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Register a new userAnisotropic upper critical field of pristine and proton-irradiated single crystals of the magnetically ordered superconductor RbEuFe$_4$As$_4$
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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.
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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.
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.
We investigate the effect of Ni doping on the Fe-site in single crystals of the magnetic superconductor RbEuFe$_4$As$_4$ for doping concentrations of up to 4%. A clear suppression in the superconducting transition temperature is observed in specific heat, resistivity and magnetization measurements. Upon Ni-doping, the resistivity curves shift up in a parallel fashion indicating a strong increase of the residual resistivity due to scattering by charged dopand atoms while the shape of the curve and thus the electronic structure appears largely unchanged. The observed step $Delta C/T_c$ at the superconducting transition decreases strongly for increasing Ni doping in agreement with expectations based on a model of multi-band superconductivity and strong inter-band pairing. The upper critical field slopes are reduced upon Ni doping for in- as well as out-of-plane fields leading to a small reduction in the superconducting anisotropy. The specific heat measurements of the magnetic transition reveal the same BKT behavior close to the transition temperature $T_m$ for all doping levels. The transition temperature is essentially unchanged upon doping. The in to out-of-plane anisotropy of Eu-magnetism observed at small magnetic fields is unaltered as compared to the undoped compound. All of these observations indicate a decoupling of the Eu magnetism from superconductivity and essentially no influence of Ni doping on the Eu magnetism in this compound.
We report an unusual enhancement of the magnetic induction in single crystals of the magnetic superconductor RbEuFe$_4$As$_4$ , highlighting the interplay between superconducting and magnetic subsystems in this material. Contrary to the conventional Meissner expulsion of magnetic flux below the superconducting transition temperature, we observe a substantial boost of the magnetic flux density upon approaching the magnetic transition temperature, Tm. Direct imaging of the flux evolution with a magneto-optical technique, shows that the magnetic subsystem serves as an internal magnetic flux pump, drawing Abrikosov vortices from the surface, while the superconducting subsystem controls their conveyance into the bulk of the magnetic superconductor via a peculiar self-organized critical state.
We report detailed specific heat measurements on the recently discovered magnetic superconductor RbEuFe$_4$As$_4$. We investigated the superconducting transition at 37K and extract the phase boundary for in and out-of plane fields resulting in an anisotropy ratio of 1.8. An unusual cusp-like feature in the calorimetric data near 14.9K marks the onset of a magnetic phase. Studying the effect of small fields along the crystallographic $c$ axis, we resolve a shift in the cusp position moving to lower temperatures. For in-plane fields the cusp rapidly disappears and a broad shoulder that shifts to higher temperatures. We are able to reproduce our measured calorimetry data quantitatively by Monte-Carlo simulations of an anisotropic easy-plane 2D Heisenberg model. We can thus show that (i) the spins are preferably in plane, (ii) the cusp in specific heat is due to a Berezinskii-Kosterlitz-Thouless (BKT) transition, and (iii) the high-temperature hump in higher fields marks a crossover from a paramagnetically disordered to an ordered state. The extracted phase and crossover boundaries from experiment and simulations agree very well.
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