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Register a new userAnalysis of the London penetration depth in Ni-doped CaKFe4As4
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We report combined experimental and theoretical analysis of superconductivity in CaK(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ (CaK1144) for $x=$0, 0.017 and 0.034. To obtain the superfluid density, $rho=left(1+Delta lambda_L(T)/lambda_L(0) right)^{-2}$, the temperature dependence of the London penetration depth, $Delta lambda_L (T)$, was measured by using tunnel-diode resonator (TDR) and the results agreed with the microwave coplanar resonator (MWR) with the small differences accounted for by considering a three orders of magnitude higher frequency of MWR. The absolute value of $lambda_L (T ll T_c) approx lambda_L(0)$ was measured by using MWR, $lambda_L (mathrm{5~K}) approx 170 pm 20$ nm, which agreed well with the NV-centers in diamond optical magnetometry that gave $lambda_L (mathrm{5~K}) approx 196 pm 12$ nm. The experimental results are analyzed within the Eliashberg theory, showing that the superconductivity of CaK1144 is well described by the nodeless s$_{pm}$ order parameter and that upon Ni doping the interband interaction increases.
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The London penetration depth $lambda$ is the basic length scale for electromagnetic behavior in a superconductor. Precise measurements of $lambda$ as a function of temperature, field, and impurity scattering have been instrumental in revealing the nature of the order parameter and pairing interactions in a variety of superconductors discovered over the past decades. Here we recount our development of the tunnel-diode resonator technique to measure $lambda$ as a function of temperature and field in small single crystal samples. We discuss the principles and applications of this technique to study unconventional superconductivity in the copper oxides and other materials such as iron-based superconductors. The technique has now been employed by several groups worldwide as a precision measurement tool for the exploration of new superconductors.
The London penetration depth, $lambda(T)$, has been measured in several single crystals of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. Thermodynamic, electromagnetic, and structural characterization measurements confirm that these crystals are of excellent quality. The observed low temperature variation of $lambda(T)$ follows a power-law, $Delta lambda (T) sim T^n$ with $n=2.4 pm 0.1$, indicating the existence of normal quasiparticles down to at least $0.02T_c$. This is in contrast to recent penetration depth measurements on single crystals of NdFeAsO$_{1-x}$F$_x$ and SmFeAsO$_{1-x}$F$_x$, which indicate an anisotropic but nodeless gap. We propose that a more three-dimensional character in the electronic structure of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ may lead to an anisotropic $s-$wave gap with point nodes that would explain the observed $lambda(T)$.
Recently, compressed H$_2$S has been shown to become superconducting at 203 K under a pressure of 155 GPa. One might expect fluctuations to dominate at such temperatures. Using the magnetisation critical current, we determine the ground-state London penetration depth, $lambda_0$=189 nm, and the superconducting energy gap, $Delta_0$=27.8 meV, and find these parameters are similar to those of cuprate superconductors. We also determine the fluctuation temperature scale, $T_{textrm{fluc}}=1470$ K, which shows that, unlike the cuprates, $T_c$ of the hydride is not limited by fluctuations. This is due to its three dimensionality and suggests the search for better superconductors should refocus on three-dimensional systems where the inevitable thermal fluctuations are less likely to reduce the observed $T_c$.
We study the effects of anisotropic order parameters on the temperature dependence of London penetration depth anisotropy $gamma_lambda(T)$. After MgB$_2$, this dependence is commonly attributed to distinct gaps on multi-band Fermi surfaces in superconductors. We have found, however, that the anisotropy parameter may depend on temperature also in one-band materials with anisotropic order parameters $Delta(T,k_F)$, a few such examples are given. We have found also that for different order parameters, the temperature dependence of $Delta(T)/Delta(0)$ can be represented with good accuracy by the interpolation suggested by D. Einzel, J. Low Temp. Phys, {bf 131}, 1 (2003), which simplifies considerably the evaluation of $gamma_lambda(T)$. Of particular interest is mixed order parameters of two symmetries for which $gamma_lambda(T)$ may go through a maximum for a certain relative weight of two phases. Also, for this case, we find that the ratio $Delta_{max}(0)/T_c$ may exceed substantially the weak coupling limit of 1.76. It, however, does not imply a strong coupling, rather it is due to significantly anisotropic angular variation of $Delta$.
The temperature dependence of the in-plane, lambda_{parallel}, and interplane, lambda_{perp}, London penetration depth was measured in the metal-free all-organic superconductor beta-ET (see title) ($T_c approx$ 5.2 K). lambda_{parallel} ~T^3 up to 0.5 Tc, a power law previously observed only in materials thought to be p-wave superconductors. lambda_{perp} is larger than the sample dimensions down to the lowest temperatures (0.35 K), implying an anisotropy of lambda_{perp}/lambda_{parallel} ~ 400-800.
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