Twisted van der Waals materials have risen as a powerful platform to engineer artificial quantum matter. Artificial moire heterostructures, in general, display two length scales, the original lattice constant and the emergent moire length. Here we reveal a microscopic mechanism to engineer van der Waals multiferroics at those two length scales from the interplay of non-collinear magnetism and spin–orbit coupling, both in van der Waals monolayers [1] and twisted multilayers [2]. First, focusing on the recently isolated NiI2 multiferroic monolayer, we reveal the origin of the helimagnetic order, and the critical role of halide spin-orbit coupling in driving a ferroelectric distortion. We demonstrate that the electronic reconstruction accounting for the ferroelectric order emerges from the interplay of such a non-collinear magnetism and spin-orbit coupling. Second, we show the emergence of multiferroic order in twisted chromium trihalide bilayers, an order fully driven by the moiré pattern and absent in aligned multilayers. We show that a spin texture is generated in the moiré supercell of the twisted system as a consequence of the competition between stacking-dependent interlayer magnetic exchange and magnetic anisotropy. An electric polarization arises associated with such a non-collinear magnetic state due to the spin-orbit coupling, leading to the emergence of a local ferroelectric order following the moiré. Among the stochiometric trihalides, our results show that twisted CrBr3 bilayers give rise to the strongest multiferroic order. We further show the emergence of a strong magnetoelectric coupling, which allows the electric generation and control of magnetic skyrmions. Our results put forward van der Waals materials as a powerful platform to engineer artificial multiferroic order and electrically control exotic magnetic textures.

[1] Adolfo O Fumega and J L Lado 2022 2D Mater. 9 025010 (2022)
[2] Adolfo O. Fumega, Jose L. Lado, arXiv:2207.01416 (2022)

Progress of light-based technology has led to a paradigm shift in materials design: light, which has been commonly relegated to a mere probe of material properties, can be instead exploited to alter material properties altogether [1,2]. Central in the paradigm shift is the possibility of generating strong coupling between light and matter and in turn induce the formation of light-matter hybrid states with properties which can be controlled on demand. An intriguing route to achieve strong light-matter coupling is to embed materials inside cavities [3, 4], where the coupling is enhanced by the confinement of light in a small region of space [4]. In the talk, I will present theoretical results based on first-principles methods on three different hybrid light-matter designs. I will demonstrate the formation of exciton-light hybrid states in 2D crystals embedded in a cavity and the tunability over their energetics and brightness [5]. I will then show how the strong light-matter coupling allows for the design of a three-way exciton-phonon-photon quasiparticle which is characterized by unique features in optical response [6]. Finally, I will introduce the concept of a photo-groundstate by demonstrating that the vacuum fluctuations of light can induce a change of the collective phase from paraelectric to ferroelectric in the groundstate of SrTiO3 (see Fig. adapted from [7]), which has thus far only been achieved in out-of equilibrium strongly excited conditions [1]. These findings demonstrate the potential of cavity material engineering as a new paradigm for material manipulation.

Ultrafast control of light-matter interactions is fundamental in view of new technological frontiers, for instance in light-driven information processing and nanoscale photochemistry [1]. In this framework, we explore metal-dielectric nanocavities to achieve all-optical modulation of the light reflectance at a specific wavelength. Without the need of driving higher order effects, our system is based on linear absorption, provides large relative modulation exceeding 100% and switching bandwidths of few hundred GHz at moderate excitation fluence [2]. This archetypical system becomes even more interesting if the “gain medium” is an inorganic van der Waals bonded semiconductor, like a transition metal dichalcogenide (TMD). TMDs are subject of intense research due to their electronic and optical properties which are promising for next generation optoelectronic devices. In this context, understanding the ultrafast carrier dynamics, as well as charge and energy transfer at the interface between metals and semiconductors is crucial and yet quite unexplored. By employing a pump-push-probe scheme, we experimentally study how thermally induced ultrafast charge carrier injection affects the exciton formation dynamics in bulk WS2 [3], opening up excellent opportunities also in nano-chemistry. In fact, if an electronic transition strongly interacts with the light modes of a resonator, we can tailor the energetics and the morphology of a molecular state. By combining quantum mechanical modelling and pump-probe spectroscopy, we shed light on the ultrafast dynamics of a hybrid system composed of photo-switchable dye molecules coupled with
 anisotropic plasmonic nanoantennas, which allow us to selectively switch between two regimes where the light-matter interaction is either weak or strong [4]. Our synergistic approach is instrumental to devise new strategies for tailoring electronic states by using plasmons for applications in polaritonic chemistry on femtosecond timescales.

[1] A. N. Koya, M. Romanelli, J. Kuttruff, N. Henriksson, A. Stefancu, G. Grinblat, A. De Andres, F. Schnur, M. Vanzan, M. Marsili, M. Rahaman, A. Viejo Rodríguez, T. Tapani, H. Lin, B. D. Dana, J. Lin, G. Barbillon, R. Proietti Zaccaria, D. Brida, D. Jariwala, L. Veisz, E. Cortes, S. Corni, D. Garoli, and N. Maccaferri, “Advances in ultrafast plasmonics”, arXiv:2211.08241.
[2] J. Kuttruff, D. Garoli, J. Allerbeck, R. Krahne, A. De Luca, D. Brida, V. Caligiuri, and N. Maccaferri. “Ultrafast all-optical switching enabled by epsilon-near-zero-tailored absorption in metal-insulator nanocavities”, Commun. Phys. 3, 114, 2020.
[3] K. R. Keller, R. Rojas-Aedo, H. Zhang, P. Schweizer, J. Allerbeck, D. Brida, D. Jariwala, and N. Maccaferri.
“Sub-ps thermionic electron injection effects on exciton-formation dynamics at a van der Waals semiconductor/metal interface”, ACS Photonics 9, 2683-2690, 2022.
[4] J. Kuttruff, M. Romanelli, E. Pedrueza-Villalmanzo, J. Allerbeck, J. Fregoni, V. Saavedra-Becerril, J. Andreasson, D. Brida, A. Dmitriev, S. Corni, and N. Maccaferri. “Ultrafast dynamics of a hybrid plasmon polariton-molecular system at the weak and strong coupling regimes”, arXiv:2205.06358.

Photonic architectures provide means to manage light which can greatly improve the performance of many optoelectronic devices. These nanostructures bring novel pathways to couple more sunlight into solar cells or they can help light escape the bulk of an LED. However, traditional nanofabrication routes are costly, cumbersome and incompatible with high throughput processes. This is why in our group we work with soft nanoimprinting lithography, a scalable, fast and inexpensive way to produce nanostructures from a variety of materials. In this presentation I will show how we use our pre-patterned stamps to induce the long-range ordering of different types of colloids (gold colloids and perovskite nanocrystals) and exploit the optical properties of the resulting metasurface. The combination of templated induced assembly and metal colloids [1],enables 2D plasmonic crystals exhibiting surface plasmon lattice resonances in the visible with high quality factors (> 60), which can be applied to enable lasing from an organic dye. Similarly, nanoimprinting lithography can be applied to cesium lead halide perovskite nanocrystals [2,3] (CsPbBr₃ NCs) to produce a photonic structure that improves photoluminescence from the material and even enables a reduced threshold for ASE (amplified spontaneous emission). In addition, the use of stamps with pre-patterned chiral motifs results in the seamless production of chiral perovskite metasurfaces exhibiting high circularly polarized photoluminescence with dissymmetry factors (glum) as high as 0.3 for unmodified green or red emitting NCs.

The full blossoming of quantum technologies requires the availability of easy-to-prepare materials where quantum coherences can be effectively initiated, controlled, and exploited, preferably at ambient conditions. Solid-state multilayers of colloidally grown quantum dots (QDs) are highly promising for this task because of the possibility of assembling networks of electronically coupled QDs through the modulation of sizes, inter-dot linkers, and distances. [1–3]

In this work, the coherent dynamics of solid-state multilayers of electronically coupled colloidally grown CdSe QDs are explored by two-dimensional electronic spectroscopy (2DES). 2DES techniques have been developed starting from the beginning of 2000, and historically they have been mainly exploited for the investigation of subtle dynamic mechanisms of energy and charge transport in biological complexes. Only later, these techniques have been recognized to be particularly valuable also for the study of transport processes in artificial nanomaterials and nanodevices. [4] In particular, recently proposed ‘action-based techniques’ appear particularly suited to address coherent dynamics in real functioning devices. [5]

In this work, we present the study of semiconductor nanocrystals (‘quantum dots’) in solid-state devices by different variants of 2DES: (i) the fully noncollinear optically detected BOXCARS configuration and (ii) the fully collinear photocurrent detected setup. The time evolution of coherent superposition of states was captured at ambient conditions in both cases. We thus provide important evidence for inter-dot coherences in such solid-state materials, opening up new avenues for the effective application of these materials in
quantum technologies. [6]

MemComputing is a new physics-based approach to computation that employs time non-locality (memory) to both process and store information on the same physical location [1]. Its digital version is designed to solve combinatorial optimization problems. A practical realization of digital memcomputing machines (DMMs) can be accomplished via circuits of non-linear dynamical systems with memory engineered so that periodic orbits and chaos can be avoided. A given logic (or algebraic) problem is first mapped into this type of dynamical system whose point attractors represent the solutions of the original problem. A DMM then finds the solution via a succession of elementary avalanches (instantons) whose role is to eliminate configurations of logical inconsistency (‘‘logical defects’’) from the circuit. I will discuss the physics behind MemComputing and show many examples of its applicability to various combinatorial optimization problems, Machine Learning, and Quantum Mechanics, demonstrating its advantages over traditional approaches and even quantum computing.

Two-dimensional magnetic materials —being the true limit of miniaturization—provide extensive control over theirs structural and magnetic properties and open prodigious opportunities in spintronics and magnonics [1,2]. Thus, the need for theoretical modelling of the large system at the atomic scale arises. Taking the robust and air-stable layered A-type antiferromagnet CrSBr [3] with high Curie temperature [4], which exhibits highly anisotropic electronic transport [5] and magnetic [6,7] properties as an example, we demonstrate an efficient computational approach for modelling of thermodynamic properties and spin dynamics under applied static strain. Starting from the full spin Hamiltonian we evince high sensibility of the critical temperature to the applied strain (40K increase at 5% strain) in the framework of renormalized spin wave theory. By the means of dynamic simulation, we display that the magnetic monolayer exhibit pronounced anisotropy of magnon propagation velocity with respect to the applied strain direction. Thus, such an anisotropic behavior makes this system a promising platform to study magnons in 2D materials.

Transition metal dichalcogenides (TMDs), such as WSe₂ or MoS₂, are van der Waals materials with exceptional properties, such as chemical stability and mechanical flexibility[1], high binding energies[2], [3], high oscillator strengths and narrow photoluminescence linewidths[4]. However, by changing the crystal thickness (from bulk to monolayer), they show a transition from indirect to direct bandgap and so the monolayer is strictly required. On the other hand, Rhenium disulfide (ReS₂) is a group-VII TMDs, with attractive properties, such as exceptionally high refractive index, significant oscillator strength and optical birefringence[5]. This envolves the formation of two orthogonally polarized in–plane excitons[6] induced by ReS₂ crystallization in a distorted single-layer trigonal structure of triclinic symmetry. It is worth nothing that these exceptional optical properties persist from bulk to monolayer, making possible to exploit the high refractive index of this material and to tune the characteristics of the nanophotonic devices by changing crystal thickness. In this work, we observed that for crystals with a thickness higher than 50 nm, the excitonic transitions show a fine structure observed in previous works in literature [6]–[9],  but whose nature  is widely debated. With our work we clearly demonstrate that this fine structure is due to the longitudinal and transverse exciton, which becomes evident in ReS₂ crystals thanks to the unique combination of its exceptional high refractive index and large exciton oscillator strength that ReS₂ possesses.

By transferring ReS₂ flake on a Distributed Bragg Reflector (DBR), we also observed a polarization-dependence strong coupling between Fabry-Perot modes and both excitons in the fundamental and Rydberg excited states, resulting in the formation of middle polariton branches. These show a strong polariton-polariton interaction which results in a blueshift of the polariton mode when the polariton density inside the system increases, that clearly reflect an outstanding interest of ReS₂ as a solid-state counterpart of Rydberg atomic systems. This study offers a complete interpretation of ReS₂ optical features, making this material interesting for a plethora of photonic applications and a novel platform for the exploration of topological properties.

Perovskite nanocrystal superlattices are three-dimensional solid-state ordered ensembles of monodispersed semiconductor nanoparticles. Upon interaction with light pulses, these highly ordered systems can give rise to unexpected phenomena such as superfluorescence, a recently reported many-body quantum state in which the excitons generated in each single perovskite nanocrystal are coherently locked [1]. The possibility to realize macroscopic quantum states with high exciton densities, low energy broadening and long dephasing time, makes these systems promising platform to develop entangled multi-photon quantum light devices or quantum information storage systems [2, 3]. Among the halide perovskite nanocrystals, hybrid organic-inorganic compositions have attracted attention owing to high conversion efficiencies in solar cells [4] and the promise to maintain quantum coherence [5, 6]. For further progress in this field, relationship between cooperative phenomena, nanocrystal composition and excitonic features still need to be established.

In this work, we compare all-inorganic CsPbX₃ (X = Cl, Br or I) and hybrid FAPbX₃ (FA = formamidinium; X = Br or I) superlattices assembled from nanocrystals derived from either a direct synthesis or an aging-assisted size-selection. By combining halides and A-cation alloying, we demonstrate exciton tunability over all the visible spectrum. We employ an ultrashort resonant optical pumping scheme to study the collective states that can arise from an ordered system of quantum confined excitons interacting with laser pulses. Starting from superlattices of CsPbBr₃ nanocrystals, we demonstrate that these systems can be driven across different quantum phases by tuning the laser intensity: from the excitonic Mott insulating phase to the collective superradiant state and the metallic electron-hole liquid phase. The behavior of CsPbBr₃ is compared with that of FAPbI₃ nanocrystal superlattices to understand structure-property relationships.

The direct wet chemical synthesis of lead chalcogenide based 2D semiconductors, so-called nanoplatelets (NPLs), yields photoluminescent materials with strong excitonic effects at room temperature. [1,2,3] Here, we report a direct wet chemical approach [1] followed by a subsequent surface passivation step [2] toward bright colloidal PbSe NPLs with high emission efficiency. The NPLs exhibit excitonic features in the range of 800 - 1000 nm and photoluminescence in the range of 860 - 1510 nm, with the respective positions being seamlessly tunable by adjusting the NPLs lateral size via the reaction parameters. [2] Upon surface passivation with metal halides the NPLs exhibit a photoluminescence quantum yield of up to 60 % depending on the PL position as well as a reduced full width at half maximum of the photoluminescence by 10 %. [1] The enhanced optical properties are ascribed to a combined passivating role of both X- and Z-type binding halides and metal halides, providing an additional tool for tailoring the optical properties, colloidal stability and photostability of the PbSe NPLs. [1]
Efficient and narrow photoluminescence in the technologically desirable near-infrared and shortwave-infrared regions are vital requirements for single photon emission and applicability in quantum optics, rendering colloidal PbSe NPLs promising candidates compatible with integrated and fiber-based technology.
 

Photonic quantum technologies like quantum key distribution for safe data exchange use single photons as information carrier, provided by a single photon source. Among others, semiconductor nanomaterials have shown their capability in single photon emission.[1]

Ultrathin colloidal two-dimensional (2D) semiconductors, so-called nanoplatelets, represent a highly topical material class with optical properties including very narrow absorption and emission.[2] They exhibit strong excitonic effects caused by a few-atom layer thickness of the structures.

Lead chalcogenide-based colloidal 2D nanoplatelets are of special interest, since their band gap can be tailored over a broad near-infrared spectral range.[3,4] For example, colloidal chemistry methods yield 2D PbS nanoplatelets with an optical absorption onset at 680 nm, a thickness of only 1-2 nm and a lateral dimension of ~10 nm². Photoluminescence quantum yields of 20 % at 720 nm are obtained. However, at room temperature, exciton-phonon coupling contributes to photoluminescence linewidth broadening in the structures.[5]

Here, we show single PbS nanoplatelet photoluminescence measurements at cryogenic temperatures, revealing an extremely narrow full-width at half maximum of 0.6 meV. Due to the 2D shape of the nanoplatelet, a polarization anisotropy unknown from spherical quantum dots is observed. We will discuss the prospects of PbS nanoplatelets as interesting colloidal candidates for single photon emission.

In recent years, the study of charge transfer and excitonic properties of van der Waals matter has been a rapidly growing area of research due to their importance for ultrathin optoelectronic, photovoltaic and photocatalytic components. In such applications, quasi-particles and excitons, often act as carriers in charge, spin and energy transfer processes. These transport processes can be significantly impacted by structural complexity, reduced dimensionality, interface composition or the presence of impurities and adatoms. In this talk, I will address some of these complexities from first principles, focusing on transition-metal dichalcogenides (TMDC) and TMDC-graphene interfaces. I will discuss the role of state localization due to atomic-size defects on the optical and magnetic properties [1, 2]. Using many-body perturbation theory within the GW and Bethe-Salpeter equation approach, I will show how the excitonic picture associated with the presence of defects, which result in a reduced valley and spin selectivity due to hybridized electron-hole transitions, can lead to structurally controllable exciton magnetic response. Subsequently, I will delve into the role of symmetries in charge transfer and excitonic properties on TMDC-graphene heterobilayers containing monoatomic chalcogen vacancies [3]. I will analyze the impact of the subgap defect-based features on the microscopic dynamics and absorption features, including the interplay between the spatial and the spin degrees of freedom through the spin-orbit interaction. Finally, I will show how defects become a slow coherent transport channel for interlayer charge transfer while simultaneously stronly altering the exciton properties of the TMDC-graphene interface due to a combination of folding, screening and mixing of the optical transitions.

[1] E. Mitterreiter, B. Schuler, A. Micevic, D. Hernangómez-Pérez, et al. Nat. Comm. 12, 3822 (2021).

[2] T. Amit, D. Hernangómez-Pérez, G. Cohen, D. Y. Qiu, and S. Refaely-Abramson, Phys. Rev. B 106, L161407 (2022).

[3] D. Hernangómez-Pérez, A. Donarini, and S. Refaely-Abramson (Phys. Rev. B, accepted, Editor's Suggestion).

Large-scale, distributed quantum states are the basis for novel applications in quantum communication, quantum remote sensing or distributed quantum computing. The necessary infrastructure will be provided by distributed quantum networks [1], allowing for quantum bit transmission, processing and storage at single nodes. Sources of single photons and entangled photon states are important constituents of such networks. Epitaxially grown quantum dots (QDs) show great potential for the deterministic generation of such photon states. Here we utilize the emerging family of epitaxially grown GaAs/AlGaAs quantum dots obtained by droplet etching and nanohole infilling. They are outstanding sources of indistinguishable single photons and polarization-entangled photon pairs, with a unique combination of metrics (brightness, purity, coherence, emission rate) [2,3]. Under pulsed resonant two-photon excitation, we observe emission of polarization-entangled photon pairs with high purity of 0.99 and high entanglement fidelities up to 0.94. Bright entangled photon emission is realized by tailoring the dielectric environment around the QDs, allowing for the first implementation of a core element in quantum communication: Entanglement swapping between two pairs of photons emitted by a single quantum dot is demonstrated with a fidelity up to 0.81 [2].

Coupling these emitters to quantum memories with long coherence times enables the development of hybrid nanophotonic devices for quantum communication applications. GaAs/AlGaAs QDs exhibit very narrow wavelength distributions at rubidium based quantum memory transitions. We apply strain tuning via piezoelectric actuators to allow for reversible fine-tuning of the emission frequency. Employing active feedback then allows to stabilize the frequency of single photons to an atomic rubidium standard [4]. Another promising quantum memory candidate is Silicon-vacancy centers in diamond, as they show strong interactions with single photons via their zero phonon line at around 737nm. We report the first GaAs/AlGaAs quantum dots emitting single photons at this wavelength. Polarization entangled photons are generated via the biexciton-exciton cascade decay with a fidelity of 0.73. High single photon purity above 89% is maintained up to 80K (above liquid nitrogen temperature), rendering this system attractive for real-world implementations.

To ensure excellent quantum optical characteristics, epitaxial QDs have to be buried under thick barrier layers to maintain a distance from detrimental surface states. However, to unlock their full potential, QDs need to be integrated into scalable (hybrid) nanophotonic devices that require a vanishing distance to the surface. For instance, investigating strong coupling to optical cavity fields requires ultra-small mode volumes, and efficient interaction with plasmonic or dielectric nanoparticles can only be achieved within a few nanometers, therefore requiring surface proximity of QDs. Although single epitaxial QDs very close to the surface have been studied for over two decades. Weak and broad emissions were the result, rendering advanced quantum optical experiments infeasible. We show the complete restoration of optical properties from quantum dots grown directly on a semiconductor surface, leading to bright, ultra-stable, coherent and blinking-free single photon emission [5]. Under quasi-resonant excitation, single photons are generated with 98.8% purity, 77% indistinguishability, linewidths down to 4µeV and 99.7% persistency across 11 orders of magnitude in time. The emission is stable even after two years and when being subjected to nanomanufacturing processes. Some long-standing stumbling blocks for surface-dominated quantum dots are thereby removed, unveiling new possibilities for hybrid nano-devices and applications in quantum communication or sensing.

Since their discovery in 1981 by Alexey I. Ekimov, semiconductor nanocrystals (NCs) have seen a tremendous development and they are now exploited in the consumer electronics market. Colloidal semiconductor NCs present remarkable properties for the fabrication of a variety of photonic applications such as: light-sources, solar cells, X-rays scintillators and detectors. Colloidal semiconductor NCs are synthetized via wet chemistry approaches that offer an endless amount of freedom on the parameters that determine their optoelectronic properties such as crystal size, shape, crystallinity, chemical composition and architectures.

In this talk, I will present how we are focusing on the development of near-infrared (NIR) emitting NCs[1-4], NIR-LEDs[1] and single photon emitters (SPEs) based on isolated NC. NIR emitting NCs and their respective LEDs are of interest for a variety of optoelectronic applications such as hyperspectral and biomedical imaging, night vision and telecommunication systems. Up until now, all efficient NIR NCs are based on toxic heavy metals compositions while The European Union’s “Restriction of Hazardous Substances” (RoHS) directive limits the use of such compounds. Colloidal indium arsenide and lead-free perovskite NCs are emerging as promising candidate for NIR applications thanks to their low toxicity and recent progresses in material synthesis leading to stable and highly efficient NCs. We exploited InAs/ZnSe core-shell NCs to produce NIR-LEDs emitting in the range of 800-1000 nm,[1] while we developed cesium manganese bromide NCs that can be employed as sensitizers for broadband Vis-to-NIR downshifting.[3] Last but not least, these NCs are of interest for the fabrication of SPEs operating in the NIR; yet, study of their emission properties has just begun.

Another important property of NCs is their capability to operate as a SPE. Yet, complex small-footprint devices based on single NCs require controlled placement of isolated NCs on a planar surface to exploit NCs in the emerging fields of quantum communication, computing, and encryption, where optically pumped quantum dots are quickly gathering attention. A whole range of techniques have been explored to control the placing of colloidal NCs from solution on a solid substrate, exploiting e.g., chemical/wettability contrast between different areas of the substrate or other chemical physical interactions. We have applied capillary assembly  (a method that exploits the force exerted on NCs at the interface between the moving meniscus of the solution solvent and a solid substrate) to entrap a NC in pre-patterned holes on the surface, typically realized by means of lithographic methods such as electron beam lithography. The large area arrays we fabricate contain hundreds of single NCs, and we correlated the morphological data collected by scanning electron microscopy with optical data to unveil how the effective density of SPEs depends on an interplay between NC photoluminescence quantum yield and Poisson statistics of the nanohole filling process.

The use of the spin of the electrons is having a tremendous impact on our electronics and computing technologies. Tell­-tale examples are found in the controlled switching and reading of thin film ferromagnets, which enabled the realization of technologies such as the hard disk read head and the magnetic random-access memory. Despite the enormous progress that has been made, current devices are still based on metallic ferromagnets, which typically suffer from high-losses and are limited in frequency.

Magnetic insulators (MIs), such as rare earth garnets (R3Fe5O12; R=Y,Tm,..), have attracted a lot of interest because of their low Gilbert damping and high-frequency dynamics. Interestingly, unlike charge currents, spin currents can couple and propagate through MIs, making possible to realize spintronic devices based on these materials.

In this talk, we will discuss how we can exploit the charge to spin conversion phenomena in heavy metals for reading and writing the magnetic state of MIs [1,2], for electrically injecting and detecting magnons carrying spin information in MIs [3,4,5], and for stabilizing and driving topological magnetic textures (Néel chiral domain walls and skyrmions) in thin film magnetic oxides [6,7]. We will discuss the key role of antisymmetric chiral interactions on the emergence of non-reciprocal phenomena in both magnon transport and the dynamics of magnetic textures. We will see that the performance of garnet devices is better than their metallic counterparts and demonstrate novel device functionalities enabled by chirality that could be exploited in future memory and spin-logic applications.

*saul.velez@uam.es

The development of new technologies has been always accompanied by the access to functional materials with targeted and exceptional properties. Among these materials and looking towards the future, layered materials and metal halide perovskites outline a prospective path for their potential application in optoelectronic, spintronic and quantum technologies.[1–4] However, for their successful integration into devices and the development of new applications, it is key to understand the relationship between composition, crystal structure and optical/magnetic properties and how to control them.

Layered hybrid organic-inorganic metal halide perovskites (HOIPs) have emerged as promising materials for optoelectronic and spintronic applications, namely due to their tunable bandgap, high carrier mobility, strong spin-orbit coupling and magnetic ordering.[1,2] Indeed, HOIPs are an ideal platform for optical (photons) and magnetic (spins) tunability due to their chemical and structural versatility.[5,6] In this line, two case studies are presented. The first one is focused on modulating the optical properties, concretely the photoluminescence (PL), by strain engineering. We report the tuning of the micro-PL emission of 2D lead-bromide HOIP flakes subject to biaxial strain. To generate the mechanical strain, we placed the flakes by viscoelastic stamping on a rigid SiO₂ ring platform, leading to the formation of domes. At low temperatures, we found that a strain < 1% can change the PL emission spectrum from a single peak (unstrained) to three well-resolved peaks. Combining temperature-dependent micro-PL and Raman spectroscopy mapping and reverse mechanical engineering strain modeling, we confirm that the emergence of the two new PL peaks is related to tensile and compressive thermo-mechanically generated strain coexisting along the flake surface and thickness.[7] Our findings provide new insight into strain-based optoelectronic and sensing devices using 2D HOIPs, leveraging on the material composition selection and substrate platform design. The second case deals with the control of the magnetic properties by varying the transition metal (Cu²⁺, Mn²⁺ and Co²⁺), organic spacer (alkyl- and aryl-ammonium) and perovskite phase (Ruddlesden-Popper and Dion-Jacobson). We show that for Cu²⁺ HOIPs, an increase of in-plane anisotropy and a reduction of the interlayer distance lead to a change in their magnetic behavior from a 2D ferromagnet to a quasi-3D antiferromagnet. In contrast, the magnetism of Mn²⁺ HOIPs is intrinsically characterized by antiferromagnetic intralayer interactions. Finally, Co²⁺ crystals with a non-perovskite structure present a dominant paramagnetic behavior. Therefore, our results demonstrate that the chemical flexibility of HOIPs can be exploited to develop novel layered magnetic materials with tailored magnetic properties.[8]

How physical properties change in solids with reduced dimensionalities and symmetry-breaking elements, is a scientific question that has fascinated researchers for decades. However, only the recent advances in material synthesis and nano-characterization have triggered an atomistic study of low-dimensional and topological phenomena.  In this talk, I will discuss how the ultra-high-vacuum growth of two-dimensional and topological materials allows the realization of exotic physical model systems and, at the same time, grants the flexibility to perform atomic engineering to achieve functional material properties.

 In the first part, I will present the long-sought thin-film realization of an inversion-symmetry breaking Weyl Semimetal [1], a recently discovered topological material class featuring a linear electronic dispersion and degeneracy (Weyl) points that lead to peculiar open-loop surface states (Fermi-Arcs). In particular, I will show how topology and electronic structure are connected to epitaxy-dependent parameters such as strain, doping and surface termination [2]. With a careful preparation of in-situ heterostructures, I will discuss how these topological properties of Weyl Semimetal thin films can be exploited for applications in spin-orbitronics and topological superconductivity.

In the second part, I will focus on the van-der-Waals epitaxy of a nearly-ideal two-dimensional magnet, a CrCl₃ monolayer grown on Graphene/6H-SiC(0001). In-situ X-ray magnetic circular dichroism reveals intrinsic ferromagnetic order with easy-plane anisotropy and a 2DXY universality class [3], suggesting the first realization of a Berezinskii-Kosterlitz-Thouless (BKT) phase transition in a quasi-freestanding monolayer magnet. The important role of the van der Waals substrate interaction and the underlying crystal symmetry will be discussed, thereby highlighting routes on how to control the anisotropy of 2D magnets via growth engineering. Finally, taking advantage of our all-in-situ approach, perspectives to interface 2D magnets with other two-dimensional ferroic materials -such as ferroelectrics- will be outlined to study novel collective phenomena in van der Waals heterostructures.