Graphene and magnetism
Graphene is a one atom thick layer of graphite in which the carbon atoms are arranged in a honeycomb structure with two non-equivalent atoms. This two-dimensional system has attracted enormous interest because of its impressive properties, such as its extreme strength for its weight, its high electrical and thermal conductivity and its chemical stability. It is, however, not intrinsically magnetic!
The band structure of graphene, with its unique valley structure and Dirac neutrality point separating hole states from electron states, has led to the observation of new electronic transport phenomena such as anomalously quantized Hall effects, the absence of weak localization and the existence of a minimum conductivity, but this needs to be modified to make graphene magnetic.
The first method used to produce graphene wass repeatedly cleaving thin graphite layers using a piece of sticky tape, but only small flakes can be produced in this way and using methods derived from it. A way of producing large area graphene is by epitaxial growth on a silicon carbide (SiC) substrate. Using controlled sublimation of silicon from the SiC crystal at temperatures around 1500°C it is possible to produce large area graphene. This graphene is 1 atomic monolayer thick and can completely cover a wafer of several cm2 in size. The epitaxial growth process can take place at the silicon terminated (0001) face of the SiC crystal as well as the carbon terminated (000) face.
Our group is mainly focused on the modification of the electronic structure of graphene to induce and tune the ferromagnetic behaviour by hydrogenation of epitaxial graphene. The role of defects, adsorbates or intercalation of epitaxial graphene provide plenty of possibilities for tuning the spin-spin interactions, creating promising materials for spintronics and related applications, such as spin transistors or spin qubits due to the low intrinsic spin-orbit interaction to field-effect spin-orbit valves when heavy elements like gold or tungsten are intercalated.
“Proximity orbital and spin-orbit effects of WSe2 on bilayer graphene”
Heterostructures of two-dimensional materials can fundamentally alter their properties due to proximity effects. For example, graphene on transition metal dichalcogenides can serve as a new platform for opto-spintronics, promoting graphene towards applications.
In this project we will study the band structure of this heterostructure with microARPES, angular resolved photoemission, STM and optical techniques to measure the relevant valence electron states and in particularly the spin-orbit splitted states, trying to understand the electric field spin-orbit effect.
This research project is inspired by the recent discovery of ballistic and fully spin-polarized transport at room temperature for graphene nano-ribbons (GNRs) grown on pre-structured SiC(0001) surfaces. The origin of the observed unique properties can be governed either by the presence of the robust edge states or by strain induced spin-polarised electron states that are not necessarily located at the edges of the ribbon.
The electronic and magnetic properties of these states are the key research issue in this project which in combination with electronic charge and spin transport studies in dedicated devices will reveal crucial information about the underlying mechanism for the ballistic and magnetic behaviour. The major objective is to explore the unique possibilities of this system to realize “all-graphene” spintronic devices where the entire spin valve architecture is made in a single GNR by gating. This creates an entirely new platform for both fundamental as well as application driven research of quasi one-dimensional carbon based magnetism and spintronics. The close entanglement of both charge and electron spin within the transport channel demands the study of both signatures.
The central objective of this project is to design graphene nanostructures that are suitable for fully spin-polarized devices. In order to reach our major objective, combining the expertise and close collaboration between several groups within the EU from the fields of nanostructuring, growth, spin-resolved and other electron spectroscopies, local probe spectroscopy techniques and transport is of crucial importance.
“Scanning tunneling microscopy/spectroscopy (STM/STS) on graphene nano ribbons”.
In this project the scanning tunnelling microscope will be used to study the morphology and local electronic structure of the graphene ribbons, in particular the edge regions. The results will be compared with dedicated electronic structure calculations in which strain effects are also taken into account. In this way we try to understand the spin-polarised signatures of these ribbons.