A high brilliance vortex generator in the electron microscope

The recently discovered elctron vortex beams have many unusual properties such as helical wave fronts, a freely floating ring current, and a magnetic moment that can be many times higher than the electron spin moment. They bear promise for many new applications, but significantly higher intensity than possible with standard methods would be needed. The aim of this project is the adaptation of magnetic aberration correctors, as they are now installed in many transmission electron microscopes, for the creation of electron vortices with high brilliance, thus improving the gain of the standard technique by a factor of 5 to 10. The method was originally developed in light optics as a vortex generator for laser beams. Such a device for electrons would open new avenues for the investigation of magnetic strucutres and, more generally, structures with broken chiral symmetry in electron microscopy. The successful outcome of this project will allow new insights into the physics of helical/spinorial matter waves. A high-brilliance vortex generator that is available without the need for further instrumentation in widely available electron microscopical equipment is of immense interest for the study of technologically promising materials with chirality (such as the recent discovery of magnetic skyrmion lattices), especially in spintronics, nanomagnetism, optical metamaterials, and in nano-bioapplications.

The discovery of electron vortices in 2010 has inspired many new ideas in solid-state and in electron physics. Electron vortices are individual particles that have angular momentum and a magnetic moment independent of the electron spin. These vortex properties are quantized; they come in multiples of the smallest possible physical units - the Planck constant and the Bohr magneton, respectively. Electron vortices can be focused to tiny ring like structures down to the atomic scale, a unique feature particularly useful in electron microscopy. Applications range from manipulating nanoparticles, applying torque to nanomagnets, measuring the magnetic moment of single atoms, generating single electrons with large magnetic moment, the study of chiral structures or molecules, to the dynamics of Landau states of freely floating electrons, all in the electron microscope. Apart from a few convincing examples, the initial high expectations could not be met. It soon became clear that the main obstacle was the weak signal and the limited purity of the electron vortices available at the time. In the present project, the /2-mode conversion - a technique that originated in light optics - was adapted to the electron microscope. The mode converter transforms a beam electron into an electron vortex using a pair of magnetic quadrupole lenses of the microscope. We were able to demonstrate the generation of electron vortices of high purity and brilliance, allowing an increase in intensity by a factor of 5 to 10. Unexpectedly, the mode converter turned out to serve as a quantum logic gate that may be useful as a fundamentally new test platform for quantum computing with free electrons. The expertise gained in the project led to a surprising side result in an entirely different field: Numerical simulations with the software developed for the electron microscope showed that the magnetic Bussard ramjet - a concept for interstellar travel proposed in 1960 and further developed in a series of peer-reviewed publications that are still referenced in the literature today - cannot work.