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

Huang SM, Xu SY, Belopolski I, Lee CC, Chang G, Wang B, Alidoust N, Bian G, Neupane M, Zhang C, Jia S, Bansil A, Lin H, Hasan MZ - Nat Commun (2015)

Bottom Line: Such a semimetal not only provides a condensed matter realization of the anomalies in quantum field theories but also demonstrates the topological classification beyond the gapped topological insulators.Here, we identify a topological Weyl semimetal state in the transition metal monopnictide materials class.Our results show that in the TaAs-type materials the Weyl semimetal state does not depend on fine-tuning of chemical composition or magnetic order, which opens the door for the experimental realization of Weyl semimetals and Fermi arc surface states in real materials.

View Article: PubMed Central - PubMed

Affiliation: 1] Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore [2] Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore.

ABSTRACT
Weyl fermions are massless chiral fermions that play an important role in quantum field theory but have never been observed as fundamental particles. A Weyl semimetal is an unusual crystal that hosts Weyl fermions as quasiparticle excitations and features Fermi arcs on its surface. Such a semimetal not only provides a condensed matter realization of the anomalies in quantum field theories but also demonstrates the topological classification beyond the gapped topological insulators. Here, we identify a topological Weyl semimetal state in the transition metal monopnictide materials class. Our first-principles calculations on TaAs reveal its bulk Weyl fermion cones and surface Fermi arcs. Our results show that in the TaAs-type materials the Weyl semimetal state does not depend on fine-tuning of chemical composition or magnetic order, which opens the door for the experimental realization of Weyl semimetals and Fermi arc surface states in real materials.

No MeSH data available.


Related in: MedlinePlus

Topological Fermi arc surface states in TaAs.(a) The (001) surface states on the top surface of TaAs. (b) The same as panel a but on the bottom surface. The black and white circles denote the Weyl points, the shaded regions represent the spectral weight of some additional bulk bands near the surface region, whereas the sharp, red or blue curves show the surface states. (c,d) Schematics of the surface Fermi surfaces for the top and the bottom surfaces. (e) A close-up of the band structure on both the top and the bottom surfaces near the  point, as indicated by the orange squares in panels a and b. (f–h) Energy dispersions of the electronic structure along three momentum space cuts, as noted in panel e.
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f4: Topological Fermi arc surface states in TaAs.(a) The (001) surface states on the top surface of TaAs. (b) The same as panel a but on the bottom surface. The black and white circles denote the Weyl points, the shaded regions represent the spectral weight of some additional bulk bands near the surface region, whereas the sharp, red or blue curves show the surface states. (c,d) Schematics of the surface Fermi surfaces for the top and the bottom surfaces. (e) A close-up of the band structure on both the top and the bottom surfaces near the point, as indicated by the orange squares in panels a and b. (f–h) Energy dispersions of the electronic structure along three momentum space cuts, as noted in panel e.

Mentions: Another key signature of a Weyl semimetal is the presence of Fermi arc surface states that connect the Weyl points in pairs in the surface BZ. We present calculations of the (001) surface states in Fig. 4. We show the surface states on the top surface in Fig. 4a and the bottom surface in Fig. 4b. The black and white circles denote the Weyl points, the shaded regions represent the spectral weight of some additional bulk bands near the surface region, whereas the sharp red or blue curves show the surface states. We find surface Fermi arcs that connect Weyl points of opposite chirality in pairs. To better understand the rich structure of the Fermi arcs, we show a schematic of the surface states on the top surface in Fig. 4c and the bottom surface in Fig. 4d. We note that Fig. 4c,d are only designed to show the connectivity of the Fermi arcs and the Weyl nodes. The detailed shape of the Fermi arcs does not necessarily match up with our calculation results in Fig. 4a,b. Note that one Fermi arc connects each pair of points W1. However, two Fermi arcs connect to each projection of points W2, because they project in pairs with the same chiral charge, as discussed above. This leads to Fermi arcs that connect the points W2 in a closed loop of surface states. The largest Fermi arc loop on the top surface threads through four projected Weyl points in the surface BZ. We also note that the Fermi surface of the surface states from the top is very different from that of the bottom (Fig. 4a,b), consistent with broken inversion symmetry in this system. In addition, because the presence of a surface breaks the C4 screw symmetry, the surface states are also very different along the kx and ky directions of the surface BZ. Finally, we observe closed Fermi surfaces that do not intersect Weyl points and do not form Fermi arcs. These extra Fermi surfaces reflect how different ways of annihilating the Weyl points would give rise to an insulator with a different topological invariant (see the Discussion below). We present a particularly simple set of Fermi arcs arising near the point in Fig. 4e, including surface states from both the top and bottom surfaces. To visualize the arc nature of the surface states, we present three energy dispersion cuts along the directions indicated in Fig. 4e. Along Cut 1, shown in Fig. 4f, we see a Dirac cone connecting the bulk valence and conduction bands across the bulk bandgap, exactly like a topological insulator. Along Cut 2, shown in Fig. 4g, we see the projected bulk Weyl cones, with surface states which pass through the Weyl points. Lastly, in Cut 3, shown in Fig. 4h, we observe a full bandgap. The surface states along this cut are trivial because they do not connect across the bulk bandgap.


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

Huang SM, Xu SY, Belopolski I, Lee CC, Chang G, Wang B, Alidoust N, Bian G, Neupane M, Zhang C, Jia S, Bansil A, Lin H, Hasan MZ - Nat Commun (2015)

Topological Fermi arc surface states in TaAs.(a) The (001) surface states on the top surface of TaAs. (b) The same as panel a but on the bottom surface. The black and white circles denote the Weyl points, the shaded regions represent the spectral weight of some additional bulk bands near the surface region, whereas the sharp, red or blue curves show the surface states. (c,d) Schematics of the surface Fermi surfaces for the top and the bottom surfaces. (e) A close-up of the band structure on both the top and the bottom surfaces near the  point, as indicated by the orange squares in panels a and b. (f–h) Energy dispersions of the electronic structure along three momentum space cuts, as noted in panel e.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4490374&req=5

f4: Topological Fermi arc surface states in TaAs.(a) The (001) surface states on the top surface of TaAs. (b) The same as panel a but on the bottom surface. The black and white circles denote the Weyl points, the shaded regions represent the spectral weight of some additional bulk bands near the surface region, whereas the sharp, red or blue curves show the surface states. (c,d) Schematics of the surface Fermi surfaces for the top and the bottom surfaces. (e) A close-up of the band structure on both the top and the bottom surfaces near the point, as indicated by the orange squares in panels a and b. (f–h) Energy dispersions of the electronic structure along three momentum space cuts, as noted in panel e.
Mentions: Another key signature of a Weyl semimetal is the presence of Fermi arc surface states that connect the Weyl points in pairs in the surface BZ. We present calculations of the (001) surface states in Fig. 4. We show the surface states on the top surface in Fig. 4a and the bottom surface in Fig. 4b. The black and white circles denote the Weyl points, the shaded regions represent the spectral weight of some additional bulk bands near the surface region, whereas the sharp red or blue curves show the surface states. We find surface Fermi arcs that connect Weyl points of opposite chirality in pairs. To better understand the rich structure of the Fermi arcs, we show a schematic of the surface states on the top surface in Fig. 4c and the bottom surface in Fig. 4d. We note that Fig. 4c,d are only designed to show the connectivity of the Fermi arcs and the Weyl nodes. The detailed shape of the Fermi arcs does not necessarily match up with our calculation results in Fig. 4a,b. Note that one Fermi arc connects each pair of points W1. However, two Fermi arcs connect to each projection of points W2, because they project in pairs with the same chiral charge, as discussed above. This leads to Fermi arcs that connect the points W2 in a closed loop of surface states. The largest Fermi arc loop on the top surface threads through four projected Weyl points in the surface BZ. We also note that the Fermi surface of the surface states from the top is very different from that of the bottom (Fig. 4a,b), consistent with broken inversion symmetry in this system. In addition, because the presence of a surface breaks the C4 screw symmetry, the surface states are also very different along the kx and ky directions of the surface BZ. Finally, we observe closed Fermi surfaces that do not intersect Weyl points and do not form Fermi arcs. These extra Fermi surfaces reflect how different ways of annihilating the Weyl points would give rise to an insulator with a different topological invariant (see the Discussion below). We present a particularly simple set of Fermi arcs arising near the point in Fig. 4e, including surface states from both the top and bottom surfaces. To visualize the arc nature of the surface states, we present three energy dispersion cuts along the directions indicated in Fig. 4e. Along Cut 1, shown in Fig. 4f, we see a Dirac cone connecting the bulk valence and conduction bands across the bulk bandgap, exactly like a topological insulator. Along Cut 2, shown in Fig. 4g, we see the projected bulk Weyl cones, with surface states which pass through the Weyl points. Lastly, in Cut 3, shown in Fig. 4h, we observe a full bandgap. The surface states along this cut are trivial because they do not connect across the bulk bandgap.

Bottom Line: Such a semimetal not only provides a condensed matter realization of the anomalies in quantum field theories but also demonstrates the topological classification beyond the gapped topological insulators.Here, we identify a topological Weyl semimetal state in the transition metal monopnictide materials class.Our results show that in the TaAs-type materials the Weyl semimetal state does not depend on fine-tuning of chemical composition or magnetic order, which opens the door for the experimental realization of Weyl semimetals and Fermi arc surface states in real materials.

View Article: PubMed Central - PubMed

Affiliation: 1] Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore [2] Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore.

ABSTRACT
Weyl fermions are massless chiral fermions that play an important role in quantum field theory but have never been observed as fundamental particles. A Weyl semimetal is an unusual crystal that hosts Weyl fermions as quasiparticle excitations and features Fermi arcs on its surface. Such a semimetal not only provides a condensed matter realization of the anomalies in quantum field theories but also demonstrates the topological classification beyond the gapped topological insulators. Here, we identify a topological Weyl semimetal state in the transition metal monopnictide materials class. Our first-principles calculations on TaAs reveal its bulk Weyl fermion cones and surface Fermi arcs. Our results show that in the TaAs-type materials the Weyl semimetal state does not depend on fine-tuning of chemical composition or magnetic order, which opens the door for the experimental realization of Weyl semimetals and Fermi arc surface states in real materials.

No MeSH data available.


Related in: MedlinePlus