High-temperature superconductivity in cuprates emerges out of a highly enigmatic 'pseudogap' metal phase. The mechanism of high-temperature superconductivity is probably encrypted in the elusive relationship between the two phases, which spectroscopically is manifested as Fermi arcs-disconnected segments of zero-energy states-collapsing into d-wave point nodes upon entering the superconducting phase. Here, we reproduce this distinct cuprate phenomenology in the 5d transition-metal oxide Sr 2 IrO 4 . Using angle-resolved photoemission, we show that the clean, low-temperature phase of 6-8% electron-doped Sr 2 IrO 4 has gapless excitations only at four isolated points in the Brillouin zone, with a predominant d-wave symmetry of the gap. Our work thus establishes a connection between the low-temperature d-wave instability and the previously reported high-temperature Fermi arcs in electron-doped Sr 2 IrO 4 (ref. 1). Although the physical origin of the d-wave gap remains to be understood, Sr 2 IrO 4 is the first non-cuprate material to spectroscopically reproduce the complete phenomenology of the cuprates, thus o ering a new material platform to investigate the relationship between the pseudogap and the d-wave gap.Sr 2 IrO 4 is a single-electron-band magnetic insulator with pseudospin-1/2 moments 2,3 on a square lattice 4 . Despite very strong spin-orbit coupling inherent to 5d transition-metal elements, magnetic interactions between iridium pseudospins are predominantly of Heisenberg type 5 , with a large energy scale 6,7 as reflected in their magnon bandwidth of ∼200 meV. This one-to-one correspondence between Sr 2 IrO 4 and high-temperature superconducting (HTSC) cuprates in their lattice, electronic and magnetic structures allows us to present, in Fig. 1a,b, angle-resolved photoemission (ARPES) intensity maps of approximately 7% electron-doped Sr 2 IrO 4 recorded at the Fermi level following the standard notations used in the cuprate literature. A sign difference between the two systems in one of the tight-binding parameters 8 characterizing the hopping of an electron in a quasi-two-dimensional (2D) square lattice renders Fermi surfaces shifted by (π, π) with respect to each other in their non-interacting electron descriptions. This sign of the next-nearest hopping can be reversed by electron-hole conjugation, which means that our results on electron-doped Sr 2 IrO 4 can be directly compared to those of hole-doped cuprates 9 .

High quality WSe2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe2/BLG. We observe that a bilayer of WSe2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct-indirect band gap crossover to trilayer WSe2. In the monolayer limit, WSe2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observed among all the MX2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe2/BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional three-dimensional semiconductors, yet small as compared to other two-dimensional transition metal dichalcogennides (TMDCs) semiconductors. Finally, our finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.

We report a molecular beam epitaxial growth of Na3Bi single-crystal thin films on two different substrates—epitaxial bilayer graphene terminated 6H-SiC(0001) and Si(111). Using reflection high-energy electron diffraction, we found that the lattice orientation of the grown Na3Bi thin film was rotated by 30° respect to the surface lattice orientations of these two substrates. An in-situ angle-resolved photoemission spectroscopy clearly revealed the 3-dimensional Dirac-cone band structure in such thin films. Our approach of growing Na3Bi thin film provides a potential route for further studying its intriguing electronic properties and for fabricating it into practical devices in future.

We performed angle resolved photoemission (ARPES) studies on Cu(111) and Au(111) surface states with circularly polarized light. Existence of local orbital angular momentum (OAM) is confirmed as has been predicted to be broadly present in a system with an inversion symmetry breaking (ISB). The single band of Cu(111) surface states is found to have chiral OAM in spite of very small spin-orbit coupling (SOC) in Cu, which is consistent with theoretical prediction. As for Au(111), we observe split bands for which OAM for the inner and outer bands are parallel, unlike the Bi2Se3 case. We also performed first principles calculation and the results are found to be consistent with the experimental results. Moreover, majority of OAM is found to be from d-orbitals and a small contribution has p-orbital origin which is anti-aligned to the spins. We derive an effective Hamiltonian that incorporates the role of OAM and used it to extract the OAM and spin structures of surface states with various SOC strength. We discuss the evolution of angular momentum structures from pure OAM case to a strongly spin-orbit entangled state. We predict that the transition occurs through reversal of OAM direction at a k-point in the inner band if the system has a proper SOC strength.PACS numbers: 71.15.Mb