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The flow curves of polymers often reveal the onset of melt instabilities such as sharkskin, stick–slip, or gross melt fracture, in order of increasing shear rates. The focus of this work lies in the application of the Göttfert sharkskin option to the investigation of flow curves of styrene-butadiene rubber (SBR) compounds. The sharkskin option consists of highly sensitive pressure transducers located inside a slit die of a capillary rheometer. This tool allows the detection of in-situ pressure fluctuation characteristics of different melt instabilities. It is shown that the change of slope of the transition region in the flow curves is only linked to slip. Dynamic Mechanical Analysis (DMA) measurements furthermore show that the shear rate at which the change of slope can be observed shows the same temperature dependency as the viscous and elastic properties of the compounds.

A dilute ensemble of randomly oriented non-interacting spherical nanomagnets is considered, and its magnetization structure and ensuing neutron scattering response are investigated by numerically solving the Landau–Lifshitz equation. Taking into account the isotropic exchange interaction, an external magnetic field, a uniaxial magnetic anisotropy for the particle core, and in particular the Néel surface anisotropy, the magnetic small-angle neutron scattering cross section and pair-distance distribution function are calculated from the obtained equilibrium spin structures. The numerical results are compared with the well known analytical expressions for uniformly magnetized particles and provide guidance to the experimentalist. In addition, the effect of a particle-size distribution function is modelled.

In the search for non-Abelian anyonic zero modes for inherently fault-tolerant quantum computing, the hybridized superconductor - quantum Hall edge system plays an important role. Inspired by recent experimental realizations of this system, we describe it through a microscopic theory based on a BCS superconductor with Rashba spin-orbit coupling and Meissner effect at the surface, which is tunnel-coupled to a spin-polarized integer or fractional quantum Hall edge. By integrating out the superconductor, we arrive at an effective theory of the proximitized edge state and establish a qualitative description of the induced superconductivity. We predict analytical relations between experimentally available parameters and the key parameters of the induced superconductivity, as well as the experimentally relevant transport signatures. Extending the model to the fractional quantum Hall case, we find that both the spin-orbit coupling and the Meissner effect play central roles. The former allows for transport across the interface, while the latter controls the topological phase transition of the induced p-wave pairing in the edge state, allows for particle-hole conversion in transport for weak induced pairing amplitudes, and determines when pairing dominates over fractionalization in the proximitized fractional quantum Hall edge. Further experimental indicators are predicted for the system of a superonductor coupled through a quantum point contact with an integer or fractional quantum Hall edge, with a Pauli blockade which is robust to interactions and fractionalization as a key indicator of induced superconductivity. With these predictions we establish a more solid qualitative understanding of this important system, and advance the field towards the realization of anyonic zero modes.

Chemically processed methylammonium tin-triiodide (CH3NH3SnI3) films include Sn in different oxidation states, leading to poor stability and low power conversion efficiency of the resulting solar cells (PSCs). The development of absorbers with Sn [2+] only has been identified as one of the critical steps to develop all Sn-based devices. Here, we report on coevaporation of CH3NH3I and SnI2 to obtain absorbers with Sn being only in the preferred oxidation state [+2] as confirmed by X-ray photoelectron spectroscopy. The Sn [4+]-free absorbers exhibit smooth highly crystalline surfaces and photoluminescence measurements corroborating their excellent optoelectronic properties. The films show very good stability under heat and light. Photoluminescence quantum yields up to 4 × 10^-3 translate in a quasi Fermi-level splittings exceeding 850 meV under one sun equivalent conditions showing high promise in developing lead-free, high efficiency, and stable PSCs.

Micromagnetic small-angle neutron scattering theory is well established for analyzing spin-misalignment scattering data of bulk ferromagnets. Here, this theory is extended to allow for a global uniaxial magnetic anisotropy (texture) of the material, in addition to the already included random zero-average local anisotropy. Macroscopic cross-sections and spin-misalignment response functions are computed analytically for several practically relevant mutual anisotropy and external magnetic field orientations in both parallel and perpendicular scattering geometries for field magnitudes both above and below the rotational saturation. Some of these expressions are tested on published experimental data of magnetic-field-annealed Vitroperm and plastically-deformed Ni, allowing to determine the corresponding global uniaxial anisotropy quality factors., Comment: 22 pages, 3 figures

Cylindrically symmetric cholesteric liquid crystal elastomer (CLCE) fibers templated by tubular confinement are reported, displaying mechanochromic, thermochromic, and thermomechanical responses. The synthesis inside a sacrificial tube secures radial orientation of the cholesteric helix, and the ground state retroreflection wavelength is easily tuned throughout the visible spectrum or into the near‐infrared by varying the concentration of a chiral dopant. The fibers display continuous, repeatable, and quantitatively predictable mechanochromic response, reaching a blue shift of more than −220 nm for 180% elongation. The cylindrical symmetry renders the response identical in all directions perpendicular to the fiber axis, making them exceptionally useful for monitoring complex strains, as demonstrated in revealing local strain during tying of different knots. The CLCE reflection color can be revealed with high contrast against any background by taking advantage of the circularly polarized reflection. Upon heating, the fibers respond—fully reversibly—with red shift and radial expansion/axial contraction. However, there is no transition to an isotropic state, confirming a largely forgotten theoretical prediction by de Gennes. These fibers and the easy way of making them may open new windows for large‐scale application in advanced wearable technology andbeyond.

Using experiments and numerical simulations, we demonstrate that the curvature of microscale conduits allow programming of liquid crystal (LC) flows. Focusing on a nematic LC flowing through U- and L-shaped channels of rectangular cross-section, our results reveal that curved flow paths can trigger gradients of flow-induced director field in the transverse direction. The emergent director field feeds back into the flow field, ultimately leading to LC flows controlled by the channel curvature. This curvature-mediated flow control, identified by polarizing optical microscopy and supported by the nematofluidic solutions, offers novel concepts in LC-based microfluidic valves and throttles, wherein the throughput distribution is determined by the Ericksen number and the variations in the local curvature. Finally, this work highlights the role of deformation history on flow-induced director alignments, when the viscous and elastic effects comparable in strength., 6 pages, 4 figures

Funder: European Commission, Funder: Digital, Industry and Space, Funder: European Research Council, Graphene is an ideal platform to study the coherence of quantum interference pathways by tuning doping or laser excitation energy. The latter produces a Raman excitation profile that provides direct insight into the lifetimes of intermediate electronic excitations and, therefore, on quantum interference, which has so far remained elusive. Here, we control the Raman scattering pathways by tuning the laser excitation energy in graphene doped up to 1.05 eV. The Raman excitation profile of the G mode indicates its position and full width at half-maximum are linearly dependent on doping. Doping-enhanced electron-electron interactions dominate the lifetimes of Raman scattering pathways and reduce Raman interference. This will provide guidance for engineering quantum pathways for doped graphene, nanotubes, and topological insulators.

Drying of biologically-relevant sessile droplets, including passive systems (like DNA and proteins), as well as active microbial systems comprising bacteria and algae, have garnered considerable attention over the last decades. Distinct morphological patterns emerge when bio-colloids undergo drying, with significant potential in a range of biomedical applications, spanning bio-sensing, medical diagnostics, drug delivery, and antimicrobial resistance. This review presents a comprehensive overview of bio-colloidal droplets drying on solid substrates, focusing on the experimental progress during the last ten years. We provide a summary of the relevant properties of bio-colloids and link their composition (constituent particles, solvent, and concentrations) to the patterns emerging due to drying. We examined the drying patterns generated by passive bio-colloids (DNA, globular, fibrous, and composite proteins, plasma, serum, blood, urine, tears, saliva). This article highlights how morphological patterns are influenced by the nature of the biological entities and the solvent, micro- and global environmental conditions. Correlations between emergent patterns and the initial droplet compositions enable the detection of potential clinical abnormalities when compared with the patterns of drying droplets of healthy control samples, offering a diagnostic blueprint. Recent experimental investigations of pattern formation in the bio-mimetic and salivary drying droplets, relevant to COVID-19 are also presented. Finally, we summarize the role of biologically active agents in drying process, including bacteria and algae during the drying process. The review concludes with a perspective on the next generation of research and applications based on drying droplets, enabling potential innovations and tools to study this exciting interface of physics, biology, data sciences, and machine learning., Comment: 92 pages, 22 figures

On the basis of Brown's static equations of micromagnetics, the uniaxial polarization of the scattered neutron beam of a bulk magnetic material is computed. The approach considers a Hamiltonian that takes into account the isotropic exchange interaction, the antisymmetric Dzyaloshinskii–Moriya interaction, magnetic anisotropy, the dipole–dipole interaction and the effect of an applied magnetic field. In the high-field limit, the solutions for the magnetization Fourier components are used to obtain closed-form results for the spin-polarized small-angle neutron scattering (SANS) cross sections and the ensuing polarization. The theoretical expressions are compared with experimental data on a soft magnetic nanocrystalline alloy. The micromagnetic SANS theory provides a general framework for polarized real-space neutron methods, and it may open up a new avenue for magnetic neutron data analysis on magnetic microstructures.

The small‐angle neutron scattering data of nanostructured magnetic samples contain information regarding their chemical and magnetic properties. Often, the first step to access characteristic magnetic and structural length scales is a model‐free investigation. However, due to measurement uncertainties and a restricted q range, a direct Fourier transform usually fails and results in ambiguous distributions. To circumvent these problems, different methods have been introduced to derive regularized, more stable correlation functions, with the indirect Fourier transform being the most prominent approach. Here, the indirect Fourier transform is compared with the singular value decomposition and an iterative algorithm. These approaches are used to determine the correlation function from magnetic small‐angle neutron scattering data of a powder sample of iron oxide nanoparticles; it is shown that with all three methods, in principle, the same correlation function can be derived. Each method has certain advantages and disadvantages, and thus the recommendation is to combine these three approaches to obtain robust results., Three different approaches are compared for determination of the correlation function from the small‐angle neutron scattering data of a powder sample of iron oxide nanoparticles.

Absolute photoluminescence measurements present a tool to predict the quality of photovoltaic absorber materials before finishing the solar cells. Quasi Fermi level splitting predicts the maximal open circuit voltage. However, various methods to extract quasi Fermi level splitting are plagued by systematic errors in the range of 10–20 meV. It is important to differentiate between the radiative loss and the shift of the emission maximum. They are not the same and when using the emission maximum as the “radiative” band gap to extract the quasi Fermi level splitting from the radiative efficiency, the quasi Fermi level splitting is 10 to 40 meV too low for a typical broadening of the emission spectrum. However, radiative efficiency presents an ideal tool to compare different materials without determining the quasi Fermi level splitting. For comparison with the open circuit voltage, a fit of the high energy slope to generalised Planck’s law gives more reliable results if the fitted temperature, i.e. the slope of the high energy part, is close to the actual measurement temperature. Generalised Planck’s law also allows the extraction of a non-absolute absorptance spectrum, which enables a comparison between the emission maximum energy and the absorption edge. We discuss the errors and the indications when they are negligible and when not.

Cu(In,Ga)S-2 holds the potential to become a prime candidate for use as the top cell in tandem solar cells owing to its tunable bandgap from 1.55 eV (CuInS2) to 2.50 eV (CuGaS2) and favorable electronic properties. Devices above 14% power conversion efficiency (PCE) can be achieved by replacing the CdS buffer layer with a (Zn,Mg)O or Zn(O,S) buffer layer. However, the maximum achievable PCE of these devices is limited by the necessary high heating temperatures during or after buffer deposition, as this leads to a drop in the quasi-Fermi level splitting (qFLs) and therefore the maximum achievable open-circuit voltage (V-OC). In this work, a low-temperature atomic layer deposited (Zn,Sn)O thin film is explored as a buffer layer to mitigate the drop in the qFLs. The devices made with (Zn,Sn)O buffer layers are characterized by calibrated photoluminescence and current-voltage measurements to analyze the optoelectronic and electrical characteristics. An improvement in the qFLs after buffer deposition is observed for devices prepared with the (Zn,Sn)O buffer deposited at 120 degrees C. Consequently, a device with a V-OC value above 1 V was achieved. A 14% PCE is externally measured and certified for the best solar cell. The results show the necessity of developing a low-temperature buffer deposition process to maintain and translate absorber qFLs to device V-OC.

The construction of exactly-solvable models has recently been advanced by considering integrable $T\bar{T}$ deformations and related Hamiltonian deformations in quantum mechanics. We introduce a broader class of non-Hermitian Hamiltonian deformations in a nonrelativistic setting, to account for the description of a large class of open quantum systems, which includes, e.g., arbitrary Markovian evolutions conditioned to the absence of quantum jumps. We relate the time evolution operator and the time-evolving density matrix in the undeformed and deformed theories in terms of integral transforms with a specific kernel. Non-Hermitian Hamiltonian deformations naturally arise in the description of energy diffusion that emerges in quantum systems from time-keeping errors in a real clock used to track time evolution. We show that the latter can be related to an inverse $T\bar{T}$ deformation with a purely imaginary deformation parameter. In this case, the integral transforms take a particularly simple form when the initial state is a coherent Gibbs state or a thermofield double state, as we illustrate by characterizing the purity, R\'enyi entropies, logarithmic negativity, and the spectral form factor. As the dissipative evolution of a quantum system can be conveniently described in Liouville space, we further discuss the spectral properties of the Liouvillians, i.e., the dynamical generators associated with the deformed theories. As an application, we discuss the interplay between decoherence and quantum chaos in non-Hermitian deformations of random matrix Hamiltonians and the Sachdev-Ye-Kitaev model., Comment: 28 pages, 6 figures

The long-wavelength physics of monolayer graphene in the presence of periodic strain fields has a natural chiral scattering network description. When the strain field varies slowly compared to the graphene lattice and the effective magnetic length of the induced valley pseudomagnetic field, the low-energy physics can be understood in terms of valley-polarized percolating domain-wall modes. Inspired by a recent experiment, we consider a strain field with threefold rotation and mirror symmetries but without twofold rotation symmetry, resulting in a system with the connectivity of the oriented kagome network. Scattering processes in this network are captured by a symmetry-constrained phenomenological $S$ matrix. We analyze the phase diagram of the kagome network, and show that the bulk physics of the strained graphene can be qualitatively captured by the network when we account for a percolation transition at charge neutrality. We also discuss the limitations of this approach to properly account for boundary physics., Comment: 13 pages; 11 figures

We demonstrate that the Berry curvature of an isolated Bloch miniband in two-dimensional superlattices can be probed by the dressed linear optical response when a uniform static field is applied to the system. In particular, when the static field is sufficiently strong such that full Bloch oscillations occur before the crystal momentum relaxes to equilibrium, the optical response of the dressed system becomes resonant at the Bloch frequencies. The latter are in the THz regime when the superlattice periodicity is of the order of 10 nm. Using a band-projected semiclassical theory, we define a dressed optical conductivity and find that the height of the resonances in the dressed Hall conductivity are proportional to the Fourier components of the Berry curvature. We illustrate our results with a low-energy model on an effective honeycomb lattice., Comment: 5 + 4 pages, 3 figures

We develop the theory of an Andreev junction, which provides a method to probe the intrinsic topology of the Fermi sea of a two-dimensional electron gas (2DEG). An Andreev junction is a Josephson $\pi$ junction proximitizing a ballistic 2DEG, and exhibits low-energy Andreev bound states that propagate $\textit{along}$ the junction. It has been shown that measuring the nonlocal Landauer conductance due to these Andreev modes in a narrow linear junction leads to a topological Andreev rectification (TAR) effect characterized by a quantized conductance that is sensitive to the Euler characteristic $\chi_F$ of the 2DEG Fermi sea. Here we expand on that analysis and consider more realistic device geometries that go beyond the narrow linear junction and fully adiabatic limits considered earlier. Wider junctions exhibit additional Andreev modes that contribute to the transport and degrade the quantization of the conductance. Nonetheless, we show that an appropriately defined $\textit{rectified conductance}$ remains robustly quantized provided large momentum scattering is suppressed. We verify and demonstrate these predictions by performing extensive numerical simulations of realistic device geometries. We introduce a simple model system that demonstrates the robustness of the rectified conductance for wide linear junctions as well as point contacts, even when the nonlocal conductance is not quantized. Motivated by recent experimental advances, we model devices in specific materials, including InAs quantum wells, as well as monolayer and bilayer graphene. These studies indicate that for sufficiently ballistic samples observation of the TAR effect should be within experimental reach., Comment: Main: 18 pages, 18 figures. Supplement: 1 appendix

Lead vanadate PbVO3 is a polar crystal with a P4mm space group at ambient conditions. It is isostructural with the model soft-mode driven ferroelectric PbTiO3, but differs from it by the so-called 'supertetragonal' elongation of its unit cell. In this paper, we report a combined study of the lattice dynamics of PbVO3 by Raman spectroscopy at room temperature and first-principle calculations. All zone-center transverse optical (TO) phonon modes are identified by polarized, angle-dependent Raman spectroscopy and assigned as follows: E modes at 136, 269, 374 and 508 cm-1, A1 modes at 188, 429 and 874 cm-1 and B1 mode at 319 cm-1. The calculations confirm the experimental symmetry assignment and allow to obtain the longitudinal (LO) phonons wavenumbers. Besides, we analyze the mode eigenvectors in detail, in order to identify the atomic displacements associated with each mode and compare them with PbTiO3. In spite of their differences in chemistry and strain, the phonon eigenvectors are found to be remarkably comparable in both compounds. We discuss the position of the ferroelectric soft mode in PbVO3 as compared to PbTiO3. A sizeable splitting of the B1+E modes appears as a characteristic feature of supertetragonal phases. The peculiarity of the vanadyl V-O bond frequency in PbVO3 is also addressed., 11 pages, 8 figures, 6 Tables, Supplemental Information 3 figures and 6 Tables, under review

Using supervised and unsupervised machine learning (ML) on features generated from nonadiabatic (NA) molecular dynamics (MD) trajectories under the classical path approximation, we demonstrate that mutual information with the NA Hamiltonian can be used for feature selection and model simplification. Focusing on CsPbI3, a popular metal halide perovskite, we observe that the chemical environment of a single element is sufficient for predicting the NA Hamiltonian. The conclusion applies even to Cs, although Cs does not contribute to the relevant wave functions. Interatomic distances between Cs and I or Pb and the octahedral tilt angle are the most important features. We reduce a typical 360-parameter ML force-field model to just a 12-parameter NA Hamiltonian model, while maintaining a high NA-MD simulation quality. Because NA-MD is a valuable tool for studying excited state processes, overcoming its high computational cost through simple ML models will streamline NA-MD simulations and expand the ranges of accessible system size and simulation time. The work was supported by U.S. National Science Foundation Grant CHE-1900510.

Quantum speed limits (QSLs) identify fundamental time scales of physical processes by providing lower bounds on the rate of change of a quantum state or the expectation value of an observable. We introduce a generalization of QSL for unitary operator flows, which are ubiquitous in physics and relevant for applications in both the quantum and classical domains. We derive two types of QSLs and assess the existence of a crossover between them, that we illustrate with a qubit and a random matrix Hamiltonian, as canonical examples. We further apply our results to the time evolution of autocorrelation functions, obtaining computable constraints on the linear dynamical response of quantum systems out of equilibrium and the quantum Fisher information governing the precision in quantum parameter estimation., 14 pages, 4 figures

We demonstrate that biomechanical forcing plays an important role as the driving force behind the dynamical self-regulation of cell size (or length) in growing bacterial colonies. In our experiments, the measured elongation rate decreases over time and depends on the areal packing density around each cell. To describe this phenomenon theoretically, we devise a cell-resolved model which includes as its key ingredient a force opposed to the growth process, accounting for mechano-self-regulation. Our model is analyzed analytically by a coarse-grained dynamical density functional theory and solved by cell-based computer simulations to predict how the strength of mechano-self-regulation depends on the bacterial size, the location in the colony and the local environment. The microscopic nature of this approach allows to quantify the effect of biomechanical interactions on the structure, composition and dynamical features of growing bacterial colonies.

Cu(In,Ga)S2 is a chalcopyrite material suitable as the higher bandgap top cell in tandem applications in next generation multijunction solar cells. This owes primarily to the tunability of its bandgap from 1.5 eV in CuInS2 to 2.45 eV in CuGaS2, and its relative stability over time. Currently, a major hinderance to the potential use of Cu(In,Ga)S2 in tandem capacity remains a deficient single-junction device performance in the form of low open-circuit voltage (VOC) and low efficiency. Aside interfacial recombination which leads to losses in the completed Cu(In,Ga)S2 solar cell, deficiencies stems from a low optoelectronic quality of the Cu(In,Ga)S2 absorber quantified by the quasi-Fermi level splitting (QFLS) and which serves as the upper limit of VOC achievable by a solar cell device. In this thesis, the QFLS is compared with the theoretical VOC (SQ-VOC) in the radiative limit, and “SQ-VOC deficit” is defined to compare the difference between SQ-VOC and QFLS as a comparable measure of the optoelectronic deficiency in the absorber material. In contrast to the counterpart Cu(In,Ga)Se2 absorber which has produced highly efficient solar cell devices, the Cu(In,Ga)S2 absorber still suffers from a high SQ-VOC deficit. However, SQ-VOC deficit in Cu(In,Ga)S2 can be reduced by growing the absorbers under Cu-deficient conditions. For the effective use of Cu(In,Ga)S2 as the top cell in tandem with Si or Cu(In,Ga)Se2 as the bottom cell, an optimum bandgap of 1.6-1.7 eV is required, and this is realized in absorbers with Ga content up to [Ga]/([Ga]+[In]) ratio of 0.30-0.35. However, the increase of Ga in Cu-poor Cu(In,Ga)S2 poses a challenge to the structural and optoelectronic quality of the absorber, resulting from the formation of segregated Ga phases with steep Ga/bandgap gradient which constitutes a limitation to the quality of the Cu(In,Ga)S2 absorber layer with a highSQ-VOC deficit and low open circuit voltage and overall poor performance of the finalized solar cell. In this work, the phase segregation in Cu(In,Ga)S2 has been circumvented by employing higher substrate temperatures and adapting the Ga flux during the first-stage of deposition when growing the Cu(In,Ga)S2 absorbers. A more homogenous Cu(In,Ga)S2 phase and improved Ga/bandgap gradient is achieved by optimizing the Ga flux at higher substrate temperature to obtain a Cu(In,Ga)S2 absorber with high optoelectronic quality and low SQ-VOC deficit. Additionally, the variation of the Cu-rich phase when growing the Cu(In,Ga)S2 absorber layers was found to not only alter the notch profile and bandgap minimum of the absorbers, but also influence the optoelectronic quality of the absorber. Shorter Cu-rich phase in the absorbers led to narrower notch profile and higher bandgap. Ultimately, several steps in the three-stage deposition method used for processing the Cu(In,Ga)S2 absorbers were revised to enhanced the overall quality of the absorbers. Consequently, the SQ-VOC deficit in high bandgap Cu(In,Ga)S2 absorbers is significantly reduced, leading to excellent device performance. This thesis also examines the temperature- and compositional-related optoelectronic improvement in pure Cu-rich CuInS2 absorbers without Ga, where improvement in QFLS was initially linked to a reduction of nonradiative recombination channels with higher deposition temperatures and increase in Cu content. Findings through photoluminescence decay measurements show that the origin of the improved QFLS in CuInS2 is rather linked to changes in doping levels with variations of deposition temperature and Cu content. Finally, in order to understand and gain insight into the influence of Ga in Cu(In,Ga)S2, the electronic structure of CuGaS2 absorbers was investigated in dependence of excitation intensity and temperature by low temperature photoluminescence measurements. A shallow donor level and three acceptor levels were detected. It was found that similar acceptor levels in CuInSe2 and CuGaSe2 which are otherwise shallow become deeper in CuGaS2. These deep defects serve as nonradiative recombination channels and their appearance in the Ga-containing compound is be detrimental to the optoelectronic quality of Cu(In,Ga)S2 absorbers as Ga content is increased therefore limiting the optimum performance of Cu(In,Ga)S2 devices.

The progressive trend to miniaturize samples presents a challenge to materials characterization techniques in terms of both lateral resolution and chemical sensitivity. The latest generation of focused ion beam (FIB) platforms has allowed to advance in a variety of different fields, including nanotechnology, geology, soil, and life sciences. State-of-the-art ultra-high resolution electron microscopy (EM) devices coupled with secondary ion mass spectrometry (SIMS) systems have enabled to perform in-situ morphological and chemical imaging of micro- and even nanosized objects to better understand materials by studying their properties correlatively. However, SIMS images are prone to artefacts induced by the sample topography as the sputtering yield changes with respect to the primary ion beam incidence angle. Knowing the exact sample topography is crucial to understand SIMS images. Moreover, using non-reactive primary ions (Ne+) produced in a gas field ion source (GFIS) allows to image in SIMS with an excellent lateral resolution of < 20 nm, but it comes with a lower ionization probability compared to reactive sources (e.g., Cs+) and due to small probe sizes only a limited number of atoms are sputtered, resulting in low signal statistics. This thesis focused first on taking advantage of high-resolution in-situ EM-SIMS platforms for applications in specific research fields and to go beyond traditional correlative 2D imaging workflows by developing adapted methodologies for 3D surface reconstruction correlated with SIMS (3D + 1). Applying this method to soil microaggregates and sediments allowed not only to enhance their visualization but also to acquire a deeper understanding of materials’ intrinsic transformation processes, in particular the organic carbon sequestration in soil biogeochemistry. To gain knowledge of the influence of the topography on surface sputtering, using model samples the change of the sputtering yield under light ion bombardment (He+, Ne+) for different ranges of incidence angles of the primary ion beam was studied experimentally. This data was compared to Monte Carlo simulation results and fitted with existing sputtering model functions. We showed thus that these models developed and studied for heavier ions (Ar+, Cs+) are also applicable to light ions (He+, Ne+). Additionally, an algorithm used to correct SIMS images with respect to topographical artefacts resulting from local changes of the sputtering yield was presented. Finally, the contribution of oxygen on positive SI yields was studied for non-reactive primary ions (25 keV Ne+) under high primary ion current densities (up to 10^20 ions/(cm2 ∙ s)). It was shown that in order to maximize and maintain a high ionization probability oxygen needs to be provided continuously to the surface. Secondary ion signal enhancement of up to three orders of magnitude were achieved for silicon, opening the doors for SIMS imaging at both highest spatial resolution and high sensitivity.

Static soliton bound states in nonlinear systems are investigated analytically and numerically in the framework of the parametrically driven and damped nonlinear Schrödinger equation. We find that the ordinary differential equations, which determine bound soliton solutions, can be transformed into the form resembling the Schrödinger-like equations for eigenfunctions with fixed eigenvalues. We assume that a nonlinear part of the equations is close to the reflectionless potential well occurring in the scattering problem associated with the integrable equations. We show that symmetric two-hump soliton solution is quite well described analytically by the three-soliton formula with the fixed soliton parameters, depending on the strength of parametric pumping and the dissipation constant.

We study the phenomenon of quantum friction in a system consisting of a polarizable atom moving at a constant speed parallel to a metallic plate. The metal is described using a charged hydrodynamic model for the electrons. This model featuring long-range interactions is appropriate for a clean metal in a temperature range where scattering due to Coulomb interactions dominates over the scattering of electron by impurities. We find that a quantum friction force between the atom and the metal surface exists even in the absence of intrinsic damping in the metal, but that it only starts once the velocity of the atom exceeds the effective speed of sound in the metal. We argue that this condition can be fulfilled most easily in metals with nearly empty or nearly filled bands. We make quantitative predictions for the friction force to the second and fourth order in the atomic polarizability, and show that the threshold behavior persists to all orders of the perturbation theory.

An anisotropic interlayer force field that describes the interlayer interactions in molybdenum disulfide (MoS2) is presented. The force field is benchmarked against density functional theory calculations for both bilayer and bulk systems within the Heyd–Scuseria–Ernzerhof hybrid density functional approximation, augmented by a nonlocal many-body dispersion treatment of long-range correlation. The parametrization yields good agreement with the reference calculations of binding energy curves and sliding potential energy surfaces for both bilayer and bulk configurations. Benchmark calculations for the phonon spectra of bulk MoS2 provide good agreement with experimental data, and the calculated bulk modulus falls in the lower part of experimentally measured values. This indicates the accuracy of the interlayer force field near equilibrium. Under external pressures up to 20 GPa, the developed force field provides a good description of compression curves. At higher pressures, deviations from experimental data grow, signifying the validity range of the developed force field.

With a record power conversion efficiency of 23.35% and a low carbon footprint, Cu(In,Ga)Se2 remains as one of the most suitable solar energy materials to assist in the mitigation of the climate crisis we are currently facing. The progress seen in the last decade of Cu(In,Ga)Se2 advancement, has been made possible by the development of postdeposition treatments (PDTs) with heavy alkali metals. PDTs are known to affect both surface and bulk properties of the absorber, resulting in an improvement of the solar cell parameters open-circuit voltage, short-circuit current density and fill factor. Even though the beneficial effects of PDTs are not questioned, the underlying mechanisms responsible for the improvement, mainly the one related to the open-circuit voltage, are still under discussion. Although such improvement has been suggested to arise from a suppression of bulk recombination, the complex interplay between alkali metals and grain boundaries has complicated the labour to discern what exactly in the bulk material is profiting the most from the PDTs. In this regard, the development of this thesis aims at investigating the effects of PDTs on the bulk properties of Cu(In,Ga)Se2 single crystals, i.e., to study the effects of alkali metals in the absence of grain boundaries. Most of the presented analyses are based on photoluminescence, since this technique allows to get access to relevant information for solar cells such as the quasi-Fermi level splitting and the density of tail states directly from the absorber layer, and without the need of complete devices. This work is a cumulative thesis of three scientific publications obtained from the results of the different studies carried out. Each publication aims at answering important questions related to the intrinsic properties of Cu(In,Ga)Se2 and the effects of PDTs. The first publication presents a thorough investigation on the effects of a single heavy alkali metal species on the optoelectronic properties of Cu(In,Ga)Se2. In the case of polycrystalline absorbers, the effects of potassium PDTs in the absence of sodium have been previously attributed to the passivation of grain boundaries and donor-like defects. The obtained results, however, suggest that potassium incorporated from a PDT can act as a dopant in the absence of grain boundaries and yield an improvement in quasi-Fermi level splitting of up to 30 meV in Cu-poor CuInSe2, where a type inversion from N-to-P is triggered upon potassium incorporation. This observation led to the second paper, where a closer look was taken to how the carrier concentration and electrical conductivity of alkali-free Cu-poor CuInSe2 is affected by the incorporation of gallium in the solid solution Cu(In,Ga)Se2. The results obtained suggest that the N-type character of CuInSe2 can remain as such until the gallium content reaches the critical concentration of 15-19%, where the N-to-P transition occurs. A model based on the trends in formation energies of donor and acceptor-like defects is presented to explain the experimental results. The conclusions drawn in this paper shed light on why CuGaSe2 cannot be doped N-type like CuInSe2. Since a decreased density of tail states as a result of reduced band bending at grain boundaries had been previously pointed out as the mechanism behind the improvement of the open-circuit voltage after postdeposition treatments, the third publication focusses on how compositional variations and alkali incorporation affect the density of tail states of Cu(In,Ga)Se2 single crystals. The results presented in this paper suggest that increasing the copper and reducing the gallium content leads to the reduction of tail states. Furthermore, it is observed that tail states in single crystals are similarly affected by the addition of alkali metals as in the case of polycrystalline absorbers, which demonstrates that tail states arise from grain interior properties and that the role of grain boundaries is not as relevant as it was thought. Finally, an analysis of the voltage losses in high-efficiency polycrystalline and single crystalline solar cells, suggested that the doping effect caused by the alkalis affects the density of tail states through the reduction of electrostatic potential fluctuations, which are reduced due to a decrease in the degree of compensation. By taking the effect of doping on tail sates into account, the entirety of the VOC losses in Cu(In,Ga)Se2 is described. The findings presented in this thesis explain the link between tail states and open circuit voltage losses and demonstrate that the effects of alkali metals in Cu(In,Ga)Se2 go beyond grain boundary passivation. The results presented shed light on the understanding of tail states, VOC losses and the intrinsic properties of Cu(In,Ga)Se2, which is a fundamental step in this technology towards the development of more efficient devices.

We study electronic transport in Weyl semimetals with spatially varying nodal tilt profiles. We find that the flow of electrons can be guided precisely by judiciously chosen tilt profiles. In a broad regime of parameters, we show that electron flow is described well by semiclassical equations of motion similar to the ones governing gravitational attraction. This analogy provides a physically transparent tool for designing tiltronic devices like electronic lenses. The analogy to gravity circumvents the notoriously difficult full-fledged description of inhomogeneous solids. A comparison to microscopic lattice simulations shows that it is only valid for trajectories sufficiently far from analogue black holes. We finally comment on the Berry curvature-driven transverse motion and relate the latter to spin precession physics., Replaced with revised version for SciPost resubmission. 38 pages, 12 figures

The seemingly simple step of molding a cholesteric liquid crystal into spherical shape, yielding aCholesteric Spherical Reflector(CSR), has profound optical consequences that open a range of opportunities for potentially transformative technologies. The chiral Bragg diffraction resulting from the helical self-assembly of cholesterics becomes omnidirectional in CSRs. This turns them into selective retroreflectors that are exceptionally easy to distinguish—regardless of background—by simple and low-cost machine vision, while at the same time they can be made largely imperceptible to human vision. This allows them to be distributed in human-populated environments, laid out in the form of QR-code-like markers that help robots and Augmented Reality (AR) devices to operate reliably, and to identify items in their surroundings. At the scale of individual CSRs, unpredictable features within each marker turn them into Physical Unclonable Functions (PUFs), of great value for secure authentication. Via the machines reading them, CSR markers can thus act as trustworthy yet unobtrusive links between the physical world (buildings, vehicles, packaging,…) and its digital twin computer representation. This opens opportunities to address pressing challenges in logistics and supply chain management, recycling and the circular economy, sustainable construction of the built environment, and many other fields of individual, societal and commercial importance.

Liquid Crystal Elastomers (LCEs) are an exciting category of material that has tremendous application potential across a variety of fields, owing to their unique properties that enable both sensing and actuation. To some, LCEs are simply another type of Shape Memory Polymer, while to others they are an interesting on-going scientific experiment. In this visionary article, we bring an interdisciplinary discussion around creative and impactful ways that LCEs can be applied in the Built Environment to support kinematic and kinetic buildings and situational awareness. We focus particularly on the autonomy made possible by using LCEs, potentially removing needs for motors, wiring and tubing, and even enabling fully independent operation in response to natural environment variations, requiring no power sources. To illustrate the potential, we propose a number of concrete application scenarios where LCEs could offer innovative solutions to problems of great societal importance, such as autonomous active ventilation, heliotropic solar panel systems which can also remove snow or sand autonomously, and invisible coatings with strain mapping functionality, alerting residents in case of dangerous (static or dynamic) loads on roofs or windows, as well as assisting building safety inspection teams after earthquakes.

Water and power related resources (energy sources and required material) are both critical and crucial resources that have become even more and more strategic as a result of climate change and geopolitics. By making a large store of salty water available, desalination appears to be a viable solution to the water crisis already affecting 40% of the population today. However, because existing desalination procedures are power intensive and rely on non-renewable energy resources, their power use at large scale is unsustainable. Alternative techniques exist that are promising in terms of environmental impact, but not yet competitive in terms of fresh water outflow and energy efficiency. The focus of this work is on one of these alternatives, Air-Gap Membrane Distillation (AGMD), which was chosen because it relies on low-grade heat that is easy to collect from solar radiations or from industrial waste heat. This technique mimics the water cycle, thanks to the use of a membrane, allowing to bring the hot and cold water streams closer together. As a result, the temperature difference that drives evaporation is strengthened and the process accelerated. However, the development of a boundary layer at the membrane interface reduces this temperature difference and thus decreases the overall performance of the process. Thus this technique still requires some improvements to become industrially attractive, in terms of fresh water outflow per kWh and energy use. The goal of this thesis work is to contribute to AGMD energy efficiency and output flow enhancement by leveraging both experimental and theoretical considerations. A test facility characterizing the boundary layer based on a Schlieren method as well as an adapted AGMD module were designed and built. By interacting with the boundary layer, the laser allows the observation of the continuous temperature profile in the hot water channel of a at sheet AGMD module. The measurement can be performed in close proximity to the membrane and under a variety of operational conditions (inlet hot and cold temperatures, inlet velocities). In parallel, the fresh water outflow corresponding to these experimental conditions can be measured. Moreover, the experimental layout opens the way for further observations of the AGMD process from a different angle - such as concentration profiles or experimentation in the air-gap - with very little addition. The overall experimental set-up has eventually been used to produce a first set of data over a range of temperature (60-75◦C), which is then interpreted thanks to a custom algorithm deriving the temperature profiles and boundary layer thicknesses. A three dimensional heat and mass transfer model for AGMD (3DH&MT) - previously developed in the research team - has been used to numerically reproduce the experimental conditions and compare the results. The comparison showed promising results as the temperature gradients at the membrane interface and fresh water outflows present similar orders of magnitude and trends. The accuracy of the experiment can be further increased through several adaptations in the set-up. This 3DH&MT model could be used to simulate more complex AGMD module designs, such as spiral modules in order to optimize the operating conditions and the overall shape of the AGMD module to enhance its performances. Finally, in the aim of improving the energy efficiency and fresh water outflow of the AGMD process, spacers are usually added in the individual channels to boost mixing and thus reduce the boundary layer thickness, which improves evaporation flux. Two novel spacer geometries inspired by current industrial mixing state of the art and nature have been proposed and investigated, yielding interesting results for two distinct applications. One is particularly well-suited to maximizing mixing regardless of the energy used, hence improving the energy efficiency of the process. The second is optimal for minimizing energy consumption while maintaining a decent mixing result, thus enhancing the fresh water outflow of the process. A couple of indicators have also been proposed to assess the mixing performance of more complex 3D geometries. Overall this work broadens the current AGMD research by providing an experimental test-bench enabling the continuous temperature profile measurement, and the validation of a 3D heat and mass transfer model. Moreover, interesting tracks for improving the design of spacers are proposed in order to minimize the AGMD process's energy efficiency resistance. AGMD is an extremely promising water treatment technique since it is applicable to a broader range of waters than just seawater. The test equipment described in this work is sufficiently adaptable to investigate this potential as well as variants of AGMD processes that might boost its attractiveness. As it is based on readily available materials and technologies, it may be used anywhere and its reliance on a naturally available energy flow (solar radiation) makes it attractive in isolated regions.

We aim to identify the control principles governing the adaptable formation of non-equilibrium structures in actomyosin networks. We build a phenomenological model and predict that biasing the energy dissipated by molecular motors should effectively renormalize the motor-mediated interactions between actin filaments. Indeed, using methods from large deviation theory, we demonstrate that biasing energy dissipation is equivalent to modulating the motor rigidity and results in an aster-to-bundle transition. From the simulation statistics, we extract a relation between the biasing parameter and the corresponding normalized motor rigidity. This work elucidates the relationship between energy dissipation, effective interactions, and pattern formation in active biopolymer networks, providing a control principle of cytoskeletal structure and dynamics., 6 pages, 2 figures

Caloric materials are suggested as energy-efficient refrigerants for future cooling devices. They could replace the greenhouse gases used for decades in our air conditioners, fridges, and heat pumps. Among the four types of caloric materials (electro, baro, elasto, magneto caloric), electrocaloric materials are more promising as applying large electric fields is much simpler and cheaper than the other fields. The research in the last years has been focused on looking for electrocaloric materials with high thermal responses. However, the energy efficiency crucial for future replacement of the vapor compression technology has been overlooked. The intrinsic efficiency of electrocaloric has been barely studied. In the present dissertation, we will study the efficiency of EC materials defined as materials efficiency. It is the ratio of the reversible electrocaloric heat to the reversible electrical work required to drive this heat. In this work, we will study the materials efficiency of the benchmark lead scandium tantalate in different shapes (bulk ceramic and multilayer capacitors). A comparison to other caloric materials is presented in this dissertation. Our work gives more insights on the figure merit of materials efficiency to further improve the efficiency of our devices.

The thesis is focused on developing spectral selective coatings (SSC) composed of multilayer cermets and periodic array of resonating omega structures, turning them to behave like metamaterials, while showing high thermal stability up to1000°C. The developed SSC is intended to be used for the concentrated solar power (CSP) applications. With the aim of achieving highest possible absorbance in the visible region of the spectrum and highest reflectance in the infrared region of the spectrum. The thesis highlights the numerical design, the synthesis and optical characterization of the SSC of approximately 500 nm thickness. A bottom-up approach was adopted for the preparation of a stack with alternate layers, consisting of a distribution of Titanium Nitride (TiN) nanoparticles with a layer of Aluminum Nitride (AlN) on top. The TiN nanoparticles, laid on a Silicon substrate by wet chemical method, are coated with conforming layer of AlN, via Plasma-enhanced Atomic Layer Deposition (PE-ALD). The control of the morphology at the nanoscale is fundamental for tuning the optical behaviour of the material. For this reason, two composites were prepared. One starting with TiN dispersion made with dry TiN powder and deionized water, and the other with ready-made TiN dispersion. Nano-structured metamaterial based absorbers have many benefits over conventional absorbers, such as miniaturisation, adaptability and frequency tuning. Dealing with the current challenges of producing the new metamaterial based absorber with optimal nanostructure design along with its synthesis within current nano-technological limits, we were capable of turning the cermets into metamaterial. A periodic array of metallic omega structures was patterned on top of both the composites I and II, by using e-beam lithography technique. Parameters, such as the size of TiN nanoparticles, the thickness of AlN thin film and the dimensions of the omega structure were all revealed by the numerical simulations, performed using Wave-Optics module in COMSOL Multiphysics. The work showcased clearly compares the two kinds of composites, using scanning electron microscope, X-ray photoelectron spectroscopy(XPS) and electrical conductivity measurement. The improvement in the optical performance of the SSC after the inclusion of metallic omega structures in the uppermost layer of the two composites has been thoroughly investigated for light absorption boosting. In addition, the optical performance of the two prepared composites and the metamaterial is used as a means of validating the computational model.

Computational studies have shown that one or more positrons can stabilize two repelling atomic anions through the formation of two-center positronic bonds. In the present work, we study the energetic stability of a system containing two positrons and three hydride anions, namely 2\ce{e^+[H^{3-}_3]}. To this aim, we performed a preliminary scan of the potential energy surface of the system with both electrons and positron in a spin singlet state, with a multi-component MP2 method, that was further refined with variational and diffusion Monte Carlo calculations, and confirmed an equilibrium geometry with \ce{D_${3h}$} symmetry. The local stability of 2\ce{e^+[H^{3-}_3]} is demonstrated by analyzing the vertical detachment and adiabatic energy dissociation channels. Bonding properties of the positronic compound, such as the equilibrium interatomic distances, force constants, dissociation energies, and bonding densities are compared with those of the purely electronic \ce{H^+_3} and \ce{Li^+_3} systems. Through this analysis, we find compelling similarities between the 2\ce{e^+[H^{3-}_3]} compound and the trilithium cation. Our results strongly point out the formation of a non-electronic three-center two-positron bond, analogous to the well known three-center two-electron counterparts, which is fundamentally distinct from the two-center two-positron bond [D. Bressanini, \textit{J. Chem. Phys.} \textbf{155}, 054306 (2021)], thus extending the concept of positron bonded molecules., Submitted to Chemical Science

In order to improve the accuracy of molecular dynamics simulations, classical force fields are supplemented with a kernel-based machine learning method trained on quantum-mechanical fragment energies. As an example application, a potential-energy surface is generalised for a small DNA duplex, taking into account explicit solvation and long-range electron exchange--correlation effects. Study of the corrected potential energy versus extension shows that leading classical DNA models have excessive stiffness with respect to stretching. This discrepancy is found to be common across multiple forcefields. The quantum correction is in qualitative agreement to the experimental thermodynamics for larger DNA double helices, providing a candidate explanation for the general and long-standing discrepancy between single molecule stretching experiments and classical calculations of DNA stretching. The new dataset of quantum calculations and the associated Kernel Modified Molecular Dynamics (KMMD) method should be of general utility in biomolecular simulations. KMMD is made available as part of the AMBER22 simulation software., Comment: Publisher postprint (with minor corrections) is open access at J Chem Phys https://doi.org/10.1063/5.0094727

Active systems, which are driven out of equilibrium by local non-conservative forces, exhibit unique behaviors and structures with potential utility for the design of novel materials. An important and difficult challenge along the path towards this goal is to precisely predict how the structure of active systems is modified as their driving forces push them out of equilibrium. Here, we use tools from liquid-state theories to approach this challenge for a classic minimal active matter model. First, we construct a nonequilibrium mean-field framework which can predict the structure of systems of weakly interacting particles. Second, motivated by equilibrium solvation theories, we modify this theory to extend it with surprisingly high accuracy to systems of strongly interacting particles, distinguishing it from most existing similarly tractable approaches. Our results provide insight into spatial organization in strongly interacting out-of-equilibrium systems., 7 pages, 2 figures. arXiv admin note: substantial text overlap with arXiv:2012.10441

9 pags., 5 figs., 2 tabs., In a scenario of spontaneous symmetry breaking in finite time, topological defects are generated at a density that scales with the driving time according to the Kibble-Zurek mechanism (KZM). Signatures of universality beyond the KZM have recently been unveiled: The number distribution of topological defects has been shown to follow a binomial distribution, in which all cumulants inherit the universal power-law scaling with the quench rate, with cumulant rations being constant. In this work, we analyze the role of boundary conditions in the statistics of topological defects. In particular, we consider a lattice system with nearest-neighbor interactions subject to soft antiperiodic, open, and periodic boundary conditions implemented by an energy penalty term. We show that for fast and moderate quenches, the cumulants of the kink number distribution present a universal scaling with the quench rate that is independent of the boundary conditions except for an additive term, which becomes prominent in the limit of slow quenches, leading to the breaking of power-law behavior. We test our theoretical predictions with a one-dimensional scalar theory on a lattice., F.J.G.-R. acknowledges financial support from European Commission FET-Open project AVaQus GA 899561.

The following work investigates the development of heat pumps that exploit electrocaloric effects in Pb(Sc,Ta)03 (PST) multilayer capacitors (MLCs). The electrocaloric effect refers to reversible thermal changes in a material upon application (and removal) of an electric field. Electrocaloric cooling is interesting because 1) it has the potential to be more efficient than competing technologies, such as vapour-compression systems, and 2) it does not compel the use of greenhouse gases, which is crucial in order to slow down global warming and mitigate the effects of climate change. The continuous progress in the field of electrocalorics has promoted the creation of several electrocaloric based heat pump prototypes. Despite the different designs and working principles utilized, these prototypes have struggled to maintain temperature variations as large as 10 K, discouraging their industrial development. In this work, bespoke PST-MLCs exhibiting large electrocaloric effects near room temperature were embodied in a novel heat pump with the motivation to surpass the 10 K-barrier. The experimental design of the heat pump was based on the outcome of a numerical model. After implementing some of the modifications suggested by the latter, consistent temperature spans of 13 K at 30 °C were reported, with cooling powers of 12 W / kg. Additional simulations predicted temperature spans as large as 50 K and cooling powers in the order of 1000 W / kg, if a new set of plausible modifications were to be put in place. Similarly, these very same PST-MLCs samples were implemented into pyroelectric harvesters revisiting Olsen's pioneering work from 1980. The harvested energies were found to be as large as 11.2 J, with energy densities reaching up to 4.4 J / cm3 of active material, when undergoing temperature oscillations of 100 K under electric fields applied of 140-200 kV / cm. These findings are two and four times, respectively, larger than the best reported values in the literature. The results obtained in this dissertation are beyond the state-of-the-art and show that 1) electrocaloric heat pumps can indeed achieve temperature spans larger than 10 K, and 2) pyroelectric harvesters can generate electrical energy in the Joule-range. Moreover, numerical models indicated that there is still room for improvement, especially when it comes to the power of these devices. This should encourage the development of these kinds of electrocaloric- and pyroelectric-based applications in the near future.

We show that Coulomb drag in hydrodynamic bilayer systems leads to additional viscosity terms in the hydrodynamic equations, i.e., the drag and drag-Hall viscosities, besides the well-known kinematic and Hall viscosities. These new viscosity terms arise from a change of the stress tensor due to the interlayer Coulomb interactions. All four viscosity terms are tunable by varying the applied magnetic field and the electron densities in the two layers. At certain ratios between the electron densities in the two layers, the drag viscosity dramatically changes the longitudinal transport resulting in a negative drag conductivity., 4 pages + Appendix, 4 figures