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Sökning: WFRF:(Wohlfeld K.)

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1.
  • Fumagalli, R., et al. (författare)
  • Mobile orbitons in Ca2CuO3: Crucial role of Hund's exchange
  • 2020
  • Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 101:20
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigate the CuL3 edge resonant inelastic x-ray scattering (RIXS) spectra of a quasi-1D antiferromagnet Ca2CuO3. In addition to the magnetic excitations, which are well-described by the two-spinon continuum, we observe two dispersive orbital excitations, the 3d(xy) and the 3d(yz) orbitons. We carry out a quantitative comparison of the RIXS spectra, obtained with two distinct incident polarizations, with a theoretical model. We show that any realistic spin-orbital model needs to include a finite, but realistic, Hund's exchange J(H) approximate to 0.5 eV. Its main effect is an increase in orbiton velocities, so that their theoretically calculated values match those observed experimentally. Even though Hund's exchange also mediates some interaction between spinon and orbiton, the picture of spin-orbit separation remains intact and describes orbiton motion in this compound.
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2.
  • Martinelli, L., et al. (författare)
  • Collective Nature of Orbital Excitations in Layered Cuprates in the Absence of Apical Oxygens
  • 2024
  • Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 132:6
  • Tidskriftsartikel (refereegranskat)abstract
    • We have investigated the 3d orbital excitations in CaCuO2 (CCO), Nd2CuO4 (NCO), and La2CuO4 (LCO) using high-resolution resonant inelastic x-ray scattering. In LCO they behave as well-localized excitations, similarly to several other cuprates. On the contrary, in CCO and NCO the dxy orbital clearly disperses, pointing to a collective character of this excitation (orbiton) in compounds without apical oxygen. We ascribe the origin of the dispersion as stemming from a substantial next-nearest-neighbor (NNN) orbital superexchange. Such an exchange leads to the liberation of the orbiton from its coupling to magnons, which is associated with the orbiton hopping between nearest neighbor copper sites. Finally, we show that the exceptionally large NNN orbital superexchange can be traced back to the absence of apical oxygens suppressing the charge transfer energy.
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