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Resonant inelastic X-ray scattering (RIXS) is a powerful bulk-sensitive photon-in/photon-out spectroscopic probe of the electronic structure with atomic and orbital sensitivity. It is an ideal method for studying excitations from the electronic ground state in correlated transition metal oxides, being directly sensitive to charge-, orbital- and spin-degrees of freedom. Ultra-high resolution instrumentation for RIXS is available at the ADvanced RESonant Spectroscopies (ADRESS) beamline of the Swiss Light Source at the Paul Scherrer Institut, being optimized for soft X-rays with variable polarization between 0.4 and 1.8 keV [1]. The SAXES (Super Advanced X-ray Emission Spectrograph) RIXS spectrometer of the ADRESS beamline has a resolving power of ca. 12000 for 1 keV. It allows varying the scattering geometry between incident and inelastically scattered X-rays in order to study low-energy excitations as a function of momentum transfer. In this talk we present high-resolution and momentum dependent RIXS studies of magnetic and electronic excitations in the quasi one-dimensional spin systems Sr14Cu24O41 and Sr2CuO3.
Understanding complex quantum systems that show exotic phenomena due to strong electron-electron correlations and quantum fluctuations is of pivotal importance in contemporary condensed matter physics. Sr2CuO3 is a quasi one-dimensional corner-sharing single-chain compound possessing the nearly ideal properties of the one-dimensional antiferromagnetic Heisenberg spin-½ model. RIXS at the Cu L3-edge probes the dynamical spin structure factor complementary to Inelastic Neutron Scattering. The momentum transfer dispersion of the Cu L3-RIXS signal along the chain direction reveals that the main spectral weight follows the lower onset of the two-spinon continuum and that the magnetic spectrum is gapless at the Brillouin zone center. A line shape analysis of the RIXS response allows testing the limits of two-spinon dynamical spin structure factor calculations within the Bethe Ansatz. Two kinds of collective modes occur within the orbital excitation energy range, one with the same periodicity as the two-spinon spectrum and one of opposite periodicity. These results are discussed in the context of spin-charge separation, an important characteristic of the one-dimensional spin-½ Heisenberg chain. Sr14Cu24O41, which consists of two different copper-oxide layers: the so-called spin-ladders and spin-chains, is known to display a range of intriguing phenomena. Among others the chains develop a charge density wave at low temperatures, whereas the ladders possess a quantum mechanical spin liquid ground state with a finite spin gap. With Cu L3-RIXS we map out the dispersion of two-triplon excitations, the elementary magnetic excitations in the ladders, achieving excellent sensitivity over almost the full Brillouin-zone and directly determining the two-triplon energy gap [2]. Using O K RIXS, oxygen site-sensitive hole-excitations are studied, giving insight into the character of the holes in the ladders and the chains. References [1] G. Ghiringhelli et al., Rev. Sci. Instrum. 77, 113108 (2006); V. N. Strocov et al., http://sls.web.psi.ch/view.php/beamlines/adress/index.html. [2] J. Schlappa, T. Schmitt, F. Vernay, V. Strocov et al., Phys. Rev. Lett. 103, 047401 (2009). |
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