Oral: Using scanning tunneling spectroscopy to determine the energy dispersion of spin excitations

Conventional methods to measure the energy-momentum relations of collective spin excitations involve probing bulk crystals with particles such as neutrons, photons or electrons, which carry a well defined momentum. In the case of finite-size spin lattices, on the contrary, spin excitations do not have a well defined momentum due to the lack of translation symmetry, and the spin excitations are measured with an eminently local probe, using for example inelastic electron spectroscopy (IETS) with a scanning tunnel microscope (STM).

In this talk we discuss under which conditions the STM-IETS spectra can be Fourier-transformed to yield energy-momentum relations. We relate the success of this approach to the degree to which spin excitations in finite-size chains form standing waves, i.e. linear superposition of two traveling waves. We show how the energy dispersion of magnons in ferromagnets and triplons in valence bond crystals can be inferred with STM-IETS, but for spin excitations in the Heisenberg S = ½ model, known as spinons, it cannot.

Recent works have successfully applied this procedure to map the excitations of a bond alternating Heisenberg model realized on a chain of Clar’s goblets (a topologically frustrated nanographene) . Others have attempted to use the same techniques to map the excitations of an Olympecene chain (whose effective spin model is the homogeneous antiferromagnetic spin-1/2 Heisenberg model) with far less success .

The goal of this talk is to lay the fundamental theoretical aspects behind this experimental technique, as well as to shed light and give insight about the kind of system where its application can be successful.