Research Lucian Covaci

Abstract

DELIGHT aims at excellence of Europe in nanoscience and impact in research and development at the highest level. The project focuses on multifunctional nanomaterials based on colloidal particles, organic/inorganic perovskites, and organic and biomaterials. Design and fabrication of these materials for state-of-the-art applications requires a high level of interdisciplinarity with expertise from chemistry, physics, material science, engineering, nanofabrication and biology, combined with the most advanced spectroscopy tools. The scientific objectives of DELIGHT are to establish a platform of highly versatile functional nanomaterials, with the use of machine learning and artificial intelligence for material/device development and characterization. The focus is on multifunctional hybrids, heterostructures, and assemblies, and to fully exploit their potential for catalysis, energy, lighting, plasmonics, and theranostics. The research is organized in 3 work packages (WPs) that target nanomaterial development, functional composites and in-depth characterization, and device applications. Social and training objectives are the education of young researchers in Europe on the highest level, with emphasis on interdisciplinarity that is fundamental in modern nanoscience, the advancement of technological know-how that enables a sustainable and eco-friendly modern society, and promotion of gender equality in the scientific landscape at all levels. These goals are implemented in a WP dedicated to training, organizing lectures, workshops, technology transfer, and outreach and dissemination events. DELIGHT assembled an academic team of outstanding excellence, which links key players in the EU working on state-of-the-art nanomaterials with world leading universities in the US, Canada, and Argentina that are known for their unique scientific and technological capabilities and efficient technology transfer.

Researcher(s)

• Promoter: Bals Sara
• Co-promoter: Covaci Lucian
• Co-promoter: Milosevic Milorad
• Co-promoter: Van Aert Sandra
• Co-promoter: Verbruggen Sammy

Research team(s)

• Electron microscopy for materials research (EMAT)

Project type(s)

• Research Project

Abstract

DYNASTY's primary objective is to build in the European South East, and in particular in the Foundation for Research and Technology Hellas (FORTH) in Crete, a significant pole of attraction for nanomaterials researchers and scientists. This will be accomplished through joint research activities and partnering with two well-established European research teams, which are in the forefront of nanomaterials research. The activities will contribute in scientific production that will motivate and attract young scientists in nanomaterials (e.g. 2D materials) science and technology. The partners include: (a) the University of Antwerp (UA) with strong expertise in advanced Electron Microscopy for Materials Science and in Condensed Matter Theory (the EMAT and CMT groups, respectively), which are both part of the UA NANOlab Center of Excellence (Belgium) (b) and the National Institute of Applied Sciences (INSA- University of Toulouse), with deep expertise in advanced spectroscopic characterization techniques of 2D materials. The activities involve training through cross-lab visits, workshops, short courses, joint conferences, and well-designed communication activities to attract young scientists at FORTH. All teams will provide their expertise and collaborate to build advanced Imaging and Spectroscopy expertise at FORTH (combining non-linear and time-resolved optical spectroscopies) that will provide precise fine structural analysis of 2D materials and their heterostructures. By the end of the three-year project, FORTH will gain advanced skills in nanomaterials characterization and knowhow in nanoelectronic devices fabrication. As a result, DYNASTY will create a collaborative platform for widening experimental networks among nanomaterials labs in Europe, enabling local teams to produce excellent interdisciplinary nanoscience, currently lacking in Greece.

Researcher(s)

• Promoter: Verbeeck Johan
• Co-promoter: Bals Sara
• Co-promoter: Covaci Lucian
• Co-promoter: Milosevic Milorad

Research team(s)

• Electron microscopy for materials research (EMAT)

Project type(s)

• Research Project

Abstract

Novel materials that couple advanced magnetic and electronic properties are paramount to sustain the hunger of the modern society for advanced consumer electronics and Internet of Things, yet reduce the energy consumption and environmental impact. To satisfy the rather versatile needs of wearable, flexible, integrable, bio-compatible, ever smarter, and low power electronics, the paradigm shift is needed - towards tailored heterostructures, where different functionalities of the constituents are strongly coupled into a multifunctional hybrid. However, such strong interaction between different materials is challenging to realize, as much as their heterostructures are difficult to grow with sufficient control and quality. In this project, we will pursue the stacks of atomically-thin 2D materials as the most versatile yet fully controllable path towards shapeable magnetoelectronics by design. With properties broadly tunable by external mechanical, electric and magnetic stimuli, 2D materials are crystalline systems that nearly ideally connect the simulation environment to their practical behavior and measured quantities. To understand the deeply quantum phenomena behind the flexo-magnetoelectric coupling in 2D heterostructures, yet bridge them over to observables of practical value at micrometer scale, we formed a consortium of leading Belgian teams for suited multiscale simulations, the pioneer of 2D materials in UK for experimental validation, and imec as technology outlet.

Researcher(s)

• Promoter: Milosevic Milorad
• Co-promoter: Covaci Lucian
• Co-promoter: Partoens Bart
• Co-promoter: Peeters Francois

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

Recent experimental and theoretical efforts in the shaping and texturing of the electron wave-function take as an example periodic arrangement of atoms in a crystal that appear in nature. For example, as observed in semiconductors, electrons acquire new properties depending on the types of atoms and their arrangement in the lattice: the spectrum becomes gapped, electrons and holes acquire effective masses, etc. In efforts to mimic this behavior and with the purpose of tuning it at will, researchers have created through various means periodic potentials for two-dimensional electron gas systems. These can be either the states formed at the interface of two semiconductors, the surface state in metals, or the naturally confined electrons in two-dimensional materials like graphene. Based on very recent experimental developments, we propose to theoretically study a sandwich-like configuration containing patterned graphene gates, imposing a periodic potential on a bilayer graphene active layer. The main goal is to artificially create and tune lattices with peculiar properties, otherwise not easily found in nature: Lieb, kagome or dice lattices that show flat electronic bands with topological properties and show great potential for novel physics. This proposal is situated in the context of a collaboration with an experimental group working on building such devices. The planned close interaction will provide input on realistic gate configurations, possibility to validate our approach, to model electric transport measurements in the presence of magnetic fields and to predict gate configurations which realize the propose flat topological bands.

Researcher(s)

• Promoter: Covaci Lucian
• Fellow: Aerts Emile

Research team(s)

• Electron microscopy for materials research (EMAT)

Project type(s)

• Research Project

Abstract

From the moment it was isolated as a 2D material, graphene has become a remarkable subject of research, exhibiting novel phenomena that extend to almost any domain within condensed matter physics and physical chemistry. Recently, this was further extended with the discovery of 'magic-angle graphene', in which twisted bilayer graphene (TBG) with nearly flat bands was observed to behave as a high-temperature superconductor - the Physics World 2018 Breakthrough of the Year. However, TBG remains extremely challenging to fabricate which, together with intrinsic constraints on tunability, limit further research on the electron correlation phenomena emerging from its flat bands. Here we propose to explore an alternative system, based on periodic lattices of strained nanobubbles in single-layer graphene, which host similar flat bands to those in TBG, with the advantage of being much more tunable (e.g. allowing for even flatter bands) and scalable (crucial for further fundamental studies as well as eventual applications). The fabrication is based on an original approach that combines ultra-low energy ion implantation (a unique technique developed by the consortium) and state-of-the-art nanofabrication. The tunability of the fabrication approach, together with the unique expertise of the consortium on theoretical tools for electronic structure calculations of such systems, will allow us to produce specific electron correlation phenomena (superconductivity and magnetism) by design.

Researcher(s)

• Promoter: Covaci Lucian
• Co-promoter: Peeters Francois

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

Periodic structures and flat bands realized experimentally in two-dimensional (2D) materials have recently proven to be a fertile ground for novel physics. I will take advantage of existing expertise and collaborations at CMT research group in order to propose periodically strained configurations of 2D materials, e.g. graphene, transition metal dichalcogenides or phosphorene, for the purpose of exploring novel opto-electronic phenomena related to (flat) electronic mini-bands or excitonic bands. To do so, I will first use numerical simulations to investigate how strong periodic strain modulations of several types can be engineered in 2D materials. Then, I will assess how these different types of modulations introduce band renormalization and how the latter, in its turn, affects optical and electronic properties of the 2D crystals in monolayer and multilayer form. In doing so, I will also be able to relate the role of external effects, such as applied electric fields, to the opto-electronic properties of these strained crystals. The external fields and periodic strains can function as a tuning knob for the opto-electronic response. Finally, I will investigate more deeply how periodic strain fields affect its excitonic properties. In this project, I will make use of the close collaborations of the CMT group with various experimental groups worldwide. The research is theoretical in nature, but I will repeatedly link my results to experiments to maximize impact of the research.

Researcher(s)

• Promoter: Partoens Bart
• Co-promoter: Covaci Lucian
• Fellow: Jorissen Bert

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

The European Commission has just launched a €1 billion Flagship-scale initiative in Quantum Technology, within the European H2020 research and innovation framework programme. This initiative aims to place Europe at the forefront of the second quantum revolution, with quantum information, communication and computing at heart, as already unfolding in USA under push by Microsoft and Google. Both latter companies see superconducting hybrid devices as a base for viable quantum technology of the future. This project is aimed at positioning Flanders as a home for realistic theoretical simulations of such devices. At present, numerous experiments around the world are performed on superconducting hybrids with special topological properties, such that they may stabilize exotic Majorana fermions -a quasiparticle obeying non-Abelian statistics, thereby useful for fault-tolerant quantum computing. As no experimental setup is ideally perfect, the convincingly proven signature of the Majorana fermion is still missing. Furthermore, additional aspects appear that are not covered by simplistic models. Therefore, simulations based on realistic parametrizations and geometries are absolutely necessary for improving the theoretical understanding of ongoing experimental efforts, for convincingly confirming the detection and manipulation of Majorana, and to design quantum devices that can reliably replace current technology. The advanced simulations in this project are fully in service of that goal.

Researcher(s)

• Promoter: Milosevic Milorad
• Co-promoter: Covaci Lucian
• Fellow: Zhang Lingfeng

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

Soon after the formulation of the Dirac equation (1928), which describes relativistic particles, it was predicted that for a high charge Z of the nucleus the atom becomes unstable, leading to the phenomenon of atomic collapse. Because of the large required Z>170 value scientists were never able to verify it experimentally. However, the discovery of graphene and the fact that its charge carriers mimic relativistic (quasi-)particles opened up a new window on atomic collapse, which was recently observed experimentally in graphene. Using this recent observation as motivation, we will theoretically investigate the atomic collapse phenomenon in graphene and other Dirac-like materials having very different energy dispersions. We will study how the various differences between these materials influence the atomic collapse phenomenon and study how this phenomenon can be tuned by external electric and magnetic fields. The purpose of this proposal is two fold: 1) to study atomic collapse in different Dirac-like materials, which will give us fundamental information and understanding about atomic collapse at the relativistic level, and 2) to investigate the influence of atomic collapse on the transport of charge carriers in Dirac-like materials, providing us with very important information needed for the development of future applications .

Researcher(s)

• Promoter: Peeters Francois
• Co-promoter: Covaci Lucian
• Fellow: Van Pottelberge Robbe

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

Two-dimensional (2D) materials are currently a very important topic in materials science due to their unique properties and high crystal quality. An important property of these materials is that they can be stacked on top of each other regardless of the mismatch between the unit cells and with almost any twist angle between the two lattices. This is thanks to the weak van der Waals interaction that acts between different layers. However, researchers have found that the properties of these stacked structures can be very different from its constituents, they not only dependent on the choice of 2D materials used for its construction but are also significantly influenced by the orientation of the two lattices. A difference in lattice constant and/or misorientation of the two lattices results in the appearance of a periodic superlattice structure called moiré pattern. Thus, the types of 2D materials used for stacking and the period of moiré pattern can be in principle used for the design of novel materials with desirable properties. In this project we will focus on the formation of moiré patterns as generated by stacking two monolayers on top of each other and their consequences on the different physical properties of the heterostructure. The effect of internal and external applied strain will be considered.

Researcher(s)

• Promoter: Peeters Francois
• Co-promoter: Covaci Lucian
• Fellow: Milovanovic Slavisa

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

Electromechanical effects, such as piezo- and flexoelectricity, are a consequence of the coupling of an applied electric field to the strain and the strain gradient, respectively. These effects are expected to be strongly enhanced in two dimensional materials (2D), first, due to the reduction in lattice symmetries in the 2D limit, and second, due to the superior elastic properties, allowing strains even up to 10% in some cases. Furthermore, 2D materials are fully flexible and bendable, thus ushering a new era of flexible opto-electronic devices. In this proposal, we will first investigate the fundamental flexoelectric properties of a wide variety of 2D materials by using a combination of analytical and ab-initio approaches. Important questions related to the magnitude of the coupling coefficients, the effect of phonon anharmonicity and the identification of materials with optimal electro- and mechanical properties will be answered. Subsequently we will model specific strain configurations as out-of-plane (ripples, folds, kirigami) and in-plane geometries (patterned layers, heterostructures, etc.). These are of significant importance because, as opposed to bulk electromechanical effects, modifications at the nanoscale in 2D materials greatly affect their optoelectronic properties. As concrete examples we will investigate the possibility of creating flexotransistors or flexo-photovoltaic devices.

Researcher(s)

• Promoter: Peeters Francois
• Co-promoter: Covaci Lucian

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

The exploration of novel low dimensional atomically thin materials is very important for a future generation of flexible nanoelectronics, optoelectronics, and energy storage devices. Among these, graphene has demonstrated a wide range of properties including, high electrical and thermal conductivity, and optical transparency. Due to the semiconducting nature of transition metal dichalcogenides, they are also becoming promising candidates. More recently, high frequency devices containing few layer black phosphorous have been demonstrated. Combining these materials in heterostructures would lead to a many-fold enhancement in their functionalities. In this proposal, with the combined effort of the two teams, prototype devices containing 2D heterostructures will be investigated. A deep understanding of the stability and electronic properties of heterostructures, investigated by the Chinese team with the use of ab-inito simulations will be coupled to effective models of prototype devices, either at tight binding or continuum level, led by the Belgian team. Systems comprised of vertical and in-plane heterostructures will be used to propose candidate devices taking advantage of either the charge or spin degrees of freedom. Of special interest are also tunable opto-electronic and excitonic effects. It is expected that this collaborative effort will lead to both a fundamental understanding of optoelectronic processes and the modeling of specific nano- and microelectronic devices.

Researcher(s)

• Promoter: Peeters Francois
• Co-promoter: Covaci Lucian

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

Metallic transition metal dichalcogenide (TMD) monolayers are promising ultrathin materials which have the potential to complete the range of graphene-related materials by offering tunable metallic phases with strong spin-orbit coupling. Many of them can be achieved by small structural deformations and doping of Group 6 TMDs and thus could thus be used as electrode materials within a single monolayer, resulting in a very low contact resistance. Experimental study of metallic TMDs is difficult as these phases are often metastable or rely on very subtle structural modifications. Thus, a careful theoretical investigation is imperative before complex experimental studies should be pursued. This consortium will investigate metallic TMD structures, including intrinsically metallic phases, metastable metallic phases, and external factors to trigger semiconductor-metal transitions such as doping, defects and strain. Special attention will be given to spin-orbit splitting and ways to control them. Computer simulations will range from band-structure calculations of small unit cells to rather complex systems, including heterostructures, doped and defected systems up to grain boundaries. Conclusions on the suitability of these materials in practical application will be further confirmed by explicit transport calculations and device simulations. While most calculations can be carried out using state-of-the-art software, some method developments are necessary and will be carried out here. Numerical methods that scale linearly with the system size, O(N), will be developed by using a polynomial expansion of the components of the conductivity tensor. These will allow for simulations of large unit cells in the presence of disorder and the calculation of spin- and valley- dependent contributions. It will become therefore suitable to describe the Spin and Valley Hall effects in realistic models of TMDs.

Researcher(s)

• Promoter: Peeters Francois
• Co-promoter: Covaci Lucian
• Co-promoter: Sahin Hasan

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

Due to their impact on fundamental physics and possible applications in low-power electronics, superconducting ultra-thin films with thickness ranging from one to a few atomic layers have recently attracted tremendous interest. Their superconducting properties are strongly influenced by the thickness, geometry and structure of the film due to the quantum confinement effects on atomistic scale. Since last years, such ultra-thin films can be grown experimentally, in clean crystalline form, and tuned with atomic precision. Numerous novel electronic properties were observed and even prototype field-effect transistors were realized. However, most of the novel properties are not precisely understood from theoretical standpoint. In this project, we will therefore study the effects of atomic engineering by state-of-the-art Bogoliubov-de Gennes numerical simulations of ultrathin superconductors, with the hope to reveal the impact of atomic edge steps, disorder, and substrate choices on the superconducting condensate and its electronic structure. Emerging new topological states (including vortices, fractional vortices, and skyrmions) will be considered in the presence of magnetic field and electric current. This project will ultimately provide a comprehensive review of possible properties and how to achieve them in scanning tunneling microscope (STM) experiments on these fascinating materials.

Researcher(s)

• Promoter: Milosevic Milorad
• Co-promoter: Covaci Lucian
• Fellow: Zhang Lingfeng

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

The transport properties of graphene on boron-nitride or on other graphene layers are strongly modified due to an induced periodic potential. Because of a weak inter-layer coupling (van der Waals), the relative mismatch becomes an important tuning parameter. We will numerically investigate the influence of substrate strain and rotation on transport. This study will provide a theoretical understanding of recent experiments by the Manchester group.

Researcher(s)

• Promoter: Covaci Lucian

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

The key questions we plan to answer in this project are: - How does strain affect electronic correlations? - Can one mechanically induce or manipulate magnetism in graphene? - Can different correlated states be stabilized through strain engineering?

Researcher(s)

• Promoter: Peeters Francois
• Co-promoter: Covaci Lucian

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

One of the main objectives is to develop efficient Monte Carlo methods, which can rigorously describe thermal (classical) phase fluctuations in unconventional superconductors. Although the core of these methods is generic, we will develop specific formulations for the different symmetries of the superconducting order parameters.

Researcher(s)

• Promoter: Peeters Francois
• Fellow: Covaci Lucian

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

We will study two peculiar configurations of graphene sheets, both involving broken translational symmetry: nano-pore graphene, a mesh of interconnected graphene ribbons which shows great potential in spintronics applications due to the appearance of edge magnetism; inhomogeneous strain in multilayer graphene for which pseudo-magnetic fields and pseudo-Landau levels are predicted and will greatly influence the electronic properties of the material. We will develop efficient numerical codes running on clusters of graphical processing units (GPUs).

Researcher(s)

• Promoter: Covaci Lucian

Research team(s)

• Condensed Matter Theory

Project website

Project type(s)

• Research Project

Abstract

With possible applications in carbon based electronics and spintronics, nano- pore graphene (NPG) has great potential. We will study NPG theoretically and computationally with the use of microscopic tight-binding Hamiltonians. Focus will be on effect of defects in the nano-pore lattice and distributions of pore sizes and shapes on the band structure and magnetism of this material. Numerical codes will be developed to run on graphical processing units (GPU). The acquisition of a high-end GPU will benefit other researchers in the Condensed Matter Theory group at UA by introducing them to GPU computing and providing the opportunity to run codes on the new machine.

Researcher(s)

• Promoter: Covaci Lucian

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

The present project proposes to numerically solve the quantum mechanical mean-field equations describing superconductivity at a microscopic level. We will refine a novel method in order to consider various inhomogeneous situations: presence of impurities, surfaces, interfaces and/or magnetic fields. We will then apply this method to problems of interest related to nanoscale superconductivity.

Researcher(s)

• Promoter: Peeters Francois
• Co-promoter: Milosevic Milorad
• Fellow: Covaci Lucian

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project