Spectroscopic view of ultrafast charge carrier dynamics in single- and bilayer transition metal dichalcogenide semiconductors

The quasiparticle spectra of atomically thin semiconducting transition metal dichalcogenides (TMDCs) and their response to an ultrafast optical excitation critically depend on interactions with the underlying substrate. Here, we present a comparative time- and angle-resolved photoemission spectroscopy (TR-ARPES) study of the transient electronic structure and ultrafast carrier dynamics in the single- and bilayer TMDCs MoS2 and WS2 on three different substrates: Au(111), Ag(111) and graphene/SiC. The photoexcited quasiparticle bandgaps are observed to vary over the range of 1.9-2.5 eV between our systems. The transient conduction band signals decay on a sub-50 fs timescale on the metals, signifying an efficient removal of photoinduced carriers into the bulk metallic states. On graphene, we instead observe a fast timescale on the order of 170 fs, followed by a slow dynamics for the conduction band decay in MoS2. These timescales are explained by Auger recombination involving MoS2 and in-gap defect states. In bilayer TMDCs on metals we observe a complex redistribution of excited holes along the valence band that is substantially affected by interactions with the continuum of bulk metallic states.

Spectroscopic Studies on Semiconducting Interfaces with Giant Spin Splitting

Emmanouil Frantzeskakis

The application of an external magnetic field can lift the spin degeneracy of electronic states through its interaction with the electronic magnetic moment. A closely-related phenomenon is the Rashba-Bychkov (RB) effect where symmetry breaking at surfaces or interfaces gives rise to an electric field which is in turn seen as an effective magnetic field in the electrons' rest frame. The resulting k-dependent energy splitting of spin-polarized electronic states has been observed on various metal surfaces but the effect is much larger in artificially-grown surface alloys; such as the BiAg2 grown at the surface of Ag(111). The spin splitting magnitude observed in these systems might be very useful in spintronics applications since it could decrease the spin precession time in a spin transistor and distinguish between the extrinsic and intrinsic spin Hall effects. Nevertheless, their metallic character poses serious obstacles in the exploitation of the RB effect due to the presence of spin-degenerate electronic states at the Fermi level which would dominate transport experiments. We have used angle-resolved photoelectron spectroscopy (ARPES) to explore the RB effects on various artificially grown structures, formed on semiconducting substrates. The interplay of quantum confinement and giant RB splitting on a trilayer Si(111)-Ag-BiAg2 system reveals the formation of a complex spin-dependent structure, which can be externally tuned by varying the Ag layer thickness. This provides a means to tailor the electronic structure and spin polarization near the Fermi level, with potential applications on Si-compatible spintronic devices. Moreover, we have discovered a giant spin splitting in a true semiconducting system, namely the Si(111)-Bi trimer phase. The size of the RB parameters is comparable to those of metallic surface alloys. Using theoretical models we have identified the peculiar band topology as the origin of the giant spin splitting on the Bi/Si(111) system. All our findings are supported by relativistic first-principles calculations. Finally, a chapter of this thesis manuscript is devoted to the description of phenomeno-logical theoretical simulation, which can capture the experimental results related to the RB effect on low-dimensional systems. A parallel experimental project is discussed in a separate chapter. It has been focused on the band topology of the novel p(2 × 2) reconstruction of the Pt(111)-Ag-Bi trilayer. We investigated the symmetry properties of the interface states by varying the amount of Ag. ARPES results present the electronic signature of a strain-related structural transition.

EPFL2010

Illuminating the dark corridor in graphene: Polarization dependence of angle-resolved photoemission spectroscopy on graphene

Isabella Gierz, Klaus Kern

We have used s- and p-polarized synchrotron radiation to image the electronic structure of epitaxial graphene near the (K) over bar point by angle-resolved photoemission spectroscopy (ARPES). Part of the experimental Fermi surface is suppressed due to the interference of photoelectrons emitted from the two equivalent carbon atoms per unit cell of graphene's honeycomb lattice. Here we show that, by rotating the polarization vector, we are able to illuminate this dark corridor giving access to the complete experimental Fermi surface. Our measurements are supported by first-principles photoemission calculations, which reveal that the observed effect persists in the low-photon-energy regime.

2011

Ultrafast photodoping and effective Fermi-Dirac distribution of the Dirac particles in Bi2Se3

Helmuth Berger, Philippe Bugnon, Alberto Crepaldi, Marco Grioni, Klaus Kern

We exploit time- and angle-resolved photoemission spectroscopy to determine the evolution of the out-of-equilibrium electronic structure of the topological insulator Bi2Se3. The response of the Fermi-Dirac distribution to ultrashort IR laser pulses has been studied by modeling the dynamics of hot electrons after optical excitation. We disentangle a large increase in the effective temperature (T*) from a shift of the chemical potential (mu*), which is consequence of the ultrafast photodoping of the conduction band. The relaxation dynamics of T* and mu* are k independent and these two quantities uniquely define the evolution of the excited charge population. We observe that the energy dependence of the nonequilibrium charge population is solely determined by the analytical form of the effective Fermi-Dirac distribution.

Amer Physical Soc2012

Spectroscopic Studies on Semiconducting Interfaces with Giant Spin Splitting

Emmanouil Frantzeskakis

EPFL2010