Electronic structure information obtained with photoemission-based measurements provides a keystone in understanding emerging quantum materials, and the unique application potential they provide. This effort is focused on advancing understanding of photoemission measurements of quantum materials and developing measurement advances to provide information in the time-domain and at nanoscale spatial dimensions.
Photoemission-based methods that interrogate solid state electronic structure, in particular the k-dependent electronic energy band structure, have played a pivotal role in identifying and understanding key features of quantum materials. Band structure information from Angle-Resolved PhotoEmission Spectroscpy (ARPES) provided the first evidence for the d-wave symmetry in cuprate superconductors, the first illustration of Dirac character in graphene, and the first verification of the existence of topologically protected states in topological insulator (TI) and Weyl semi-metal systems. In this effort, measurements are being developed that push beyond the static band structure to provide a deeper understanding of the quasiparticle dynamics in these emerging materials, building on laser-based time-resolved techniques employed previously for investigating exciton dynamics in donor-acceptor heterojunctions.
In addition, a need to push band structure measurements to nanoscale spatial dimensions arises, not only from increasingly small device dimensions, but also from the inherent heterogeneity of many materials. Many “emergent” materials are characterized by coexistence of competing interactions, such as ferroelectric, magnetic, piezoelectric, etc. correlations, often leading to the formation of complex spatial phases, as in multiferroics with nanoscale dimensions. Electronic heterogeneity can also arise with doping or compositional variation in TI’s to modify the Fermi level or to drive a TI-to-trivial insulator phase transition, or in mixed insulator-metal phases near Mott transitions. Measurement platforms to address these issues are being pursued. One direction involves the development of Photoemission Electron Microscopy (PEEM) capabilities. A close collaboration on internal proposals with staff in the Nanoscale Device Characterization Division in PML over several years led to the acquisition of a commercial PEEM instrument in PML. This instrument will provide angle-integrated electronic structure information on length-scales of 10’s of nm via formation of energy filtered photoemission images employing an electron-optics column where the sample is the cathode element.
In addition to angle-integrated imaging, PEEM can also provide spatially resolved ARPES by imaging in the back-focal plane. This, however, requires insertion of an aperture, limiting spatial resolution in this mode to micron-scale dimensions. Alternatively, higher resolution ARPES information has been demonstrated at several synchrotron facilities using focused photon sources employing zone plates to reach spatial resolution below 100 nm. A further goal is to enable this measurement with lab-based laser excitation. Direct transfer of the zone plate format employed at synchrotrons to produce focused excitation for high spatial resolution ARPES with laser-based photoemission is not easily achieved due to the longer photon wavelength. Different approaches to overcome the barriers to achieving laser-based ≈ 100 nm ARPES are being explored.