Query: semimetal 

title: Discovery of a Weyl fermion semimetal and topological Fermi arcs
abstract: Weyl physics emerges in the laboratory Weyl fermions massless particles with half integer spin were once mistakenly thought to describe neutrinos. Although not yet observed among elementary particles, Weyl fermions may exist as collective excitations in so called Weyl semimetals. These materials have an unusual band structure in which the linearly dispersing valence and conduction bands meet at discrete "Weyl points." Xu et al. used photoemission spectroscopy to identify TaAs as a Weyl semimetal capable of hosting Weyl fermions. In a complementary study, Lu et al. detected the characteristic Weyl points in a photonic crystal. The observation of Weyl physics may enable the discovery of exotic fundamental phenomena. Science, this issue p. 613 and 622 Angle resolved photoemission is used to detect the topological surface states and bulk dispersion of the compound tantalum arsenide.  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 fermion nodes, consistent with their topological character. Our work opens the field for the experimental study of Weyl fermions in physics and materials science.

title: Helicity dependent photocurrents in the chiral Weyl semimetal RhSi
abstract: The CPGE spectrum and optical conductivity in RhSi are measured and discussed, comparing to theoretical predictions. Weyl semimetals are crystals in which electron bands cross at isolated points in momentum space. Associated with each crossing point (or Weyl node) is a topological invariant known as the Berry monopole charge. The circular photogalvanic effect (CPGE) whereby circular polarized light generates a helicity dependent photocurrent, is a notable example of a macroscopic property that emerges directly from the topology of the Weyl semimetal band structure. Recently, it was predicted that the amplitude of the CPGE associated with optical transitions near a Weyl node is proportional to its monopole charge. In chiral Weyl systems, nodes of opposite charge are nondegenerate, opening a window of wavelengths where the CPGE resulting from uncompensated Berry charge can emerge. Here, we report measurements of CPGE in the chiral Weyl semimetal RhSi, revealing a CPGE response in an energy window that closes at 0.65 eV, in agreement with the predictions of density functional theory.

title: Topological semimetal and Fermi arc surface states in the electronic structure of pyrochlore iridates
abstract: We investigate novel phases that emerge from the interplay of electron correlations and strong spin orbit interactions. We focus on describing the topological semimetal, a three dimensional phase of a magnetic solid, and argue that it may be realized in a class of pyrochlore iridates (such as \mathrm{Y}}_{2}$Ir$}_{2}$O$}_{7} based on calculations using the \text{LDA}+U$ method. This state is a three dimensional analog of graphene with linearly dispersing excitations and provides a condensed matter realization of Weyl fermions that obeys a two component Dirac equation. It also exhibits remarkable topological properties manifested by surface states in the form of Fermi arcs, which are impossible to realize in purely two dimensional band structures. For intermediate correlation strengths, we find this to be the ground state of the pyrochlore iridates, coexisting with noncollinear magnetic order. A narrow window of magnetic `axion' insulator may also be present. An applied magnetic field is found to induce a metallic ground state.

title: Colossal mid infrared bulk photovoltaic effect in a type I Weyl semimetal
abstract: Broadband, efficient and fast conversion of light to electricity is crucial for sensing and clean energy. The bulk photovoltaic effect (BPVE) is a second order nonlinear optical effect that intrinsically converts light into electrical current. Here, we demonstrate a large mid infrared BPVE in microscopic devices of the Weyl semimetal TaAs. This discovery results from combining recent developments in Weyl semimetals, focused ion beam fabrication and theoretical works suggesting a connection between BPVE and topology. We also present a detailed symmetry analysis that allows us to separate the shift current response from photothermal effects. The magnitude and wavelength range of the assigned shift current may impact optical detectors, clean energy and topology, and demonstrate the utility of Weyl semimetals for practical applications.A large bulk photovoltaic effect is observed in the type I Weyl semimetal TaAs, and attributed to the diverging Berry curvature of the Weyl nodes.

title: Magnetic Weyl semimetal phase in a Kagome crystal
abstract: Magnetic Weyl semimetals Weyl semimetals (WSMs) materials that host exotic quasiparticles called Weyl fermions must break either spatial inversion or time reversal symmetry. A number of WSMs that break inversion symmetry have been identified, but showing unambiguously that a material is a time reversal breaking WSM is tricky. Three groups now provide spectroscopic evidence for this latter state in magnetic materials (see the Perspective by da Silva Neto) Belopolski et al. probed the material Co2MnGa using angle resolved photoemission spectroscopy, revealing exotic drumhead surface states. Using the same technique, Liu et al. studied the material Co3Sn2S2, which was complemented by the scanning tunneling spectroscopy measurements of Morali et al. These magnetic WSM states provide an ideal setting for exotic transport effects. Science, this issue p. 1278, p. 1282, p. 1286; see also p. 1248 Photoemission and scanning tunneling spectroscopies provide direct evidence for the existence of magnetic Weyl semimetals. Weyl semimetals are crystalline solids that host emergent relativistic Weyl fermions and have characteristic surface Fermi arcs in their electronic structure. Weyl semimetals with broken time reversal symmetry are difficult to identify unambiguously. In this work, using angle resolved photoemission spectroscopy, we visualized the electronic structure of the ferromagnetic crystal Co3Sn2S2 and discovered its characteristic surface Fermi arcs and linear bulk band dispersions across the Weyl points. These results establish Co3Sn2S2 as a magnetic Weyl semimetal that may serve as a platform for realizing phenomena such as chiral magnetic effects, unusually large anomalous Hall effect and quantum anomalous Hall effect.

title: Fermi arc diversity on surface terminations of the magnetic Weyl semimetal Co3Sn2S2
abstract: Magnetic Weyl semimetals Weyl semimetals (WSMs) materials that host exotic quasiparticles called Weyl fermions must break either spatial inversion or time reversal symmetry. A number of WSMs that break inversion symmetry have been identified, but showing unambiguously that a material is a time reversal breaking WSM is tricky. Three groups now provide spectroscopic evidence for this latter state in magnetic materials (see the Perspective by da Silva Neto) Belopolski et al. probed the material Co2MnGa using angle resolved photoemission spectroscopy, revealing exotic drumhead surface states. Using the same technique, Liu et al. studied the material Co3Sn2S2, which was complemented by the scanning tunneling spectroscopy measurements of Morali et al. These magnetic WSM states provide an ideal setting for exotic transport effects. Science, this issue p. 1278, p. 1282, p. 1286; see also p. 1248 Photoemission and scanning tunneling spectroscopies provide direct evidence for the existence of magnetic Weyl semimetals. Bulk surface correspondence in Weyl semimetals ensures the formation of topological "Fermi arc" surface bands whose existence is guaranteed by bulk Weyl nodes. By investigating three distinct surface terminations of the ferromagnetic semimetal Co3Sn2S2, we verify spectroscopically its classification as a time reversal symmetry broken Weyl semimetal. We show that the distinct surface potentials imposed by three different terminations modify the Fermi arc contour and Weyl node connectivity. On the tin (Sn) surface, we identify intra Brillouin zone Weyl node connectivity of Fermi arcs, whereas on cobalt (Co) termination, the connectivity is across adjacent Brillouin zones. On the sulfur (S) surface, Fermi arcs overlap with nontopological bulk and surface states. We thus resolve both topologically protected and nonprotected electronic properties of a Weyl semimetal.

title: An ultrafast symmetry switch in a Weyl semimetal
abstract: Topological quantum materials exhibit fascinating properties1 3, with important applications for dissipationless electronics and fault tolerant quantum computers4,5. Manipulating the topological invariants in these materials would allow the development of topological switching applications analogous to switching of transistors6. Lattice strain provides the most natural means of tuning these topological invariants because it directly modifies the electron ion interactions and potentially alters the underlying crystalline symmetry on which the topological properties depend7 9. However, conventional means of applying strain through heteroepitaxial lattice mismatch10 and dislocations11 are not extendable to controllable time varying protocols, which are required in transistors. Integration into a functional device requires the ability to go beyond the robust, topologically protected properties of materials and to manipulate the topology at high speeds. Here we use crystallographic measurements by relativistic electron diffraction to demonstrate that terahertz light pulses can be used to induce terahertz frequency interlayer shear strain with large strain amplitude in the Weyl semimetal WTe2, leading to a topologically distinct metastable phase. Separate nonlinear optical measurements indicate that this transition is associated with a symmetry change to a centrosymmetric, topologically trivial phase. We further show that such shear strain provides an ultrafast, energy efficient way of inducing robust, well separated Weyl points or of annihilating all Weyl points of opposite chirality. This work demonstrates possibilities for ultrafast manipulation of the topological properties of solids and for the development of a topological switch operating at terahertz frequencies.Terahertz light pulses induce transitions between a topological and a trivial phase in the Weyl semimetal WTe2 through an interlayer shear strain.

title: Zero Field Nernst Effect in a Ferromagnetic Kagome Lattice Weyl Semimetal Co3 Sn2 S2.
abstract: The discovery of magnetic topological semimetals has recently attracted significant attention in the field of topology and thermoelectrics. In a thermoelectric device based on the Nernst geometry, an external magnet is required as an integral part. Reported is a zero field Nernst effect in a newly discovered hard ferromagnetic kagome lattice Weyl semimetal Co3 Sn2 S2 A maximum Nernst thermopower of 3 uV K 1 at 80 K in zero field is achieved in this magnetic Weyl semimetal. The results demonstrate the possibility of application of topological hard magnetic semimetals for low power thermoelectric devices based on the Nernst effect and are thus valuable for the comprehensive understanding of transport properties in this class of materials.

title: Colossal Bulk Photovoltaic Effect in a Weyl Semimetal
abstract: Broadband, efficient and fast conversion of light to electricity is crucial for sensing and clean energy. Here we reveal the largest observed bulk photo voltaic effect (BPVE) an intrinsic mechanism predicted to be ultrafast and exceed the Shockley Quiesser limit. This discovery results from combining recent developments in the connection of BPVE to topology, Weyl semimetals and focused ion beam fabrication. Our room temperature observation of the first BPVE in the mid IR, is enabled by microscopic devices of the Weyl semimetal TaAs. Detailed symmetry analysis enables unambiguous separation of this response from competing photothermal effects. The size and wavelength range of the shift current offers new opportunities in optical detectors, clean energy, and topology, while directly demonstrating the utility of Weyl semimetals for applications.

title: Experimental discovery of Weyl semimetal TaAs
abstract: Weyl semimetals are a class of materials that can be regarded as three dimensional analogs of graphene breaking time reversal or inversion symmetry. Electrons in a Weyl semimetal behave as Weyl fermions, which have many exotic properties, such as chiral anomaly and magnetic monopoles in the crystal momentum space. The surface state of a Weyl semimetal displays pairs of entangled Fermi arcs at two opposite surfaces. However, the existence of Weyl semimetals has not yet been proved experimentally. Here we report the experimental realization of a Weyl semimetal in TaAs by observing Fermi arcs formed by its surface states using angle resolved photoemission spectroscopy. Our first principles calculations, matching remarkably well with the experimental results, further confirm that TaAs is a Weyl semimetal.