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Register a new userCoexistence of Supercondcutivity and Magnetism in LaFeAs(O0.94F0.06) Probed by Muon Spin Relaxation
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Recent discovery of oxypnictide superconductor LaFeAs(O,F) (LFAO-F) with the critical temperature (Tc) of 26 K and succeeding revelation of much increased Tc upon substitution of La for other rare earth elements (such as Sm, leading to ~43 K) and application of pressure for LFAO-F (~ 43 K) has triggered broad interest in the mechanism yielding relatively high Tc in this new class of compounds. While they share a feature with high-Tc cuprates that superconductivity occurs upon carrier doping to pristine compound which exhibits magnetism, they also resemble the heavy-fermion compounds in the sense that superconductivity appears in the vicinity of magnetic phase. Investigation of electronic states near the boundary between these two phases might provide some useful information on the mechanism of superconductivity, as it has been proved to be the case in many exotic superconductors. Here we show by muon experiment in the LFAO-F compound that a macroscopic phase separation into superconducting and spin glass-like magnetic phases occurs at x=0.06 that is near the phase boundary, where both the magnetism and superconductivity develop simultaneously below a common Tc ~ 18 K. This accordance strongly suggests intimate relationship between magnetism and superconductivity typically found in heavy-fermion systems near the quantum critical point.
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We report muon-spin relaxation measurements on SrFeAsF, which is the parent compound of a newly discovered iron-arsenic-fluoride based series of superconducting materials. We find that this material has very similar magnetic properties to LaFeAsO, such as separated magnetic and structural transitions (TN = 120 K, Ts = 175 K), contrasting with SrFe2As2 where they are coincident. The muon oscillation frequencies fall away very sharply at TN, which suggests that the magnetic exchange between the layers is weaker than in comparable oxypnictide compounds. This is consistent with our specific heat measurements, which find that the entropy change S = 0.05 J/mol/K largely occurs at the structural transition and there is no anomaly at TN.
We report results of a muon spin relaxation study of slow magnetic fluctuations in the pseudogap phase of underdoped single-crystalline YBa$_{2}$Cu$_{3}$O$_{y}$, $y = 6.77$ and 6.83. The dependence of the dynamic muon spin relaxation rate on applied magnetic field yields the rms magnitude~$Bmathrm{_{loc}^{rms}}$ and correlation time~$tau_c$ of fluctuating local fields at muon sites. The observed relaxation rates do not decrease with decreasing temperature~$T$ below the pseudogap onset at $T^ast$, as would be expected for a conventional magnetic transition; both $Bmathrm{_{loc}^{rms}}$ and $tau_c$ are roughly constant in the pseudogap phase down to the superconducting transition. Corresponding NMR relaxation rates are estimated to be too small to be observable. Our results put strong constraints on theories of the anomalous pseudogap magnetism in YBa$_{2}$Cu$_{3}$O$_{y}$.
Using the transverse field muon spin relaxation technique we measure the temperature dependence of the magnetic field penetration depth $lambda$, in the Na$_{x}$CoO$_{2}cdot y$H$_{2}$O system. We find that $lambda,$ which is determined by superfluid density $n_{s}$ and the effective mass $m^{ast}$, is very small and on the edge of the TF-$mu$SR sensitivity. Nevertheless, the results indicate that the order parameter in this system has nodes and that it obeys the Uemura relation. By comparing $lambda$ with the normal state electron density we conclude that $m^{ast}$ of the superconductivity carrier is 70 times larger than the mass of bare electrons.
$^{75}$As and $^{139}$La NMR results of LaFeAs(O$_{1-x}$F$_x$) ($x$=0, 0.025, and 0.04) were reported. Upon F-doping, the tetragonal-to-orthorhombic structural phase transition temperature $T_S$, antiferromagnetic transition temperature $T_N$ and internal magnetic field $mu_0H_{rm int}$ are gradually reduced for $x<0.04$. However, at $x=0.04$, $T_N$ is abruptly suppressed to be 30 K along with a tiny $mu_0H_{rm int}$, which is distinct from the continuous disappearance of the ordered phases in the Ba122 systems of Ba(Fe,Co)$_2$As$_2$ and BaFe$_2$(As,P)$_2$. The anisotropy of the spin-lattice relaxation rate $T_1^{-1}$, $(T_1)^{-1}_{Hparallel ab}/(T_1)^{-1}_{Hparallel c}$, in the paramagnetic phase of $x = 0$ and 0.025 is constant ($sim 1.5$), but increases abruptly below $T_S$ due to the enhancement of $(T_1)^{-1}_{Hparallel ab}$ by the slowing down of magnetic fluctuations. This indicates that the tetragonal-to-orthorhombic structural distortion enhances the anisotropy in the spin space via magnetoelastic coupling and/or spin-orbit interaction.
Copper metaborate CuB$_2$O$_{4}$ was studied by muon spin relaxation measurements in order to clarify its static and dynamic magnetic properties. The time spectra of muon spin depolarization suggest that the local fields at the muon site contain both static and fluctuating components in all ordered phases down to 0.3 K. In the weak ferromagnetic phase (20 K~$>T>$~9.3 K), the static component is dominant. On the other hand, upon cooling the fluctuating component becomes dominant in the incommensurate helix phase (9.3K > T > 1.4K). The dynamical fluctuations of the local fields persist down to 0.3K, where a new incommensurate phase (T < 1.4K) is expected to appear. This result suggests that spins fluctuate even at T to 0. We propose two possible origins of the remnant dynamical spin fluctuations: frustration of the exchange interactions and the dynamic behavior of the soliton lattice.
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