Session L21: 1144 Iron Based Superconductors

Structure and superconductivity in the 1144 type compounds of AeAFe$_{\mathrm{4}}$As$_{\mathrm{4}}$ (Ae $=$ Ca, Sr, A $=$ K, Rb, Cs)

The Fe-based superconductors$^{\mathrm{\thinspace }}$discovered in 2008 have attracted significant research interest, because of their rich material variety as well as their high superconducting transition temperatures (T$_{\mathrm{c}})$. A large number of related Fe-based superconductors with various types of crystal structures have been found to date. Here, we report new-structure-type Fe-based superconductors AeAFe$_{\mathrm{4}}$As$_{\mathrm{4}}$ (Ae $=$ Ca, Sr, A $=$ K, Rb, or Cs), which can be regarded as hybrid phases between AeFe$_{\mathrm{2}}$As$_{\mathrm{2}}$ and AFe$_{\mathrm{2}}$As$_{\mathrm{2}}$. Unlike solid solutions such as (Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}})$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ and (Sr$_{\mathrm{1-x}}$Na$_{\mathrm{x}})$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$, the Ae and A do not occupy crystallographically equivalent sites, owing to large differences between their ionic radii. Rather, the Ae and A layers are inserted alternately between the Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ layers in the c-axis direction in AeAFe$_{\mathrm{4}}$As$_{\mathrm{4}}$ (AeA1144). The ordering of the Ae and A layers causes a change in space group from I4/mmm to P4/mmm, which is clearly apparent in powder X-ray diffraction patterns. AeA1144 is the first known structure among not only Fe-based superconductors, but also other materials. The AeA1144 is formed as a stoichiometric compound. Therefore, each AeA1144 has its own T$_{\mathrm{c}}$ of approximately 31$-$36 K.   

Superconductivity and Ferromagnetism in AEuFe4As4 (A $=$ Rb and Cs).

Superconductivity (SC) and ferromagnetism (FM) are mutually antagonistic collective phenomena in solids, and therefore, bulk SC rarely coexists with FM in a single material. Here we report evidence of coexistence of bulk SC and full FM in 1144-type iron pnictides~AEuFe4As4 (A$=$Rb and Cs), variants of EuFe2As2 in which~every alternate Eu layer is replaced by non-magnetic Rb/Cs layer. Both materials show bulk SC at 35-36 K and Eu-spin ferromagnetic ordering at ca. 15 K. The robustness of SC and FM in AEuFe4As4 suggests that the expected mutual suppression between SC and FM can be minimized via a certain mechanism, which may shed light on the mechanism of iron-based SC.   

Anisotropic physical properties of single phase, single-crystalline CaKFe$_4$As$_4$

An overview of the synthesis and anisotropic thermodynamic and transport properties of single-crystalline, single-phase CaKFe$_4$As$_4$ will be presented [1]. The samples were grown out of a high-temperature, quaternary melt. Temperature-dependent measurements of x-ray diffraction, anisotropic electrical resistivity, elastoresistivity, thermoelectric power, Hall effect, magnetization, specific heat, and $^{57}$Fe M\"ossbauer spectroscopy measurements, combined with field-dependent measurements of electrical resistivity and field and pressure-dependent measurements of magnetization indicate that CaKFe$_4$As$_4$ is an ordered, stoichiometric, Fe-based superconductor with a superconducting critical temperature, $T_c = 35.0 \pm 0.2$ K. Other than superconductivity, there is no indication of any other phase transition for 1.8K $\leq T \leq$ 300 K. All of these thermodynamic and transport data reveal striking similarities to those found for optimally or slightly overdoped (Ba$_{1−x}$K$_x$)Fe$_2$As$_2$, suggesting that stoichiometric CaKFe$_4$As$_4$ is intrinsically close to what is referred to as “optimally-doped” on a generalized phase diagram for Fe-based superconductors. The anisotropic superconducting upper critical field, Hc2(T), of CaKFe$_4$As$_4$, determined up to $\sim 65$ T will be discussed in some detail. A comparison with CaFe$_2$As$_2$ and KFe$_2$As$_2$ will be presented. \\ \\ 1. W. R. Meier, T. Kong, U. S. Kaluarachchi, V. Taufour, N. H. Jo, G. Drachuck, A. E. B\"ohmer, S. M. Saunders, A. Sapkota, A. Kreyssig, M. A. Tanatar, R. Prozorov, A. I. Goldman, Fedor F. Balakirev, Alex Gurevich, S. L. Bud'ko, and P. C. Canfield, Phys. Rev. B 94, 064501 (2016).   

Visualizing the vortex lattice in stoichiometric high Tc CaKFe4As4 superconductor

Many single crystalline iron based superconductors can be understood by considering a s-wave superconducting order parameter that changes sign in two different parts of the Fermi surface. Interband scattering in such s$+$- superconductors is expected to produce pair-breaking close to crystalline defects, as a consequence of the sign changes of the order parameter. Unlike other superconductors, vortex lattices in single crystals of iron based superconductors are often disordered. However, it is still unknown if vortex pinning is due to pair breaking defects as a consequence of sign changing superconductivity or to substitutional disorder as in conventional superconductors. Here I will present Scanning Tunneling Microscopy studies in single crystals of stoichiometric CaKFe4As4. The superconducting critical temperature is very high, of Tc$=$ 35 K, a value comparable to that found near optimal substitution levels in other non-stoichiometric iron based systems. I will discuss evidence for two-gap superconducting behavior and sign changing superconductivity. I will also show that the vortex lattice is hexagonal but becomes disordered after a few lattice spacings. Results of imaging individual vortices, show that the spatial variation of the superconducting order parameter within the vortex core has the same length scale for the two parts of the Fermi surface. Images of the whole vortex lattice show that disorder in the vortex positions appears because of pair breaking induced by scattering by defects.   

Electronic structure and superconducting gap in CaKFe$_{\mathrm{4}}$As$_{\mathrm{4}}$

In my talk I will utilize the density functional theory and high resolution angle resolved photoemission spectroscopy to study the electronic properties of CaKFe$_{\mathrm{4}}$As$_{\mathrm{4}}$. In contrast to related CaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$ compounds, CaKFe$_{\mathrm{4}}$As$_{\mathrm{4}}$ has high Tc of 35K at stochiometric composition. This presents unique opportunity to study properties of high temperature superconductivity of iron arsenic superconductors in absence of doping or substitution. The Fermi surface consists of three hole pockets at $\Gamma $ and two electron pockets at the $M$ point. The values of the superconducting gap are nearly isotropic, but significantly different for each of the FS sheets. Most importantly one finds that the overall momentum dependence of the gap magnitudes plotted across the entire Brillouin zone displays a strong deviation from the simple cos(kx)cos(ky) functional form of the gap function, proposed in the scenario of the Cooper-pairing driven by a short range antiferromagnetic exchange interaction. Instead, the maximum value of the gap is observed for FS sheets that are closest to the ideal nesting condition in contrast to the previous observations in some other ferropnictides. These results provide strong support for the multiband character of superconductivity in CaKFe$_{\mathrm{4}}$As$_{\mathrm{4}}$, in which Cooper pairing forms on the electron and the hole bands interacting via dominant interband repulsive interaction, enhanced by FS nesting.