G. F. Chen

Observation of Dirac Cone Electronic Dispersion in BaFe2As2

We performed an angle-resolved photoemission spectroscopy study of BaFe2As2, which is the parent compound of the so-called 122 phase of the iron-pnictide high-temperature superconductors. We reveal the existence of a Dirac cone in the electronic structure of this material below the spin-density-wave temperature, which is responsible for small spots of high photoemission intensity at the Fermi level. Our analysis suggests that the cone is slightly anisotropic and its apex is located very near the Fermi level, leading to tiny Fermi surface pockets. The bands forming the cone show an anisotropic leading edge gap away from the cone that suggests a nodal spin-density-wave description.

Transport and anisotropy in single-crystalline SrFe2As2 and A(0.6)K(0.4)Fe(2)As(2) (A=Sr, Ba) superconductors

We have successfully grown high-quality single crystals of SrFe2As2 and A(0.6)K(0.4)Fe(2)As(2)(A=Sr, Ba) using flux method. The resistivity, specific heat, and Hall coefficient have been measured. For parent compound SrFe2As2, an anisotropic resistivity with rho(c)/rho(ab) as large as 130 is obtained at low temperatures. A sharp drop in both in-plane and out-plane resistivities due to the spin-density-wave (SDW) instability is observed below 200 K. The angular dependence of in-plane magnetoresistance shows twofold symmetry with field rotating within ab plane below SDW transition temperature. This is consistent with a stripe-type spin ordering in SDW state. In K-doped A(0.6)K(0.4)Fe(2)As(2)(A=Sr, Ba), the SDW instability is suppressed and the superconductivity appears with T-c above 35 K. The rather low anisotropy in upper critical field between H parallel to ab and H parallel to c indicates that interplane coupling plays an important role in hole-doped Fe-based superconductors.

Doping evolution of antiferromagnetic order and structural distortion in LaFeAsO(1-x)F(x)

We use neutron scattering to study the structural distortion and antiferromagnetic (AFM) order in LaFeAsO(1-x)F(x) as the system is doped with fluorine (F) to induce superconductivity. In the undoped state, LaFeAsO exhibits a structural distortion, changing the symmetry from tetragonal (space group P4/nmm) to orthorhombic (space group Cmma) at 155 K, and then followed by an AFM order at 137 K. Doping the system with F gradually decreases the structural distortion temperature, but suppresses the long range AFM order before the emergence of superconductivity. Therefore, while superconductivity in these Fe oxypnictides can survive in either the tetragonal or the orthorhombic crystal structure, it competes directly with static AFM order.

Nanoscale phase separation of antiferromagnetic order and superconductivity in K0.75Fe1.75Se2

We report an in-plane optical spectroscopy study on the iron-selenide superconductor K0.75Fe1.75Se2. The measurement revealed the development of a sharp reflectance edge below Tc at frequency much smaller than the superconducting energy gap on a relatively incoherent electronic background, a phenomenon which was not seen in any other Fe-based superconductors so far investigated. Furthermore, the feature could be noticeably suppressed and shifted to lower frequency by a moderate magnetic field. Our analysis indicates that this edge structure arises from the development of a Josephson-coupling plasmon in the superconducting condensate. Together with the transmission electron microscopy analysis, our study yields compelling evidence for the presence of nanoscale phase separation between superconductivity and magnetism. The results also enable us to understand various seemingly controversial experimental data probed from different techniques.

Two superconducting gaps in LaFeAsO0.92F0.08 revealed by As-75 nuclear quadrupole resonance

We report As-75 nuclear quadrupole resonance studies on superconducting oxypnictide LaFeAsO0.92F0.08 (T-c=23 K). The temperature dependence of the spin-lattice relaxation rate (1/T-1) decreases below T-c without a coherence (Hebel-Slichter) peak and shows a temperature dependence that is not simple power law nor exponential. We show that the result can be understood in terms of two superconducting gaps of either d- or +/- s-wave symmetry, with the larger gap Delta(1)similar to 4k(B)T(c) and the smaller one Delta(2)similar to 1.5k(B)T(c). Our result suggests that the multiple-gap feature is universal in the oxypnictide superconductors, which is probably associated with the multiple electronic band structures in this class of materials. We also find that 1/T1T above T-c increases with decreasing temperature, which suggests weak magnetic fluctuations in the normal state.

Static magnetic order and superfluid density of RFeAs(O,F) (R=La,Nd,Ce) and LaFePO studied by muon spin relaxation: Unusual similarities with the behavior of cuprate superconductors

J. P. Carlo, Y. J. Uemura, ∗ T. Goko, 2, 3 G. J. MacDougall, J. A. Rodriguez, W. Yu, G. M. Luke, Pengcheng Dai, N. Shannon, S. Miyasaka, S. Suzuki, S. Tajima, G. F. Chen, W. Z. Hu, J. L. Luo, and N. L. Wang Department of Physics, Columbia University, New York, New York 10027, USA Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., V6T 2A3, Canada Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA H H Wills Physics Laboratory, University of Bristol, BS8 1TL Bristol, United Kingdom Department of Physics, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, Peoples Republic of China (Dated: May 14, 2008)

Fermi surface and band renormalization of Sr(1-x)K(x)Fe(2)As(2) from angle-resolved photoemission spectroscopy

High-resolution angle-resolved photoemission measurements have been carried out on the (Sr,K) Fe(2)As(2) superconductor (T(c) = 21 K). Three holelike Fermi-surface sheets are resolved around the Gamma point. One electronlike Fermi surface and strong Fermi spots are observed near the M(pi,pi) point which are different from a round electron pocket expected from band calculations. This discrepancy and the observed band renormalization ask for further theoretical effort in understanding the electronic structure of the Fe-based compounds.

As-75 NMR study of single crystals of the heavily overdoped pnictide superconductors Ba1-xKxFe2As2 (x=0.7 and 1)

We performed As-75 NMR studies on two overdoped high-quality Ba1-xKxFe2As2 (x=0.7 and 1.0) single crystals. In the normal states, we found a dramatic increase in the spin-lattice relaxation (1/T-75(1)) from the x =0.7 to the x=1.0 samples. In KFe2As2, the ratio of 1/(T1TKn2)-T-75, where K-75(n) is the Knight shift, increases as temperature drops. These results indicate the existence of another type of spin fluctuations in KFe2As2 which is accustomed to being treated as a simple Fermi liquid. In the superconducting state, the temperature scalings of 1/T-75(1) below T-c in the overdoped samples are significantly different from those in the under or optimally doped ones. A power-law scaling behavior 1/(T1T)-T-75 similar to T-0.5 is observed, which indicates universal strong low-energy excitations in the overdoped hole-type superconductors.

Raman phonons of α -FeTe and Fe 1.03 Se 0.3 Te 0.7 single crystals

The polarized Raman scattering spectra of nonsuperconducting

Two-magnon Raman scattering in A(0.8)Fe(1.6)Se(2) systems (A = K, Rb, Cs, and Tl): Competition between superconductivity and antiferromagnetic order

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