Investigation of interactions in two-dimensional electron gases with angular resolved photoemission spectroscopy

In this thesis, angular resolved photoemission spectroscopy (ARPES) is used to study the electronic structure of different two-dimensional electron systems (2DES). This technique is very surface sensitive and the most direct method to probe the surface band structure of a system. In addition to the band dispersion also signatures from interactions can be detected with ARPES. A careful analysis of peak position and linewidth gives access to the complex self-energy Sigma, which describes the energy renormalization and the lifetime of the electrons due to many-body interactions. By using circularly polarized light it is possible to excite electrons selectively and to extract additional information about the electronic states in the surface. 2DESs exist typically either at interfaces or at surfaces. We put our focus on the surface states of topological insulators and investigate Bi2Te2Se and TiTe1.5Se0.5. The first is known to be a topological insulator with the Fermi energy located in the bulk band gap. A time-dependent doping behavior is observed and we show that the Coulomb interaction in this material is strongly dependent on the charge carrier density and not negligible for very low concentrations. TiTe1.5Se0.5 was predicted to be a topological insulator. We are able to present the surface band structure, but due to the fact that ARPES can only probe the occupied states it is impossible to see the predicted topological surface states. The third system under investigation is a monolayer of epitaxial graphene on SiC(0001) decorated by small amounts of Tl adatoms. ARPES measurements at very low temperature and very low impurity concentrations show that Tl adatoms not only give rise to electron doping but have also a large effect on the quasiparticle scattering rate. The adatoms introduce disorder and act on the graphene electronic structure both as Coulomb long-range scatterers as well as short-range scatterers with a delta-like potential. We show that also for charged impurities short-range scattering can play an important role and is not always negligible. By modeling the self-energy for long- and short-range scattering, we are able to extract a strong short-range scattering potential delta=-3.2 +/- 1 eV. A detailed understanding of the underlying scattering mechanisms in graphene is very important for the development of novel impurity-graphene-based electronics. ARPES experiments are also performed on the surface alloys on Ag(111). For these measurements, we use circularly polarized light and take advantage of the fact that the interaction between light and matter changes for different polarizations. Surface alloys on Ag(111) are formed by depositing 1/3 of a monolayer of bismuth or antimony on the clean Ag(111) surface, where every third Ag atom is replaced by such an alloy atom. These systems are known for their Rashba type spin splitting and the resulting chiral spin structure in the two-dimensional (2D) band structure. We show that circular dichroism (CD) in the angular distribution of the photoelectrons depends strongly on the experimental setup and the energy of the photons. Only under special circumstances it is possible to use CD to access the orbital angular momentum and - in systems with high spin-orbit coupling - maybe even the spin texture of the surface. Because of the final state effects, interpretation of CD data is quite complicated and has to be done very cautiously.

Investigation of Two-Dimensional Electron Gases with Angular Resolved Photoemission Spectroscopy

Isabella Gierz

In this thesis we study the electronic structure of different two-dimensional (2D) electron systems with angular resolved photoemission spectroscopy (ARPES). This technique is based on the photoelectric effect and directly probes the electronic structure of a system. By carefully analyzing the measured band structure with respect to peak position and line width we can determine the complex self-energy Σ that describes the renormalization of the electron's energy and the change in lifetime due to many-body interactions. The 2D electron systems investigated in this work are surface alloys on Ag(111), bismuth trimers on Si(111) and epitaxial graphene monolayers grown on SiC(0001). Surface alloys on Ag(111) are formed by depositing 1/3 of a monolayer of bismuth, lead or antimony (alloy atoms) on the clean silver surface. Although (Bi,Pb,Sb) and Ag atoms are immiscible in the bulk they form long-range ordered surface alloys, where every third Ag atom is replaced by an alloy atom. These systems as well as the Bi trimers on Si(111) show a spin splitting of the 2D band structure due to the Rashba-Bychkov (RB) effect. The RB model states that in a symmetry broken environment (such as the surface of a semi-infinite crystal) the spin-orbit interaction will lift the spin-degeneracy of the band structure. Such a spin-split band structure bares great potential for applications in the field of spintronics, e.g. in a Datta-Das spin field effect transistor. In the present work we investigate the origin of the observed giant spin splitting in surface alloys, especially the interplay between structural parameters and the atomic spin-orbit interaction. Furthermore, we will show that it is possible to transfer these concepts to a semiconducting substrate, which is better suited for spintronics applications. The third system under investigation — graphene — is an ideally two-dimensional crystal. It consists of a single layer of carbon atoms arranged in a honeycomb lattice, and its charge carriers are confined within a plane that is just one atom thick. These charge carriers behave like massless Dirac particles and possess extremely high carrier mobilities. This makes graphene a promising material system for high-speed electronic devices. In order to reach this ambitious goal one needs reliable methods for the large-scale production of high quality graphene films. Epitaxial growth on silicon carbide (0001) substrates is the method of choice in this case, as it offers the advantage of a precise thickness control and a semiconducting substrate at the same time. However, the presence of the substrate reduces the carrier mobility of graphene's charge carriers considerably. Therefore, it is necessary to decouple the graphene layer from the substrate after epitaxial growth. A second issue that needs to be addressed, are viable doping methods for graphene. As graphene's peculiar band structure results from a sensible interplay between electrons and crystal lattice it is not an option to replace single atoms of the graphene lattice by dopants as is common practice when doping silicon. In order to preserve its band structure, graphene is usually doped by adsorbing atoms or molecules on its surface. As graphene grown on SiC is n-doped due to charge transfer from the substrate, appropriate means for p-type doping are clearly required. In this thesis, we will present a new growth method for quasi free-standing graphene on SiC(0001) and viable means for p-type doping.

EPFL2011

Spin and Charge transport in Dirac Materials

Alexander Ronald Hoyer

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.

EPFL2016

Surface and bulk electronic structure of the strongly correlated system SmB6 and implications for a topological Kondo insulator

Kazimierz Conder, Jan Hugo Dil, Joël Mesot

Recent theoretical calculations and experimental results suggest that the strongly correlated material SmB6 may be a realization of a topological Kondo insulator. We have performed an angle-resolved photoemission spectroscopy study on SmB6 in order to elucidate elements of the electronic structure relevant to the possible occurrence of a topological Kondo insulator state. The obtained electronic structure in the whole three-dimensional momentum space reveals one electron-like 5d bulk band centered at the X point of the bulk Brillouin zone that is hybridized with strongly correlated f electrons, as well as the opening of a Kondo band gap (Delta(B) similar to 20 meV) at low temperature. In addition, we observe electron-like bands forming three Fermi surfaces at the center Gamma point and boundary (X) over bar point of the surface Brillouin zone. These bands are not expected from calculations of the bulk electronic structure, and their observed dispersion characteristics are consistent with surface states. Our results suggest that the unusual low-temperature transport behavior of SmB6 is likely to be related to the pronounced surface states sitting inside the band hybridization gap and/or the presence of a topological Kondo insulating state.