Joachim Burgdörfer

Photovoltaic effect in an electrically tunable van der Waals heterojunction.

Semiconductor heterostructures form the cornerstone of many electronic and optoelectronic devices and are traditionally fabricated using epitaxial growth techniques. More recently, heterostructures have also been obtained by vertical stacking of two-dimensional crystals, such as graphene and related two-dimensional materials. These layered designer materials are held together by van der Waals forces and contain atomically sharp interfaces. Here, we report on a type-II van der Waals heterojunction made of molybdenum disulfide and tungsten diselenide monolayers. The junction is electrically tunable, and under appropriate gate bias an atomically thin diode is realized. Upon optical illumination, charge transfer occurs across the planar interface and the device exhibits a photovoltaic effect. Advances in large-scale production of two-dimensional crystals could thus lead to a new photovoltaic solar technology.

Delay in Photoemission

Defining Time-Zero When a high-energy photon hits an atom and is absorbed, the result can be the excitation and emission of an electron. This photoemission, or photoelectric effect, is generally assumed to occur instantaneously, and represents the definition of “time-zero” in clocking such ultrafast events. Schultze et al. (p. 1658, see the cover; see the Perspective by van der Hart) use ultrafast spectroscopy, with light pulses on the time scale of several tens of attoseconds, to test this assumption directly. They excite neon atoms with 100 eV photons and find that there is a small (20-attosecond) time delay between the emission of electrons from the 2s and 2p orbitals of the atoms. These results should have implications in modeling electron dynamics occurring on ultrafast time scales. Ultrafast metrology reveals a 20-attosecond delay between photoemission from different electronic orbitals in neon atoms. Photoemission from atoms is assumed to occur instantly in response to incident radiation and provides the basis for setting the zero of time in clocking atomic-scale electron motion. We used attosecond metrology to reveal a delay of 21±5 attoseconds in the emission of electrons liberated from the 2p orbitals of neon atoms with respect to those released from the 2s orbital by the same 100–electron volt light pulse. Small differences in the timing of photoemission from different quantum states provide a probe for modeling many-electron dynamics. Theoretical models refined with the help of attosecond timing metrology may provide insight into electron correlations and allow the setting of the zero of time in atomic-scale chronoscopy with a precision of a few attoseconds.

The Multiradical Character of One- and Two-Dimensional Graphene Nanoribbons

When is an acene stable? The pronounced multiradical character of graphene nanoribbons of different size and shape was investigated with high-level multireference methods. Quantitative information based on the number of effectively unpaired electrons leads to specific estimates of the chemical stability of graphene nanostructures.

Quantum path interference in the wavelength dependence of high-harmonic generation

We investigate the fundamental-wavelength dependence of high-harmonic generation yield. Superimposed on a smooth power-law dependence, we find surprisingly strong and rapid fluctuations on a fine wavelength scale, due to quantum-path interferences.

Wave-function mapping of graphene quantum dots with soft confinement.

Using low-temperature scanning tunneling spectroscopy, we map the local density of states of graphene quantum dots supported on Ir(111). Because of a band gap in the projected Ir band structure around the graphene K point, the electronic properties of the QDs are dominantly graphenelike. Indeed, we compare the results favorably with tight binding calculations on the honeycomb lattice based on parameters derived from density functional theory. We find that the interaction with the substrate near the edge of the island gradually opens a gap in the Dirac cone, which implies soft-wall confinement. Interestingly, this confinement results in highly symmetric wave functions. Further influences of the substrate are given by the known moiré potential and a 10% penetration of an Ir surface resonance into the graphene layer.

Electron-hole crossover in graphene quantum dots.

We investigate the addition spectrum of a graphene quantum-dot in the vicinity of the electron-hole crossover as a function of perpendicular magnetic field. Coulomb-blockade resonances of the 50 nm wide dot are visible at all gate voltages across the transport gap ranging from hole to electron transport. The magnetic field dependence of more than 50 states displays the unique complex evolution of the diamagnetic spectrum of a graphene dot from the low-field regime to the Landau regime with the n=0 Landau level situated in the center of the transport gap marking the electron-hole crossover. The average peak spacing in the energy region around the crossover decreases with increasing magnetic field. In the vicinity of the charge neutrality point we observe a well resolved and rich excited state spectrum.

Graphene quantum dots: Beyond a Dirac billiard

We present realistic simulations of quantum confinement effects in phase-coherent graphene quantum dots with linear dimensions of 10\char21{}40 nm. We determine wave functions and energy-level statistics in the presence of disorder resulting from edge roughness, charge impurities, or short-ranged scatterers. Marked deviations from a simple Dirac billiard for massless fermions are found. We find a remarkably stable dependence of the nearest-neighbor level spacing on edge roughness suggesting that the roughness of fabricated devices can be possibly characterized by the distribution of measured Coulomb blockade peaks.

Direct measurement and analysis of the carrier-envelope phase in light pulses approaching the single-cycle regime

We demonstrate a solid-state device capable of providing direct information about the carrier-envelope (CE) phase of ultrashort (4 fs) laser pulses. The measurement is based on multi-photon-induced photoelectron emission from a gold surface. The amount of the charge emitted from the surface gives a clear indication of phase sensitivity, as predicted by our simulations and also by a simple intuitive model. This phenomenon was used to determine the CE phase value of each laser pulse in a mode-locked, unamplified, low-energy pulse train. The inability of the commonly used f -to-2f interferometric method to measure accurately extracavity drifts of the CE phase is discussed and contrasted with the direct phase measurement method proposed here. The evolution of the CE phase upon propagation of pulses comparable in duration to the optical cycle is analysed.

Electron rescattering at metal nanotips induced by ultrashort laser pulses

We report on the first investigation of plateau and cut-off structures in photoelectron spectra from nano-scale metal tips interacting with few-cycle near-infrared laser pulses. These hallmarks of electron rescattering, well-known from atom-laser interaction in the strong-field regime, appear at remarkably low laser intensities with nominal Keldysh parameters of the order of

A comparison of singlet and triplet states for one- and two-dimensional graphene nanoribbons using multireference theory

\gtrsim 10