Intellectual Merits :

1. Spatially resolved quasiparticle tunneling spectroscopic studies of cuprate and iron-based high-temperature superconductorsNai-Chang Yeh, California Institute of Technology, DMR 0907251 Intellectual Merits: The ultimate objective of this project is to elucidate the pairing mechanism of high-Tc cuprate and ferrous superconductors by comparative studies of their low-energy quasiparticle excitations using scanning tunneling spectroscopy (STS). Here we focus on the highlights of the iron arsenides Ba(Fe1-xCox)2As2 (Co-122) single crystals with x = 0.06, 0.08 and 0.12, corresponding to under-, optimal and over-doped samples, respectively:1,2 [1] First tunneling spectral evidence (Fig.1) for two-gap superconductivity in the ferrous superconductors;1 [2] evidence for sign-changing s-wave (s) pairing symmetry from phase sensitive studies of the Fourier-transformed (FT) local density of states (LDOS) (Fig.2);1 [3] first observation of anisotropic vortices and pseudogap spectra inside the vortex cores (Fig.3);2 [4] determination of a universal relation between two magnetic resonant modes and two superconducting (SC) gaps (Fig.4).2 These findings and their comparison with the cuprates suggest that the commonalities of high-Tc superconductivity involve competing orders, spin fluctuations, and unconventional pairing symmetry with sign-changing order parameters on different parts of the Fermi surface. ,a (a) w = DM w = DG Fig.2: FT-LDOS spectra of Co-122 with x = 0.06 at constant energy w = (a)DM (left) and DG (right), where the quasiparticle interference (QPI) wave-vectors q1,2,3 are defined as the inter-Fermi pocket scattering wave-vectors shown in (b) , with the electron (hole) pockets represented by the red (blue) circles in the first Brillouin zone (k -space). For s-pair where the signs of the electron and hole pairing potentials are opposite, the QPI due to non-magnetic impurities would yield strong q2 and weak q3 intensities only at w = DM and DG, whereas pure s-pairing would yield weak q2 and strong q3 intensities. The FT-LDOS spectra in (a) are therefore supportive of s pair. Fig.1: Tunneling conductance spectra of & T-dependent two SC gaps DG and DM of Co-122: (a) underdoped x = 0.06 (TC = 14 K), (b) overdoped x = 0.12 (TC = 20 K). Representative T-dependent point spectra are shown in the left panels, and the right panels display the T-dependence of the SC gaps defined as one half of the peak-to-peak energy separations. Both gaps decrease with increasing T and vanish above TC, implying their SC origin. (b) References: 1. M.L. Teague et al., Phys. Rev. Lett. 106, 087004 (2011). 2. N.-C. Yeh et al., arXiv:1107.0697 (2011). k-space q-space

2. Spatially resolved quasiparticle tunneling spectroscopic studies of cuprate and iron-based high-temperature superconductors Nai-Chang Yeh, California Institute of Technology, DMR 0907251 Boarder Impacts: Education: In 2010 – 2011, this research project has involved two graduate students (Marcus L. Teague, Hao Chu) and two undergraduate students (Renee T. P. Wu, Jing Shi) at Caltech. Among the trainees, Jing Shi received her B.Sc. in June, 2011. Outreach: The PI chaired the Eurasia-Pacific Summer School and Workshop on Correlated Electrons at the Institute of Theoretical and Applied Physics (ITAP) in Turkey from July 4th to July 14th, 2011. The PI also lectured 9 hours at the summer school on topics of conventional, cuprate and ferrous superconductivity. Some of lecture contents involved research results supported by this NSF grant. The conference website can be found at: http://www.fen.bilkent.edu.tr/~eurasia_pacific/ Publications supported by this award since last report: Fig.3: Quasiparticle tunneling spectra of an optimally doped Co-122 sample (x = 0.08) under an applied magnetic field H = 1 Tesla: (a) Spatially varying spectra A, B and C taken at the inter-vortex, vortex-edge and vortex-center locations shown in (b). A pseudogap feature is found near the center of the vortex core (C), and the pseudogap energy is consistent with the inter-vortex SC gap, in contrast to the much larger pseudogap than SC gap found in under and optimally doped hole-type cuprate superconductors. (b) a (60  60) nm2 conductance ratio map, where two anisotropic vortices are visible. The long-to-short axis length of the anisotropic vortex core is approximately 2:1, with the short axis length comparable to the SC coherence length. (c) An enlarged (20  20) nm2 conductance ratio map over the upper left corner in (b). Δg/d Δa,g/d Wr1 Wr1 Δb x=0.08 H = 0 (T = 6K) (T = 6K) (c) (b) (a) A B A C C B A B C dI/dV (a.u.) -30 -15 0 15 30 0 X (nm) 20 0 X (nm) 60 Energy (meV) Fig.4: Inelastic quasiparticle scattering by magnetic resonant modes are manifested as “humps” at energies Wr1,2 larger than the SC gap in the tunneling spectra, where Wr1 = |Da| + |Dg/d| ~ 2|Dg/d| and Wr2 = |Db| + |Dg/d| ~ 1.5|Dg/d|. Therefore, the resonant modes are expected to scale with the SC gaps and forming a universal relation for all doping levels and all temperatures below TC. (a) Point spectrum for x = 0.12. (b) Point spectrum for x = 0.08. (c) A universal relation is verified for Wr1 vs. |Db| among three doping levels and multiple temperatures below TC. • M. L. Teague et al., Phys. Rev. Lett. 106, 087004 • (2011). • 2. C. R. Hughes et al., Phys. Rev. B 82, 13441 (2010). • 3. N.-C. Yeh et al., arXiv:1107.0697 (2011). meV (a) (b) (c) Wr1 Δb dI/dV (a.u.) W r1(T) meV Db(T) Energy (meV)