Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G16: Focus Session: Iron Based Superconductors: Local Probes |
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Sponsoring Units: DMP DCOMP Chair: Alexander Balatsky, Los Alamos National Laboratory/Nordita Room: 101AB |
Tuesday, March 3, 2015 11:15AM - 11:27AM |
G16.00001: Quasiparticle interference (QPI) as a model-free phase sensitive tool to determing the pairing symmetry in Fe based superconductors (FeBS) Igor Mazin, Peter Hirschfeld, Ilya Eremin QPI has been successfully used to identify the d-wave pairing in high-T$_c$ cuprates[1]. However, applying the same technique to FeBS has been confounded by difficulties not present in in cuprates, where QPI is dominated by several hot spots, thus calling for a more detailed analysis. This talk aims at providing such theoretical framework and clarifying some misconceptions regarding QPI in superconductors (partially pointed out before, but not generally appreciated). We emphaseize that detailed comparison of the calculated model QPI maps with the experiment, as often attempted, is hardly helpful because the result depends not only on the assumed pairing symmetry but of numerous uncontrolled approximations. Instead, we suggest that the T dependence (but not the detailed q dependence at any given T) bears clear qualitative fingerprints of the s$_\pm$ (or s$_{++}$) symmetry, independent of the details of impurity scattering, electronic structure etc. We propose an experiment that should unambiguously distinguish between the two symmetries in question. [1] Hanaguri et al, Science, 323, 923 (2009) [Preview Abstract] |
Tuesday, March 3, 2015 11:27AM - 11:39AM |
G16.00002: STM investigation of FeSe$_{1-x}$S$_{x}$ M. Iavarone, S.A. Moore, E. Lechner, J. Curtis, O.S. Volkova, A.N. Vasiliev, D.A. Chareev, G. Karapetrov FeSe has the simplest structure among the Fe-based superconductors, and this very simplicity could provide the most appropriate venue of understanding the superconducting mechanism of Fe-based superconductors. High quality FeSe single crystals were grown in evacuated quartz ampoules using a KCl/AlCl flux and the structure of tetragonal P4/nmm was demonstrated by x-ray diffraction. Low temperature STM measurements were performed on FeSe$_{1-\delta}$ and FeSe$_{1-x}$S$_{x}$ with x=0.04 and 0.09. Effects of multiband superconductivity and vortex matter as a function of doping will be presented. [Preview Abstract] |
Tuesday, March 3, 2015 11:39AM - 11:51AM |
G16.00003: Real Space Imaging of the Atomic-Scale Magnetic Structure of Fe$_{1+y}$Te Peter Wahl, Mostafa Enayat, Zhi-Xiang Sun, Udai Raj Singh, Ramakrischna Aluru, Stefan Schmaus, Alexander Yaresko, Yong Liu, Chengtian Lin, Vladimir Tsurkan, Alois Loidl, Joachim Deisenhofer High temperature superconductivity, both in cuprate as well as iron pnictide materials, occurs in close proximity to magnetically ordered phases, indicating an intimate relationship between the two. Up to now, most information on the magnetic structure of strongly correlated electron systems has been obtained by neutron scattering. Here we demonstrate real space atomic scale imaging of the magnetic structure of iron tellurium (Fe$_{1+y}$Te), the non-superconducting parent compound of the iron chalcogenides, by spin-polarized low temperature scanning tunneling microscopy. Our images of the magnetic structure reveal that magnetic order in the monoclinic phase is truly a unidirectional stripe order, whereas in the orthorhombic phase at higher excess iron concentrations ($y > 0.12$), a transition to a phase with coexistence of stripes in both directions is observed. [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:27PM |
G16.00004: Visualization of electronic nematicity in the iron pnictides Invited Speaker: Ethan Rosenthal The nematic state from which superconductivity emerges in the iron pnictides continues to confound. We use scanning tunneling microscopy (STM) and spectroscopy (STS) to image both long-range nematic order and nematic fluctuations across the doping phase diagram in Co-doped NaFeAs. We associate twinning domains with long-range order and directly visualize the temperature and doping dependence of these features. Anisotropic electronic structure is found to persist outside of the ordered nematic phase . With the aid of a novel experimental setup which combines simultaneous STS and variable, uniaxial strain, we determine the relationship between strain and nematic fluctuations which gives rise to the anisotropy. [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G16.00005: Observation of van Hove singularity and quasiparticle interference in KFe2As2 superconductors revealed by STM/STS measurements Hai-Hu Wen, Delong Fang, Zengyi Du, Zhenyu Wang, Huan Yang, Xiaxin Ding We have conducted STM/STS investigations on the KFe$_2$As$_2$ superconducting single crystals down to 0.45 K under magnetic field. Clear electronic standing waves have been observed allowing us to investigate the quasiparticle interference (QPI). Interestingly we observed a sharp peak of local density of states (LDOS) near the Fermi energy showing evidence of strongly enhanced DOS both below and above Tc. We demonstrate that this is induced by a van Hove singularity with the saddle point locating only 4 meV below the Fermi energy. Below Tc it is found that only 20\% of the normal state DOS is gapped away by superconductivity, with the major part of DOS due to VHS ungapped. Combing with the ARPES data, we find that the VHS points locate on the ($\pi$,0) point, which gives strong constraint on the gap function and pairing mechanism. In the mixed state we clearly observed the mixture of vortices and the standing waves due to quasiparticle interference, giving support to above picture. In collaboration with X. Shi, P. Richard, T. Qian and H. Ding et al. in Institute of Physics, CAS. [Preview Abstract] |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G16.00006: Local characterization of superconductivity in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ Ophir Auslaender, Yuval Lamhot, Alon Yagil, Nadav Shapira, Shigeru Kasahara, Tatsuya Watashige, Takasada Shibauchi, Yuji Matsuda We use magnetic force microscopy (MFM) to characterize superconductivity across the superconducting dome in BaFe$_2$(As$_{1-x}$P$_x$)$_2$, an isovalently doped pnictide that exhibits a peak in the penetration depth $\left(\lambda_{ab}\right)$ at optimal doping ($x_{\rm opt}$), as shown previously in sample-wide measurements. Our local measurements show a peak at $x_{\rm opt}$ and a $T_{C}$ vs. $\lambda_{ab}^{-2}$ dependence similar on both sides of $x_{opt}$. Near the underdoped edge of the dome $\lambda_{ab}$ increases sharply, suggesting that superconductivity competes with another phase. Indeed MFM vortex imaging shows correlated defects parallel to twin boundaries only in underdoped samples and not for $x\ge x_{\rm opt}$. Furthermore, in underdoped samples we report stripes parallel to twin boundaries that are visible even in the absence of vortices. [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G16.00007: ABSTRACT WITHDRAWN |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G16.00008: Antiferromagnetic Spin Fluctuations and Pseudogap Behavior in Ca(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ Studied by$^{5}$As NMR Jinfang Cui, Sheng Ran, Sergey Bud'ko, Paul Canfield, Yuji Furukawa $^{75}$As NMR measurements of single-crystalline Ca(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ have been carried out for four different doping concentration crystals (x $=$ 0.023, 0.028, 0.033, 0.059) annealed at 350$^{\circ}$C [1]. Co-doped CaFe$_{2}$As$_{2}$ is a compound in 122 family of iron-pnictide superconductors with three principle phases exhibited: paramagnetic (PM), antiferromagnetic (AFM) and superconducting (SC) states. The magnetic phase transition to AFM state occurs at T$_{\mathrm{N}}=$180K at x$=$0 and is suppressed to T$_{\mathrm{N}}=$53K for x$=$0.028, which is accompanied by a structural phase transition from tetragonal to orthorhombic phases. $^{75}$As NMR was used to study the low energy spin dynamics via Knight shift (K) and spin-lattice relaxation rate (1/T$_{1})$ measurements. From our analysis of the temperature dependence of both K and (T$_{1}$T)$^{-1}$ in x$=$0.028 (T$_{\mathrm{N}}=$53K), 0.033 (T$_{\mathrm{c}}=$9K) and 0.059 (T$_{\mathrm{c}}=$10K), we found a gradual decrease of AFM spin fluctuations below T$^{\mathrm{\ast }}=$88K for x$=$0.028, 72K for x$=$0.033 and 41K for x$=$0.059, respectively, indicating the possible pseudogap behavior in spin excitation spectrum in the system. \\[4pt] [1] S. Ran, et al., Phys. Rev. B 85, 224528 (2012) [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G16.00009: Study of Fe(Se, Te) micron-sized rings by magnetic force microscopy Gregory Polshyn, Can Zhang, Tyler Naibert, James Eckstein, Raffi Budakian The study of fluxoid states and fluxoid dynamics in mesoscopic iron-based superconducting rings is valuable for characterizing the basic properties of the superconductor, and may also provide important insight into the superconducting paring symmetry. We report the fabrications of micron-sized rings and disks from thin films of Fe(Se, Te) grown by molecular beam epitaxy. In order to study fluxoid states in rings we developed a custom-tailored version of magnetic force microscopy (MFM). This technique has a number of qualitative advantages for working with mesoscopic superconducting samples in comparison to the conventional MFM and other imaging techniques. We observed metastable fluxoid states in rings of different sizes. Thermally activated fluxoid dynamics of these states was studied and modeled. In addition, we found different regimes of interaction between Fe(Se, Te) ring and MFM tip which are explained. Possibilities of the existence of exotic vortex states and proposals for experiments to test the symmetry of the superconducting order parameter in iron based superconductors are analyzed. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G16.00010: Friedel oscillations of the superconducting gap in FeSe T. Hanaguri, T. Watashige, Y. Kohsaka, K. Iwaya, T. Machida, T. Shibauchi, R. Kobayashi, S. Kasahara, Y. Matsuda, A. B\"ohmer, T. Wolf, C. Meingast, H. v. L\"ohneysen The spatial variation of the superconducting gap of FeSe has been studied by spectroscopic-imaging scanning tunneling microscopy. The apparent gap amplitude defined by the energy separation between the quasi-particle coherence peaks exhibits spatial periodic oscillations. We found that the wavelength of the oscillations agrees with the wavelength of the normal-state quasi-particle interference pattern at the Fermi energy. Such Friedel oscillations of the superconducting gap may be related to the extremely small Fermi surfaces of FeSe in the BCS-BEC crossover~[1]. [1] S. Kasahara {\it et al.}, PNAS Early Edition, Nov. 6, 2014, doi:10.1073/pnas.1413477111 [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G16.00011: $^{75}$As and $^{59}$Co NMR studies of SrCo$_{2}$As$_{2}$ Yuji Furukawa, Vasily Ogloblichev, Abhisek Pandey, David C. Johnston After the discovery of unconventional superconductivity in Fe pnictides with the ThCr$_{2}$Si$_{2}$-type structure, much attention has been paid to the related materials AM$_{2}$As$_{2}$(A $=$ Ca, Sr, and Ba, and M $=$ Co, Ni, Mn, and Cu). We have been studying the electronic and magnetic properties of these related materials systematically. Among them, metallic SrCo$_{2}$As$_{2}$ is an interesting system~[1] because inelastic neutron scattering measurements indicate strong stripe-type antiferromagnetic correlations [2], similar to the Fe pnictide superconductors. In order to investigate the magnetic and electronic properties of SrCo$_{2}$As$_{2}$ from a microscopic point of view, we carried out $^{59}$Co and $^{75}$As NMR in the temperature range T $=$ 1.3 -- 300 K. In this talk, based on $^{59}$Co NMR data including $^{75}$As NMR results published previously [1], we discuss the characteristic magnetic fluctuations in the system and compare them with those measured from NMR data for another cobalt arsenide BaCo$_{2}$As$_{2}$. \\[4pt] [1] A. Pandey, et al., Phys. Rev. B 88, 014526 (2013).\\[0pt] [2] W. Jayasekara, et al., Phys. Rev. Lett. 111, 157001 (2013). [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G16.00012: $^{57}$Fe M\"ossbauer study of iron-silicide superconductor Lu$_2$Fe$_3$Si$_5$ Xiaoming Ma, Sheng Ran, Hua Pang, Fashen Li, Paul Canfield, Sergey Bud'ko In order to investigate the changes of the hyperfine parameters of a compound when it goes into a superconducting state from a normal state, we studied Lu$_2$Fe$_3$Si$_5$. Lu$_2$Fe$_3$Si$_5$ is a superconductor with a transition temperature, T$_C$ $\sim$ 6 K and the Fe has been proved to be non-magnetic in a previous M\"ossbauer study[1]. We performed detailed $^{57}$Fe M\"ossbauer spectra measurement on Lu$_2$Fe$_3$Si$_5$ from room temperature down to 4.4 K with particular attention paid to region near T$_C$. No clear feature that can be associated with the superconducting transition was found in the hyperfine parameters. Detailed hyperfine parameters and recoilless fraction as a function of temperature will be presented and discussed. [Preview Abstract] |
Tuesday, March 3, 2015 2:03PM - 2:15PM |
G16.00013: Specific Heat in Zero and Applied Magnetic Field of Overdoped Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Fe$_{2}$As$_{2}$: Analysis of The Multigap Behavior and Anisotropies G.R. Stewart, J.S. Kim, Yong Liu, Thomas A. Lograsso Specific heat measurements down to 0.4 K and up to H$_{\mathrm{c2}}$ on single crystals of two different compositions of overdoped Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Fe$_{2}$As$_{2}$ with T$_{\mathrm{c}}$ values of 5.9 K (x$\approx $0.93) and 8.9 K (x$\approx $0.85) are reported. The possibility of mixed/crossover behavior in the pairing symmetry in this composition range has been discussed in the literature. The zero field data analysis for both samples results in essentially the same two gap values: 2$\Delta $/kT$_{\mathrm{c}}=$0.9 and 3.5 while the field data indicate pronounced non-linearity in $\gamma $ vs H both as the lower gap is suppressed by about 1 T but also in higher fields up to H$_{\mathrm{c2}}\approx $5.5 T. Analysis of the field data to investigate the gap anisotropies will be presented. [Preview Abstract] |
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