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Register a new userAnisotropic thermodynamic and transport properties of single crystalline CaKFe$_{4}$As$_{4}$
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Single crystalline, single phase CaKFe$_{4}$As$_{4}$ has been grown out of a high temperature, quaternary melt. Temperature dependent measurements of x-ray diffraction, anisotropic electrical resistivity, elastoresistivity, thermoelectric power, Hall effect, magnetization and specific heat, 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 1.8 K $leq T leq$ 300 K. All of these thermodynamic and transport data reveal striking similarities to that found for optimally- or slightly over-doped (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 optimal-doped on a generalized, Fe-based superconductor, phase diagram. The anisotropic superconducting upper critical field, $H_{ctext{2}}(T)$, of CaKFe$_{4}$As$_{4}$ was determined up to 630 kOe. The anisotropy parameter $gamma(T)=H_{ctext{2}}^{perp}/H_{ctext{2}}^{|}$, for $H$ applied perpendicular and parallel to the c-axis, decreases from $simeq 2.5$ at $T_c$ to $simeq 1.5$ at 25 K which can be explained by interplay of paramagnetic pairbreaking and orbital effects. The slopes of $dH_{ctext{2}}^{|}/dTsimeq-44$ kOe/K and $dH_{ctext{2}}^{perp}/dT simeq-109$ kOe/K at $T_c$ yield an electron mass anisotropy of $m_{perp}/m_{|}simeq 1/6$ and short Ginzburg-Landau coherence lengths $xi_{|}(0)simeq 5.8 text{AA}$ and $xi_{perp}(0)simeq 14.3 text{AA}$. The value of $H_{ctext{2}}^{perp}(0)$ can be extrapolated to $simeq 920$ kOe, well above the BCS paramagnetic limit.
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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.
Plate-like single crystals of SrFe2As2 as large as 3x3x0.5 mm3 have been grown out of Sn flux. The SrFe2As2 single crystals show a structural phase transition from a high temperature tetragonal phase to a low temperature orthorhombic phase at To = 198 K, and do not show any sign of superconductivity down to 1.8 K. The structural transition is accompanied by an anomaly in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In an intermediate temperature range from 198 K to 160 K, single crystal X-ray diffraction suggests a coexistence of the high-temperature tetragonal and the low-temperature orthorhombic phases.
Single crystalline samples of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with $x < 0.12$ have been grown and characterized via microscopic, thermodynamic and transport measurements. With increasing Co substitution, the thermodynamic and transport signatures of the structural (high temperature tetragonal to low temperature orthorhombic) and magnetic (high temperature non magnetic to low temperature antiferromagnetic) transitions are suppressed at a rate of roughly 15 K per percent Co. In addition, for $x ge 0.038$ superconductivity is stabilized, rising to a maximum $T_c$ of approximately 23 K for $x approx 0.07$ and decreasing for higher $x$ values. The $T - x$ phase diagram for Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ indicates that either superconductivity can exist in both low temperature crystallographic phases or that there is a structural phase separation. Anisotropic, superconducting, upper critical field data ($H_{c2}(T)$) show a significant and clear change in anisotropy between samples that have higher temperature structural phase transitions and those that do not. These data show that the superconductivity is sensitive to the suppression of the higher temperature phase transition.
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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