## Ongoing projects

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Few meets many: quantum droplets stabilized by three-body interactions and polaronic probing in mixtures.

#### Abstract

The three-body problem is notoriously hard to solve, as explained to a general audiance in Liu Cixin's novels. In condensed matter physics, this problem is exacerbated by the fact that the interaction energy of a three-particle system cannot be decomposed in a sum of pairwise interactions. The part that cannot be reduced is called the three-body interaction, and is typically neglected. However, in extreme cases, the three-body interactions can become dominant over pairwise terms. In ultracold atomic gases, these interactions can stabilize superfluid droplets, which would otherwise collapse. So far, these droplets have remained elusive, but the overall goal of this project is to calculate conditions under which they could be observed, and to work together with experimentalists to finally create this system so crucial to our understanding of three-body physics. Doing so requires a unique combination of expertise in many-body physics and few-body physics, that is realized by combining my skills in three-body coupled channel calculations with my host's expertise in many-body physics. We will go beyond deriving the formation criteria for these droplets, and also propose a way to characterize their properties by using an impurity as a probe in a variety of media and mixtures of quantum fluids. The theoretical analysis and creation of the three-body interaction stabilized droplets will have an important impact on the understanding of a broad class of quantum many-body systems.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Ahmed-Braun Denise

#### Research team(s)

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- Research Project

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Ab-initio study of anharmonicity on electron-phonon coupling and polarons.

#### Abstract

The large polaron, an electron interacting with a continuum of lattice phonons, is one of the most fundamental problems of many-body physics. Most of the Hamiltonians available in the literature assume a linear electron-phonon coupling. However, this assumption is invalid in several topical materials, such as SrTiO3, PbTe, H3S, or halide perovskites. The goal of this project is to provide realistic yet analytical expressions for additional anharmonic interaction terms in the large polaron Hamiltonian, such as the 1-electron-2-phonon interaction. A derivation is proposed that allows us to write the desired Hamiltonian in terms of several unknown material parameters. A scheme is proposed to calculate these parameters using DFT. Additionally, the anharmonic polaron ground state energy, effective mass, and optical absorption spectrum will be calculated for the abovementioned anharmonic materials. The project is in a unique position between the theoretical study of model polaron Hamiltonians and the computational ab initio treatments of the polaron, which have traditionally been investigated separately in the literature. In order to successfully complete the computational part of the project, the researcher will be trained in first principles and diagrammatic monte carlo methods by the co-promotor. After the project the researcher will have a diverse and unique research profile, which has a theoretical and computational component just like the proposed project.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Houtput Matthew

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- Research Project

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Kardar-Parisi-Zhang scaling in Bose-Einstein condensates of photons.

#### Abstract

Universality means that very different physical systems have in some respects identical properties. One of the most celebrated manifestations of universality is for second order phase transitions, resulting in the same behavior of fluctuations in systems as diverse as magnets, superfluids , liquids at the critical point. These examples all share that they occur at thermal equilibrium. Systems that lack thermal equilibrium however are also ubiquitous, including living organisms. Universality turns out to be present in certain nonequilibrium systems as well, with the Kardar-Parisi-Zhang equation capturing the behavior of a surprisingly large set of them, comprising the interfaces of growing crystals, the shape of fire fronts and the fluctuation properties of lasers. The present project is devoted to the latter type of systems. We will address lasers that contain dye molecules that drive the system toward thermal equilibrium. Their nonequilibrium nature on the other hand stems from photon losses that are compensated by an excitation laser. Their deviation from thermal equilibrium being experimentally tunable, these systems are expected to be well suited for the investigation of the role of thermalization on the scaling of the fluctuations in optical systems.#### Researcher(s)

- Promoter: Wouters Michiel

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- Research Project

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Creating highly entangled quantum states in the NISQ era (CHEQS).

#### Abstract

The exponentially growing complexity of large entangled quantum systems is a curse for understanding them but also a blessing as it opens up formidable opportunities for creating novel technologies. The field of quantum sciences explores ways of exploiting quantum effects to create quantum devices that are much more powerful than their classical counterparts. The ensuing second quantum revolution is currently in full motion, and novel quantum devices are being conceived in the context of computation, telecommunication, sensing and cryptography. Major challenges for fundamental physics still remain, several of which will be addressed in this proposal that combines the knowledge and interests of six world-leading Belgian groups working in the field of quantum sciences. The central focus is on creating entanglement – the core fuel that drives all quantum power – in state of the art quantum many-body platforms and in quantum sensors. We will construct novel protocols for preparing, certifying and validating the precious entanglement and implement those ideas in state of the art experiments. As such, we will make Belgium ready for the new era of quantum technologies.#### Researcher(s)

- Promoter: Tempere Jacques
- Co-promoter: Wouters Michiel

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- Research Project

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Modeling and simulation with applications in finance, insurance and economics.

#### Abstract

The scientific target of this project is to develop, analyse and implement numerical methods for dealing with highly sophisticated mathematical models in finance and insurance, e.g., jump-diffusion models, free boundary problems, swing contracts, and high-dimensional systems. In particular, stochastic models will be implemented to tackle valuation, and network modelling will be developed to study systemic risk. This will also allow us to study the emergence of economic behaviour and opinions in local populations by using data-driven models of social networks, social influence, and opinion dynamics.#### Researcher(s)

- Promoter: Tempere Jacques

#### Research team(s)

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- Research Project

## Past projects

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Many-polaron effects in a Bose-Einstein condensate.

#### Abstract

A Bose-Einstein condensate (BEC) can be thought of as a gas of atoms which undergoes a transition into a specific phase at very low temperatures. In this new phase the atomic gas exhibits various peculiar properties such as superfluidity, quantized vortices and many other phenomena not expected in normal gases. One such interesting problem is that of an impurity (usually an atom of a different species) moving through a BEC. This impurity will disturb the gas around it and create a dip of lower density which it will have to drag along. This will modify the properties of the impurity and for example change the effective mass, analogous to a person having more trouble walking on a trampoline and dragging along the deformation in the fabric. Such an impurity together with the dip in density as a whole is called a Bose-polaron. In 2016 two experiments first realized condensates that contained many Bose-polarons and gave rise to an active discussion in the theoretical community. It has been shown that for an accurate theoretical description of the polaron additional correction terms had to be taken into account which were not present in previous discussions. This has been recently done for a description of single Bose-polarons. In this research these correction terms will be included to describe a system of many polarons which in combination has not been done before. The results found here will also be extended to other atomic gases called ultracold fermionic gases.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Ichmoukhamedov Timour

#### Research team(s)

#### Project type(s)

- Research Project

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Elementary excitations in Fermi systems: From cold atoms to condensed matter.

#### Abstract

When cooling down a system made of many interacting particles, one encounters new states of matter that display unique properties. Superconductors are metals whose resistivity suddenly drops at low temperature; similarly, superfluids are liquids or gases whose viscosity is much lower than in normal fluids. These phenomena rely on the same physical mechanism: at low temperature, the particles, instead of being distinguishable little balls, gather in a condensate, a quantum wave as big as the whole system. In systems made of fermionic particles (that is particles that cannot occupy the same quantum state, which include electrons, neutrons, protons and many atom nuclei), condensation may occur only if the particles first pair up. This kind of pair condensates are found in many different fields of physics, from solid-state physics with superconductors, to astronomy with neutron stars, and their study is equally crucial to the understanding of the fundamental laws of physics, and to the development of new technologies. I will try to better understand how these fermionic condensates in superconductors react to external perturbations, for instance laser pulses, with a special focus on the case when the perturbation has enough energy to break the pairs. Understanding this behavior will help predict the properties of the system as a function of temperature, and may help explain why some materials maintain their superconducting properties at higher temperatures than others.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Kurkjian Hadrien

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- Research Project

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Polaronic effects in superfluid Fermi gases.

#### Abstract

When a particle is placed in a medium consisting of other quantum particles, the interaction with this medium will lead to new effective properties for this particle. This was systematically studied for an electron in an ionic or polar lattice: the electron charge distorts the lattice, and the electron together with the lattice deformation is a new, heavier composite object called a polaron. This "polaronic effect" turns out to be ubiquitous in physical systems. Its most recent realization is the dressing of impurity atoms in a quantum gas of atoms cooled down to the nanokelvin regime where the gas turns into a superfluid, i.e. a state of matter exhibiting frictionless flow. The quantum gas embodiment of the polaron problem is particularly useful to study polaron physics since quantum gases are tunable and controllable to a high degree of precision by experimentalists. Hence, polarons can be brought into regimes hitherto inaccessible, where many-body theory can be tested in unprecedented ways. In that respect, superfluid Fermi gases show even more promise than superfluid Bose gases, as they possess a much richer and more tunable spectrum of elementary excitations that can dress the impurity atom. In this project, we will provide an in-depth study of the polaronic effects in these superfluid Fermi gases.This will lead not only to a better understanding of polaron physics, but also to new insights on the formation of superfluid Cooper pairs of fermionic atoms.#### Researcher(s)

- Promoter: Tempere Jacques

#### Research team(s)

#### Project type(s)

- Research Project

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Ab-initio calculations for anharmonic polarons in hydrides.

#### Abstract

Hydrogen-rich materials or "hydrides" at high pressure reveal a host of interesting properties, among which record high critical temperatures for superconductivity. This recent discovery has put high-pressure hydrides in the spotlight. In this project, we focus on an aspect that makes these materials special: their very large phonon anharmonicity. Phonons are quantized lattice vibrations of the atoms in the crystal. When at atom is displaced out of its equilibrium position, it feels a restoring force that is usually approximated by a spring pulling it back to its lattice position. For hydrides, the force is no longer spring-like, but more complicated, and this is referred to as phonon anharmonicity. The electrons feel the lattice vibrations, and form an effective composite quasiparticle called a polaron, consisting of the electron taken together with the lattice deformation it induces. We combine the expertise of the Flemish partner, polaron physics, with that of the Austrian partner, first-principles calculation of phonons and electron-phonon interaction strength, to take into account phonon anharmonicity in the description of polarons in hydrides. This will lead to a better understanding of the normal state electronic and optical properties of the interesting class of materials that are the hydrides.#### Researcher(s)

- Promoter: Tempere Jacques

#### Research team(s)

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- Research Project

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Post-quench prethermalization and thermalization dynamics in Bose gases: extension of the hierarchy of correlations method to the strongly interacting regime, multicomponent systems and finite temperature.

#### Abstract

When a gas of atoms is cooled close to absolute zero, it undergoes a transition to a Bose-Einstein condensate, a quantum mechanical state of matter characterized by frictionless flow or "superfluidity". In this project, we investigate what happens to such a superfluid when a parameter such as the interatomic interaction strength is suddenly changed or "quenched". In particular, the project focuses on how the Bose-Einstein condensate evolves towards the new equilibrium state. Several experimental observations, such as the existence of a prethermal steady state and universal dynamics, pose theoretical challenges that we plan to resolve by taking into account correlations between more than two atoms in our model. The behavior of strongly interacting ultracold atom gases is furthermore archetypical of a broad range of quantum many body systems ranging from neutron stars to superconductors. The research topic thus has many applications, and moreover touches on fundamental questions regarding the role of thermal equilibrium in quantum systems.#### Researcher(s)

- Promoter: Wouters Michiel
- Co-promoter: Tempere Jacques
- Fellow: Fernandes Lennart

#### Research team(s)

#### Project type(s)

- Research Project

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Bose-Einstein condensation of ultracold atoms out of equilibrium.

#### Abstract

Superfluids form a phase of matter, distinct from the gaseous, fluid and solid states, whose most remarkable characteristic is a vanishing viscosity. This absence of friction is a consequence of the fact that all particles move together, in analogy to the photons that come out of a laser. Lasers and superfluids share the coherence of the particles (atoms and photons respectively), but an important difference between them is that the former are driven (e.g. by an electrical current), where the latter are in thermal equilibrium. The need for the driving of the laser is a direct consequence of its usefulness as a source of coherent photons. Recently, several research activities have developed to bridge the differences between the various forms of coherent matter. From the photonic side, experiments have been performed where the photons come very close to thermal equilibrium by working with efficient thermalization mechanisms and long photon life times. From the superfluid atomic side, experiments have been performed where atom losses were induced by an electron beam, that are replenished by a nearby atomic cloud. The aim of this project is to construct theoretical descriptions of atomic superfluids that are driven away from thermal equilibrium by particle losses. Based on previous studies of photonic systems, we expect that the phase transition between the normal and coherent phases as well as their vortex properties will be modified by the atom losses.#### Researcher(s)

- Promoter: Wouters Michiel

#### Research team(s)

#### Project type(s)

- Research Project

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Many-polaron effects in a Bose-Einstein condensate.

#### Abstract

A Bose-Einstein condensate (BEC) can be thought of as a gas of atoms which undergoes a transition into a specific phase at very low temperatures. In this new phase the atomic gas exhibits various peculiar properties such as superfluidity, quantized vortices and many other phenomena not expected in normal gases. One such interesting problem is that of an impurity (usually an atom of a different species) moving through a BEC. This impurity will disturb the gas around it and create a dip of lower density which it will have to drag along. This will modify the properties of the impurity and for example change the effective mass, analogous to a person having more trouble walking on a trampoline and dragging along the deformation in the fabric. Such an impurity together with the dip in density as a whole is called a Bose-polaron. In 2016 two experiments first realized condensates that contained many Bose-polarons and gave rise to an active discussion in the theoretical community. It has been shown that for an accurate theoretical description of the polaron additional correction terms had to be taken into account which were not present in previous discussions. This has been recently done for a description of single Bose-polarons. In this research these correction terms will be included to describe a system of many polarons which in combination has not been done before. The results found here will also be extended to other atomic gases called ultracold fermionic gases.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Ichmoukhamedov Timour

#### Research team(s)

#### Project type(s)

- Research Project

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Dynamics and decay of solitons and solitonic vortices in superfluid Fermi gases.

#### Abstract

Ultracold quantum gases consist of a collection of magnetically trapped atoms cooled down to nanokelvin temperatures. At these ultralow temperatures, the laws of quantum mechanics, which are usually confined to the microscopic world of atoms and particles, now become apparent on the scale of the entire macroscopic cloud. This leads to remarkable behavior, such as flow without friction or "superfluidity". Superfluids are characterized by a complex order parameter. Phase defects in this order parameter are known as solitonic excitations, such as solitons and vortices. The former are localized density dips that propagate without changing their shape, with classical counterparts in water canals and optical fibers. The latter are quantized "whirlpools". Here, we will study these solitonic excitations in a Fermi quantum fluid by making use of a finite-temperature effective field theory that we developed specifically for these systems. Solitons in Fermi superfluids are experimentally seen to decay into vortices. We will model this decay and propose ways to stabilize solitons. We will also investigate collisions between solitonic excitations, and their spontaneous appearance when a gas is cooled rapidly. Finally, we will investigate how disorder and mixing of superfluids influence the properties of the solitonic excitations.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Van Alphen Wout

#### Research team(s)

#### Project type(s)

- Research Project

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Modelling of thermo-optical properties of hydrogen at extreme pressures.

#### Abstract

Hydrogen is the simplest element in the universe. When it is at room temperature and atmospheric pressure, hydrogen takes the form of a gas. One can cool or pressurize this gas to turn it into a solid. Under these conditions, the solid hydrogen is an electrical insulator. However, nearly a century ago, it was predicted that putting a pressure of a quarter of a million times atmospheric pressure on solid hydrogen would turn it into a metal. This material was called metallic hydrogen, and physicists have been trying to create it ever since it was predicted to exist. Theoretical predictions also indicate that metallic hydrogen is a room-temperature superconductor, meaning that it can transport electricity without losses. Additionally, it would be a very powerful rocket fuel, and it would remain metallic even when the pressure is taken off. Recently, experiments by the Silvera research group at Harvard University indicate the first creation of metallic hydrogen in the lab. However, other research groups do not agree with this claim. In the proposed research, we attempt to theoretically model the experiment used by the Silvera research group to get a correct interpretation of their results. Furthermore, we will use this model and the experimental results to estimate material parameters of metallic hydrogen. Finally, we will theoretically develop an experiment that can measure whether the metallic hydrogen in the experiments is superconducting, as predicted by theory#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Houtput Matthew

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- Research Project

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Variational quantum trajectory description of driven-dissipative systems.

#### Abstract

Variational principles are fundamental in our theoretical understanding of closed quantum systems at thermal equilibrium. For open, driven-dissipative systems, variational techniques are much less established. Classical examples of driven dissipative systems range from convection cells in hydrodynamics to electrical patterns in the heart. In recent years, progress in fabrication of electromagnetic resonators coupled to matter degrees of freedom, has spurred the theoretical interest in driven-dissipative quantum systems. An important motivation for this research is the possibility of realizing correlated quantum states with potential applications in quantum computing and quantum simulation. For the theoretical simulation of driven-dissipative quantum systems, two equivalent approaches exist: a master equation for the density matrix and a quantum trajectory equation for wave functions. These two techniques relate to each other as the diffusion equation to the Langevin equation in the theory of Brownian motion. A practical advantage of the quantum trajectory method for numerical purposes is that it works on the Hilbert space of wave functions instead of the quadratically larger Hilbert space of density matrices. A conceptual bonus is that it 'unravels' distinct macroscopic superpositions of the Schrödinger cat type and gives insight in the emergence of a classical configurations out of an entangled quantum state. In the present project, we will investigate variational approximations to the quantum trajectory dynamics. The advantage of applying the variational principle to the trajectory dynamics instead of the density matrix itself is that the unraveled states are expected to be more amenable to such a description. This expectation is borne out by a preliminary study with the Gutzwiller approximation to a photonic dimer. Encouraged by this success, we will set out to investigate various variational approximations to the quantum trajectory description of driven-dissipative quantum systems. One of the advantages of such a description is that it can be carried out even for large systems and in more than one dimension, where other numerical techniques become impractical. Access to large systems is in particular important for the descriptions of phase transitions, that only become sharp in the thermodynamic limit. The most important goal of this research project is to provide a new theoretical tool for the simulation of driven-dissipative quantum systems. We envisage to apply this technique to further our understanding of phase transitions, which can lead to new fundamental insights regarding the differences and similarities of driven-dissipative systems with respect to closed systems in thermal equilibrium.#### Researcher(s)

- Promoter: Wouters Michiel
- Fellow: Huybrechts Dolf

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Non-adiabatic dynamics of interacting quantum many-body systems

#### Abstract

Whether it's the heat produced in a car engine or the decoherence of a qubit, all losses stem from our lack of control on the microscopic degrees of freedom of the system. Since the early-days of thermodynamics, the adiabatic process has emerged as a universal way to minimize losses, leading to the concept of Carnot efficiency -- the cornerstone of modern thermodynamics. In spite of its conceptual importance, practical implications of the Carnot efficiency are limited since the maximal efficiency goes hand in hand with zero power. Similar issues appear in a seemingly different topic of adiabatic quantum computation, simulation and state preparation. Here the idea is to prepare an interesting state out of a state that is easy to prepare by slow switching a control parameter, in the hope not to excite the system. As in engines, this algorithm works in the adiabatic limit if the ramping rate is infinitesimally slow but leads to heating at finite ramping rates relevant for actual experiments. In this project we will investigate non-equilibrium phenomena in many-body quantum systems. Specifically, we intend to engineer non-adiabatic protocols that lead to the same result as the fully adiabatic protocol, albeit in finite time. For this purpose we will primarily build on the idea of transitionless driving. To deal with many-body quantum systems we will combine it with cutting edge numerical methods such as t-DMRG, phase-space methods and machine learning techniques.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Sels Dries

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- Research Project

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Unconventional atomic Fermi superfluids.

#### Abstract

In a superfluid, all particles, instead of being discernable little balls, gather in a single wave whose behaviour is very different from that of a normal fluid. When a superfluid rotates, swirls of quantized size form and arrange themselves in a lattice structure; sound propagates in a superfluid by twisting the macroscopic wave, rather than by compressing the fluid. We will study swirls and sound waves in two new kinds of superfluids recently realized in the lab using gases of atoms cooled down to nearly absolute zero and attracting each other with a strengh that can be magnetically adjusted. In a gas where 2 species of atoms are mixed in unequal proportions, the atoms of the minority form pairs with some atoms of the majority; the remaining unpaired atoms cause defects in the superfluid that self-arrange in an intriguing spatial pattern. We want to see how this pattern of defects reacts to swirls and sound waves. In a gas containing 3 species, threebody objects called trimers form. The nature of swirls and sound waves in this system is an entirely open problem, which would help understand how the collective behaviour of a fluid changes when its constituants interact 3-by-3, not just 2-by-2. Our study will answer fundamental questions relevant for many physical objects: cold gases, liquid Helium, superconductors and quarks. Meanwhile, it contributes to a better understanding of superfluids, objects nowadays widely used in the most advanced quantum technologies. -#### Researcher(s)

- Promoter: Tempere Jacques

#### Research team(s)

#### Project type(s)

- Research Project

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Dynamics and decay of solitons and solitonic vortices in superfluid Fermi gases.

#### Abstract

Ultracold quantum gases consist of a collection of magnetically trapped atoms cooled down to nanokelvin temperatures. At these ultralow temperatures, the laws of quantum mechanics, which are usually confined to the microscopic world of atoms and particles, now become apparent on the scale of the entire macroscopic cloud. This leads to remarkable behavior, such as flow without friction or "superfluidity". Superfluids are characterized by a complex order parameter. Phase defects in this order parameter are known as solitonic excitations, such as solitons and vortices. The former are localized density dips that propagate without changing their shape, with classical counterparts in water canals and optical fibers. The latter are quantized "whirlpools". Here, we will study these solitonic excitations in a Fermi quantum fluid by making use of a finite-temperature effective field theory that we developed specifically for these systems. Solitons in Fermi superfluids are experimentally seen to decay into vortices. We will model this decay and propose ways to stabilize solitons. We will also investigate collisions between solitonic excitations, and their spontaneous appearance when a gas is cooled rapidly. Finally, we will investigate how disorder and mixing of superfluids influence the properties of the solitonic excitations.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Van Alphen Wout

#### Research team(s)

#### Project type(s)

- Research Project

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Analog Gravity Models with Microcavity Polaritons.

#### Abstract

The main objective of this project, and moreover the central idea, is to bring complex and difficult to comprehend systems within the reach of realistic quantum fluid experiments. Since phenomena related to cosmology are often extremely difficult, if not impossible, to study experimentally, it can be very stimulating to translate some of the problems to realistic analog experiments. The specific quantum fluid that we will consider is a quantum degenerate Bosonic gas of exciton-polaritons in semiconductor microcavities.#### Researcher(s)

- Promoter: Wouters Michiel
- Fellow: Van Regemortel Mathias

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Polarons in oxides: hamiltonian description and ab-initio treatment

#### Abstract

Polarons are quasiparticles that originate from the coupling between charge carriers and the lattice phonon field. They are of fundamental importance for many practical applications involving charge transfer, conduction and optical excitation. The strength of the electron-phonon coupling determines shape, type, size and characteristics of the polaron: in the short-range strong-coupling regime the polaron size is similar to the lattice constant, when the coupling decreases, the polarons becomes large and a continuum approximation is possible. Although the theoretical basis of polaron physics has been established in the 1950s, a unified treatment of small and large polarons at the same level of theory is still missing. The continuum many-body Hamiltonian description of Frohlich works for large polarons, whereas the basic features of small polarons are better captured by ab-initio calculations. The first goal of this project is to reformulate, design and test an ab-initio and model Hamiltonian approach that will allow a unified theoretical description of small and large polarons within the same theoretical framework. The second objective is to apply our new framework to realistic problems in materials science, specifically, to transition metal oxides.#### Researcher(s)

- Promoter: Tempere Jacques

#### Research team(s)

#### Project website

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Dynamics and decay of solitons and solitonic vortices in superfluid Fermi gases.

#### Abstract

Ultracold quantum gases consist of a collection of magnetically trapped atoms cooled down to nanokelvin temperatures. At these record low temperatures, the laws of quantum mechanics, usually confined to the microscopic world of single atoms and particles, now dictate the behavior of the entire macroscopic cloud. This leads to remarkable behavior, such as flow without friction or "superfluidity". The main aim of this project is to investigate theoretically how solitary waves travel through a superfluid. These solitary waves, or "solitons", have classical counterparts in water canals and optical fibers, where they propagate without changing their shape. Here, we will study the solitons in a quantum fluid, where they are experimentally seen to decay into quantized whirlpools or "solitonic vortices". For this purpose, we will use and expand a finite-temperature effective field theory developed specifically for superfluid quantum gases. With it, we will model soliton decay and propose ways to stabilize solitons. We will also investigate collisions between solitons, and the interaction between solitons and vortices. We focus on fermionic superfluids, where the atoms form Cooper pairs that in turn lead to superfluidity. Two important effects frustrate this pairing: the presence of an imbalance in the populations of the pairing atoms, and the presence of disorder potentials. We will investigate how solitons behave in superfluids where these two effects hamper the superfluidity.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Van Alphen Wout

#### Research team(s)

#### Project website

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Superfluidity and superconductivity in multicomponent quantum condensates.

#### Abstract

Both superconductors and fermionic superfluids are characterized by frictionless coherent flow, respectively of electron pairs and fermionic atom pairs. Usually, there is only one 'species' of electron pair in a superductor, and analogously only one type of atomic pair in a fermionic superfluid. Recently systems with mixtures of multiple species of pairs have caught the attention of researchers, as it became clear that the interplay of the different types of pairs leads to new behavior that was not expected on the basis of systems with only one type of pair. These systems are called 'multiband' superconductors or superfluids, and in this project we will set up the theoretical tools to model their behavior from the microscopic level up to the level of the macroscopic coherent behavior. With these tools we will systematically investigate how properties (such as critical field and temperature) and important flow patterns (such as vortex matter and solitons) are affected by the multiband nature of the system, and how this multiband nature can be engineered through quantum confinement. Moreover, we seek to characterize new quantum states emerging from the coupling between the different types of pairs.#### Researcher(s)

- Promoter: Tempere Jacques
- Co-promoter: Milosevic Milorad

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Emergent Phenomena in Multicomponent Quantum Condensates.

#### Abstract

Quantum effects usually only matter at the microscopic scale. However, in superconductors and superfluids these quantum effects appear on a macroscopic level, leading to surprising properties such as frictionless or lossless flow. The macroscopic quantum state arises from the collective behavior of a large number of microscopic particles (Bose-Einstein condensation). In the case of fermionic particles these must first pair up. Neutral particles lead to superfluidity, charged ones to superconductivity. Both cases are described by the same underlying mathematical formalism. The discovery of superfluidity in magnesium diboride in 2001 marked the appearance of a new class of macroscopic quantum systems, the so-called multiband systems. They are characterized by multiple types of pairs, leading to a mixture of quantum condensates. This mixing of different types of quantum fluids within the confines of a single fluid or solid leads to a rich set of novel phenomena. Experimentally not only multiband superconductors have been realized but also multiband superfluids. The goal of the project is to study the interplay between these multiple quantum condensates and to quantify the effects of mixing. We aim to develop and extend the mathematical formalism to the multiband case, and to develop efficient solvers for the non-linear field equations characteristic for this formalism. This will be applied to study a wide range of macroscopic quantum phenomena, both for multiband superfluids and for multiband superconductors.#### Researcher(s)

- Promoter: Tempere Jacques
- Co-promoter: Vanroose Wim

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Solitons and vortex patterns in superfluid Fermi gases.

#### Abstract

Trapped clouds of atoms can be cooled down to nanokelvin temperatures and reach the quantum mechanical state known as 'superfluidity'. A superfluid can flow coherently (like a laser), and without friction (like a superconductor). Many flow phenomena that are known for classical fluids, like solitary waves and vortices, have their quantum counterparts in superfluids. The aim of this project is to study solitary waves and collections of these vortices, in dilute atomic superfluids consisting of fermionic atoms. Fermionic atoms have to pair up in order to become superfluid, so the interplay of the coherence and the interatomic interaction is paramount. This is in contrast to bosonic atoms, which can become superfluid without pairing up. The fermionic system only became experimentally available a few years ago. These experiments also reveal that the properties in dilute fermionic superfluids differ from what is known in superfluid helium and in superconductors. To do investigate these properties, we developed tailored extensions of the techniques used to described superconductors, and combine them with recent theoretical models for fermionic superfluids obtained in the TQC lab where this research will take place. With our approach, we aim to understand and explain how the differences in superflow between bosonic and fermionic superfluids arise.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Verhelst Nick

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Non-equilibrium dynamics and thermalization of quantum many-body systems.

#### Abstract

Since Kepler's observation of the regularities in the planetary motion, physics has been concerned with the study of all kinds of regularities in nature. Each of these regularities satisfies a set of rules, the laws of nature. 'The elements of the behavior which are not specified by the laws of nature are called initial conditions. These, then, together with the laws of nature, specify the behavior as far as it can be specified at all' (E.P. Wigner, Nobel lecture, 1963). When systems grow bigger the number of initial conditions grows accordingly, in contrast to the laws which remain as simple as always. New laws emerge, i.e. the laws of thermodynamics, from our ignorance of all the initial conditions. Thermodynamic equilibrium is reached, when we reach the state of maximal ignorance. The field of non-equilibrium thermodynamics is concerned precisely with the process of evolving towards equilibrium. The emergence of non-equilibrium thermodynamics from the microscopic laws has been well established in classical systems. However, nature is intrinsically quantum mechanical, and the main aim of this project is to provide a theoretical description of quantum many-body systems out of equilibrium. For this purpose, we primarily build on Wigner's description of quantum mechanics, and combine this with Feynman's path integral theory to study the time evolution, equilibration, thermalization of quantum many-body systems.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Sels Dries

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Analog Models of Gravity with Microcavity Polaritons.

#### Abstract

The main objective of this project, and moreover the central idea, is to bring complex and difficult to comprehend systems within the reach of realistic quantum fluid experiments. Since phenomena related to cosmology are often extremely difficult, if not impossible, to study experimentally, it can be very stimulating to translate some of the problems to realistic analog experiments. The specific quantum fluid that we will consider is a quantum degenerate Bosonic gas of exciton-polaritons in semiconductor microcavities.#### Researcher(s)

- Promoter: Wouters Michiel
- Fellow: Van Regemortel Mathias

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Spin-orbit coupling in ultracold Fermi gases.

#### Abstract

One of the latest achievements with ultracold atoms has been the ability to create artificial spin-orbit coupling. The goal of my project is to theoretically describe the properties of spinorbit coupling in ultracold gases, and to predict new phenomena that can be observed experimentally in the near future.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Devreese Jeroen

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Probing itinerant ferromagnetism with ultracold quantum gases.

#### Abstract

The goal of this project is to improve the description of itinerant ferromagnetism using path integral formalism. First, the formalism will be verified with existing mean-field theories. Next, fluctuations around the mean field will be taken into account. This improves the description of strong interactions. In the third part, the interplay between itinerant ferromagnetism and the competing fermionic pairing processes (i.e. superfluidity) will be studied.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Vermeyen Enya

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Investigation of strongly-correlated states of photons in the presence of artificial gauge fields.

#### Abstract

In our project we plan to investigate the interplay between the artificial gauge fields imposed on photons and the effective inter-photonic interactions with an eye to the inherenlty non-equilibrium nature of these systems. In the first place, expanding on our previous work [16,17], we will explore the possibility to excite edge-like modes in a few-particle system of photons. Obeserving the analogues of chiral edge states of the quantum Hall physics and understanding the effects of interactions would be complementary to the studies which have been conducted for non-interacting photons. In doing this, we will also try to think of simpler configurations than have been thought of by considering the state-of-the-art experimental techniques.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Umucalilar Rifat Onur

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Analog Models with Microcavity Polaritons.

#### Abstract

In the present project, we will pursue the analogy between the physics of exciton-polaritons in microcavities and cosmological phenomena. Exciton-polaritons are hybrid particles that are half matter (exciton) and half light (photon). Thanks to their hybrid nature, they combine interactions with easy optical manipulation. In particular, the microcavity polariton physics allows studying analogs of the Einstein field equations with a cosmological constant. A second topic of cosmological interest that we plan to address is the creation of particles due to changing gravitational fields.#### Researcher(s)

- Promoter: Wouters Michiel
- Fellow: Van Regemortel Mathias

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Quantum turbulence in atomic and solid state Bose-Einstein condensates.

#### Abstract

This project aims at a theoretical analysis of these quantum fluids in the turbulent regime. Theories for turbulence in superfluid helium will be adapted to account for a larger vortex core size. Interestingly, additional key observables, such as the spatial and temporal coherence, can be measured. We will develop theoretical descriptions for these quantities in order to characterize the turbulent state in these novel quantum fluids.#### Researcher(s)

- Promoter: Wouters Michiel
- Co-promoter: Tempere Jacques

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Quantum simulation of polaronic effects in quantum gases.

#### Abstract

The focus of the proposed project is on the quantum simulation and study of impurities strongly coupled to an environment, in particular a trapped quantum gas. These quantum mechanical quasi-particles play a key role in the dynamics of the macroscopic quantum liquid. This can be related to the general framework derived first for polarons in the context of solid state physics.#### Researcher(s)

- Promoter: Tempere Jacques

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Quantum mechanical effects in the optical response of metallic .

#### 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: Tempere Jacques
- Fellow: Van den Broeck Nick

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Nonlinear Transport of the Wigner Solid on a Superfluid 4He in a Quasi-One- Dimensional Channel.

#### Abstract

Our main objective is to investigate the transport properties of the Wigner solid in the quantum wire' regime, i.e., in the quasi-one-dimensional (quasi-1D) case when a typical width of the channel is comparable to the inter-electron separation. This new regime, not yet reached in experiments or studied theoretically, is expected to demonstrate new interesting physics. In this study, we will also explore similarities to other quasi-1D systems, e.g., colloids in quasi-1D channels or superconducting vortices in low-pinning narrow channels.#### Researcher(s)

- Promoter: Wouters Michiel
- Co-promoter: Misko Vyacheslav

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in semiconductor nanostructures and ultracold atomic gases.

#### Abstract

The subject of the research project is the theory of semiconductor nanostructures and ultracold atomic gases in the regime of strong light-matter coupling, with the goal to advance the theoretical models, to elucidate conceptual issues and to devise technological applications.#### Researcher(s)

- Promoter: Wouters Michiel

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Probing itinerant ferromagnetism with ultracold quantum gases.

#### Abstract

The goal of this project is to improve the description of itinerant ferromagnetism using path integral formalism. First, the formalism will be verified with existing mean-field theories. Next, fluctuations around the mean field will be taken into account. This improves the description of strong interactions. In the third part, the interplay between itinerant ferromagnetism and the competing fermionic pairing processes (i.e. superfluidity) will be studied.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Vermeyen Enya

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Lattice models for quantum gases with spin imbalance.

#### Abstract

My research consists of a theoretical study of ultracold Fermionic gases with spin imbalance. At sufficiently low temperatures, these systems can become superfluid. My main goal is to study the formation of exotic forms of superfluidity (supervloeibaarheid).#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Devreese Jeroen

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Quantum kinetics of exciton polaritons

#### Abstract

The subject of the research project is the theory of semiconductor nanostructures in the regime of strong light-matter coupling, with the goal to develop the theoretical models, to elucidate conceptual questions and to propose technological applications. In semiconductor microcavities, strong light-matter coupling results in the so-called polariton quasi-particles that are a coherent superposition of quantum well exciton and microcavity photon. Thanks to their composite nature, these bosonic quasi-particles combine significant interactions with good quantum coherence. These favorable properties have led to the first observation of Bose-Einstein condensation (BEC) in the solid state. The research on polariton BEC has developed in a lively subject of fundamental research, on the crossroad between of semiconductor physics, quantum optics and quantum gases and enjoys fruitful collaboration between theorists and experimentalists. Besides their interest from the fundamental physics side, microcavities in the strong coupling regime have high potential for technological applications, such as ultralow threshold lasing, generation of entangled photon pairs, miniaturized nonlinear optical devices and ultrafast optical memories. The two elements that make polaritons different from other realizations of quantum degenerate bose gases are the finite polariton life time and their interactions with the solid state environment. These pose great challenges for their theoretical description. Due to the finite polariton life time, the polariton gas does not reach thermodynamic equilibrium. As a consequence, the steady state cannot be found by minimizing a free energy. Instead, the kinetics has to be modeled. The nonequilibrium character also raises conceptual questions related to the meaning of superfluidity, because standard treatments rely on thermodynamic arguments. We plan to attack the polariton quantum kinetics with methods based on quasi-probability distributions developed in quantum optics. These distributions can be sampled with stochastic classical fields, using Monte Carlo techniques. I have developed effective models of this type before, but these exploratory studies contained quite drastic approximations. It is the aim of the present research project to go beyond these simplifications and to develop a full model for the kinetics of a quantum degenerate polariton gas that interacts with its solid state environment. Applications of the theoretical model will include among others a quantitative study of the long range spatial coherence, density fluctuations, the dynamics of the formation of coherence, the shape of the condensate state and its coherence in the presence of periodic or disordered potentials and the polarization state of polariton condensates. In addition, we will consider the application of the developed formalisms to study different physical systems. One promising example is a nanocavity with an embedded quantum dot, where recently the controversial observation of lasing in the strong coupling regime was reported. In addition, we will seek to make conceptual progress in the domain of polariton superfluidity. Our model will contain all the ingredients to make a microscopic calculation of the superfluid fraction. Such a calculation is important, because of the two dimensional nature of the polariton fluid: from the analogy with the two-dimensional bose gas at thermodynamic equilibrium, it is expected that the transition from the incoherent to coherent state is of the Berezinskii-Kosterlitz-Thouless type, characterized by a jump in the superfluid fraction. Finally, the directions of technological applications that we think of are based on the polarization dynamics, that was recently exploited to construct an ultrafast all optical spin memory.#### Researcher(s)

- Promoter: Wouters Michiel

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Stochastic modeling with applications in financial markets.

#### Abstract

This is a fundamental research project financed by the Research Foundation - Flanders (FWO). The project was subsidized after selection by the FWO-expert panel.#### Researcher(s)

- Promoter: Tempere Jacques

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Localization and response properties of one or more impurities in a Bose-Einstein condensate.

#### Abstract

It has been shown that the system of an impurity in a Bose-Einstein condensate can be described by the Frölich Hamiltonian. This generic model has many applications in different branches of physics. One of these is the well known polaron in solid state physics which consists of an electron coupled to the phonons of the crystal. Despite the many applications there are still some important outstanding issues about the strong coupling regime of the model. The main reason these haven't been solved yet is that the strong coupling regime doesn't appear in solids. We look at this particular model in an ultra cold gas because in these it is possible to very accurately tune some of the aspects of the experiment, as for example the interaction strength between particles. This will allow us to make measurements in the regime of strong coupling for the first time. The purpose of the project is to develop the theoretical description for this system. We will calculate the response properties for arbitrary coupling and also the effect of multiple impurities. Doing the predictions we will always keep the experiment in mind which will allow us to probe the strong coupling properties of the model.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Casteels Wim

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Electronic structure of a nano-shell.

#### 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

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Lattice models for quantum gases with spin imbalance.

#### Abstract

This is a fundamental research project financed by the Research Foundation - Flanders (FWO). The project was subsidized after selection by the FWO-expert panel.#### Researcher(s)

- Promoter: Tempere Jacques
- Fellow: Devreese Jeroen

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Matching the functional properties of nanoparticles and nanowires.

#### Abstract

This project aims at a continuing integration and optimalization of the unique expertise available in the WOG consortium, regarding the preparation, the structural and physical characterization and the theoretical modeling of nanowires and nanoparticles, forming the building blocks in the developmment of nanotechnological applications. Specifically, theoretical modeling techniques will be developed to describe and model the functional properties of interacting and non-interacting nanoparticles and -wires.#### Researcher(s)

- Promoter: Tempere Jacques

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Strong-coupling phenomena for impurity atoms interacting with condensate fluctuations.

#### Abstract

In short, cold atomic gases appear as a new and potent way to investigate an essential problem in many-body physics: particles (strongly) coupled to a bath of bosons. This basic problem falls precisely in the expertise of our research group, so we do not want to miss out on this opportunity to valorize previous results. We plan to develop the path-integral treatment so that it describes the polaronic effect in cold atomic gases on three levels: ground state properties, response properties, and multipolaron effects.#### Researcher(s)

- Promoter: Tempere Jacques

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Dynamic effects in coupled superconductor-ferromagnet nanosystems.

#### Abstract

The main objective of this project is to investigate experimentally and theoretically the physical properties of coupled, mutually influencing, superconductor-ferromagnet nanosystems. We will study the different contributions of the ubiquitous electromagnetic coupling and the more fragile exchange coupling. This will include the back-coupling from the superconductor to the (soft) magnetic system. A complementary theoretical approach will be developed by studying more general systems like cold atomic gases, for which the physical description can be mapped onto the S/F hybrids systems and vice versa.#### Researcher(s)

- Promoter: Tempere Jacques

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Optical confinement phenomena in plasmonic nanomaterials with predesigned electromagnetic properties.

#### Abstract

The principal objective of this project is to investigate and control the optical confinement phenomena through mastering the plasmonic excitations in individual metallic nanoparticles and study negative refractive index materials composed of nano-engineered arrays of metallic and superconducting nanoresonators. This goal will be achieved by using the following nanoengineered systems, which also eventually will improve the limitations of existing NRI: (i) to broaden considerably the negative magnetic permeability (¿<0) interval, by using nanostructured multilayers and nanostructured hybrid structures [metal/dielectric/metal]n and single layers where several resonance frequencies will be superimposed. The resonant frequencies in a multilayer will be the same within the layer but different from layer to layer. In a single plane version a 2D superposition of the cells with different resonances will be used. (ii) to increase the frequency for which the negative index occurs up to a visible range. Different techniques can be implemented to achieve this goal as is highlighted in the section Design and Methodology. (iii) to reduce absorption at the frequencies below the superconducting gap, these systems will be made from both normal metallic and superconducting films.#### Researcher(s)

- Promoter: Tempere Jacques

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Path-integral techniques for prizing of financial options.

#### Abstract

In this project we develop and apply many-body path integral techniques to derive prizing formulae for financial options for which till now no analytic prizing formulae are available. This is achieved by transferring recent breakthroughs in path integration applied from the context of many-body physics to the context of financial models with stochastic volatility.#### Researcher(s)

- Promoter: Tempere Jacques

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Excitons in semiconductor nanocrystals as energy suppliers for optoelectronic and biomedical applications.

#### 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

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Superfluid properties of mixtures of ultracold atomic gases.

#### 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

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Dynamic magneto-transport properties in quantum dots woth electron-phonon interaction.

#### Abstract

#### Researcher(s)

- Promoter: Brosens Fons
- Fellow: Putteneers Katrijn

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Superfluidity of ultracold atomic Fermi gases.

#### Abstract

#### Researcher(s)

- Promoter: Brosens Fons
- Fellow: Wouters Michiel

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FWO Visiting Postdoctoral Fellowship. (Arkady Shanenko)

#### Abstract

#### Researcher(s)

- Promoter: Brosens Fons
- Fellow: Shanenko Arkady

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Hybrid systems on nanostructures.

#### 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

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Superfluidity of fermionic atoms in a magnetic trap.

#### Abstract

In this project, a theoretical investigation will be made on the artificial interactions between fermionic atoms in a magnetic trap, and how these interactions affect the properties of the gas as concerns the stability of the gas in the trap and the realisation of fermionic superfluidity. The properties of the superfluid state of the trapped fermionic atoms will be investigated and properties suited for the detection of this superfluidity will be identified.#### Researcher(s)

- Promoter: Tempere Jacques

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Influence of crystal defects and interface roughness on the magnetism of mesoscopic ferromagnets and dilute magnetic alloys.

#### Abstract

#### Researcher(s)

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

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#### Project type(s)

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Confinement effects in nanostructured superconductors.

#### 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

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#### Project type(s)

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Application of new methods for confined many-fermion and many-boson systems to confined atomic gases and multielectron bubbles in helium.

#### 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)

#### Project type(s)

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Bipolaron in Polaron Environment.

#### 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)

#### Project type(s)

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Statistics and dynamics of confined fermion systems.

#### Abstract

#### Researcher(s)

- Promoter: Lemmens Lucien

#### Research team(s)

#### Project type(s)

- Research Project

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Statistics and dynamics of confined fermion systems.

#### 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)

#### Project type(s)

- Research Project

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Path-integral treatment of interacting many-boson and many-fermion systems.

#### 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)

#### Project type(s)

- Research Project

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Self-assembled nanostructured materials for electronic and optoelectronic applications (NANOMAT).

#### Abstract

#### Researcher(s)

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

#### Research team(s)

#### Project type(s)

- Research Project

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Superconductors with nanengineered periodic pinning arrays.

#### 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)

#### Project type(s)

- Research Project

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Electron-electron and electron-phonon interaction in metals, semiconductors and two-dimensional systems.

#### 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

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Confinement phenomena in nanostructured superconductors.

#### 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)

#### Project type(s)

- Research Project

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Nanostructures : electronic, magnetic and optical phenomena

#### 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)

#### Project type(s)

- Research Project