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 two types of high-Tc superconductors, the cuprates and the iron pnictides. Using spatially resolved scanning tunneling spectroscopy (STS), we have found important similarities and differences between these two systems. Here we focus on the highlights of the cuprates, including: 1) direct evidence for competing orders (e.g. spin-density, charge-density and pair-density waves, SDW, CDW, PDW) coexisting with superconductivity (SC) in the ground state; and 2) attributing the presence (absence) of the pseudogap (PG) above Tc in hole- (electron-) doped cuprates to a competing-order energy gap larger (smaller) than the SC gap, as detailed in Refs. [1] – [4]. The empirical evidences include: a PG inside the vortex cores, (Fig.1), and two distinct types of wave vectors found in the Fourier-transformed (FT) local density of states (LDOS), (see Fig.2). The energy-dependent quasiparticle interference (QPI) wave vectors are due to the SC excitations, and the energy-independent wave vectors are related to competing orders. (a) (b) (c) Y-123 vortex image Y-123 ky w (meV) w (meV) Fig.2:(a) Vortex-state FT-LDOS in the first Brillouin zone (BZ), integrated over all energy. The circles indicate CDW/SDW/PDW. (b) Wave-vectors connecting CDW and SDW excitations over the Fermi surface. (c) Wave-vectors connecting QPI states. (d) Energy (w) independent wave-vectors. (e) w-dependent QPI. (c) (b) w (meV) w (meV) (a) FT-LDOS of YBa2Cu3O7 at H = 5 T & T = 6 K Fig.1: Vortex-state tunneling conductance spectra of cuprate superconductors at T = 6 K: (a) Vortex images of hole-doped YBa2Cu3O7 (Y-123) at H = 4.5 T. (b) Representative inter- and intra-vortex spectra of Y-123 at H = 2 T, showing a PG energy VCO inside the vortex core with VCO > DSC, the SC gap. (c) Representative inter- and intra-vortex spectra of electron-doped La0.1Sr0.9CuO2 (La-112) at H = 1 T, showing VCO inside the vortex core smaller than Deff. (e) (d) References: [1] N.-C. Yeh & A.D. Beyer, int. J. Mod. Phys. 23, 4543 (2009) [2] N.-C. Yeh et al., J. Supercond. Nov. Mag. 23, 757 (2010). [3] A.D. Beyer et al., Europhys. Lett. 87, 37005 (2009). [4] M.L. Teague et al., Europhys. Lett. 85, 17004 (2009).

2. Spatially resolved quasiparticle tunneling spectroscopic studies of cuprate and iron-based high-temperature superconductorsNai-Chang Yeh, California Institute of Technology, DMR 0907251 Fig.4: Schematic illustrations of varying tunneling conductance (G) obtained by using regular STM (with Pt/Ir tip) and SP-STM (with Cr-coated tip) on a ferromagnetic La0.7Ca0.3MnO3 (LCMO) thin film under different magnetic fields (H = 0, -0.3 T, 3.0 T). Here the coercive field for the Cr-tip is ~ 1.2 T and that for the LCMO is ~ 0.1 T, so that the two external fields chosen lead to either anti-parallel or parallel magnetizations between the tip and the sample, known as the “spin-valve” configurations for F-I-F junctions. The conductance G is modeled by considering the density of states (DOS) of the majority (Maj.) and minority (Min.) bands of the tip and of the sample, as well as the spin selectivity under different magnetic fields. The last row shows real tunneling conductance maps at energy = U+, consistent with our model. Boarder Impacts: Education: This research project has involved three graduate students (A. D. Beyer, M. L. Teague, C. R. Hughes) and three undergraduate students (G. P. Lockhart, G. K. Drayna, J. Shi) at Caltech. Outreach: The PI was invited by the Institute of Theoretical and Applied Physics (ITAP) in Turkey to lecture at the summer schools for European and Turkish graduate students and postdoctoral scholars in both 2009 (60 hours on “Advanced Condensed Matter Field Theory”) and 2010 (15 hours on “Physics of Fundamental Effects in Condensed Matter Physics: Theory & Experiment”). Some of the lectures covered topics related to the NSF-funded research. The lecture notes for 2009 are available at: http://www.its.caltech.edu/~yehgroup/ITAP_2009/ Instrumentation: The PI’s group has incorporated spin-polarized (SP) tunneling capabilities into the homemade cryogenic STM system. The SP-STM technique has been successfully applied to studying the intrinsic electronic heterogeneity in colossal magnetoresistive manganites. Figure 4 illustrates an example of manifesting the spin-valve effect of a F-I-F junction (F: ferromagnetic metal, I: insulator, which is the vacuum gap for STM junctions) by means of STM and SP-STM under different magnetic fields. Inhomogeneous conductance map for H = 0 Ref.: Hughes et al (arXiv:1004.1448) Homogeneous conductance G G-maps over (20x50) nm2 sample area and T = 6 K (H = 0) (H = 0) (H = -0.3 T) (H = 3.0 T) Higher G Lower G