Spin- and Angle-Resolved Photoemission Spectroscopy
With angle-resolved Photoemission spectroscopy (ARPES), we can measure the electronic band structure of surfaces and interfaces by simultaneously measuring the kinetic energy and momentum distribution of photoelectrons using a CCD-detector.
The FERRUM spin detector which uses the same hemispherical electron analyzer can additionally probe the spin structure. This spin detector is based on very low energy electron diffraction (VLEED) which makes it one of the most efficient detectors, it is combined with a magnetic lens system which enables measuring all three spin directions. (by rotating the spin of the photoelectrons to the spin sensitive axis of the detector).
- Energy resolution: 20 meV
- Momentum resolution: 0.003 Å -1
- 6-axis automatized sample positioning
- Sample cooling to 12.5 K
Photoemission Electron Microscopy (PEEM)
A Photoemission Electron Microscope (PEEM) images the lateral distribution of photoelectrons that are generated when a conducting surface of a solid is illuminated with light. Using electrons rather than light to image a surface, the spatial resolution of PEEM is typically a few tens of nanometers. The electrostatic lens system allows an on-the-fly adjustment of microscope by changing the lens voltages. The field-of-view can be varied from only a few to hundreds of micrometers or alternatively, the electron emission angles instead of positions can be imaged to access the electronic band structure (Momentum Microscopy).
As electron detectors, our PEEMs use either a flourescent screen and camera in combination with a double-hemispherical energy filter or a time-of-flight based delay-line detector (DLD). With the latter, we can detect electron energies along with their 2D positions. In this way, we can take 3D images in parallel, giving us energy-resolved images or electronic band structures.
Our studies with PEEM are focussed on time-resolved experiments using femtosecond laser pulses as a light source. We study the ultrafast dynamics of plasmonic and photonic near-fields at nanostructured surfaces and excited electrons generated in metals and semiconductors.
• Spatial resolution <40 nm
• Momentum resolution < 20 m Å-1
• Energy resolution <70 meV
• Laser Irradiation under grazing incidence (65°) or near-normal incidence (~4°)
Scanning tunneling microscope (STM) / Atomic force microscope (AFM)
We employ a variable temperature Scanning Tunnelling (STM) Microscope and Atomic Force Microscope (AFM) to investigate the surfaces and adsorption properties of various types of materials.
- For topographic measurements, STM can be used in either constant tunnelling current mode or constant height mode.
- Scanning Tunnelling Spectroscopy (STS) enables us to acquire local information about the electronic structure of the surface. Extending this measurement mode to a desired scan area, a comprehensive DOS map is captured (dI/dV Map)
- AFM is mostly operated in two modes; Contact mode and Non-contact mode. An additional advantage of AFM is the possibility of measuring non-conductive samples.
· Maximum scan range: 10 µm x 10 µm
· Temperature variation: 50 K - 650 K
· Spatial resolution on lateral < 0.1 nm
A shared preparation and transfer chamber with the PEEM facilitates the possibility of joint projects for a more comprehensive analysis.
Magneto Optical Analysis
We investigate ultrafast magnetization dynamics in an all-optical time-resolved pump-probe setup. The magnetization dynamics is measured at varying time delays of the probe pulse to the exciting pump pulse. At each time step we analyze the change of the polarization and intensity of the reflected probe pulse induced by the magneto-optical Kerr-effect (MOKE). This reveals the full temporal behaviour of the magnetization after an optical excitation.
By carefully analyzing the occurring Kerr-rotation and Kerr-ellipticity due to the MOKE it is possible to disentangle the magnetic signals of each magnetic layer in multilayer thin film systems.
Additionally in our experiments we are able to tune the wavelength of the exciting or probing-pulse by the use of an optical-parametric-amplifier (OPA), which grants us access to photon-energy-dependent effects.
Using high harmonic generation (HHG) as probe beam, we enter the XUV-range which allows us to measure magnetization dynamics element sensitive in magnetic alloys.
Besides those time-resolved methods the MOKE can also be utilized as an imaging-tool for magnetic domains. Hence we use a so called Kerr-microscope to investigate samples on their capability to reverse their magnetization all-optically, revealing their possible application in new data storage functionalities.