G. D. Gu

Electronic Structure of the Topological Insulator Bi2Se3 Using Angle-Resolved Photoemission Spectroscopy: Evidence for a Nearly Full Surface Spin Polarization

We performed high-resolution spin- and angle-resolved photoemission spectroscopy studies of the electronic structure and the spin texture on the surface of Bi2Se3, a model TI. By tuning the photon energy, we found that the topological surface state is well separated from the bulk states in the vicinity of kz = Z plane of the bulk Brillouin zone. The spin-resolved measurements in that region indicate a very high degree of spin polarization of the surface state, ~0.75, much higher than previously reported. Our results demonstrate that the topological surface state on Bi2Se3 is highly spin polarized and that the dominant factors limiting the polarization are mainly extrinsic.

Identification of a new form of electron coupling in the Bi2Sr2CaCu2O8 superconductor by laser-based angle-resolved photoemission spectroscopy.

Laser-based angle-resolved photoemission measurements with superhigh resolution have been carried out on an optimally doped Bi(2)Sr(2)CaCu(2)O(8) high temperature superconductor. New high energy features at approximately 115 meV and approximately 150 meV, in addition to the prominent approximately 70 meV one, are found to develop in the nodal electron self-energy in the superconducting state. These high energy features, which cannot be attributed to electron coupling with single phonon or magnetic resonance mode, point to the existence of a new form of electron coupling in high temperature superconductors.

Magnetoinfrared Spectroscopy of Landau Levels and Zeeman Splitting of Three-Dimensional Massless Dirac Fermions in ZrTe(5).

We present a magnetoinfrared spectroscopy study on a newly identified three-dimensional (3D) Dirac semimetal ZrTe(5). We observe clear transitions between Landau levels and their further splitting under a magnetic field. Both the sequence of transitions and their field dependence follow quantitatively the relation expected for 3D massless Dirac fermions. The measurement also reveals an exceptionally low magnetic field needed to drive the compound into its quantum limit, demonstrating that ZrTe(5) is an extremely clean system and ideal platform for studying 3D Dirac fermions. The splitting of the Landau levels provides direct, bulk spectroscopic evidence that a relatively weak magnetic field can produce a sizable Zeeman effect on the 3D Dirac fermions, which lifts the spin degeneracy of Landau levels. Our analysis indicates that the compound evolves from a Dirac semimetal into a topological line-node semimetal under the current magnetic field configuration.

Raman-scattering measurements and theory of the energy-momentum spectrum for underdoped Bi2Sr2CaCuO(8+δ) superconductors: evidence of an s-wave structure for the pseudogap.

S. Sakai , S. Blanc, M. Civelli, Y. Gallais, M. Cazayous , M.-A. Méasson, J. S. Wen, Z. J. Xu, G. D. Gu, G. Sangiovanni , Y. Motome, K. Held, A. Sacuto, A. Georges , and M. Imada Centre de Physique Théorique, École Polytechnique, CNRS, 91128 Palaiseau, France, Department of Applied Physics, University of Tokyo, Hongo, Tokyo 113-8656, Japan, JST-CREST, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan, Laboratoire Matériaux et Phénom̀ enes Quantiques (UMR 7162 CNRS), Université Paris Diderot-Paris 7, Bat. Condorcet, 75205 P aris Cedex 13, France, Laboratoire de Physique des Solides, Université Paris-Su d, CNRS, UMR 8502, F-91405 Orsay Cedex, France, Matter Physics and Materials Science, Brookhaven National Laboratory (BNL), Upton, NY 11973, USA, Institut für Theoretische Physik und Astrophysik, Univer sität Würzburg, Am Hubland, D-97074 Würzburg, Germany, Institute for Solid State Physics, Vienna University of Tec hnology, 1040 Vienna, Austria, Collège de France, 11 place Marcelin Berthelot, 75005 Pari s, France, DPMC, Université de Genève, 24 quai Ernest Ansermet, CH-1 211 Genève, Suisse (Dated: May 2, 2014)

Electronic correlations and unusual superconducting response in the optical properties of the iron chalcogenide FeTe0.55Se0.45

The in-plane complex optical properties of the iron-chalcogenide superconductor FeTe{sub 0.55}Se{sub 0.45} have been determined above and below the critical temperature T{sub c} = 14 K. At room temperature the conductivity is described by a weakly interacting Fermi liquid; however, below 100 K the scattering rate develops a frequency dependence in the terahertz region, signaling the increasingly correlated nature of this material. We estimate the dc conductivity {sigma}{sub dc}(T {ge} T{sub c}) = 3500 {+-} 400 {Omega}{sup -1} cm{sup -1} and the superfluid density {rho}{sub s0} = 9 {+-} 1 x 10{sup 6} cm{sup -2}, which places this material close to the scaling line {rho}{sub s0}/8 = 8.1{sigma}{sub dc}T{sub c} for a BCS dirty-limit superconductor. Below T{sub c} the optical conductivity reveals two gap features at {Delta}{sub 1,2} = 2.5 and 5.1 meV.

Optimizing the superconducting transition temperature and upper critical field of Sn1-xInxTe

Sn(1-x)In(x)Te is a possible candidate for topological superconductivity. Previous work has shown that substitution of In for Sn in the topological crystalline insulator SnTe results in superconductivity, with the transition temperature, Tc, growing with In concentration. We have performed a systematic investigation of Sn(1-x)In(x)Te for a broad range of x, synthesizing single crystals (by a modified floating zone method) as well as polycrystalline samples. The samples have been characterized by x-ray diffraction, resistivity, and magnetization. For the single crystals, the maximum Tc is obtained at x=0.45 with a value of of 4.5 K, as determined by the onset of diamagnetism.

Collapse of the normal-state pseudogap at a Lifshitz transition in the Bi(2)Sr(2)CaCu(2)O(8+δ) cuprate superconductor.

We report a fine tuned doping study of strongly overdoped Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} single crystals using electronic Raman scattering. Combined with theoretical calculations, we show that the doping, at which the normal-state pseudogap closes, coincides with a Lifshitz quantum phase transition where the active holelike Fermi surface becomes electronlike. This conclusion suggests that the microscopic cause of the pseudogap is sensitive to the Fermi surface topology. Furthermore, we find that the superconducting transition temperature is unaffected by this transition, demonstrating that their origins are different on the overdoped side.

High Energy Dispersion Relations for the High Temperature Bi2Sr2CaCu2O8 Superconductor from Laser-Based Angle-Resolved Photoemission Spectroscopy

Wentao Zhang, Guodong Liu, Jianqiao Meng, Lin Zhao, Haiyun Liu, Xiaoli Dong, Wei Lu, J. S. Wen, Z. J. Xu, G. D. Gu, T. Sasagawa, Guiling Wang, Yong Zhu, Hongbo Zhang,Yong Zhou, Xiaoyang Wang, Zhongxian Zhao, Chuangtian Chen, Zuyan Xu and X. J. Zhou 1,∗ National Laboratory for Superconductivity, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama Kanagawa, Japan Laboratory for Optics, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China (Dated: January 16, 2008)

Enhanced charge stripe order of superconducting La2 and #8722;xBaxCuO4 in a magnetic field

The effect of a magnetic field on the charge stripe order in La(2-x)Ba(x)CuO(4) has been studied by means of high energy (100 keV) x-ray diffraction for charge carrier concentrations ranging from strongly underdoped to optimally doped. We find that charge stripe order can be significantly enhanced by a magnetic field applied along the c-axis, but only at temperatures and dopings where it coexists with bulk superconductivity at zero field. The field also increases stripe correlations between the planes, which can result in an enhanced frustration of the interlayer Josephson coupling. Close to the famous x=1/8 compound, where zero field stripe order is pronounced and bulk superconductivity is suppressed, charge stripe order is independent of a magnetic field. The results imply that static stripe order and three-dimensionally coherent superconductivity are competing ground states.

Charge order, metallic behavior, and superconductivity in La2-xBaxCuO4 with x=1/8.

The ab-plane optical properties of a cleaved single crystal of La2-xBaxCuO4 for x=1/8 (Tc approximately =2.4 K) have been measured over a wide frequency and temperature range. The low-frequency conductivity is Drude-like and shows a metallic response with decreasing temperature. However, below approximately =60 K, corresponding to the onset of charge-stripe order, there is a rapid loss of spectral weight below about 40 meV. The behavior is quite different from that typically associated with the pseudogap in the normal state of the cuprates. Instead, the gapping of the normal-state single-particle excitations looks surprisingly similar to that observed in superconducting La2-xSrxCuO4, including the presence of a residual Drude peak with reduced weight.