Dr. Freek Massee, Peter Sprau, Andrey Kostin

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.


[Top Left] Constant-current topographic image of the iron-based superconductor Ca(Fe1-xCox)2As2 taken at V0=-50mV and Iz=10pA on a 71.0-nm square FOV. (Inset) A high-resolution topograph in a smaller FOV (4.2-nm square) taken at V0=-5mV and Iz=10pA=100pA. The red and blue arrows indicate the atoms of 1x2 configuration. The orange arrows indicate the first indications of unidirectional a axis-oriented electronic nanostructures.
[Top Right] The current map I(r,E)=+50meV. (Inset) Autocorrelation analysis of this image; the centers of self-similarity peaks (red dotted lines) are separated by ∼8aFe-Fe.
[Bottom Left] The current image, I(r,E)=50meV. The static unidirectional domains clearly change directionality by 90ˆ across the twin boundary. (Insets) Autocorrelations; center of self-similarity peaks (gray dotted lines) are separated by ∼8aFe-Fe.
[Bottom Right] (movie) Fourier transform of the conductance maps, FFT{g(r,E)}=g(q,E), as a function of energy.

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.

From transport studies we know that the anisotropy is actually very weak at x=0, grows towards a maximum near x∼4%, and disappears beyond x=xc a situation now widely observed in these materials. The higher resistivity (quasi-insulating) axis is always the ferromagnetic b-axis, as also detected in high frequency transport studies. However, these anisotropic transport characteristics cannot be due merely to crystal orthorhombicity. Firstly, they are minimal at zero doping where orthorhombicity is a maximum. Further, the residual resistivity, as well as the resistivity anisotropy, increase ∼linearly with Co dopant density, and atomic substitutions outside the FeAs plane generate very weak transport anisotropy.Moreover the resistivity maximum along the crystal b-axis develops with increasing concentration of dopant atoms; this 'nematicity' vanishes when the `parent' phase disappears near the maximum superconducting Tc. The interplay between the electronic structure surrounding each dopant atom, quasiparticle scattering therefrom, and the transport nematicity has therefore become a pivotal focus of research into these materials.

By directly visualizing the atomic-scale electronic structure, we first showed that substituting Co for Fe atoms in underdoped iron-based superconductor Ca(Fe1-xCox)2As2 generates a dense population of identical anisotropic impurity states. Second, by imaging their surrounding interference patterns, we demonstrate that these impurity states scatter quasiparticles in a highly anisotropic manner, with the maximum scattering rate concentrated along the b-axis. These data show that the anisotropic scattering by dopant-induced impurity states can contribute strongly to the transport nematicity; they also yield simple explanations for the enhancement of the nematicity proportional to the dopant density and for the occurrence of the highest resistivity along the b-axis.

These results were published in Nature Physics 9, 220 (2013).


[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.

If strong electron-electron interactions between neighboring Fe atoms mediate the Cooper pairing in iron-pnictide superconductors, then specific and distinct anisotropic superconducting energy gaps Δi(k) should appear on the different electronic bands i. We introduced intra-band Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of Δi(k) in such materials. We identify three hole-like bands and determine the , magnitude and relative orientations of their Δi(k). Such data could play a key role in identifying the Cooper pairing mechanism of iron-based superconductivity.

These results have been published in Science 336, 563.


[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.

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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.

We recently upgraded our cryostat so that it can operate between 250mK and 20K, with a hold time at the lowest temperature of about 5 days. Additionally, we installed a new Dewar and a 9 Tesla magnet.


[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.

* Representative scientists investigating iron based superconductors with a spectroscopic imaging STM

Prof. Paul Canfield - Condensed Matter Physics Group, Ames Lab, Iowa State University
Prof. Greg Boebinger - National High Magnetic Field Lab, Florida State University
Dr. Hiroshi Eisaki - National Institute of Advanced Industrial Science and Technology (AIST), Japan