L. X. Yang

Nodeless superconducting gap in AxFe2Se2 (A=K,Cs) revealed by angle-resolved photoemission spectroscopy

Pairing symmetry is a fundamental property that characterizes a superconductor. For the iron-based high-temperature superconductors, an s(±)-wave pairing symmetry has received increasing experimental and theoretical support. More specifically, the superconducting order parameter is an isotropic s-wave type around a particular Fermi surface, but it has opposite signs between the hole Fermi surfaces at the zone centre and the electron Fermi surfaces at the zone corners. Here we report the low-energy electronic structure of the newly discovered superconductors, A(x)Fe(2)Se(2) (A=K,Cs) with a superconducting transition temperature (Tc) of about 30 K. We found A(x)Fe(2)Se(2) (A=K,Cs) is the most heavily electron-doped among all iron-based superconductors. Large electron Fermi surfaces are observed around the zone corners, with an almost isotropic superconducting gap of ~10.3 meV, whereas there is no hole Fermi surface near the zone centre, which demonstrates that interband scattering or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. Thus, the sign change in the s(±) pairing symmetry driven by the interband scattering as suggested in many weak coupling theories becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is probably a better description.

Evolution of the Fermi surface of Weyl semimetals in the transition metal pnictide family

Topological Weyl semimetals (TWSs) represent a novel state of topological quantum matter which not only possesses Weyl fermions (massless chiral particles that can be viewed as magnetic monopoles in momentum space) in the bulk and unique Fermi arcs generated by topological surface states, but also exhibits appealing physical properties such as extremely large magnetoresistance and ultra-high carrier mobility. Here, by performing angle-resolved photoemission spectroscopy (ARPES) on NbP and TaP, we directly observed their band structures with characteristic Fermi arcs of TWSs. Furthermore, by systematically investigating NbP, TaP and TaAs from the same transition metal monopnictide family, we discovered their Fermiology evolution with spin-orbit coupling (SOC) strength. Our experimental findings not only reveal the mechanism to realize and fine-tune the electronic structures of TWSs, but also provide a rich material base for exploring many exotic physical phenomena (for example, chiral magnetic effects, negative magnetoresistance, and the quantum anomalous Hall effect) and novel future applications.

Electronic Identification of the Parental Phases and Mesoscopic Phase Separation of KxFe2-ySe2 Superconductors

F. Chen, 1 M. Xu,1 Q. Q. Ge, 1 Y. Zhang, 1, ∗ Z. R. Ye,1 L. X. Yang,1 Juan Jiang, 1 B. P. Xie,1 R. C. Che, 2 M. Zhang, 3 A. F. Wang, 3 X. H. Chen, 3 D. W. Shen, 4 X. M. Xie,4 M. H. Jiang, 4 J. P. Hu, 5 and D. L. Feng1, † 1State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People’s Republic of China 2Department of Materials Science, and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People’s Republic of China 3Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China 4State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 20005 5Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA

Electronic-Structure-Driven Magnetic and Structure Transitions in Superconducting NaFeAs Single Crystals Measured by Angle-Resolved Photoemission Spectroscopy

C. He, Y. Zhang, B. P. Xie, X. F. Wang, L. X. Yang, B. Zhou , F. Chen, M. Arita, K. Shimada, H. Namatame, M. Taniguchi, X. H. Chen, J. P. Hu and D. L. Feng Department of Physics, Surface Physics Laboratory (National Key Laboratory), and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People’s Republic of China 2 Department of Physics, University of science and technology of China, Hefei, Anhui 230027, People’s Republic of China Hiroshima Synchrotron Radiation Center and Graduate School of Science, Hiroshima University, Hiroshima 739-8526, Japan. and 4 Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA (Dated: January 18, 2010)

Novel Mechanism of a Charge Density Wave in a Transition Metal Dichalcogenide

The charge density wave (CDW) is usually associated with Fermi surfaces nesting. We here report a new CDW mechanism discovered in a 2H-structured transition metal dichalcogenide, where the two essential ingredients of the CDW are realized in very anomalous ways due to the strong-coupling nature of the electronic structure. Namely, the CDW gap is only partially open, and charge density wave vector match is fulfilled through participation of states of the large Fermi patch, while the straight Fermi surface sections have secondary or negligible contributions.

Evolution of the electronic structure of 1T-Cu(x)TiSe(2).

The electronic structure of a new charge-density-wave system or superconductor, 1T-Cu(x)TiSe(2), has been studied by photoemission spectroscopy. A correlated semiconductor band structure is revealed for the undoped case, which resolves a long-standing controversy in the system. With Cu doping, the charge-density wave is suppressed by the raising of the chemical potential, while the superconductivity is enhanced by the enhancement of the density of states, and possibly suppressed at higher doping by the strong scattering.

Observation of unusual topological surface states in half-Heusler compounds LnPtBi (Ln=Lu, Y)

Topological quantum materials represent a new class of matter with both exotic physical phenomena and novel application potentials. Many Heusler compounds, which exhibit rich emergent properties such as unusual magnetism, superconductivity and heavy fermion behaviour, have been predicted to host non-trivial topological electronic structures. The coexistence of topological order and other unusual properties makes Heusler materials ideal platform to search for new topological quantum phases (such as quantum anomalous Hall insulator and topological superconductor). By carrying out angle-resolved photoemission spectroscopy and ab initio calculations on rare-earth half-Heusler compounds LnPtBi (Ln=Lu, Y), we directly observe the unusual topological surface states on these materials, establishing them as first members with non-trivial topological electronic structure in this class of materials. Moreover, as LnPtBi compounds are non-centrosymmetric superconductors, our discovery further highlights them as promising candidates of topological superconductors.

Superconducting coherence peak in the electronic excitations of a single-layer Bi2Sr1.6La0.4CuO6+delta cuprate superconductor.

J. Wei, Y. Zhang, H. W. Ou, B. P. Xie, D. W. Shen, J. F. Zhao, L. X. Yang, M. Arita, K. Shimada, H. Namatame, M. Taniguchi, Y. Yoshida, H. Eisaki, and D. L. Feng ∗ Department of Physics, Surface Physics Laboratory (National Key Laboratory) and Advanced Materials Laboratory, Fudan University, Shanghai 200433, P. R. China Hiroshima Synchrotron Radiation Center and Graduate School of Science, Hiroshima University, Hiroshima 739-8526, Japan National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, 3058568, Japan (Dated: February 2, 2008)

Surface and bulk electronic structures of LaOFeAs studied by angle resolved photoemission spectroscopy

The electronic structure of LaOFeAs, a parent compound of iron-arsenic superconductors, is studied by angleresolved photoemission spectroscopy. By examining its dependence on photon energy, polarization, sodium dosing and the counting of Fermi surface volume, both the bulk and the surface contributions are identified. We find that a bulk band moves toward high binding energies below structural transition, and shifts smoothly across the spin density wave transition by about 25 meV. Our data suggest the band reconstruction may play a crucial role in the spin density wave transition, and the structural transition is driven by the short range magnetic order. For the surface states, both the LaO-terminated and FeAs-terminated components are revealed. Certain small band shifts are verified for the FeAs-terminated surface states in the spin density wave state, which is a reflection of the bulk electronic structure reconstruction. Moreover, sharp quasiparticle peaks quickly rise at low temperatures, indicating of drastic reduction of the scattering rate. A kink structure in one of the surface band is shown to be possibly related to the electron-phonon interactions.

Surface and bulk electronic structures of LaFeAsO studied by angle-resolved photoemission spectroscopy

Chang Liu, 2 Yongbin Lee, A. D. Palczewski, 2 J. -Q. Yan, Takeshi Kondo, 2 B. N. Harmon, 2 R. W. McCallum, 3 T. A. Lograsso, and A. Kaminski 2 Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA (Dated: June 7, 2010)