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Register a new userNodal superconductivity coexists with low-moment static magnetism in single-crystalline tetragonal FeS: A muon spin relaxation and rotation study
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We report muon spin relaxation and rotation ($mu$SR) measurements on hydrothermally-grown single crystals of the tetragonal superconductor~FeS, which help to clarify the controversial magnetic state and superconducting gap symmetry of this compound. $mu$SR time spectra were obtained from 280~K down to 0.025~K in zero field (ZF) and applied fields up to 20 mT. In ZF the observed loss of initial asymmetry (signal amplitude) and increase of depolarization rate~$Lambda_mathrm{ZF}$ below 10~K indicate the onset of static magnetism, which coexists with superconductivity below $T_c$. Transverse-field $mu$SR yields a muon depolarization rate $sigma_mathrm{sc} propto lambda_{ab}^{-2}$ that clearly shows a linear dependence at low temperature, consistent with nodal superconductivity. The $s{+}d$-wave model gives the best fit to the observed temperature and field dependencies. The normalized superfluid densities versus normalized temperature for different fields collapse onto the same curve, indicating the superconducting gap structure is independent of field. The $T=0$ in-plane penetration depth $lambda_{ab}$(0) = 198(3) nm.
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The recent observation of superconductivity with critical temperatures up to 55 K in the FeAs based pnictide compounds marks the first discovery of a non copper-oxide based layered high-Tc superconductor (HTSC) [1-3]. It has raised the suspicion that these new materials share a similar pairing mechanism to the cuprates, since both exhibit superconductivity following charge doping of a magnetic parent material. Here we present a muon spin rotation study on SmFeAsO1-xFx (x=0-0.30), which shows that static magnetism persists well into the superconducting regime. The analogy with the cuprates is quite surprising since the parent compounds appear to have different magnetic ground states: itinerant spin density wave for the pnictides contrasted with the Mott-Hubbard insulator in the cuprates. Our findings suggest that proximity to magnetic order and associated soft magnetic fluctuations, rather than the strong electronic correlations in the vicinity of a Mott-Hubbard-metal-to-insulator transition, may be the key ingredients of HTSC.
We present a muon spin rotation (muSR) study of the magnetic and superconducting properties of single crystals of electron-doped BaFe2-xCoxAs2 with x=0.08, 0.20, and 0.25 (Tc=9, 25 and 20K) and of polycrystalline hole-doped Pr1-xSrxFeAsO with x=0 and 0.2 (Tc=15 K). In the former series we observe some interesting parallels with the electron doped SmFeAsO1-xFx 1111-type system [A.J. Drew et al., to appear in Nature Materials 2009 and arXiv:0807.4876]. In particular, we obtain evidence that strongly disordered static magnetism coexists with superconductivity on a microscopic scale in underdoped samples and even at optimum doping there is a slowing down (or enhancement) of dynamic magnetic correlations below Tcapprox25K. To the contrary, for the hole-doped Pr1-xSrxFeAsO samples we obtain evidence for a mesoscopic phase segregation into regions with nearly unperturbed AF order and others that are non magnetic and most likely superconducting. The observed trend resembles the one that was previously reported for hole-doped Ba1-xKxFe2As2 [A.A. Aczel et al., Phys. Rev. B 78, 214503 (2008); J.T. Park et al., arXiv:0811.2224] and thus seems to be fairly common in these hole doped systems.
We present a low-energy muon-spin-rotation study of the magnetic and superconducting properties of YBa2Cu3O7/PrBa2Cu3O7 trilayer and bilayer heterostructures. By determining the magnetic-field profiles throughout these structures we show that a finite superfluid density can be induced in otherwise semiconducting PrBa2Cu3O7 layers when juxtaposed to YBa2Cu3O7 electrodes while the intrinsic antiferromagnetic order is unaffected.
In the present work we demonstrate that C-doped Zr$_{5}$Pt$_{3}$ is an electron-phonon superconductor (with critical temperature T$_mathrm{C}$ = 3.7,K) with a nonsymmorphic topological Dirac nodal-line semimetal state, which we report here for the first time. The superconducting properties of Zr$_{5}$Pt$_{3}$C$_{0.5}$ have been investigated by means of magnetization and muon spin rotation and relaxation ($mu$SR) measurements. We find that at low temperatures the depolarization rate is almost constant and can be well described by a single-band $s-$wave model with a superconducting gap of $2Delta(0)/k_mathrm{B}T_mathrm{C}$ = 3.84, close to the value of BCS theory. From transverse field $mu$SR analysis we estimate the London penetration depth $lambda_{L}$ = 469 nm, superconducting carrier density $n_{s}$ = 2$times$10$^{26}$ $m^{-3}$, and effective mass m$^{*}$ = 1.584 $m_{e}$. Zero field $mu$SR confirms the absence of any spontaneous magnetic moment in the superconducting ground state. To gain additional insights into the electronic ground state of C-doped Zr$_5$Pt$_3$, we have also performed first-principles calculations within the framework of density functional theory (DFT). The observed homogenous electronic character of the Fermi surface as well as the mutual decrease of $T_mathrm{C}$ and density of states at the Fermi level are consistent with the experimental findings. However, the band structure reveals the presence of robust, gapless fourfold-degenarate nodal lines protected by $6_{3}$ screw rotations and glide mirror planes. Therefore, Zr$_5$Pt$_3$ represents a novel, unprecedented condensed matter system to investigate the intricate interplay between superconductivity and topology.
The magnetic response of CaK(Fe$_{0.949}$Ni$_{0.051}$)$_4$As$_4$ was investigated by means of the muon-spin rotation/relaxation. The long-range commensurate magnetic order sets in below the N{e}el temperature $T_{rm N}= 50.0(5)$~K. The density-functional theory calculations have identified three possible muon stopping sites. The experimental data were found to be consistent with only one type of magnetic structure, namely, the long-range magnetic spin-vortex-crystal order with the hedgehog motif within the $ab-$plane and the antiferromagnetic stacking along the $c-$direction. The value of the ordered magnetic moment at $Tapprox3$ K was estimated to be $m_{rm Fe}=0.38(11)$ $mu_{rm B}$ ($mu_{rm B}$ is the Bohr magneton). A microscopic coexistence of magnetic and superconducting phases accompanied by a reduction of the magnetic order parameter below the superconducting transition temperature $T_{rm c}simeq 9$ K is observed. Comparison with 11, 122, and 1144 families of Fe-based pnictides points to existence of correlation between the reduction of the magnetic order parameter at $Trightarrow 0$ and the ratio of the transition temperatures $T_{rm c}/T_{rm N}$. Such correlations were found to be described by Machidas model for coexistence of itinerant spin-density wave magnetism and superconductivity [Machida, J. Phys. Soc. Jpn. 50, 2195 (1981) and Budko et al., Phys. Rev. B 98, 144520 (2018)].
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