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Register a new userAnisotropic magnetoresistance and upper critical fields up to 63 T in CaKFe$_4$As$_4$ single crystals
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We report the temperature dependencies of the upper critical fields $H_{ctext{2}}^{text{c}}(T)$ parallel to the c-axis and $H_{ctext{2}}^{text{ab}}(T)$ parallel to the ab-plane of single crystalline CaKFe$_4$As$_4$ inferred from the measurements of the temperature-dependent resistance in static magnetic fields up to 14 T and magnetoresistance in pulsed fields up to 63 T. We show that the observed decrease of the anisotropy parameter $gamma(T)=H_{ctext{2}}^{text{ab}}/H_{ctext{2}}^{text{c}}$ from $simeq 2.5$ at $T_c$ to $simeq 1.5$ at 25 K can be explained by interplay of paramagnetic pairbreaking and orbital effects in a multiband theory of $H_{c2}$. The slopes of $dH_{ctext{2}}^{text{c}}/dTsimeq-4.4$ T/K and $dH_{ctext{2}}^{text{ab}}/dT simeq-10.9$ T/K at $T_c$ yield an electron mass anisotropy of $m_{ab}/m_csimeq 1/6$ and short coherence lengths $xi_csimeq 5.8,text{AA}$ and $xi_{ab}simeq 14.3,text{AA}$. The behavior of $H_{ctext{2}}(T)$ turns out to be similar to that of the optimal doped (Ba,K)Fe$_2$As$_2$, with $H_{ctext{2}}^{text{ab}}(0)$ extrapolating to $simeq 92$ T, well above the BCS paramagnetic limit.
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We present a study of the upper critical field, H$_{c2}$, of pristine and proton-irradiated RbEuFe$_4$As$_4$ crystals in pulsed magnetic fields of up to 65 T. The data for H$_{c2}$ reveal pronounced downwards curvature, particularly for the in-plane field orientation, and a superconducting anisotropy that decreases with decreasing temperature. These features are indicative of Pauli paramagnetic limiting. For the interpretation of these data, we use a model of a clean single-band superconductor with an open Fermi surface in the shape of a warped cylinder, which includes strong paramagnetic limiting. Fits to the data reveal that the in-plane upper critical field is Pauli paramagnetic limited, while the out-of-plane upper critical field is orbitally limited and that the orbital and paramagnetic fields have opposite anisotropies. A consequence of this particular combination is the unusual inversion of the anisotropy, $H_{c2}^{ab} < H_{c2}^c$, of the irradiated sample at temperatures below 10 K. The fits also yield an in-plane Maki parameter, $alpha_M^{110} approx$ 2.6, exceeding the critical value for the formation of the Fulde-Ferrell-Larkin-Ovchinnikov state. Nevertheless, the current measurements did not reveal direct evidence for the occurrence of this state.
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 upper critical field of multiband superconductors is an important quantity that can reveal the details about the nature of the superconducting pairing. Here we experimentally map out the complete upper critical field phase diagram of a stoichiometric superconductor, CaKFe$_4$As$_4$, up to 90T for different orientations of the magnetic field and at temperatures down to 4.2K. The upper critical fields are extremely large, reaching values close to ~3$T_c$ at the lowest temperature, and the anisotropy decreases dramatically with temperature leading to essentially isotropic superconductivity at 4.2K. We find that the temperature dependence of the upper critical field can be well described by a two-band model in the clean limit with band coupling parameters favouring intraband over interband interactions. The large Pauli paramagnetic effects together with the presence of the shallow bands is consistent with the stabilization of an FFLO state at low temperatures in this clean superconductor.
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.
We investigate the anisotropic superconducting and magnetic properties of single-crystal RbEuFe$_4$As$_4$ using magnetotransport and magnetization measurements. We determine a magnetic ordering temperature of the Eu-moments of $T_m$ = 15 K and a superconducting transition temperature of $T_c$ = 36.8 K. The superconducting phase diagram is characterized by high upper critical field slopes of -70 kG/K and -42 kG/K for in-plane and out-of-plane fields, respectively, and a surprisingly low superconducting anisotropy of $Gamma$ = 1.7. Ginzburg-Landau parameters of $kappa_c sim 67$ and $kappa_{ab} sim 108$ indicate extreme type-II behavior. These superconducting properties are in line with those commonly seen in optimally doped Fe-based superconductors. In contrast, Eu-magnetism is quasi-two dimensional as evidenced by highly anisotropic in-plane and out-of-plane exchange constants of 0.6 K and $<$ 0.04 K. A consequence of the quasi-2D nature of the Eu-magnetism are strong magnetic fluctuation effects, a large suppression of the magnetic ordering temperature as compared to the Curie-Weiss temperature, and a cusp-like anomaly in the specific heat devoid of any singularity. Magnetization curves reveal a clear magnetic easy-plane anisotropy with in-plane and out-of-plane saturation fields of 2 kG and 4 kG.
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