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Robust Edge Photocurrent Response on Layered Type II Weyl Semimetal Wte2

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Robust Edge Photocurrent Response on Layered Type II Weyl Semimetal Wte2 ARTICLE https://doi.org/10.1038/s41467-019-13713-1 OPEN Robust edge photocurrent response on layered type II Weyl semimetal WTe2 Qinsheng Wang 1,2, Jingchuan Zheng1,2, Yuan He1,2, Jin Cao1, Xin Liu3, Maoyuan Wang1, Junchao Ma3, Jiawei Lai 3, Hong Lu3, Shuang Jia3,4, Dayu Yan5, Youguo Shi5, Junxi Duan1,2, Junfeng Han1,2, Wende Xiao 1,2, Jian-Hao Chen 3,4, Kai Sun 6, Yugui Yao1,2* & Dong Sun 3,4* Photosensing and energy harvesting based on exotic properties of quantum materials and new operation principles have great potential to break the fundamental performance limit of conventional photodetectors and solar cells. Weyl semimetals have demonstrated novel optoelectronic properties that promise potential applications in photodetection and energy harvesting arising from their gapless linear dispersion and Berry field enhanced nonlinear optical effect at the vicinity of Weyl nodes. In this work, we demonstrate robust photocurrent generation at the edge of Td-WTe2, a type-II Weyl semimetal, due to crystalline-symmetry breaking along certain crystal fracture directions and possibly enhanced by robust fermi-arc type surface states. This edge response is highly generic and arises universally in a wide class of quantum materials with similar crystal symmetries. The robust and generic edge current response provides a charge separation mechanism for photosensing and energy harvesting over broad wavelength range. 1 Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement (MOE), School of Physics, Beijing Institute of Technology, Beijing, China. 2 Micronano Centre, Beijing Key Lab of Nanophotonics & Ultrafine Optoelectronic Systems, Beijing Institute of Technology, Beijing, China. 3 International Center for Quantum Materials, School of Physics, Peking University, Beijing, China. 4 Collaborative Innovation Center of Quantum Matter, Beijing, China. 5 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, China. 6 Department of Physics, University of Michigan, Ann Arbor, MI 48109-1040, USA. *email: [email protected]; [email protected] NATURE COMMUNICATIONS | (2019) 10:5736 | https://doi.org/10.1038/s41467-019-13713-1 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-13713-1 fficient separation of photo-excited electron–hole pairs is an Results fi T essential ingredient for various highly applicable elds such Edge photocurrent response of WTe2. d-WTe2 with crystal E 1 2 as photo sensing , solar energy harvesting and photo cat- structure shown in Fig. 1a is proposed as an inversion-symmetry alysis3. For this purpose, imbalanced momentum distribution of breaking type-II Weyl semimetal27. It belongs to space group of non-equilibrium photo-excited charge carriers, either from Pmn21(No. 31) and point group C2v, which consists a twofold C population imbalance or velocity imbalance in k-space, is rotation symmetry with screw rotation axis 2 along the crys- ^ required. Technically, this can be achieved by breaking inversion tallographic c-axis, and mirror symmetry with mirror plane (Ma) symmetry either internally or externally in devices. In conven- perpendicular to the crystallographic ^a-axis and glide mirror tional semiconductors with inversion symmetry, such as silicon, symmetry with mirror plane (Mb) perpendicular to the crystal- ^ GaAs and graphene, applying a bias voltage over the electrodes lographic b-axis. In the first Brillion zone, WTe2 has 8 Weyl k = externally is a straightforward approach to break the inversion nodes laying on c 0 surface (see below). These Weyl nodes symmetry of the device. Advanced approaches of inversion come in two quartets located 0.052 eV and 0.058 eV above symmetry breaking through external ways can also be achieved by the Fermi level, all of which are type-II Weyl nodes that exist at introducing a built-in electric field in a PN junction4, Schottky the boundaries between electron and hole pockets27. Due to the 5 junction or at interface with other materials, or domains pro- inversion symmetry breaking, WTe2 demonstrates large second cessing different Seebeck coefficients through photo thermo- order photocurrent response in Weyl semimetals with oblique electric effect (PTE)6. Although the above schemes have been light excitation25, but for normal incident light excitation with ^ successfully demonstrated for photo-excited charge separation, electric fields lying in the ^a À b plane, the planar components of a there are fundamental limitations on device fabrication and C T = rank-3 tensor with the 2v symmetries are all null: αβγ 0, due specific applicable circumstances. For example, applying an ^ to the existence of in-plane inversion symmetry in the ^a À b external bias on detectors based on semi-metallic materials such as Dirac/Weyl semimetals is not feasible because the external bias plane. fi In a typical scanning photocurrent measurement (SPCM) can induce a signi cant current, termed as the dark current, in fi fi conducting materials even in the absence of light1,7,8. PN junc- shown in Fig. 1b, 633-nm pulse ltered from a ber white light super-continuum source is used to excite a 6-nm thick two- tions can efficiently separate photoexcited electron–hole pairs, but its application is limited by the requirement of doping, which can terminal WTe2 device under un-biased condition at room temperature. According to the scanning photocurrent response be very challenging for certain materials. On the other hand, the inversion symmetry breaking can shown in Fig. 1c, d, the photocurrent is mainly generated either along the interface of WTe2-metal contact or along the edge of naturally occur in some classes of crystals, which could lead to fl efficient charge separation of photo-excited carriers and result in the WTe2 akes. The response at the WTe2-contact interface is 9 expected, which has been well understood from previously studies shift current response . This phenomenon, which is often referred 26,28 to as bulk photovoltaic effect (BPVE) or anomalous photovoltaic on similar devices based on other 2D layered semimetals . The 10,11 charge separation is induced by the interplay of built-in electric effect , is due to the fact that the real space position of electron fi shifts a little as the expected electron positions of the ground state eld and PTE effect at the interface in absence of external voltage bias. When the excitation is away from the contact area, the major and the excited state are slightly different after photoexcitation fi under homogenous photo illumination in non-centrosymmetric charge separation mechanisms (e.g., PTE, built-in electric eld, photo Dember) all vanish, thus the PC response should be crystal. Previously, this effect is mainly demonstrated in semi- negligible because all the planar components of rank-3 tensor are conductors with non-centrosymmetric structure such as zero under normal incident light excitation. This is consistent LiNbO 12 and BaTiO 13. Nevertheless, most of these crystals have 3 3 with the experimental observation that no photocurrent is large bandgaps, which limit their applications in low-energy observed over the region away from the metal contact (Fig. 1d). photo detection and energy harvesting. Weyl semimetals with However, we have observed clear photocurrent response of 27 μA inversion symmetry breaking provide a promising class of − – W 1 at the edge of the device which is totally unexpected. Such material candidates via this route14 17. The gapless linear- robust edge photocurrent response, to the best of our knowledge, dispersed band structure of Weyl semimetal allows broadband has never been observed without applying an external magnetic wavelength operation extendable to far-infrared or even Ter- field in non-chiral materials29,30. ahertz range18,19. In addition, the monopole-type of Berry cur- vature around the Weyl nodes can enhance the nonlinear optical effects at the vicinity of Weyl nodes20–23, which leads to a large photocurrent response over a broad wavelength range24. How- Symmetry analysis. To further investigate the edge current response of Td-WTe2, eight devices are fabricated with edges ever, the shift current response in inversion-symmetry-breaking ^ Weyl semimetals is canceled under the light incident geometry fracturing along different crystal orientations within ^a À b plane. that the surface perpendicular to the incident light still preserves The SPCM results of three representative devices are shown in the in-plane inversion symmetry. For example, in well-established Fig. 2. The in-plane fracture is random during the mechanical C Type-II Weyl semimetals with 2v crystal point group, such as exfoliation process, but for WTe2, it has the tendency to fracture WTe , MoTe , and TaIrTe 25,26, the second order nonlinear shift along certain in-plane crystallography orientations31. The most 2 2 4 ^ current response vanishes if light incidence direction is along the preferable fracture direction within ^a À b plane is along the tung- crystallographic ^c-axis25. sten chain direction (<100>). Experimentally, the crystal orientation In this work, we experimentally show that the photo excited of the edge can be identified by polarization resolved Raman electron–hole pairs can be separated efficiently along certain spectroscopy, the ratio of Raman peaks at 164 and 212 cm−1 C edges due to the broken of 2v symmetry upon fracturing along reaches maximum when the laser polarization is parallel to the certain crystallographic directions, which provides robust edge <100> direction as shown in Supplementary note 132. current response in Type-II Weyl semimetal WTe2 under normal An obvious feature from Fig. 2 is that not all edges have incidence. Interestingly, although the survival of the photocurrent photocurrent responses, photocurrent generation only happens response is solely determined by the edge fracture direction from on edges along certain crystal orientations. Even within one a pure crystal symmetry consideration, the observed responsivity device, some edges exhibit photo responses while the other edges possibly relates to the fermi-arc type surface states.
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