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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 phase transitions of an inversion breaking Weyl semimetal.(a) A Weyl semimetal can be understood as an intermediate phase between a trivial insulator and a topological insulator as a function of a tuning parameter m. The grey circles represent the band touchings at the critical point, each of which is composed of two degenerate Weyl nodes. The black and white circles are the Weyl nodes with positive and negative chiral charges. The blue lines are the topological surface states. (b) The calculated Fermi surface of TaAs near the  point. (c) In the Weyl semimetal TaAs, electrons exhibit an unusual path in real and momentum (z−kx−ky) space under an external magnetic field along the z direction. The orange arrows show the real-space motion of the electrons between the top and the bottom surfaces. The blue and red arrows show the electron's momentum space trajectories tracing out the constant energy contour of the Fermi arcs on the surfaces. (d) In the topological insulator Bi2Se3, an electron tracing out a surface state constant energy contour will not encounter a bulk state and the wavefunction will always remain localized on the same surface. (e) An image of TaAs single crystals we have grown.
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f5: Topological phase transitions of an inversion breaking Weyl semimetal.(a) A Weyl semimetal can be understood as an intermediate phase between a trivial insulator and a topological insulator as a function of a tuning parameter m. The grey circles represent the band touchings at the critical point, each of which is composed of two degenerate Weyl nodes. The black and white circles are the Weyl nodes with positive and negative chiral charges. The blue lines are the topological surface states. (b) The calculated Fermi surface of TaAs near the point. (c) In the Weyl semimetal TaAs, electrons exhibit an unusual path in real and momentum (z−kx−ky) space under an external magnetic field along the z direction. The orange arrows show the real-space motion of the electrons between the top and the bottom surfaces. The blue and red arrows show the electron's momentum space trajectories tracing out the constant energy contour of the Fermi arcs on the surfaces. (d) In the topological insulator Bi2Se3, an electron tracing out a surface state constant energy contour will not encounter a bulk state and the wavefunction will always remain localized on the same surface. (e) An image of TaAs single crystals we have grown.

Mentions: Next, we provide some general comments on the nature of the phase transition between a trivial insulator, a Weyl semimetal and a topological insulator, illustrated in Fig. 5a. When a system with inversion symmetry undergoes a topological phase transition between a trivial insulator and a topological insulator, the bandgap necessarily closes at a Kramers' point. If we imagine moving the system through the phase transition by tuning a parameter m, then there will be a critical point where the system is gapless. In a system which breaks inversion symmetry, there will instead be a finite range of m where the system remains gapless, giving rise to a Weyl semimetal phase. In this way, the Weyl semimetal phase can be viewed as an intermediate phase between a trivial insulator and a topological insulator, where the bulk bandgap of a trivial insulator closes and Weyl points of opposite chiral charge nucleate from the bulk band touchings. As m is varied, the Weyl points thread surface states through the surface BZ and eventually annihilate each other, allowing the bulk bandgap to reopen with a complete set of surface states, giving rise to a topological insulator. This understanding of a Weyl semimetal as an intermediate phase between a trivial insulator and a topological insulator offers some insight into the closed Fermi surfaces we find in TaAs around the point of the top surface and the point of the bottom surface. We propose that these surface states reflect the topological invariant we would find if we annihilated the Weyl points to produce a bulk insulator. We consider the bottom surface, and annihilate the Weyl points in pairs in the obvious way to remove all surface states along . Then, we can annihilate the remaining Weyl points to produce two concentric Fermi surfaces around the point. Since this is an even number of surface states, we find that this way of annihilating the Weyl points gives rise to a trivial insulator. If, instead, we annihilate the remaining Weyl points to remove the Fermi arc connecting them, only the closed Fermi surface would be left around , giving rise to a topological insulator. A similar analysis applies to the top surface.


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 phase transitions of an inversion breaking Weyl semimetal.(a) A Weyl semimetal can be understood as an intermediate phase between a trivial insulator and a topological insulator as a function of a tuning parameter m. The grey circles represent the band touchings at the critical point, each of which is composed of two degenerate Weyl nodes. The black and white circles are the Weyl nodes with positive and negative chiral charges. The blue lines are the topological surface states. (b) The calculated Fermi surface of TaAs near the  point. (c) In the Weyl semimetal TaAs, electrons exhibit an unusual path in real and momentum (z−kx−ky) space under an external magnetic field along the z direction. The orange arrows show the real-space motion of the electrons between the top and the bottom surfaces. The blue and red arrows show the electron's momentum space trajectories tracing out the constant energy contour of the Fermi arcs on the surfaces. (d) In the topological insulator Bi2Se3, an electron tracing out a surface state constant energy contour will not encounter a bulk state and the wavefunction will always remain localized on the same surface. (e) An image of TaAs single crystals we have grown.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f5: Topological phase transitions of an inversion breaking Weyl semimetal.(a) A Weyl semimetal can be understood as an intermediate phase between a trivial insulator and a topological insulator as a function of a tuning parameter m. The grey circles represent the band touchings at the critical point, each of which is composed of two degenerate Weyl nodes. The black and white circles are the Weyl nodes with positive and negative chiral charges. The blue lines are the topological surface states. (b) The calculated Fermi surface of TaAs near the point. (c) In the Weyl semimetal TaAs, electrons exhibit an unusual path in real and momentum (z−kx−ky) space under an external magnetic field along the z direction. The orange arrows show the real-space motion of the electrons between the top and the bottom surfaces. The blue and red arrows show the electron's momentum space trajectories tracing out the constant energy contour of the Fermi arcs on the surfaces. (d) In the topological insulator Bi2Se3, an electron tracing out a surface state constant energy contour will not encounter a bulk state and the wavefunction will always remain localized on the same surface. (e) An image of TaAs single crystals we have grown.
Mentions: Next, we provide some general comments on the nature of the phase transition between a trivial insulator, a Weyl semimetal and a topological insulator, illustrated in Fig. 5a. When a system with inversion symmetry undergoes a topological phase transition between a trivial insulator and a topological insulator, the bandgap necessarily closes at a Kramers' point. If we imagine moving the system through the phase transition by tuning a parameter m, then there will be a critical point where the system is gapless. In a system which breaks inversion symmetry, there will instead be a finite range of m where the system remains gapless, giving rise to a Weyl semimetal phase. In this way, the Weyl semimetal phase can be viewed as an intermediate phase between a trivial insulator and a topological insulator, where the bulk bandgap of a trivial insulator closes and Weyl points of opposite chiral charge nucleate from the bulk band touchings. As m is varied, the Weyl points thread surface states through the surface BZ and eventually annihilate each other, allowing the bulk bandgap to reopen with a complete set of surface states, giving rise to a topological insulator. This understanding of a Weyl semimetal as an intermediate phase between a trivial insulator and a topological insulator offers some insight into the closed Fermi surfaces we find in TaAs around the point of the top surface and the point of the bottom surface. We propose that these surface states reflect the topological invariant we would find if we annihilated the Weyl points to produce a bulk insulator. We consider the bottom surface, and annihilate the Weyl points in pairs in the obvious way to remove all surface states along . Then, we can annihilate the remaining Weyl points to produce two concentric Fermi surfaces around the point. Since this is an even number of surface states, we find that this way of annihilating the Weyl points gives rise to a trivial insulator. If, instead, we annihilate the remaining Weyl points to remove the Fermi arc connecting them, only the closed Fermi surface would be left around , giving rise to a topological insulator. A similar analysis applies to the top surface.

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