Isabella Gierz

Atomic hole doping of graphene.

The application of graphene in nanoscale electronic devices requires the deliberate control of the density and character of its charge carriers. We show by angle-resolved photoemission spectroscopy that substantial hole doping in the conical band structure of epitaxial graphene monolayers can be achieved by the adsorption of bismuth, antimony, or gold. In the case of gold doping the Dirac point is shifted into the unoccupied states. Atomic doping presents excellent perspectives for large scale production.

Snapshots of non-equilibrium Dirac carrier distributions in graphene

The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point. It has been proposed that even in the absence of a bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by impact ionization has been discussed in the context of light harvesting applications. However, all of these effects are difficult to test quantitatively by measuring the transient optical properties alone, as these only indirectly reflect the energy- and momentum-dependent carrier distributions. Here, we use time- and angle-resolved photoemission spectroscopy with femtosecond extreme-ultraviolet pulses to directly probe the non-equilibrium response of Dirac electrons near the K-point of the Brillouin zone. In lightly hole-doped epitaxial graphene samples, we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. Whereas excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find any indication of carrier multiplication, questioning the applicability of graphene for light harvesting.

Electronic decoupling of an epitaxial graphene monolayer by gold intercalation

The application of graphene in electronic devices requires large-scale epitaxial growth. The presence of the substrate, however, usually reduces the charge-carrier mobility considerably. We show that it is possible to decouple the partially sp(3)-hybridized first graphitic layer formed on the Si-terminated face of silicon carbide from the substrate by gold intercalation, leading to a completely sp(2)-hybridized graphene layer with improved electronic properties.

Silicon surface with giant spin splitting.

We demonstrate a giant Rashba-type spin splitting on a semiconducting substrate by means of a Bi-trimer adlayer on a Si(111) wafer. The in-plane inversion symmetry is broken inducing a giant spin splitting with a Rashba energy of about 140 meV, much larger than what has previously been reported for any semiconductor heterostructure. The separation of the electronic states is larger than their lifetime broadening, which has been directly observed with angular resolved photoemission spectroscopy. The experimental results are confirmed by relativistic first-principles calculations.

Optically induced coherent transport far above Tc in underdoped YBa2Cu3O6+δ

We report on a photo-induced transient state of YBa2Cu2O6+x in which transport perpendicular to the Cu-O planes becomes highly coherent. This effect is achieved by excitation with mid-infrared optical pulses, tuned to the resonant frequency of apical oxygen vibrations, which modulate both lattice and electronic properties. Below the superconducting transition temperature Tc, the equilibrium signatures of superconducting interlayer coupling are enhanced. Most strikingly, the optical excitation induces a new reflectivity edge at higher frequency than the equilibrium Josephson plasma resonance, with a concomitant enhancement of the low frequency imaginary conductivity. Above Tc, the incoherent equilibrium conductivity becomes highly coherent, with the appearance of a reflectivity edge and a positive imaginary conductivity that increases with decreasing frequency. These features are observed up to room temperature in YBa2Cu2O6.45 and YBa2Cu2O6.5. The data above Tc can be fitted by hypothesizing that the light re-establishes a transient superconducting state over only a fraction of the solid, with a lifetime of a few picoseconds. Non-superconducting transport could also explain these observations, although one would have to assume transient carrier mobilities near 10^4 cm^2/(V.sec) at 100 K, with a density of charge carriers similar to the below Tc superfluid density. Our results are indicative of highly unconventional non-equilibrium physics and open new prospects for optical control of complex solids.

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

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 graphenes 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.

Population inversion in monolayer and bilayer graphene

The recent demonstration of saturable absorption and negative optical conductivity in the Terahertz range in graphene has opened up new opportunities for optoelectronic applications based on this and other low dimensional materials. Recently, population inversion across the Dirac point has been observed directly by time- and angle-resolved photoemission spectroscopy (tr-ARPES), revealing a relaxation time of only ∼130 femtoseconds. This severely limits the applicability of single layer graphene to, for example, Terahertz light amplification. Here we use tr-ARPES to demonstrate long-lived population inversion in bilayer graphene. The effect is attributed to the small band gap found in this compound. We propose a microscopic model for these observations and speculate that an enhancement of both the pump photon energy and the pump fluence may further increase this lifetime.

Structural influence on the Rashba-type spin splitting in surface alloys

The Bi/Ag(111), Pb/Ag(111), and Sb/Ag(111) surface alloys exhibit a two-dimensional band structure with a strongly enhanced Rashba-type spin splitting, which is in part attributed to the structural asymmetry resulting from an outward relaxation of the alloy atoms. In order to gain further insight into the spin splitting mechanism, we have experimentally determined the outward relaxation of the alloy atoms in these surface alloys using quantitative low-energy electron diffraction. The structure plays an important role in the size of the spin splitting as it dictates the potential landscape, the symmetry as well as the orbital character. Furthermore, we discuss the band ordering of the Pb/Ag(111) surface alloy as well as the reproducible formation of Sb/Ag(111) surface alloys with unfaulted (face-centered cubic) and faulted (hexagonally close-packed) top layer stacking.

Graphene Sublattice Symmetry and Isospin Determined by Circular Dichroism in Angle-Resolved Photoemission Spectroscopy

The Dirac-like electronic structure of graphene originates from the equivalence of the two basis atoms in the honeycomb lattice. We show that the characteristic parameters of the initial state wave function (sublattice symmetry and isospin) can be determined using angle-resolved photoemission spectroscopy (ARPES) with circularly polarized synchrotron radiation. At a photon energy of hν = 52 eV, transition matrix element effects can be neglected allowing us to determine sublattice symmetry and isospin with high accuracy using a simple theoretical model.

Phonon-Pump Extreme-Ultraviolet-Photoemission Probe in Graphene: Anomalous Heating of Dirac Carriers by Lattice Deformation

We modulate the atomic structure of bilayer graphene by driving its lattice at resonance with the in-plane E1u lattice vibration at 6.3μm. Using timeand angle-resolved photoemission spectroscopy (tr-ARPES) with extreme ultra-violet (XUV) pulses, we measure the response of the Dirac electrons near the K-point. We observe that lattice modulation causes anomalous carrier dynamics, with the Dirac electrons reaching lower peak temperatures and relaxing at faster rate compared to when the excitation is applied away from the phonon resonance or in monolayer samples. Frozen phonon calculations predict dramatic band structure changes when the E1u vibration is driven, which we use to explain the anomalous dynamics observed in the experiment. 1 ar X iv :1 41 1. 38 88 v1 [ co nd -m at .m es -h al l] 1 4 N ov 2 01 4