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Research on Iron-based superconductivity The discovery of iron-based superconductors (FeSC) by the Hosono Group in 2008 has sparked a great deal of research effort aimed at understanding their properties and underlying physics. These materials allow, for the first time, to contrast and compare completely different families of high-temperature superconductors, and potentially to find the 'key ingredient' necessary for their existence. And indeed, striking analogies exist, such as the qualitatively similar phase diagram and the shared proximity of a magnetically ordered phase. Since high-temperature superconductors are strongly correlated electron systems, local measurements are especially significant. Our research has concentrated on four main areas: |
Discovery of Nematic Electronic Structure in the "Parent" State of the Iron-Based Superconductor Ca(Fe1-xCox)2As2 For this project, we used spectroscopic imaging scanning tunneling microscopy (SI-STM) to study the electronic structure of the iron-based superconductor CaFe1.94Co0.06As2, in the underdoped state from which high temperature superconductivity emerges. This was the first time the electronic structure of the parent state of any FeSC was imaged, and it lead to the unexpected discovery: The delocalized electronic states detectable by quasiparticle interference imaging are dispersive along the b-axis only, and are consistent with a with an apparent q≈±2π/8aFe-Fe folding along the a-axis. All these effects rotated through 90° at orthorhombic twin-boundaries indicating that they are bulk properties. As none of these phenomena are expected due merely to crystal symmetry, underdoped ferropnictides may exhibit a more complicated electronic nematic state than originally expected. The electronic nematicity in underdoped FeScs has subsequently been confirmed by many different probes, including DC-transport, AC-transport, ARPES, and magnetic torque measurements. Yet, the microscopic origin of the electronic nematicity is not fully understood. These results were published in Science 327, 181. |
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C2 anisotropic Impurity states, QPI and nematic transport Starting from the discovery of the electronic nematicity in underdoped FeSC, we wanted to investigate the microscopic origin of these and related phenomena, in particular the strikingly anisotropic electronic transport that was subsequently discovered after detwinning techniques were developed. |
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[Left] a. 48x48nm2 topographic image of the Ca-122 surface, taken simultaneously with the measurements depicted in (b-d). The horizontal lines stem from a surface reconstruction and do not influence our ability to visualize the correct FeAs electronic structure; the orientations of crystal axes are identified. b. The simultaneously recorded non-dispersive component in electronic structure as determined the current map I(r,E) at E=-37meV. The inset shows the characteristic non-dispersive electronic structure environment of a typical Co dopant atom: it is observed directly to be a "dimer" shaped electronic impurity state. c. The autocorrelation of I(r,-37meV) shows three peaks separated by the characteristic length scale of 22Å. Note that apart from the triple-peak, the AC{I(r,-37meV)} signal is low, proving that 22Å is the only persistent length scale in the image. e. Inset, the proposed impurity state with the Fe-lattice for comparison. The lower half show simulations with n=5 and n=47. f. A simulation with n=1000 anisotropic impurity states in the same 48x48nm2 FOV; their centers are randomly distributed but they are all aligned with the a-axis. This 'glassy' pattern of overlapping anisotropic impurity states looks similar to the data shown in b-d. The autocorrelation (g) of the image in f support the validity of our deduction that the static electronic disorder consists of a-axis oriented electronic anisotropic impurity states only. [Right] A schematic phase diagram with increasingly nematic transport is indicated by blue, yellow and red colors, where the latter marks strongest transport anisotropy. To emphasize the directional dependence of the scattering probability from our QPI data, we plot in the inset the cumulative scattering probability in a given direction Θ=∠(k,k'). This is obtained by integrating over the radial coordinate: W(Θ)=∫dq W(Θ, |q|) in a. Note the strong scattering maxima along the b-axis. |
Anisotropic Energy Gaps of Iron-Based Superconductivity from Intraband Quasiparticle Interference in LiFeAs This project concentrates on the mechanism that leads to Cooper pairing in high-temperature superconductors.
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[Top] Anisotropic gaps as measured by SI-STM. [Bottom Left] Simulation of the h3 band with an anisotropic Gap and Fermi surface. [Bottom Center] Our Bogoliubov quasiparticle interference data agrees well with the model. [Bottom Right] To quantitatively extract the gap as a function of angle, we measure QPI maxima along the minimum gap direction. |
The conductance map ∼LDOS(r, E) measured at 1.2K. Clear oscillations in the density of states are visible, with wavevectors that change with energy. Note the In-gap state at ∼1.4meV. * What you can try when you don't see the above movies running : |
250mK - 9 Tesla Spectroscopic Imaging STM for Iron-Pnictide Studeis For an experiment as described, one measures typically ∼50 energy points, on a ∼512x512 point r-grid, in total that is >20 millions datapoints that have to be taken in a low temperature cryostat and in ultra-high vacuum. Thus, measuring the individual spectra in short time and with a high signal to noise ratio is imperative. A rigid head design and a thorough vibration insulation as well as well-designed electronics are a necessity. |
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[Top Left] Three different views of the STM head and the 250mK to 20K cryostat. [Top Right] STM head and cryostat. (A,B), Schematic drawing of the STM head (CAD drawing by C. Taylor). (C,D) Photograph of the cryostat and head. For instrumental details, see Rev. Sci. Instrum. 70, 1459. [Bottom] Two stage vibration insulation. |
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* Representative scientists investigating iron based superconductors with a spectroscopic imaging STM |
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