Morten Willatzen

Bio: Morten Willatzen is an academic researcher from Chinese Academy of Sciences. The author has contributed to research in topics: Quantum dot & Boundary value problem. The author has an hindex of 32, co-authored 268 publications receiving 4349 citations. Previous affiliations of Morten Willatzen include Center for Excellence in Education & Technical University of Denmark.

Electronic properties of semiconductor nanowires.

Abstract: This paper provides a review of the state-of-the-art electronic-structure calculations of semiconductor nanowires. Results obtained using empirical k.p, empirical tight-binding, semi-empirical pseudopotential, and with ab initio methods are compared. For conciseness, we will restrict our detailed discussions to free-standing plain and modulated nanowires. Connections to relevant experimental data, particularly band gaps and polarization anisotropy, will be made since these results depend crucially on the electronic properties. For completeness, a brief review on the synthesis of nanowires is included.

The k p Method: Electronic Properties of Semiconductors

Abstract: I Homogeneous Crystals.- One-Band Model.- Perturbation Theory#x2014 Valence Band.- Perturbation Theory #x2013 Kane Models.- Method of Invariants.- Spin Splitting.- Strain.- II Nonperiodic Problem.- Shallow Impurity States.- Magnetic Effects.- Electric Field.- Excitons.- Heterostructures: Basic Formalism.- Heterostructures: Further Topics.- Conclusion.

Nonlinear gain suppression in semiconductor lasers due to carrier heating

Abstract: A simple model is presented for carrier heating in semiconductor lasers from which the temperature dynamics of the electron and hole distributions can be calculated. Analytical expressions for two new contributions to the nonlinear gain coefficient, in are derived, which reflect carrier heating due to stimulated emission and free carrier absorption. In typical cases, carrier heating and spectral holeburning are found to give comparable contributions to nonlinear gain suppression. The results are in good agreement with recent measurements on InGaAsP laser diodes. >

A general dynamic simulation model for evaporators and condensers in refrigeration. Part I: moving-boundary formulation of two-phase flows with heat exchange

Abstract: A mathematical model describing the transient phenomena of two-phase flow heat exchangers is presented. Based on the one-dimensional partial-differential equations representing mass and energy conservation (leaving out momentum equations assuming pressure drops to be negligible) and a tube-wall energy equation, a simpler set of ordinary-differential equations is formulated by integrating separately over the three zones (liquid, two-phase, and vapor) generally present in a heat exchanger. Special attention is given to the study of large transients induced by on/off control of vapor-compression cooling systems.

Bandstructures of conical quantum dots with wetting layers

Abstract: The influence of wetting-layer states on quantum-dot states and vice versa is analysed numerically for electrons in the conduction band in the general case with arbitrary kinetic energy in the plane of the quantum-well wetting layer. Since the analysed quantum dot is embedded in a barrier material with different properties, the effective mass approximation methodology leads to a Schrodinger model with discontinuous coefficients. This complicates the analysis and, in addition, requires a special attention to the formulation of boundary conditions for the entire structure, consisting of the quantum dot with wetting layer embedded in a barrier material. In the present paper, the complete model is formulated and solved numerically via a variational approach based on finite element approximations and Arnoldi iterations. By analysing different geometrical configurations, we demonstrate that the ground eigenstate of the entire structure can be considerably affected by the presence of the wetting layer. The dependency demonstrated between eigenstates of the 'pure' quantum dots and the quantum-well wetting layers indicates that a conventional analysis of quantum-dot structures without accounting for wetting layers may not be sufficient for an adequate characterization of quantum dots as active regions in future electronic and optical devices.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Stochastic Processes in Physics and Chemistry

Abstract: N G van Kampen 1981 Amsterdam: North-Holland xiv + 419 pp price Dfl 180 This is a book which, at a lower price, could be expected to become an essential part of the library of every physical scientist concerned with problems involving fluctuations and stochastic processes, as well as those who just enjoy a beautifully written book. It provides an extensive graduate-level introduction which is clear, cautious, interesting and readable.

Stochastic Differential Equations

Abstract: We explore in this chapter questions of existence and uniqueness for solutions to stochastic differential equations and offer a study of their properties. This endeavor is really a study of diffusion processes. Loosely speaking, the term diffusion is attributed to a Markov process which has continuous sample paths and can be characterized in terms of its infinitesimal generator.