Pressure-induced inhomogeneous chiral-spin ground state in FeGe.

(57)Fe nuclear forward scattering on the chiral magnet FeGe reveals an extremely large precursor phase region above the helimagnetic ordering temperature T(C)(p) and beyond the pressure-induced quantum phase transition at 19 GPa. The decrease of the magnetic hyperfine field ⟨B(hf)⟩ with pressure is accompanied by a large increase of the width of the distribution of ⟨B(hf)⟩, indicating a strong quasistatic inhomogeneity of the magnetic states in the precursor region. Hyperfine fields of the order of 4 T (equivalent to a magnetic moment µ(Fe)≈0.4µ(B)) persist up to 28.5 GPa. No signatures of magnetic order have been found at about 31 GPa. The results, supported by ab initio calculations, suggest that chiral magnetic precursor phenomena, such as an inhomogeneous chiral-spin state, are vastly enlarged due to increasing spin fluctuations as FeGe is tuned to its quantum phase transition.

Strong ferromagnetism at the surface of an antiferromagnet caused by buried magnetic moments.

Carrying a large, pure spin magnetic moment of 7 µB per atom in the half-filled 4f shell, divalent europium is an outstanding element for assembling novel magnetic devices in which a two-dimensional electron gas may be polarized due to exchange interaction with an underlying magnetically-active Eu layer. Here we show that the Si-Rh-Si surface trilayer of the antiferromagnet EuRh2Si2 bears a surface state, which exhibits an unexpected and large spin splitting controllable by temperature. The splitting sets in below ~32.5 K, well above the ordering temperature of the Eu 4f moments (~24.5 K) in the bulk, indicating a larger ordering temperature in the topmost Eu layers. The driving force for the itinerant ferromagnetism at the surface is the aforementioned exchange interaction. Such a splitting may also be induced into states of functional surface layers deposited onto the surface of EuRh2Si2 or similarly ordered magnetic materials with metallic or semiconducting properties.

Surface adatom conductance filtering in scanning tunneling spectroscopy of co-doped BaFe2As2 iron pnictide superconductors.

We establish in a combination of ab initio theory and experiments that the tunneling process in scanning tunneling microscopy or spectroscopy on the A-122 iron pnictide superconductors-in this case BaFe(2-x)Co(x)As(2)-involves a strong adatom filtering of the differential conductance from the near-E(F) Fe-3d states, which in turn originates from the topmost subsurface Fe layer of the crystal. The calculations show that the dominance of surface Ba-related tunneling pathways leaves fingerprints found in the experimental differential conductance data, including large particle-hole asymmetry and energy-dependent contrast inversion in conductance maps.

Electronic confinement and ordering instabilities in colossal magnetoresistive bilayer manganites.

We present angle-resolved photoemission studies of (La{1-z}Pr{z}){2-2x}Sr{1+2x}Mn{2}O{7} with x=0.4 and z=0.1, 0.2, and 0.4 along with density functional theory calculations and x-ray scattering data. Our results show that the bilayer splitting in the ferromagnetic metallic phase of these materials is small, if not completely absent. The charge carriers are therefore confined to a single MnO{2} layer, which in turn results in a strongly nested Fermi surface. In addition to this, the spectral function also displays clear signatures of an electronic ordering instability well below the Fermi level. The increase of the corresponding interaction strength with z and its magnitude of ∼400 meV make the coupling to a bare phonon highly unlikely. Instead we conclude that fluctuating order, involving electronic and lattice degrees of freedom, causes the observed renormalization of the spectral features.

Optical study of LaO0.9F0.1FeAs: evidence for a weakly coupled superconducting state.

We have studied the reflectance of the recently discovered superconductor LaO0.9F0.1FeAs in a wide energy range from the far infrared to the visible regime. We report on the observation of infrared active phonons, the plasma edge, and possible interband transitions. On the basis of this data and the reported in-plane penetration depth lambda{L}(0)=254 nm [H. Luetkens, Phys. Rev. Lett. 101, 097009 (2008)] a disorder sensitive relatively small value of the total electron-boson coupling constant lambda{tot}=lambda{e-ph}+lambda{e-sp} approximately 0.6+/-0.35 can be estimated adopting an effective single-band picture.

Anomalous quasiparticle renormalization in Na0.73CoO2: role of interorbital interactions and magnetic correlations.

We report an angular resolved photoemission study of NaxCoO2 with x approximately 0.73 where it is found that the renormalization of the quasiparticle (QP) dispersion changes dramatically upon a rotation from GammaM to GammaK. The comparison of the experimental data to the calculated band structure reveals that the quasiparticle renormalization is most pronounced along the GammaK direction, while it is significantly weaker along the GammaM direction. We discuss the observed anisotropy in terms of multiorbital effects and point out the relevance of magnetic correlations for the band structure of NaxCoO2 with x approximately 0.75.

Stripe correlations in Na(0.75)CoO2.

We present a combined high-energy x-ray diffraction and local-density approximation study of the sodium ordering in Na(0.75)CoO2. The obtained results rule out previously proposed Na-ordering models and provide strong evidence for the formation of sodium-density stripes in this material. The local-density approximation calculations prove that the sodium-density stripes lead to a sizable dip in the density of the Co states at the Fermi level, pointing to band structure effects as a driving force for the stripe formation. This indicates that the sodium ordering is connected to stripelike charge correlations within the CoO2 layers, leading to an astonishing similarity between the doped cuprates and the NaxCoO2 compounds.