BEGIN:VCALENDAR VERSION:2.0 PRODID:-//DTU.dk//NONSGML DTU.dk//EN CALSCALE:GREGORIAN BEGIN:VEVENT DTSTART:20200408T120000Z DTEND:20200407T220000Z SUMMARY:Online PhD-defence by Johannes Handberg Juul Martiny: Valley physics and disorder phenomena in 2D materials X-ALT-DESC;FMTTYPE=text/html:
Signing up as audience please contact Mads Brandbyge: mabr@dtu.dk
\nProfessor Antti-Pekka Jauho, DTU Physics
\nDr. Kristen Kaasbjerg, DTU Physics
\nProfessor Annica Black-Schaffer, Uppsala University, Sweden
\nProfessor Oded Zilberberg, ETH Zürich, Switzerland
Associate Professor Thomas Olsen, DTU Physics
\nThe successful isolation in 2004 of the first 2D material graphene, a single layer of carbon
\natoms, has opened up new pathways for both fundamental research into condensed matter
\nat the nanoscale and the development of entirely new technologies. Among these new
\npossibilities is the option of transferring information using a degree of freedom other than
\nthe electron charge, and in this manner redefining conventional electronics. In graphene
\nsuch a degree of freedom exists in the form of distinct momentum states of electrons in
\ntwo unique ”valleys” of the electronic band structure. Electrons in graphene can thus be
\ndistinguished by their so-called valley index. Storing and transferring information can be
\naccomplished by selective manipulation of electrons based on their valley index, setting
\nup currents, not of charge, but of valley polarization. Such currents are expected to be
\nprotected from the effects of most common sources of disorder in the nanoscale system, a
\nmajor advantage over conventional charge-based electronics.
\nIn this thesis we consider how the valley degree of freedom can be manipulated in graphene
\nthrough engineering of the nanoscale system. We suggest an approach to inducing currents
\nof valley polarization in the graphene sheet which can be controlled by an external po-
\ntential, and demonstrate how such tunability of the resulting tunable filtering of electrons
\nbased on their valley index predicts a clear signature in experiment. We go on to discuss
\nthe effects of disorder in realistic nanostructured systems, outlining both the robustness of
\nour results to moderate levels of imperfections and the possibility of new regimes of valley
\nfiltering in the strongly disordered system.
\nFurthermore, we extend our studies of disorder to include impurities on the surface of the
\nhigh-temperature superconductor FeSe, wherein recent experimental evidence indicates
\nthat local magnetism can be nucleated around defect sites. We model such impurity-
\ninduced magnetism in a microscopic model of FeSe and predict the formation and un-
\nderlying symmetries of the local magnetism. Finally, we derive the expected signature of
\nthese symmetries in experiment and compare our findings with recent scanning tunneling
\nmicroscopy measurements.