Transition from Coulomb Diamonds to Checkerboard-like Spectroscopies in a Mesoscopic Quantum Hall Interferometer.

We report low temperature ( 100 mK) magnetotransport, scanning gate microscopy and scanning gate spectroscopy measurements in an In0:7Ga0:3As=In0:52Al0:48As quantum point contact (QPC). The magnetoresistance of the QPC shows oscillations in the vicinity of in- teger quantum Hall states. We attribute these magnetoresistance oscillations to the formation of an electron interferometer around a small, disorder-induced quantum Hall island located within the constriction. The magnetic eld B tunes the edge states conguration in the QPC, leading to dierent signatures in the transport measurements. Interestingly, near the Landau level lling factor = 3, the spectroscopy measurements performed on the quantum Hall interferometer, as a function of B or scanning gate tip voltage, exhibit a smooth transition from Coulomb diamonds to a checkerboard pattern

Accurate effective masses from first principles

The accurate ab initio description of effective masses is of key interest in the design of materials with high mobility. However, up to now, they have been calculated using finite-difference estimation of density functional theory (DFT) electronic band curvatures. To eliminate the numerical noise inherent to finite-difference and obtain an approach that is more suitable for material design using high throughput computing, we develop a method allowing to obtain the curvature of DFT bands using Density-Functional Perturbation Theory (DFPT), taking a change of wavevector as a perturbation. Also, the inclusion of G$_0$W$_0$ corrections to DFT bands in our method will be presented

Electronic properties of double-wall carbon nanotubes and the effect of functionalization

Although promising for many electronic applications, further understanding of carbon nanotubes systems are required for practical designs. A difficulty currently hindering further development in this field is the considerable degradation of transport properties in a single-wall carbon nanotube (SWCNT) when it is subjected to ambient conditions or functionalized. Double-wall carbon nanotubes (DWCNT) could solve this problem, by allowing the outer tube to be functionalized while the inner tube would retain a pristine structure and it's promising electronic properties. However, our understanding of interactions between the tubes and their consequences on the system's electronic properties is still incomplete. In this presentation, we investigate those interactions using density-functional theory (DFT) calculations. In particular, we investigate separately the effects of structural deformations, Fermi energy realignment and electronic orbital overlap on the band structure of DWCNT. The effects of functionalization will also be addressed

Zero-point motion effect on the bandgap of diamond: validation of codes

Verification and validation of codes, as well as new theoretical methods, are of utmost importance if one wants to provide reliable results. In this work we present a rigorous and careful study of all the quantities that enters into the calculation of the zero point motion renormalization of the direct band gap of diamond due to electron-phonon coupling. This study has been done within the Allen-Heine-Cardona (AHC) formalism as implemented into Abinit and Yambo on top of Quantum Espresso. We aim at quantifying the agreement between the codes for the different quantities of interest. This study shows that one can get less than $10^{-5}Ha/at$ differences on the total energy, 0.07 cm$^{-1}$ on the phonon frequencies, 0.5\% on the electron-phonon matrix elements and less than 4 meV on the zero-point motion renormalization. At the LDA level, the converged direct bandgap renormalization in diamond due to electron-phonon coupling in the AHC formalism is -409 meV (reduction of the band gap)[1]. [1] S. Poncé et al., arXiv:1309.0729 [cond-mat.mtrl-sci] and submitted for publication in Comput. Mat. Science (2013)

First-Principles Study of Frequency-Dependent Resonant Raman Scattering

A resonance phenomenon appears in the Raman response when the exciting light has frequency close to electronic transitions. Unlike for molecules and for graphene, the theoretical prediction of such frequency-dependent Raman response of crystalline systems has remained a challenge. Indeed, many Raman intensity first-principle calculations are nowadays done at vanishing light frequency, using static Density-Functional Perturbation Theory, thus neglecting the frequency dependence and excitonic effects. Recently, we proposed a finite-difference method for the computation of the first-order frequency-dependent Raman intensity [1], with excitonic effects described by the Bethe-Salpeter equation. We found these to be crucial for the accurate description of the experimental enhancement for laser photon energies around the gap. In this work, we generalize this approach to the more complex second-order Raman intensity, with phonon losses coming from the entire Brillouin zone. Interestingly, even without excitonic effects, one is able to capture the main relative changes in the frequency-dependent Raman spectrum at fixed laser frequencies. The excitonic effects are discussed. [1] Y. Gillet, M. Giantomassi, X. Gonze, Phys. Rev. B 88, 094305 (2013)

Tailoring ballistic injection

Efficient electron injection into nanoscale devices is difficult to experimentally characterize at the local scale. We discuss here how little modifications of the electrostatic potential landscape in the vicinity of a nanodevice injection leads can drastically enhance electron transmission, by tuning semi-classical trajectories and directly re-orienting charge flow in the desired paths. In this context, quantum rings (QRs) appear as interesting geometries since, in a semiclassical view, most electrons bounce against the hard-wall potential of the central QR antidot directly after injection. We found that a local partial depletion of the QR close to this hard-wall can counter-intuitively ease ballistic electron flow. On the contrary, local charge accumulation can focus the flow on the hard wall potential and increase back-scattering. Simulating current density distributions in the ring gives insights on these peculiar transmission conditions. Using a voltage-polarised scanning gate to tune in situ the ballistic electron flow in a QR patterned from a high mobility 2D electron system, we find a remarkable direct experimental confirmation of this particular phenomenology