Spin and Charge transport in Dirac Materials

Abstract

The digitalization of our world is proceeding with every new technical product placed on the market. By making them smarter, e.g. linking them together and equipping them with sensors, a vast mass of data is collected. Processing and storing this information is particularly challenging as it should be done in real time and, ideally with a handheld device. This requires logic devices to be very fast, low on power consumption and rather small. The latter requirement is leading towards a transition from the classical physics domain to a domain where quantum effects like tunneling and interference start to dominate the behavior. Thus for miniaturization to continue, it is necessary to adapt new techniques that work in the quantum regime. One promising approach to achieve all of this is to exploit the spin degree of freedom an electron possesses, opening up the field of spin electronics (spintronics). As first principle operation of such devices has been demonstrated with silicon, new materials may possess attributes that better suite the requirements of spintronics devices. Graphene, a two dimensional carbon allotrope, is such a candidate as it shows low spin orbit coupling, which theoretical allows high spin life times to be achieved. Furthermore the 2D nature of graphene allows to control and tailor the electronic and spin properties by external means such as proximity and chemical functionalization. On this basis low temperature magnetotransport measurements are performed to explore the effect of functionalization of graphene with 4-nitrophenyl diazonium tetrafluoroborate within this thesis. Basic measurements in Hall bar geometry show an introduction of disorder due to the functionalization. Additional weak localization, a quantum effect that gives information on the phase coherence length of the electrons, is observed after functionalization and used to explore the electronic properties of the tailored graphene. To address the spin properties of graphene its spin bath needs to be controlled and manipulated. This is done within this thesis by all electrical means and requires the fabrication of spin valve structures on graphene. Two strategies to inject spin polarized currents into graphene are pursued within this framework. First spin valve measurements on functionalized graphene are promising regarding the manipulation of spin properties. The generation of the spin polarized currents is still a big issue. Traditionally, ferromagnets are used as leads to fulfill this task. Latest studies show that topological insulators (TI), a new class of materials, could be better suited to fulfill this task as they give rise to fully spin polarized currents. Conceptually these materials possess a band gap that is crossed by spin polarized bands. Crystals of nanometer size of Tin(II)telluride, a potential TI, are grown in a PVD process and electrically contacted to look for fingerprints of these unusual states. Charge and spin transport experiments show the presence of two types of charge carriers and a high intrinsic p-doping. Finally, a new 3^He system is set up within this thesis and a concept for low noise measurements is implemented that integrates a home build signal generation and amplification stage.