Supersymmetric Boundary Conditions for the N=2 Sigma Model

We clarify the discussion of N=2 supersymmetric boundary conditions for the classical d=2, N=(2,2) Non-Linear Sigma Model on an infinite strip. Our conclusions about the supersymmetric cycles match the results found in the literature. However, we find a constraint on the boundary action that is not satisfied by many boundary actions used in the literature.Comment: 23 pages, no figures. minor changes in text, a reference adde

Using Blogs to Foster Inquiry, Collaboration, and Feedback in Pre-Service Teacher Education

This chapter presents a critical case study on the use of information technology in a pre-service teacher education program. The authors integrated Weblogs (blogs) into two constructivist-oriented teacher preparation courses with the goal of helping students learn to think like a teacher through enhanced inquiry, collaboration, and feedback. The authors found that, through the use of blogs, pre-service teaching candidates grew in their abilities to reflect on their own teaching and to provide constructive comments to peers. The authors’ experience also indicated that while instructor and peer feedback via blogs was valuable, it functioned best when paired with face-to-face meetings between the instructors and students. They discussed design principles for combining online and face-to-face environments and offer possibilities for the expanded use of blogs in pre-service teacher education

Avalanches and Self-Organized Criticality in Superconductors

We review the use of superconductors as a playground for the experimental study of front roughening and avalanches. Using the magneto-optical technique, the spatial distribution of the vortex density in the sample is monitored as a function of time. The roughness and growth exponents corresponding to the vortex landscape are determined and compared to the exponents that characterize the avalanches in the framework of Self-Organized Criticality. For those situations where a thermo-magnetic instability arises, an analytical non-linear and non-local model is discussed, which is found to be consistent to great detail with the experimental results. On anisotropic substrates, the anisotropy regularizes the avalanches

Band gap and band parameters of InN and GaN from quasiparticle energy calculations based on exact-exchange density-functional theory

We have studied the electronic structure of InN and GaN employing G0W0 calculations based on exact-exchange density-functional theory. For InN our approach predicts a gap of 0.7 eV. Taking the Burnstein-Moss effect into account, the increase of the apparent quasiparticle gap with increasing electron concentration is in good agreement with the observed blue shift of the experimental optical absorption edge. Moreover, the concentration dependence of the effective mass, which results from the non-parabolicity of the conduction band, agrees well with recent experimental findings. Based on the quasiparticle band structure the parameter set for a 4x4 kp Hamiltonian has been derived.Comment: 3 pages including 3 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Electronic properties of lanthanide oxides from the GW perspective

A first-principles understanding of the electronic properties of f -electron systems is currently regarded as a great challenge in condensed-matter physics because of the difficulty in treating both localized and itinerant states on the same footing by the current theoretical approaches, most notably density-functional theory (DFT) in the local-density or generalized gradient approximation (LDA/GGA). Lanthanide sesquioxides (Ln2O3) are typical f -electron systems for which the highly localized f states play an important role in determining their chemical and physical properties. In this paper, we present a systematic investigation of the performance of many-body perturbation theory in the GW approach for the electronic structure of the whole Ln2O3 series. To overcome the major failure of LDA/GGA, the traditional starting point for GW, for f -electron systems, we base our GW calculations on Hubbard U corrected LDA calculations (LDA+U). The influence of the crystal structure, the magnetic ordering, and the existence of metastable states on the electronic band structures are studied at both the LDA+U and the GW level. The evolution of the band structure with increasing number of f electrons is shown to be the origin for the characteristic structure of the band gap across the lanthanide sesquioxide series. A comparison is then made to dynamical mean-field theory (DMFT) combined with LDA or hybrid functionals to elucidate the pros and cons of these different approaches

GaN/AlN Quantum Dots for Single Qubit Emitters

We study theoretically the electronic properties of $c$-plane GaN/AlN quantum dots (QDs) with focus on their potential as sources of single polarized photons for future quantum communication systems. Within the framework of eight-band k.p theory we calculate the optical interband transitions of the QDs and their polarization properties. We show that an anisotropy of the QD confinement potential in the basal plane (e.g. QD elongation or strain anisotropy) leads to a pronounced linear polarization of the ground state and excited state transitions. An externally applied uniaxial stress can be used to either induce a linear polarization of the ground-state transition for emission of single polarized photons or even to compensate the polarization induced by the structural elongation.Comment: 6 pages, 9 figures. Accepted at Journal of Physics: Condensed Matte

Exploring the random phase approximation: Application to CO adsorbed on Cu(111)

The adsorption of CO on the Cu(111) surface is investigated in the random phase approximation (RPA) as formulated within the adiabatic connection fluctuation-dissipation theorem. The RPA adsorption energy is obtained by adding a "local exchange-correlation correction" that is extrapolated from cluster calculations of increasing size, to the Perdew-Burke-Ernzerhof (PBE) value for the extended system. In comparison to density-functional theory calculations with the generalized gradient functionals PBE and AM05 and the hybrid functionals PBE0 and HSE03, we find a hierarchy of improved performance from AM05/PBE to PBE0/HSE03, and from PBE0/HSE03 to RPA, both in terms of the absolute adsorption energy as well as the adsorptionenergy difference between the atop and the hollow fcc sites. In particular, the very weak atop site preference at the PBE0/HSE03 level is further stabilized by about 0.2 eV in the RPA. The mechanism behind this improvement is analyzed in terms of the GW density of states that gives a spectral representation en par with the RPA formalism for the total energy

Controlling polarization at insulating surfaces: Quasiparticle calculations for molecules adsorbed on insulator films

By means of quasiparticle-energy calculations in the G0W0 approach, we show for the prototypical insulator-semiconductor system NaCl=Ge(001) that polarization effects at the interfaces noticeably affect the excitation spectrum of molecules adsorbed on the surface of the NaCl films. The magnitude of the effect can be controlled by varying the thickness of the film, offering new opportunities for tuning electronic excitations in, e.g., molecular electronics or quantum transport. Polarization effects are visible even for the excitation spectrum of the NaCl films themselves, which has important implications for the interpretation of surface science experiments for the characterization of insulator surfaces

New Perspective on Formation Energies and Energy Levels of Point Defects in Nonmetals

We propose a powerful scheme to accurately determine the formation energy and thermodynamic charge transition levels of point defects in nonmetals. Previously unknown correlations between defect properties and the valence-band width of the defect-free host material are identified allowing for a determination of the former via an accurate knowledge of the latter. These correlations are identified through a series of hybrid density-functional theory computations and an unbiased exploration of the parameter space that defines the Hyde-Scuseria-Ernzerhof family of hybrid functionals. The applicability of this paradigm is demonstrated for point defects in Si, Ge, ZnO, and ZrO2

Exciting prospects for solids: Exact-exchange based functionals meet quasiparticle energy calculations

Focussing on spectroscopic aspects of semiconductors and insulators we will illustrate how quasiparticle energy calculations in the G0W0 approximation can be successfully combined with density-functional theory calculations in the exact-exchange optimised e ective potential approach (OEPx) to achieve a first principles description of the electronic structure that overcomes the limitations of local or gradiant corrected DFT functionals (LDA and GGA)