Research Fons Brosens

Abstract

This project represents a research agreement between the UA and on the onther hand IWT. UA provides IWT research results mentioned in the title of the project under the conditions as stipulated in this contract.

Researcher(s)

• Promoter: Brosens Fons
• Fellow: Putteneers Katrijn

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

In this project we will investigate excitons in semiconductor nanocrystals, which allow for their employment as efficient photosensitizers of rare earth ions and active oxigen molecules for applications in optoelectronics and biomedicine. We will combine our unique experimental and theoretical expertise to unravel the process of selective exciton formation and energy transfer to molecules that interact with semiconductor nanocrystals.

Researcher(s)

• Promoter: Brosens Fons

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

The first aim of this project is to set up a microscopic quantum-kinetic theory for the dilute Bose gas that -is capable of making a distinction between the superfluid and dissipative dynamics ; -is valid at arbitrary temperatures ; -is in agreement with experimental observations; -is satisfying all necessary conservation laws and the second principle of thermodynamics.

Researcher(s)

• Promoter: Tempere Jacques
• Co-promoter: Brosens Fons

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

This project aims to measure inelastic interactions of fast electrons with plasmons in order to obtain phaserelations. It combines two well known techniques in electron microscopy: electron energy loss spectroscopy and electron holography. Traditional electron holography operates with elastically scattered electrons to obtain phase information of the exit wave near the object. In this project, we will use holography to obtain the phase relations in experiments with inelastic electrons. In this case, the reference wave traditionally used in holography needs to be replaces by a beam which underwent the same inelastic excitation in order to still have some coherence between reference wave and exit wave near the sample. In the theoretical part of this project we study the inelastic interaction of electrons with plasmon and how this can be linked to the experiments. We make use of the time dependent Hartree Fock aproach. This theory uses the equations of motion aproach of the general density matrix (essentially the Fourier transform of the MDFF). Special attention is put to the off-diagonal elements of the density matrix that contain information on correlation and coherence which can be closely linked to the experimental results.

Researcher(s)

• Promoter: Verbeeck Johan
• Co-promoter: Brosens Fons

Research team(s)

• Electron microscopy for materials research (EMAT)

Project type(s)

• Research Project

Abstract

Researcher(s)

• Promoter: Brosens Fons
• Fellow: Putteneers Katrijn

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Researcher(s)

• Promoter: Brosens Fons
• Fellow: Wouters Michiel

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Researcher(s)

• Promoter: Brosens Fons
• Fellow: Shanenko Arkady

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

This project is located in the area of nanoscience, i.e. the study of new physical phenomena emerging when the sample size is reduced below 100 nm.

Researcher(s)

• Promoter: Brosens Fons

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Researcher(s)

• Promoter: Brosens Fons
• Co-promoter: Fomin Vladimir

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

In this research project the confinement phenomena of the magnetic flux and of the superconducting condensate (order parameter ?) will be investigated. On one hand we will focus on the finite geometrical confinement in small individual superconducting islands with different shapes (disc, square, triangle, line), where the effects of confinement on ? and the interaction between a little amount of flux lines will be studied. On the other hand the flux confinement will be realized in systems with a lattice of controlled artificial pinning centers, such as holes (antidots) or magnetic dots. Theoretically as well as experimentally there have already been significant efforts focused on the optimization of the flux pinning on defects of different types and measurements. By systematically varying the measurements, the geometry, the type and the distribution of the pinning centers, the conditions for optimized pinning and critical parameters will be studied.

Researcher(s)

• Promoter: Brosens Fons
• Co-promoter: Peeters Francois

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

This project focuses on two specific and complementary confined many-body systems: atoms in magnetic traps and electrons in a multielectron bubble. These timely subjects are ideally suited to apply and develop the many-body methods introduced during my previous research and based on the expertise present in the laboratory TFVS where I propose to perform this project. Previous publications, establishing the methods to be used and exploring the systems to be studied testify to the realizabily of the proposed project. For both systems, collaborations with leading experimental groups will enrich and valorize the theoretical results.

Researcher(s)

• Promoter: Brosens Fons
• Fellow: Tempere Jacques

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

The spinless bipolaron will be investigated in a polar environment, where it should be treated in interaction with a bath of polarons. As a consequence, polaron densities exist in which the spinless bipolaron is stable in a polaron environment, whereas it would be unstable according to the standard criterion. In this project the following topics will be studied: (i) spatial correlations in the bipolaron-polaron stability criterion, (ii) thermodynamic properties of the polaron-bipolaron mixture, (iii) deviations in the bipolaron-polaron and polaron-polaron interaction from the Coulomb type of interaction.

Researcher(s)

• Promoter: Brosens Fons
• Fellow: Shanenko Arkady

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

The recent trends in research and the experiments to come in the domain of the confined atomic fermion gasses and electron bubbles will allow researchers in the near future to observe many-fermion systems in regimes and under physical conditions which up to now were not accessible. These new developments will certainly extend the frontiers of our knowledge of these systems. The aim of the present project is to develop a theoretical framework for these experiments, to predict the new phases and phenomena which will come within reach of the experimentalists, and to formulate the theory of many-electron systems for regimes and under conditions which hitherto were essentially left unconsidered.

Researcher(s)

• Promoter: Brosens Fons
• Co-promoter: Devreese Jozef
• Co-promoter: Tempere Jacques

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Within the TFVS a model was developed to describe analytically a system of N harmonically interacting identical particles (both fermions and bosons), confined in a parabolical potential, subject or not to an external magnetical field. In this project we will study with this theory the effect of the interaction between particles with different spin. This study will provide us with the zeroth order system that will be extended by variational and perturbative methods to describe more realistic systems like quantum dots, mesoscopic stuctures and superconducting clusters. In a first stage of the project we will study the thermodynamics and statistical correlation functions of a system consisting of unpolarised fermions. The second stage will aim at a further investigation of the interaction of the unpolarised fermions with a phonon bath. To arrive at a description of more realistic systems in a third fase we will take into acount the influence of non-parabolic confining potentials and of the Coulomb interaction between the particles which cannot be described in an analytical way. For this purpose we will use variational methods, in particular, the Jensen-Feynman inequality.

Researcher(s)

• Promoter: Brosens Fons
• Co-promoter: Devreese Jozef
• Fellow: Wouters Michiel

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Researcher(s)

• Promoter: Devreese Jozef
• Promoter: Peeters Francois
• Co-promoter: Brosens Fons
• Co-promoter: Peeters Francois

Research team(s)

• Condensed Matter Theory
• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Recently a path integral method was developed in the research group for Theoretical Solid State Physics at the UIA to describe analytically a system of identical particles in a parabolic confining well, subject to a harmonic interparticle interaction or a magnetic field [1]. Within the framework of the path integral formalism, an expression for the generating function of the partition function was derived, which enables to calculate the canonical partition function recursively for a system with quadratic two body interactions, even in the presence of a magnetic field [2]. This is a starting-point for the exact treatment of thermodynamical properties, such as the internal energy, the heat capacity, and correlation functions, e.g., density, pair correlation function. The goal of the present project is to examine the effect of an interaction between particles with spin degrees of freedom within this theory, which was developed both for bosons and fermions. We investigate the formation of coherent states and pairs, and the effects resulting from the finite amount of particles. This study provides a model that will serve as a trial system to examine more realistic systems like quantum dots, mesoscopic structures and superconducting clusters by perturbative and variational methods.

Researcher(s)

• Promoter: Brosens Fons
• Fellow: Hameeuw Kris

Research team(s)

Project type(s)

• Research Project

Abstract

We plan, first, to use the path-integral approach for the study of the electron energy spectra in metallic nanostructures for a fixed number of electrons with up- and down-spins. The role of the size of the structures and the geometry of confinement will be investigated. Second, we will derive the phonon spectra characteristic for those structures, using the methods, which have been used earlier in TFVS to investigate the optical spectra of quantum dots. Third, we will analyze the effect of the electron-phonon interaction on the electron correlations in metallic nanostructures, again on the basis of the path-integral treatment of fermi- and bose-systems. In particular, we will investigate the onset of superconductivity as a function of the number of electrons in metallic nanostructures. The surface-induced anisotropy of impurity spins in dilute magnetic alloys will be studied for mesoscopic grains of various shape, taking into account the roughness of their surfaces. Magnetization phenomena in polycrystalline mesoscopic samples of dilute magnetic alloys will be theoretically analyzed for different shape and size of the grains. We plan to study mesoscopic hybrid structures of the following three types: semiconductor/superconductor hybrid structures; superconductor/ferromagnet hybrid structures; dilute magnetic alloy/superconductor hybrid structures.

Researcher(s)

• Promoter: Devreese Jozef
• Co-promoter: Brosens Fons

Research team(s)

Project type(s)

• Research Project

Abstract

New types of periodic pinning arrays (PPA) will be designed and their properties will be investigated in order to study flux confinement phenomena by these structures, the enhancement of critical parameters they induce and the possibilities of using PPA in novel flux devices. New facilities will be used which further reduce the nanostructure length scale down to 30 nm. Furthermore, the integration of heavy ion irradiation through a special mask at the second nanostructuring stage will allow the formation of pinning centers of radius close to 10 nm. By combining these state of the art techniques with the knowledge and expertise accumulated in Flanders and China, an efficient design and analysis of these new structures will be carried out. The theoretical work executed by the group TFVS will allow for interpreting the experimental observations in real time. Using the Ginzburg-Landau formalism, the spatial vortex structure and the critical fields will be calculated for the superconducting structures with PPA.

Researcher(s)

• Promoter: Brosens Fons
• Promoter: Devreese Jozef

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

The project aims at the theoretical study of the dynamics of electrons, including exchange and correlation interactions. The frequency and wave vector dependent dielectric function is calculated, and applied on many-body problems in various materials, a.o. in metals, semiconductors and heterojunctions.

Researcher(s)

• Promoter: Brosens Fons
• Fellow: Brosens Fons

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Theoretical study of the transport properties of mesoscopic systems and artificial nanostructures consisting of semiconductors and superconductors. Central in this study is that those systems consist of a finite number of degrees of freedom.

Researcher(s)

• Promoter: Peeters Francois
• Co-promoter: Brosens Fons
• Co-promoter: Devreese Jozef
• Co-promoter: Lemmens Lucien

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

In this research project the confinement phenomena of the magnetic flux and of the superconducting condensate (order parameter ?) will be investigated. On one hand we will focus on the finite geometrical confinement in small individual superconducting islands with different shapes (disc, square, triangle, line), where the effects of confinement on ? and the interaction between a little amount of flux lines will be studied. On the other hand the flux confinement will be realized in systems with a lattice of controlled artificial pinning centers, such as holes (antidots) or magnetic dots. Theoretically as well as experimentally there have already been significant efforts focused on the optimization of the flux pinning on defects of different types and measurements. By systematically varying the measurements, the geometry, the type and the distribution of the pinning centers, the conditions for optimized pinning and critical parameters will be studied.

Researcher(s)

• Promoter: Brosens Fons
• Promoter: Devreese Jozef
• Co-promoter: Peeters Francois

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Recently a path integral method was developed in the research group for Theoretical Solid State Physics at the UIA to describe analytically a system of identical particles in a parabolic confining well, subject to a harmonic interparticle interaction or a magnetic field [1]. Within the framework of the path integral formalism, an expression for the generating function of the partition function was derived, which enables to calculate the canonical partition function recursively for a system with quadratic two body interactions, even in the presence of a magnetic field [2]. This is a starting-point for the exact treatment of thermodynamical properties, such as the internal energy, the heat capacity, and correlation functions, e.g., density, pair correlation function. The goal of the present project is to examine the effect of an interaction between particles with spin degrees of freedom within this theory, which was developed both for bosons and fermions. We investigate the formation of coherent states and pairs, and the effects resulting from the finite amount of particles. This study provides a model that will serve as a trial system to examine more realistic systems like quantum dots, mesoscopic structures and superconducting clusters by perturbative and variational methods.

Researcher(s)

• Promoter: Brosens Fons
• Co-promoter: Devreese Jozef
• Fellow: Hameeuw Kris

Research team(s)

Project type(s)

• Research Project

Abstract

In order to investigate the effects of the fermion or boson statistics on the thermodynamics of systems with a finite number of interacting identical particles, a harmonic model was recently introduced which is quantum mechanically exactly soluble with path integral techniques. This model provides the trial action for a variational or perturbative treatment of realistic systems, and was tested extensively by comparison with experimantal results in Bose-Einstein condensates. In the present project the model will be extended to take also the electron-phonon interaction into account, resulting in a trial action for the treatment of a gas of electrons in a phonon bath. This model will provide a generalization to a polaron gas of Feynman's well-known polaron treatment.

Researcher(s)

• Promoter: Brosens Fons
• Fellow: Saeys Dirk

Research team(s)

Project type(s)

• Research Project

Abstract

This project is located in the area of nanoscience, i.e. the study of new physical phenomena emerging when the sample size is reduced below 100 nm. Nanostructures will be prepared by evaporation and cluster deposition techniques and will be characterized by e.g. scanning probe microscopy. The physical analysis will focus on metallic clusters, spin dependent transport and magnetic properties, optical properties, 2DEGs and quantum dots and theoretical modelling.

Researcher(s)

• Promoter: Brosens Fons
• Promoter: Devreese Jozef

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

I. Confinement-effects in superconducting nanostructures: a theory will be developed which allows the analysis of the folloing superconducting structures: isolated loops and arrays of antidota of various geometry (square, circular...); loops with attached contact wires and with different geometry... II. Kondo-effect in nanostructures: we will analyse the influence on the magnetic susceptibility of the surface-induced magnetic anisotropy and of the shape of the structure. III. Quantization-effects in quantum dots: we aim at the study of non-adiabaticity in the optical absorption, photoluminescence and photoluminescence excitation for nanocrystals with strong electron-phonon interaction and for nanocrystals embedded in strongly polar matrices.

Researcher(s)

• Promoter: Devreese Jozef
• Co-promoter: Brosens Fons

Research team(s)

Project type(s)

• Research Project

Abstract

We developed a new method for including the statistics of identical particles into the Feynman-Kac functional. By restricting the paths in imaginary time to the state space a subdomain of the configuration space, with appropriate boundary conditions (absorption for fermions, reflection for bosons) the propagator was shown to remain positive. Therefore it does not suffer form the sign problem for fermions which hinders the accurate numerical treatment of many-electrons systems.

Researcher(s)

• Promoter: Brosens Fons

Research team(s)

Project type(s)

• Research Project

Abstract

Researcher(s)

• Promoter: Brosens Fons
• Fellow: Foulon Sven

Research team(s)

Project type(s)

• Research Project

Abstract

For distinguishable particles it is well known that Brownian motion and a Feynman-Kac functional can be used to calculate the path integral for a general class of scalar potentials. This method is generalised to treat identical particles (fermions as well as bosons) with numerical and analytical techniques.

Researcher(s)

• Promoter: Brosens Fons
• Co-promoter: Devreese Jozef

Research team(s)

Project type(s)

• Research Project

Abstract

Researcher(s)

• Promoter: Brosens Fons

Research team(s)

Project type(s)

• Research Project

Abstract

A description of the electronic properties in systems of reduced dimension and dimensionality is proposed: an optical study of the dielectric response in ul tra-thin metal films; flux-dynamics and multi-polaron stability in 2-dimensional superconductors; quantisa tion effects in quantum wells (in the normal and in the superconducting phase).

Researcher(s)

• Promoter: Devreese Jozef
• Co-promoter: Brosens Fons

Research team(s)

Project type(s)

• Research Project

Abstract

The project aims at the theoretical description of the electronic properties of the zero-dimensional electron gas in quantum dots, and of the two-dimensional electron gas in metallic films and superlattices, and in the CuO2 layers of the high-temperature superconductors.

Researcher(s)

• Promoter: Devreese Jozef
• Co-promoter: Brosens Fons
• Co-promoter: Peeters Francois

Research team(s)

Project type(s)

• Research Project

Abstract

On the basis of Kac's formulation of the Feynman path-integral, we developed a new numerical technique to take the exchange interaction into account in a random walk process. The method will be applied to calculate the groundstate of the electron gas.

Researcher(s)

• Promoter: Brosens Fons

Research team(s)

Project type(s)

• Research Project

Abstract

The path-integral formulation allows to exactly eliminate the phonons in the description of the polaron, but the resulting path-integral for the electron (with retarded interaction) is not analytically tractable. To solve this path-integral numerically, the Quantum-Monte-Carlo procedure will be fine-tuned for vector and parallel execution.

Researcher(s)

• Promoter: Brosens Fons

Research team(s)

Project type(s)

• Research Project

Abstract

The project aims at the theoretical description of the electronic properties of the zero-dimensional electron gas in quantum dots, and of the two-dimensional electron gas in metallic films and super lattices, and in the CuO2 layers of the high-temperature superconductors

Researcher(s)

• Promoter: Devreese Jozef
• Co-promoter: Brosens Fons
• Co-promoter: Peeters Francois

Research team(s)

Project type(s)

• Research Project

Abstract

The project aims at the theoretical study of the dynamics of electrons, including exchange and correlation interactions. The frequency and wave vector dependent dielectric function is calculated, and applied on many-body problems in various materials, a.o. in metals, semiconductors and heterojunctions.

Researcher(s)

• Promoter: Brosens Fons
• Fellow: Brosens Fons

Research team(s)

Project type(s)

• Research Project