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Tuning the valley and chiral quantum state of Dirac electrons with high magnetic fields

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, which is 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 electrons’ chirality and ideally control the electron pseudospin. 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 Laboratoire National des Champs Magnétiques Intenses (LNCMI) in Grenoble and the High Field Magnet Laboratory (HFML) in Nijmegen, achieved this goal.

Scientists at the National Graphene Institute of the University of Manchester 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 are 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.

The possibility to control the electron pseudospin in such so-called “Van-der-Waals heterostructures” opens up exciting perspectives to build and operate new electronic devices for valleytronics, a new scientific field where researchers aim to store information in the pseudospins of electrons.



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