Department of Physics, National Kapodistrian University of Athens www.phys.uoa.gr | Physics Laboratory, Department of Education, ASPETE edu.aspete.gr/en/physics-lab | Hellenic Society for the Science and Technology of Condensed Matter www.hsstcm.eu |

*Computational Materials Science Laboratory
Department of Materials Science and Engineering, University of Ioannina, 45110 Ioannina, Greece
*

The second generation of β-type Ti-based alloys has been promising for replacing the widely used TiAl6V4 implants due to their low Young moduli, high corrosion resistance and minimal cytotoxicity while their enrichment with well-known antibacterial elements like Ga, Cu and Ag might to cause antibiofilm activity.
This work presents Density Functional Theory calculations using Siesta or Vasp software on Ti-based alloys aiming to reveal the electronic origin of the structural 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. The calculated 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. The enrichment of β-TiNb with selective elements like In and Sn might decrease the Young modulus while Ga and Ag might provide antibacterial characteristics. 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.

*Department of Physics, School of Applied Mathematical and Physical Sciences, National Technical University of Athens*

The existence of so-called many body interactions among the electrons of any physical system makes it impossible to solve the corresponding quantum-mechanical equations with full accuracy. The Density Functional Theory (DFT) approach has been arguably the most successful method to deal with the very difficult challenge of probing the properties of materials at the atomic-scale, as it typically provides results which are in very good agreement with pertinent experimental data. The first part of this talk will outline the basic framework of DFT calculations, both in terms of the underlying theory and with regard to practical implementation. The second part will present representative recent DFT studies on a wide range of materials which are used in state-of-the-art electronic and optoelectronic devices.

*School of Physics, Department of Condensed Matter and Materials Physics, Aristotle University of Thessaloniki, 54124 Greece.*

Recently several machine learning-based methods have been introduced in order to build interatomic potentials. Among these approaches, the ones displaying the highest degree of accuracy involve training the potentials on ab initio and essentially on Density Functional Theory based data. In striving for this objective, it is essential to give thorough attention to three key aspects: the data acquisition process, data representation, and the machine learning methodology.

* ITCP-Department of Physics, University of Crete, Heraklion, 71003, Greece
Institute for Electronic structure and Lasers (IESL-FORTH), Heraklion, 71110, Greece*

In the framework of non-Hermitian photonics, we present recent results regarding Anderson localization in open systems and the interplay of robustness and sensitivity in non-Hermitian topological lattices that exhibit higher order exceptional points. Emphasis on the computational aspects will be given. The first part of the talk will be devoted to localization problems in non-Hermitian disordered media. A generalization of the concept of plane waves [1-3] and a novel way of propagation [4,5] that is possible only in non-Hermitian systems will be presented. In the second part of the talk, we will examine the interplay between topological protection and non-Hermiticity around higher order exceptional points and their relation to the underlying theory of pseudospectra [6-9].

- [1] K.G. Makris, A. Brandstotter, P. Ambichl, Z.H. Musslimani, and S. Rotter, "Wave propagation through disordered media without backscattering and intensity variations", Light Sci. Appl. 6, e17035 (2017).
- [2] E. Rivet, A. Brandstotter, K.G. Makris, H. Lissek, S. Rotter, and R. Fleury, "Constant- pressure sound waves in nonHermitian disordered media", Nat. Phys. 14, 479 (2018).
- [3] K.G. Makris, I. Kresic, A. Brandstötter, and S. Rotter, "Scattering-free channels of invisibility across non-Hermitian media", Optica 7, 619 (2020).
- [4] A. Tzortzakakis, K.G. Makris, A. Szameit, and E. N. Economou, "Transport and spectral features in non-Hermitian open systems", Phys. Rev. Research 3, 013208 (2021).
- [5] A. Leventis, K.G. Makris, and E. N. Economou, "Non-Hermitian jumps in disordered lattices", Phys. Rev. B 106, 064205 (2022).
- [6] S. Xia, D. Kaltsas, D. Song, I. Komis, J. Xu, A. Szameit, H. Buljan, K. G. Makris, and Z. Chen, "Nonlinear tuning of PT symmetry and non-Hermitian topological states", Science 372, 72 (2021).
- [7] I. Komis, D. Kaltsas, S. Xia, H. Buljan, Z. Chen, and K. G. Makris, "Robustness versus sensitivity in non-Hermitian topological lattices probed by pseudospectra", Phys. Rev. Research 4, 043219 (2022).
- [8] K. G. Makris, L. Ge, and H. E. Türeci, "Anomalous transient amplification of waves in non-normal photonic media", Phys. Rev. X 4, 041044 (2014).
- [9] K. G. Makris, "Transient growth and dissipative exceptional points", Phys. Rev. E 104, 054218 (2021)

*School of Electrical and Computer Engineering, National Technical University of Athens, Athens GR-15780, Greece*

Unconventional superconductivity has been recently detected in a two-dimensional (2D) superlattice created by stacking two sheets of
graphene that are twisted relatively to each other at "magic" angles leading to flat electronic band structure [1]. This major discovery
has been theoretically interpreted and given several follow-up findings concerning strong many-body correlations and transitions
between superconducting, insulating or metallic phases. In fact, magic angles in graphene bilayers ignited a new approach to manipulate
the electronic properties of 2D media labeled as "twistronics" [2] and has returned several premiums like quantized anomalous Hall
Effect and Dirac revival transitions. Remarkably, suitable relative rotations in bilayers have provided setups serving a broad range of
applications such as gate-controlled quantum interference via Josephson junctions and manipulation of radiative heat transfer via exotic
directional energy transport.

A key question to be addressed would be: "How twistronics behave in the presence of nonlinearities?" Indeed, with nonlinear substances,
the respective systems may acquire multiple stable outputs dependent on the past values of their excitation, even though their inputs at
the present time are identical. In this way, the setup reacts differently judging from the history of controlling parameters and develops
memory utilities for them. In this talk, instead of being restricted to monolayers of 2D media, generalized metasurfaces char acterized
by anisotropic complex conductivities are considered being coupled via the electromagnetic waves going back and forth. Importantly,
the role of controlling parameters will be played by the rotation angles (twists) of the optical axes and multistability with respect to them
will be demonstrated. Therefore, a nonlinear analog to the electromagnetic twistronics developed in [3] will be presented, with emphasis
on the polarization engineering possibilities that arise. The formulated nonlinear boundary value problem is treated by im plementing
the analytical techniques elaborated in [4], where a detection for all possible states of the system is achieved simply by finding roots of
a real transcendental equation within a close interval.

Keywords: Magic angles, Multistability, Nonlinear metasurfaces, Twistronics.

- [1] Y. Cao, V. Fatemi, S. Fang, K. Watanabe, T. Taniguchi, E. Kaxiras, and P. Jarillo-Herrero, "Unconventional superconductivity in magic-angle graphene superlattices", Nature (2018).
- [2] S. Carr, D. Massatt, S. Fang, P. Cazeaux, M. Luskin, and E. Kaxiras, "Twistronics: Manipulating the electronic properties of two-dimensional layered structures through their twist angle'", Phys. Rev. B (2017).
- [3] C. Valagiannopoulos, "Electromagnetic Analog to Magic Angles in Twisted Bilayers of Two-Dimensional Media", Phys. Rev. Appl. (2022).
- [4] C. Valagiannopoulos, "Multistability in Coupled Nonlinear Metasurfaces", IEEE Trans. Antennas Propag. (2022).

*Computational Materials Science Laboratory
Department of Materials Science and Engineering, University of Ioannina, 45110 Ioannina, Greece
Institute of Materials Science and Computing, University Research Center of Ioannina, 45110 Ioannina, Greece*

Interactions between light and graphene carriers are in the heart of graphene-based
optoelectronic applications [1]. They manifest graphene's optical, electrical, and thermal
properties as well as the interplay between them, including ultrafast processes such as nonequilibrium carrier excitation, saturable absorption, hot carrier generation, and electron-phonon
cooling [2]. Here we present our self-consistent multi-physics framework, taking into
consideration all the above, for the simulation and design of graphene-based photodetectors,
modulators, and sensors. The excellent comparison with experimental measurements for
selected devices provides validation and confidence in our results. We study integrated
photodetectors at λ = 1550 nm in 3 different architectures based on the photo-thermoelectric
effect: asymmetric source-drain contacts, graphene split-gate pn-junction, plasmonic slot pnjunction. These devices operate at zero bias and under proper optimization can reach ~A/W
responsivity and ~100 GHz operation speed [3]. We also study various free-space applications in
a wide range of frequencies from NIR to THz, including a MIR graphene/Si Schottky
photodetector [2,4] operating in the thermionic regime, a MIR pn-junction photodetector based on
coupling between plasmons in metallic bow-tie antennas and hyperbolic phonon-polaritons in
the hBN encapsulation [5], and a THz self-induced ultrafast absorption modulation of a Salisbury
screen type of device based on graphene [6]. Finally, we show how graphene, critically coupled
to a dielectric cavity, can be used in the saturable absorption regime as a nonlinear modulator
and extreme pulse shaper, and propose a unique scheme for experimentally studying ultrafast
phenomena without the need for ultrafast optics, i.e., by just using ps pulses. The presented
theoretical framework, validated by experiments, can be used to design different optoelectronic
devices reliably and realistically, across a broadband spectral regime, using a plethora of
materials alongside graphene, including vdW heterostructures.

*Acknowledgement
The research leading to these results received funding from the EU H2020 Projects Graphene Flagship
(881603), Plasmoniac (871391), and KAUST Project LASEMAL (OSR-2020-CRG9-4347).*

- [1] M. Massicotte et al, Nanoscale 13, 8376 (2021)
- [2] S. Doukas et al, Phys. Rev. B 105, 115417 (2022)
- [3] I. Vangelidis et al, ACS Photonics 9, 1992 (2022)
- [4] S. Doukas et al, Appl. Phys. Lett. 121, 051103 (2022)
- [5] S. Castilla, et al, Nature Comm 11, 4872 (2020)
- [6] A.D Koulouklidis et al, ACS Photonics 9, 3075 (2022)

*School of Mathematics, Statistics and Actuarial Science
University of Essex, Colchester CO4 3SQ, United Kingdom*

In this talk I will discuss some of the most powerful methods in the field of Computational Statistical Physics used for unravelling critical and universal behaviours in complex and disordered spin models on the lattice. These methods are an asset for the study of systems with rough free-energy landscapes and when combined with theoretical approaches, such as the renormalisation group and finite-size scaling, can produce robust results to longstanding problems in the field. Selected results will also be presented, mostly focusing on the problem of universality violations in the random-field Ising model but also in the determination of the order of the transition in spin-1 models under the presence of a chemical potential.

*Department of Materials Science and Engineering University of Ioannina, 45110 Ioannina, Greece
*

Micromagnetism is a continuous medium theory in which the magnetic state of a particular magnetic body is described by the spatial dependence of the magnetization vector that has a length equal to the saturation magnetization MS and a direction which is a function of position within the material m(r). The function m(r) is determined by minimizing the total free energy which is a functional of m(r) and its derivatives. The presence of local minima in which the magnetic state can be trapped yields the well-known hysteretic effects that are typically observed in magnets. The continuous medium approach is justified by the fact that, at least in technologically useful magnetic materials, the quantum exchange interactions are strong enough so that the vector m(r) does not vary much over distances shorter than the so-called exchange length, which is of the order of 3-5 nm and therefore much larger than the atomic distances.

The fast dynamics of magnetization are described by the Landau-Lifshitz-Gilbert (LLG) equation. This contains a precession term that relates the frequency precession to the strength of the local effective field, and a damping term that qualitatively expresses how many precession cycles the magnetization will perform until it finally relaxes along the direction of the local field.
Examples of the application of such simulations to the optimization of cobalt nanowire-polymer composite magnets and microwave assisted reversal will be given.

*School of Physics, Department of Solid State Physics, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece
Institute of Chemical Engineering Sciences, Foundation for Research and Technology Hellas (FORTH – ICE/HT), Patras 26504, Greece *

Phonons have a profound influence on various physical processes within solids, impacting, among others, electron mobility, optical properties, thermodynamics, superconductivity, phase transitions, thermal conductivity, and thermal expansion.
In this lecture we explore the framework of lattice dynamics using phenomenological and ab-initio derived interatomic potentials to calculate physical observables such as Raman and Infrared active mode eigenfrequencies and eigenvectors, phonon dispersion curves, and density-of-states.

Concerning two-dimensional (2D) materials emphasis will be given in Raman spectroscopy, a pivotal experimental technique to investigate first- and second order phonon modes, offering unique insights into number of layers, electron or hole doping, structural disorder, edges, and electron-phonon coupling. Through experimental and theoretical examples, we gain insight on intralayer and low-frequency interlayer Raman modes such as breathing and shear modes in graphene, boron nitride, and transition metal dichalcogenides. Finally, the influence of mechanical strain both theoretically and experimentally on the vibrational spectrum of selected 2D materials will be discussed.

*Condensed Matter Physics Laboratory
Department of Physics, University of Thessaly*

The aim of the present lecture is to familiarize students and researchers with the basics of the Molecular Dynamics (MD) method employed in simulations of condensed matter physics systems, with focus on fluids. The foundation on statistical mechanics is briefly presented, based on the classical approach of interatomic interactions. Such atomistic approaches provide fundamental perspectives, and thereby a means to determine important macroscopic parameters solely from the atomistic structure of the material. Results on fluids will be presented at the nanoscale, covering fluid transport properties calculation across scales, as well as implications that appear during ion and other unwanted substance removal from water inside nano-conduits. Future applications that also employ current machine learning techniques, both numerical and symbolic, will be highlighted.

Webpage credits: Ioannis Remediakis and Niki Sorogas