High-energy magnetic excitations in overdoped La2−x_{2-x}Srx_{x}CuO4_{4} studied by neutron and resonant inelastic X-ray scattering

We have performed neutron inelastic scattering and resonant inelastic X-ray scattering (RIXS) at the Cu-$L_3$ edge to study high-energy magnetic excitations at energy transfers of more than 100 meV for overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.25$ ($T_c=15$ K) and $x=0.30$ (non-superconducting) using identical single crystal samples for the two techniques. From constant-energy slices of neutron scattering cross-sections, we have identified magnetic excitations up to ~250 meV for $x=0.25$. Although the width in the momentum direction is large, the peak positions along the (pi, pi) direction agree with the dispersion relation of the spin-wave in the non-doped La$_{2}$CuO$_{4}$ (LCO), which is consistent with the previous RIXS results of cuprate superconductors. Using RIXS at the Cu-$L_3$ edge, we have measured the dispersion relations of the so-called paramagnon mode along both (pi, pi) and (pi, 0) directions. Although in both directions the neutron and RIXS data connect with each other and the paramagnon along (pi, 0) agrees well with the LCO spin-wave dispersion, the paramagnon in the (pi, pi) direction probed by RIXS appears to be less dispersive and the excitation energy is lower than the spin-wave of LCO near (pi/2, pi/2). Thus, our results indicate consistency between neutron inelastic scattering and RIXS, and elucidate the entire magnetic excitation in the (pi, pi) direction by the complementary use of two probes. The polarization dependence of the RIXS profiles indicates that appreciable charge excitations exist in the same energy range of magnetic excitations, reflecting the itinerant character of the overdoped sample. A possible anisotropy in the charge excitation intensity might explain the apparent differences in the paramagnon dispersion in the (pi, pi) direction as detected by the X-ray scattering.Comment: 7 pages, 7 figure

A magnetic model for the incommensurate I phase of spin-Peierls systems

A magnetic model is proposed for describing the incommensurate I phase of spin-Peierls systems. Based on the harmonicity of the lattice distortion, its main ingredient is that the distortion of the lattice adjusts to the average magnetization such that the system is always gapful. The presence of dynamical incommensurabilities in the fluctuation spectra is also predicted. Recent experimental results for CuGeO_3 obtained by NMR, ESR and light scattering absorption are well understood within this model.Comment: 8 pages, 3 figures, Latex with EPL style files all include

Conductivity landscape of highly oriented pyrolytic graphite surface containing ribbons and edges

We present an extensive study on electrical spectroscopy of graphene ribbons and edges of highly oriented pyrolytic graphite (HOPG) using atomic force microscope (AFM). We have addressed in the present study two main issues, (1) How does the electrical property of the graphite (graphene) sheet change when the graphite layer is displaced by shear forces? and (2) How does the electrical property of the graphite sheet change across a step edge? While addressing these two issues we observed, (1) variation of conductance among the graphite ribbons on the surface of HOPG. The top layer always exhibits more conductance than the lower layers, (2) two different monolayer ribbons on the same sheet of graphite shows different conductance, (3) certain ribbon/sheet edges show sharp rise in current, (4) certain ribbons/sheets on the same edge shows both presence and absense of the sharp rise in the current, (5) some lower layers at the interface near a step edge shows a strange dip in the current/conductance (depletion of charge). We discuss possible reasons for such rich conducting landscape on the surface of graphite.Comment: 13 pages, 9 figures. For better quality figures please contact autho

Quantum simulation of a Fermi-Hubbard model using a semiconductor quantum dot array

Interacting fermions on a lattice can develop strong quantum correlations, which lie at the heart of the classical intractability of many exotic phases of matter. Seminal efforts are underway in the control of artificial quantum systems, that can be made to emulate the underlying Fermi-Hubbard models. Electrostatically confined conduction band electrons define interacting quantum coherent spin and charge degrees of freedom that allow all-electrical pure-state initialisation and readily adhere to an engineerable Fermi-Hubbard Hamiltonian. Until now, however, the substantial electrostatic disorder inherent to solid state has made attempts at emulating Fermi-Hubbard physics on solid-state platforms few and far between. Here, we show that for gate-defined quantum dots, this disorder can be suppressed in a controlled manner. Novel insights and a newly developed semi-automated and scalable toolbox allow us to homogeneously and independently dial in the electron filling and nearest-neighbour tunnel coupling. Bringing these ideas and tools to fruition, we realize the first detailed characterization of the collective Coulomb blockade transition, which is the finite-size analogue of the interaction-driven Mott metal-to-insulator transition. As automation and device fabrication of semiconductor quantum dots continue to improve, the ideas presented here show how quantum dots can be used to investigate the physics of ever more complex many-body states

Polarization phenomena in hyperon-nucleon scattering

We investigate polarization observables in hyperon-nucleon scattering by decomposing scattering amplitudes into spin-space tensors, where each component describes scattering by corresponding spin-dependent interactions, so that contributions of the interactions in the observables are individually identified. In this way, for elastic scattering we find some linear combinations of the observables sensitive to particular spin-dependent interactions such as symmetric spin-orbit (LS) interactions and antisymmetric LS ones. These will be useful to criticize theoretical predictions of the interactions when the relevant observables are measured. We treat vector analyzing powers, depolarizations, and coefficients of polarization transfers and spin correlations, a part of which is numerically examined in $\Sigma^{+} p$ scattering as an example. Total cross sections are studied for polarized beams and targets as well as for unpolarized ones to investigate spin dependence of imaginary parts of forward scattering amplitudes.Comment: 15 pages, 8 figure

High magnetic field induced charge density wave states in a quasi-one dimensional organic conductor

We have measured the high field magnetoresistence and magnetization of quasi-one- dimensional (Q1D) organic conductor (Per)2Pt(mnt)2 (where Per = perylene and mnt = maleonitriledithiolate), which has a charge density wave (CDW) ground state at zero magnetic field below 8 K. We find that the CDW ground state is suppressed with moderate magnetic fields of order 20 T, as expected from a mean field theory treatment of Pauli effects[W. Dieterich and P. Fulde, Z. Physik 265, 239 - 243 (1973)]. At higher magnetic fields, a new, density wave state with sub-phases is observed in the range 20 to 50 T, which is reminiscent of the cascade of field induced, quantized, spin density wave phases (FISDW) observed in the Bechgaard salts. The new density wave state, which we tenatively identify as a field induced charge density wave state (FICDW), is re-entrant to a low resistance state at even higher fields, of order 50 T and above. Unlike the FISDW ground state, the FICDW state is only weakly orbital, and appears for all directions of magnetic field. Our findings are substantiated by electrical resistivity, magnetization, thermoelectric, and Hall measurements. We discuss our results in light of theoretical work involving magnetic field dependent Q1D CDW ground states in high magnetic fields [D. Zanchi, A. Bjelis, and G. Montambaux, Phys. Rev. B 53, (1996)1240; A. Lebed, JETP Lett. 78,138(2003)].Comment: 16 pages, 5 figure

Optical properties of photonic crystal slabs with asymmetrical unit cell

Using the unitarity and reciprocity properties of the scattering matrix, we analyse the symmetry and resonant optical properties of the photonic crystal slabs (PCS) with complicated unit cell. We show that the reflectivity is not changed upon the 180deg-rotation of the sample around the normal axis, even in PCS with asymmetrical unit cell. Whereas the transmissivity becomes asymmetrical if the diffraction or absorption are present. The PCS reflectivity peaks to unity near the quasiguided mode resonance for normal light incidence in the absence of diffraction, depolarisation, and absorptive losses. For the oblique incidence the full reflectivity is reached only in symmetrical PCS.Comment: 5 pages, 2 Postscript figure