Temperature dependence of the electronic structure of Sr14Cu24O41 studied by resonant inelastic X-ray scattering

We report a resonant inelastic X-ray scattering (RIXS) study of charge excitations in the two-leg ladder Sr₁₄Cu₂₄O₄₁. RIXS spectra at 1–5 eV are found to be dependent on temperature. An intraband excitation of the ladder, which appears as a continuum intensity below the Mott gap, decreases in intensity with temperature. Because the intraband excitation is related to the dynamics of doped holes in the ladder, its decrease of the intraband excitation is attributed to the reduction of the mobile holes in the ladder at low temperature.     

Electronic states in polypyrrole

Electron energy loss spectra of valence excitations in polypyrrole doped with boron tetrafluoride, undoped, and subsequently redoped with arsenic pentafluoride were measured. Energy loss peaks associated with intraband and interband excitations which show characteristically different momentum dependence were observed. Intraband peaks exhibit weak negative dispersion while interband excitations show strong positive dispersion. This suggests that interband excitations occur between wide energy bands (～5 eV) in long polymetric molecules while intraband excitations may be due to intrinsic or extrinsic intermolecular charge transfer transitions in a narrow band (～0.5 eV).     

Resonant inelastic x-ray scattering at the oxygen K resonance of NiO: nonlocal charge transfer and double-singlet excitations

We report high-resolution polarization-dependent resonant inelastic x-ray scattering (RIXS) at the O K resonance of NiO showing a rich excitation spectrum. We perform multisite Ni6O19 cluster model calculations, revealing that solid state effects are substantial. We identify a nonlocal charge transfer excitation at 4-5 eV and double-singlet creation at 1.75 eV, both exhibiting significant scattering geometry dependence. Apart from an intense band of local charge transfer excitations (above 5 eV) also dd excitations at 1 eV are observed. Finally, we point out that O K RIXS of correlated metal oxides allows a quantitative and consistent determination of the charge transfer energy delta and the Hund coupling energy J(H).     

Localized electronic excitations in NiO studied with resonant inelastic X-Ray scattering at the Ni M threshold: evidence of spin flip

We studied the neutral electronic excitations of NiO localized at the Ni sites by measuring the resonant inelastic x-ray scattering (RIXS) spectra at the Ni M2,3 edges. The good energy resolution allows an unambiguous identification of several spectral features due to excitations. The dependence of the RIXS spectra on the excitation energy gives evidence of local spin flip and yields a value of 125 +/- 15 meV for the antiferromagnetic exchange interaction. Accurate crystal field parameters are also obtained.     

Optical self-energy in graphene due to correlations

In highly correlated systems one can define an optical self-energy in analogy to its quasiparticle (QP) self-energy counterpart. This quantity provides useful information on the nature of the excitations involved in inelastic scattering processes. Here we calculate the self-energy of the intraband optical transitions in graphene originating in the electron-electron interaction (EEI) as well as electron-phonon interaction (EPI). Although optics involves an average over all momenta (k) of the charge carriers, the structure in the optical self-energy is nevertheless found to mirror mainly that of the corresponding quasiparticles for k equal to or near the Fermi momentum k(F). Consequently, plasmaronic structures which are associated with momenta near the Dirac point at k = 0 are not important in the intraband optical response. While the structure of the electron-phonon interaction (EPI) reflects the sharp peaks of the phonon density of states, the excitation spectrum associated with the electron-electron interaction is in comparison structureless and flat and extends over an energy range which scales linearly with the value of the chemical potential. We introduce a method whereby detailed quantitative information on such excitation spectra can be extracted from optical data. Modulations seen on the edge of the interband optical conductivity as it rises towards its universal background value are traced to structure in the quasiparticle self-energies around k(F) of the lower Dirac cone associated with the occupied states.     

Influence of intraband motion on the interband excitation and high harmonic generation

Tunnelling, acceleration, and collision of electrons are the basic events in the process of high harmonic generation(HHG) in strong-field interaction with atoms.However, the periodic array of atoms in semiconductor structure makes three steps become interatomic coherent process which leads to complicated carrier dynamics and two sources of high harmonic emission: interband polarization and intraband current.The difference of features of high harmonic generation between semiconductors and atoms is strongly linked to the unique presence of intraband motion which manifests itself a nontrivial role in intertwined two dynamics.Here, we review recent experimental and theoretical advances of understanding coupled interband and intraband mechanisms of HHG in semiconductors.Particularly we focus on the influence of intraband motion on the interband excitation, and on the subsequent HHG emission and attosecond pulse generation.     

Defect state model for localized excitations in LiF

We find that a defect state treatment of localized excitations in LiF within the local density functional formalism accounts remarkably well for the observed experimental (core plus optical gap) excitations — in contrast to the failure of the one-electron band model. We show that when electron relaxation, self-interaction and charge polarization effects are taken into account by treating the excitation as a localized points defect, the improved band model predicts the correct excitation and interband states.     

Revealing the insulating gap in α'-NaV (2)O(5) with resonant inelastic x-ray scattering

We measured the low energy excitation spectrum of α'-NaV (2)O(5) across its charge ordering and crystallographic phase transition with resonant inelastic x-ray scattering (RIXS) at the V L(3) edge. Exploiting the polarization dependence of the RIXS signal and the high resolution of the data, we reveal the excitation across the insulating gap at 1?eV and identify the excitations from occupied 3d(xy) bonding orbitals to unoccupied bonding 3d(xy) and 3d(yz)/3d(xz) orbitals. Furthermore we observe a progressive change of the electronic structure of α'-NaV (2)O(5) induced by soft x-ray irradiation, with the appearance of features characteristic of sodium deficient Na(x)V (2)O(5) (x?相似文献   

Collective mode dispersions of organic chain compounds

We investigate the collective mode dispersions for the tight-binding dielectric matrix with two one-dimensional electron bands per donor and acceptor chains, and the three-dimensional long-range Coulomb electron-electron interaction within the random phase approximation. The hybridized collective modes are the result of the strong coupling between the intraband plasmon and the interband dipolar modes due to strong dipole Coulomb interactions. Our calculations show the existence of the low-energy renormalized plasmon mode above the electron-hole quasi-continuum in the long wavelength limit. The obtained modes are brought into correspondence with the optical data of quasi-one-dimensional organic conductor tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ). Namely, the renormalized plasmon and the dipolar mode are assigned to the observed excitations at respective energy scales of roughly 10 meV and 0.75 eV, explaining why lower excitation is eliminated while higher excitation persists below the temperature of the Peierls phase transition.     

Unraveling the nature of charge excitations in La₂CuO₄ with momentum-resolved Cu K-edge resonant inelastic x-ray scattering

The results of model calculations using exact diagonalization reveal the orbital character of states associated with different Raman loss peaks in Cu K-edge resonant inelastic x-ray scattering (RIXS) from La?CuO?. The model includes electronic orbitals necessary to highlight the nonlocal Zhang-Rice singlet, charge transfer, and d-d excitations, as well as states with apical oxygen 2p(z) character. The dispersion of these excitations is discussed with prospects for resonant final state wave-function mapping. A good agreement with experiments emphasizes the substantial multiorbital character of RIXS profiles in the energy transfer range 1-6 eV.     

Theory of RIXS in strongly correlated electron systems: Mott gap excitations in cuprates

We theoretically examine the momentum dependence of resonant inelastic X-ray scattering (RIXS) spectrum for one-dimensional and two-dimensional cuprates based on the single-band Hubbard model with realistic parameter values. The spectrum is calculated by using the numerical diagonalization technique for finite-size clusters. We focus on excitations across the Mott gap and clarify spectral features coming from the excitations as well as the physics behind them. Good agreement between the theoretical and existing experimental results clearly demonstrates that the RIXS is a potential tool to study the momentum-dependent charge excitations in strongly correlated electron systems.     

Intraband absorption and emission of light in quantum wells and quantum dots

High-resolution spectroscopy in the mid-infrared spectral range is used to study electronic transitions between size-quantization subbands in stepped quantum wells under picosecond interband excitation. The contributions from intersubband and intrasubband absorption of light are separated by using the difference in time profiles of the absorption coefficient for these cases. For stepped quantum wells, spontaneous interband luminescence and superluminescence are studied for different excitation levels. For structures with quantum dots, the intraband absorption spectra for n-and p-type structures and the spectra of photoinduced intraband absorption and emission (for polarized radiation) for undoped structures are studied.     

Mott gap excitations and resonant inelastic x-ray scattering in doped cuprates

Predictions are made for the momentum- and carrier-dependent degradation of the Mott gap upon doping in high-T(c) cuprates as would be observed in Cu K-edge resonant inelastic x-ray scattering (RIXS). The two-dimensional Hubbard model with second- and third-nearest-neighbor hopping terms has been studied by numerical exact diagonalization. Special emphasis is placed on the particle-hole asymmetry of the Mott gap excitations. We argue that the Mott gap excitations observed by RIXS are significantly influenced by the interaction between charge carriers and antiferromagnetic correlations.     

Microscopic analysis of the coherent optical generation and the decay of charge and spin currents in semiconductor heterostructures

The coherent optical injection and temporal decay of spin and charge currents in semiconductor heterostructures is described microscopically, including excitonic effects, many-body Coulomb correlations, and the carrier LO-phonon coupling on the second-order Born-Markov level, as well as nonperturbative light-field-induced intraband and interband excitations. A nonmonotonic dependence of the currents on the intensities of the laser beams is predicted. Enhanced damping of the spin current relative to the charge current is obtained as a consequence of Coulomb scattering.     

Charge-transfer excitations in the model superconductor HgBa(2)CuO(4+delta)

We report a Cu -edge resonant inelastic x-ray scattering (RIXS) study of charge-transfer excitations in the 2-8 eV range in the structurally simple compound HgBa(2)CuO(4+delta) at optimal doping (T(c)=96.5 K). The spectra exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of weakly dispersive (<0.5 eV) electron-hole excitations, including a mode at 2 eV. The observation of this 2 eV excitation suggests the existence of a remnant charge-transfer gap deep in the superconducting phase. Quite generally, our results, which include additional data for the Mott insulator La(2)CuO(4), demonstrate the importance of exploring the incident photon-energy dependence of the RIXS cross section.     

Theoretical study of broadband near-field optical spectrum of twisted bilayer graphene

We theoretically study the broadband near-field optical spectrum of twisted bilayer graphene (TBG) at various twist angles near the magic angle using two different models. The spectrum at low Fermi energy is characterized by a series of peaks that are almost at the same energies as the peaks in the far-field optical conductivity of TBG. When the Fermi energy is near a van Hove singularity, an additional strong peak appears at finite energy in the near-field spectrum, which has no counterpart in the optical conductivity. Based on a detailed calculation of the plasmon dispersion, we show that these spectroscopic features are associated with interband and intraband plasmons, which can provide critical information about the local band structure and plasmonic excitations in TBG. The near-field peaks evolve systematically with the twist angle, so they can serve as fingerprints for identifying the spatial dependent twist angle in TBG samples. Our findings pave the way for future experimental studies of the novel optical properties of TBG in the nanoscale.     

Theory of resonant inelastic X-ray scattering spectrum for Ni impurities in cuprates

We perform the numerically exact diagonalization calculation for small Cu-O clusters with a Ni impurity site, representing the Ni-substituted cuprate, to examine the single-particle excitation spectra as well as the resonant inelastic X-ray scattering (RIXS) spectra. We clarify relations between low-energy electronic structures near the Ni site and excitations seen in the RIXS spectra.     

Single and double orbital excitations probed by resonant inelastic x-ray scattering

The dispersion of the elusive elementary excitations of orbital ordered systems, orbitons, has escaped detection so far. The recent advances in resonant inelastic x-ray scattering (RIXS) techniques have made it, in principle, a powerful new probe of orbiton dynamics. We compute the detailed traces that orbitons leave in RIXS for an e{g} orbital ordered system, using the ultrashort core-hole lifetime expansion for RIXS. We observe that both single- and double-orbiton excitations are allowed, where the former, at lower energy, have sharper features. The rich energy- and momentum-dependent intensity variations that we observe make clear that RIXS is an ideal method to identify and map out orbiton dispersions.     

Low energy electronic excitations in the layered cuprates studied by copper L3 resonant inelastic x-ray scattering

We have measured the resonant inelastic x-ray scattering (RIXS) spectra at the Cu L3 edge in a variety of cuprates. Exploiting a considerably improved energy resolution (0.8 eV) we recorded significant dependencies on the sample composition and orientation, on the scattering geometry, and on the incident photon polarization. The RIXS final states correspond to two families of electronic excitations, having local (dd excitations) and nonlocal (charge-transfer) character. The dd energy splitting can be estimated with a simple crystal field model. The RIXS at the L3 edge demonstrates here a great potential, thanks to the resonance strength and to the large 2p spin-orbit splitting.     

Optical spectroscopy of graphene: From the far infrared to the ultraviolet

The unique electronic structure of graphene leads to several distinctive optical properties. In this brief review, we outline the current understanding of two general aspects of optical response of graphene: optical absorption and light emission. We show that optical absorption in graphene is dominated by intraband transitions at low photon energies (in the far-infrared spectral range) and by interband transitions at higher energies (from mid-infrared to ultraviolet). We discuss how the intraband and interband transitions in graphene can be modified through electrostatic gating. We describe plasmonic resonances arising from the free-carrier (intraband) response and excitonic effects that are manifested in the interband absorption. Light emission, the reverse process of absorption, is weak in graphene due to the absence of a band gap. We show that photoluminescence from hot electrons can, however, become observable either through femtosecond laser excitation or strong electrostatic gating.