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ABSTRACT: We report a resonant inelastic x-ray scattering (RIXS) study of the doping dependence of charge-transfer excitations in La2−xSrxCuO4. The momentum 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 samples, 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 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.
Phys. Rev. B. 09/2004; 70(9).
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ABSTRACT: 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. B
04/2004;
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ABSTRACT: We report a resonant inelastic x-ray scattering study of electronic excitations in Li2CuO2, an insulating compound comprised of ribbons of edge-sharing copper-oxygen chains. Three excitations, which show little dependence on momentum transfer, are observed in our measurements. The lowest-energy excitation at ∼2.1eV is dispersionless and is attributed to a localized d-d orbital excitation. We also observe two excitations at ∼5.4eV and ∼7.6eV which we assign to charge-transfer excitations. These high-energy excitations are also dispersionless along the copper-oxygen chain direction. However, in each case we observe a small energy dispersion along the direction perpendicular to the copper-oxygen ribbons, suggesting a significant interchain coupling in this system. We also discuss the possible implications of ferromagnetic nearest-neighbor intrachain coupling on the charge excitation spectra.
Phys. Rev. B. 04/2004; 69(15).
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C S Nelson,
J P Hill,
Doon Gibbs,
M Rajeswari,
A Biswas,
S Shinde,
R L Greene,
T Venkatesan,
A J Millis,
F Yokaichiya,
C Giles,
D Casa, C T Venkataraman,
T Gog
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ABSTRACT: We report the characterization of the crystal structure, low-temperature charge and orbital ordering, transport and magnetization of Pr0.6Ca0.4MnO3 films grown on LaAlO3, NdGaO3 and SrTiO3 substrates, which provide compressive (LaAlO3) and tensile (NdGaO3 and SrTiO3) strain. The films are observed to exhibit different crystallographic symmetries from the bulk material and the low-temperature ordering is found to be more robust under compressive—as opposed to tensile—strain. In fact, bulk-like charge and orbital ordering is not observed in the film grown on NdGaO3, which is the substrate that provides the least amount of measured, but tensile, strain. This result suggests the importance of the role played by the Mn–O–Mn bond angles in the formation of charge and orbital ordering at low temperatures. Finally, in the film grown on LaAlO3, a connection between the lattice distortion associated with orbital ordering and the magnetization is reported.
Journal of Physics Condensed Matter 12/2003; 16(1):13. · 2.55 Impact Factor
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ABSTRACT: The theoretical formulation of x-ray resonant magnetic scattering from rough surfaces and interfaces is given for the diffuse (off-specular) scattering, and general expressions are derived in both the Born approximation (BA) and the distorted-wave Born approximation (DWBA) for both single and multiple interfaces. We also give in the BA the expression for off-specular magnetic scattering from magnetic domains. For this purpose, structural and magnetic interfaces are defined in terms of roughness parameters related to their height-height correlation functions and the correlations between them. The results are generalized to the case of multiple interfaces, as in the case of thin films or multilayers. Theoretical calculations for each of the cases are illustrated as numerical examples and compared with experimental data of mangetic diffuse scattering from a Gd/Fe multilayer. Comment: 28 pages, 5 figures
05/2003;
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ABSTRACT: We report a resonant inelastic x-ray scattering study of electronic excitations in Li_2CuO_2, an insulating compound comprised of ribbons of edge-sharing copper-oxygen chains. Three excitations, which show little dependence on momentum transfer, are observed in our measurements. The lowest energy excitation at ~2.1 eV is dispersionless and is attributed to a localized d-d orbital excitation. We also observe two excitations at ~5.4 eV and ~7.6 eV which we assign to charge-transfer excitations. These high-energy excitations are also dispersionless along the copper-oxygen chain direction. However, in each case we observe a small energy dispersion along the direction perpendicular to the copper-oxygen ribbons, suggesting a significant interchain coupling in this system. We also discuss the possible implications of ferromagnetic nearest-neighbor intrachain coupling on the charge excitation spectra. Comment: 7 pages, 5 figures
04/2003;
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C. S. Nelson,
J. P. Hill,
Doon Gibbs,
M. Rajeswari,
A. Biswas,
S. Shinde,
R. L. Greene,
T. Venkatesan,
A. J. Millis,
F. Yokaichiya,
C. Giles,
D. Casa, C. T. Venkataraman,
T. Gog
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ABSTRACT: We report the characterization of the crystal structure, low-temperature charge and orbital ordering, transport, and magnetization of Pr_{0.6}Ca_{0.4}MnO_{3} films grown on LaAlO_{3}, NdGaO_{3}, and SrTiO_{3} substrates, which provide compressive (LaAlO_{3}) and tensile (NdGaO_{3} and SrTiO_{3}) strain. The films are observed to exhibit different crystallographic symmetries than the bulk material, and the low-temperature ordering is found to be more robust under compressive-- as opposed to tensile-- strain. In fact, bulk-like charge and orbital ordering is not observed in the film grown on NdGaO_{3}, which is the substrate that provides the least amount of nominal and measured, but tensile, strain. This result suggests the importance of the role played by the Mn--O--Mn bond angles in the formation of charge and orbital ordering at low temperatures. Finally, in the film grown on LaAlO_{3}, a connection between the lattice distortion associated with orbital ordering and the onset of antiferromagnetism is reported. Comment: 12 pages, 7 figures
03/2003;
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ABSTRACT: 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 approximately 2.2 eV at the zone enter, and a dispersion of approximately 1 eV. The spectral weight of this mode becomes dramatically smaller around (pi, pi). The second peak shows a smaller dispersion ( approximately 0.5 eV) with a zone-center energy of approximately 3.9 eV. We argue that these are both highly dispersive exciton modes damped by the presence of the electron-hole continuum.
Physical Review Letters 11/2002; 89(17):177003. · 7.37 Impact Factor
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ABSTRACT: 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 figures
06/2002;
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C. S. Nelson,
M. v. Zimmermann,
J. P. Hill,
Doon Gibbs,
V. Kiryukhin,
T. Y. Koo,
S-W Cheong,
D. Casa,
B. Keimer,
Y Tomioka,
Y Tokura,
T. Gog, C. T. Venkataraman
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ABSTRACT: 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 figures
11/2000;
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ABSTRACT: We report on the development of a circularly polarized x-ray microprobe in the intermediate energy range from 5 to 10 keV. In this experiment linearly polarized synchrotron radiation was circularly polarized by means of a Bragg-diffracting diamond phase retarder and subsequently focused down to a spot size of about 4×2 μm2 by a Fresnel zone plate. The properties of the microprobe were characterized, and the technique was applied to the two-dimensional mapping of magnetic domains in HoFe2. © 2000 American Institute of Physics.
Review of Scientific Instruments 05/2000; 71(6):2386-2390. · 1.37 Impact Factor
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ABSTRACT: We have fabricated and characterized the performance of a monolithic Ge Bragg–Laue phase retarder capable of simultaneously producing both right- and left-handed circularly polarized x rays. The energy range of operation of the phase retarder is between 50 and 100 keV making it well suited to the measurement of spin-dependent Compton profiles within the impulse approximation, primarily because of the increased momentum resolution and larger Compton scattering cross section available at these higher incident energies. Although the phase retarder was optimized for operation at 86 keV, it can produce highly circularly polarized x rays over a substantial energy range. The performance of the phase retarder was tested via magnetic Compton scattering measurements on an Fe sample at the undulator A of the Advanced Photon Source. It was found to perform well in terms of flux and degree of circular polarization thereby greatly reducing the data collection times required for this inherently weak scattering process. © 1998 American Institute of Physics.
Review of Scientific Instruments 04/1998; 69(5):1970-1973. · 1.37 Impact Factor
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ABSTRACT: An elliptical multipole wiggler (EMW), designed to produce helicity-switching circularly polarized radiation over a wide energy range, has been installed at the Advanced Photon Source (APS). Spectral flux and polarization measurements were performed on the EMW in the photon energy range from 10 to 100 keV for different values of the EMW deflection parameter in the vertical plane. Absolute flux and degree of linear polarization measurements were accomplished by utilizing scattering from a gas in combination with an energy-dispersive detector. In addition, the circular polarization of the beam was confirmed by performing magnetic Compton scattering experiments on a well-characterized iron sample. The obtained results are in reasonable agreement with the calculated values. © 1997 American Institute of Physics.
AIP Conference Proceedings. 06/1997; 417(1):49-54.
