Titus Neupert

Discovery of a Weyl fermion state with Fermi arcs in niobium arsenide

We report the discovery of Weyl semimetal NbAs featuring topological Fermi arc surface states.

Topological nodal-line fermions in spin-orbit metal PbTaSe2

Topological semimetals can support one-dimensional Fermi lines or zero-dimensional Weyl points in momentum space, where the valence and conduction bands touch. While the degeneracy points in Weyl semimetals are robust against any perturbation that preserves translational symmetry, nodal lines require protection by additional crystalline symmetries such as mirror reflection. Here we report, based on a systematic theoretical study and a detailed experimental characterization, the existence of topological nodal-line states in the non-centrosymmetric compound PbTaSe2 with strong spin-orbit coupling. Remarkably, the spin-orbit nodal lines in PbTaSe2 are not only protected by the reflection symmetry but also characterized by an integer topological invariant. Our detailed angle-resolved photoemission measurements, first-principles simulations and theoretical topological analysis illustrate the physical mechanism underlying the formation of the topological nodal-line states and associated surface states for the first time, thus paving the way towards exploring the exotic properties of the topological nodal-line fermions in condensed matter systems.

Signatures of the Adler–Bell–Jackiw chiral anomaly in a Weyl fermion semimetal

Weyl semimetals provide the realization of Weyl fermions in solid-state physics. Among all the physical phenomena that are enabled by Weyl semimetals, the chiral anomaly is the most unusual one. Here, we report signatures of the chiral anomaly in the magneto-transport measurements on the first Weyl semimetal TaAs. We show negative magnetoresistance under parallel electric and magnetic fields, that is, unlike most metals whose resistivity increases under an external magnetic field, we observe that our high mobility TaAs samples become more conductive as a magnetic field is applied along the direction of the current for certain ranges of the field strength. We present systematically detailed data and careful analyses, which allow us to exclude other possible origins of the observed negative magnetoresistance. Our transport data, corroborated by photoemission measurements, first-principles calculations and theoretical analyses, collectively demonstrate signatures of the Weyl fermion chiral anomaly in the magneto-transport of TaAs.

Fractional Quantum Hall States at Zero Magnetic Field

We present a simple prescription to flatten isolated Bloch bands with a nonzero Chern number. We first show that approximate flattening of bands with a nonzero Chern number is possible by tuning ratios of nearest-neighbor and next-nearest-neighbor hoppings in the Haldane model and, similarly, in the chiral-π-flux square lattice model. Then we show that perfect flattening can be attained with further range hoppings that decrease exponentially with distance. Finally, we add interactions to the model and present exact diagonalization results for a small system at 1/3 filling that support (i) the existence of a spectral gap, (ii) that the ground state is a topological state, and (iii) that the Hall conductance is quantized.

Drumhead surface states and topological nodal-line fermions in TlTaSe2

A topological nodal-line semimetal is a new condensed matter state with one-dimensional bulk nodal lines and two-dimensional drumhead surface bands. Based on first-principles calculations and our effective k . p model, we propose the existence of topological nodal-line fermions in the ternary transition- metal chalcogenide TlTaSe2. The noncentrosymmetric structure and strong spin-orbit coupling give rise to spinful nodal-line bulk states which are protected by a mirror reflection symmetry of this compound. This is remarkably distinguished from other proposed nodal-line semimetals such as Cu3NPb(Zn) in which nodal lines exist only in the limit of vanishing spin-orbit coupling. We show that the drumhead surface states in TlTaSe2, which are associated with the topological nodal lines, exhibit an unconventional chiral spin texture and an exotic Lifshitz transition as a consequence of the linkage among multiple drumhead surface-state pockets.

Disentanglement of Surface and Bulk Rashba Spin Splittings in Noncentrosymmetric BiTeI

BiTeI has a layered and non-centrosymmetric structure where strong spin-orbit interaction leads to a giant spin splitting in the bulk bands. Here we present high-resolution angle-resolved photoemission (ARPES) data in the UV and soft x-ray regime that clearly disentangle the surface from the bulk electronic structure. Spin-resolved UV-ARPES measurements on opposite, nonequivalent surfaces show identical spin structures, thus clarifying the surface state character. Soft x-ray ARPES data clearly reveal the spindle-torus shape of the bulk Fermi surface, induced by the spin-orbit interaction. PACS numbers: 71.20.Nr, 71.70.Ej, 79.60.Bm 1 ar X iv :1 20 4. 21 96 v1 [ co nd -m at .m tr lsc i] 1 0 A pr 2 01 2 The breaking of inversion symmetry and its influence on the spin structure of surface states under action of spin–orbit interaction (SOI) has been extensively studied in recent years [1, 2]. The main finding is that the surface states become spin-split according to the Rashba model [3] resulting in two spin-polarized concentric Fermi contours. The lack of inversion symmetry in the bulk crystal structure is expected to induce a spin splitting with a more complex bandand spin-structure. Combined with strong SOI the Fermi surface can take the shape of a torus [4]. For non-centrosymmetric superconductors such as for example CePt3Si [5] this peculiar band structure is expected to result in topologically protected spin polarized edge states reminiscent of Majorana modes [6]. Recently, an ARPES and spin-resolved ARPES study by Ishizaka et al. [7] proposed that the semiconductor BiTeI features a very large spin-splitting, arising from the broken inversion symmetry in the crystal bulk and a strong SOI. Theoretical work based on the perturbative k ·p formalism linked the unusually large spin splitting in BiTeI to the negative crystal field splitting of the top valence bands [8]. Optical transition measurements [9] are in accordance with the giant bulk spin-splitting of the gap defining valence and conduction bands predicted by first principle calculations [7, 8]. In addition it was shown in recent theoretical work that BiTeI can become a topological insulator under action of hydrostatic pressure [10], and thus is closely related to non-centrosymmetric topological superconductors. The present study provides first band mapping of a system without bulk inversion symmetry and giant SOI by the example of BiTeI, featuring a three-dimensional Rashba splitting of the bulk bands. Further it is shown that the Rashba-split state observed for this material in the UV photon energy regime is not a quantum well state [7] but rather a surface state, using a simple symmetry argument based on spin-resolved ARPES (SARPES) measurements, which is confirmed by first principle calculations. All measurements were performed at the Swiss Light Source of the Paul-Scherrer-Institut. The SARPES data was measured with the Mott polarimeter at the COPHEE endstation [11] of the Surface and Interface Spectroscopy beamline at a photon energy of 24 eV. The spin-integrated data at photon energies 20-63 eV were taken at the high-resolution ARPES endstation at the same beamline. The soft x-ray ARPES data were taken at the SX-ARPES endstation of the ADRESS beamline at photon energies of 310-850 eV. All spin-integrated measurements were performed at a sample temperature of 11 K and a base pressure lower than 10−10 mbar, the SARPES data was taken at 20 K.

New type of Weyl semimetal with quadratic double Weyl fermions.

Significance We predict a new Weyl semimetal candidate. This is critically needed for this rapidly developing field as TaAs is the only known Weyl semimetal in nature. We show that SrSi2 has many new and novel properties not possible in TaAs. Our prediction provides a new route to studying the elusive Weyl fermion particles originally considered in high-energy physics by tabletop experiments. Weyl semimetals have attracted worldwide attention due to their wide range of exotic properties predicted in theories. The experimental realization had remained elusive for a long time despite much effort. Very recently, the first Weyl semimetal has been discovered in an inversion-breaking, stoichiometric solid TaAs. So far, the TaAs class remains the only Weyl semimetal available in real materials. To facilitate the transition of Weyl semimetals from the realm of purely theoretical interest to the realm of experimental studies and device applications, it is of crucial importance to identify other robust candidates that are experimentally feasible to be realized. In this paper, we propose such a Weyl semimetal candidate in an inversion-breaking, stoichiometric compound strontium silicide, SrSi2, with many new and novel properties that are distinct from TaAs. We show that SrSi2 is a Weyl semimetal even without spin–orbit coupling and that, after the inclusion of spin–orbit coupling, two Weyl fermions stick together forming an exotic double Weyl fermion with quadratic dispersions and a higher chiral charge of ±2. Moreover, we find that the Weyl nodes with opposite charges are located at different energies due to the absence of mirror symmetry in SrSi2, paving the way for the realization of the chiral magnetic effect. Our systematic results not only identify a much-needed robust Weyl semimetal candidate but also open the door to new topological Weyl physics that is not possible in TaAs.

Floquet Fractional Chern Insulators

Fractional Chern insulators are theoretically predicted states of electronic matter with emergent topological order. They exhibit the same universal properties as the fractional quantum Hall effect, but dispose of the need to apply a strong magnetic field. However, despite intense theoretical work, an experimental realization for these exotic states of matter is still lacking. Here we show that doped graphene turns into a fractional Chern insulator, when irradiated with high-intensity circularly polarized light. We derive the effective steady state band structure of light-driven graphene using Floquet theory and subsequently study the interacting system with exact numerical diagonalization. The fractional Chern insulator state equivalent to the 1/3 Laughlin state appears at 7/12 total filling of the honeycomb lattice (1/6 filling of the upper band). The state also features spontaneous ferromagnetism and is thus an example of the spontaneous breaking of a continuous symmetry along with a topological phase transition.

Fractional topological liquids with time-reversal symmetry and their lattice realization

We present a class of time-reversal-symmetric fractional topological liquid states in two dimensions that support fractionalized excitations. These are incompressible liquids made of electrons, for which the charge Hall conductance vanishes and the spin Hall conductance needs not be quantized. We then analyze the stability of edge states in these two-dimensional topological fluids against localization by disorder. We find a

Evidence for superconductivity with broken time-reversal symmetry in locally noncentrosymmetric SrPtAs

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