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Register a new userAnisotropy induced vortex lattice rearrangement in CaKFe$_4$As$_4$
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The magnetic penetration depth anisotropy $gamma_lambda=lambda_{c}/lambda_{ab}$ ($lambda_{ab}$ and $lambda_{c}$ are the in-plane and the out-of-plane components of the magnetic penetration depth) in a CaKFe$_4$As$_4$ single crystal sample (the critical temperature $T_{rm c}simeq 35$ K) was studied by means of muon-spin rotation ($mu$SR). $gamma_lambda$ is almost temperature independent for $Tlesssim 20$ K ($gamma_lambdasimeq 1.9$) and it reaches $simeq 3.0$ by approaching $T_{rm c}$. The change of $gamma_lambda$ induces the corresponding rearrangement of the flux line lattice (FLL), which is clearly detected via enhanced distortions of the FLL $mu$SR response. Comparison of $gamma_lambda$ with the anisotropy of the upper critical field ($gamma_{H_{rm c2}}$) studied in Phys. Rev B {bf 94}, 064501 (2016), reveals that $gamma_lambda$ is systematically higher than $gamma_{H_{rm c2}}$ at low-temperatures and approaches $gamma_{H_{rm c2}}$ for $T rightarrow T_{rm c}$. The anisotropic properties of $lambda$ are explained by the multi-gap nature of superconductivity in CaKFe$_4$As$_4$ and are caused by anisotropic contributions of various bands to the in-plane and the out-of-plane components of the superfluid density.
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We report the temperature-pressure phase diagram of CaKFe$_4$As$_4$ established using high pressure electrical resistivity, magnetization and high energy x-ray diffraction measurements up to 6 GPa. With increasing pressure, both resistivity and magnetization data show that the bulk superconducting transition of CaKFe$_4$As$_4$ is suppressed and then disappears at $p$ $gtrsim$ 4 GPa. High pressure x-ray data clearly indicate a phase transition to a collapsed tetragonal phase in CaKFe$_4$As$_4$ under pressure that coincides with the abrupt loss of bulk superconductivity near 4 GPa. The x-ray data, combined with resistivity data, indicate that the collapsed tetragonal transition line is essentially vertical, occuring at 4.0(5) GPa for temperatures below 150 K. Band structure calculations also find a sudden transition to a collapsed tetragonal state near 4 GPa, as As-As bonding takes place across the Ca-layer. Bonding across the K-layer only occurs for $p$ $geq$ 12 GPa. These findings demonstrate a new type of collapsed tetragonal phase in CaKFe$_4$As$_4$: a half-collapsed-tetragonal phase.
We use polarized inelastic neutron scattering to study the spin-excitations anisotropy in the bilayer iron-based superconductor CaKFe$_4$As$_4$ ($T_c$ = 35 K). In the superconducting state, both odd and even $L-$modulations of spin resonance have been observed in our previous unpolarized neutron scattering experiments (T. Xie {it et al.} Phys. Rev. Lett. {bf 120}, 267003 (2018)). Here we find that the high-energy even mode ($sim 18$ meV) is isotropic in spin space, but the low-energy odd modes consist of a $c-$axis polarized mode around 9 meV along with another partially overlapped in-plane mode around 12 meV. We argue that such spin anisotropy is induced by the spin-orbit coupling in the spin-vortex-type fluctuations of this unique compound. The spin anisotropy is strongly affected by the superconductivity, where it is weak below 6 meV in the normal state and then transferred to higher energy and further enhanced in the odd mode of spin resonance below $T_c$.
Measurements of the London penetration depth and tunneling conductance in single crystals of the recently discovered stoicheometric, iron - based superconductor, CaKFe$_4$As$_4$ (CaK1144) show nodeless, two effective gap superconductivity with a larger gap of about 6-9 meV and a smaller gap of about 1-4 meV. Having a critical temperature, $T_{c,onset}approx$35.8 K, this material behaves similar to slightly overdoped Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ (e.g. $x=$0.54, $T_c approx$ 34 K)---a known multigap $s_{pm}$ superconductor. We conclude that the superconducting behavior of stoichiometric CaK1144 demonstrates that two-gap $s_{pm}$ superconductivity is an essential property of high temperature superconductivity in iron - based superconductors, independent of the degree of substitutional disorder.
The temperature dependence of the in-plane magnetic penetration depth ($lambda_{ab}$) in an extensively characterized sample of superconducting CaKFe$_4$As$_4$ ($T_{rm c}simeq35$ K) was investigated using muon-spin rotation ($mu$SR). A comparison of $lambda_{ab}^{-2}(T)$ measured by $mu$SR with the one inferred from ARPES data confirms the presence of multiple gaps at the Fermi level. An agreement between $mu$SR and ARPES requires the presence of additional bands, which are not resolved by ARPES experiments. These bands are characterized by small supercondcting gaps with an average zero-temperature value of $Delta_{0} =$ 2.4(2) meV. Our data suggest that in CaKFe$_4$As$_4$ the $s^pm$ order parameter symmetry acquires a more sophisticated form by allowing a sign change not only between electron and hole pockets, but also within pockets of similar type.
We employ polarization-resolved Raman spectroscopy to study multi-band stoichiometric superconductor CaKFe$_4$As$_4$. The B$_{2g}$ symmetry Raman response shows no signatures of Pomeranchuk-like electronic nematic fluctuations which is observed for many other Fe-based superconductors. In the superconducting state, we identify three pair-breaking peaks at 13.8, 16.9 and 21 meV and full spectral weight suppression at low energies. The pair-breaking peak energies in Raman response are about 20% lower than twice the gap energies as measured by single-particle spectroscopy, implying a sub-dominant $d$-wave symmetry interaction. We analyze the superconductivity induced phonon self-energy effects and give an estimation of weak electron-phonon coupling constant $lambda^Gamma$=0.0015.
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