Correlated states in twisted double bilayer graphene

Electron-electron interactions play an important role in graphene and related systems and can induce exotic quantum states, especially in a stacked bilayer with a small twist angle(1-7). For bilayer graphene where the two layers are twisted by the 'magic angle', flat band and strong many-body effects lead to correlated insulating states and superconductivity(4-7). In contrast to monolayer graphene, the band structure of untwisted bilayer graphene can be further tuned by a displacement field(8-10), providing an extra degree of freedom to control the flat band that should appear when two bilayers are stacked on top of each other. Here, we report the discovery and characterization of displacement field-tunable electronic phases in twisted double bilayer graphene. We observe insulating states at a half-filled conduction band in an intermediate range of displacement fields. Furthermore, the resistance gap in the correlated insulator increases with respect to the in-plane magnetic fields and we find that the g factor, according to the spin Zeeman effect, is 2, indicating spin polarization at half-filling. These results establish twisted double bilayer graphene as an easily tunable platform for exploring quantum many-body states. Placing two Bernal-stacked graphene bilayers on top of each other with a small twist angle gives correlated states. As the band structure can be tuned by an electric field, this platform is a more varied setting to study correlated electrons.

Time-independent, high electron mobility in thin PC61BM films: Relevance to organic photovoltaics

Jacques-Edouard Moser

Ultrafast optical probing of electric field by means of electroabsorption combined with conventional photocurrent measurements was employed to investigate the drift and mobility dynamics of photo-generated charge carriers in the pristine PC61BM film and in the blend with a merocyanine dye. Electrons passed a 40 nm thick PC61BM film within a few picoseconds with time-independent and weakly dispersive mobility. The electron mobility is 1 cm2/(V s) at 1 MV/cm and an estimate of the zero-field mobility yields 5 ⋅ 10−2 cm2/(V s). The initial electron mobility in the blend is of the order of 10−2 cm2/(V s) and decreases rapidly. We conclude that electron motion in PC61BM based organic bulk hetero-junction solar cells is limited by barriers between PC61BM domains rather than by intrinsic PC61BM properties.

Elsevier2014

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

Carola Strasser

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.

EPFL2015

Dynamics of Photocarrier Separation in MAPbI3 Perovskite Multigrain Films under a Quasistatic Electric Field

Jacques-Edouard Moser, Arun Aby Paraecattil, Vytenis Pranculis, Jelissa Risse, Joël Teuscher

Applying time-resolved electroabsorption spectroscopy for the first time to methylammonium lead triiodide perovskite (MAPbI3) thin films under reverse bias, we monitored optically the ultrafast evolution of the local counter-electric field produced by the drift of photogenerated electrons and holes in opposite directions. Under an externally applied electric field of |E| < 10^5 V cm–1, the carriers were found to reach a separation of 40 nm within ∼1 ps. This distance corresponds to the average dimensions of crystalline grains in the active film, at the boundaries of which charges were trapped. An intragrain average carrier drift mobility of μ± = 23 cm^2 V–1 s–1 was inferred. Subsequent charge detrapping, migration through the entire film, and accumulation at its insulated surfaces caused a blue shift of the perovskite absorption edge that arose within tens of picoseconds, owing to a trap-limited electron drift mobility μn = 6 cm^2 V–1 s–1. Charge recombination was entirely suppressed between field-separated photocarriers generated at initial densities of n0 ≤ 2 × 10^16 cm–3. Accumulation of electrons at the interface between a mesoporous TiO2 electron-transport layer and a multigrain MAPbI3 film was also observed, which was indicative of delayed charge injection through a poor contact junction.

American Chemical Society2016

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

Carola Strasser

EPFL2015