Spectroscopic evidence for the gapless electronic structure in bulk ZrTe5

Accepted Manuscript Title: Spectroscopic Evidence for the Gapless Electronic Structure in Bulk ZrTe5 Author: L. Shen M.X. Wang S.C. Sun J. Jiang X. Xu T. Zhang Q.H. Zhang Y.Y. Lv S.H. Yao Y.B. Chen M.H. Lu Y.F. Chen C. Felser B.H. Yan Z.K. Liu L.X. Yang Y.L. Chen PII: DOI: Reference:

S0368-2048(16)30150-5 http://dx.doi.org/doi:10.1016/j.elspec.2016.10.007 ELSPEC 46597

To appear in:

Journal of Electron Spectroscopy and Related Phenomena

Received date: Revised date: Accepted date:

10-8-2016 13-10-2016 18-10-2016

Please cite this article as: L.Shen, M.X.Wang, S.C.Sun, J.Jiang, X.Xu, T.Zhang, Q.H.Zhang, Y.Y.Lv, S.H.Yao, Y.B.Chen, M.H.Lu, Y.F.Chen, C.Felser, B.H.Yan, Z.K.Liu, L.X.Yang, Y.L.Chen, Spectroscopic Evidence for the Gapless Electronic Structure in Bulk ZrTe5, Journal of Electron Spectroscopy and Related Phenomena http://dx.doi.org/10.1016/j.elspec.2016.10.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Spectroscopic Evidence for the Gapless Electronic Structure in Bulk ZrTe5 L. Shen1*, M. X. Wang2*, S. C. Sun1*, J. Jiang2,3, X. Xu1, T. Zhang1, Q. H. Zhang1, Y. Y. Lv4, S. H. Yao4, Y. B. Chen4, M. H. Lu4, Y. F. Chen4, C. Felser5, B. H. Yan5, Z. K. Liu2, L. X. Yang1† and Y. L. Chen1,2,6†

1

State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics and Collaborative Innovation Center of Quantum Matter, Tsinghua University, Beijing, China 2 School of Physical Science and Technology, ShanghaiTech University and CAS-Shanghai Science Research Center, Shanghai, China 3 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 4 National Laboratory of Solid State Microstructures, Department of Materials Science and Engineering, Nanjing University, Nanjing, China 5 Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany 6 Department of Physics, Clarendon Laboratory, University of Oxford Parks Road, Oxford, OX1 3PU, UK †

Correspondence should be addressed to: [email protected], [email protected]

* These authors contributed equally to this work

Research Highlights of “Spectroscopic Evidence for the Gapless Electronic Structure in Bulk ZrTe5”   

We present comprehensive ARPES and STM studies of bulk ZrTe5. We observe a gapless electronic structure of ZrTe5. We observe edge states near the step of terraces on the surface of ZrTe5.

Abstract Recently, transition metal pentatellurides MTe5 (M = Zr, Hf) have inspired intensive research effort. Being predicted to be quantum spin Hall insulators (QSHI) with the bulk gap up to hundreds of meVs, it could lead to promising applications at unprecedented high temperature compared with previously discovered QSHI (e.g. HgTe/CdTe or InAs/GaSb quantum wells). However, the experimental works soon followed illustrated considerable discrepancies regarding to whether MTe5 compounds possess a full bulk gap, making their topological nature (topological insulators or Dirac semimetals) illusive. In this work, combining investigations of angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM), we systematically studied the electronic properties of ZrTe5. In intrinsic samples, we observed little evidence for the existence of topological surface states or large bulk gap. With bulk and surface doping to adjust the position of the Fermi-level, ARPES spectra indicate gapless and highly linear dispersions at the valance band top, in consistence with the STM measurements that show a Vshaped total density of states near the Fermi-level (i.e. suggesting a gapless nature of the electronic structure of ZrTe5). Moreover, near the terrace edge on the surface, we observed non-zero DOS, indicating the existence of edge states. Keywords: Topological Dirac semimetal; Gapless electronic structure; Angle-resolved photoemission spectroscopy; Scanning tunneling spectroscopy.

1. Introduction Topological quantum materials (TQMs) have attracted enormous attention due to their great implication on fundamental physics and broad application potential. After extensive exploration in the last decade, TQMs have extended from topological insulators (TIs)[1, 2] to topological semimetals (TSs), such as topological Dirac semimetals (TDSs)[3-8], topological Weyl semimetals[9-15] and topological nodal line semimetals[16-18]. TIs are characterized by a full bulk band gap with in-gap surface Dirac fermions of helical spin structures [1, 2]; while in contrast, TSs are characterized by the linear touching of the bulk conduction and valance bands [3-18]. In the case of discrete touching, 3D bulk fermions are formed (e.g. in TDSs and TWSs), making TSs the analogue to “3D graphene”. In addition to their linear bulk dispersions, TSs can also possess nontrivial topological surface states, forming either closed or unclosed surface Fermi arcs on the Fermi surface (FS) [3, 4, 8-15]. Due to these unusual electronic structure, TSs encapsulate various intriguing properties, such as chiral magnetic anomaly, negative magnetoresistance, anomalous quantum hall effect, ultrafast carrier mobility and unsaturated magnetoresistance [19-25], thus providing promising electronics and spintronics application potentials for the future. Recently, transition metal pentatellurides MTe5 (M = Zr, Hf) have inspired much research interests due to their unusual electronic properties[26]. In the monolayer limit, ZrTe5 was predicted to be a quantum spin hall insulator with a large bulk gap, providing a promising material basis for the future device applications at high temperatures[26]. In bulk ZrTe5, however, different recent investigations show considerable discrepancy regarding to its exact electronic structures. Theoretically, it has been predicted that the bulk ZrTe5 resides near the transition boundary between a strong TI and weak TI, depending on the strength of interlayer coupling[26]. Experimentally, on one hand, transport measurements provide evidences that suggest ZrTe5 as a TDS, such as linear dependence of optical conductivity with the frequency of excitation light, inter-Landau-level transitions due to the existence of Dirac fermions in magneto-optical measurements, quantum

oscillations with non-trivial Berry phase, and more prominently, the observation of chiral magnetic effect in magneto-resistivity measurements[27-31]. On the other hand, some recent angle-resolved photoemission (ARPES) and scanning tunneling spectroscopy (STS) studies indicate the existence of a large bulk band gap (which evolves with temperature) despite the inconsistency of the absolute value of the gap [32-34]. To further complicate the problem, there exists a puzzling uncertainty regarding to the evolution of the band structure of ZrTe5 over the temperature [32, 33, 35-37]. In order to extract the real electronic structure of ZrTe5, we carried out comprehensive highresolution ARPES measurements with broad photon energy range (to cover the whole BZ in the kz direction) and STM measurements. In the ARPES measurements, although the large energy scale features of the bands are in general consistence with previous ab-initio calculation[26], we did not observe topological surface states which are typically significant in the ARPES spectra in TIs[3841]. By bulk and surface doping, we successfully tuned the Fermi energy (EF) through and observed the valance band top at the bulk  point, which showed a Dirac like linear dispersion. Consistent with the ARPES measurements, our STS measurement with high statistics provided evidence of Vshaped density of states (DOS) near EF, also suggesting a gapless electronic structure of bulk ZrTe5, similar to that in Na3Bi[42]. In addition to the measurement in the bulk, we also conducted STS investigation on the DOS at the edge of the terraces on the surface of ZrTe5, and observed non-zero DOS around EF, indicating the existence of edge states. These results are in overall consistence with the bulk sensitive transport measurements, and suggest ZrTe5 as a TDS[27-31].

2. Experiment details ZrTe5 crystallizes into a layered structure with space group of Cmcm (63) as shown in Fig. 1(a). In its orthorhombic unit cell, there are two ZrTe5 layers offset relatively half the unit cell along a direction, forming A-B-A-B type stacking along b direction in the bulk ZrTe5[43]. The different layers are coupled by weak van der Waals force, making it easy to cleave along (010) direction. In each ZrTe5 layer, there are trigonal prismatic ZrTe3 chains (Fig. 1(b) and (c)) running along a

direction. The neighbouring trigonal prismatic chains rotate 180 degrees about the chains axes and are interconnected by zigzag Te chains, forming wrinkling ZrTe5 layers. The 3D BZ is a nonregular hexagonal prism and the projected surface BZ is a rectangle as shown in Fig. 1(d). High-quality single crystal ZrTe5 and Ta doped ZrTe5 were grown by the chemical vapor transport method with I2 as the transport additive. Polycrystalline samples of ZrTe5 and Ta doped ZrTe5 were synthesized from high purity (99.999%) Zr, Ta and Te powders by solid state reaction in a fused silica tube under vacuum of about 4 × 10−6 mbar at about 500 °C for seven days. The obtained polycrystalline is then mixed with I2 (about 5 mg/L) and the mixture is further loaded into a evacuated quartz tube and placed in a two-zone furnace with a typical temperature gradient from 520 °C to 450 °C. After over 10 days of sample growth, single crystal ZrTe5 was successfully obtained with a typical size of about 35 × 0.5 × 1 mm3, and the size of Ta doped ZrTe5 is smaller than that of ZrTe5 crystal. ARPES measurements were performed at beamline 10.0.1 of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory, USA and beamline I05 of Diamond light source (DLS), UK. The samples were measured under ultra-high vacuum below 3 × 10-11 Torr at ALS and 1 × 1010

Torr at DLS. Data were collected by Scienta R4000 analyzers at 10 K sample temperature. The

total convolved energy and angle resolutions were 20 meV and 0.2°, respectively. In order to obtain a fresh and clean surface, the samples were cleaved in situ along its natural (010) cleavage plane. The surface dosing was conducted in situ by evaporating alkali metals directly onto the sample surface at measurement temperature. Scanning tunneling microscopy/spectroscopy (STM/STS) experiments were carried out in ultrahigh vacuum (UHV) below 1 × 10-10 Torr. ZrTe5 single crystals were glued to highly doped silicon wafers and cleaved in situ at room temperature after a thorough out-gas. Cleaved samples were transferred to a cryogenic stage kept at 77 K for STM/STS experiments. Chemically etched

tungsten tips were used for both imaging and tunneling spectroscopy and lock-in technique was employed to obtain dI/dV curves. A modulation voltage of 5 mV at 991 Hz generated by a lock-in amplifier was applied to the sample alongside with the DC sample bias.

3. Results and discussion Figures 1(e), (f) show the STM surface topography of our pristine ZrTe5 crystal cleaved in situ under ultra-high vacuum. We obtained large-area atomic flat and clean surface with terraces between different ZrTe5 layers. With atomic resolution, we can clearly observe quasi-one dimensional chains of dumbbell units on the top layer. The distance between two dumbbell units is about 0.39 nm, while the distance between chains is about 1.35 nm, in consistence with the lattice constants of a = 3.9876 Å and c = 13.052 Å, as schematically shown in the inset of Fig. 1(f) . Figure 1(g) shows the core level photoemission spectrum collected on pristine ZrTe5. The sharp Te 4d peaks suggest high quality and clean surface of our sample. The absence of the characteristic peaks of Zr is due to the low cross-section of Zr core levels of 80 eV photons. The broad FS mapping (Fig. 1(h)) over multiple BZs also confirmed the periodicity and dimension of the BZ along (010) surface. Figure 2 shows the overall electronic structure of pristine ZrTe5. From the 3D plot of the band structure in Fig. 2(a), we observed sharp dispersions along both kx and ky directions. The constantenergy-contours evolve from a point-like pocket at the centre of BZ at EF to a rich texture (consisted of inter-connected rhombuses) at high binding energies (Fig. 2(b), (c)). Figures 2(d)-(g) present the band dispersions along high symmetry directions. Along both 𝛤̅ 𝑌̅ and 𝛤̅ 𝑋̅ directions, the general shape of the band dispersions at broad energy is in good general agreement with previous ab initio calculation[26]. The slightly blurry and continuous spectral weight distribution near the tip of the valance band top could be due to the kz broadening effect[44] as we will discuss below in more ̅ directions with dispersion details. The dispersions are slightly anisotropic along 𝛤̅ 𝑋̅,𝛤̅ 𝑌̅ and 𝛤̅ 𝑀 slopes of about 6.6 eV·Å, 5.4 eV·Å and 5.9 eV·Å, as extracted from Figs. 2(d)-(f), respectively.

The overall band dispersions show strong anisotropic along kx and ky directions. There is an additional band besides the main band with band top near 0.5 eV along𝛤̅ 𝑋̅, which smoothly evolves ̅ 𝑌̅ (Fig. 2(g)). towards the BZ boundary and forms an M-like feature along 𝑀 As can be clearly seen in Fig. 2, the pristine sample is slightly p-doped and the size of the band gap (if there is any) cannot be observed. In order to investigate the evolution from the bulk valance and conduction band and whether there is a bulk gap, we need to tune up the EF. We first achieved this by bulk doping in ZrTe5 with Ta. In Fig. 3, we present the band structure of Ta-doped ZrTe5 with a nominal composition of Ta0.12Zr0.88Te5. Clearly, the EF is successfully shifted up (by ~120 meV), which can be seen by the comparison between the constant energy contour plots at different binding energies (Fig. 3(b), (c)), ̅ (Fig. 3(g)) the dispersion around the 𝛤̅ point (Fig. 3(a), (d)-(f)), and the M-like feature along 𝑌̅𝑀 with their counterparts in Fig. 2. With the up-shift of EF, the FS of the conduction band is now visible (see Figs. 3(a)-(c)), which forms networks of rhombuses evolved from the point like FS near the top of the valance band (see Fig. 2(c) (i) and Fig. 3(c) (ii)). The lift of the EF in Ta0.12Zr0.88Te5 also allows us to study the band structure between the narrow energy region between the valance and the conduction bands, which does not reveal a bulk band gap of hundreds of meVs, as will be discussed below. We first zoom into the energy region between EB = 0~150 meV, we can clearly see the band dispersion that forms continuous evolving band contour evolution (Fig. 3(h)), and their relatively blurry spectra indicating the kz dispersion (i.e. of the bulk nature), which motivated us to carry out the photon energy dependent ARPES measurement to investigate the kz dependent band structures. As can be seen in Fig. 4, the photon energy dependent ARPES measurements clearly show the periodicity of the ARPES spectra along the kz direction (Figs. 4(a)-(c)), despite the relatively weak

inter-layer coupling in bulk ZrTe5. The clear kz dispersion thus helped us determine the locations of  and  (indicated by the red and black dashed lines respectively in Figs. 4(a)-(c)). We can then perform the measurements at different kz locations with ease. As two examples, the dispersions cutting through the andpointsare shown in Figs. 4(d), (e), respectively. In Fig. 4(d), the valence band clearly disperses crossing EF. On the contrary, near the  point, there is only reminiscent spectra intensity below EF (Figs. 4(e)), leaving an “apparent” energy gap of about 150 meV (the spectral weight between EF and the valance band top in Fig. 4(e) is likely due to the kz broadening effect [44]). The clear dispersion along kz and the apparent gap can be observed at different kz other than the  point (e.g. in Fig. 4(e)), thus naturally explained the apparent gap observed in previous laser-ARPES results [32] and demonstrated the importance of the photon energy (i.e. kz) dependent ARPES measurements in the full understanding of the complete band structure of ZrTe5. In addition to the bulk doping, we can further tune up EF by in situ doping alkali metals on the sample surface, as can be seen in Fig. 5. The bulk bands further shift towards high binding energies and the band top near the  point shows up after surface dosing for 220 s (Fig. 5(c)). Noticeably, the dispersion near the band top resembles a Dirac-cone like structure (Fig. 5(c)), indicating the gapless nature of ZrTe5. We noticed that previous ARPES measurement revealed a large energy gap (~100 meV) near the  point, in contrast to our observation here. However, previous measurement was conducted at 200 K, which may be affected by the temperature considering the strong temperature dependence of the energy gap[32, 33]. We also observe certain distribution of spectral weight near 200 meV, which might be related to the surface modification and is beyond the scope of this work. In order to further confirm the gapless behaviour of the electronic structure of ZrTe5, we also conducted STS measurements on pristine ZrTe5 in Fig. 6. Figure 6(a) shows a typical STM

topography image indicating a step edge on the flat surface of in situ cleaved ZrTe5 surface. The step height is about 0.7 nm, in consistence with the thickness of one ZrTe5 structural layer along b direction. We thus conducted the STS measurement at different location along the line (indicated by the black arrow in Fig 6(a)) that cuts through the step edge. The results are assembled in Fig. 6(b), (c). It can be clearly seen that the obtained STS spectra clearly separate into two groups depending on their proximity from the step edge – where the spectra taken away from the step edge show rather flat total DOS (i.e. dI/dV, see black curves in Fig. 6(b)) between 0-100 mV tip bias, the STS spectra near the step edge (red curves in Fig. 6(b)) show clearly higher density, indicating the existence of the edge states, in consistence with a previous work[33, 34]. We now focus back on the DOS of the STS away from the step edge that should represent the bulk electronic structures and check if it agrees with the ARPES measurements. Interestingly, although at the first sight, there seems an apparent gap of ~100 meV between the valance and conduction bands at positions far away from the step (Figs. 6(b), (c)). However, if we zoom-into the spectrum and obtain data with very high statistics, we immediately noticed that the “flat” part of the spectra in fact shows a V-shaped DOS curves (Fig. 6(d)) with non-zero DOS except at the lowest dip of the STS spectrum, which resembles the STS spectrum in the canonical TDS Na3Bi[42], and is in consistence with the integrated DOS from our ARPES measurements. Considering the clean sample surface and the independence of the STS spectrum with the measurement position away from the step, we argue that the observed V-shaped DOS is not due to the impurity states and reflects the intrinsic property of bulk ZrTe5, thus again confirmed the gapless electronic structure in ZrTe5. Note: After this manuscript was submitted, we noted a similar observation of V-shaped DOS near the EF in the bulk ZrTe5 in Ref.[45].

4. Conclusion

In conclusion, we have systematically investigated the electronic structure of ZrTe5 with systematic ARPES and STM/STS studies, both of which indicate the gapless nature of this compound and suggest ZrTe5 as a TDS (in consistence with the transport measurements[27-31]) rather than a TI (as suggested by some other previous studies[32-34]).

Acknowledgement Y. L. C. acknowledges the support of the EPSRC Platform Grant (Grant No.EP/M020517/1) and Hefei Science Center CAS (2015HSC-UE013). M. X. W. acknowledges the support by Shanghai Sailing Program (16YF1407800). C. F. acknowledges the financial support by the ERC Advanced Grant (No. 291472 “Idea Heusler”). J. J. acknowledges the support of the NRF, Korea through the SRC center for Topological Matter (No. 2011-0030787).

References [1] M.Z. Hasan, C.L. Kane, Colloquium: Topological insulators, Rev. Mod. Phys., 82 (2010) 3045-3067. [2] X.-L. Qi, S.-C. Zhang, Topological insulators and superconductors, Rev. Mod. Phys., 83 (2011) 1057-1110. [3] Z. Wang, Y. Sun, X.-Q. Chen, C. Franchini, G. Xu, H. Weng, X. Dai, Z. Fang, Dirac semimetal and topological phase transitions in A3Bi (A=Na, K, Rb), Phys. Rev. B, 85 (2012) 195320. [4] Z. Wang, H. Weng, Q. Wu, X. Dai, Z. Fang, Three-dimensional Dirac semimetal and quantum transport in Cd3As2, Phys. Rev. B, 88 (2013) 125427. [5] Z.K. Liu, B. Zhou, Y. Zhang, Z.J. Wang, H.M. Weng, D. Prabhakaran, S.K. Mo, Z.X. Shen, Z. Fang, X. Dai, Z. Hussain, Y.L. Chen, Discovery of a Three-Dimensional Topological Dirac Semimetal, Na3Bi, Science, 343 (2014) 864-867. [6] Z.K. Liu, J. Jiang, B. Zhou, Z.J. Wang, Y. Zhang, H.M. Weng, D. Prabhakaran, S.K. Mo, H. Peng, P. Dudin, T. Kim, M. Hoesch, Z. Fang, X. Dai, Z.X. Shen, D.L. Feng, Z. Hussain, Y.L. Chen, A stable three-dimensional topological Dirac semimetal Cd3As2, Nat. Mater., 13 (2014) 677-681. [7] M. Neupane, S.-Y. Xu, R. Sankar, N. Alidoust, G. Bian, C. Liu, I. Belopolski, T.-R. Chang, H.-T. Jeng, H. Lin, A. Bansil, F. Chou, M.Z. Hasan, Observation of a threedimensional topological Dirac semimetal phase in high-mobility Cd3As2, Nat. Commun., 5 (2014) 3786. [8] S.-Y. Xu, C. Liu, S.K. Kushwaha, R. Sankar, J.W. Krizan, I. Belopolski, M. Neupane, G. Bian, N. Alidoust, T.-R. Chang, H.-T. Jeng, C.-Y. Huang, W.-F. Tsai, H. Lin, P.P.

Shibayev, F.-C. Chou, R.J. Cava, M.Z. Hasan, Observation of Fermi arc surface states in a topological metal, Science, 347 (2015) 294-298. [9] X. Wan, A.M. Turner, A. Vishwanath, S.Y. Savrasov, Topological semimetal and Fermi-arc surface states in the electronic structure of pyrochlore iridates, Phys. Rev. B, 83 (2011) 205101. [10] H. Weng, C. Fang, Z. Fang, B.A. Bernevig, X. Dai, Weyl Semimetal Phase in Noncentrosymmetric Transition-Metal Monophosphides, Phys. Rev. X, 5 (2015) 011029. [11] S.-M. Huang, S.-Y. Xu, I. Belopolski, C.-C. Lee, G. Chang, B. Wang, N. Alidoust, G. Bian, M. Neupane, C. Zhang, S. Jia, A. Bansil, H. Lin, M.Z. Hasan, A Weyl Fermion semimetal with surface Fermi arcs in the transition metal monopnictide TaAs class, Nat. Commun., 6 (2015) 7373. [12] S.-Y. Xu, I. Belopolski, N. Alidoust, M. Neupane, G. Bian, C. Zhang, R. Sankar, G. Chang, Z. Yuan, C.-C. Lee, S.-M. Huang, H. Zheng, J. Ma, D.S. Sanchez, B. Wang, A. Bansil, F. Chou, P.P. Shibayev, H. Lin, S. Jia, M.Z. Hasan, Discovery of a Weyl fermion semimetal and topological Fermi arcs, Science, 349 (2015) 613-617. [13] B.Q. Lv, H.M. Weng, B.B. Fu, X.P. Wang, H. Miao, J. Ma, P. Richard, X.C. Huang, L.X. Zhao, G.F. Chen, Z. Fang, X. Dai, T. Qian, H. Ding, Experimental Discovery of Weyl Semimetal TaAs, Phys. Rev. X, 5 (2015) 031013. [14] L.X. Yang, Z.K. Liu, Y. Sun, H. Peng, H.F. Yang, T. Zhang, B. Zhou, Y. Zhang, Y.F. Guo, M. Rahn, D. Prabhakaran, Z. Hussain, S.K. Mo, C. Felser, B. Yan, Y.L. Chen, Weyl semimetal phase in the non-centrosymmetric compound TaAs, Nat. Phys., 11 (2015) 728732.

[15] Z.K. Liu, L.X. Yang, Y. Sun, T. Zhang, H. Peng, H.F. Yang, C. Chen, Y. Zhang, Y.F. Guo, D. Prabhakaran, M. Schmidt, Z. Hussain, S.K. Mo, C. Felser, B. Yan, Y.L. Chen, Evolution of the Fermi surface of Weyl semimetals in the transition metal pnictide family, Nat. Mater., 15 (2016) 27-31. [16] Y. Kim, B.J. Wieder, C.L. Kane, A.M. Rappe, Dirac Line Nodes in InversionSymmetric Crystals, Phys. Rev. Lett., 115 (2015) 036806. [17] H. Weng, Y. Liang, Q. Xu, R. Yu, Z. Fang, X. Dai, Y. Kawazoe, Topological nodeline semimetal in three-dimensional graphene networks, Phys. Rev. B, 92 (2015) 045108. [18] M. Neupane, I. Belopolski, M.M. Hosen, D.S. Sanchez, R. Sankar, M. Szlawska, S.Y. Xu, K. Dimitri, N. Dhakal, P. Maldonado, P.M. Oppeneer, D. Kaczorowski, F. Chou, M.Z. Hasan, T. Durakiewicz, Observation of topological nodal fermion semimetal phase in ZrSiS, Phys. Rev. B, 93 (2016) 201104. [19] A.A. Zyuzin, A.A. Burkov, Topological response in Weyl semimetals and the chiral anomaly, Phys. Rev. B, 86 (2012) 115133. [20] D.T. Son, B.Z. Spivak, Chiral anomaly and classical negative magnetoresistance of Weyl metals, Phys. Rev. B, 88 (2013) 104412. [21] X. Huang, L. Zhao, Y. Long, P. Wang, D. Chen, Z. Yang, H. Liang, M. Xue, H. Weng, Z. Fang, X. Dai, G. Chen, Observation of the Chiral-Anomaly-Induced Negative Magnetoresistance in 3D Weyl Semimetal TaAs, Phys. Rev. X, 5 (2015) 031023. [22] J. Xiong, S.K. Kushwaha, T. Liang, J.W. Krizan, M. Hirschberger, W. Wang, R.J. Cava, N.P. Ong, Evidence for the chiral anomaly in the Dirac semimetal Na3Bi, Science, 350 (2015) 413-416.

[23] F. Arnold, C. Shekhar, S.-C. Wu, Y. Sun, R.D. dos Reis, N. Kumar, M. Naumann, M.O. Ajeesh, M. Schmidt, A.G. Grushin, J.H. Bardarson, M. Baenitz, D. Sokolov, H. Borrmann, M. Nicklas, C. Felser, E. Hassinger, B. Yan, Negative magnetoresistance without well-defined chirality in the Weyl semimetal TaP, Nat. Commun., 7 (2016). [24] A.C. Potter, I. Kimchi, A. Vishwanath, Quantum oscillations from surface Fermi arcs in Weyl and Dirac semimetals, Nat. Commun., 5 (2014) 5161. [25] C. Shekhar, V. Süss, M. Schmidt, Mobility induced unsaturated high linear magnetoresistance

in

transition-metal

monopnictides

Weyl

semimetals,

arXiv:1606.06649v1, (2016). [26] H. Weng, X. Dai, Z. Fang, Transition-Metal Pentatelluride ZrTe5 and HfTe5: A Paradigm for Large-Gap Quantum Spin Hall Insulators, Phys. Rev. X, 4 (2014) 011002. [27] R.Y. Chen, S.J. Zhang, J.A. Schneeloch, C. Zhang, Q. Li, G.D. Gu, N.L. Wang, Optical spectroscopy study of the three-dimensional Dirac semimetal ZrTe5, Phys. Rev. B, 92 (2015) 075107. [28] R.Y. Chen, Z.G. Chen, X.Y. Song, J.A. Schneeloch, G.D. Gu, F. Wang, N.L. Wang, Magnetoinfrared Spectroscopy of Landau Levels and Zeeman Splitting of ThreeDimensional Massless Dirac Fermions in ZrTe5, Phys. Rev. Lett., 115 (2015) 176404. [29] X. Yuan, C. Zhang, Y. Liu, C. Song, S. Shen, X. Sui, J. Xu, H. Yu, Z. An, J. Zhao, H. Yan, F. Xiu, Observation of quasi-two-dimensional Dirac fermions in ZrTe5, arXiv:1510.00907v1, (2015). [30] G. Zheng, J. Lu, X. Zhu, W. Ning, Y. Han, H. Zhang, J. Zhang, C. Xi, J. Yang, H. Du, K. Yang, Y. Zhang, M. Tian, Transport evidence for the three-dimensional Dirac semimetal phase in ZrTe5, Phys. Rev. B, 93 (2016) 115414.

[31] Q. Li, D.E. Kharzeev, C. Zhang, Y. Huang, I. Pletikosic, A.V. Fedorov, R.D. Zhong, J.A. Schneeloch, G.D. Gu, T. Valla, Chiral magnetic effect in ZrTe5, Nat. Phys., 12 (2016) 550-554. [32] Y. Zhang, C. Wang, L. Yu, G. Liu, A. Liang, J. Huang, S. Nie, Y. Zhang, B. Shen, J. Liu, H. Weng, L. Zhao, G. Chen, X. Jia, C. Hu, Y. Ding, S. He, L. Zhao, F. Zhang, S. Zhang, F. Yang, Z. Wang, Q. Peng, X. Dai, Z. Fang, Z. Xu, C. Chen, X.J. Zhou, Electronic Evidence of Temperature-Induced Lifshitz Transition and Topological Nature in ZrTe5, arXiv:1602.03576v1, (2016). [33] R. Wu, J.Z. Ma, S.M. Nie, L.X. Zhao, X. Huang, J.X. Yin, B.B. Fu, P. Richard, G.F. Chen, Z. Fang, X. Dai, H.M. Weng, T. Qian, H. Ding, S.H. Pan, Evidence for Topological Edge States in a Large Energy Gap near the Step Edges on the Surface of ZrTe5, Phys. Rev. X, 6 (2016) 021017. [34] X.-B. Li, W.-K. Huang, Y.-Y. Lv, K.-W. Zhang, C.-L. Yang, B.-B. Zhang, Y.B. Chen, S.-H. Yao, J. Zhou, M.-H. Lu, L. Sheng, S.-C. Li, J.-F. Jia, Q.-K. Xue, Y.-F. Chen, D.-Y. Xing, Experimental Observation of Topological Edge States at the Surface Step Edge of the Topological Insulator ZrTe5, Phys. Rev. Lett., 116 (2016) 176803. [35] D.N. McIlroy, S. Moore, Z. Daqing, J. Wharton, B. Kempton, R. Littleton, M. Wilson, T.M. Tritt, C.G. Olson, Observation of a semimetal–semiconductor phase transition in the intermetallic ZrTe 5, Journal of Physics: Condensed Matter, 16 (2004) L359. [36] A. Sterzi, A. Crepaldi, M. Diego, F. Cilento, M. Zacchigna, P. Bugnon, H. Berger, A. Magrez, M. Grioni, F. Parmigiani, G. Manzoni, Ultrafast Optical Control of the Electronic Properties of ZrTe5, Phys. Rev. Lett., 115 (2015) 207402.

[37] L. Moreschini, J.C. Johannsen, H. Berger, J. Denlinger, C. Jozwiack, E. Rotenberg, K.S. Kim, A. Bostwick, M. Grioni, Nature and topology of the low-energy states in ZrTe5, Phys. Rev. B, 94 (2016) 081101. [38] Y.L. Chen, J.G. Analytis, J.H. Chu, Z.K. Liu, S.K. Mo, X.L. Qi, H.J. Zhang, D.H. Lu, X. Dai, Z. Fang, S.C. Zhang, I.R. Fisher, Z. Hussain, Z.X. Shen, Experimental Realization of a Three-Dimensional Topological Insulator, Bi2Te3, Science, 325 (2009) 178-181. [39] D. Hsieh, D. Qian, L. Wray, Y. Xia, Y.S. Hor, R.J. Cava, M.Z. Hasan, A topological Dirac insulator in a quantum spin Hall phase, Nature, 452 (2008) 970-974. [40] Y.L. Chen, Z.K. Liu, J.G. Analytis, J.H. Chu, H.J. Zhang, B.H. Yan, S.K. Mo, R.G. Moore, D.H. Lu, I.R. Fisher, S.C. Zhang, Z. Hussain, Z.X. Shen, Single Dirac Cone Topological Surface State and Unusual Thermoelectric Property of Compounds from a New Topological Insulator Family, Phys. Rev. Lett., 105 (2010) 266401. [41] Z.K. Liu, Y.L. Chen, J.G. Analytis, S.K. Mo, D.H. Lu, R.G. Moore, I.R. Fisher, Z. Hussain, Z.X. Shen, Robust topological surface state against direct surface contamination, Physica. E. Low. Dimens. Syst. Nanostruct., 44 (2012) 891-894. [42] J. Hellerstedt, M.T. Edmonds, N. Ramakrishnan, C. Liu, B. Weber, A. Tadich, K.M. O’Donnell, S. Adam, M.S. Fuhrer, Electronic Properties of High-Quality Epitaxial Topological Dirac Semimetal Thin Films, Nano Lett., 16 (2016) 3210-3214. [43] S. Furuseth, L. Brattas, A. Kjekshus, The Crystal Structure of HfTe5, Acta. Chem. Scand, 27 (1973) 2367-2374. [44] Y. Chen, Studies on the electronic structures of three-dimensional topological insulators by angle resolved photoemission spectroscopy, Front. Phys., 7 (2012) 175-192.

[45] G. Manzoni, L. Gragnaniello, G. Autès, T. Kuhn, A. Sterzi, F. Cilento, M. Zacchigna, V. Enenkel, I. Vobornik, L. Barba, F. Bisti, P. Bugnon, A. Magrez, V. N.Strocov, H. Berger, O.V. Yazyev, M. Fonin, F. Parmigiani, A. Crepaldi, Evidence for a Strong Topological Insulator Phase in ZrTe5, arXiv:1608.03433v1, (2016).

Fig. 1. Basic characteristics of ZrTe5. (a)-(c) Perspective view (a), side view (b) and top view (c) of the crystal structure of ZrTe5. (d) The bulk and surface projected (blue shaded rectangle) Brillouin zone (BZ) of ZrTe5. (e) Large-area STM topography image obtained on the sample surface after in situ cleavage (bias voltage = -2.5 V, Tunneling current = 100 pA ). (f) Atomicresolution STM image on the sample surface. The inset shows the consistency between STM image and the top-layer crystal structure. (bias voltage = -0.3V, Tunneling current = 300pA). (g) Core level photoemission spectrum on ZrTe5 showing characteristic Te 4d peaks. (h) FS mapping over multiple BZs obtained by integrating ARPES intensity in an energy window of 20 meV around EF.

Fig. 2. General electronic structure of pristine ZrTe5. (a) 3D illustration of the band structure with a linear dispersion near the center of the surface BZ. (b) Stacking plot of constant-energy contours showing the evolution of the band structure with binding energy. (c) Separated plots of constantenergy contours at selected binding energies. (d)-(f) Band dispersions along high-symmetry directions as marked on the top of the panels. Data were collected using 75 eV photons with linear horizontal polarization.

Fig. 3. General electronic structure of Ta-doped ZrTe5 with nominal composition of Ta0.12Zr0.88Te5. (a) 3D illustration of the band structure with a linear dispersion near the center of the surface BZ. (b) Stacking plot of constant-energy contours showing the evolution of the band structure with binding energy in a large energy range. (c) Separated plots of constant-energy contours at selected binding energies. (d)-(g) Band dispersions along high-symmetry directions as marked on the top of the panels. (h) Stacking plot of constant-energy contours showing the evolution of the band structure within a small binding energy range near EF. Data were collected using 75 eV photons with linear horizontal polarization.

Fig. 4. Photon energy dependent ARPES measurements on Ta0.12Zr0.88Te5 in a large energy range. (a)-(c) Constant-energy contours on kx-kz plane at (a) EF, (b) 200 meV, and (c) 450 meV. (d) (i) ARPES intensity image spectrum, (ii)the corresponding momentum distribution curves, and (iii) the energy distribution curve near the point. (e) The same as (d) but near the Z point. Data were collected with linear horizontally polarized photons.

Fig. 5. Evolution of the electronic structure of Ta0.12Zr0.88Te5 with surface K dosing ((a)-(c)) measured at photon energy of 55 eV (near the  point) at 10 K.

Fig. 6. STM/STS measurements on the pristine ZrTe5. (a) STM topography image with a step (bias voltage = -1.6 V, tunneling current = 100 pA). (b), (c) The dI/dV curves collected along the solid black arrow in panel (a). (d) A typical dI/dV curve far away from the step showing V-shape total density of state around EF.