Ramifications of optical pumping on the interpretation of time-resolved photoemission experiments on graphene

In pump-probe time and angle-resolved photoemission spectroscopy (TR-ARPES) experiments the presence of the pump pulse adds a new level of complexity to the photoemission process in comparison to conventional ARPES. This is evidenced by pump-induced vacuum space-charge effects and surface photovoltages, as well as multiple pump excitations due to internal reflections in the sample-substrate system. These processes can severely affect a correct interpretation of the data by masking the out-of-equilibrium electron dynamics intrinsic to the sample. In this study, we show that such effects indeed influence TR-ARPES data of graphene on a silicon carbide (SiC) substrate. In particular, we find a time- and laser fluence-dependent spectral shift and broadening of the acquired spectra, and unambiguously show the presence of a double pump excitation. The dynamics of these effects is slower than the electron dynamics in the graphene sample, thereby permitting us to deconvolve the signals in the time domain. Our results demonstrate that complex pump-related processes should always be considered in the experimental setup and data analysis. (C) 2015 Elsevier B.V. All rights reserved.

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

Ultrafast electron dynamics in epitaxial graphene investigated with time- and angle-resolved photoemission spectroscopy

Alberto Crepaldi, Marco Grioni, Jens Christian Johannsen

In order to exploit the intriguing optical properties of graphene it is essential to gain a better understanding of the light-matter interaction in the material on ultrashort timescales. Exciting the Dirac fermions with intense ultrafast laser pulses triggers a series of processes involving interactions between electrons, phonons and impurities. Here we study these interactions in epitaxial graphene supported on silicon carbide (semiconducting) and iridium (metallic) substrates using ultrafast time-and angle-resolved photoemission spectroscopy (TR-ARPES) based on high harmonic generation. For the semiconducting substrate we reveal a complex hot carrier dynamics that manifests itself in an elevated electronic temperature and an increase in linewidth of the p band. By analyzing these effects we are able to disentangle electron relaxation channels in graphene. On the metal substrate this hot carrier dynamics is found to be severely perturbed by the presence of the metal, and we find that the electronic system is much harder to heat up than on the semiconductor due to screening of the laser field by the metal.

Iop Publishing Ltd2015

Spectroscopic Studies on Semiconducting Interfaces with Giant Spin Splitting

Emmanouil Frantzeskakis

EPFL2010