When viewed as an elementary particle, the electron has spin and charge. When binding to the atomic nucleus, it also acquires an angular momentum quantum number corresponding to the quantized atomic orbital it occupies. Even if electrons in solids form bands and delocalize from the nuclei, in Mott insulators they retain their three fundamental quantum numbers: spin, charge and orbital. The hallmark of one-dimensional physics is a breaking up of the elementary electron into its separate degrees of freedom. The separation of the electron into independent quasi-particles that carry either spin (spinons) or charge (holons) was first observed fifteen years ago. Here we report observation of the separation of the orbital degree of freedom (orbiton) using resonant inelastic X-ray scattering on the one-dimensional Mott insulator Sr2CuO3. We resolve an orbiton separating itself from spinons and propagating through the lattice as a distinct quasi-particle with a substantial dispersion in energy over momentum, of about 0.2 electronvolts, over nearly one Brillouin zone.

We investigate magnetic excitations in the spin-ladder compound Sr 14 Cu 24 O 41 using high-resolution Cu L 3 edge resonant inelastic x-ray scattering (RIXS). Our findings demonstrate that RIXS couples to two-triplon collective excitations. In contrast to inelastic neutron scattering, the RIXS cross section changes only moderately over the entire Brillouin zone, revealing high sensitivity also at small momentum transfers, allowing determination of the two-triplon energy gap as 100 AE 30 meV. Our results are backed by calculations within an effective Hubbard model for a finite-size cluster, and confirm that optical selection rules are obeyed for excitations from this spherically symmetric quantum spin-liquid ground state. DOI: 10.1103/PhysRevLett.103.047401 PACS numbers: 78.70.En, 71.10.Pm, 75.25.+z, 75.30.Ds Collective excitations in strongly correlated electron materials remain a pivotal challenge in contemporary solid state physics. It is widely debated whether magnetic excitations provide the pairing interaction in the hightemperature and unconventional superconductors [1,2]. From that perspective quantum spin systems attract considerable interest. While most such materials, e.g., the cuprate superconductors, exhibit enormous complexity, the two-leg spin ladder is easier to tract theoretically [3][4][5][6]. It consists of two parallel chains (legs) with a transverse (rung) exchange coupling. This system features a singlet ground state and dispersive triplet excitations (triplons), that both have quantum mechanical origin without any classical counterpart. To date, mainly two techniques have been established as momentum-and energy-resolved probes of the dispersion of collective excitations: angleresolved photoelectron spectroscopy and inelastic neutron scattering (INS) for charge and spin degrees of freedom, respectively [7,8]. Because of the latest instrumental improvements [9,10], the energy scale of magnetic exchange is becoming readily accessible for resonant inelastic x-ray scattering (RIXS) [11][12][13][14], which is promising to give information on both, spin and charge degrees of freedom, and in addition is an element-specific technique. Furthermore, RIXS requires only small sample volumes (<0:1 mm 3 ). Recent RIXS studies were performed on long-range ordered magnets with spin-wave excitations [15][16][17].In this Letter, we report a study of the two-leg quantum spin ladder Sr 14 Cu 24 O 41 [18,19] by means of momentumresolved high-resolution RIXS at the Cu L 3 edge. Given that Cu L 3 scattering experiments have been already shown to contain valuable information about the charge degrees of freedom [20], an outstanding question we would like to address here is: how can RIXS provide information on magnetic excitations from a quantum ground state. In the ladder system of Sr 14 Cu 24 O 41 no symmetry breaking occurs-neither in spin-nor in real-space-in contrast to, e.g., a magnetically ordered state, where both symmetries are broken and the direction of the ordered moments dictates the quantization axi...

Amorphous materials represent a large and important emerging area of material's science. Amorphous oxides are key technological oxides in applications such as a gate dielectric in Complementary metal-oxide semiconductor devices and in Silicon-Oxide-NitrideOxide-Silicon and TANOS (TaN-Al 2 O 3 -Si 3 N 4 -SiO 2 -Silicon) flash memories. These technologies are required for the high packing density of today's integrated circuits. Therefore the investigation of defect states in these structures is crucial. In this work we present X-ray synchrotron measurements, with an energy resolution which is about 5-10 times higher than is attainable with standard spectrometers, of amorphous alumina. We demonstrate that our experimental results are in agreement with calculated spectra of amorphous alumina which we have generated by stochastic quenching. This first principles method, which we have recently developed, is found to be superior to molecular dynamics in simulating the rapid gas to solid transition that takes place as this material is deposited for thin film applications. We detect and analyze in detail states in the band gap that originate from oxygen pairs. Similar states were previously found in amorphous alumina by other spectroscopic methods and were assigned to oxygen vacancies claimed to act mutually as electron and hole traps. The oxygen pairs which we probe in this work act as hole traps only and will influence the information retention in electronic devices. In amorphous silica oxygen pairs have already been found, thus they may be a feature which is characteristic also of other amorphous metal oxides.stochastic quench | X-ray absorption spectroscopy | ab initio | coating D espite early attempts to describe the fundamental electronic properties of noncrystalline semiconductors (1-5), experimental and theoretical knowledge of localized states in the gap of amorphous semiconductors and insulators is still limited. General features of the electronic structure of amorphous semiconductors are quite well known, such as the broad distribution of coordinations and the lack of long range order that induces valence and conduction band tails in the band gap (6). However, the origin of these states is less explored experimentally (7,8) and theoretical investigations are mainly limited to the crystalline polymorphs (9-11). Amorphous Alumina (am-Al 2 O 3 ) is currently one of the key technological amorphous materials, where one promising application of am-Al 2 O 3 is as a high-k dielectric in transistors (12). The use of am-Al 2 O 3 in TANOS (TaN-Al 2 O 3 -Si 3 N 4 -SiO 2 -Silicon) flash memories, which are currently investigated for gigabite and terabite scale flash memories, puts even higher demands on alumina as a current-blocking high-k dielectric.From optical absorption and photoluminescence, states related to F-centers (9, 10) and impurities have been identified in the band gap of am-Al 2 O 3 down to 3.18 and 3.25 eV relative to the valence band edge (13,14). In another study, electronbeam induced states in the am-Al ...

We have performed 2p X-ray absorption (XAS) and 2p3d resonant X-ray emission (RXES) experiments on a CoO bulk single crystal as well as on 4.2 nm CoO nanocrystals. The single crystal data were measured with linearly polarized incident X-rays in the scattering and perpendicular to the scattering plane. An unprecedented total experimental resolution of 100 meV allowed the first Xray observation of the CoO 4 T 1g ( 4 F) manifold that occurs 120 meV above ground state. Detailed theoretical modeling was performed to assess the tetragonal crystal field splitting, spin− orbit, and superexchange parameters for both the single crystal and the nanocrystals. We show that 2p XAS is mainly sensitive to the octahedral field and 3d spin−orbit coupling, while the 4 T 1g ( 4 F) manifold in 2p3d RXES probes the tetragonal distortion and the superexchange interactions with high sensitivity. We observe that the nanocrystals have a reduced cubic crystal field splitting and a broadened 4 T 1g ( 4 F) RXES manifold that we ascribe to a larger average Co−O distance and an increased magnetic exchange in the nanocrystals. We demonstrate that further improvement of the RXES experimental resolution would not only allow for better disentanglement of the tetragonal distortion, spin−orbit, and superexchange interactions, but it would also allow observing anti-Stokes features in the RXES spectrum.