Search results for " GAP"
showing 10 items of 811 documents
Excitonic absorption and Urbach's tail in bismuth sulfide single crystals
1988
The absorption coefficient of bismuth sulfide single crystals has been measured through more than four orders of magnitude and in the range of energies from 1.25 to 1.70 eV. A detailed study as a function of temperature has been carried out from 29 to 300 K. An Urbach tail for low values of absorption has been found. This tail and its temperature evolution fit the expression for ionic materials. An excitonic region appears at low temperature and the shape of the exciton peak is Gaussian, which corresponds to a strong exciton-phonon coupling. The exciton binding energy is estimated (28±3 meV) and then the energy gap at 29 K is obtained (Eg=1.523±0.003 eV). The fundamental electronic transiti…
Mass generation in Yang-Mills theories *
2017
In this talk we review recent progress on our understanding of the nonperturbative phenomenon of mass generation in non-Abelian gauge theories, and the way it manifests itself at the level of the gluon propagator, thus establishing a close contact with a variety of results obtained in large-volume lattice simulations. The key observation is that, due to an exact cancellation operating at the level of the Schwinger-Dyson equations, the gluon propagator remains rigorously massless, provided that the fully-dressed vertices of the theory do not contain massless poles. The inclusion of such poles activates the well-known Schwinger mechanism, which permits the evasion of the aforementioned cancel…
Excitation levels and magic numbers of small parahydrogen clusters (N⩽40)
2008
The excitation energies of parahydrogen clusters have been systematically calculated by the diffusion Monte Carlo technique in steps of one molecule from 3 to 40 molecules. These clusters possess a very rich spectra, with angular momentum excitations arriving up to L=13 for the heavier ones. No regular pattern can be guessed in terms of the angular momenta and the size of the cluster. Clusters with N=13 and 36 are characterized by a peak in the chemical potential and a large energy gap of the first excited level, which indicate the magical character of these clusters. From the calculated excitation energies the partition function has been obtained, thus allowing for an estimate of thermal e…
Pressure dependence of the exciton absorption and the electronic subband structure of aGa0.47In0.53As/Al0.48In0.52As multiple-quantum-well system
1992
We have measured the optical absorption of a ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/${\mathrm{Al}}_{0.48}$${\mathrm{In}}_{0.52}$As multiple quantum well at 10 K for pressures up to 7 GPa. The energies of optical transitions between heavy- and light-hole subbands and electron levels of the wells show a blueshift with pressure similar to the bulk lowest direct band gap. We observe a decrease with pressure of the energy splitting between heavy- and light-hole subbands with the same quantum number n. From the analysis of the absorption line shape, we have obtained the pressure dependences of exciton binding energies, oscillator strengths, and linewidths. These results are interpreted in…
Topological Insulators in Ternary Compounds with a Honeycomb Lattice
2010
One of the most exciting subjects in solid state physics is a single layer of graphite which exhibits a variety of unconventional novel properties. The key feature of its electronic structure are linear dispersive bands which cross in a single point at the Fermi energy. This so-called Dirac cone is closely related to the surface states of the recently discovered topological insulators. The ternary compounds, such as LiAuSe and KHgSb with a honeycomb structure of their Au-Se and Hg-Sb layers feature band inversion very similar to HgTe which is a strong precondition for existence of the topological surface states. In contrast to graphene with two Dirac cones at K and K' points, these material…
Band structure tuning of Heusler compounds: Spin- and momentum-resolved electronic structure analysis of compounds with different band filling
2019
Physical review / B 103(5), 054407 (2021). doi:10.1103/PhysRevB.103.054407
Measurement and control of electron wave packets from a single-electron source
2015
We report an experimental technique to measure and manipulate the arrival-time and energy distributions of electrons emitted from a semiconductor electron pump, operated as both a single-electron source and a two-electron source. Using an energy-selective detector whose transmission we control on picosecond time scales, we can measure directly the electron arrival-time distribution and we determine the upper bound to the distribution width to be 30 ps. We study the effects of modifying the shape of the voltage wave form that drives the electron pump, and show that our results can be explained by a tunneling model of the emission mechanism. This information was in turn used to control the em…
Fate of the false Mott-Hubbard transition in two dimensions
2014
We have studied the impact of non-local electronic correlations at all length scales on the Mott-Hubbard metal-insulator transition in the unfrustrated two-dimensional Hubbard model. Combining dynamical vertex approximation, lattice quantum Monte-Carlo and variational cluster approximation, we demonstrate that scattering at long-range fluctuations, i.e., Slater-like paramagnons, opens a spectral gap at weak-to-intermediate coupling -- irrespectively of the preformation of localized or short-ranged magnetic moments. This is the reason, why the two-dimensional Hubbard model is insulating at low enough temperatures for any (finite) interaction and no Mott-Hubbard transition is observed.
The growth of atomically rough 4He crystals
1986
We have studied the growth of atomically rough bcc and hcp4He crystals from the superfluid phase for temperaturesT>0.9 K. The growth coefficient displays a temperature dependence which can be represented bym 4 K∝ $$e^{\Delta E/k_B T} $$ . The parameter ΔE is found to be in close agreement with the energy gap of rotons, suggesting that these thermal excitations dominate the growth kinetics. Besides, the absolute value of the growth coefficient depends on crystal orientation, with an anisotropy for the hcp phase of about a factor of 2.5 between the $$\left\{ {10\bar 10} \right\}$$ and {0001} planes.
Band Tails in a Disordered System
1993
In crystalline solids electronic excitations have a band structure. Energy intervals, in which excitations occur, are separated by band gaps, where the density of electronic states vanishes. At the band edge the density-of-states (DOS) has power law singularities, so-called van Hove singularities.