PEAXIS: The new end-station for Photo Electron Analysis and X-ray Inelastic Spectroscopy at BESSY II
PEAXIS is dedicated to soft x-ray spectroscopy studies on the electronic structure of novel energy materials. It offers capabilities for soft X-ray Resonant Inelastic X-ray Scattering (RIXS) and Angular Resolved Photo Emission Spectroscopy (ARPES) on liquid samples housed in microfluidic devices, bulk and thin-film solid state samples. RIXS in particular can be used to study the local electronic structure at element and orbital specific atomic sites and yields wavevector-resolved information about dynamics of collective excitations.
As part of the mission of EM-AMCT, thermoelectric materials are the main focus of research.
Fig. 1: Layout of the new PEAXIS spectrometer, which was built by the company PREVAC, Rogów, Polen. The station is equipped with an electron energy analyzer (violet) and a RIXS spectrometer (blue). Various sample manipulators (red) can be mounted to the sample chamber (green) according to different sample environments. The RIXS spectrometer includes a grating chamber (dark blue), a five-meter RIXS arm (light blue) and a CCD detector (orange). The distance between sample and grating as well as between grating and detector is variable. For q-dependent measurements the RIXS arm can be continuously rotated around the sample in a range of 106.5°.
- PEAXIS is the first RIXS end-station at BESSY II that allows measurements with high energy resolution in combination with a continuous variation of the scattering angle under UHV conditions [1,2] for wavevector-resolved studies.
- A modular sample environment allows investigations in the temperature range between 20 K and 1000 K. The low-temperature manipulator is well matched to the requirements for magnetic systems, while catalytic studies can be performed in an intermedium temperature range and thermoelectric materials can be studied in operando conditions at high temperature.
- Simultaneous ARPES capabilities.
- With the soft X-ray penetration depth typically ranging from 100 nm to 10 μm in the energy range (200 eV up to 1200 eV) RIXS at PEAXIS is considered to be bulk sensitive.
Goals within EMR activities
In the field of thermoelectrics a better fundamental understanding of electronic states and their coupling on other elementary excitations like phonons shall lead to novel material design strategies. For catalytic systems, operando studies on the interaction of materials and light shall point towards solar fuel technologies for environment-friendly energy production.
Unique applications of RIXS
Electron-phonon coupling: The intensity of phononic excitations in RIXS spectra is directly proportional to the electron-phonon coupling strength, a parameter not directly accessible with other techniques. This unique feature is of particular interest for studies on thermoelectric materials where coupling of lattice degrees of freedom to the electronic structure have a direct influence on the thermoelectric performance.
Dispersive excitations at high energies: Inelastic measurements with photons provide access to the high energy part of dispersions considered beneficial for microscopic modeling of elementary excitations.
Nanostructured and thin film samples: RIXS allows dynamical studies on small-quantity samples to elucidate the effect of finite-size, nanostructuring and low dimensionality on material properties respectively electronic states.
Magnetic excitations: RIXS can also be used to study magnetic excitations in a variety of materials ranging from model materials for fundamental research to functional energy materials, e.g. spin-caloric systems. It is a complementary technique to the more traditional method of inelastic neutron scattering and by combining these two methods a much deeper understanding of the physics of magnetic systems can be obtained. In particular RIXS is able to probe parts of the Brillouin zone where the neutron cross-section is too weak to measure e.g. around the G-point. RIXS is also able to measure higher energy excitations e.g. in the cuprates, which form the basis of many exotic high temperature superconductors and quantum magnets, the excitations can extend beyond 100 meV becoming inaccessible to most neutron spectrometers. Furthermore orbital excitations can be probed by RIXS which can identify the specific element responsible for the orbital order which neutrons are unable to do, thus RIXS can be used for crystal field studies. This is particularly important in the emerging field of 4d and 5d oxides where both spin orbit coupling and crystal field plays an important role and gives rise to exotic states.
 K. Lieutenant, T. Hofmann, C. Schulz, M. V. Yablonskikh, K. Habicht, E. F. Aziz, Design concept of the high-resolution end-station PEAXIS at BESSY II: Wide-Q-range RIXS and XPS measurements on solids, solutions, and interfaces, J. Electron Spectrosc. Relat. Phenom. 210 (2016) 54–65.
 K. Lieutenant, T. Hofmann, C. Zendler, C. Schulz, E. F. Aziz, K. Habicht, Numerical optimization of a RIXS spectrometer using raytracing simulations, J. Phys. Conf. Ser. 738/1 (2016) 012104.