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Department of Materials Science, University of Patras www.matersci.upatras.gr | Hellenic Society for the Science and Technology of Condensed Matter www.hsstcm.eu | |
Department of Education, School of Pedagogical & Technological Education (ASPETE)
Magnetic skyrmions (Sk) are spatially confined vortex-like spin structures with topological characteristics, that can be stabilized in non-centrosymmetric magnetic systems, as for example in bulk B20 compounds (MnSi, MnGe), transition metal (FM) - heavy metal (HM) interfaces, frustrated magnets, etc. A sizable antisymmetric exchange interaction (Dzyaloshinskii-Moriya, DMI) that competes with the symmetric exchange (Heisenberg) is a prerequisite for their stabilization. Two-dimensional magnetic skyrmions formed in the FM layer of a FM-HM nanostrip, constitute a promising building block for future spintronic devices, as they can be used to encode bits in racetrack-based devices. When an electric current is applied parallel to the FM-HM interface of a racetrack, an in-plane spin-current is generated due the strong spin-orbit coupling in the HM and provides the required spin (orbit) torque (SOT) that causes drift of the skyrmion along the nanostrip. An issue in developing skyrmion-based racetrack memory devices is the occurrence of a transverse component of Sk drift, caused by the magnetic Magnus force that arises due to skyrmion chirality. This phenomenon (Skyrmion Hall Effect, SkHE) leads to skyrmion motion towards the lateral free edges of the nanostrip and can cause the eventual annihilation of the skyrmion, when the driving current is high enough. Various proposals to overcome the SkHE for FM skyrmions include the transverse modulation of the nanostrip anisotropy via graded alloying or varying nanostrip thickness, racetracks composed of antiferromagnetically-coupled pairs of FM-HM nanostrips and also, nanostrips without free side boundaries (nanotubes, nanorings). In the present talk, we focus in the latter case and discuss recent results from numerical micrormagnetic studies of the static properties (shape, size, topological charge) and current-driven motion of FM skyrmions in cylindrical nanotubes and planar nanorings.
Department of Materials Science and Technology, University of Crete and Institute of Electronic Structure and Laser (IESL), Foundation for Research and Technology Hellas (FORTH)
Triggered by graphene, research on two-dimensional (2D) materials is growing at an explosive rate over the last decade. Single layers of atomic thickness produced by exfoliation and other fabrication techniques, often display distinct properties, different from their 3D counterparts. Ranging from insulators and semiconductors to conductors, 2D crystals exhibit exciting low-dimensional physics and show great promise for applications. Among them, semiconducting transition metal dichalcogenide (TMD) monolayers have become one of the hottest topics of 2D materials research. They have direct bandgaps, striking excitonic properties, and, in addition to charge and spin, their electrons possess a valley degree of freedom. Robust metallic states at the edges of TMD nanostructures are observed in quasi-one-dimensional (1D) and zero-dimensional (0D) structures such as nanoribbons and nanoflakes (dots), respectively. TMDs unique optoelectronic properties depend on composition, dimensionality, strain, defects, chemical modification and nanostructuring, so that they may be engineered for specific applications. When combined in heterostructures with graphene and/or other materials, they offer new opportunities in nanotechnology. Transistors, solar cells, light-emitting devices, sensors, and inexpensive catalysts are some of the technological applications already demonstrated using TMDs but their wide, commercial use is not likely in the near future. Great progress in the field in a short time period has raised great expectations and, at the same time, many fundamental and practical issues. Theory, accurate modeling and simulation, contribute decisively to the effort of resolving them. Detailed theoretical understanding of these systems, besides interpreting and guiding experiments, provides predictions and guidelines for improved design and functionality of 2D materials and nanostructures. We will discuss theoretical and computational methods used to understand these systems, from simple models to atomistic simulations and first principles calculations. We will present theoretical results for 2D, quasi-1D and 0D nanostructures and heterostructures, including the effects of strain, defects, adsorbed atoms and molecules. We will address theoretical/computational challenges and potential for applications in optoelectronics and catalysis for energy applications.
Department of Chemistry, Aristotle University of Thessaloniki
A respectable portion of modern day research in materials sciences involves the examination of charge transfer that occurs in molecular structures upon excitation under illumination. Considerable efforts have gone in designing stuctures with desired behavior in processes in which the transfer of charge occurs. Time-dependent density functional theory has been proven a usefull tool in quantifying and visualizing the transfer of charge that occurs upon excitation of a molecular system. With this talk we will examine some of the techniques that can be employed with the aim to a) make some quantitative predictions and b) visualise results, of the transfer of charge. To this end we will employ freely available as well as commercial codes, and walk through some of these techniques in such a manner that the audience should be able to reproduce the presented results in full.
Department Materials Science and Engineering, University of Ioannina
Metallic implants require materials with low Young moduli, high corrosion resistance and minimal cytotoxicity. The a'-type Ti-Nb alloys have been suggested as promising materials as biocompatible and more suitable Young modulus for replacingthe widely used TiAl6V4 implants. This work presents computational studies on Ti-based alloys aiming to reveal the electronic origin of the structural, magnetic and mechanical properties, for the design of materials with predefined properties, even antibacterial, suitable for hard tissue implant applications. Ab-initio results reveal the electronic rules for the a'-Ti instability that are related to the electronic band structure characteristics along the phonon critical directions and the electronic occupation at the Fermi level. In Ti-based alloys: Nb composition, phase coordination number and sp dopants enrich these electronic rules. The mechanical stability conditions and the elastic constants predict the a'-TiNb stabilization only for Nb-rich compositions and the known w-shape Young modulus curve in agreement with the experimental data. Implant-associated infections caused by bacterial biofilms represent a serious clinical problem. Being highly resistant to conventional antibiotic treatments, it becomes urgent to find new solutions to tackle this form of infection, such as innovative materials with effective antimicrobial properties. Binary TiNb alloys are promising antibacterial implant materials especially upon addition of elements with antibacterial features like Ga, Cu and Ag. Ab initio calculation along with large scale simulations will be used to study these alloys as well as biological processes like Phagocytosis where monocytes engulf bacteria, The results of this work could be of use in the design of antibacteria, low rigidity a'-typeTi-alloys with non-toxic additions, suitable for orthopedic and orthodontics applications. Acknowledgements This work is supported by the Bioremia (H2020-MSCA-ITN-2019, No 861046, 2020-2024) and BioTiNet (FP7-PEOPLE-2010-ITN No 264635, 2011-2014) projects. The authors would like to thank Prof. G. Evangelakis, Assoc. Prof. Papageorgiou, Dr N. Panagiotopoulos and Dr. A. Gebert for fruitful collaboration and discussions.
Department of Physics, University of Athens
We discuss charge transfer and transport in (i) atomic wires made of bioelements, e.g., carbynes, dicyanopolyynes, and, as a didactic example, benzene, (ii) biopolymers like DNA or RNA. Transfer means that a carrier, generated at a specific site, by oxidation or reduction, moves, time passing, to more favorable sites, without application of external voltage. Transport implies application of external voltage or other gradient between electrodes. Nucleic acids are important for life and have nanotechnological applications. Using the four-letter alphabet [Guanine (G), Cytosine (C), Adenine (A), Thymine (T)], we construct periodic or aperiodic sequences (quasi periodic, fractal, random, natural). Atomic carbon wires or carbynes are atom-thick nanowires, conceptually divided into cumulenic (equidistant carbons) and pollynic (alternating shorter and longer distances). We mainly use analytical and numerical Tight Binding (TB) as well as Density Functional Theory (DFT) related methods like RT-TDDFT [Real-Time Time-Dependent DFT] to study electronic structure and charge transfer and transport. We use benzene as a didactic example. We discuss its symmetry and its electronic structure via different TB variants, using: (a) carbon 2pz orbitals, (b) hydrogen 1s orbital, carbon three sp2 and 2pz orbitals, (c) all valence orbitals. We observe how a hole created at a CH site is transferred within this ring. We extract ionization energy and HOMO-LUMO gap. In DFT, we use bases 3-21G, 6-31G*, 6-311G*, cc-pVDZ, cc-pVTZ with Mulliken and L?wdin population analysis. We optimize geometry, check convergence of C-C and C-H distances and relevant angles, extract ionization energy, electron affinity and energy gap, analyze normal modes. By CDFT, we create a hole and observe how it is transferred along the ring. We compare TB and DFT results between them as well as with the experiment.
Department of Materials Science, University of Patras
In the last decade, the light-matter interaction of quantum emitters with photonic nanostructures has been pushing the boundaries of cavity quantum electrodynamics into new regimes; in particular, in the strongcoupling limit, it may lead to numerous important phenomena in nanophotonics and applications in quantum technologies. A basic phenomenon under strong light-matter interaction of a quantum emitter with its nanophotonic environment is the coherent exchange of energy between the quantum emitter and the photonic nanostructure, leading to reversible, non-Markovian, spontaneous emission dynamics of the quantum emitter. This phenomenon has been predicted when a quantum emitter is coupled to various nanophotonic platforms, including plasmonic nanostructures, epsilon-and-mu near-zero media, two-dimensional semiconductors, ferromagnetic or ferrimagnetic nanoparticles, as well as topological photonic structures. In this talk, we discuss the dynamical evolution of the spontaneous emission of a two- and three-level quantum emitters near nanophotonic structures, with focus on low-dimensional semiconducting materials. We present the reversible population dynamics of the excited state of a two-level quantum emitter and explore the dynamics of the open system in the non-Markovian limit. In particular, we quantify the non-Markovian spontaneous emission dynamics using different non-Markovianity measures and the quantum speed limit in order to investigate the potential quantum speed-up of the dynamics under non-Markovian evolution. In case of a V-type three-level quantum emitter, high-degree quantum interference in its spontaneous emission in conjunction with strong light-matter coupling, resulting in non-Markovian dynamics, features a rich spectrum of population dynamics phenomena, depending on the energy of the quantum emitter with respect to the energies of the exciton-polariton resonances of the nanophotonic structure and the initial state of the quantum emitter.
Department of Physics, National Technical University of Athens
In topological phases of matter, properties like conductivity and optical reflectivity are not determined by the crystalline order but by the topological order of the electronic states. Such states of matter are the integer and fractional quantum Hall states [1], as well as the recently discovered topological insulators [2]. A typical example of an integer quantum Hall state is graphene under the influence of a periodic magnetic field while topological insulators are binary alloys based on Bi, e.g., Bi1-xSbx. Here we propose that we can have topological states of photons in certain metamaterial structures operating as photonic analogues of the above electronic states of matter. We show in particular, that a gyrotropic (chiral) medium supporting a longitudinal-wave excitation exhibits a Dirac point in the corresponding photon dispersion lines. By breaking the time-reversal symmetry in such a medium, the dispersion relation resembles the energy dispersion of a spin-polarized two-dimensional electron gas with Rashba spin-orbit coupling. The resulting split bands of the dispersion relation correspond to nonzero Chern numbers implying the existence of nontrivial topological states of the electromagnetic field [3] in similarity to the integer quantum Hall effect. Topological photonic band structures can also emerge in two-dimensional electromagnetic lattices of metamaterial components without the application of an external magnetic field. The topological nature of the band structure manifests itself by the occurrence of exceptional points in the band structure or by the emergence of one-way guided modes. Based on an EM network with nearly flat frequency bands of nontrivial topology, we propose a coupled-cavity lattice made of superconducting transmission lines and cavity QED components which is described by the Janes-Cummings-Hubbard model and can serve as simulator of the fractional quantum Hall effect [4]. We also show that a tetragonal lattice of weakly interacting particles with uniaxial electromagnetic response is the photonic counterpart of topological crystalline insulators, a new topological phase of atomic band insulators [4]. Namely, the frequency band structure stemming from the interaction of resonant modes of the individual cavities exhibits an omnidirectional band gap within which gapless surface states emerge for finite slabs of the lattice [5].
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