Magnetic field dependence of charge stripe order in La2-xBaxCuO4 (x~1/8)

We have carried out a detailed investigation of the magnetic field dependence of charge ordering in La2-xBaxCuO4 (x~1/8) utilizing high-resolution x-ray scattering. We find that the charge order correlation length increases as the magnetic field greater than ~5T is applied in the superconducting phase (T=2K). The observed unusual field dependence of the charge order correlation length suggests that the static charge stripe order competes with the superconducting ground state in this sample.Comment: 4 pages, 4 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.

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

Resonant inelastic x-ray scattering study of charge excitations in La2CuO4

We report a resonant inelastic x-ray scattering study of the dispersion relations of charge transfer excitations in insulating La$_2$CuO$_4$. These data reveal two peaks, both of which show two-dimensional characteristics. The lowest energy excitation has a gap energy of $\sim 2.2$ eV at the zone center, and a dispersion of $\sim 1$ eV. The spectral weight of this mode becomes dramatically smaller around ($\pi$, $\pi$). The second peak shows a smaller dispersion ($\sim 0.5$ eV) with a zone-center energy of $\sim 3.9$ eV. We argue that these are both highly dispersive exciton modes damped by the presence of the electron-hole continuum.Comment: 5 pages, 3 figure

Correlated Polarons in Dissimilar Perovskite Manganites

We report x-ray scattering studies of broad peaks located at a (0.5 0 0)/(0 0.5 0)-type wavevector in the paramagnetic insulating phases of La_{0.7}Ca_{0.3}MnO_{3} and Pr_{0.7}Ca_{0.3}MnO_{3}. We interpret the scattering in terms of correlated polarons and measure isotropic correlation lengths of 1-2 lattice constants in both samples. Based on the wavevector and correlation lengths, the correlated polarons are found to be consistent with CE-type bipolarons. Differences in behavior between the samples arise as they are cooled through their respective transition temperatures and become ferromagnetic metallic (La_{0.7}Ca_{0.3}MnO_{3}) or charge and orbitally ordered insulating (Pr_{0.7}Ca_{0.3}MnO_{3}). Since the primary difference between the two samples is the trivalent cation size, these results illustrate the robust nature of the correlated polarons to variations in the relative strength of the electron-phonon coupling, and the sensitivity of the low-temperature ground state to such variations.Comment: 13 pages, 6 figure

Development of an instrument for measuring different types of cognitive load

According to cognitive load theory, instructions can impose three types of cognitive load on the learner: intrinsic load, extraneous load, and germane load. Proper measurement of the different types of cognitive load can help us understand why the effectiveness and efficiency of learning environments may differ as a function of instructional formats and learner characteristics. In this article, we present a ten-item instrument for the measurement of the three types of cognitive load. Principal component analysis on data from a lecture in statistics for PhD students (n = 56) in psychology and health sciences revealed a three-component solution, consistent with the types of load that the different items were intended to measure. This solution was confirmed by a confirmatory factor analysis of data from three lectures in statistics for different cohorts of bachelor students in the social and health sciences (ns = 171, 136, and 148), and received further support from a randomized experiment with university freshmen in the health sciences (n = 58)

Holographic analysis of diffraction structure factors

We combine the theory of inside-source/inside-detector x-ray fluorescence holography and Kossel lines/x ray standing waves in kinematic approximation to directly obtain the phases of the diffraction structure factors. The influence of Kossel lines and standing waves on holography is also discussed. We obtain partial phase determination from experimental data obtaining the sign of the real part of the structure factor for several reciprocal lattice vectors of a vanadium crystal.Comment: 4 pages, 3 figures, submitte

Orbital Correlations in Doped Manganites

We review our recent x-ray scattering studies of charge and orbital order in doped manganites, with specific emphasis on the role of orbital correlations in Pr_1-xCa_xMnO_3. For x=0.25, we find an orbital structure indistinguishable from the undoped structure with long range orbital order at low temperatures. For dopings 0.3<x<0.5, we find scattering consistent with a charge and orbitally ordered CE-type structure. While in each case the charge order peaks are resolution limited, the orbital order exhibits only short range correlations. We report the doping dependence of the correlation length and discuss the connection between the orbital correlations and the finite magnetic correlation length observed on the Mn^3+ sublattice with neutron scattering techniques. The physical origin of these domains, which appear to be isotropic, remains unclear. We find that weak orbital correlations persist well above the phase transitions, with a correlation length of 1-2 lattice constants at high temperatures. Significantly, we observe similar correlations at high temperatures in La_0.7Ca_0.3MnO_3, which does not have an orbitally ordered ground state, and we conclude that such correlations are robust to variations in the relative strength of the electron-phonon coupling.Comment: 22 pagegs, 7 figure