Chirality of graphene electrons manipulated by high magnetic fields

An international group of physicists has observed and manipulated the chirality of graphene electrons by using the high magnetic fields of the High Field Magnet Laboratory at Radboud University and the Laboratoire National des Champs Magnétiques Intenses in Grenoble. The finding is in particular interesting for valleytronics, a new scientific field which aims to store information in pseudospins. Science published the results on August 5.

Electrons have various basic properties: they have a charge, a spin, and on top of that can have an additional degree of freedom: their pseudospin or valley degeneracy, which corresponds to their location in the atomic structure. In graphene, the truly two-dimensional form of carbon, two types of electrons with an opposite pseudospins exist. It is either parallel or antiparallel to its direction of propagation and is therefore referred to as chirality. However, measuring an electron’s chirality has proven to be quite tricky. And to base calculations on it, for example when developing a future quantum computer, scientists need to be able to observe and manipulate the electrons’ chirality. In the current publication, scientists from the the University of Manchester, the University of Nottingham and two labs of the European High Magnetic Field Laboratories (EMFL), the High Field Magnet Laboratory (HFML) in Nijmegen and the Laboratoire National des Champs Magnétiques Intenses (LNCMI) in Grenoble, achieved both.

They constructed special structures of stacked graphene with a thin layer of boron nitride in between. The hexagonal graphene molecules were almost perfectly aligned, but not completely. Electrons can flow from the top graphene layer to the bottom one by quantum mechanical tunnelling though the insulation boron nitride layer between them. However, since the graphene layers a slightly misaligned this current flow is difficult because electrons have to displace laterally from one molecule to the next. High magnetic fields of up to 30 Tesla, applied in the plane of the graphene, can help the electrons make this ‘jump’, by giving them a so-called Lorentz boost, i.e. by changing their direction of movement slightly when passing through the boron nitride. Additionally, this makes it possible to select, depending on the angle of the magnetic field, one specific chirality which can tunnel easily thought the boron nitride whereas the other one is supressed.

‘Making these specific graphene structures, a kind of tiny electronic sandwiches, is a complicated and time-consuming process’, says Uli Zeitler, a physicist at the HFML. ‘The samples in this study were made at the National Graphene Institute of the University of Manchester, and we subsequently exposed them to a high magnetic field of 30 tesla. Currently, there are some ten to twenty two-dimensional materials that resemble graphene and can be stacked to so-called van-der-Waals heterostructures. These allow us to access and to control properties in a way not possible in traditional semiconductors, and that is why they have our attention. By building and investigating these sandwiches, in the end we can build small electronic devices with specific properties. This is interesting for e.g. valleytronics, a new scientific field where researchers aim to store information in the pseudospins of electrons, which may have important implications on a future quantum computer.’

Reference:
J. R. Wallbank et al.,
Tuning the valley and chiral quantum state of Dirac electrons in van der Waals heterostructures;
Science 355, 575 (2016)
DOI: 10.1126/science.aaf4621