PACS numbers: * Electronic address: stockert@cpfs.mpg.de 1The origin of unconventional superconductivity, including high-temperature and heavy-fermion superconductivity, is still a matter of controversy. Spin excitations instead of phonons are thought to be responsible for the formation of Cooper pairs. Using inelastic neutron scattering, we present the first in-depth study of the magnetic excitation spectrum in momentum and energy space in the superconducting and the normal states of CeCu 2 Si 2 . A clear spin excitation gap is observed in the superconducting state. We determine a lowering of the magnetic exchange energy in the superconducting state, in an amount considerably larger than the superconducting condensation energy. Our findings identify the antiferromagnetic excitations as the major driving force for superconducting pairing in this prototypical heavy-fermion compound located near an antiferromagnetic quantum critical point.While conventional superconductivity (SC) is generally incompatible with magnetism, magnetic excitations seem to play an important role in the Cooper pair formation of unconventional superconductors such as the high-T c cuprates or the low-T c organic and heavyfermion (HF) superconductors. Since the discovery of SC in CeCu 2 Si 2 1 , antiferromagnetic (AF) spin excitations have been proposed as a viable mechanism for SC 2-4 . The discovery of SC at the boundary of AF order in CePd 2 Si 2 5 has pushed this notion into the framework of AF quantum criticality 6 . Unfortunately, such quantum critical points (QCPs) proximate to HF superconductors typically arise under pressure, which makes it difficult to probe their magnetic excitation spectrum.Here, we report a detailed study of the magnetic excitations in CeCu 2 Si 2 , which exhibits SC below T c ≈ 0.6 K. This prototypical HF compound is ideally suited for our purpose, since SC here is in proximity to an AF QCP already at ambient pressure (cf. Fig. 1(a)).As displayed in Fig. 1(b) CeCu 2 Si 2 crystallises in a structure with body-centred tetragonal symmetry and is one of the best studied HF superconductors and well characterised by low-temperature transport and thermodynamic measurements 7 . Moreover, those measurements in the field-induced normal state have already provided evidence that the QCP in this compound is of the three-dimensional (3D) spin-density-wave (SDW) type 8 . The spatial anisotropy of the spin fluctuations in superconducting CeCu 2 Si 2 was measured at T = 0.06 K and at an energy transfer ω = 0.2 meV and is shown in Fig. 1(c). These magnetic correlations display only a small anisotropy (a factor of 1.5) in the correlation lengths 2 between the [110] and the [001] direction. Therefore, these quite isotropic spin fluctuations are in line with thermodynamic and transport measurements exhibiting C/T = γ 0 − a √ T or ρ − ρ 0 = AT α , α = 1 − 1.5 8,9 , and strongly support a three-dimensional quantum critical SDW scenario 10 . We are able to identify the magnetic excitations in the normal state of paramagnetic, ...

The material class of rare earth nickelates with high Ni3+ oxidation state is generating continued interest due to the occurrence of a metal-insulator transition with charge order and the appearance of non-collinear magnetic phases within this insulating regime. The recent theoretical prediction for superconductivity in LaNiO3 thin films has also triggered intensive research efforts. LaNiO3 seems to be the only rare earth nickelate that stays metallic and paramagnetic down to lowest temperatures. So far, centimeter-sized impurity-free single crystal growth has not been reported for the rare earth nickelates material class since elevated oxygen pressures are required for their synthesis. Here, we report on the successful growth of centimeter-sized LaNiO3 single crystals by the floating zone technique at oxygen pressures of up to 150 bar. Our crystals are essentially free from Ni2+ impurities and exhibit metallic properties together with an unexpected but clear antiferromagnetic transition.

The magnetic excitations in the cuprate superconductors might be essential for an understanding of high-temperature superconductivity. In these cuprate superconductors the magnetic excitation spectrum resembles an hour-glass and certain resonant magnetic excitations within are believed to be connected to the pairing mechanism, which is corroborated by the observation of a universal linear scaling of superconducting gap and magnetic resonance energy. So far, charge stripes are widely believed to be involved in the physics of hour-glass spectra. Here we study an isostructural cobaltate that also exhibits an hour-glass magnetic spectrum. Instead of the expected charge stripe order we observe nano phase separation and unravel a microscopically split origin of hour-glass spectra on the nano scale pointing to a connection between the magnetic resonance peak and the spin gap originating in islands of the antiferromagnetic parent insulator. Our findings open new ways to theories of magnetic excitations and superconductivity in cuprate superconductors.

We have studied a EuFe 2 As 2 single crystal by neutron diffraction under magnetic fields up to 3.5 T and temperatures down to 2 K. A field induced spin reorientation is observed in the presence of a magnetic field along both the a and c axes, respectively. Above critical field, the ground state antiferromagnetic configuration of Eu 2+ moments transforms into a ferromagnetic structure with moments along the applied field direction. The magnetic phase diagram for Eu magnetic sublattice in EuFe 2 As 2 is presented. A considerable strain (∼0.9%) is induced by the magnetic field, caused by the realignment of the twinning structure. Furthermore, the realignment of the twinning structure is found to be reversible with the rebound of magnetic field, which suggested the existence of magnetic shape-memory effect. The Eu moment ordering exhibits close relationship with the twinning structure. We argue that the Zeeman energy in combined with magnetic anisotropy energy is responsible for the observed spin-lattice coupling.PACS numbers: 74.70. Xa, 75.30.Kz, 75.80.+q The recent discovery of iron pnictide superconductors has triggered extensive research on their physical properties and mechanism of high temperature superconductors [1][2][3]. All iron pnictides are found to be of layered structure in nature. For undoped iron pnictides, the chains of parallel Fe spins within the FeAs layers couple antiferromagnetically in the ab plane of the orthorhombic lattice with an antiparallel arrangement along the c axis [4][5][6]. This antiferromagnetic (AFM) order in the parent compounds is likely due to a spin-densitywave (SDW) instability caused by Fermi surface nesting [7]. Similar to the high T c cuprate superconductors, the undoped iron pnictides are not superconducting under ambient pressure and show an antiferromagnetic SDW order. Upon carrier doping, the magnetic order is suppressed and superconductivity emerges concomitantly [8,9]. EuFe 2 As 2 is a peculiar member of the iron arsenide AFe 2 As 2 family since the A site is occupied by Eu 2+ , which is an S-state (orbital angular momentum L = 0) rare-earth ion possessing a 4f 7 structure with the total electron spin S = 7/2. Here we report a single crystal neutron diffraction measurement on EuFe 2 As 2 under a magnetic field up to 3.5 T. The spin reorientation of Eu moments is observed upon an applied magnetic field parallel to both a and c axes of the orthorhombic structure, while the Fe SDW order persists at high magnetic fields. Interestingly, the application of a magnetic field changes the twinning population in EuFe 2 As 2 and the redistribution of the domain population is found to be associated with the evolution of the magnetic order of Eu moments, which indicates the existence of a giant spin-lattice coupling effect. A single crystal of EuFe 2 As 2 was grown by the Sn-flux method [10]. It was in shape of a platelet with approximate dimensions of 5 × 5 × 1 mm 3 . Single crystal neutron scattering measurements were performed on the thermal neutron two-axis diffractometer...

Frustrated quasidoublets without time-reversal symmetry can host highly unconventional magnetic structures with continuously distributed order parameters even in a single-phase crystal. Here, we report the comprehensive thermodynamic and neutron diffraction investigation on the single crystal of TmMgGaO4, which entails non-Kramers Tm 3+ ions arranged on a geometrically perfect triangular lattice. The crystal electric field (CEF) randomness caused by the site-mixing disorder of the nonmagnetic Mg 2+ and Ga 3+ ions, merges two lowest-lying CEF singlets of Tm 3+ into a ground-state (GS) quasidoublet. Well below Tc ∼ 0.7 K, a small fraction of the antiferromagnetically coupled Tm 3+ Ising quasidoublets with small inner gaps condense into two-dimensional (2D) up-up-down magnetic structures with continuously distributed order parameters, and give rise to the columnar magnetic neutron reflections below µ0Hc ∼ 2.6 T, with highly anisotropic correlation lengths, ξ ab ≥ 250a in the triangular plane and ξc < c/12 between the planes. The remaining fraction of the Tm 3+ ions remain nonmagnetic at 0 T and become uniformly polarized by the applied longitudinal field at low temperatures. We argue that the similar model can be generally applied to other compounds of non-Kramers rare-earth ions with correlated GS quasidoublets.