A Dirac fermion in a topological Dirac semimetal is a quadruple-degenerate quasi-particle state with a relativistic linear dispersion. Breaking either time-reversal or inversion symmetry turns this system into a Weyl semimetal that hosts double-degenerate Weyl fermion states with opposite chiralities. These two kinds of quasi-particles, although described by a relativistic Dirac equation, do not necessarily obey Lorentz invariance, allowing the existence of so-called type-II fermions. Recent theoretical discovery of type-II Weyl fermions evokes the prediction of type-II Dirac fermions in PtSe 2 -type transition metal dichalcogenides, expecting an experimental confirmation. Here, we report an experimental realization of type-II Dirac fermions in PdTe 2 by angle-resolved photoemission spectroscopy combined with ab-initio band calculations. Our experimental finding makes the first example that has both superconductivity and type-II Dirac fermions, which turns the topological material research into a new phase.

An explicit connection between the electronic structure and the anisotropic high conductivity of delafossite-type PdCoO2 has been established by angle-resolved photoemission spectroscopy (ARPES) and core-level x-ray photoemission spectroscopy. The ARPES spectra show that a large hexagonal electronlike Fermi surface (FS) consists of very dispersive Pd 4d states. The carrier velocity and lifetime are determined from the ARPES data, and the conductivity is calculated by a solution of the Boltzmann equation, which demonstrates that the high anisotropic conductivity originates from the high carrier velocity, the large two-dimensional FS, and the long lifetime of the carriers.

We report the first case of the successful measurements of a localized spin antiferromagnetic transition in delafossite-type PdCrO2 by angle-resolved photoemission spectroscopy (ARPES). This demonstrates how to circumvent the shortcomings of ARPES for investigation of magnetism involved with localized spins in limited size of two-dimensional crystals or multi-layer thin films that neutron scattering can hardly study due to lack of bulk compared to surface. Also, our observations give direct evidence for the spin ordering pattern of Cr3+ ions in PdCrO2 suggested by neutron diffraction and quantum oscillation measurements, and provide a strong constraint that has to be satisfied by a microscopic mechanism for the unconventional anomalous Hall effect recently reported in this system.

No abstract

Brief Reports are accounts of completed research which, while meeting the usual Physical Review standards of scientific quality, do not warrant regular articles. A Brief Report may be no longer than four printed pages and must be accompanied by an abstract. The same publication schedule as for regular articles is followed, and page proofs are sent to authors.The high-binding-energy structure in the Eu 3d core-level x-ray photoelectron spectra of divalent Eu compounds, which has been ascribed as the "shakeup" satellite, is shown to be due to the multiplet structure of 3d4f i (bar denotes a hole) final-state configuration. Hence, the apparent inconsistency between this so-called high-energy probe and low-energy probes for the valence of these Eu compounds is resolved.Rare-earth metals and their compounds have been the subject of extensive study because of the valence Auctuation phenomena' and heavy fermion properties, which originate from the interaction between highly correlated 4f electrons and delocalized conduction electrons. Corelevel x-ray photoelectron spectroscopy (XPS) has been very valuable in studying electronic structures of these compounds. Core-level spectra can be usually understood by the following Anderson impurity Hamiltonian including the Coulomb interaction between the core hole and 4f electrons: ' XH = g f Ftp, +"tp"de+[a& -U&, (1 n, )]q&+ -pt v= 1 + f [V(e)tp" tp"+ V(e)*tp, +, tp, ]dE + UJJ g~n~n E+~n P)V where e describes the energy of the conduction-band state, e& is the energy of the 4f level, and E, describes the energy of a core level. The 4f level has N&-fold degeneracy, whose quantum numbers are denoted by p, v. The hopping between the 4f level and the conduction state is described by V(e) and the Coulomb interaction between 4f electrons is given by UI&, where in the simple model multiplet effects are neglected. U&, is the Coulomb interaction between a core hole and an f electron. Thus while the~f")and~f" + ' ) states are nearly degenerate in energy in the initial state, they have an energy separation of U&, in the final state of photoemission, because the energy of the~f ") state is pulled down by the amount n U&, (n is the number of f electrons) by the core hole created in photoionization.This gives rise to the satellite structure in the deep core-level XPS spectra, whose intensity depends on the strength of V(e) and the energy position of e&. Therefore, by analyzing deep core-level XPS spectra, we can get information of the valence electronic structure. For this purpose 3d core-level spectra usually used because 4d core-level XPS shows very complicated multiplet structures due to the strong interaction between the 4d hole and unfilled 4f shell (they have the same principle quantum number n =4). Many Ce compounds and several Yb compounds have been studied by the core-level XPS technique, and they are shown to be consistent with physical properties measured by so-called low-energy probe techniques such as Mossbauer isomer shift and the magnetic susceptibility. ' However, for some Eu co...

No abstract

PACS 73.20.-r -Electron states at surfaces and interfaces PACS 79.60.-i -Photoemission and photoelectron spectra PACS 71.20.-b -Electron density of states and band structure of crystalline solidsAbstract -We have experimentally demonstrated the robustness and immunity of the surface states to surface impurities and disorder in topological insulator (TI) Bi0.9Sb0.1 in comparison with those of semi-metallic Sb(111) surface by measuring the scattering rates of the quasiparticles (QP) via angle-resolved photoemission spectroscopy. The surface impurities on TIs increase the scattering rate of the QPs but their effects are extrinsic and limited regardless of their magnetic properties, while in the semimetallic Sb case they open new scattering channels making the surface states fragile.