A number of interesting properties of graphene and graphite are postulated to derive from the peculiar bandstructure of graphene. This bandstructure consists of conical electron and hole pockets that meet at a single point in momentum (k) space--the Dirac crossing, at energy $E_{D} = \hbar \omega_{D}$. Direct investigations of the accuracy of this bandstructure, the validity of the quasiparticle picture, and the influence of many-body interactions on the electronic structure have not been addressed for pure graphene by experiment to date. Using angle resolved photoelectron spectroscopy (ARPES), we find that the expected conical bands are distorted by strong electron-electron, electron-phonon, and electron-plasmon coupling effects. The band velocity at $E_{F}$ and the Dirac crossing energy $E_{D}$ are both renormalized by these many-body interactions, in analogy with mass renormalization by electron-boson coupling in ordinary metals. These results are of importance not only for graphene but also graphite and carbon nanotubes which have similar bandstructures.

We present a self-consistent analysis of the photoemission spectral function A(k, w) of graphene monolayers grown epitaxially on SiC(0001). New information derived from spectral intensity anomalies (in addition to linewidths and peak positions) confirms that sizeable kinks in the electronic dispersion at the Dirac energy ED and near the Fermi level EF arise from many-body interactions, not single-particle effects such as substrate bonding or extra bands. The relative electron-phonon scattering rate from phonons at different energy scales evolves with doping. The electron-phonon coupling strength is extracted and found to be much larger (~3.5-5 times) than predicted.

The new Microscopic and Electronic Structure Observatory (MAESTRO) at the Advanced Light Source (ALS) in Berkeley provides X-rays of variable polarization, produced by an elliptically polarized undulator (EPU), for angle resolved photoemission (ARPES) and photoemission electron microscopy (PEEM) experiments. The interpretation of photoemission data, in particular of dichroism effects in ARPES, requires the precise knowledge of the exact polarization state. Numerical simulations show that the first harmonics of the EPU at MAESTRO provides soft X-rays of almost 100% on axis polarization. However, the higher harmonics as well as the downstream optical elements of the beamline, have a considerable impact on the polarization of the light delivered to the experimental end-station. Employing a simple reflective polarimeter, the polarization is characterized for variable EPU and beamline settings and the overall degree of polarization in the MAESTRO end-stations is estimated to be on the order of 83%.

The nature of the coupling leading to superconductivity in layered materials such as high-Tc superconductors and graphite intercalation compounds (GICs) is still unresolved. In both systems, interactions of electrons with either phonons or other electrons or both have been proposed to explain superconductivity. In the high-Tc cuprates, the presence of a Van Hove singularity (VHS) in the density of states near the Fermi level was long ago proposed to enhance the many-body couplings and therefore may play a role in superconductivity. Such a singularity can cause an anisotropic variation in the coupling strength, which may partially explain the so-called nodal-antinodal dichotomy in the cuprates. Here we show that the topology of the graphene band structure at dopings comparable to the GICs is quite similar to that of the cuprates and that the quasiparticle dynamics in graphene have a similar dichotomy. Namely, the electron-phonon coupling is highly anisotropic, diverging near a saddle point in the graphene electronic band structure. These results support the important role of the VHS in layered materials and the possible optimization of Tc by tuning the VHS with respect to the Fermi level.

Angle-Resolved Photoemission Spectroscopy (ARPES) is a premier technique for understanding the electronic excitations in conductive, crystalline matter, in which the induced photocurrent is collected and dispersed in energy and angle of emission to reveal the energy- and momentum-dependent single particle spectral function A(k,ω). So far, ARPES in a magnetic field has been precluded due to the need to preserve the electron paths between the sample and detector. In this paper we report progress towards “magnetoARPES”, a variant of ARPES that can be conducted in a magnetic field. It is achieved by applying a microscopic probe beam (≲10 μm) to a thinned sample mounted upon a special sample holder that generates magnetic field confined to a thin layer near the sample surface. In this geometry we could produce ARPES in magnetic fields up to around ±100 mT. The magnetic fields can be varied from purely in-plane to nearly purely out-of-plane, by scanning the probe beam across different parts of the device. We present experimental and simulated data for graphene to explore the aberrations induced by the magnetic field. These results demonstrate the viability of the magnetoARPES technique for exploring symmetry breaking effects in weak magnetic fields.

Scandium nitride (ScN) has recently attracted much attention for its potential applications in thermoelectric energy conversion, as a semiconductor in epitaxial metal/semiconductor superlattices, as a substrate for GaN growth, and alloying it with AlN for 5G technology. This study was undertaken to better understand its stoichiometry and electronic structure. ScN (100) single crystals 2 mm thick were grown on a single crystal tungsten (100) substrate by a physical vapor transport method over a temperature range of 1900-2000 °C and a pressure of 20 Torr. The core level spectra of Sc 2p3/2,1/2 and N 1s were obtained by x-ray photoelectron spectroscopy (XPS). The XPS core levels were shifted by 1.1 eV toward higher values as the [Sc]:[N] ratio varied from 1.4 at 1900 °C to ∼1.0 at 2000 °C due to the higher binding energies in stoichiometric ScN. Angle-resolved photoemission spectroscopy measurements confirmed that ScN has an indirect bandgap of ∼1.2 eV.

The high-mobility conducting interface (CI) between LaAlO3 (LAO) and SrTiO3 (STO) has revealed many fascinating phenomena, including exotic magnetism and superconductivity. But, the formation mechanism of the CI has not been conclusively explained. Here, using in situ angle-resolved photoemission spectroscopy, we elucidated the mechanisms for the CI formation. In as-grown samples, we observed a built-in potential (Vbi) proportional to the polar LAO thickness starting from the first unit cell (UC) with CI formation appearing above 3 UCs. However, we found that the Vbi is removed by synchrotron ultraviolet (UV)-irradiation; The built-in potential is recovered by oxygen gas (O2(g))-exposure. Furthermore, after UV-irradiation, the CI appears even below 3UC of LAO. Our results demonstrate not only the Vbi-driven CI formation in as-grown LAO/STO, but also a new route to control of the interface state by UV lithographic patterning or other surface modification.