Electrides are a fascinating class of materials in which electrons confined in empty spaces in a crystal behave as anions, but are not bound to any ions. They can be classified according to the topology of the anionic electrons as either 0D (in a cavity), 1D (in a channel), or 2D (in a layer). The highly anisotropic electronic distribution is expected to give rise to a variety of anomalous behaviors. A breakthrough occurred in 2013 with the demonstration that Ca2N is a 2D electride . Ca2N is an excellent metal with 2D transport characteristics, a large carrier concentration derived from the anionic electrons, and electron mobility higher than graphene, and yet thin films are optically transparent. These extraordinary properties stimulated growing interest in electrides in recent years. High throughput computational screening has so far predicted that more than 90 compounds should be electrides, including alkaline and rare earth nitrides and carbides, early transition-metal halides, and others. Many have been synthesized but their physical properties are largely unexplored.
Understanding the unusual electronic structure of electrides is of fundamental interest. They generally have low work functions and the bands occupied by the anionic electrons lie near the Fermi level, making them favorable for achieving band inversions. This is predicted to lead to a variety of topological phases, including nodal-line and nodal-sheet semimetals, quantum spin Hall systems, and quantum anomalous Hall insulators . They are also predicted to display anomalous plasmonic behavior, with an energy that is highly anisotropic and independent of electron density .
Electrides are also of interest for their potential in applications. In addition to their extraordinary transport properties, their highly anisotropic plasmonic behavior shows promise for realizing novel optical devices based on plasmon polaritons or coupled plasmon-phonon polaritons. These could lead to sub-diffraction optical confinement and enhanced light-matter interactions, along with active tunability via electric field induced doping. Many electrides can be exfoliated to obtain 2D nanosheets and their behavior can be studied as a function of layer thickness. The exquisite sensitivity of polaritonic materials to their dielectric environment also creates an opportunity for controlling nanophotonic devices by integrating them with quantum materials, that can have tunable and nanoscale structure in their dielectric properties, such as at metal-insulator or colossal magnetoresistance transitions.
This MPI-UBC collaboration will carry out a program of fundamental experimental studies to understand the underlying physics of electrides. Samples will be provided by Yuri Grin and Peter Höhn (MPI-CPFS, Dresden). H. Tjeng (MPI-CPFS, Dresden) will carry out ARPES and x-ray spectroscopy to unravel the electronic structure and valence states of the electrides. S. Dierker and G. Sawatzky (QMI-UBC) will employ momentum resolved EELS to explore the collective electronic excitations in the electrides, including the predicted anomalous plasmonic behavior. S. Dierker (QMI-UBC) will explore the optical properties of the electrides using Raman, Infrared, Ellipsometry, and s-SNOM, including evaluating their potential for applications.
 Lee, et al, “Dicalcium nitride as a two-dimensional electride with an anionic electron layer” Nature 494, 336–340 (2013).
 Hirayama, et al, “Electrides as a New Platform of Topological Materials” Phys. Rev. X 8, 031067 (2018).
 J. Wang, et al, “Anomalous Dirac Plasmons in 1D Topological Electrides” Phys. Rev. Lett. 123, 206402 (2019).
S. Dierker (UBC)
G. Sawatzky (UBC)
H. Tjeng (MPI Dresden)