A new beamline system offers insights into electron behaviour

ARPES measurements of thin films of EuO grown on I05 uncover unusual electron interactions

The joint development of a new, miniaturised molecular-beam epitaxy (MBE) system on the I05 beamline by the University of Oxford and Createc GmbH (Erligheim, Germany) was the catalyst for research into the electronic structure of thin films of Europium monoxide (EuO). The highly 3D nature and air-sensitivity of EuO makes it hard to study by angle-resolved photoemission from single crystals, but the new facility allows a freshly-grown surface to be transferred directly to the beamline, under good vacuum conditions. This allowed Jon Riley, studying for a PhD with Diamond and the University of St Andrews, to start experimenting with doping EuO films with Gadolinium (Gd). On its own, EuO is a ferromagnetic insulator, but Gd doping adds extra electrons. The experimental results, together with supporting theoretical work from researchers at the University of Oxford, have recently been published in Nature Communications and point to an intriguing polaronic state, arising due to the coupling of the induced charge carriers to the conduction-electron plasmon excitations.
     
Doping-induced crossover from phononic to plasmonic polarons in EuO.
Doping-induced crossover from phononic to plasmonic polarons in EuO.

Soviet physicist Igor Tamm introduced the concept of phonons in 1932, as a collective excitation of atoms or molecules in condensed matter. These characteristic vibration modes of the crystal lattice can couple to electrons as they travel through a crystal, making them move as if they are heavier and less mobile. A pronounced electron-phonon coupling is known to mediate the formation of so-called polarons, a composite quasiparticle formed of an electron and a phonon cloud. Previous research has shown that polarons have a significantly enhanced effective mass, which can impose fundamental limits on electron mobility in semiconductors (important in, for example, transistors). It has also been suggested that they play an important role in certain unconventional superconducting states.

Tuning such polaronic states is therefore important, but phonons are typically only weakly modified under easily accessible experimental conditions. This research showed that polarons can also be formed via a coupling to collective plasmon excitations of an electron gas, which provides a much more tuneable system. (Plasmons are quasiparticles associated with a local collective oscillation of charge density.)
 
The research team performed in situ Angle-Resolved Photoemission Spectroscopy (ARPES) on samples with increasing concentrations of Gd (and hence increasing electron concentrations). Professor Phil King from the University of St Andrews, and a co-supervisor of this study, explains that 

 Diamond is the ideal place to carry out this research, which requires the ability to fabricate high-quality thin-film samples, to transfer these to the measurement chamber without them ever leaving an ultra-high vacuum environment, and then to perform ARPES measurements at carefully chosen photon energies to enable probing the relevant electronic states of interest.”

The team discovered two distinct regimes of how the electrons behave in differently-doped EuO samples. At lower doping concentrations, the measurements showed how distinct loss-features could be observed separated by the energy of a known phonon mode of this material. This points to a phonon polaron state, similar to that observed in other materials with strong electron-phonon coupling.

The most exciting results, however, were observed when increasing the electron density. They showed how new loss features emerge whose energy separation is equal to that of the plasmon modes in the system. The plasmon mode is a collective excitation of the electron gas itself, with a characteristic frequency that grows with increasing carrier density. This renders the coupling to the plasmon modes highly tuneable, and indicates that a new state – a plasmonic polaron – can be found here.
 
Dr. Moritz Hoesch of DESY, who co-supervised the study commented: “Being able to make the material and then measure it has been very exciting. Due to surface ageing after a day the timing of sample growth and measurements had to be just right. Diamond proved the perfect environment for this task. The huge flexibility of tuning the data acquisition conditions to be just right for our questions provided an unmeasurable advantage of the synchrotron radiation data over any lab based spectroscopy. This has been a great team work involving the best equipment of its kind for each work step.”
 
Electron-phonon coupling can cause a superconducting instability (see: Discovering the secrets of oxide electronics). The same could well be true of this new electron-plasmon coupling, at least for non-ferromagnetic materials. If so then it might produce a highly tuneable environment, and be of considerable interest for condensed matter theories considering unconventional superconducting states. Excitingly, the plasmonic polarons observed here could well be found in many other materials systems, suggesting intriguing doping-dependent many-body interactions remain to be found in a host of systems. The next steps are to search for these and better understand their implications. Prof. Thorsten Hesjedal of the University of Oxford, who also co-supervised this work and led the development of the MBE on the I05 beamline, said: “The development of a miniaturized oxide MBE system for the ARPES beamline has proven to be a huge success. Owing to the limited space around the end station of a beamline, we had to rethink and redesign all components of a standard MBE system – an effort that paid off and will allow us to explore novel oxides for electronic applications in the future.”
 
To find out more about the I05 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Cephise Cacho: cephise.cacho@diamond.ac.uk
 

Related publications

Riley JM et al. Crossover from lattice to plasmonic polarons of a spin-polarised electron gas in ferromagnetic EuO. Nature Communications 9, 2305 (2018). DOI: 10.17630/4e82a731-57c6-4cf5-b8c2-841486b8dbde.
 
Baker AA et al. An ultra-compact, high-throughput molecular beam epitaxy growth system. Review of Scientific Instruments 86, 043901 (2015). DOI: 10.1063/1.491700