Many of the key properties of graphene samples relates to their topological electronic properties, as the pseudospin and Berry phase. We have shown that, analyzing quasi-particle interference patterns (QPI) surrounding impurities, it is possible to use STM to directly access these two entities, pseudospin and Berry phase, in graphene samples.

Measuring graphene’s Berry phase at B=0T

Our measurements prove that the induced magnetic moments couple strongly at very long distances following a particular rule: magnetic moments sum-up or neutralize critically depending on the relative H-H adsorption sites. Furthermore, we achieve the controlled manipulation of single H atoms, which enable us to selectively tune the collective magnetic properties of chosen graphene regions.

The video shows the evolution of momentum and pseudo-spin of incoming and reflected electronic waves on the H atom as function of the tip position (purple dot)

It is usually thought that measuring the Berry phase requires applying electromagnetic forces. Contradicting this belief, we show that graphene’s Berry phase can be measured, using STM, in absence of any external magnetic field.

We have been able to visualize graphene’s Berry phase, by measuring a topological singularity in the QPI patterns near a hydrogen atom grafted on a graphene surface, a method that could apply to other materials in the search of new topological sates

Nature 574, (2019) Article

Graphene’s pseudospin and quasiparticle chirality

Many of the tantalizing electronic properties of graphene can be understood as due to the conservation of pseudospin and quasiparticle chirality, two entities that have no equivalence in any other two-dimensional system. They are responsible, for example, of the new ‘chiral’ quantum Hall effects (QHE) observed for monolayer and bilayer exfoliated graphene, which are the most direct evidence for Dirac fermions in graphene. Our work shows how pseudospin and quasiparticle chirality can be experimentally probed at the nanoscale by means of STM, since they are reflected in the quasiparticle interference processes that take place in graphene. Our STM data, complemented by theoretical calculations, demonstrate that the quasiparticles in epitaxial graphene on SiC(0001) have the chirality predicted for ideal (free standing) graphene, which proves the Dirac character of the quasiparticles in this system.

Physical Review Letters 101, 206802 (2008); Article
Phys. Rev: B 086, 45444, (2012); Article
Comptes Rendus Physique 17, (2016); Article