RESUMO
We report the first experimental detection of x-ray magnetochiral dichroism in magnetoelectric Cr2O3. This dichroism, which does not require any polarized x-ray beam, is related to the time-reversal odd part of the optical activity tensor dominated by electric dipole-electric quadrupole E1E2 interference terms. The experiments were carried out using either a single crystal or a powdered pellet required to grow a single antiferromagnetic domain by magnetoelectric annealing. This new element (edge) specific spectroscopy offers unique access to the atomic orbital anapole moment Omega-z.
RESUMO
Electric quadrupole transitions are revealed through the angular dependence of X-ray absorption spectra in a cubic crytal. Data collection was achieved at the iron K-edge in pyrite (FeS2) by using the angular moment method developed for X-ray Natural Circular Dichoism (XNCD) measurements. The natural linear dichroism (XNLD) was found to be around 0.5% of the edge jump. Experimental results are compared with monelectronic and multilectronic calculations. Calculations allow to quantitatively determine the proportion of quadrupolar transitions in the pre-edge structure and the anisotropy is explained in terms of structural and electronic parameters.
RESUMO
We report the first observation of a nonreciprocal x-ray linear dichroism caused by the time-reversal odd, real part zeta of the complex gyrotropy tensor zeta(*) which is dominated by electric dipole-electric quadrupole E1E2 interference terms. A nonreciprocal transverse anisotropy was observed in the low temperature insulating phase of a Cr doped V2O3 Mott crystal when a single antiferromagnetic domain was grown by magnetoelectric annealing along the hexagonal c axis. This new element (edge) specific spectroscopy could nicely complement x-ray magnetic circular dichroism which is silent for antiferromagnetic materials.
RESUMO
X-ray natural circular dichroism (XNCD) has been recently detected in the XANES region for uniaxial and biaxial gyrotropic crystals. Chiral-EXAFS (chi-EXAFS) spectra are reported for the first time over a wider energy range and are analysed in terms of multiple-scattering paths of relevant symmetry. For such heavily absorbing single crystals as lithium iodate, paratellurite or potassium titanyl phosphate, the differential absorption between left-handed and right-handed circularly polarized X-ray photons cannot be measured in transmission but gyrotropy effects can still be detected in fluorescence excitation spectra. Whereas XNCD and fluorescence-detected X-ray natural circular dichroism spectra are strictly identical for uniaxial crystals, it has been established that this was true only to the first order for biaxial crystals such as potassium titanyl phosphate.
