Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U37: Focus Session: Fe-based Superconductors: Tunneling Spectroscopy |
Hide Abstracts |
Sponsoring Units: DMP DCOMP Chair: Laura Greene, University of Illinois Room: 345/346 |
Thursday, March 21, 2013 11:15AM - 11:27AM |
U37.00001: STM on LiFeAs - Momentum Resolved Superconducting Gap Structure, Electron-Boson Interactions and Charge Susceptibilities in a Prototypical Iron-Based Superconductor A.W. Rost, M.P. Allan, T.-M. Chuang, F. Massee, K. Lee, M. Fischer, Y. Xie, K. Kihou, C.-H. Lee, A. Iyo, H. Eisaki, A.P. Mackenzie, E.-A. Kim, D.J. Scalapino, J.C. Davis Tunneling spectroscopy on strong coupling superconductors has been one of the key experiments confirming the phonon-mediated mechanism of superconductivity. In the last two decades it has become possible using STM to access this information in real space with atomic resolution. One of the most important aspects of these developments is the ability to extract momentum space resolved information from Fourier-Transform STM measurements. Here we will demonstrate using our recent data on LiFeAs how this technique allows access to a range of fundamental properties of the electronic excitation spectrum. In particular I will show that it is now in principle possible to access momentum space resolved information not only on the superconducting gap structure but also on quantities such as electron-boson interactions and geometric information on `nesting' vectors giving rise to peaks in the charge susceptibility. The resulting `fingerprint' of the mechanism driving superconductivity goes well beyond the information obtained in traditional tunneling experiments and has the potential of being a key experimental tool in the study of the mechanism of unconventional superconductors. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U37.00002: Tunneling Spectroscopy in Iron Pnictides to Track Orbital Splitting and Spin Density Waves Nachum Plonka, Alexander Kemper, Thomas Devereaux, Siegfried Graser, Arno Kampf In iron-based superconductors, nematicity has been reported in transport measurements and a broad range of spectroscopies, including angle-resolved photoemission, neutron scattering, and scanning tunneling spectroscopy (STS). Several theories have attributed these observed anisotropies of broken tetragonal symmetry to either pure spin physics or unequal occupation of the iron d-electron orbitals, referred to as orbital ordering. We use realistic multi-orbital tight-binding Hamiltonians and T-matrix formalism to explore the effects of non-magnetic impurities in an orbitally split and spin density wave (SDW) state. In each of these, the local density of states around the impurity in both position space and Fourier-transformed quasiparticle interference (QPI) have very specific signatures that may be observable in STS. These allow one to identify and track the evolution of orbital splitting and SDW gaps in regimes that have not previously been explored. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U37.00003: Electronic Inhomogeneity and Vortex Disorder in Superconducting Sr$_{0.75}$K$_{0.25}$Fe$_{2}$As$_{2}$ Can-Li Song, Yi Yin, Martin Zech, Tess Williams, Michael Yee, Gen-Fu Chen, Jian-Lin Luo, Nan-Lin Wang, Eric W. Hudson, Jennifer E. Hoffman We characterize the surface structure, superconducting, and vortex properties in the hole-doped superconductor Sr$_{0.75}$K$_{0.25}$Fe$_{2}$As$_{2}$ (underdoped, $T_{c}=$32 K) by scanning tunneling microscopy. A 1 $\times$ 2 surface reconstruction and inhomogeneous superconducting gap with clear coherence peaks are universally found on the dominant Sr/K-terminated surfaces. Rarer patches of As termination show no reconstruction and no gap. The superconducting gap energy $\Delta $ anti-correlates with both the zero bias conductance and coherence peak strength with a characteristic length scale of $\sim$ 3 nm. Isotropic single-quantum vortices with short-range hexagonal order are imaged at 9 T magnetic field. By fitting the vortex-induced subgap density of states, the coherence length $\xi \sim$ 2.8 nm is found to be comparable to the length scale of $\Delta $ variations. We suggest that the vortices are strongly pinned by nanoscale electronic inhomogeneity arising from K clustering. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U37.00004: Visualizing the microscopic coexistence of spin density wave and superconductivity in underdoped NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As Peng Cai, Xiaodong Zhou, Wei Ruan, Aifeng Wang, Xianhui Chen, Dung-Hai Lee, Yayu Wang Although the origin of high Tc superconductivity in the iron pnictides is still under debate, it is widely believed that magnetic interactions or fluctuations play an important role in triggering Cooper pairing. Because of the relevance of magnetism to pairing, the question of whether long range spin magnetic order can coexist with superconductivity microscopically has attracted strong interests. The available experimental methods used to answer this question are either bulk probes or local ones without control of probing position, thus the answers range from mutual exclusion to homogeneous coexistence. In this talk we present STM studies of the local electronic structures of an underdoped NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As near the spin density wave (SDW) and superconducting (SC) phase boundary. Spatially resolved spectroscopy directly reveal both the SDW and SC gap features at the same atomic location, providing compelling evidence for the microscopic coexistence of the two phases. The strengths of the SDW and SC features are shown to anti correlate with each other, indicating the competition of the two orders. The microscopic coexistence clearly indicates that Cooper pairing occurs when portions of the Fermi surface are already gapped by the SDW order. [1]P. Cai, et al., arxiv:1208.3842(2012) [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U37.00005: Conductance spectra of the Fe111 compounds in the normal and superconducting states Hamood Arham, W.K. Park, L.H. Greene, D.Y. Chung, D. Bugaris, M.G. Kanatzidis We use quasiparticle scattering spectroscopy (QPS), also known as point contact spectroscopy, to study Co doped NaFeAs. A conductance enhancement is observed in the normal state of NaFeAs with an onset temperature $\sim$ 95 K. Our previous work on the electron and hole doped Fe122 compounds revealed that a conductance enhancement in the normal state is only observed for those compounds that have an in-plane resistive anisotropy. This enhancement is caused by the non-Fermi liquid behavior of these compounds due to orbital fluctuations. (Arham et al. PRB 85, 214515 (2012); Lee et al. arXiv:1110.5917). Our initial results indicate that the same conditions hold true for the Fe111 compounds as well. QPS is effective in detecting strong electron correlations (hybridization gap, Fano resonance, orbital fluctuations) in the normal state of a variety of strongly correlated electron systems that exhibit the ubiquitous `domed' phase diagram. The need for some kind of a microscopic theory that explains how QPS detects strong electron correlations will be discussed. This work is supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the US DOE, Office of Science, Award No. DE-AC0298CH1088. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U37.00006: Doping dependence of the gap of cobalt doped BaFe$_{2}$As$_{2}$ from Point Contact Spectroscopy John Timmerwilke, Brendan Faeth, J.S. Kim, G.R. Stewart, Amlan Biswas Point-contact spectroscopy (PCS) is a unique method which has been used for investigating the gap/s of various superconductors including the iron based superconductors. PCS measurements are capable of systematically identifying the size and number of gaps in a superconductor, certain features of various gap symmetries and gap anisotropy. We have performed a-b plane point contact measurements on single crystal Ba(Fe$_{1-x}$Co$_{x)2}$As$_{2}$ samples in the under, optimal, and over-doped cases. Previously we had shown clear evidence of two full gaps in the optimally-doped case. The under and over-doped crystals do not show such definitive evidence of two gaps. The changes in anisotropy and weight of the gaps for these dopings will be presented. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U37.00007: Similarities in the Tunneling Spectral Dip in FeAs-based and Cuprate Superconductors John Zasadzinski, Liam Coffey, Omid Ahmadi, Ken Gray, David Hinks Recent STS measurements on LiFeAs revealed an above-gap spectral dip feature in the superconducting state that diminished in size with increasing T and disappeared at Tc. We argue that such a feature mimics conventional strong coupling effects and bears a striking resemblance to dip features found in cuprates such as Bi2212. In all cases, the estimated boson energy, $\Omega$, lies within the superconducting gap, 2$\Delta $, suggesting a spin exciton, and is $\sim$ 5k$_{\mathrm{B}}$Tc, consistent with the resonance mode found in neutron scattering. The doping dependence of the dip in Bi2212 break junctions is reviewed and it is shown that fits of the tunneling data can be achieved using an Eliashberg formalism. The electron-boson spectral function is dominated by a sharp peak at $\Omega $. These results indicate that the two classes of superconductors have a similar pairing interaction of electrons coupled to a spin fluctuation spectrum renormalized by superconductivity. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U37.00008: Temperature-concentration phase diagram and multigap superconductivity revealed by soft point-contact spectroscopy in (Ca $_{\mathrm{1-x}}$ La$_{\mathrm{x}})_{10}$(Pt$_3$As$_8)$(Fe$_2$As$_2)_5$ Ni Ni, Eunsung Park, Warren E. Straszheim, Xin Lu, Darrick J. Williams, Makariy A. Tanatar, Ruslan Prozorov, Eric D. Bauer, Filip Ronning, Joe D. Thompson, Robert J. Cava Sizable single crystals of the superconducting iron-pnictide system (Ca $_{\mathrm{1-x}}$ La$_{\mathrm{x}})_{10}$(Pt$_{3}$As$_{\mathrm{8}})$(Fe$_{2}$As$_{2})_{5}$ (x$=$0 to 0.182) have been grown and characterized by X-ray, microscopic, resistivity, Hall coefficient, susceptibility and specific heat measurements. Features in magnetic susceptibility, specific heat and two kinks in the derivative of the electrical resistivity around 100 K in the x$=$0 compound support the existence of decoupled structural and magnetic phase transitions. With La doping, the structural/magnetic phase transitions are suppressed and a dome of superconductivity with a maximal T$_{\mathrm{c}}$ up to 23 K is observed in the temperature-concentration phase diagram. Soft point-contact spectroscopy was performed on the optimally doped sample of x$=$0.145. By fitting the multigap Blonder-Tinkham-Klapwijk(BTK) model to the data, three gaps with $\Delta _{1}=$1 meV, $\Delta_{2}=$8 meV and $\Delta _{3}=$27 meV are revealed. Acknowledgement: Work at Los Alamos was performed under the auspices of the US DOE. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U37.00009: Surface investigation of Ca$_{1-x}$Pr$_{x}$Fe$_{2}$As$_{2}$ by scanning tunneling microscopy Dennis Huang, Ilija Zeljkovic, Can-Li Song, Bing Lv, Ching-Wu Chu, Jennifer E. Hoffman Rare-earth-doped CaFe$_{2}$As$_{2}$ exhibits small volume-fraction superconductivity up to 49 K of unknown origin [1,2]. We use scanning tunneling microscopy to locally probe possible sources of this phase in Ca$_{1-x}$Pr$_{x}$Fe$_{2}$As$_{2}$. We encounter three kinds of surface morphologies and infer their chemical identities with local work function measurements. We also image Pr$^{3+}$ dopants as positive-energy resonances in tunneling conductance and examine their relationship with an observed inhomogeneous spectral gap. [1] B. Lv, L. Denga, M. Goocha, F. Weia, Y. Suna, J. K. Meena, Y.-Y. Xuea, B. Lorenza, and C.-W. Chu, Proc. Nat. Acad. Sci. 108, 15705 (2011). [2] S. R. Saha, N. P. Butch, T. Drye, J. Magill, S. Ziemak, K. Kirshenbaum, P. Y. Zavalij, J. W. Lynn, and J. Paglione, Phys. Rev. B 85, 024525 (2012) [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U37.00010: Identification of surface terminations of iron pnictides with low-temperature STM/STS Jihui Wang, Ang Li, Jihua Ma, Zheng Wu, Jiaxin Yin, Bing Lv, C.W. Chu, A. Sefat, M. McGuire, B. Sales, D. Mandrus, Chenglin Zhang, Pengcheng Dai, Rongying Jin, Jiandi Zhang, E.W. Plummer, Genfu Chen, Hong Ding, Shuheng H. Pan The alkaline-earth metal iron pnictide superconductor AEFe2As2 (AE$=$Ca, Sr, Ba) have been studied extensively with modern surface techniques, such as scanning tunneling microscopy/spectroscopy (STM/STS) and Angle Resolved Photoemission Spectroscopy (ARPES). Yet the surface termination upon cleaving is still controversial. Hence, the interpretation of those results of STM/STS and reconcile with results of other surface techniques tend to be challenging. We have performed a systematic low-temperature STM/STS study on a series of (Ca,Na)Fe2As2, (Ba,K)Fe2As2, Ba(Fe,Co)2As2, and BaFe2(As,P)2. We found that, with cryogenic cleaving method, all three crystalline atomic layers can be revealed and identified. We will discuss their identities and their implications. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U37.00011: Observation of orbital governed surface selection of superconducting gap in iron Pnictides with low temperature STM/S Jiaxin Yin, Ang Li, Zheng Wu, Jihui Wang, Jian Li, Chin-Sen Ting, Chenglin Zhang, Pengcheng Dai, ChangQing Jin, Hong Ding, Shuheng H. Pan The strong anisotropy of orbitals plays important roles in strongly correlated electron systems. For iron pnictides, due to their layered structure, overlaping of iron 3d with arsenic 4p orbitals is essential in the pairing mechanism. To reveal such physics, Ba(K)Fe2As2 and LiFeAs are the ideal candidates owing to their integrity in the Fe-As layer. We have used low temperature scanning tunneling microscopy/spectroscopy (STM/STS) to investigate the orbital physics in Ba0.6K0.4Fe2As2 and LiFeAs at atomic level. By comparing the STM/S results on these two materials and referring to the results of angle resolved photoemission spectroscopy (ARPES), we found the phenomenon of surface dependent selection of superconducting gaps. We discuss the implications of these observations with the orbital physics in these materials. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U37.00012: (1x2) Surface Reconstruction for Ca(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_2$As$_2$: Spin-Charge-Lattice-Coupling Guorong Li, Liangbo Liang, V.B. Nascimento, Xiaobo He, A.B. Karki, Yimin Xiong, Vincent Meunier, Rongying Jin, Jiandi Zhang, E.W. Plummer Low energy electron diffraction (LEED) and density functional theory (DFT) have been utilized to investigate the surface structure for the stripe 1x2 phase of Ca(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ iron pnictides, for x $=$ 0 and x $=$ 0.075. Quantitative structural analysis of LEED-I(V) using the fractional spots of the 1x2 phase on both parent and doped samples gives a similar surface structure with a termination layer of half Ca atoms. The surface Ca layer has a large inward relaxation about 0.5 Angstrom and the underneath As-Fe-As layer displays a buckling distortion of about 0.07 Angstrom. DFT calculations show significant charge rearrangements at the surface, which is driven by spin charge coupling, verified by freezing the structure and reducing the magnetic moment to zero. The role of spin-charge coupling in determining the surface reconstruction will be elucidated by self-consistent calculations of the structure as a function of the magnetic moment. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U37.00013: Surface structure and electronic properties in Ca$_{10}$(Pt$_{4}$As$_{8}$)(Fe$_{2}$As$_{2}$)$_{5}$ Jisun Kim, Guorong Li, Amar Karki, Jiandi Zhang, Rongying Jin, E.W. Plummer Among Iron-based superconductors, a new family of Ca$_{10}$(Pt$_{n}$As$_{8}$)(Fe$_{2}$As$_{2}$)$_{5}$ with n= 3 (``10-3-8'') or n=4 (``10-4-8'') is unique owing to the existence of Pt$_{n}$As$_{8}$ layer. This sets them with different electronic properties than the rest of Iron-based superconductors. By cleaving 10-4-8 single crystals (T$_{c}$ $\sim$ 34 K) in the ultra-high vacuum, we are able to observe three surfaces: Ca layer, FeAs layer, and Pt$_{4}$As$_{8}$ layer. Scanning tunneling microscope (STM) reveals both the topology and electronic density of individual layers. We discuss the implications of our results with the combination of bulk electronic properties. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U37.00014: Multi-band model analysis of transport properties of Ba(FeAs)$_2$ Huynh Khuong, Yoichi Tanabe, Takahiro Urata, Satoshi Heguri, Takanori Kida, Masayuki Hagiwara, Katsumi Tanigaki In iron pnitides, unique energetic band topology and interband antiferromagnetic scatterings are the main sources of rich physics, including multiband superconductivity and Dirac cones quantum states [1, 2]. Despite its importance, the band structure of iron pnictides is not fully understood, especially in terms of transport phenomena. In this meeting, we present that the tranport properties of Ba(FeAs)$_2$, a typical iron pnictide compound, are strongly affected by the shape of Fermi surfaces and the high mobility ($\mu$) in the Dirac cones. From magnetic-field ($B$) dependencies of the conductivity tensor under $B < 50$ T, we successfully extracted a spectrum of carrier number as a function of $\mu$. Whereas the hole side of the spectra is purely characterized by parabolic hole pockets, the electron side shows interesting effects originating from partly concave Fermi pockets as well as the very high $\mu$ (50,000 cm$^2$V$^{-1}$s$^{-1}$) of the Dirac carriers. Our observations are also in a good agreement with the first principles band calculations and experimental spectroscopic observations on its Fermi surface [3, 4]. [1] K. Kuroki et al, PRL 101 (2008) [2] Ran et al, PRB 79 (2009) [3] Yin et al, Nat.Phys. 7 (2011) [4] T. Shimojima, PRL 104 (2010) [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700