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Register a new userStrong Pauli paramagnetic effect in the upper critical field of KCa$_2$Fe$_4$As$_4$F$_2$
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Recently, 12442 system of Fe-based superconductors has attracted considerable attention owing to its unique double-FeAs-layer structure. A steep increase in the in-plane upper critical field with cooling has been observed near the superconducting transition temperature, $T_c$, in KCa$_2$Fe$_4$As$_4$F$_2$ single crystals. Herein, we report a high-field investigation on upper critical field of this material over a wide temperature range, and both out-of-plane ($H|c$, $H_{c2}^{c}$) and in-plane ($H|ab$, $H_{c2}^{ab}$) directions have been measured. A sublinear temperature-dependent behavior is observed for the out-of-plane $H_{c2}^{c}$, whereas strong convex curvature with cooling is observed for the in-plane $H_{c2}^{ab}$. Such behaviors could not be described by the conventional Werthamer--Helfand--Hohenberg (WHH) model. The data analysis based on the WHH model by considering the spin aspects reveals a large Maki parameter $alpha=9$, indicating that the in-plane upper critical field is affected by a very strong Pauli paramagnetic effect.
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We find evidence that the newly discovered Fe-based superconductor KCa$_2$Fe$_4$As$_4$F$_2$ ($T_c~=~33.36(7)$~K) displays multigap superconductivity with line nodes. Transverse field muon spin rotation ($mu$SR) measurements show that the temperature dependence of the superfluid density does not have the expected behavior of a fully-gapped superconductor, due to the lack of saturation at low temperatures. Moreover, the data cannot be well fitted using either single band models or a multiband $s$-wave model, yet are well described by two-gap models with line nodes on either one or both of the gaps. Meanwhile the zero-field $mu$SR results indicate a lack of time reversal symmetry breaking in the superconducting state, but suggest the presence of magnetic fluctuations. These results demonstrate a different route for realizing nodal superconductivity in iron-based superconductors. Here the gap structure is drastically altered upon replacing one of the spacer layers, indicating the need to understand how the pairing state is tuned by changes of the asymmetry between the pnictogens located either side of the Fe planes.
We report synthesis, crystal structure and physical properties of a quinary iron-arsenide fluoride KCa$_2$Fe$_4$As$_4$F$_2$. The new compound crystallizes in a body-centered tetragonal lattice (with space group $I4/mmm$, $a$ = 3.8684(2) {AA}, c = 31.007(1) {AA}, and $Z$ = 2), which contains double Fe$_2$As$_2$ conducting layers separated by insulating Ca$_2$F$_2$ layers. Our measurements of electrical resistivity, dc magnetic susceptibility and heat capacity demonstrate bulk superconductivity at 33 K in KCa$_2$Fe$_4$As$_4$F$_2$.
We present a systematic study of electrical resistivity, Hall coefficient, magneto-optical imaging, magnetization, and STEM analyses of KCa${_2}$Fe${_4}$As${_4}$F${_2}$ single crystals. Sharp diamagnetic transition and magneto-optical imaging reveal homogeneity of single crystal and prominent Bean-like penetrations of vortices. Large anisotropy of electrical resistivity, with ${rho _c / rho _{ab}}$ > 100, and semiconductor-like ${rho _c}$ suggest that the electronic state is quasi two-dimensional. Hall effect measurements indicate that KCa${_2}$Fe${_4}$As${_4}$F${_2}$ is a multiband system with holes as main carriers. Magnetization measurements reveal significantly larger J$_c$ compared with that in other iron-based superconductors with different values of J$_c$ depending on the direction of magnetic field. Origin of these J$_c$ characteristics is discussed based on microstructural observations using STEM. In addition, further enhancement of J$_c$ in KCa${_2}$Fe${_4}$As${_4}$F${_2}$ for future application is demonstrated in terms of heavy-ion irradiation.
We report resistance and elastoresistance measurements on (Ba$_{0.5}$K$_{0.5}$)Fe$_2$As$_2$, CaKFe$_4$As$_4$, and KCa$_2$Fe$_4$As$_4$F$_2$. The Fe-site symmetry is $D_{2d}$ in the first compound but $C_{2v}$ in the latter two, which lifts the degeneracy of the Fe $d_{xz}$ and $d_{yz}$ orbitals. The temperature dependence of the resistance and elastoresistance is similar between the three compounds. Especially, the [110] elastoresistance is enhanced with decreasing temperature irrespective of the Fe-site symmetry. This appears to be in conflict with recent Raman scattering studies on CaKFe$_4$As$_4$, which suggest the absence of nematic fluctuations. We consider possible ways of reconciliation and suggest that the present result is important in elucidating the origin of in-plane resistivity anisotropy in iron-based superconductors.
We use ultrafast optical spectroscopy to study the nonequilibrium quasiparticle relaxation dynamics of the iron-based superconductor KCa$_2$Fe$_4$As$_4$F$_2$ with $T_c=33.5$ K. Our results reveal an evident pseudogap ($Delta_{PG}$ = 2.4 $pm$ 0.1 meV) below $T^*approx 50$ K but prior to the opening of a superconducting gap ($Delta_{SC}$(0) $approx$ 4.3 $pm$ 0.1 meV). Measurements under high pump fluence real two distinct coherent phonon oscillations with frequencies of 1.95 and 5.51 THz, respectively. The high-frequency mode corresponds to the $c-$axis polarized vibrations of As atoms ($A_{1g}$ mode) with a nominal electron-phonon coupling constant $lambda_{A_{1g}}$ = 0.194 $pm$ 0.02. Below $T_c$, the temperature dependence of both frequency and damping rate of $A_{1g}$ mode clearly deviate from the description of optical phonon anharmonic effects. These results suggest that the pseudogap is very likely a precursor of superconductivity, and the electron-phonon coupling may play an essential role in the superconducting pairing in KCa$_2$Fe$_4$As$_4$F$_2$.
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