Paul Scherrer Institute Team Observes Charge Fractionalisation in Common Metal Alloy

A research team led by the Paul Scherrer Institute has observed the fractionalisation of electronic charge in an iron-based metallic ferromagnet, a phenomenon that could have potential applications in electronic devices. The team used spectroscopy to observe the behavior of electrons in the ferromagnet when illuminated by a laser. The researchers discovered this effect in Fe3Sn2, a compound of iron and tin, arranged in a lattice structure that reduces electron kinetic energies and promotes interaction. The study, which also involved the University of Geneva, EPFL, and ETH Zurich, was published in the journal Nature.

Spectroscopic Observation of Charge Fractionalisation in Ferromagnet

A research team spearheaded by the Paul Scherrer Institute has made a significant breakthrough in the field of quantum mechanics by spectroscopically observing the fractionalisation of electronic charge in an iron-based metallic ferromagnet. This discovery, published in the journal Nature, is not only of fundamental importance but also holds potential for future applications in electronic devices due to its occurrence in an alloy of common metals at accessible temperatures.

The Phenomenon of Charge Fractionalisation

The fundamental unit of charge, according to basic quantum mechanics, is the electron charge, which is quantised and unbreakable. However, exceptions have been observed where electrons collectively arrange themselves as if they were split into independent entities, each possessing a fraction of the charge. This phenomenon, known as charge fractionalisation, has been experimentally observed since the early 1980s with the Fractional Quantum Hall Effect. In this effect, the conductance of a system in which electrons are confined to a two-dimensional plane is observed to be quantised in fractional units of charge, rather than integer units.

The Role of Flat Bands in Charge Fractionalisation

To fractionalise charges, electrons need to be taken to a state where they stop following normal rules. In conventional metals, electrons typically move through the material, generally ignoring each other apart from the occasional bump. However, in some materials, certain extreme conditions can push electrons to start interacting and behaving collectively. Flat bands are regions in the electronic structure of a material where the electrons all lie in the same energy state, i.e., where they have nearly infinite effective masses. Here, electrons are too heavy to escape each other and strong interactions between electrons reign. These flat bands can lead to phenomena including exotic forms of magnetism or topological phases such as fractional quantum Hall states.

Kagome Lattice Structure and Charge Fractionalisation

The research team achieved charge fractionalisation in a different way, without the application of a strong magnetic field. They created a lattice structure that reduces electron kinetic energies and allows them to interact. Such a lattice is the Japanese woven bamboo “kagome” mat, which characterises atomic layers in a surprisingly large number of chemical compounds. They made their discovery in Fe3Sn2, a compound consisting only of the common elements iron (Fe) and tin (Sn) assembled according to the kagome pattern of corner sharing triangles.

Spectroscopic Observation and Future Implications

The researchers used laser angle resolved photoemission spectroscopy (laser ARPES) at the University of Geneva with a very small beam diameter, which allowed them to probe the local electronic structure of the material at an unprecedented resolution. They observed a dispersive band interacting with a flat band, predicted to exist by colleagues from EPFL. The interaction between flat and dispersive bands allows new phases of matter to emerge, such as “marginal” metals where electrons do not travel much further than their quantum wavelength and peculiar superconductors. This observation provides direct spectroscopic evidence of charge fractionalisation, which holds exciting prospects for both fundamental research and potential applications in electronic devices.