Charge-transfer exciton in La2CuO4 probed with resonant inelastic x-ray scattering

We report a high-resolution resonant inelastic x-ray scattering study of La2CuO4. A number of spectral features are identified that were not clearly visible in earlier lower-resolution data. The momentum dependence of the spectral weight and the dispersion of the lowest energy excitation across the insulating gap have been measured in detail. The temperature dependence of the spectral features was also examined. The observed charge transfer edge shift, along with the low dispersion of the first charge transfer excitation are attributed to the lattice motion being coupled to the electronic system. In addition, we observe a dispersionless feature at 1.8 eV, which is associated with a d-d crystal field excitation.Comment: 5 pages, 4 figure

Excitonic quasiparticles in a spin-orbit Mott insulator

In condensed matter systems, out of a large number of interacting degrees of freedom emerge weakly coupled particles, in terms of which most physical properties are described. For example, Landau quasiparticles (QP) determine all electronic properties of a normal metal. The lack of identification of such QPs is major barrier for understanding myriad exotic properties of correlated electrons, such as unconventional superconductivity and non-Fermi liquid behaviours. Here, we report the observation of a composite particle in a Mott insulator Sr2IrO4---and exciton dressed with magnons---that propagates with the canonical characteristics of a QP: a finite QP residue and a lifetime longer than the hopping time scale. The dynamics of this charge-neutral bosonic excitation mirrors the fundamental process of the analogous one-hole propagation in the background of ordered spins, for which a well-defined QP has never been observed. The much narrower linewidth of the exciton reveals the same intrinsic dynamics that is obscured for the hole and is intimately related to the mechanism of high temperature superconductivity.Comment: submitted versio

Cognitive Load Theory: Advances in Research on Worked Examples, Animations, and Cognitive Load Measurement

The contributions to this special issue document some recent advances of cognitive load theory, and are based on contributions to the Third International Cognitive Load Theory Conference (2009), Heerlen, The Netherlands. The contributions focus on developments in example-based learning, amongst others on the effects of integrating worked examples in cognitive tutoring systems; specify the effects of transience on cognitive load and why segmentation may help counteract these effects in terms of the role of time in working memory load; and discuss the possibilities offered by electroencephalography (EEG) to provide a continuous and objective measure of cognitive load. This article provides a short introduction to the contributions in this issue

Quartz-based flat-crystal resonant inelastic x-ray scattering spectrometer with sub-10 meV energy resolution

Continued improvement of the energy resolution of resonant inelastic x-ray scattering (RIXS) spectrometers is crucial for fulfilling the potential of this technique in the study of electron dynamics in materials of fundamental and technological importance. In particular, RIXS is the only alternative tool to inelastic neutron scattering capable of providing fully momentum resolved information on dynamic spin structures of magnetic materials, but is limited to systems whose magnetic excitation energy scales are comparable to the energy resolution. The state-of-the-art spherical diced crystal analyzer optics provides energy resolution as good as 25 meV but has already reached its theoretical limit. Here, we demonstrate a novel sub-10meV RIXS spectrometer based on flat-crystal optics at the Ir-L$_3$ absorption edge (11.215$\sim$ keV) that achieves an analyzer energy resolution of 3.9$\sim$meV, very close to the theoretical value of 3.7$\sim$meV. In addition, the new spectrometer allows efficient polarization analysis without loss of energy resolution. The performance of the instrument is demonstrated using longitudinal acoustical and optical phonons in diamond, and magnon in Sr$_3$Ir$_2$O$_7$. The novel sub-10$\sim$meV RIXS spectrometer thus provides a window into magnetic materials with small energy scales

Resonant inelastic X-ray scattering study of overdoped La2−x_{2-x}Srx_{x}CuO4_{4}

Resonant inelastic x-ray scattering (RIXS) at the copper K absorption edge has been performed for heavily overdoped samples of La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x= 0.25$ and 0.30. We have observed the charge transfer and molecular-orbital excitations which exhibit resonances at incident energies of $E_i= 8.992$ and 8.998 keV, respectively. From a comparison with previous results on undoped and optimally-doped samples, we determine that the charge-transfer excitation energy increases monotonically as doping increases. In addition, the $E_i$-dependences of the RIXS spectral weight and absorption spectrum exhibit no clear peak at $E_i = 8.998$ keV in contrast to results in the underdoped samples. The low-energy ($\leq 3$ eV) continuum excitation intensity has been studied utilizing the high energy resolution of 0.13 eV (FWHM). A comparison of the RIXS profiles at $(\pi ~0)$ and $(\pi ~\pi)$ indicates that the continuum intensity exists even at $(\pi ~\pi)$ in the overdoped samples, whereas it has been reported only at $(0 ~0)$ and $(\pi ~0)$ for the $x=0.17$ sample. Furthermore, we also found an additional excitation on top of the continuum intensity at the $(\pi ~\pi)$ and $(\pi ~0)$ positions.Comment: 7 pages, 7 figure

Properties of charge density waves in La2−x_{2-x}Bax_{x}CuO4_4

We report a comprehensive x-ray scattering study of charge density wave (stripe) ordering in $\rm La_{2-x}Ba_xCuO_4 (x \approx 1/8)$, for which the superconducting $T_c$ is greatly suppressed. Strong superlattice reflections corresponding to static ordering of charge stripes were observed in this sample. The structural modulation at the lowest temperature was deduced based on the intensity of over 70 unique superlattice positions surveyed. We found that the charge order in this sample is described with one-dimensional charge density waves, which have incommensurate wave-vectors (0.23, 0, 0.5) and (0, 0.23, 0.5) respectively on neighboring $\rm CuO_2$ planes. The structural modulation due to the charge density wave order is simply sinusoidal, and no higher harmonics were observed. Just below the structural transition temperature, short-range charge density wave correlation appears, which develops into a large scale charge ordering around 40 K, close to the spin density wave ordering temperature. However, this charge ordering fails to grow into a true long range order, and its correlation length saturates at $\sim 230\AA$, and slightly decreases below about 15 K, which may be due to the onset of two-dimensional superconductivity.Comment: 11 pages, 9 figure

Doping dependence of charge-transfer excitations in La_{2-x}Sr_xCuO_4

We report a resonant inelastic x-ray scattering (RIXS) study of the doping dependence of charge-transfer excitations in $\rm La_{2-x}Sr_xCuO_4$. The mome ntum dependence of these charge excitations are studied over the whole Brillouin zone in underdoped (x=0.05) and optimally doped (x=0.17) samples, and compared with that of the undoped (x=0) sample. We observe a large change in the RIXS spectra between the x=0 and x=0.17 sample, while the RIXS spectra of the x=0.05 sample are similar to that of the x=0 sample. The most prominent effect of doped-holes on the charge excitation spectra is the appearance of a continuum of intensity, which exhibits a strong momentum-dependence below 2 eV. For the x=0.17 sample, some of the spectral weight from the lowest-lying charge-transfer excitation of the undoped compound is transferred to the continuum intensity below the gap, in agreement with earlier optical studies. However, the higher energy charge-transfer excitation carries significant spectral weight even for the x=0.17 sample. The doping dependence of the dispersion of this charge-transfer excitation is also discussed and compared with recent theoretical calculations.Comment: 7 pages, 6 figures, to appear in Phys. Rev.