abovEF — Measuring the electronic structure above the Fermi level

Understanding the full electronic structure of solids requires the exploration of both occupied and unoccupied states. While angle-resolved photoemission spectroscopy (ARPES) has revolutionized our knowledge of the electronic bands below the Fermi level, its inverse counterpart, angle-resolved inverse photoemission spectroscopy (ARIPES), remains largely undeveloped due to intrinsic technical limitations. As a result, the unoccupied states — essential to describe phenomena such as unconventional superconductivity or topological phases — are still experimentally elusive.

The abovEF project aims to build the first high-resolution ARIPES instrument capable of mapping electronic states above the Fermi level with energy resolution better than 30 meV and momentum resolution of 0.1 Å⁻¹, roughly an order of magnitude beyond the current state of the art. This breakthrough will be achieved by transferring to ARIPES the technological advances that have revolutionized Resonant Inelastic X-ray Scattering (RIXS).

The new laboratory will combine ARPES, ARIPES, and potentially Electron Energy Loss Spectroscopy (EELS) capabilities in a single ultra-high-vacuum setup operating at cryogenic temperatures. This unique configuration will allow the direct comparison of occupied and unoccupied band dispersions in the same sample and under identical experimental conditions.

Scientifically, abovEF will open unprecedented access to the unoccupied electronic states of correlated and low-dimensional systems, enabling a complete reconstruction of their band structure across the Fermi level. The first targets will include high-Tc superconducting cuprates, where the coexistence of superconducting and pseudogap phases raises fundamental questions on particle–hole symmetry and pairing mechanisms, and two-dimensional quantum materials such as FeSe and transition-metal dichalcogenides, which exhibit tunable superconductivity, charge density waves, and topological transitions. By directly probing the dispersion and symmetry of the empty states, ARIPES will provide crucial insight into electron–phonon coupling, band renormalization, and many-body interactions that govern emergent quantum phases.

Ultimately, the project will establish a new paradigm for unoccupied-state spectroscopy of solids, enabling the direct visualization of the electronic structure “above Fermi” and providing unprecedented understanding of the interplay between electronic correlations, topology, and superconductivity in quantum materials.

The abovEF project is fully funded by the second call of the Fondo Italiano per la Scienza (FIS2) program as an Advanced Grant mainly devoted to the construction of a brand new laboratory. The project has officially started in September 2025.