Z. Guguchia

Coexistence of low-moment magnetism and superconductivity in tetragonal FeS and suppression of T-c under pressure

We report local probe (mu SR) measurements on the recently discovered tetragonal FeS superconductor which has been predicted to be electronically very similar to superconducting FeSe. Most remarkably, we find that low-moment (10(-2)-10(-3) mu B) disordered magnetism with a transition temperature of T-N approximate to 20 K microscopically coexists with bulk superconductivity below T-c = 4.3(1) K. From transverse field mu SR we obtain an in-plane penetration depth lambda(ab)(0) = 223(2) nm for FeS. The temperature dependence of the corresponding superfluid density lambda(-2)(ab)(T) indicates a fully gapped superconducting state and is consistent with a two gap s-wave model. Additionally, we find that the superconducting T-c of FeS continuously decreases for hydrostatic pressures up to 2.2 GPa.

Pressure-induced electronic phase separation of magnetism and superconductivity in CrAs

The recent discovery of pressure (p) induced superconductivity in the binary helimagnet CrAs has raised questions on how superconductivity emerges from the magnetic state and on the mechanism of the superconducting pairing. In the present work the suppression of magnetism and the occurrence of superconductivity in CrAs were studied by means of muon spin rotation. The magnetism remains bulk up to pu2009u20093.5u2009kbar while its volume fraction gradually decreases with increasing pressure until it vanishes at pu2009u20097u2009kbar. At 3.5u2009kbar superconductivity abruptly appears with its maximum Tcu2009u20091.2u2009K which decreases upon increasing the pressure. In the intermediate pressure region (3.5u2009u2009pu2009u20097u2009kbar) the superconducting and the magnetic volume fractions are spatially phase separated and compete for phase volume. Our results indicate that the less conductive magnetic phase provides additional carriers (doping) to the superconducting parts of the CrAs sample thus leading to an increase of the transition temperature (Tc) and of the superfluid density (ρs). A scaling of ρs with as well as the phase separation between magnetism and superconductivity point to a conventional mechanism of the Cooper-pairing in CrAs.

Direct evidence for a pressure-induced nodal superconducting gap in the Ba0.65Rb0.35Fe2As2 superconductor.

Identifying the superconducting (SC) gap structure of the iron-based high-temperature superconductors (Fe-HTS’s) remains a key issue for the understanding of superconductivity in these materials. In contrast to other unconventional superconductors, in the Fe-HTS’s both d-wave and extended s-wave pairing symmetries are close in energy, with the latter believed to be generally favored over the former. Probing the proximity between these very different SC states and identifying experimental parameters that can tune them, are of central interest. Here we report high-pressure muon spin rotation experiments on the temperature-dependent magnetic penetration depth λ (T ) in the optimally doped Fe-HTS Ba0.65Rb0.35Fe2As2. At ambient pressure this material is known to be a nodeless s-wave superconductor. Upon pressure a strong decrease of λ (0) is observed, while the SC transition temperature remains nearly constant. More importantly, the low-temperature behavior of 1/λ2 (T ) changes from exponential saturation at zero pressure to a power-law with increasing pressure, providing unambiguous evidence that hydrostatic pressure promotes nodal SC gaps. Comparison to microscopic models favors a d-wave over a nodal s+−-wave pairing as the origin of the nodes. Our results provide a new route of understanding the complex topology of the SC gap in Fe-HTS’s.The superconducting gap structure in iron-based high-temperature superconductors (Fe-HTSs) is non-universal. In contrast to other unconventional superconductors, in the Fe-HTSs both d-wave and extended s-wave pairing symmetries are close in energy. Probing the proximity between these very different superconducting states and identifying experimental parameters that can tune them is of central interest. Here we report high-pressure muon spin rotation experiments on the temperature-dependent magnetic penetration depth in the optimally doped nodeless s-wave Fe-HTS Ba0.65Rb0.35Fe2As2. Upon pressure, a strong decrease of the penetration depth in the zero-temperature limit is observed, while the superconducting transition temperature remains nearly constant. More importantly, the low-temperature behaviour of the inverse-squared magnetic penetration depth, which is a direct measure of the superfluid density, changes qualitatively from an exponential saturation at zero pressure to a linear-in-temperature behaviour at higher pressures, indicating that hydrostatic pressure promotes the appearance of nodes in the superconducting gap.

Superconductivity and magnetism in RbxFe2−ySe2: Impact of thermal treatment on mesoscopic phase separation

An extended study of the superconducting and normal-state properties of various as-grown and post-annealed Rb

Ferromagnetic quantum critical point avoided by the appearance of another magnetic phase in LaCrGe3 under pressure

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Spin-lattice coupling induced weak dynamical magnetism in EuTiO3 at high temperatures

Fe

Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning

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Evidence for strong lattice effects as revealed from huge unconventional oxygen isotope effects on the pseudogap temperature in La 2 − x Sr x CuO 4

Se

Magnetic states of MnP: muon-spin rotation studies

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Probing the pairing symmetry in the over-doped Fe-based superconductorBa0.35Rb0.65Fe2As2as a function of hydrostatic pressure

single crystals is presented. Magnetization experiments evidence that annealing of Rb