key: cord-0103725-cfgvozmz
authors: Xing, Ying; Shao, Zhibin; Ge, Jun; Wang, Jinhua; Zhu, Zengwei; Liu, Jun; Wang, Yong; Zhao, Zhiying; Yan, Jiaqiang; Mandrus, David; Yan, Binghai; Liu, Xiong-Jun; Pan, Minghu; Wang, Jian
title: Surface Superconductivity in the type II Weyl Semimetal TaIrTe4
date: 2018-05-28
journal: nan
DOI: nan
sha: f7e279f22c25997902b126c819562095ceca42bd
doc_id: 103725
cord_uid: cfgvozmz

The search for unconventional superconductivity in Weyl semimetal materials is currently an exciting pursuit, since such superconducting phases could potentially be topologically nontrivial and host exotic Majorana modes. The layered material TaIrTe4 is a newly predicted time-reversal invariant type II Weyl semimetal with minimum Weyl points. Here, by a systematical study based on electrical transport measurements at ultralow temperature and in ultrahigh magnetic field, we discover that TaIrTe4 crystals exhibit quasi-one-dimensional (quasi-1D) superconductivity with an onset transition temperature (Tc) up to 1.54 K, and show strong Shubnikov de Haas quantum oscillations. The normalized upper critical field h*(T/Tc) behavior indicates that the discovered superconductivity is unconventional with the p-wave pairing. The superconductivity is observed to be uniform on the sample surface with a V-shaped superconducting gap by scanning tunneling spectroscopy (STS). The STS further visualizes Fermi arc surface states that are consistent with the previous angle-resolved photoemission spectroscopy results. Both the transport and STS measurements reveal that the superconductivity occurs in the surface states and exhibits the quasi-1D features. Our results suggest that TaIrTe4 is a promising platform to explore topological superconductivity.

* minghupan@hust.edu.cn † jianwangphysics@pku.edu.cn

The search for unconventional superconductivity in Weyl semimetal materials is currently an exciting pursuit, since such superconducting phases could potentially be topologically nontrivial and host exotic Majorana modes. The layered material TaIrTe 4 is a newly predicted time-reversal invariant type II Weyl semimetal with minimum Weyl points. Here, by a systematical study based on electrical transport measurements at ultralow temperature and in ultrahigh magnetic field, we discover that TaIrTe 4 crystals exhibit quasi-one-dimensional (quasi-1D) superconductivity with an onset transition temperature (T c ) up to 1.54 K, and show strong Shubnikov de Haas quantum oscillations. The normalized upper critical field h * (T/T c ) behavior indicates that the discovered superconductivity is unconventional with the p-wave pairing. The superconductivity is observed to be uniform on the sample surface with a V-shaped superconducting gap by scanning tunneling spectroscopy (STS). The STS further visualizes Fermi arc surface states that are consistent with the previous angle-resolved photoemission spectroscopy results. Both the transport and STS measurements reveal that the superconductivity occurs in the surface states and exhibits the quasi-1D features. Our results suggest that TaIrTe 4 is a promising platform to explore topological superconductivity.

Weyl semimetals, which possess nodal points in the bulk and Fermi arc states in the surface, have generated considerable research interest in the recent years [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] . The chirality of Weyl fermions is responsible for a few novel transport phenomena, such as the chiral anomaly. On the surface of Weyl semimetals, universal signatures of topological Fermi arcs in quasi-particle interference were theoretically predicted [11] and experimentally observed by scanning tunneling microscopy [12, 13] . Moreover, the theoretical studies have shown that the presence of superconductivity in Weyl semimetal may lead to a bunch of novel topological phases, including the time-reversal invariant topological superconductor [14] , Fulde-Ferrell-Larkin-Ovchinnikov superconductors [15] [16] [17] , and chiral non-Abelian Majorana fermions protected by second Chern numbers [18] . These predictions suggest that turning a Weyl semimetal into superconducting state may provide a promising way to explore topological superconductivity and Majorana modes, which can be applied to topological quantum computation [19] [20] [21] .

Experimentally, superconductivity has been observed in both type I and type II Weyl semimetals, such as tip induced superconductivity on TaAs [22, 23] , pressure induced superconductivity on TaP [24] , as well as T d phase WTe 2 (pressure driven) [25, 26] and MoTe 2 crystals (without pressure) [27, 28] . However, in these Weyl semimetals, the number of Weyl points is 8 or even 24, more than the minimal number of Weyl points allowed for a time-reversal invariant Weyl semimetal, which leads to complicated band structures and hinders the further studies. Therefore, to observe superconductivity in simpler Weyl semimetals possessing minimal number of Weyl points is highly desired.

Following the first-principle calculations by K. Koeperni et al. [29] , TaIrTe 4 hosts only four well-separated Weyl points, which is the minimal number in a Weyl semimetal with timereversal symmetry. The Fermi arcs connecting Weyl nodes of opposite chirality in TaIrTe 4 extend to about ⅓ of the surface Brillouin zone in the b direction. This large momentum-space separation makes TaIrTe 4 quite favorable for exploring the Fermi arcs spectroscopically and the important transport properties. The exotic surface states supporting the quasi-one-dimensional (quasi-1D) Fermi arcs have been observed by angle-resolved photoemission spectroscopy (ARPES) [30] . Fermi arcs as well as Weyl nodes in the bulk of TaIrTe 4 have been identified directly by pump-probe ARPES [31] . The Weyl points and Fermi arcs are found to live at 50 ~ 100 meV above Fermi energy. If the noncentronsymmetric Weyl material TaIrTe 4 can be superconducting, it would stimulate further investigations on the superconductivity in topological materials and long-sought-after topological superconductors.

In this work, we perform electrical transport studies of the ternary compound TaIrTe 4 single crystal down to 0.06 K with a high magnetic field up to 54.5 T. The unconventional superconductivity with quasi-1D characteristics coexisting with strong Shubnikov de Haas (SdH) oscillations is discovered in TaIrTe 4 single crystals. The detected superconductivity is further demonstrated by the scanning tunneling microscopy and spectroscopy (STM and STS) studies at ultralow temperatures. The observed unconventional superconductivity and the nontrivial topological properties revealed by quantum oscillations demonstrate that TaIrTe 4 might be a candidate for topological superconductors.

Single crystals of TaIrTe 4 were synthesized from excess Te flux. The crystal is needle-like morphology and grows preferentially along the [100] direction (the length direction). The width direction is along the [010] and the cleavage surface of the crystal is the (001) plane. The high structural quality of the sample was confirmed by X-ray diffraction (XRD), high-resolution scanning transmission electron microscopy (HRSTEM) and STM. 

More than 10 samples are studied and all samples exhibit consistent results. Figure 2(a) shows the resistivity of sample 1 (S1) as a function of temperature (T) from 2 K to 300 K. The resistivity exhibits metallic-like behavior and tends to saturate at 10 K with a residual resistivity ratio (RRR) of 6.8 (the resistivity at room temperature over the resistivity at 2 K). Interestingly, when upon further cooling, an evident resistivity drop appears at about 1.54 K ( Fig. 2(b) ). When applying perpendicular magnetic field (B//c axis), the resistivity drop shifts to lower temperatures as the field increases and completely suppressed at around 0.4 T. This is a typical superconducting behavior although no zero resistance is observed down to 0.06 K and the proportion of resistivity drop is ~ 44% (Fig. 2(b) ). The diamagnetic signal from the superconductivity is hard to be detected due to the residual resistance below T c . 2(e)) were carried out at various temperatures from 0.08 to 2.0 K. It is evident that superconductivity at B//a axis varies differently from the other two directions. For example, B c2 is around 0.5 T at 0.1 K for both B//c axis and B//b axis, substantially smaller than B c > 1.5 T for the B//a axis situation ( Fig. 2 (f)). Since zigzag Ta-Ir chains along a direction of TaIrTe 4 , the difference of B c2 may originate primarily from the anisotropy of the sample, which causes quasi-1D superconductivity [33, 34] . Besides the observed anisotropic superconductivity, when the temperature is above T c , the pronounced anisotropic MR at 2 K up to 15 T is also detected, which further confirms the anisotropic characteristic of TaIrTe 4 , as shown in Fig. S3 . For quasi-1D superconductors, below T c phase slips can give rise to broad superconducting transition with the residual resistance [35] , which may explain our observations. Another possible scenario is that the superconductivity occurs in the surface states which are helical states for a time-reversal invariant Weyl semimetal with dispersions along a-axis, leading to the quasi-1D p-wave superconducting phases. Similar resistivity drops are observed in two other TaIrTe 4 samples, with T c (where resistivity starts to drop) from 1.19 K to 1.38 K (Fig. S4) , confirming the observed superconductivity in TaIrTe 4 crystals. Further measurements show that the T c of different regions in the same TaIrTe 4 sample exhibits consistent superconducting behavior, which excludes the macroscopic superconducting phase separation in TaIrTe 4 crystals (Fig. S5 ).

The reduced critical field h* equals B c2 /[T c (-dB c2 /dT| T=Tc )], which is calculated to compare with known models for s-wave superconductors (Werthamer-Helfand-Hohenberg theory, WHH, h*(0) ≈ 0.7 [36] ) and spin-triplet p-wave superconductors (h*(0) ≈ 0.8 [37] ). B c2 is defined as the field above which the TaIrTe 4 sample becomes the normal state. Obviously, the h*(T/T c ) relation is close to that of a polar p-wave state, suggesting the possibility of unconventional superconducting paring symmetry in TaIrTe 4 as shown in the inset of Fig. 2(c) . Critical current (I c ) is another key factor of superconductors. Figure S6 increasing, the sample is gradually tuned from superconducting state to normal state. The I c is suppressed by both temperature and magnetic field, which provides another evidence of superconductivity in TaIrTe 4 . In addition, the existence of p-wave superconductivity is backed by the essential symmetry consideration, in which both the bulk and surface of the studied material breaks the inversion symmetry and thus, allow the spin-triplet pairing [38] .

Next we investigate the MR at ultrahigh pulsed magnetic field. Figure 3 give the complexity and anisotropy of Fermi surface, similar to the previous report by torque measurements [39] .

To further demonstrate and explore the observed superconductivity in TaIrTe 4 single crystals, STM and STS were performed at the (001) surface of TaIrTe 4 single crystals. Samples were cleaved in situ at room temperature under the vacuum with pressure better than 1×10 -10 torr.

The cleaved sample was quickly transferred into a Unisoko-1300 STM system for ultralow temperature measurements down to 0.4 K. Figure 4 Fig. 4(b) . The spectrum exhibits a V-shaped superconducting gap (Δ) of 1.2 meV defined by half the distance between the two conductance peaks. Such a V-shape indicates a gapless nodal structure in the gap function as a signature of the p-wave superconductivity. The superconducting gap is also observed uniformly on the whole cleaved surface (see Fig. S9 ). We have macroscopically changed the locations of STM scanning. Similar topographic images and superconductivity were observed reproducibly. Figure 4(c) shows the temperature evolution of dI/dV spectra measured from 0.4 K to 1.10 K. As the temperature increases, the dip at zero bias is reduced and the gap almost vanishes near 1.10 K, which agrees with the onset T c (1.19 K ~ 1.54 K for different samples) obtained from the electrical transport measurements. The BCS ratio 2Δ(0)/k B T c (k B is the Boltzmann constant)) is estimated about 18.4, which is much larger than that of weak coupling BCS superconductors and reminiscent of the possibility of topological superconductivity [13, 40, 41] . Magnetic field dependence of dI/dV spectra are shown in Fig. 4(d) . The superconducting gap decreases with the increasing field and almost vanishes near 0.25 T. This agrees very well with the B c2 from transport measurements (see Fig. 2(f) ). Therefore, our STS results confirm the superconductivity in TaIrTe 4 .

Furthermore, we locate a terrace edge that is perpendicular with the direction of 1D atomic rows and perform a line spectroscopic survey along a 1D atomic row by crossing the broken end (blue dashed line in lower panel of Fig. 4(f) ). The dI/dV spectra along the 1D Ta-Ir chain show that superconducting gap becomes smaller and shallower by approaching to the broken end and finally almost vanishes. It is worth mentioning that superconductivity can be observed on every terrace of the sample. The crucial dependence of the superconductivity on defects in the Ta-Ir chains again suggests that the pairing order might be unconventional, in contrast to the conventional s-wave order which is stable against to defects.

Quasiparticle interference (QPI) based on spectroscopic-imaging scanning tunneling microscopy, has shown success in identifying the topological surface states of topological insulator [42, 43] and topological Fermi arc states of Weyl semimetals TaAs [44] , MoTe 2 [12] , [13] and Mo 0.66 W 0.34 Te 2 [45] . In the surface Brillouin zone, the extremal pairs of k i and k f on a two-dimensional constant energy contour, where k i and k f are the initial and final wavevectors, contribute dominantly to the spatial interference pattern of the local density of states. Topological Fermi arcs in QPI on the surface of Weyl semimetal have been proposed theoretically [11] . The features in Fourier transform of dI/dV mapping correspond to the scattering vector q= k i -k f of the extremal pairs. (Fig. 5(g) ). For a pair of topological Fermi arcs, three scattering wavevectors ( Fig. 5(h) ), labelled q 1 , q 2 and q 3 , might be expected to appear in QPI. Among them, q 2 is forbidden due to the requirement of the time-reversal symmetry in the system [29] . q 3 does not correspond to the observed features discussed above because this vector has very small length in k space and stays very close to the center. The scattering wavevectors should generate visible features of four arcs, which indicated by yellow arrows in Fig. 5(g) . Such features are clearly resolved at 80 mV, which is consistent with previous prediction. The existence of such a pattern beyond the trivial surface states (0 mV) excludes the possibility of trivial surface states as the origin. This is a direct and strong experimental evidence for the existence of the topological surface states.

As we mentioned above, both the low proportion of resistivity drop and undetectable diamagnetic signal in TaIrTe 4 indicate the tiny superconducting volume fraction, and do not support the bulk superconductivity. Our STS results show uniform superconducting gap on the surface of TaIrTe 4 . More importantly, it is found that the I c remains almost the same when reducing the thickness of sample 3 from 30 μm to 6 μm (see Fig. S6 ). This observation provides a strong evidence of the surface superconductivity, rather than the bulk superconductivity, since the I c of a bulk superconductor decreases proportionately as the thickness decreases. Further, the surface superconductivity is also supported by the magnitude of the critical current density. In particular, if treating the system as a bulk superconductor, the calculated critical current density is only J c ~ 42.5 A/cm 2 at 0.3 K, which is considerably smaller than that in the typical bulk superconductors ~10 4 A/cm 2 and type II superconductors ~10 4 -10 8 A/cm 2 (Fig. S6) . Instead, if the superconductivity originates from one unit cell (1.304 nm) in the surface, we find that J c is ~ 9.6×10 5 A/cm 2 , which is a reasonable magnitude for a surface or film superconductor. As a comparison, we note that for 2D indium film grown on Si substrate, the critical current density J c is ~ 6×10 5 A/cm 2 at 1.8 K [46] . For two atomic layer Ga film grown on GaN substrate, J c is ~ 1×10 6 A/cm 2 at 2 K [47] , and for four atomic layer Pb film grown on Si substrate the J c is ~ 4.7×10 5 A/cm 2 at 2 K [48] . With these results our observations exclude scenario of the bulk superconductivity, but can be explained with the surface superconductivity. with Ni film indicates the magnetic property of Ni film (Fig. S12(c) ). The magnetic Ni film has little effect on the superconductivity of TaIrTe 4 ( Fig. S12(a) and (b) ), which supports the p-wave like or topological superconductivity from the surface state together with the fitting for critical field vs temperature behavior and the detected Fermi arc surface state.

In conclusion, we have observed the novel superconductivity in type II Weyl semimetal The experimental evidences on surface superconductivity in TaIrTe 4 are summarized as Table I . 

Superconducting resistance drops which can be suppressed by increasing magnetic field, temperature and current (Fig.2, Fig. s6 )

Superconducting gap which can be suppressed by increasing magnetic field and temperature (Fig.4) Homogeneity of superconductivity XRD (Fig.1a) , STEM (Fig.1b) , STM (Fig.4a,Fig.S2 )

Evidence of high quality single crystal Multi-electrodes transport results (Fig.S5) Exclude macroscopic phase separation Uniform superconducting gap in whole surface (Fig.S9) Exclude minority phase region The superconductivity comes from the surface state.

Low proportion of resistivity drop (Fig.2) Indicate low superconducting volume fraction and exclude the possibility of bulk superconductivity.

Diamagnetic signal is too small to be detected Indicate low superconducting volume fraction and exclude the possibility of bulk superconductivity.

Nearly thickness-independent critical current. (Fig. S6) Exclude the possibility of bulk superconductivity

Low critical current density (Fig. S6) Exclude the possibility of bulk superconductivity

The superconducting gap is detected by surface sensitive STM (Fig.4, Fig.s9 and Fig.s10 )

The sample surface is superconducting.

The possibility of nontrivial topological superconductivity Fermi arc states observed by STS (Fig.S5) Topological non-trivial surface state Ferromagnetic Ni film has little effect on the superconductivity of TaIrTe 4 (Fig.S12) Suggesting the possibility of topological superconducting paring symmetry in TaIrTe 4 h*(T/T c ) relation is close to that of a polar p-wave state (Fig. 2c) Suggesting the possibility of topological superconducting paring symmetry in TaIrTe 4 Superconducting gap becomes smaller and shallower by approaching to the broken end (Fig. 4f) Suggesting that the pairing order might be unconventional Quasi 1D feature STEM (Fig.1b) , STM (Fig.4a,Fig.S2 )

Quasi-1D structure Non-zero resistance at low temperature (Fig.2b) Quantum phase slip from quasi-1D superconducting system

The anisotropy of upper critical field (Fig.2f) and magnetoresistance (Fig.S3) Quasi-1D superconductivity and characteristic Fermi arc states observed by STS (Fig.S5) Quasi-1D behavior on the surface

Topological semimetal and Fermi-arc surface states in the electronic structure of pyrochlore iridates

Weyl Semimetal in a Topological Insulator Multilayer

Chern Semimetal and the Quantized Anomalous Hall Effect in HgCr2Se4

Weyl Semimetal Phase in Noncentrosymmetric Transition-Metal Monophosphides

A Weyl Fermion semimetal with surface Fermi arcs in the transition metal monopnictide TaAs class

Discovery of a Weyl fermion semimetal and topological Fermi arcs

Observation of Weyl nodes in TaAs

Experimental Discovery of Weyl Semimetal TaAs

Signatures of the Adler-Bell-Jackiw chiral anomaly in a Weyl fermion semimetal

Observation of the Chiral-Anomaly-Induced Negative Magnetoresistance in 3D Weyl Semimetal TaAs

Universal signatures of Fermi arcs in quasiparticle interference on the surface of Weyl semimetals

Experimental observation of topological Fermi arcs in type-II Weyl semimetal MoTe 2

Nontrivial superconductivity in topological MoTe 2−x S x crystals

Time-reversal-invariant topological superconductivity in doped Weyl semimetals

Superconductivity of doped Weyl semimetals: Finitemomentum pairing and electronic analog of the 3He-A phase

Odd-parity superconductivity in Weyl semimetals

The Fulde-Ferrell-Larkin-Ovchinnikov state for ultracold fermions in lattice and harmonic potentials: a review

Non-Abelian Majorana Modes Protected by an Emergent Second Chern Number

Non-Abelian anyons and topological quantum computation

Non-Abelian statistics and topological quantum information processing in 1D wire networks

Non-Abelian Majorana Doublets in Time-Reversal-Invariant Topological Superconductors

Discovery of tip induced unconventional superconductivity on Weyl semimetal

Mesoscopic superconductivity and high spin polarization coexisting at metallic point contacts on Weyl semimetal TaAs

Concurrence of superconductivity and structure transition in Weyl semimetal TaP under pressure

Superconductivity emerging from a suppressed large magnetoresistant state in tungsten ditelluride

Pressure-driven dome-shaped superconductivity and electronic structural evolution in tungsten ditelluride

Prediction of Weyl semimetal in orthorhombic MoTe 2

Superconductivity in Weyl semimetal candidate MoTe 2

TaIrTe 4 : A ternary type-II Weyl semimetal

Experimental realization of type-II Weyl state in noncentrosymmetric TaIrTe 4

Signatures of a time-reversal symmetric Weyl semimetal with only four Weyl points

Metal-metal vs tellurium-tellurium bonding in WTe 2 and its ternary variants TaIrTe 4 and NbIrTe 4

Superconductivity in Quasi-One-Dimensional K 2 Cr 3 As 3 with Significant Electron Correlations

Superconductivity and Quantum Oscillations in Crystalline Bi Nanowire

Quantum phase slips in superconducting nanowires

Temperature and Purity Dependence of the Superconducting Critical Field, Hc2. III. Electron Spin and Spin-Orbit Effects

p-wave superconductors in magnetic fields

Superconductivity without Inversion Symmetry: MnSi versus CePt 3 Si

Magnetotransport and de Haas-van Alphen measurements in the type-II Weyl semimetal TaIrTe 4

Experimental signature of topological superconductivity and Majorana zero modes on β-Bi 2 Pd thin films

Dirac-fermion-induced parity mixing in superconducting topological insulators

Topological surface states protected from backscattering by chiral spin texture

Experimental Demonstration of Topological Surface States Protected by Time-Reversal Symmetry

Quasiparticle interference of the Fermi arcs and surface-bulk connectivity of a Weyl semimetal

Atomic-Scale Visualization of Quasiparticle Interference on a Type-II Weyl Semimetal Surface

Macroscopic superconducting current through a Silicon surface reconstruction with Indium adatoms: Si(111)-(√7×√3)-In

Detection of a superconducting phase in a two-atom Layer of hexagonal Ga film grown on semiconducting GaN(0001)

Interface-induced Zeeman-protected superconductivity in ultrathin crystalline lead films

R(I) data of three TaIrTe 4 samples with different thickness (30 μm, 6 μm)

Quantum oscillations in TaIrTe 4 single crystals (S5) with magnetic field perpendicular to the ac plane (B//b). (a) Magnetic field (up to 54.5 T) dependence of resistivity at different temperatures. (b) Second derivative of the ρ(B) data in (a) vs