Final Report Summary - LASER-ARPES (Laser based photoemission: revolutionizing the spectroscopy of correlated electrons)
Within this ERC project we developed a laser based angle resolved photoemission system (laser-ARPES) for the investigation of the electronic structure of complex materials with unprecedented resolution. During the last decade, ARPES emerged as a standard technique to map electronic excitations in momentum space. It uses monochromatic ultraviolet light to excite photoelectrons from crystallin solids and measures their intensity distribution as a function of kinetic energy and emission angle. This gives direct access to the single particle spectral function and thus to the direction of motion, velocity and scattering rate of electrons as they travel through a material. This information is fundamental to most theoretical models describing the macroscopic properties of solids, such as electrical and thermal conductivity including superconductivity, magnetic susceptibility or, more general, performance in device applications.
Conventionally, such experiments have to be performed at synchrotron light sources. This increases the costs substantially and greatly limits access to the technique. Moreover, synchrotron light is intrinsically broadband and thus needs to be monochromatized for ARPES experiments. This process is inefficient and even the latest synchrotron beamlines can only provide a limited photon flux in the extremely narrow bandwidth required for ultra-high-resolution measurements.
Within this project, we demonstrated that an alternative laser-based UV light source can largely overcome these limitations. To this end, we developed a laboratory based ARPES system that uses a compact amplified diode laser with two resonant frequency doublers as excitation source. This laser can provide two orders of magnitude higher flux in a two orders of magnitude narrower bandwidth than the latest synchrotron ARPES beamlines. First test experiments with this new source were performed on the sp-surface state on Cu(111), a famous model system for electronic structure studies investigated by many authors over the last two decades. Surprisingly, already these first experiments revealed qualitative differences to earlier data. They allowed us to resolve for the first time a minute lifting of the spin degeneracy in this state, which arises from the broken inversion symmetry at the surface confirming the immense potential of this new technique for unprecedented insight into electronic structures.
Conventionally, such experiments have to be performed at synchrotron light sources. This increases the costs substantially and greatly limits access to the technique. Moreover, synchrotron light is intrinsically broadband and thus needs to be monochromatized for ARPES experiments. This process is inefficient and even the latest synchrotron beamlines can only provide a limited photon flux in the extremely narrow bandwidth required for ultra-high-resolution measurements.
Within this project, we demonstrated that an alternative laser-based UV light source can largely overcome these limitations. To this end, we developed a laboratory based ARPES system that uses a compact amplified diode laser with two resonant frequency doublers as excitation source. This laser can provide two orders of magnitude higher flux in a two orders of magnitude narrower bandwidth than the latest synchrotron ARPES beamlines. First test experiments with this new source were performed on the sp-surface state on Cu(111), a famous model system for electronic structure studies investigated by many authors over the last two decades. Surprisingly, already these first experiments revealed qualitative differences to earlier data. They allowed us to resolve for the first time a minute lifting of the spin degeneracy in this state, which arises from the broken inversion symmetry at the surface confirming the immense potential of this new technique for unprecedented insight into electronic structures.