A permanent magnet retains a substantial fraction of its saturation magnetization in the absence of an external magnetic field. Realizing magnetic remanence in a single atom allows for storing and processing information in the smallest unit of matter. We show that individual holmium (Ho) atoms adsorbed on ultrathin MgO(100) layers on Ag(100) exhibit magnetic remanence up to a temperature of 30 kelvin and a relaxation time of 1500 seconds at 10 kelvin. This extraordinary stability is achieved by the realization of a symmetry-protected magnetic ground state and by decoupling the Ho spin from the underlying metal by a tunnel barrier.

One of the most intensely studied scenarios of high-temperature superconductivity (HTS) postulates pairing by exchange of magnetic excitations. Indeed, such excitations have been observed up to optimal doping in the cuprates. In the heavily overdoped regime, neutron scattering measurements indicate that magnetic excitations have effectively disappeared, and this has been argued to cause the demise of HTS with overdoping. Here we use resonant inelastic X-ray scattering, which is sensitive to complementary parts of reciprocal space, to measure the evolution of the magnetic excitations in La(2-x)Sr(x)CuO4 across the entire phase diagram, from a strongly correlated insulator (x = 0) to a non-superconducting metal (x = 0.40). For x = 0, well-defined magnon excitations are observed. These magnons broaden with doping, but they persist with a similar dispersion and comparable intensity all the way to the non-superconducting, heavily overdoped metallic phase. The destruction of HTS with overdoping is therefore caused neither by the general disappearance nor by the overall softening of magnetic excitations. Other factors, such as the redistribution of spectral weight, must be considered.

Applying time-dependent photoemission we unravel the graphene growth process on a metallic surface by chemical vapor deposition (CVD). Graphene CVD growth is in stark contrast to the standard growth process of two-dimensional films because it is self-limiting and stops as soon as a monolayer graphene has been synthesized. Most importantly, a novel phase of metastable graphene was discovered that is characterized by permanent and simultaneous construction and deconstruction. The high quality and large area graphene flakes are characterized by angle-resolved photoemission proofing that they are indeed monolayer and cover the whole 1×1 cm Nickel substrate. These findings are of high relevance to the intensive search for reliable synthesis methods for large graphene flakes of controlled layer number. 1 arXiv:0904.3220v2 [cond-mat.mtrl-sci]

The evolution of electronic (spin and charge) excitations upon carrier doping is an extremely important issue in superconducting layered cuprates and the knowledge of its asymmetry between electron-and hole-dopings is still fragmentary. Here we combine X-ray and neutron inelastic scattering measurements to track the doping dependence of both spin and charge excitations in electron-doped materials. Copper L 3 resonant inelastic X-ray scattering spectra show that magnetic excitations shift to higher energy upon doping. Their dispersion becomes steeper near the magnetic zone centre and they deeply mix with charge excitations, indicating that electrons acquire a highly itinerant character in the doped metallic state. Moreover, above the magnetic excitations, an additional dispersing feature is observed near the G-point, and we ascribe it to particle-hole charge excitations. These properties are in stark contrast with the more localized spin excitations (paramagnons) recently observed in hole-doped compounds even at high doping levels.

Experimental evidence on high-T c cuprates reveals ubiquitous charge density wave (CDW) modulations 1-10 , which coexist with superconductivity. Although the CDW had been predicted by theory 11-13 , important questions remain about the extent to which the CDW influences lattice and charge degrees of freedom and its characteristics as functions of doping and temperature. These questions are intimately connected to the origin of the CDW and its relation to the mysterious cuprate pseudogap 10,14 . Here, we use ultrahigh-resolution resonant inelastic X-ray scattering to reveal new CDW character in underdoped Bi 2.2 Sr 1.8 Ca 0.8 Dy 0.2 Cu 2 O 8+δ . At low temperature, we observe dispersive excitations from an incommensurate CDW that induces anomalously enhanced phonon intensity, unseen using other techniques. Near the pseudogap temperature T * , the CDW persists, but the associated excitations significantly weaken with an indication of CDW wavevector shift. The dispersive CDW excitations, phonon anomaly, and analysis of the CDW wavevector provide a comprehensive momentumspace picture of complex CDW behaviour and point to a closer relationship with the pseudogap state.With sufficient energy resolution, resonant inelastic X-ray scattering (RIXS) can be an ideal probe for revealing the CDW excitations in cuprates. By tuning the incident photon energy to the Cu L 3 -edge (Fig. 1a), the resonant absorption and emission processes can leave the system in excited final states, which couple to a variety of excitations arising from orbital, spin, charge, and lattice degrees of freedom 15 . Thus, information of these elementary excitations in energy and momentum space can be deduced from analysing the RIXS spectra as functions of the energy loss and the momentum transfer of the photons (Fig. 1a). This is highlighted by the pivotal role that RIXS has recently played in revealing orbital and magnetic excitations in cuprates [16][17][18][19][20] . In addition, RIXS provided the first X-ray scattering evidence for an incommensurate CDW in (Y,Nd)Ba 2 Cu 3 O 6+δ (ref. 4), owing to energy resolution that separated the quasi-elastic CDW signal (bright spot in Fig. 1b, limited by the instrumental resolution ∼130 meV) from other intense higher-energy excitations. Notably this quasi-elastic signal is asymmetric with respect to zero energy loss (Fig. 1c), which indicates the possible existence of additional low-energy excitations near the CDW wavevector (Q CDW ).In this work, we exploit the newly commissioned ultrahighresolution RIXS instrument at the European Synchrotron Radiation Facility to reveal these low-energy excitations near the CDW. We choose the double-layer cuprate Bi 2.2 Sr 1.8 Ca 0.8 Dy 0.2 Cu 2 O 8+δ (Bi2212), whose electronic structure has been extensively studied by surface-sensitive spectroscopy, such as scanning tunnelling microscopy 21 and angle-resolved photoemission 22 , and in which a short-range CDW order was recently reported 7,8 . With improved energy resolution up to 40 meV, we see additional features in the pre...

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High temperature cuprate superconductors consist of stacked CuO2 planes, with primarily two dimensional electronic band structures and magnetic excitations [1,2], while superconducting coherence is three dimensional. This dichotomy highlights the importance of out-of-plane charge dynamics, believed to be incoherent in the normal state [3,4], yet lacking a comprehensive characterization in energy-momentum space. Here, we use resonant inelastic x-ray scattering (RIXS) with polarization analysis to uncover the pure charge character of a recently discovered collective mode in electron-doped cuprates [5-7]. This mode disperses along both the in-and, importantly, out-of-plane directions, revealing its three dimensional nature. The periodicity of the out-of-plane dispersion corresponds to the CuO2 plane distance rather than the crystallographic c-axis lattice constant, suggesting that the interplane Coulomb interaction drives the coherent out-of-plane charge dynamics. The observed properties are hallmarks of the long-sought acoustic plasmon, predicted for layered systems [8-13] and argued to play a substantial role in mediating high temperature superconductivity [13-15]. The charge dynamics of systems with periodically stacked quasi-two dimensional (2D) conducting planes are drastically affected in the presence of poorly screened interplane Coulomb interactions. In a simple layered electron gas model with conducting planes separated by dielectric spacers [8-10], the dispersion of plasmons, the collective electronic modes of the charge dynamics, evolves from optical-like to acoustic-like as a function of out-of-plane momentum qz [Fig. 1(a)], a behavior distinct from that in either pure 2D or isotropic 3D systems.For superconducting cuprates, similar charge dynamics have been postulated since they consist of conducting CuO2 planes stacked along the c-axis with poor out-of-plane screening [11][12][13]. While plasmons were observed in various spectroscopic studies at the Brillouin zone center [4,16,17] and by transmission electron energy loss spectroscopy (EELS) typically exploring in-plane