Online PhD-defence by Johannes Handberg Juul Martiny: Valley physics and disorder phenomena in 2D materials

The defence will be carried out as an online video conference

Signing up as audience please contact Mads Brandbyge:


Professor Antti-Pekka Jauho, DTU Physics


Dr. Kristen Kaasbjerg, DTU Physics

Evaluation Board

Professor Annica Black-Schaffer, Uppsala University, Sweden
Professor Oded Zilberberg, ETH Zürich, Switzerland


Associate Professor Thomas Olsen, DTU Physics


The successful isolation in 2004 of the first 2D material graphene, a single layer of carbon
atoms, has opened up new pathways for both fundamental research into condensed matter
at the nanoscale and the development of entirely new technologies. Among these new
possibilities is the option of transferring information using a degree of freedom other than
the electron charge, and in this manner redefining conventional electronics. In graphene
such a degree of freedom exists in the form of distinct momentum states of electrons in
two unique ”valleys” of the electronic band structure. Electrons in graphene can thus be
distinguished by their so-called valley index. Storing and transferring information can be
accomplished by selective manipulation of electrons based on their valley index, setting
up currents, not of charge, but of valley polarization. Such currents are expected to be
protected from the effects of most common sources of disorder in the nanoscale system, a
major advantage over conventional charge-based electronics.
In this thesis we consider how the valley degree of freedom can be manipulated in graphene
through engineering of the nanoscale system. We suggest an approach to inducing currents
of valley polarization in the graphene sheet which can be controlled by an external po-
tential, and demonstrate how such tunability of the resulting tunable filtering of electrons
based on their valley index predicts a clear signature in experiment. We go on to discuss
the effects of disorder in realistic nanostructured systems, outlining both the robustness of
our results to moderate levels of imperfections and the possibility of new regimes of valley
filtering in the strongly disordered system.
Furthermore, we extend our studies of disorder to include impurities on the surface of the
high-temperature superconductor FeSe, wherein recent experimental evidence indicates
that local magnetism can be nucleated around defect sites. We model such impurity-
induced magnetism in a microscopic model of FeSe and predict the formation and un-
derlying symmetries of the local magnetism. Finally, we derive the expected signature of
these symmetries in experiment and compare our findings with recent scanning tunneling
microscopy measurements.



Wed 08 Apr 20


DTU Fysik


Online Defence
For participation contact Mads Brandbyge