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We report an ultrafast lattice dynamics investigation of the topological Dirac semimetal Cd3As2. A coherent phonon beating among three evenly spaced A1g optical phonon modes (of frequencies 1.80, 1.96, and 2.11 THz, respectively) is unambiguously observed. The two side modes originate from the counter helixes composing Cd vacancies. Significantly, such helix vacancy-induced phonon (HVP) modes experience prominent extra waning in their ultrafast dynamics as temperature increases, which is immune to the central mode. Above 200 K, the HVP becomes inactive, which may potentially affect the topological properties. Our results in the lattice degree of freedom suggest the indispensable role of temperature in considering topological properties of such quantum materials. © 2017 American Physical Society.

Two-dimensional topological insulators with a large bulk band gap are promising for experimental studies of quantum spin Hall effect and for spintronic device applications. Despite considerable theoretical efforts in predicting large-gap two-dimensional topological insulator candidates, none of them have been experimentally demonstrated to have a full gap, which is crucial for quantum spin Hall effect. Here, by combining scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy, we reveal that ZrTe5 crystal hosts a large full gap of ~100 meV on the surface and a nearly constant density of states within the entire gap at the monolayer step edge. These features are well reproduced by our first-principles calculations, which point to the topologically nontrivial nature of the edge states.

In 1929, H. Weyl proposed that the massless solution of the Dirac equation represents a pair of a new type of particles, the so-called Weyl fermions. However, their existence in particle physics remains elusive after more than eight decades. Recently, significant advances in both topological insulators and topological semimetals have provided an alternative way to realize Weyl fermions in condensed matter, as an emergent phenomenon: when two non-degenerate bands in the three-dimensional momentum space cross in the vicinity of the Fermi energy (called Weyl nodes), the low-energy excitations behave exactly as Weyl fermions. Here we report the direct observation in TaAs of the long-sought-after Weyl nodes by performing bulk-sensitive soft X-ray angle-resolved photoemission spectroscopy measurements. The projected locations at the nodes on the (001) surface match well to the Fermi arcs, providing undisputable experimental evidence for the existence of Weyl fermionic quasiparticles in TaAs. © 2015 Macmillan Publishers Limited. All rights reserved.

We demonstrate, using first-principles calculations, that the electronic structure of FeSe1-xTex(x=0.5) is topologically nontrivial and characterized by an odd Z2 invariant and Dirac cone type surface states, in sharp contrast to the end member FeSe (x=0). This topological state is induced by the enhanced three-dimensionality and spin-orbit coupling due to Te substitution (compared to FeSe), and characterized by a band inversion at the Z point of the Brillouin zone, which is confirmed by our ARPES measurements. The results suggest that the surface of FeSe0.5Te0.5 may support a nontrivial superconducting channel in proximity to the bulk. © 2015 American Physical Society.

We have investigated the spin texture of surface Fermi arcs in the recently discovered Weyl semimetal TaAs using spin- and angle-resolved photoemission spectroscopy. The experimental results demonstrate that the Fermi arcs are spin polarized. The measured spin texture fulfills the requirement of mirror and time-reversal symmetries and is well reproduced by our first-principles calculations, which gives strong evidence for the topologically nontrivial Weyl semimetal state in TaAs. The consistency between the experimental and calculated results further confirms the distribution of chirality of the Weyl nodes determined by first-principles calculations. © 2015 American Physical Society.

We used angle-resolved photoemission spectroscopy to reveal directly the momentum distribution of the superconducting gap in FeTe 1-xSe x, which has the simplest structure of all Fe-based superconductors. We found isotropic superconducting gaps on all Fermi surfaces whose sizes can be fitted by a single gap function derived from a strong coupling approach, promoting local antiferromagnetic exchange interactions as a serious candidate for the pairing origin. © 2012 American Physical Society.

In order to determine the orbital characters on the various Fermi surface pockets of the Fe-based superconductors Ba 0.6K 0.4Fe 2As 2 and FeSe 0.45Te 0.55, we introduce a method to calculate photoemission matrix elements. We compare our simulations to experimental data obtained with various experimental configurations of beam orientation and light polarization. We show that the photoemission intensity patterns revealed from angle-resolved photoemission spectroscopy measurements of Fermi surface mappings and energy-momentum plots along high-symmetry lines exhibit asymmetries carrying precious information on the nature of the states probed, information that is destroyed after the data symmetrization process often performed in the analysis of angle-resolved photoemission spectroscopy data. Our simulations are consistent with Fermi surfaces originating mainly from the d xy, d xz, and d yz orbitals in these materials. © 2012 American Physical Society.

We have performed an angle-resolved photoemission spectroscopy study of the new iron-based superconductor K0.8Fe1.7Se2 (Tc∼30K). Clear band dispersion is observed with the overall bandwidth renormalized by a factor of 2.5 compared to our local density approximation calculations, indicating relatively strong correlation effects. Only an electronlike band crosses the Fermi energy, forming a nearly circular Fermi surface (FS) at M (π, 0). The holelike band at Γ sinks ∼90meV below the Fermi energy, with an indirect band gap of 30 meV, to the bottom of the electronlike band. The observed FS topology in this superconductor favors (π, π) inter-FS scattering between the electronlike FSs at the M points, in sharp contrast to other iron-based superconductors which favor (π, 0) inter-FS scattering between holelike and electronlike FSs. © 2011 American Physical Society.

We have performed a systematic photoemission study of the chemical-potential shift as a function of carrier doping in a pnictide system based on BaFe2As2. The experimentally determined chemical-potential shift is consistent with the prediction of a rigid band shift picture by renormalized first-principle band calculations. This leads to an electron-hole asymmetry (EHA) in the Fermi surface (FS) nesting condition due to different effective masses for different FS sheets, which can be calculated from the Lindhard function of susceptibility. This built-in EHA, which matches well the observed asymmetric superconducting domes in the phase diagram, strongly supports FS quasinesting driven superconductivity in iron pnictides. © 2011 American Physical Society.

La mine Beaufor se situe dans la partie Sud-Est de la ceinture de roches vertes de l'Abitibi et est contenu dans le Batholite de Bourlamaque. Le gisement est actuellement limité au Nord par la faille Perron et au Sud par la faille Sud. Le contact Est correspond au contact lithologique entre le Batholite de Bourlamaque et les roches volcaniques de la Formation de Dubuisson. La mine Beaufor exploite une minéralisation aurifère de type filonien orogénique. Les veines aurifères, composées de quartz, carbonate, tourmaline et pyrite, sont encaissées dans des corridors altérés en chlorite et silice du Batholite de Bourlamaque. Une association spatiale existe entre ces veines et des dykes de diorite. Cinq failles d'importance sont connues à la mine Beaufor. Il s'agit des failles Beaufor, Perron, Centrale, Sud et Cisaillement Ouest. Ces failles sont grossièrement de direction Est- Ouest. Les failles Beaufor et Perron s'inclinent à 80° vers le Nord alors que les failles Centrale, Sud et Cisaillement Ouest s'inclinent généralement à 60-70° vers le Sud. De nombreuses failles de direction E-W, NE-SW, NO-SE et N-S affectent le gisement. Finalement, de nombreuses failles associées à des couloirs de déformation ductile d'orientation N070, présentant généralement un pendage abrupt vers le SE, recoupent l'ensemble du gisement. Toutes les unités lithologiques, à l'exception de dykes aplitiques, sont affectées par une fabrique d'orientation similaire aux couloirs de déformation ductile d'orientation N070. La présence de cette fabrique s'intensifie à l'approche des zones de faille. La foliation dans l'intrusif est définie par un allongement des plagioclases et de la chlorite ainsi que par la recristallisation du quartz dans les zones intensément déformées. La mine Beaufor se trouve dans un environnement relativement homogène, soit entièrement encaissée dans le Batholite de Bourlamaque. L'objectif du projet de recherche est de déterminer quelles sont les failles qui ont régi la mise en p

We performed an angle-resolved photoemission spectroscopy study of BaFe2As2, which is the parent compound of the so-called 122 phase of the iron-pnictide high-temperature superconductors. We reveal the existence of a Dirac cone in the electronic structure of this material below the spin-density-wave temperature, which is responsible for small spots of high photoemission intensity at the Fermi level. Our analysis suggests that the cone is slightly anisotropic and its apex is located very near the Fermi level, leading to tiny Fermi surface pockets. The bands forming the cone show an anisotropic leading edge gap away from the cone that suggests a nodal spin-density-wave description. © 2010 The American Physical Society.

We observed a novel orbital selective Mott transition in Ca1.8Sr0.2RuO4 by angle-resolved photoemission. While two sets of dispersing bands and the Fermi surface associated with the doubly degenerate dyz and dzx orbitals are identified, the Fermi surface associated with the wider dxy band is missing as a consequence of selective Mott localization. Our theoretical calculations demonstrate that this orbital selective Mott transition is mainly driven by the combined effects of interorbital carrier transfer, superlattice potential, and orbital degeneracy, whereas the bandwidth difference plays a less important role. © 2009 The American Physical Society.

We have performed a high-resolution angle-resolved photoelectron spectroscopy study on the newly discovered superconductor Ba0.6K 0.4Fe2As2 (Tc=37 K). We have observed two superconducting gaps with different values: a large gap (Δ∼12 meV) on the two small hole-like and electron-like Fermi surface (FS) sheets, and a small gap (∼6 meV) on the large hole-like FS. Both gaps, closing simultaneously at the bulk transition temperature (Tc), are nodeless and nearly isotropic around their respective FS sheets. The isotropic pairing interactions are strongly orbital dependent, as the ratio 2Δ/kBTc switches from weak to strong coupling on different bands. The same and surprisingly large superconducting gap due to strong pairing on the two small FSs, which are connected by the (π, 0) spin-density-wave vector in the parent compound, strongly suggests that the pairing mechanism originates from the inter-band interactions between these two nested FS sheets. © 2008 Europhysics Letters Association.