## Abstract

A Weyl semimetal is a crystal which hosts Weyl fermions as emergent quasiparticles and admits a topological classification that protects Fermi arc surface states on the boundary of a bulk sample. This unusual electronic structure has deep analogies with particle physics and leads to unique topological properties. We report the experimental discovery of a Weyl semimetal, TaAs. Using photoemission spectroscopy, we directly observe Fermi arcs on the surface, as well as the Weyl fermion cones and Weyl nodes in the bulk of TaAs single crystals. We find that Fermi arcs terminate on the Weyl nodes, consistent with their topological character. Our work opens the field for the experimental study of Weyl fermions in physics and materials science.

Weyl fermions have long been known in quantum field theory, but have not been observed as a fundamental particle in nature (*1*–*3*). Recently, it was understood that a Weyl fermion can emerge as a quasiparticle in certain crystals, Weyl fermion semimetals (*1*–*22*). Despite being a gapless metal, a Weyl semimetal is characterized by topological invariants, broadening the classification of topological phases of matter beyond insulators. Specifically, Weyl fermions at zero energy correspond to points of bulk band degeneracy, Weyl nodes, which are associated with a chiral charge that protects gapless surface states on the boundary of a bulk sample. These surface states take the form of Fermi arcs connecting the projection of bulk Weyl nodes in the surface Brillouin zone (BZ) (*6*). A band structure like the Fermi arc surface states would violate basic band theory in an isolated two-dimensional system and can only arise on the boundary of a three-dimensional sample, providing a dramatic example of the bulk-boundary correspondence in a topological phase. In contrast to topological insulators where only the surface states are interesting (*21*, *22*), a Weyl semimetal features unusual band structure in the bulk and on the surface. The Weyl fermions in the bulk are predicted to provide a condensed matter realization of the chiral anomaly, giving rise to a negative magnetoresistance under parallel electric and magnetic fields, unusual optical conductivity, non-local transport and local non-conservation of ordinary current (*5*, *12*–*16*). At the same time, the Fermi arc surface states are predicted to show unconventional quantum oscillations in magneto-transport, as well as unusual quantum interference effects in tunneling spectroscopy (*17*–*19*). The prospect of the realization of these phenomena has inspired much experimental and theoretical work. (*1*–*22*).

Here we report the experimental realization of a Weyl semimetal in a single crystalline material tantalum arsenide, TaAs. Utilizing the combination of the vacuum ultraviolet (low-photon-energy) and soft X-ray (SX) angle-resolved photoemission spectroscopy (ARPES), we systematically and differentially study the surface and bulk electronic structure of TaAs. Our ultraviolet (low-photon-energy) ARPES measurements, which are highly surface sensitive, demonstrate the existence of the Fermi arc surface states, consistent with our band calculations presented here. Moreover, our SX-ARPES measurements, which are reasonably bulk sensitive, reveal the three-dimensional linearly dispersive bulk Weyl cones and Weyl nodes. Furthermore, by combining the low-photon-energy and SX-ARPES data, we show that the locations of the projected bulk Weyl nodes correspond to the terminations of the Fermi arcs within our experimental resolution. These systematic measurements demonstrate TaAs as a Weyl semimetal.

## The material system and theoretical considerations

Tantalum arsenide, TaAs, is a semimetallic material that crystalizes in a body-centered tetragonal lattice system (Fig. 1A) (*23*). The lattice constants are and , and the space group is (, ), as consistently reported in previous structural studies (*23*–*25*). The crystal consists of interpenetrating Ta and As sub-lattices, where the two sub-lattices are shifted by , . Our diffraction data matches well with the lattice parameters and the space group (*26*). The scanning tunneling microscopic (STM) topography (Fig. 1B) clearly resolves the (001) square lattice without any obvious defect. From the topography, we obtain a lattice constant . Electrical transport measurements on TaAs confirmed its semimetallic transport properties and reported negative magnetoresistance suggesting the anomalies due to Weyl fermions (*23*).

We discuss the essential aspects of the theoretically calculated bulk band structure (*9*, *10*) that predicts TaAs as a Weyl semimetal candidate. Without spin-orbit coupling, calculations (*9*, *10*) show that the conduction and valence bands interpenetrate (dip into) each other to form four 1D line nodes (closed loops) located on the and planes (shaded blue in Figs. 1, C and E). Upon the inclusion of spin-orbit coupling, each line node loop is gapped out and shrinks into six Weyl nodes that are away from the and mirror planes (Fig. 1E, small filled circles). In our calculation, in total there are 24 bulk Weyl cones (*9*, *10*), all of which are linearly dispersive and are associated with a single chiral charge of (Fig. 1E). We denote the 8 Weyl nodes that are located on the brown plane () as W1 and the other 16 nodes that are away from this plane as W2. At the (001) surface BZ (Fig. 1F), the 8 W1 Weyl nodes are projected in the vicinity of the surface BZ edges, and . More interestingly, pairs of W2 Weyl nodes with the same chiral charge are projected onto the same point on the surface BZ. Therefore, in total there are 8 projected W2 Weyl nodes with a projected chiral charge of , which are located near the midpoints of the and the lines. Because the chiral charge is a projected value, the Weyl cone is still linear (*9*). The number of Fermi arcs terminating on a projected Weyl node must equal its projected chiral charge. Therefore, in TaAs, two Fermi arc surface states must terminate on each projected W2 Weyl node.

## Surface electronic structure of TaAs

We carried out low-photon-energy ARPES measurements to explore surface electronic structure of TaAs. Figure 1H presents an overview of the (001) Fermi surface map. We observe three types of dominant features, namely a crescent-shaped feature in the vicinity of the midpoint of each or line, a bowtie-like feature centered at the point, and an extended feature centered at the point. We find that the Fermi surface and the constant energy contours at shallow binding energies (Fig. 2A) violate the symmetry, considering the features at and points. In the crystal structure of TaAs, where the rotational symmetry is implemented as a screw axis that sends the crystal back into itself after a rotation and a translation by along the rotation axis, such an asymmetry is expected in calculation. The crystallinity of (001) surface in fact breaks the rotational symmetry. We now focus on the crescent-shaped features. Their peculiar shape suggests the existence of two arcs and their termination points in -space seem to coincide with the surface projection of the W2 Weyl nodes. Because the crescent feature consists of two non-closed curves, it can either arise from two Fermi arcs or a closed contour, however, the decisive property that clearly distinguishes one case from the other is the way in which the constant energy contour evolves as a function of energy. As shown in Fig. 2F, in order for the crescent feature to be Fermi arcs, the two non-closed curves have to move (disperse) in the same direction as one varies the energy (*26*). We now provide ARPES data to show that the crescent features in TaAs indeed exhibit this “co-propagating” property. To do so, we single out a crescent feature as shown in Figs. 2, B and E and show the band dispersions at representative momentum space cuts, Cut I and Cut II, as defined in Fig. 2E. The corresponding dispersions are shown in Figs. 2, C and D. The evolution (dispersive “movement”) of the bands as a function of binding energy can be clearly read from the slope of the bands in the dispersion maps, and is indicated in Fig. 2E by the white arrows. It can be seen that the evolution of the two non-closed curves are consistent with the co-propagating property. In order to further visualize the evolution of the constant energy contour throughout space, we use surface state constant energy contours at two slightly different binding energies, namely and . Figure 2G shows the difference between these two constant energy contours, namely , where is the ARPES intensity. The -space regions in Fig. 2G that have negative spectral weight (red) correspond to the constant energy contour at , whereas those regions with positive spectral weight (blue) corresponds to the contour at . Thus one can visualize the two contours in a single map. The alternating “red - blue - red - blue” sequence for each crescent feature in Fig. 2G shows the co-propagating property, consistent with Fig. 2F. Furthermore, we note that there are two crescent features, one located near the axis and the other near the axis, in Fig. 2G. The fact that we observe the co-propagating property for two independent crescent features which are rotated with respect to each other further shows that this observation is not due to artifacts, such as a misalignment while performing the subtraction. The above systematic data reveal the existence of Fermi arcs on the (001) surface of TaAs. Just like one can identify a crystal as a topological insulator by observing an odd number of Dirac cone surface states, we emphasize that our data here are sufficient to identify TaAs as a Weyl semimetal because of bulk-boundary correspondence in topology.

Theoretically, the co-propagating property of the Fermi arcs is unique to Weyl semimetals because it arises from the nonzero chiral charge of the projected bulk Weyl nodes (*26*), which in this case is . Therefore, this property distinguishes the crescent Fermi arcs not only from any closed contour but also from the double Fermi arcs in Dirac semimetals (*27*, *28*) because the bulk Dirac nodes do not carry any net chiral charges (*26*). After observing the surface electronic structure containing Fermi arcs in our ARPES data, we are able to slightly tune the free parameters of our surface calculation and obtain a calculated surface Fermi surface that reproduces and explains our ARPES data (Fig. 1G). This serves as an important cross-check that our data and interpretation are self consistent. Specifically, our surface calculation indeed also reveals the crescent Fermi arcs that connect the projected W2 Weyl nodes near the midpoints of each or line (Fig. 1G). In addition, our calculation shows the bowtie surface states centered at the point, also consistent with our ARPES data. According to our calculation, these bowtie surface states are in fact Fermi arcs (*26*) associated with the W1 Weyl nodes near the BZ boundaries. However, our ARPES data cannot resolve the arc character since the W1 Weyl nodes are too close to each other in momentum space compared to the experimental resolution. Additionally, we note that the agreement between the ARPES data and the surface calculation upon the contour at the point can be further improved by fine-optimizing the surface parameters. In order to establish the topology, it is not necessary for the data to have a perfect correspondence with the details of calculation because some changes in the choice of the surface potential allowed by the free parameters do not change the topology of the materials, as is the case in topological insulators (*21*, *22*). In principle, Fermi arcs can coexist with additional closed contours in a Weyl semimetal (*6*, *9*), just as Dirac cones can coexist with additional trivial surface states in a topological insulator (*21*, *22*). Particularly, establishing one set of Weyl Fermi arcs is sufficient to prove a Weyl semimetal (*6*). This is achieved by observing the crescent Fermi arcs as we show here by our ARPES data in Fig. 2, which is further consistent with our surface calculations.

## Bulk measurements

We now present bulk-sensitive SX-ARPES (*29*) data, which reveal the existence of bulk Weyl cones and Weyl nodes. This serves as an independent proof of the Weyl semimetal state in TaAs. Figure 3B shows the SX-ARPES measured Fermi surface at (note that none of the Weyl nodes are located on the plane). We emphasize that the clear dispersion along the direction (Fig. 3B) firmly shows that our SX-ARPES predominantly images the bulk bands. SX-ARPES boosts the bulk-surface contrast in favor of the bulk band structure, which can be further tested by measuring the band dispersion along the axis in the SX-ARPES setting. This is confirmed by the agreement between the ARPES data (Fig. 3B) and the corresponding bulk band calculation (Fig. 3A). We now choose an incident photon energy (i.e., a value) that corresponds to the -space location of W2 Weyl nodes and map the corresponding ** Fermi surface. As shown in Fig. 3C, the Fermi points that are located away from the or axes are the W2 Weyl nodes. In Fig. 3D, we clearly observe two linearly dispersive cones that correspond to the two nearby W2 Weyl nodes along Cut 1. The -space separation between the two W2 Weyl nodes is measured to be , which is consistent with both the bulk calculation and the separation of the two terminations of the crescent Fermi arcs measured in Fig. 2. The linear dispersion along the out-of-plane direction for the W2 Weyl nodes is shown by our data in Fig. 3E. Additionally, we also observe the W1 Weyl cones in Figs. 3G-I. Notably, our data shows that the energy of the bulk W1 Weyl nodes is lower than that of the bulk W2 Weyl nodes, which agrees well with our calculation shown in Fig. 3J and an independent modeling of the bulk transport data on TaAs (***23*).

In general, in a spin-orbit coupled bulk crystal, point-like linear band crossings can either be Weyl cones or Dirac cones. Because the observed bulk cones in Figs. 3C,D are located neither at Kramers' points nor on a rotational axis, they cannot be identified as bulk Dirac cones and have to be Weyl cones according to topological theories (*6*, *28*). Therefore, our SX-ARPES data alone, proves the existence of bulk Weyl nodes. The agreement between the SX-ARPES data and our bulk calculation, which only requires the crystal structure and the lattice constants as inputs, provides further cross-check.

## Bulk-surface correspondence

Finally, we show that the -space locations of the surface Fermi arc terminations match with the projection of the bulk Weyl nodes on the surface BZ. We superimpose the SX-ARPES measured bulk Fermi surface containing W2 Weyl nodes (Fig. 3C) onto the low-photon-energy ARPES Fermi surface containing the surface Fermi arcs (Fig. 2A) to-scale. From Fig. 4A we see that all the arc terminations and projected Weyl nodes match with each other within the -space region that is covered in our measurements. To establish this point quantitatively, in Fig. 4C, we show the zoomed-in map near the crescent Fermi arc terminations, from which we obtain the -space location of the terminations to be at . Figure 4D shows the zoomed-in map of two nearby W2 Weyl nodes, from which we obtain the -space location of the W2 Weyl nodes to be at . In our bulk calculation, the -space location of the W2 Weyl nodes is found to be at . Since the SX-ARPES bulk data and the low-photon-energy ARPES surface data are completely independent measurements using two different beamlines, the fact that they match well provides another piece of evidence of the topological nature (the surface-bulk correspondence) of the Weyl semimetal state in TaAs. In figs. S5-S7, we further show that the bulk Weyl cones can also be observed in our low-photon-energy ARPES data, although their spectral weight is much lower than the surface state intensities that dominate the data. Our demonstration of the Weyl fermion semimetal state in and Fermi arc surface metals paves the way (*30*) for the realization of many fascinating topological quantum phenomena.

## Supplementary Materials

www.sciencemag.org/cgi/content/full/science.aaa9297/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S10

## References and Notes

- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵Materials and methods are available as supplementary materials on
*Science*Online. - ↵
- ↵
- ↵
- ↵
- ↵
- ↵
**Acknowledgments:**Work at Princeton University and Princeton-led synchrotron-based ARPES measurements were supported by the Gordon and Betty Moore Foundations EPiQS Initiative through Grant GBMF4547 (Hasan). First-principles band structure calculations at National University of Singapore were supported by the National Research Foundation, Prime Minister's Office, Singapore under its NRF fellowship (NRF Award No. NRF-NRFF2013-03). Single crystal growth was supported by National Basic Research Program of China (Grant Nos. 2013CB921901 and 2014CB239302) and characterization by U.S. DOE DE-FG-02-05ER46200. F.C.C acknowledges the support provided by MOST-Taiwan under project number 102-2119-M-002-004. We gratefully acknowledge J. D. Denlinger, S. K. Mo, A. V. Fedorov, M. Hashimoto, M. Hoesch, T. Kim, and V. N. Strocov for their beamline assistance at the Advanced Light Source, the Stanford Synchrotron Radiation Lightsource, the Diamond Light Source, and the Swiss Light Source. We also thank D. Huse, I. Klebanov, T. Neupert, A. Polyakov, P. Steinhardt, H. Verlinde, and A. Vishwanath for discussions. R.S. and H.L. acknowledge visiting scientist support from Princeton University. A patent application is being prepared on behalf of the authors on the discovery of a Weyl semimetal