Nicolas Ubrig,Department of Quantum Matter Physics, University of Geneva

Nicolas Ubrig, jeudi 24 mai 2018, amphi Urbain à14h00

Valley and spin physics in atomically thin van der Waals materials

The discovery of graphene has seeded a new field of research on two dimensional materials that
is still growing at an impressive pace. Quickly the scientific community came to realize that other
materials with two dimensional crystal structure can offer complementary properties. The strong
confinement of charge carriers in these low dimensional materials and the availability of many
different electronic states including semiconductors, semimetals, topological insulators, magnetic
materials, and superconductors lead to the emergence of new physical phenomena of interest for
a broad scientific community.
For instance, in monolayers of group VI transition metal dichalcogenides (TMDs) a new degree
of freedom has been identified by the presence of two degenerate valleys with equal and opposite
Berry curvature. It has been predicted that, when illuminated with circularly polarized light,
interband transitions generate an unbalanced nonequilibrium population of electrons and holes
in these valleys, resulting in a finite Hall voltage at zero magnetic field when a current flows
through the system. This is the so-called valley Hall effect that has recently been observed
experimentally. W show that this effect is mediated by photogenerated neutral excitons and
charged trions and not by interband transitions generating independent electrons and holes. We
further demonstrate an experimental strategy, based on wavelength dependent spatial mapping
of the Hall voltage, which allows the exciton and trion contributions to the valley Hall effect to
be discriminated in the measurement.
Another emerging class of materials giving access to new physical phenomena are magnetic
layered van der Waals crystals, where recently the first observation of 2D ferromagnetism in
Cr2Ge2Te6 and CrI3 was reported. Of particular interest in compounds possessing
semiconducting properties is the interplay between magnetism and transport, which has
remained entirely unexplored. Here we present first magneto-transport measurements on thin
exfoliated CrI3 crystals above and below the ferromagnetic phase transition at Tc = 61 K. We find
that conduction in the direction perpendicular to the crystalline planes- mediated by electrons
tunneling through the bandgap of CrI3- exhibits variations as a function of applied field that
correspond to a magnetoresistance as large as 10 000 %. The evolution of the measured
magnetoresistance with magnetic field and temperature reveals that the phenomenon originates
from multiple transitions to different magnetic states. Our findings demonstrate the presence of
an extremely strong coupling between charge carriers and magnetism in magnetic van der Waals
semiconductors, and show that the study of transport can unveil phenomena not apparent in
common magnetization measurements.

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