Techniques
Emmy Noether Research Group
Different experimental approaches based on cutting-edge synchrotron radiation techniques are combined to explore correlated electron behavior in different energy regimes ranging from several eV down to meV. This is an important step in establishing a complete physical picture of the complex electronic quantum systems under study. We are also engaged in the development and improvement of experimental capabilities.
We employ a number of experimental techniques based on synchrotron light. Some of them are well established, like x-ray absorption spectroscopy and non-resonant x-ray scattering. Others, like resonant elastic and resonant inelastic x-ray scattering as well as high-resolution low energy angular resolved photoemission are new non-standard methods:
- resonant elastic x-ray scattering (REXS)
Electronic superlattices typically involve only the valence states, i.e., only tiny fractions of the total charge density. Orbital order phenomena, for example, even only involve the spatial charge distribution of the valence electrons. Conventional non-resonant x-ray scattering experiments, which probe the total charge density, are rather insensitive to such an ordering, while neutron scattering techniques do not couple to the charge density at all.
REXS is a new technique that can be regarded as a combination of x-ray absorption spectroscopy (XAS) and x-ray diffraction (XRD): XAS provides the sensitivity to specific valence states at a given lattice site, while the XRD part provides the information about the long-range ordering of these electronic states. By choosing the appropriate edges and polarization, REXS can yield information about electronic superlattices that can not been obtained otherwise.
- high-resolution angle-resolved photoemission (HR-ARPES)
Angular resolved photoelectron spectroscopy (ARPES) is a unique and well established tool to study the electronic structure of a correlated electron system near the Fermi level. State-of-the-art ARPES experiments nowadays make effective use of synchrotron radiation sources by studying spectra as a function of excitation energy and polarization. However, at conventional excitation energies of about 50eV, this technique is very surface sensitive and the separation of surface and bulk properties can be difficult. One of the latest innovations in the field of ARPES enhances the bulk sensitivity significantly: Since the mean free path of photoelectrons increases strongly at low kinetic energies, ARPES experiments at excitation energies below 10 eV become much more bulk sensitive. For instance, at energies about 6 eV the mean free path of the photoelectrons is about 50 Å. Until recently, however, synchrotron-based ARPES experiments at these extremely low photon energies have not been possible.
In a combined effort, the IFW Dresden and BESSY have put together their expertise in ARPES endstations on the one hand and beamline design on the other hand to setup a unique experiment at BESSYII. This beamline combines the outstanding possibilities of synchrotron based ARPES (variable polarization and energy) with very low excitation energies.
- resonant inelastic x-ray scattering (RIXS) in combination with electron energy loss spectroscopy (EELS)
Inelastic neutron scattering (INS) yields detailed information about collective spin excitations in terms of the dynamical structure factor Sspin(q,ω) (q, ω: momentum and energy transfer). As a result, our understanding of magnetic phenomena has been improved tremendously. Similarly, a detailed knowledge of collective charge excitations in terms of Scharge(q,ω), especially at finite q, would be of great value. Inelastic x-ray scattering (IXS) in the non-resonant limit and electron energy loss spectroscopy (EELS) can probe Scharge(q,ω). However, the corresponding cross sections for IXS are generally small, while the EELS signal decreases rapidly with q and suffers from multiple scattering at larger momentum transfers. Optical spectroscopies, which can also be used to probe Scharge(q, ω) of a solid, are confined to q≈0. In other words, the experimental access to collective charge excitations at finite q has been very limited so far.
RIXS is a new experimental technique that can strongly enhance the sensitivity to electronic excitations and enables to measure at finite q. The creation of a core-hole in the intermediate state of the scattering process shakes up the electronic system and the probability of creating a charge excitation is strongly increased.
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