Spin and charge relaxation in graphene

Abstract

In this thesis, we investigate spin and charge relaxation mechanisms in graphene by means of room temperature and low temperature transport experiments. In the beginning of the thesis in chapter 1 an introduction to graphene and its properties is given. The unique band structure is introduced and possible spin-orbit terms are discussed, which can arise when graphene is put in proximity to a transition metal dichalcogen layer. Apart from charge and spin transport in graphene, a basic introduction on thermal transport is also given, focussing on low temperatures. Chapter 2 gives an overview of the most important fabrication techniques used to prepare the devices studied in this thesis. The fundamental cornerstones of electrical transport measurements and set-up specific information are also shown. In chapter 3 nanomagnets are characterized using magnetic force microscopy and photoemission electron microscopy. In addition to the characterization of the magnetic domain structure, the latter technique is also used to study the role of a chemical vapour deposited (CVD) hexagonal boron nitride (hBN) layer in protecting the ferromagnetic nanostructures from oxidation. In the end of the chapter, the CVD hBN itself is characterized employing various techniques. Throughout this thesis CVD hBN is used as a tunnel barrier for two purposes. As a first example superconducting tunnel spectroscopy of graphene is presented in chapter 4. This technique, which gives access to the energy distribution function, is used to study electron thermal transport in the electron and phonon cooled regime. In chapter 5 the CVD hBN is used as a tunnel barrier for electrical spin injection into graphene. The spin transport properties of graphene devices are studied at room and low temperatures, where similar contribution of the Dyakonov-Perel and Elliott-Yafet spin relaxation mechanism were found. In addition, high resistance tunnel contacts show opposite spin injection polarizations, which are tunable by bias voltage. In the end of the chapter a possible route to characterize the influence of magnetic moments on the spin transport is presented. An alternative method to create a spin current in graphene is presented in chapter 6. High frequency magnetic fields are used to excite a ferromagnetic contact into ferromagnetic resonance, where a spin current is injected into the graphene channel. The inverse spin-Hall effect in platinum is used to detect the spin current travelling through the graphene channel. Our approach of a transmission line allows simple on-chip integration and a broadband excitation. The low spin-orbit coupling (SOC) in graphene is ambivalent as it on one side allows for very long spin relaxation times but at the same time graphene lacks a strong electric field tunability of spin relaxation time. In addition to spintronics applications, topological states have been predicted in graphene due to spin-orbit coupling. In chapter 7, the spin-orbit coupling arising from the proximity to a WSe2 crystal is investigated using quantum interference phenomena. A strong valley-Zeeman SOC is found in these structure, which leads to a strong asymmetry in spin relaxation of in-plane and out-of-plane spins. A novel contactless characterization method is presented in chapter 8. An encapsulated pn-junction is capacitively coupled to a superconducting resonator operating at high frequency. As an example we used this scheme to extract the quantum capacitance and the charge relaxation resistance of a graphene pn-junction without the need for electrical contacts. Quantum-Hall measurements on a bilayer graphene pnp-junction presented in chapter 9 shed light onto equilibration phenomena of different Landau levels, revealing spin dependent edge state equilibration.