H phase for pressures loaded together with He as pressure transmission medium into a hole in a stainless up to 40 GPa. steel gasket preindented to B80mm with an initial diameter of 300 mm. Low temperature data were collected in a He-flow cryostat. Single-crystal data have been collected by a vertical-acting o-axis rotation, with an integrated step scan of 0.5 Discussion and a counting time of 1 s per frame. Diffraction intensities have been recorded For MoTe , the superconducting behaviour in the low-P region with a Mar555 flat-panel detector. Diffraction data have been processed and analysed with CrysAlisPro-171.37.35 and Jana2006 software. Pressures were clearly differs from that in the high-P region. Under quite low P, measured with the ruby fluorescence method . the sharp increase in T is concomitant with a strong suppression of the structural transition, which is reminiscent of observations DFT calculations. DFT calculations were performed using the Vienna Ab-initio for other superconductors with various kinds of competing phase 38,39 Simulation Package with projected augmented wave potential . The exchange transitions. The drastic increase of the T occurs within the T c d and correlation energy was considered at the generalized gradient approximation phase, which is shown by DFT calculations to be a Weyl 40 level for the geometry optimization , and the electronic structure was calculated semimetal (Supplementary Fig. 9a and Supplementary Note 2) using the hybrid functional (HSE06) . Spin–orbital coupling was included in all with a band structure around the Fermi level, which is extremely calculations. Van der Waals corrections were included via a pair-wise force field of 24,34 the Grimme method . In the lattice relaxation, the volumes were fixed while lattice sensitive to changes in the lattice constants . Thus, one can constants and atomic positions were optimized. The pressure was derived by expect that dramatic structural and electronic instabilities emerge fitting the total energy dependence on the volume with the Murnaghan equation . in the low-P region, which may account for the strong After checking the k convergence, the 24  12  8 and 7 5 3 k-meshes with enhancement of T . At higher pressures, the topologically trivial Gaussian-type smearing were used for the generalized gradient approximation (Supplementary Fig. 10) and HSE06 calculations, respectively. The band structures, (due to inversion and time reversal symmetry) 1T phase density of states and Fermi surfaces were interpolated in a dense k-mesh of (Supplementary Fig. 9b and Supplementary Note 2) remains 200  200 200 using the maximally localized Wannier functions extracted from stable in the whole temperature range. Although within this phase HSE06 calculations. T still continues to increase up to its maximum value, the rate of the increase is significantly lower and this growth is naturally References explained by the increase of the electronic density of states 1. Wilson, J. A. & Yoffe, A. D. The Transition Metal Dichalcogenides. Discussion at the Fermi level in the 1T phase (Supplementary Fig. 9c). and interpretation of the observed optical, electrical and structural properties. Adv. Phys. 18, 193–335 (1969). Thorough exploration of superconductivity in MoTe from both 2. Klemm, R. A. Pristine and intercalated transition metal dichalcogenide experimental and theoretical perspectives is required. superconductors. Phys. C 514, 86–94 (2015). 3. Morris, R. C., Coleman, R. V. & Bhandari, R. Superconductivity and Methods magnetoresistance in NbSe . Phys. Rev. B 5, 895–901 (1972). Single-crystal growth. 1T -MoTe crystals were grown via chemical vapour 4. Morosan, E. et al. Superconductivity in Cu TiSe . Nat. Phys. 2, 544–550 (2006). 2 x 2 transport using polycrystalline MoTe powder and TeCl as a transport additive . 2 4 5. Moncton, D. E., Axe, J. D. & DiSalvo, F. J. Neutron scattering study of the Molar quantities of Mo (Sigma Aldrich 99.99%) were ground in combination with charge-density wave transitions in 2H-TaSe and 2H-NbSe . Phys. Rev. B 16, 2 2 purified Te pieces (Alfa Aesar 99.99%), pressed into pellets and heated in an 801–819 (1977). evacuated quartz tube at 800 C for 7 days. Crystals were obtained by sealing 1 g of 6. Qian, X., Liu, J., Fu, L. & Li, J. Quantum spin Hall effect in two-dimensional this powder and TeCl (3 mg ml ) in a quartz ampoule, which was then flushed transition metal dichalcogenides. Science 346, 1344–1347 (2014). with Ar, evacuated, sealed and heated in a two-zone furnace. Crystallization was 7. Xu, X., Yao, W., Xiao, D. & Heinz, T. F. Spin and pseudospins in layered conducted from (T ) 1,000 to (T ) 900 C. The quartz ampoule was then quenched 2 1 transition metal dichalcogenides. Nat. Phys. 10, 343–350 (2014). in ice water to yield the high-temperature monoclinic phase. The obtained crystals 8. Bates, J. B., Gruzalski, G. R., Dudney, N. J., Luck, C. F. & Yu, X. Rechargeable were silver-gray and rectangular in shape. 2H-MoTe crystals were grown using a thin-film lithium batteries. Solid State Ion. 70/71, 619–628 (1994). similar method, but without quenching. 9. Li, Y. et al. MoS nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction. J. Am. Chem. Soc. 133, 7296–7299 (2011). 10. Zhang, Y. J., Oka, T., Suzuki, R., Ye, J. T. & Iwasa, Y. Electrically switchable Structural and transport measurements at ambient pressure. The structures of chiral light-emitting transistor. Science 344, 725–728 (2014). the MoTe crystals were investigated using SXRD with Mo K radiation. To analyse 2 a 11. Lin, Y.-C., Dumcenco, D. O., Huang, Y.-S. & Suenaga, K. Atomic mechanism of the atomic structure of the material, HAADF-STEM was performed. The the semiconducting-to-metallic phase transition in single-layered MoS . Nat. dependence of the electrical resistivity r on temperature T was measured using a 2 Nanotechnol. 9, 391–396 (2014). conventional four-probe method (low-frequency alternating current, Physical 12. Clarke, R., Marseglia, E. & Hughes, H. P. A low-temperature structural phase Property Measurement System (PPMS), Quantum Design). Temperatures down to 0.08 K were achieved using a home-built adiabatic demagnetization stage. The transition in b-MoTe . Philos. Mag. B 38, 121–126 (1978). pulsed magnetic field experiments were conducted at the Dresden High Magnetic 13. Puotinen, D. & Newnhan, R. E. The crystal structure of MoTe . Acta Field Laboratory (Helmholtz-Zentrum Dresden-Rossendorf, HLD-HZDR). Crystallogr. 14, 691–692 (1961). 6 NATURE COMMUNICATIONS | 7:11038 | DOI: 10.1038/ncomms11038 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11038 ARTICLE 14. Zandt, T., Dwelk, H., Janowitz, C. & Manzke, R. Quadratic temperature 39. Kresse, G. & Furthmu¨ller, J. Efficiency of ab-initio total energy calculations for dependence up to 50 K of the resistivity of metallic MoTe . J. Alloys Compd. metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 6, 442, 216–218 (2007). 15–50 (1996). 15. Brown, B. E. The crystal structures of WTe and high-temperature MoTe . Acta 40. Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation 2 2 Crystallogr. 20, 268–274 (1966). made simple. Phys. Rev. Lett. 77, 3865–3868 (1996). 16. Ali, M. N. et al. Large, non-saturating magnetoresistance in WTe . Nature 514, 41. Heyd, J., Scuseria, G. E. & Ernzerhof, M. Hybrid functionals based on a 205–208 (2014). screened Coulomb potential. J. Chem. Phys. 118, 8207–8215 (2003). 17. Ali, M. N. et al. Correlation of crystal quality and extreme magnetoresistance of 42. Grimme, S. Semiempirical GGA-type density functional constructed with a WTe . Europhys. Lett. 110, 67002 (2015). long-range dispersion correction. J. Comput. Chem. 27, 1787–1799 (2006). 18. Pan, X.-C. et al. Pressure-driven dome-shaped superconductivity and electronic 43. Murnaghan, F. D. The compressibility of media under extreme pressures. Proc. structural evolution in tungsten ditelluride. Nat. Commun. 6, 7805 (2015). Natl Acad. Sci. USA 30, 244–247 (1944). 19. Kang, D. et al. Superconductivity emerging from suppressed large 44. Marzari, N. & Vanderbilt, D. Maximally localized generalized Wannier magnetoresistant state in WTe . Nat. Commun. 6, 7804 (2015). functions for composite energy bands. Phys. Rev. B 56, 12847–12865 20. Zhu, Z. et al. Quantum oscillations, thermoelectric coefficients, and the fermi (1997). surface of semimetallic WTe . Phys. Rev. Lett. 114, 176601 (2015). 21. Soluyanov, A. et al.Type II Weyl Semimetals. Preprint at http://arxiv.org/abs/ Acknowledgements 1507.01603 (2015). Y.Q. acknowledges financial support from the Alexander von Humboldt Foundation. 22. Riflikova´, M., Martonˇa´k, R. & Tosatti, E. Pressure-induced gap closing and We would like to thank C. Klausnitzer, M. Nicklas and R. Koban for their help with high- metallization of MoSe and MoTe . Phys. Rev. B 90, 035108 (2014). 2 2 pressure magnetic measurements. This work was financially supported by the Deutsche 23. Fu, L. & Kane, C. L. Superconducting proximity effect and Majorana fermions Forschungsgemeinschaft (DFG, Project No. EB 518/1-1 of DFG-SPP 1666 ‘Topological at the surface of a topological insulator. Phys. Rev. Lett. 100, 096407 (2008). Insulators’) and by a European Research Council (ERC) Advanced Grant, No. (291472) 24. Sun, Y., Wu, S.-C., Ali, M. N., Felser, C. & Yan, B. Prediction of the Weyl ‘Idea Heusler’. semimetal in the orthorhombic MoTe . Phys. Rev. B 92, 161107 (2015). 25. Cho, G. Y., Bardarson, J. H., Lu, Y.-M. & Moore, J. E. Superconductivity of doped Weyl semimetals: Finite-momentum pairing and electronic analog of the Author contributions He-A phase. Phys. Rev. B 86, 214514 (2012). B.Y. and C.F. conceived the project. M.N.A., C.R.R. and V.S. prepared the samples and 26. Wei, H., Chao, S.-P. & Aji, V. Odd-parity superconductivity in Weyl performed XRD structural characterization. E.P., P.W. and R.H. performed TEM studies. semimetals. Phys. Rev. B 89, 014506 (2014). W.S. performed ambient pressure transport measurements and Meiner effect 27. Hosur, P., Dai, X., Fang, Z. & Qi, X.-L. Time-reversal-invariant topological measurements at low pressures. C.S., T.F. and E.K. performed magneto-transport superconductivity in doped Weyl semimetals. Phys. Rev. B 90, 045130 (2014). measurements at ambient pressure. Y.Q., P.G.N., O.B. and S.A.M. performed 28. Jian, S.-K., Jiang, Y.-F. & Yao, H. Emergent Spacetime Supersymmetry in 3D high-pressure electrical resistivity, Raman spectroscopy and magnetic susceptibility Weyl Semimetals and 2D Dirac Semimetals. Phys. Rev. Lett. 114, 237001 measurements. M.H. performed high-pressure SXRD studies. S.C.W., Y.S. and B.Y. (2015). carried out the theoretical calculations. All authors discussed the results of the studies. 29. Keum, D. H. et al. Bandgap opening in few-layered monoclinic MoTe . Nat. 2 Y.Q., B.Y., W.S. and S.A.M. co-wrote the paper. All authors commented on the Phys. 11, 482–487 (2015). manuscript. 30. Hughes, H. P. & Friend, R. H. Electrical resistivity anomaly in b-MoTe . J. Phys. C Solid State Phys. 11, L103–L105 (1978). 31. Hulliger, F. Crystal Chemistry of Chalcogenides and Pnictides of the Transition Additional information Elements. in Structure and Bonding, Vol. 4, 83–229 (Springer-Verlag, 1968). Supplementary Information accompanies this paper at http://www.nature.com/ 32. Suderow, H., Tissen, V. G., Brison, J. P., Martı´nez, J. L. & Vieira, S. Pressure naturecommunications induced effects on the Fermi surface of superconducting 2H-NbSe . Phys. Rev. Competing financial interests: The authors declare no competing financial interests. Lett. 95, 117006 (2005). 33. Mu¨ller, K. H. et al. The upper critical field in superconducting MgB . J. Alloys Reprints and permission information is available online at http://npg.nature.com/ Compd. 322, L10–L13 (2001). reprintsandpermissions/ 34. Wang, Z. et al. MoTe : Weyl and Line Node Topological Metal. Preprint at http://arxiv.org/abs/1511.07440 (2015). How to cite this article: Qi, Y. et al. Superconductivity in Weyl semimetal candidate 35. Fourcaudot, G., Gourmala, M. & Mercier, J. Vapor phase transport and crystal MoTe . Nat. Commun. 7:10038 doi: 10.1038/ncomms11038 (2016). growth of molybdenum trioxide and molybdenum ditelluride. J. Cryst. Growth 46, 132–135 (1979). This work is licensed under a Creative Commons Attribution 4.0 36. Mao, H. K., Xu, J. & Bell, P. M. Calibration of the ruby pressure gauge to 800 International License. The images or other third party material in this kbar under quasi-hydrostatic conditions. J. Geophys. Res. 91, 4673–4676 (1986). article are included in the article’s Creative Commons license, unless indicated otherwise 37. Merlini, M. & Hanfland, M. Single-crystal diffraction at megabar conditions by in the credit line; if the material is not included under the Creative Commons license, synchrotron radiation. High Press. Res. 33, 511–522 (2013). users will need to obtain permission from the license holder to reproduce the material. 38. Kresse, G. & Hafner, J. Ab initio molecular dynamics for open-shell transition To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ metals. Phys. Rev. B 48, 13115–13118 (1993). NATURE COMMUNICATIONS | 7:11038 | DOI: 10.1038/ncomms11038 | www.nature.com/naturecommunications 7 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nature Communications Springer Journals

Loading next page...
 
/lp/springer-journals/superconductivity-in-weyl-semimetal-candidate-mote2-H4fU4fLamP

References (64)

Publisher
Springer Journals
Copyright
Copyright © 2016 by The Author(s)
Subject
Science, Humanities and Social Sciences, multidisciplinary; Science, Humanities and Social Sciences, multidisciplinary; Science, multidisciplinary
eISSN
2041-1723
DOI
10.1038/ncomms11038
Publisher site
See Article on Publisher Site

Abstract

ARTICLE Received 10 Sep 2015 | Accepted 15 Feb 2016 | Published 14 Mar 2016 DOI: 10.1038/ncomms11038 OPEN Superconductivity in Weyl semimetal candidate MoTe 1 1 2 1 1 1 Yanpeng Qi , Pavel G. Naumov , Mazhar N. Ali , Catherine R. Rajamathi , Walter Schnelle , Oleg Barkalov , 3 1 1 1 1 1 1 Michael Hanfland , Shu-Chun Wu , Chandra Shekhar , Yan Sun , Vicky Su¨ , Marcus Schmidt , Ulrich Schwarz , 4 4 4 5 5 4 2 Eckhard Pippel , Peter Werner , Reinald Hillebrand , Tobias Fo¨rster , Erik Kampert , Stuart Parkin , R.J. Cava , 1 1,6 1 Claudia Felser , Binghai Yan & Sergey A. Medvedev Transition metal dichalcogenides have attracted research interest over the last few decades due to their interesting structural chemistry, unusual electronic properties, rich intercalation chemistry and wide spectrum of potential applications. Despite the fact that the majority of related research focuses on semiconducting transition-metal dichalcogenides (for example, MoS ), recently discovered unexpected properties of WTe are provoking strong interest 2 2 in semimetallic transition metal dichalcogenides featuring large magnetoresistance, pressure-driven superconductivity and Weyl semimetal states. We investigate the sister compound of WTe , MoTe , predicted to be a Weyl semimetal and a quantum spin Hall 2 2 insulator in bulk and monolayer form, respectively. We find that bulk MoTe exhibits superconductivity with a transition temperature of 0.10 K. Application of external pressure dramatically enhances the transition temperature up to maximum value of 8.2 K at 11.7 GPa. The observed dome-shaped superconductivity phase diagram provides insights into the interplay between superconductivity and topological physics. 1 2 Max Planck Institute for Chemical Physics of Solids, No¨thnitzer Strae 40, 01187 Dresden, Germany. Department of Chemistry, Princeton University, 3 4 Princeton, New Jersey 08544, USA. European Synchrotron Radiation Facility, BP 220, 38043 Grenoble, France. Max Planck Institute of Microstructure Physics, 06120 Halle, Germany. Dresden High Magnetic Field Laboratory (HLD-EMFL), Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany. Max Planck Institute for the Physics of Complex Systems, 01187 Dresden, Germany. Correspondence and requests for materials should be addressed to B.Y. (email: Yan@cpfs.mpg.de) or to S.A.M. (email: Sergiy.Medvediev@cpfs.mpg.de). NATURE COMMUNICATIONS | 7:11038 | DOI: 10.1038/ncomms11038 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11038 ransition metal dichalcogenides (TMDs) have attracted The discovery of superconductivity in WTe is apparently tremendous attention due to their rich physics and contradictory to previous theoretical predictions , which claim 1–11 Tpromising potential applications . TMDs share the that 2H TMDs may become superconducting at high P, but the same formula, MX , where M is a transition metal (for 1T phases will not. Thus the investigation of other TMDs for the example, Mo or W) and X is a chalcogenide atom (S, Se appearance of superconductivity under pressure is of big interest. and Te). These compounds typically crystallize in many Molybdenum ditelluride (MoTe ) is unique among the TMDs 0 0 structures, including 2H-, 1T-, 1T - and T -type lattices. The since it is the only material that can be grown in both 2H and 1T most common structure is the 2H phase, where M atoms are forms, allowing for direct examination of this theory. If super- trigonal-prismatically coordinated by the chalcogenide atoms. conductivity exists in 1T -MoTe , it may allow the topological These planes then stack on one other with van der Waals gaps edge states to also become superconducting because of the inbetween. In contrast, the 1T structure corresponds to proximity effect in a bulk superconductor. This would open up a octahedral coordination of M. The 1T phase is a monoclinic new platform for the study of topological superconductivity, lattice that can be interpreted as a distortion of the 1T phase by which has potential application in quantum computation . the formation of in-plane M–M bonds, resulting in a pseudo- Regarding the recently anticipated Weyl semimetal phase in MoTe hexagonal layer with zigzag metal chains. Finally, the T phase is (ref. 24), discovery of superconductivity may introduce a new 0 25–27 very similar to the 1T phase, but the layers stack in a direct pathway for the exploration of topological superconductivity fashion, resulting in a higher-symmetry orthorhombic structure. along with emergent space-time supersymmetry . Depending on the synthesis technique, the same composition of Here, we report on the transport properties of the 2H, 1T and T MX can crystallize in a variety of structures with very different polytypes of MoTe under various applied P.Wefind that 2 2 electronic properties. For example, MoTe exists in 2H, 1T and T -MoTe exhibits superconductivity with T ¼ 0.10 K, according 2 d 2 c 12–14 T structures , while WTe has commonly been observed in to electrical resistivity (r) measurements. Application of relatively d 2 the T structure . The 2H and 1T compounds are primarily low pressures below 1 GPa dramatically enhances the T ,and a d c semiconducting, whereas the 1T and T compounds are typically dome-shaped T –P phase diagram is observed with maximum d c semimetallic. T ¼ 8.2 K at 11.7 GPa; this is B80 times larger than the ambient Very recently, semimetallic TMDs have attracted considerable pressure value. In contrast, we do not observe any traces of attention because of the discovery of salient quantum phenom- superconductivity in the 2H phase, even when it becomes metallic ena. For instance, T  WTe has been found to exhibit an under P. We assume that the extreme sensitivity of the super- d 2 16,17 extremely large magnetoresistance , pressure (P)-driven conductivity to P is a consequence of the unique electronic structure. superconductivity (highest resistive transition temperature Thus, MoTe presents the opportunity to study the interaction of T E7 K at 16.8 GPa) (refs 18,19), and a large and linear Nernst topological physics and superconductivity in a bulk material. effect . Further, this material has been theorized to constitute the first example of a type-II Weyl semimetal . Moreover, the Results 1T -MX monolayer has been predicted to be a two-dimensional Structure and transport properties at ambient pressure. 6 0 topological insulator . Prior physical properties measurements, synthesized 1T -MoTe a b (000) (011) (022) (002) (004) [100] d c 1T’ T 1T' 302 304 306 308 310 312 Volume (Å ) Figure 1 | MoTe crystal structure. (a) HAADF-STEM image of 1T -MoTe along the [100] zone (scale bar, 0.5 nm). The red rectangle shows HAADF 2 2 simulated image, and the red and blue spheres in the yellow rectangle represent Te and Mo atoms, respectively. (b) Corresponding electron diffraction 0 0 images. (c)1T and T -MoTe crystal structures. (d) Energy-volume dependence for 1T and T phases from DFT calculations. d 2 d 2 NATURE COMMUNICATIONS | 7:11038 | DOI: 10.1038/ncomms11038 | www.nature.com/naturecommunications E –E (meV) d NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11038 ARTICLE samples were structurally characterized (Fig. 1) using single- crystal x-ray diffraction (SXRD) and high-angle annular dark-field scanning transmission electron microscopy (HAADF- MoTe STEM). The atomic arrangement of the 1T structure was Cooling Warming determined using high-resolution HAADF-STEM images and diffraction patterns, as shown in Fig. 1a,b and Supplementary Fig. 1a,b. The crystal structures of 1T and T -MoTe are sketched d 2 1.2 in Fig. 1c. At room temperature, the crystals exhibit the expected monoclinic 1T -MoTe structure, while the SXRD measurements 1.0 at 120 K indicate a transition into the orthorhombic T structure. The 1T -MoTe structure crystallizes in the P2 /m space group 2 1 with lattice parameters of a¼ 6.320 Å, b ¼ 3.469 Å, c¼ 13.86 Å 0.8 and b¼ 93.917; these results are consistent with the previously 200 220 240 260 280 Temperature (K) reported structure . The Raman spectra at ambient P contain two characteristic peaks (Supplementary Fig. 1c), which are due 0 50 100 150 200 250 300 to the A and B vibrational modes of the 1T -MoTe structure; g g 2 Temperature (K) this is also in agreement with a previous report . A full structural solution was obtained for the orthorhombic T phase at 0.4 120 K, the refined parameters are given in Supplementary Tables 1 and 2. Temperature dependence of electrical resistivity of MoTe 0.3 down to a minimum temperature of T ¼ 0.08 K at ambient min pressure is presented in Fig. 2. In contrast to the 2H phase, which displays semiconducting behaviour, 1T -MoTe is semimetallic in nature. At zero field, the room-temperature resistivity is r¼ 1.0 0.2 5  7 10 O m, which decreases to 2.8 10 O m at 0.25 K, yielding a residual resistance ratio (RRR) E36. At TE250 K an anomaly with thermal hysteresis (Fig. 2a, inset) is observed, 0.1 which is associated with the first-order structural phase transition 0 14,30 from the 1T to the T polytype . A range of magneto- transport properties has been measured at zero pressure on our 0.0 MoTe crystals (Supplementary Figs 2–4 and Supplementary 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Note 1). From Hall effect measurements, MoTe shows dominant Temperature (K) electron-type transport. Within a single-band model the electron 19  3 concentration n is estimated to 5 10 cm at 2 K and Figure 2 | Resistivity of 1T -MoTe at ambient pressure. e 2 20  3 8  10 cm at 300 K (Supplementary Fig. 2), which is close to (a) Temperature-dependent resistivity at near zero pressure. Inset: anomaly reported values . In addition, T -MoTe gradually becomes with hysteresis observed at B250 K. This hysteresis is associated with the d 2 superconducting below TB0.3 K (the onset of transition), while structural phase transition from 1T -MoTe to T -MoTe .(b) Resistivity 2 d 2 zero resistance is observed at T ¼ 0.10 K (Fig. 2b). Note that, detail from 0.08 to 1.2 K. Superconductivity is observed with onset at although potential superconductivity atB0.25 K in MoTe has E0.25 K and zero resistance at T ¼ 0.10 K. 2 c been briefly mentioned in the literature , no related data have been published. stabilizes the monoclinic 1T structure. Increase of P at room temperature results in enhancement of monoclinic distortion (increase of the monoclinic angle b). In an isothermal run at 1T –T structural transition under pressure. It is well known that high pressure can effectively modify lattice structures and the 135 K the reversible orthorhombic T to monoclinic 1T transition is observed at E0.8 GPa (E0.4 GPa) at pressure corresponding electronic states in a systematic fashion. Hence, we measured r(T) for the same 1T -MoTe single crystal at various increase (decrease) (Fig. 4c). Thus, application of P well below 1 GPa decreases the temperature of structural transition to below pressure values P (Fig. 3). Figure 3a shows the typical r(T) curves for P up to 34.9 GPa. For increasing P, the metallic characteristic 135 K. Furthermore, at PE1.5 GPa, the 1T structure remains becomes stronger and r decreases over the entire temperature stable down to at least 80 K. The quantitative discrepancy in the T values derived from structural and resistivity data is most likely range. At low pressures, resistance curves exhibit an anomaly at a temperature T , associated with the monoclinic 1T –orthorhom- due to nonhydrostatic pressure conditions in the resistivity measurements, and the thermal hysteresis since the resistivity bic T structural phase transition similarly to the ambient pres- sure data. With pressure increase, the resistivity anomaly becomes curves are recorded with increasing temperatures. The stability of MoTe in different phases can be explained less pronounced whereas the temperature of anomaly T is sig- s 2 nificantly shifted to lower T and disappears completely above using total energy calculations within density-functional theory (DFT). The optimized lattice constants are very close to 4 GPa. Thus, the application of P tends to stabilize the monoclinic phase. In addition, the Raman spectra recorded at room tem- experimental values for both phases, as shown in Supplementary Fig. 6 and Supplementary Table 3. After evaluating the total perature under different pressures (Fig. 4a) contain only two 0 29 characteristic peaks for the 1T -structure A and B modes . The energies of the two phases at ambient pressure, we found that the g g T phase exhibits slightly lower energy (0.5 meV per formula frequencies of both vibrational modes increase gradually with no discontinuities as P increases (Fig. 4b) indicating the absence of unit) than the 1T phase. This is consistent with the fact that the major structural phase transition in the whole studied pressure low- and high-T phases are T and 1T , respectively, without external pressure. As the 1T phase can be obtained by sliding range at room temperature. The SXRD data (Fig. 4c and Supplementary Fig. 5) also indicate that application of pressure between layers of the T phase, the former exhibits a slightly NATURE COMMUNICATIONS | 7:11038 | DOI: 10.1038/ncomms11038 | www.nature.com/naturecommunications 3 –6 Resistivity (10 Ω m) –6 Resistivity (10 Ω m) –6 Resistivity (10 Ω m) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11038 0.8 GPa MoTe 34.9 GPa 0 50 100 150 200 250 300 Temperature (K) bc 0.4 0.20 34.9 GPa 0.3 0.15 4.4 GPa 0.2 0.10 0.8 GPa 11.7 GPa 7.2 GPa 0.1 0.05 11.7 GPa 21.2 GPa 27.1 GPa 0.0 0.00 46 8 10 02468 10 Temperature (K) Temperature (K) de 3.0 T 0.3 11.7 GPa 1.1 GPa Fitting 0.2 0 T 1.6 T 0.4 T 0.1 0.8 T 0.0 0 02 4 6 8 10 02 46 8 Temperature (K) Temperature (K) Figure 3 | Transport properties of 1T -MoTe as a function of pressure. (a) Electrical resistivity as a function of temperature for pressures of 0.76 34.9 GPa. The anomaly associated with the structural transition is completely suppressed with increasing pressure. (b,c) Electrical resistivity as a function of temperature for pressures of 0.7 11.7 and 11.7 34.9 GPa, respectively. Clear electrical resistivity drops and zero-resistance behaviour are apparent. T increases under increasing pressure and a dome-shaped superconducting phase in pressure–temperature space is observed for the maximum superconducting transition temperature corresponding to T ¼ 8.2 K at 11.7 GPa. (d) Temperature dependence of resistivity under different magnetic fields of up to 3 T at 11.2 GPa. (e) Temperature dependence of MoTe upper critical field H . T is defined as temperature at which resistivity drops to 90% of its 2 c2 c 1 þ a residual value in normal state. The red curve is the best least squares fit of the equation H (T)¼ H *(1—T/T ) to the experimental data. c2 c2 c 4 NATURE COMMUNICATIONS | 7:11038 | DOI: 10.1038/ncomms11038 | www.nature.com/naturecommunications –6 –6 Resistivity (10 Ω m) Resistivity (10 Ω m) –6 Resistivity (10 Ω m) –6 Resistivity (10 Ω m) H (T) 0 c2 Temperature (K) NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11038 ARTICLE ab MoTe 0.8 GPa 3.5 GPa 010 20 30 Pressure (GPa) 8.8 GPa 14.8 GPa 295 K 21.2 GPa 135 K 135 K 0.0 0.5 1.0 1.5 2.0 150 200 250 300 350 –1 Pressure (GPa) Raman shift (cm ) 0 0 Figure 4 | High-pressure Raman spectroscopy and structural studies of 1T -MoTe . (a) Pressure-dependent Raman signals for 1T -MoTe at room 2 2 temperature. The Raman spectra contain two characteristic peaks due to the A and B vibrational modes of the 1T -MoTe structure. (b) Frequencies of A g g 2 g and B modes as function of pressure. The frequencies of both vibrational modes increase gradually and continuously as the pressure increases. (c) Pressure dependence of the monoclinic angle b obtained from SXRD studies. Isothermal compression at room temperature (red filled squares) shows increase of the monoclinic distortion with pressure, whereas reversible orthorhombic T –monoclinic 1T transition is observed in isothermal compression (filled blue circles)/decompression (open blue circles) run at 135 K. The values of Raman frequencies in b and monoclinic angle in c at each pressure are average values obtained from several Raman spectra (XRD patterns) collected from different areas across the sample. The error bars for Raman frequencies in b and monoclinic angle in c due to s.d. are smaller than the symbols size. smaller equilibrium volume than the latter, as also revealed from 300 25 the lattice parameters measured via SXRD. As illustrated by the T (resistivity) MoTe energy-volume profile in Fig. 1d, external pressure will stabilize T (XRD) T (run 1) the 1T phase with the smaller volume (and correspondingly T (run 2) higher density) by increasing the shift between neighbouring c T (decompression) layers. T (shielding effect) The dome-shaped superconductivity behaviour. Our pressure studies have revealed that the T is very sensitive to pressure. That is, T increases dramatically to 5 K at relatively low pressures 5 Superconductivity below 1 GPa, before beginning a slower increase to a maximum T of 8.2 K at 11.7 GPa (Figs 3b and 5). Beyond this pressure, T 0 0 decreases and no superconductivity with T 41.5 K is found at c 0 5 101520253035 P434.9 GPa (Fig. 3c). Remarkably, the drastic increase of T at Pressure (GPa) low pressures is associated with a sharp decrease of the 1T –T Figure 5 | MoTe electronic phase diagram. The black and green squares structural phase transition temperature T . Subsequently at higher represent the structural phase transition temperature T obtained from pressures, T still increases to its maximum value with increasing resistivity and single-crystal synchrotron x-ray diffraction data. The red, P but with significantly lower rate. Our findings demonstrate that blue and olive circles represent the T extracted from various electrical the strong enhancement of T at relatively low P is associated with resistance measurements, and the magenta triangles represent the T suppression of the 1T –T structural phase transition. All the determined from the magnetization measurements. The error bars deduced characteristic temperatures in the above experimental results are from resistivity measurements values of T (red, olive and blue solid circles) summarized in the T–P phase diagram in Fig. 5. A dome-shaped due to s.d. of resistivity values (Methods section) are smaller than the superconducting phase boundary is obtained for MoTe , with a symbols size. sharp slope towards the zero-P end of the diagram. The bulk character of the superconductivity is confirmed by observations of the magnetic shielding effect in the low pressure low pressure range (Fig. 5). Further, we conducted resistivity range and at 7.5 GPa (Supplementary Fig. 7). The onset measurements in the vicinity of T for various external magnetic temperatures of the diamagnetism are consistent with that of fields. As can be seen in Fig. 3d, the zero-resistance-point T the resistivity drop and confirm the drastic increase of T in the under P¼ 11.2 GPa is gradually suppressed with increasing field. NATURE COMMUNICATIONS | 7:11038 | DOI: 10.1038/ncomms11038 | www.nature.com/naturecommunications 5 Intensity (arb. u.) –1 Raman shift (cm ) Monoclinic angle  (degrees) Temperature (K) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11038 Deviating from the Werthamer–Helfand–Hohenberg theory Experimental details of high-pressure measurements. A non-magnetic diamond anvil cell was used for r measurements under P values of up to 40 GPa. based on the single-band model, the upper critical field, H (T), c2 A cubic BN/epoxy mixture was used for the insulating gaskets and Pt foil was of MoTe has a positive curvature close to T (H ¼ 0), as shown in 2 c employed in the electrical leads. The diameters of the flat working surface of the Fig. 3e. This is similar to the behaviours of both WTe (ref. 18) diamond anvil and the sample chamber were 500 and 200 mm, respectively. The and NbSe (ref. 32). The experimental H (T) data can be initial sample thickness was E40 mm. Electrical resistivity at zero magnetic field 2 c2 was measured using the dc current in van der Pauw technique in a customary described within the entire T/T range by the expression 1þ a cryogenic setup (lowest achievable temperature 1.5 K). The resistivity values were H (T)¼ H *(1—T/T ) (refs 18,33). The fitting parameter c2 c2 c defined as an average of five successive measurements at constant temperature. H * ¼ 4.0 T can be considered as the upper limit for the upper c2 Resistivity measurements in magnetic field were performed on PPMS. Pressure was critical field H (0), which yields a Ginzburg–Landau coherence measured using the ruby scale by measuring the luminescence from small chips c2 of ruby placed in contact with the sample. length x (0) of B9 nm. The corresponding data obtained at GL Magnetization was measured on MoTe (m¼ 3.1 mg) in a pressure cell P¼ 1.1 GPa is also shown in Fig. 3e. It is also worth noting that (m¼ 170 mg) for Pr0.7 GPa and TZ0.5 K (Quantum Design Magnetic Property our estimated value of H (0) is well below the Pauli-Clogston c2 Measurement System (MPMS), iQuantum He insert). Shielding (after zero-field limit. cooling) and Meiner effect curves (in field-cooling) were recorded. We repeated the high-pressure experiments using different The high-P Raman spectra were recorded using a customary micro-Raman spectrometer with a HeNe laser as the excitation source and a single-grating crystal flakes. Similar superconducting behaviour with almost spectrograph with 1 cm resolution. Raman scattering was calibrated using Ne identical T was observed. For comparison with 1T -MoTe ,we c 2  1 lines with an uncertainty of 1cm . also measured r(T) for the 2H-MoTe single crystal at various High-pressure diffraction experiments have been performed at ID09A pressure values. We found a pressure-induced metallization at synchrotron beamline using monochromatic x-ray beam (E ¼ 30 keV, l ¼ 0.413 Å) 2 37 focused to 15 10mm on the sample . We used a membrane-driven high- 15 GPa (Supplementary Fig. 8), which is consistent with previous pressure cell equipped with Boehler-Almax seats and diamond anvil design, theoretical predictions . However, in contrast, we did not detect allowing an opening cone of 64. The culet size was 600 mm and the sample was any signature of superconductivity in the 2H phase for pressures loaded together with He as pressure transmission medium into a hole in a stainless up to 40 GPa. steel gasket preindented to B80mm with an initial diameter of 300 mm. Low temperature data were collected in a He-flow cryostat. Single-crystal data have been collected by a vertical-acting o-axis rotation, with an integrated step scan of 0.5 Discussion and a counting time of 1 s per frame. Diffraction intensities have been recorded For MoTe , the superconducting behaviour in the low-P region with a Mar555 flat-panel detector. Diffraction data have been processed and analysed with CrysAlisPro-171.37.35 and Jana2006 software. Pressures were clearly differs from that in the high-P region. Under quite low P, measured with the ruby fluorescence method . the sharp increase in T is concomitant with a strong suppression of the structural transition, which is reminiscent of observations DFT calculations. DFT calculations were performed using the Vienna Ab-initio for other superconductors with various kinds of competing phase 38,39 Simulation Package with projected augmented wave potential . The exchange transitions. The drastic increase of the T occurs within the T c d and correlation energy was considered at the generalized gradient approximation phase, which is shown by DFT calculations to be a Weyl 40 level for the geometry optimization , and the electronic structure was calculated semimetal (Supplementary Fig. 9a and Supplementary Note 2) using the hybrid functional (HSE06) . Spin–orbital coupling was included in all with a band structure around the Fermi level, which is extremely calculations. Van der Waals corrections were included via a pair-wise force field of 24,34 the Grimme method . In the lattice relaxation, the volumes were fixed while lattice sensitive to changes in the lattice constants . Thus, one can constants and atomic positions were optimized. The pressure was derived by expect that dramatic structural and electronic instabilities emerge fitting the total energy dependence on the volume with the Murnaghan equation . in the low-P region, which may account for the strong After checking the k convergence, the 24  12  8 and 7 5 3 k-meshes with enhancement of T . At higher pressures, the topologically trivial Gaussian-type smearing were used for the generalized gradient approximation (Supplementary Fig. 10) and HSE06 calculations, respectively. The band structures, (due to inversion and time reversal symmetry) 1T phase density of states and Fermi surfaces were interpolated in a dense k-mesh of (Supplementary Fig. 9b and Supplementary Note 2) remains 200  200 200 using the maximally localized Wannier functions extracted from stable in the whole temperature range. Although within this phase HSE06 calculations. T still continues to increase up to its maximum value, the rate of the increase is significantly lower and this growth is naturally References explained by the increase of the electronic density of states 1. Wilson, J. A. & Yoffe, A. D. The Transition Metal Dichalcogenides. Discussion at the Fermi level in the 1T phase (Supplementary Fig. 9c). and interpretation of the observed optical, electrical and structural properties. Adv. Phys. 18, 193–335 (1969). Thorough exploration of superconductivity in MoTe from both 2. Klemm, R. A. Pristine and intercalated transition metal dichalcogenide experimental and theoretical perspectives is required. superconductors. Phys. C 514, 86–94 (2015). 3. Morris, R. C., Coleman, R. V. & Bhandari, R. Superconductivity and Methods magnetoresistance in NbSe . Phys. Rev. B 5, 895–901 (1972). Single-crystal growth. 1T -MoTe crystals were grown via chemical vapour 4. Morosan, E. et al. Superconductivity in Cu TiSe . Nat. Phys. 2, 544–550 (2006). 2 x 2 transport using polycrystalline MoTe powder and TeCl as a transport additive . 2 4 5. Moncton, D. E., Axe, J. D. & DiSalvo, F. J. Neutron scattering study of the Molar quantities of Mo (Sigma Aldrich 99.99%) were ground in combination with charge-density wave transitions in 2H-TaSe and 2H-NbSe . Phys. Rev. B 16, 2 2 purified Te pieces (Alfa Aesar 99.99%), pressed into pellets and heated in an 801–819 (1977). evacuated quartz tube at 800 C for 7 days. Crystals were obtained by sealing 1 g of 6. Qian, X., Liu, J., Fu, L. & Li, J. Quantum spin Hall effect in two-dimensional this powder and TeCl (3 mg ml ) in a quartz ampoule, which was then flushed transition metal dichalcogenides. Science 346, 1344–1347 (2014). with Ar, evacuated, sealed and heated in a two-zone furnace. Crystallization was 7. Xu, X., Yao, W., Xiao, D. & Heinz, T. F. Spin and pseudospins in layered conducted from (T ) 1,000 to (T ) 900 C. The quartz ampoule was then quenched 2 1 transition metal dichalcogenides. Nat. Phys. 10, 343–350 (2014). in ice water to yield the high-temperature monoclinic phase. The obtained crystals 8. Bates, J. B., Gruzalski, G. R., Dudney, N. J., Luck, C. F. & Yu, X. Rechargeable were silver-gray and rectangular in shape. 2H-MoTe crystals were grown using a thin-film lithium batteries. Solid State Ion. 70/71, 619–628 (1994). similar method, but without quenching. 9. Li, Y. et al. MoS nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction. J. Am. Chem. Soc. 133, 7296–7299 (2011). 10. Zhang, Y. J., Oka, T., Suzuki, R., Ye, J. T. & Iwasa, Y. Electrically switchable Structural and transport measurements at ambient pressure. The structures of chiral light-emitting transistor. Science 344, 725–728 (2014). the MoTe crystals were investigated using SXRD with Mo K radiation. To analyse 2 a 11. Lin, Y.-C., Dumcenco, D. O., Huang, Y.-S. & Suenaga, K. Atomic mechanism of the atomic structure of the material, HAADF-STEM was performed. The the semiconducting-to-metallic phase transition in single-layered MoS . Nat. dependence of the electrical resistivity r on temperature T was measured using a 2 Nanotechnol. 9, 391–396 (2014). conventional four-probe method (low-frequency alternating current, Physical 12. Clarke, R., Marseglia, E. & Hughes, H. P. A low-temperature structural phase Property Measurement System (PPMS), Quantum Design). Temperatures down to 0.08 K were achieved using a home-built adiabatic demagnetization stage. The transition in b-MoTe . Philos. Mag. B 38, 121–126 (1978). pulsed magnetic field experiments were conducted at the Dresden High Magnetic 13. Puotinen, D. & Newnhan, R. E. The crystal structure of MoTe . Acta Field Laboratory (Helmholtz-Zentrum Dresden-Rossendorf, HLD-HZDR). Crystallogr. 14, 691–692 (1961). 6 NATURE COMMUNICATIONS | 7:11038 | DOI: 10.1038/ncomms11038 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11038 ARTICLE 14. Zandt, T., Dwelk, H., Janowitz, C. & Manzke, R. Quadratic temperature 39. Kresse, G. & Furthmu¨ller, J. Efficiency of ab-initio total energy calculations for dependence up to 50 K of the resistivity of metallic MoTe . J. Alloys Compd. metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 6, 442, 216–218 (2007). 15–50 (1996). 15. Brown, B. E. The crystal structures of WTe and high-temperature MoTe . Acta 40. Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation 2 2 Crystallogr. 20, 268–274 (1966). made simple. Phys. Rev. Lett. 77, 3865–3868 (1996). 16. Ali, M. N. et al. Large, non-saturating magnetoresistance in WTe . Nature 514, 41. Heyd, J., Scuseria, G. E. & Ernzerhof, M. Hybrid functionals based on a 205–208 (2014). screened Coulomb potential. J. Chem. Phys. 118, 8207–8215 (2003). 17. Ali, M. N. et al. Correlation of crystal quality and extreme magnetoresistance of 42. Grimme, S. Semiempirical GGA-type density functional constructed with a WTe . Europhys. Lett. 110, 67002 (2015). long-range dispersion correction. J. Comput. Chem. 27, 1787–1799 (2006). 18. Pan, X.-C. et al. Pressure-driven dome-shaped superconductivity and electronic 43. Murnaghan, F. D. The compressibility of media under extreme pressures. Proc. structural evolution in tungsten ditelluride. Nat. Commun. 6, 7805 (2015). Natl Acad. Sci. USA 30, 244–247 (1944). 19. Kang, D. et al. Superconductivity emerging from suppressed large 44. Marzari, N. & Vanderbilt, D. Maximally localized generalized Wannier magnetoresistant state in WTe . Nat. Commun. 6, 7804 (2015). functions for composite energy bands. Phys. Rev. B 56, 12847–12865 20. Zhu, Z. et al. Quantum oscillations, thermoelectric coefficients, and the fermi (1997). surface of semimetallic WTe . Phys. Rev. Lett. 114, 176601 (2015). 21. Soluyanov, A. et al.Type II Weyl Semimetals. Preprint at http://arxiv.org/abs/ Acknowledgements 1507.01603 (2015). Y.Q. acknowledges financial support from the Alexander von Humboldt Foundation. 22. Riflikova´, M., Martonˇa´k, R. & Tosatti, E. Pressure-induced gap closing and We would like to thank C. Klausnitzer, M. Nicklas and R. Koban for their help with high- metallization of MoSe and MoTe . Phys. Rev. B 90, 035108 (2014). 2 2 pressure magnetic measurements. This work was financially supported by the Deutsche 23. Fu, L. & Kane, C. L. Superconducting proximity effect and Majorana fermions Forschungsgemeinschaft (DFG, Project No. EB 518/1-1 of DFG-SPP 1666 ‘Topological at the surface of a topological insulator. Phys. Rev. Lett. 100, 096407 (2008). Insulators’) and by a European Research Council (ERC) Advanced Grant, No. (291472) 24. Sun, Y., Wu, S.-C., Ali, M. N., Felser, C. & Yan, B. Prediction of the Weyl ‘Idea Heusler’. semimetal in the orthorhombic MoTe . Phys. Rev. B 92, 161107 (2015). 25. Cho, G. Y., Bardarson, J. H., Lu, Y.-M. & Moore, J. E. Superconductivity of doped Weyl semimetals: Finite-momentum pairing and electronic analog of the Author contributions He-A phase. Phys. Rev. B 86, 214514 (2012). B.Y. and C.F. conceived the project. M.N.A., C.R.R. and V.S. prepared the samples and 26. Wei, H., Chao, S.-P. & Aji, V. Odd-parity superconductivity in Weyl performed XRD structural characterization. E.P., P.W. and R.H. performed TEM studies. semimetals. Phys. Rev. B 89, 014506 (2014). W.S. performed ambient pressure transport measurements and Meiner effect 27. Hosur, P., Dai, X., Fang, Z. & Qi, X.-L. Time-reversal-invariant topological measurements at low pressures. C.S., T.F. and E.K. performed magneto-transport superconductivity in doped Weyl semimetals. Phys. Rev. B 90, 045130 (2014). measurements at ambient pressure. Y.Q., P.G.N., O.B. and S.A.M. performed 28. Jian, S.-K., Jiang, Y.-F. & Yao, H. Emergent Spacetime Supersymmetry in 3D high-pressure electrical resistivity, Raman spectroscopy and magnetic susceptibility Weyl Semimetals and 2D Dirac Semimetals. Phys. Rev. Lett. 114, 237001 measurements. M.H. performed high-pressure SXRD studies. S.C.W., Y.S. and B.Y. (2015). carried out the theoretical calculations. All authors discussed the results of the studies. 29. Keum, D. H. et al. Bandgap opening in few-layered monoclinic MoTe . Nat. 2 Y.Q., B.Y., W.S. and S.A.M. co-wrote the paper. All authors commented on the Phys. 11, 482–487 (2015). manuscript. 30. Hughes, H. P. & Friend, R. H. Electrical resistivity anomaly in b-MoTe . J. Phys. C Solid State Phys. 11, L103–L105 (1978). 31. Hulliger, F. Crystal Chemistry of Chalcogenides and Pnictides of the Transition Additional information Elements. in Structure and Bonding, Vol. 4, 83–229 (Springer-Verlag, 1968). Supplementary Information accompanies this paper at http://www.nature.com/ 32. Suderow, H., Tissen, V. G., Brison, J. P., Martı´nez, J. L. & Vieira, S. Pressure naturecommunications induced effects on the Fermi surface of superconducting 2H-NbSe . Phys. Rev. Competing financial interests: The authors declare no competing financial interests. Lett. 95, 117006 (2005). 33. Mu¨ller, K. H. et al. The upper critical field in superconducting MgB . J. Alloys Reprints and permission information is available online at http://npg.nature.com/ Compd. 322, L10–L13 (2001). reprintsandpermissions/ 34. Wang, Z. et al. MoTe : Weyl and Line Node Topological Metal. Preprint at http://arxiv.org/abs/1511.07440 (2015). How to cite this article: Qi, Y. et al. Superconductivity in Weyl semimetal candidate 35. Fourcaudot, G., Gourmala, M. & Mercier, J. Vapor phase transport and crystal MoTe . Nat. Commun. 7:10038 doi: 10.1038/ncomms11038 (2016). growth of molybdenum trioxide and molybdenum ditelluride. J. Cryst. Growth 46, 132–135 (1979). This work is licensed under a Creative Commons Attribution 4.0 36. Mao, H. K., Xu, J. & Bell, P. M. Calibration of the ruby pressure gauge to 800 International License. The images or other third party material in this kbar under quasi-hydrostatic conditions. J. Geophys. Res. 91, 4673–4676 (1986). article are included in the article’s Creative Commons license, unless indicated otherwise 37. Merlini, M. & Hanfland, M. Single-crystal diffraction at megabar conditions by in the credit line; if the material is not included under the Creative Commons license, synchrotron radiation. High Press. Res. 33, 511–522 (2013). users will need to obtain permission from the license holder to reproduce the material. 38. Kresse, G. & Hafner, J. Ab initio molecular dynamics for open-shell transition To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ metals. Phys. Rev. B 48, 13115–13118 (1993). NATURE COMMUNICATIONS | 7:11038 | DOI: 10.1038/ncomms11038 | www.nature.com/naturecommunications 7

Journal

Nature CommunicationsSpringer Journals

Published: Mar 14, 2016

There are no references for this article.