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.

How coherent quasiparticles emerge by doping quantum antiferromagnets is a key question in correlated electron systems, whose resolution is needed to elucidate the phase diagram of copper oxides. Recent resonant inelastic X-ray scattering (RIXS) experiments in hole-doped cuprates have purported to measure high-energy collective spin excitations that persist well into the overdoped regime and bear a striking resemblance to those found in the parent compound, challenging the perception that spin excitations should weaken with doping and have a diminishing effect on superconductivity. Here we show that RIXS at the Cu L 3 -edge indeed provides access to the spin dynamical structure factor once one considers the full influence of light polarization. Further we demonstrate that high-energy spin excitations do not correlate with the doping dependence of T c , while low-energy excitations depend sensitively on doping and show ferromagnetic correlations. This suggests that high-energy spin excitations are marginal to pairing in cuprate superconductors.

We investigate the spectral properties of a hole moving in a two-dimensional Hubbard model for strongly correlated t2g electrons. Although superexchange interactions are Ising-like, a quasione-dimensional coherent hole motion arises due to effective three-site terms. This mechanism is fundamentally different from the hole motion via quantum fluctuations in the conventional spin model with SU(2) symmetry. The orbital model describes also propagation of a hole in some eg compounds, and we argue that orbital degeneracy alone does not lead to hole self-localization.[Published in Phys. Rev. Lett. 100, 066403 (2008).]PACS numbers: 71.10. Fd, 72.10.Di, 72.80.Ga, One of the fundamental problems in solid state physics consists in understanding the motion of an electron or hole coupled to the other degrees of freedom in a material. In many cases, the other degrees of freedom (spin, orbital or phonon excitations) can increase the mass of the carrier and possibly localize it. For example, the undoped parent compound of high-T c cuprates is an antiferromagnetic (AF) Mott insulator due to electron-electron repulsion. A hole doped into it was at first thought to be localized, because its movement would disturb the AF background and thus cost energy [1]. Only two decades later it was found out that quantum spin fluctuations heal the background and lead to a coherent hole motion [2, 3], see . This shows, how important it is to critically assess any approximation used and to identify possible mechanisms of the coherent hole motion.Novel aspects of localization occur in orbital models. The superexchange (SE) is then no longer SU (2) field splits t 2g orbitals in d 1 or d 2 systems. If one of the three t 2g orbitals is either empty (d 1 ) or fully occupied (d 2 ), the remaining two can form the AO order, as e.g. in the planes of Sr 2 VO 4 [14] with possible weak FM order [15]. In addition to t 2g compounds, this "t 2g model" also describes e g orbitals in the above mentioned fluorides, where a crystal field induces d z 2 −x 2 /d z 2 −y 2 -type AO order [10,11], so both quantum fluctuations and interorbital hopping are quenched and cannot generate coherent quasiparticle (QP) propagation [9], shown in Fig. 1.In this Letter, we show that a hole doped in a state with alternating t 2g orbitals is not confined but finds a way to move coherently via three-site effective hopping terms arising from SE, i.e., even in a model with strictly nearest-neighbor hopping. For the present orbital model, long-range hopping is not expected to be important, because it is straightforward to verify that: (i) the second neighbor hoppings flip the orbital flavor [16], so they do not contribute to QP dispersion, while (ii) the third neighbor hoppings which conserve the orbital flavor are considerably smaller than the three-site terms for realistic parameters. These latter SE terms are often neglected [17], but here they play a central role and determine QP propagation. This finding contradicts naive expectations of absence of coherent hole motion for the I...