H P Hughes

Kramers-Kronig analysis of the reflectivity spectra of 2H-MoS2, 2H-MoSe2 and 2H-MoTe2

By extrapolating to 0 and 30 eV experimental data in the energy range from approximately 1-1.3 to 14 eV a Kramers-Kronig analysis of the reflectivity spectrum at room temperature from the basal plane (E perpendicular to c) of single crystals of 2H-MoS2, 2H-MoSe2 and 2H-MoTe2 has been performed. The optical data so obtained have been discussed and interpreted in terms of the proposed band model for these materials.

Photo-emission study of in situ intercalation of TiS2 with Ag

The authors present angle-resolved ultraviolet photo-emission data on thin films of Ag evaporated onto the surface of TiS2. They find evidence for epitaxial growth of Ag on TiS2 and subsequent intercalation at room temperature and above. With submonolayer Ag coverage intercalation was indicated also at 220K. Changes in spectra were observed that can be understood as intercalation-induced changes in the TiS2 band structure. In particular, the appearance of a prominent new valence band structure was found to indicate formation of Ag-intercalated TiS2.

Electronic structure of ordered Cs and K overlayers on graphite: Direct observation of complete charge transfer

Abstract To elucidate further the electronic changes occurring during the initial stages of intercalation, we have made the first studies of ordered Cs and K overlayers on single crystal graphite using angle-resolved ultraviolet photoelectron spectroscopy (ARUPS). In this Communication, we clearly demonstrate that in the case of a (2 × 2) R 0° overlayer of Cs or K, the outer alkali metal s electron is completely transferred to the graphite π ∗ -conduction bands. We find no evidence for alkali-like s states at the Fermi level.

Determination of observable conduction band symmetry in angle-resolved electron spectroscopies: Non-symmorphic space groups

Abstract In angle-resolved photoemission, selection rules restrict the observavle final state symmetry. The rules for the initial state in inverse photoemission are the same. For cubic groups they are known (Hermanson [1]). In non-symmorphic space groups new rules apply. Here we show how to determine the observable final states for this class of space groups and work out explicitly the example of the graphite structure. The mathematical concepts are supported by direct experimental evidence obtained in normal photoemission from graphite.

Observation of direct p-d overlap in VSe2

The authors have demonstrated using photo-electron spectroscopy that the p and d valence bands in 1T-VSe2 overlap at the centre of the Brillouin zone to produce a small hole pocket. They also show that the holes can contribute significantly to bulk transport properties such as the Hall coefficient and resistivity, and that in this weak charge-density wave material there is no direct manifestation at room temperature of the charge-density-wave instability.

Electron correlations in an electron bilayer at finite temperature: Landau damping of the acoustic plasmon

We report angle-resolved Raman scattering observations of the temperature-dependent Landau damping of the acoustic plasmon in an electron bilayer system realized in a GaAs double-quantum-well structure. Corresponding calculations of the charge-density excitation spectrum of the electron bilayer using forms of the random-phase approximation (RPA), and the static local field formalism of Singwi, Tosi, Land and Sjolander (STLS) extended to incorporate non-zero electron temperature T e and phenomenological damping, are also presented. The STLS calculations include details of the temperature dependence of the intra- and inter-layer local field factors and pair correlation functions. Good agreement between experiment and the various theories is obtained for the acoustic plasmon energy and damping for T e T F /2, where T F is the Fermi temperature. However, contrary to current expectations, all of the calculations show significant departures from our experimental data for T e T F /2. From this, we go on to demonstrate unambiguously that real local field factors fail to provide a physically accurate description of exchange correlation behaviour in low-dimensional electron gases. Our results suggest instead that one must resort to a dynamical local field theory, characterized by a complex field factor to provide a more accurate description.

Photoemission and magnetic effects in NaCrS2

X-ray and angle-resolved ultraviolet photoemission data on the layered compound NaCrS2 are presented and related to the electronic structure. The data indicate that the Cr 3d electrons are essentially localised and give rise to a multiplet splitting of the Cr 3s emission line; the magnitude of the splitting is discussed in terms of the magnetic properties of the material and the net spin moment of the Cr ions.

Photo-emission studies of AgxTaS2

The authors present angle-integrated photo-emission spectra of the Ta 4f levels of pure 2H-TaS2 and of AgxTaS2, obtained at high resolution using synchrotron radiation, and of the valence-band regions. The data throw light on the band picture of the intercalated samples, and on the extent of the charge transfer from the Ag+ intercalant to the host lattice. The samples are shown to be inhomogeneous, with some regions including no Ag+ ions while in others the d(z2) conduction band of the TaS2 host appears to be virtually filled due to charge transfer from the intercalant.

3s multiplets in X-ray photoemission from layered magnetic intercalates

The authors report X-ray photoemission measurements of the intercalant 3s core levels in the magnetic layer compounds Mn13/NbS2 and Fe13/NbS2. Multiplet splittings of 6.1 eV (Mn 3s) and 4.5 eV (Fe 3s) are observed, which suggest, in agreement with previous magnetic studies, that the intercalant ions adopt 2+ ionisation states; there is some evidence for hybridisation of the intercalant 3d orbitals with orbitals on the surrounding atoms, particularly in Fe13/NbS2.

Tunable, Strongly Non parabolic Confinement in a Quasi-One-Dimensional Electron Gas Formed by Epitaxial Regrowth.

A new type of quasi-one-dimensional electron gas (Q1DEG) characterised by a tunable, strongly non parabolic confining potential in the lateral direction has been produced, using molecular beam epitaxy to grow a high-mobility heterostructure on a (311)A GaAs substrate selectively etched to expose (100) facets. The electron gas on the (100) facets is confined in one dimension by the two-dimensional hole gases on the (311)A facets, forming a p-n-p structure. Infrared cyclotron resonance (CR), magneto resistance and electron-beam-induced current (EBIC) measurements have been made on these Q1DEGs for various biases (Vh) between the hole and electron gases to investigate its effects on the confinement potential. Two different regimes are present. For Vh>-1.9 V, a strong peak attributable to a confined magnetoplasmon (CPM), together with its higher-frequency harmonics, are observed in the CR spectra. For Vh<-1.9 V, a new mode appears and the CPM no longer fits the experimental data. The anomalous dependence of the resonance frequency on carrier density, together with the EBIC images, provides some understanding of the tunable, non parabolic nature of the confining potential.