We report local probe (µSR) 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 µB) disordered magnetism with a transition temperature of TN ≈ 20 K microscopically coexists with bulk superconductivity below Tc = 4.3(1) K. From transverse field µSR we obtain an in-plane penetration depth λ ab (0) = 223(2) nm for FeS. The temperature dependence of the corresponding superfluid density λ −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 Tc of FeS continuously decreases for hydrostatic pressures up to 2.2 GPa. 74.62.Fj, 76.75.+i arXiv:1602.01987v1 [cond-mat.supr-con]

Two-dimensional magnetic systems with continuous spin degrees of freedom exhibit a rich spectrum of thermal behaviour due to the strong competition between fluctuations and correlations. When such systems incorporate coupling via the anisotropic dipolar interaction, a discrete symmetry emerges, which can be spontaneously broken leading to a low-temperature ordered phase. However, the experimental realisation of such two-dimensional spin systems in crystalline materials is difficult since the dipolar coupling is usually much weaker than the exchange interaction. Here we realise two-dimensional magnetostatically coupled XY spin systems with nanoscale thermally active magnetic discs placed on square lattices. Using low-energy muon-spin relaxation and soft X-ray scattering, we observe correlated dynamics at the critical temperature and the emergence of static long-range order at low temperatures, which is compatible with theoretical predictions for dipolar-coupled XY spin systems. Furthermore, by modifying the sample design, we demonstrate the possibility to tune the collective magnetic behaviour in thermally active artificial spin systems with continuous degrees of freedom.

Prior to the development of pulsed lasers, one assigned a single local temperature to the lattice, the electron gas, and the spins. With the availability of ultrafast laser sources, one can now drive the temperature of these reservoirs out of equilibrium. Thus, the solid shows new internal degrees of freedom characterized by individual temperatures of the electron gas T_{e}, the lattice T_{l} and the spins T_{s}. We demonstrate an analogous behavior in the spin polarization of a ferromagnet in an ultrafast demagnetization experiment: At the Fermi energy, the polarization is reduced faster than at deeper in the valence band. Therefore, on the femtosecond time scale, the magnetization as a macroscopic quantity does not provide the full picture of the spin dynamics: The spin polarization separates into different parts similar to how the single temperature paradigm changed with the development of ultrafast lasers.

We report a comprehensive muon spin rotation (µSR) study of the prototypical magnetoelectric antiferromagnet Cr2O3. We find the positively charged muon (µ + ) occupies several distinct interstitial sites, and displays a rich dynamic behavior involving local hopping, thermally activated site transitions and the formation of a charge-neutral complex composed of a muon and an electron polaron. The discovery of such a complex has implications for the interpretation of µSR spectra in a wide range of magnetic oxides, and opens a route to study the dopant characteristics of interstitial hydrogen impurities in such materials. We address implications arising from implanting a µ + into a linear magnetoelectric, and discuss the challenges of observing a local magnetoelectric effect generated by the charge of the muon.

We report muon spin rotation (µSR) and magnetization measurements under pressure on Fe 1+δ Se1-xSx with x ≈ 0.11. We find an extended dome of long range magnetic order above p ≈ 0.6 GPa spanning a pressure range between previously reported separated magnetic phases. The magnetism initially competes with coexisting superconductivity leading to a local maximum and minimum of the superconducting Tc(p). The maximum of Tc corresponds to the onset of magnetism while the minimum coincides with the pressure of strongest competition. A shift of the maximum of Tc(p) for a series of single crystals with x up to 0.14 roughly extrapolates to a putative magnetic and superconducting state at ambient pressure for x ≥ 0.2.

The recently synthesized ThFeAsN iron-pnictide superconductor exhibits a T c of 30 K, the highest of the 1111-type series in absence of chemical doping. To understand how pressure affects its electronic properties, we carried out microscopic investigations up to 3 GPa via magnetization, nuclear magnetic resonance, and muon-spin rotation experiments. The temperature dependence of the 75 As Knight shift, the spin-lattice relaxation rates, and the magnetic penetration depth suggest a multi-band s ± -wave gap symmetry in the dirty limit, while the gap-to-T c ratio ∆/k B T c hints at a strong-coupling scenario. Pressure modulates the geometrical parameters, thus reducing T c , as well as T m , the temperature where magnetic-relaxation rates are maximized, both at the same rate of approximately -1.1 K/GPa. This decrease of T c with pressure is consistent with band-structure calculations, which relate it to the deformation of the Fe 3d z 2 orbitals. * nbarbero@phys.ethz.ch

The rare-earth chromium(II) oxyselenides RECrSeO (RE = La-Nd) were synthesized in eutectic NaI/KI fluxes, and their crystal structures were determined by single-crystal and powder X-ray diffraction (PbHgClO-type, C2/m, Z = 2). The magnetic structure of LaCrSeO was solved and refined from neutron powder diffraction data. Main building blocks are chains of edge-sharing CrSeO octahedra linked together by two edge-sharing ORECr tetrahedra forming infinite ribbons. The Jahn-Teller instability of divalent Cr (d) leads to structural phase transitions at 200 and 130 K in LaCrSeO and CeCrSeO, respectively. RECrSeO are Curie-Weiss paramagnetic above T ≈ 14-17 K. Neutron powder diffraction reveals anti-ferromagnetic ordering of the Cr moments in LaCrSeO below T = 12.7(3) K with an average ordered moment of 3.40(4) μ/Cr at 4 K, which was confirmed by muon spin rotation experiments.

Time-and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser Review of Scientific Instruments 91, 013109 (2020);