A Weyl semimetal (WSM) is a novel topological phase of matter [1][2][3][4][5][6][7][8][9][10][11][12][13], in which Weyl fermions (WFs) arise as pseudo-magnetic monopoles in its momentum space. The chirality of the WFs, given by the sign of the monopole charge, is central to the Weyl physics, since it directly serves as the sign of the topological number [5,12] and gives rise to exotic properties such as Fermi arcs [5,9,11] and the chiral anomaly [12][13][14][15][16]. Despite being the defining property of a WSM, the chirality of the WFs has never been experimentally measured. Here, we directly detect the chirality of the WFs by measuring the photocurrent in response to circularly polarized mid-infrared light. The resulting photocurrent is determined by both the chirality of WFs and that of the photons. Our results pave the way for realizing a wide range of theoretical proposals [12,13,[17][18][19][20][21][22][23][24][25][26] for studying and controlling the WFs and their associated quantum anomalies by optical and electrical means. More broadly, the two chiralities, analogous to the two valleys in 2D materials [27,28], lead to a new degree of freedom in a 3D crystal with potential novel pathways to store and carry information.infrared pump and a soft X-ray probe, which is technically very challenging. On the other hand, optical experiments on WSMs have remained very limited [36,37], although they are promising approaches to achieve these goals [12]. In this paper, we detect the chirality of the WFs in the WSM TaAs by measuring its mid-infrared photocurrent response. Circularly polarized light induced photocurrents, also called the circular photogalvanic effect (CPGE), have been previously measured in other systems [29-31] but have not been experimentally studied in WSMs.We first discuss the theoretical picture of the CPGE for optical transitions from the lower part of the Weyl cone to the upper part [17]. There are two independent factors important for the CPGE here. The first is the chirality selection rule (Figs. 1c,d). For a right circularly polarized (RCP) light propagating along +ẑ and a χ = +1 WF, the optical transition is allowed on the +k z side but forbidden on the −k z side [17]. The second is the Pauli blockade, which is only present when chemical potential is away from the Weyl node. FIG. 4: Detection and manipulation of chiral Weyl fermions by optical means. a, Our calculations (see SI.II.3) showĴ THY = χ W 1klight ×ĉ. b, By measuring the direction of the currentJ and knowing the polarization and propagation direction of the light, we obtainĴ EXP =k light ×ĉ from data. By comparing theory and data, we obtain χ W 1 = +1. The χ W 1 = +1 determined by the photocurrent agrees with that predicted by first-principles ( Fig. 1j), further confirming our experimental detection of WF chirality. c-e, Comparison between the chirality degree of freedom of the WFs and valley degree of freedom in gapped Dirac system. c, In a gapped Dirac system, an optical excitation with a particular handedness can only populate ...

Based on the ab initio calculations, we show that MoTe2, in its low-temperature orthorhombic structure characterized by an X-ray diffraction study at 100 K, realizes 4 type-II Weyl points between the N -th and N +1-th bands, where N is the total number of valence electrons per unit cell. Other WPs and nodal lines between different other bands also appear close to the Fermi level due to a complex topological band structure. We predict a series of strain-driven topological phase transitions in this compound, opening a wide range of possible experimental realizations of different topological semimetal phases. Crucially, with no strain, the number of observable surface Fermi arcs in this material is 2 -the smallest number of arcs consistent with time-reversal symmetry.

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We report the synthesis and crystal structure of a new high-temperature form of Ca3P2. The crystal structure was determined through Rietveld refinements of synchrotron powder x-ray diffraction data. This form of Ca3P2 has a crystal structure of the hexagonal Mn5Si3 type, with a Ca ion deficiency compared to the ideal 5:3 stoichiometry. This yields a stable, charge-balanced compound of Ca2+ and P3−. We also report the observation of a secondary hydride phase, Ca5P3H, which again is a charge-balanced compound. The calculated band structure of Ca3P2 indicates that it is a three-dimensional Dirac semimetal with a highly unusual ring of Dirac nodes at the Fermi level. The Dirac states are protected against gap opening by a mirror plane in a manner analogous to what is seen for graphene.

Nodal‐line semimetals (NLSs) represent a new type of topological semimetallic phase beyond Weyl and Dirac semimetals in the sense that they host closed loops or open curves of band degeneracies in the Brillouin zone. Parallel to the classification of type‐I and type‐II Weyl semimetals, there are two types of NLSs. The type‐I NLS phase has been proposed and realized in many compounds, whereas the exotic type‐II NLS phase that strongly violates Lorentz symmetry has remained elusive. First‐principles calculations show that Mg 3 Bi 2 is a material candidate for the type‐II NLS. The band crossing is close to the Fermi level and exhibits the type‐II nature of the nodal line in this material. Spin–orbit coupling generates only a small energy gap (≈35 meV) at the nodal points and does not negate the band dispersion of Mg 3 Bi 2 that yields the type‐II nodal line. Based on this prediction, Mg 3 Bi 2 single crystals are synthesized and the presence of the type‐II nodal lines in the material is confirmed. The angle‐resolved photoemission spectroscopy measurements agree well with the first‐principles results below the Fermi level and thus strongly suggest Mg 3 Bi 2 as an ideal material platform for studying the as‐yet unstudied properties of type‐II nodal‐line semimetals.

We present transition metal-embedded (T@Ga n ) endohedral Gaclusters as a favorable structural motif for superconductivity and develop empirical, molecule-based, electron counting rules that govern the hierarchical architectures that the clusters assume in binary phases. Among the binary T@Ga n endohedral cluster systems, Mo 8 Ga 41 , Mo 6 Ga 31 , Rh 2 Ga 9 , and Ir 2 Ga 9 are all previously known superconductors. The well-known exotic superconductor PuCoGa 5 and related phases are also members of this endohedral gallide cluster family. We show that electron-deficient compounds like Mo 8 Ga 41 prefer architectures with vertex-sharing gallium clusters, whereas electron-rich compounds, like PdGa 5 , prefer edge-sharing cluster architectures. The superconducting transition temperatures are highest for the electron-poor, corner-sharing architectures. Based on this analysis, the previously unknown endohedral cluster compound ReGa 5 is postulated to exist at an intermediate electron count and a mix of corner sharing and edge sharing cluster architectures. The empirical prediction is shown to be correct and leads to the discovery of superconductivity in ReGa 5 . The Fermi levels for endohedral gallide cluster compounds are located in deep pseudogaps in the electronic densities of states, an important factor in determining their chemical stability, while at the same time limiting their superconducting transition temperatures. Although one can analyze the superconductivity, once discovered, through materials physics-based "k-space" pictures based on Fermi surfaces, energy band dispersions, and effective interactions, often it is chemists, whose viewpoint is instead from "real space" rather than k-space, who find such superconductors in the first place (1, 2). Given the difficulty in making extrapolations between the physics of superconductivity and the chemical stability of compounds that will be superconducting, there are as many strategies for finding new superconductors as there are researchers looking for them (3-5). Most such search strategies fail, because the interactions that give rise to superconductivity can also lead to competing electronic states or can be strong enough to tear potential compounds apart (6, 7).One chemical perspective for increasing the odds of finding superconductivity is to postulate that it runs in structural families. The perovskites are a well-known example of this in metal oxides, and in intermetallic compounds, the "122" ThCr 2 Si 2 structure type is a good example (8-10). It is the discovery of these new structural families of superconductors that often leads, sometimes slowly or sometimes quickly, to advances in new superconducting materials. Here we show that a previously unappreciated chemical family, the endohedral gallium cluster phases, is a favored chemical family for superconductivity. Further, we analyze the occurrence and hierarchical structures of such phases from a molecular perspective and then use that perspective to predict the existence and structure of a previously un...

Two superconductors with a new chiral noncentrosymmetric crystal structure and Hc2 values above the Pauli limit.

We report a reinvestigation of superconducting Sn1−xInxTe at both low and high In doping levels. Considering the system over a broad composition range in a single study allows us to characterize a significant change in the properties as a function of x : the system evolves from a weakly coupled p-type superconductor to a strongly coupled n-type superconductor with increasing indium content. Hall Effect measurements show that the carrier density does not vary monotonically with Indium content; a change from p-type to n-type is observed near 10% In-doping. This is contrary to expectations dictating that In should be a p-type dopant in semiconducting SnTe because it has one less valance electron than Sn. A crystallographic search for point defects at high x indicates that the material remains ideal NaCl-type over a wide composition range. Density functional theory calculations for In-doped SnTe support a picture where In does not act as a trivial hole dopant, but instead forms a distinct, partly filled In 5s -Te 5p hybridized state centered around EF , which is very different from what is seen for other nominal hole dopants such as Na, Ag, and vacant Sn sites.