We investigated electronic structure of 5d transition-metal oxide Sr 2 IrO 4 using angle-resolved photoemission, optical conductivity, and x-ray absorption measurements and first-principles band calculations. The system was found to be well described by novel effective total angular momentum J eff states, in which relativistic spin-orbit (SO) coupling is fully taken into account under a large crystal field. Despite of delocalized Ir 5d states, the J eff -states form so narrow bands that even a small correlation energy leads to the J eff = 1/2 Mott ground state with unique electronic and magnetic behaviors, suggesting a new class of the J eff quantum spin driven correlated-electron phenomena.

We investigated the temperature-dependent evolution of the electronic structure of the J eff = 1 2 Mott insulator Sr 2 IrO 4 using optical spectroscopy. The optical conductivity spectra ͑ ͒ of this compound has recently been found to exhibit two d-d transitions associated with the transition between the J eff = 1 2 and J eff = 3 2 bands due to the cooperation of the electron correlation and spin-orbit coupling. As the temperature increases, the two peaks show significant changes resulting in a decrease in the Mott gap. The experimental observations are compared with the results of first-principles calculation in consideration of increasing bandwidth. We discuss the effect of the temperature change in the electronic structure of Sr 2 IrO 4 in terms of local lattice distortion, excitonic effect, electron-phonon coupling, and magnetic ordering.

We report a resonant inelastic x-ray scattering study of charge excitations in the quasi-onedimensional Mott insulator SrCuO2. We observe a continuum of low-energy excitations, the onset of which exhibits a small dispersion of ∼ 0.4 eV. Within this continuum, a highly dispersive feature with a large sinusoidal dispersion (∼ 1.1 eV) is observed. We have also measured the optical conductivity, and studied the dynamic response of the extended Hubbard model with realistic parameters, using a dynamical density-matrix renormalization group method. In contrast to earlier work, we do not find a long-lived exciton, but rather these results suggest that the excitation spectrum comprises a holon-antiholon continuum together with a broad resonance.PACS numbers: 78.70. Ck, 71.10.Fd, 75.10.Pq The separation of spin and charge degrees of freedom is one of the most important and fascinating properties of electrons in strongly correlated systems in one dimension. In particular, it is well known that in the one-dimensional (1D) Hubbard model, the low-energy physics is dominated by collective excitations of decoupled charge and spin degrees of freedom called holons and spinons, respectively [1]. Experimentally, if one creates a hole by removing an electron, this hole is expected to decay into a spinon and a holon, which can be studied with angle resolved photoemission spectroscopy (ARPES) [2]. The situation is different for so-called "particle-hole" probes, such as optical spectroscopy, resonant inelastic x-ray scattering (RIXS), and electron energy loss spectroscopy (EELS). In these experiments, the total charge is conserved in the scattering process, so that an electron is simply moved from one site to another, creating a hole and a doubly occupied site. The decay of the hole creates a holon and a spinon, while the double-occupancy decays into an antiholon and a spinon. Since photons and electrons strongly couple to the charge sector, the behavior of holon-antiholon pairs can be studied with these particle-hole probes [3,4].The so-called corner-sharing chain cuprates Sr 2 CuO 3 and SrCuO 2 are both charge-transfer insulators; that is, they have insulating gaps of ∼ 2 eV arising from strong electron correlations. Since their crystal structure is highly anisotropic, the electronic structure remains 1D over a wide temperature range. Only at a very low temperature does magnetic order set in, due to the small interchain coupling (T N ≈ 2 K for SrCuO 2 [5]). Based on the commonly-used measure of quasi-one-dimensionality, T N /J ∼ 10 −3 , these compounds can be regarded as among the best realizations of quasi-1D systems [6] In this Letter, we report a detailed study of the momentum dependence of the low-energy charge excitations in SrCuO 2 , utilizing the RIXS technique. We observe a continuum of excitations arising from the creation of particle and hole pairs. Within this continuum, a welldefined spectral feature with a large sinusoidal dispersion (∼ 1.1 eV) is observed. We have also measured the optical conductivity σ(ω) and carri...

We investigate the kinetic roughening of Ar + ion-sputtered Pd(001) surface both experimentally and theoretically. In situ real-time x-ray reflectivity and in situ scanning tunneling microscopy show that nanoscale adatom islands form and grow with increasing sputter time t. Surface roughness, W (t), and lateral correlation length, ξ(t), follows the scaling laws, W (t) ∼ t β and ξ(t) ∼ t 1/z with the exponents β ≃ 0.20 and 1/z ≃ 0.20, for ion beam energy ε = 0.5 keV, which is inconsistent with the prediction of the Kuramoto-Sivashinsky (KS) model. We thereby extend the KS model by applying the Sigmund theory of sputter erosion to the higher order, O(∇ 4 , h 2 ), where h is surface height, and derive a new term of the form ∇ 2 (∇h) 2 which plays an indispensable role in describing the observed morphological evolution of the sputtered surface. 64.60.Cn, 79.20.Rf Recently, the observation of ordered nanostructures such as ripples and two-dimensional patterns on ion-sputtered surfaces has attracted much attention due to the demonstration of the possibility of fabrication of ordered nanoscale structures in a relatively easy and affordable way [1,2,3,4,5,6, 7]. Such experimental results have motivated extensive theoretical investigations aiming to understand the mechanism of the morphological evolution of ion-sputtered surfaces. A linear model, proposed by Bradley and Harper (BH) [8], has been successful in predicting the formation of the ripple structure. The wavelength, orientation and amplitude of the ripples can be predicted in terms of experimental parameters such as the incident angle of the ion beam and substrate temperature [9]. The BH theory, however, fails to explain a number of experimental observations such as the saturation of the ripple amplitude [10,11,12], or the appearance of kinetic roughening [13,14]. To remedy such shortcomings, the noisy Kuramoto-Sivashinsky (KS) equation [15,16] was introduced based on the Sigmund theory of sputter erosion [17]. In addition to the linear terms of the BH model, it contains a nonlinear term proportional to (∇h) 2 , known as the Kardar-ParisiZhang (KPZ) term [18], where h is surface height. Due to the nonlinear term, the surface roughness (or the ripple amplitude), which was growing exponentially with increasing sputter time in the linear model, changes to the type following a power law and eventually saturates to a constant value [19]. Although the KS model seems to be successful in offering insights for understanding the nonlinear behavior of sputtereroded surfaces, it has not been convincing yet because the detailed properties of the kinetic roughening predicted by the KS equation have not been fully tested experimentally. Kinetic roughening behavior is described by the scaling theory [20].The surface roughness of the sam-The roughness and growth exponents α and β are related via scaling relations to give the dynamic exponent, z = α/β, which determines the scaling of the saturation time with the system size L. Below, we will deal with the inverse dynamic expon...

We have investigated the electronic structure of meta-stable perovskite Ca 1-x Sr x IrO 3 (x = 0, 0.5, and 1) thin films using transport measurements, optical spectroscopy, and first-principles calculations. We artificially fabricated the perovskite phase of Ca 1-x Sr x IrO 3 , which has a hexagonal or post perovskite crystal structure in bulk form, by growing epitaxial thin films on perovskite GdScO 3 substrates using epi-stabilization technique. The transport properties of the perovskite Ca 1-x Sr x IrO 3 films systematically changed from nearly insulating (or semi-metallic) for x = 0 to bad metallic for x = 1. Due to the extended wavefunctions, 5d electrons are usually delocalized. However, the strong spin-orbit coupling in Ca 1-x Sr x IrO 3 results in the formation of effective total angular momentum J eff = 1/2 and 3/2 states, which puts Ca 1-x Sr x IrO 3 in the vicinity of a metal-insulator phase boundary. As a result, the electrical properties of the Ca 1-x Sr x IrO 3 films are found to be sensitive to x and strain.