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Register a new userBulk superconductivity in undoped T-La$_{1.9}$Y$_{0.1}$CuO$_4$ probed by muon spin rotation
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The Meissner effect has been directly demonstrated by depth-resolved muon spin rotation measurements in high-quality thin films of the T-structured cuprate, T-La$_{1.9}$Y$_{0.1}$CuO$_4$, to confirm bulk superconductivity ($T_csimeq21$ K) in its {sl undoped} state. The gradual expelling of an external magnetic field is observed over a depth range of $sim$100 nm in films with a thickness of 275(15) nm, from which the penetration depth is deduced to be 466(22) nm. Based on this result, we argue that the true ground state of the parent compound of the $n$-type cuprates is not a Mott insulator but a strongly correlated metal with colossal sensitivity to apical oxygen impurities.
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We have investigated the effects of magnetic Ni and nonmagnetic Zn impurities on the superconductivity in undoped T-La$_{1.8}$Eu$_{0.2}$CuO$_4$ (T-LECO) with the Nd$_2$CuO$_4$-type structure, using the polycrystalline bulk samples, to clarify the pairing symmetry. It has been found that both suppression rates of the superconducting transition temperature $T_mathrm{c}$ by Ni and Zn impurities are nearly the same and are very similar to those in the optimally doped and overdoped regimes of hole-doped T-La$_{2-x}$Sr$_{x}$CuO$_4$ with the K$_2$NiF$_4$-type structure. These results strongly suggest that the superconductivity in undoped T-LECO is of the $d$-wave symmetry and is mediated by the spin fluctuation.
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.
Superfluid density ($n_s$) in the mixed state of an iron pnictide superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ is determined by muon spin rotation for a sample with optimal doping ($x=0.4$). The temperature dependence of $n_s$ is perfectly reproduced by the conventional BCS model for s-wave paring, where the order parameter can be either a single-gap with $Delta=8.35(6)$ meV [$2Delta/k_BT_c=5.09(4)$], or double-gap structure with $Delta_1=12$ meV (fixed) [$2Delta_1/k_BT_c=7.3$] and $Delta_2=6.8(3)$ meV [$2Delta_2/k_BT_c=4.1(2)$]. The latter is consistent with the recent result of angle-resolved photo-emssion spectroscopy. The large gap parameters ($2Delta/k_BT_c$) indicate extremely strong coupling of carriers to bosons that mediate the Cooper pairing.
Superconductors usually display either type-I or type-II superconductivity and the coexistence of these two types in the same material, for example at different temperatures is rare in nature. We the employed muon spin rotation (muSR) technique to unveil the superconducting phase diagram of the dodecaboride ZrB12 and obtained clear evidence of both type-I and type-II characteristics. Most importantly, we found a region showing unusual behavior where the usually mutually exclusive muSR signatures of type-I and type-II superconductivity coexist. We reproduced that behavior in theoretical modeling that required taking into account multiple bands and multiple coherence lengths, which suggests that material has one coherence length larger and another smaller than the magnetic field penetration length (the type-1.5 regime). At stronger fields, a footprint of the type-II mixed state showing square flux-line lattice was also obtained using neutron diffraction.
Local magnetic field distribution B(r) in the mixed state of a boride superconductor, YB6, is studied by muon spin rotation (muSR). A comparative analysis using the modified London model and Ginzburg-Landau (GL) model indicates that the GL model exhibits better agreement with muSR data at higher fields, thereby demonstrating the importance of reproducing the field profile near the vortex cores when the intervortex distance becomes closer to the GL coherence length. The temperature and field dependence of magnetic penetration depth ($lambda$) does not show any hint of nonlocal effect nor of low-lying quasiparticle excitation. This suggests that the strong coupling of electrons to the rattling motion of Y ions in the boron cage suggested by bulk measurements gives rise to a conventional superconductivity with isotropic s-wave pairing. Taking account of the present result, a review is provided for probing the anisotropy of superconducting order parameters by the slope of $lambda$ against field.
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