In quantum field theory, Weyl fermions are relativistic particles that travel at the speed of light and strictly obey the celebrated Lorentz symmetry. Their low-energy condensed matter analogs are Weyl semimetals, which are conductors whose electronic excitations mimic the Weyl fermion equation of motion. Although the traditional (type I) emergent Weyl fermions observed in TaAs still approximately respect Lorentz symmetry, recently, the so-called type II Weyl semimetal has been proposed, where the emergent Weyl quasiparticles break the Lorentz symmetry so strongly that they cannot be smoothly connected to Lorentz symmetric Weyl particles. Despite some evidence of nontrivial surface states, the direct observation of the type II bulk Weyl fermions remains elusive. We present the direct observation of the type II Weyl fermions in crystalline solid lanthanum aluminum germanide (LaAlGe) based on our photoemission data alone, without reliance on band structure calculations. Moreover, our systematic data agree with the theoretical calculations, providing further support on our experimental results.

}, keywords = {arcs, crystal, dirac semimetals, electron, energy-bands, hall, mote2, phase}, issn = {2375-2548}, doi = {10.1126/sciadv.1603266}, author = {Xu, Su-Yang and Alidoust, Nasser and Chang, Guoqing and Lu, Hong and Singh, Bahadur and Belopolski, Ilya and Sanchez, Daniel S. and Zhang, Xiao and Bian, Guang and Zheng, Hao and Husanu, Marious-Adrian and Bian, Yi and Huang, Shin-Ming and Hsu, Chuang-Han and Chang, Tay-Rong and Jeng, Horng-Tay and Bansil, Arun and Neupert, Titus and Strocov, Vladimir N. and Lin, Hsin and Jia, Shuang and Hasan, M. Zahid} } @article {singh_stable_2017, title = {Stable charge density wave phase in a 1T-{TiSe}2 monolayer}, journal = {Phys. Rev. B}, volume = {95}, number = {24}, year = {2017}, note = {WOS:000404469700003}, month = {06/2017}, pages = {245136}, abstract = {Charge density wave (CDW) phases are symmetry-reduced states of matter in which a periodic modulation of the electronic charge frequently leads to drastic changes of the electronic spectrum, including the emergence of energy gaps. We analyze the CDW state in a 1T-TiSe2 monolayer within a density functional theory framework and show that, similarly to its bulk counterpart, the monolayer is unstable towards a commensurate 2x2 periodic lattice distortion (PLD) and CDWat low temperatures. Analysis of the electron and phonon spectrum establishes the PLD as the stable T = 0 K configuration with a narrow band gap, whereas the undistorted and semimetallic state is stable only above a threshold temperature. The lattice distortions as well as the unfolded and reconstructed band structure in the CDW phase agree well with experimental results. We also address evidence in our results for the role of electron-electron interactions in the CDW instability of 1T-TiSe2 monolayers.

}, keywords = {diselenide, metal, spectrum, tise2, transition}, issn = {2469-9950}, doi = {10.1103/PhysRevB.95.245136}, author = {Singh, Bahadur and Hsu, Chuang-Han and Tsai, Wei-Feng and Pereira, Vitor M. and Lin, Hsin} } @article {ghosh_electric-field_2016, title = {Electric-field tunable {Dirac} semimetal state in phosphorene thin films}, journal = {Phys. Rev. B}, volume = {94}, number = {20}, year = {2016}, note = {WOS:000388466200006}, month = {11/2016}, pages = {205426}, abstract = {We study the electric-field tunable electronic properties of phosphorene thin films, using the framework of density functional theory. We show that phosphorene thin films offer a versatile material platform to study two-dimensional Dirac fermions on application of a transverse electric field. Increasing the strength of the transverse electric field beyond a certain critical value in phosphorene thin films leads to the formation of two symmetry protected gapless Dirac fermions states with anisotropic energy dispersion. The spin-orbit coupling splits each of these Dirac states into two spin-polarized Dirac cones which are also protected by nonsymmorphic crystal symmetries. Our study shows that the position as well as the carrier velocity of the spin-polarized Dirac cone states can be controlled by the strength of the external electric field.

}, keywords = {2-dimensional materials, augmented-wave method, gap, layer black phosphorus, pseudospins, spin, strain, topological insulators, transition-metal dichalcogenides}, issn = {2469-9950}, doi = {10.1103/PhysRevB.94.205426}, author = {Ghosh, Barun and Singh, Bahadur and Prasad, R. and Agarwal, Amit} } @article {singh_role_2016, title = {Role of surface termination in realizing well-isolated topological surface states within the bulk band gap in {TlBiSe}2 and {TlBiTe}2}, journal = {Physical Review B}, volume = {93}, number = {8}, year = {2016}, note = {WOS:000369405900007}, month = {02/2016}, pages = {085113}, abstract = {Electronic structures associated with the flat (polar) Se/Te- or Tl-terminated surfaces of TlBiSe2 and TlBiTe2 are predicted to harbor not only Dirac cone states, but also trivial dangling bond states near the Fermi energy. However, the latter, trivial states have never been observed in photoemission measurements. In order to address this discrepancy, we have carried out ab initio calculations for various surfaces of TlBiSe2 and TlBiTe2. A rough nonpolar surface with an equal number of Se/Te and Tl atoms in the surface atomic layer is found to destroy the trivial dangling bond states, leaving only the Dirac cone states in the bulk energy gap. The resulting energy dispersions of the Dirac states are in good accord with the corresponding experimental dispersions in TlBiSe2 as well as TlBiTe2. We also show that in the case of flat, Se terminated, high-index (221) and (112) surfaces of TlBiSe2, the trivial surface states shift energetically below the Dirac node and become well separated from the Dirac cone states.

}, doi = {10.1103/PhysRevB.93.085113}, author = {Singh, Bahadur and Lin, Hsin and Prasad, R. and Bansil, A.} } @article {singh_topological_2014, title = {Topological phase transition and quantum spin Hall state in {TlBiS}2}, journal = {Journal of Applied Physics}, volume = {116}, number = {3}, year = {2014}, month = {07/2014}, pages = {033704}, abstract = {We have investigated the bulk and surface electronic structures and band topology of {TlBiS}2 as a function of strain and electric field using ab-initio calculations. In its pristine form, {TlBiS}2 is a normal insulator, which does not support any non-trivial surface states. We show however that a compressive strain along the (111) direction induces a single band inversion with Z2 = (1;000), resulting in a Dirac cone surface state with a large in-plane spin polarization. Our analysis shows that a critical point lies between the normal and topological phases where the dispersion of the 3D bulk Dirac cone at the \Γ-point becomes nearly linear. The band gap in thin films of {TlBiS}2 can be tuned through an out-of-the-plane electric field to realize a topological phase transition from a trivial insulator to a quantum spin Hall state. An effective k \· p model Hamiltonian is presented to simulate our first-principles results on {TlBiS}2.

}, keywords = {Band gap, Dirac equation, Electric fields, Insulating thin films, Surface states}, issn = {0021-8979, 1089-7550}, doi = {10.1063/1.4890226}, url = {http://scitation.aip.org/content/aip/journal/jap/116/3/10.1063/1.4890226}, author = {Singh, Bahadur and Lin, Hsin and Prasad, R. and Bansil, A.} } @article {singh_topological_2013, title = {Topological phase transition and two-dimensional topological insulators in Ge-based thin films}, journal = {Physical Review B}, volume = {88}, number = {19}, year = {2013}, month = {11/2013}, pages = {195147}, abstract = {We discuss possible topological phase transitions in Ge-based thin films of Ge(Bix{\textless}math{\textgreater}{\textless}msub{\textgreater}{\textless}mrow{\textgreater}{\textless}/mrow{\textgreater}{\textless}mi \_moz-math-font-style=\"italic\"{\textgreater}x{\textless}/mi{\textgreater}{\textless}/msub{\textgreater}{\textless}/math{\textgreater}Sb1\−x{\textless}math{\textgreater}{\textless}msub{\textgreater}{\textless}mrow{\textgreater}{\textless}/mrow{\textgreater}{\textless}mrow{\textgreater}{\textless}mn{\textgreater}1{\textless}/mn{\textgreater}{\textless}mo{\textgreater}\−{\textless}/mo{\textgreater}{\textless}mi \_moz-math-font-style=\"italic\"{\textgreater}x{\textless}/mi{\textgreater}{\textless}/mrow{\textgreater}{\textless}/msub{\textgreater}{\textless}/math{\textgreater})2{\textless}math{\textgreater}{\textless}msub{\textgreater}{\textless}mrow{\textgreater}{\textless}/mrow{\textgreater}{\textless}mn{\textgreater}2{\textless}/mn{\textgreater}{\textless}/msub{\textgreater}{\textless}/math{\textgreater}Te4{\textless}math{\textgreater}{\textless}msub{\textgreater}{\textless}mrow{\textgreater}{\textless}/mrow{\textgreater}{\textless}mn{\textgreater}4{\textless}/mn{\textgreater}{\textless}/msub{\textgreater}{\textless}/math{\textgreater} as a function of layer thickness and Bi concentration x{\textless}math{\textgreater}{\textless}mi \_moz-math-font-style=\"italic\"{\textgreater}x{\textless}/mi{\textgreater}{\textless}/math{\textgreater} using the first-principles density functional theory framework. The bulk material is a topological insulator at x=1.0{\textless}math{\textgreater}{\textless}mrow{\textgreater}{\textless}mi \_moz-math-font-style=\"italic\"{\textgreater}x{\textless}/mi{\textgreater}{\textless}mo{\textgreater}={\textless}/mo{\textgreater}{\textless}mn{\textgreater}1.0{\textless}/mn{\textgreater}{\textless}/mrow{\textgreater}{\textless}/math{\textgreater} with a single Dirac cone surface state at the surface Brillouin zone center, whereas it is a trivial insulator at x=0{\textless}math{\textgreater}{\textless}mrow{\textgreater}{\textless}mi \_moz-math-font-style=\"italic\"{\textgreater}x{\textless}/mi{\textgreater}{\textless}mo{\textgreater}={\textless}/mo{\textgreater}{\textless}mn{\textgreater}0{\textless}/mn{\textgreater}{\textless}/mrow{\textgreater}{\textless}/math{\textgreater}. Through a systematic examination of the band topologies, we predict that thin films of Ge(Bix{\textless}math{\textgreater}{\textless}msub{\textgreater}{\textless}mrow{\textgreater}{\textless}/mrow{\textgreater}{\textless}mi \_moz-math-font-style=\"italic\"{\textgreater}x{\textless}/mi{\textgreater}{\textless}/msub{\textgreater}{\textless}/math{\textgreater}Sb1\−x{\textless}math{\textgreater}{\textless}msub{\textgreater}{\textless}mrow{\textgreater}{\textless}/mrow{\textgreater}{\textless}mrow{\textgreater}{\textless}mn{\textgreater}1{\textless}/mn{\textgreater}{\textless}mo{\textgreater}\−{\textless}/mo{\textgreater}{\textless}mi \_moz-math-font-style=\"italic\"{\textgreater}x{\textless}/mi{\textgreater}{\textless}/mrow{\textgreater}{\textless}/msub{\textgreater}{\textless}/math{\textgreater})2{\textless}math{\textgreater}{\textless}msub{\textgreater}{\textless}mrow{\textgreater}{\textless}/mrow{\textgreater}{\textless}mn{\textgreater}2{\textless}/mn{\textgreater}{\textless}/msub{\textgreater}{\textless}/math{\textgreater}Te4{\textless}math{\textgreater}{\textless}msub{\textgreater}{\textless}mrow{\textgreater}{\textless}/mrow{\textgreater}{\textless}mn{\textgreater}4{\textless}/mn{\textgreater}{\textless}/msub{\textgreater}{\textless}/math{\textgreater} with x=0.6{\textless}math{\textgreater}{\textless}mrow{\textgreater}{\textless}mi \_moz-math-font-style=\"italic\"{\textgreater}x{\textless}/mi{\textgreater}{\textless}mo{\textgreater}={\textless}/mo{\textgreater}{\textless}mn{\textgreater}0.6{\textless}/mn{\textgreater}{\textless}/mrow{\textgreater}{\textless}/math{\textgreater}, 0.8, and 1.0 are candidates for two-dimensional ({2D)} topological insulators, which would undergo a {2D} topological phase transition as a function of x{\textless}math{\textgreater}{\textless}mi \_moz-math-font-style=\"italic\"{\textgreater}x{\textless}/mi{\textgreater}{\textless}/math{\textgreater}. A topological phase diagram for Ge(Bix{\textless}math{\textgreater}{\textless}msub{\textgreater}{\textless}mrow{\textgreater}{\textless}/mrow{\textgreater}{\textless}mi \_moz-math-font-style=\"italic\"{\textgreater}x{\textless}/mi{\textgreater}{\textless}/msub{\textgreater}{\textless}/math{\textgreater}Sb1\−x{\textless}math{\textgreater}{\textless}msub{\textgreater}{\textless}mrow{\textgreater}{\textless}/mrow{\textgreater}{\textless}mrow{\textgreater}{\textless}mn{\textgreater}1{\textless}/mn{\textgreater}{\textless}mo{\textgreater}\−{\textless}/mo{\textgreater}{\textless}mi \_moz-math-font-style=\"italic\"{\textgreater}x{\textless}/mi{\textgreater}{\textless}/mrow{\textgreater}{\textless}/msub{\textgreater}{\textless}/math{\textgreater})2{\textless}math{\textgreater}{\textless}msub{\textgreater}{\textless}mrow{\textgreater}{\textless}/mrow{\textgreater}{\textless}mn{\textgreater}2{\textless}/mn{\textgreater}{\textless}/msub{\textgreater}{\textless}/math{\textgreater}Te4{\textless}math{\textgreater}{\textless}msub{\textgreater}{\textless}mrow{\textgreater}{\textless}/mrow{\textgreater}{\textless}mn{\textgreater}4{\textless}/mn{\textgreater}{\textless}/msub{\textgreater}{\textless}/math{\textgreater} thin films is presented to help guide their experimental exploration.

}, doi = {10.1103/PhysRevB.88.195147}, url = {http://link.aps.org/doi/10.1103/PhysRevB.88.195147}, author = {Singh, Bahadur and Lin, Hsin and Prasad, R. and Bansil, A.} }