The multimarket contacts theory; an application to Italian banks

The multimarket contact hypothesis holds that more contacts between firms competing in the same markets may induce more collusion. This paper tests the hypothesis for the Italian banking market, analysing the behaviour of the largest Italian banks from 1990 to 1996. Market rivalry is gauged by changes in loan market shares and interest rates in each Italian province. Different measures of multimarket contacts are built. We estimate the effects of increasing multimarket contacts, concentration indicators, banksÂ’ costs and loan growth on variations in market shares and interest rates. No support is found for the multimarket contact hypothesis. Geographical overlap in banking is positively correlated with changes in market shares, confirming the thesis of an overall increase in competition within the Italian banking system. Greater multimarket links also seem to correspond to lower lending rates.banks, antitrust policy, multimarket contacts, panel data

Morphological instability of core-shell metallic nanoparticles

Bimetallic nanoparticles (often known as nanoalloys) with core-shell arrangement are of special interest in several applications, such as in optics, catalysis, magnetism and biomedicine. Despite wide interest in applications, the physical factors stabilizing the structures of these nanoparticles are still unclear to a great extent, especially for what concerns the relationship between geometric structure and chemical ordering pattern. Here global-optimization searches are performed in order to single out the most stable chemical ordering patterns corresponding to the most important geometric structures, for a series of weakly miscible systems, including AgCu, AgNi, AgCo and AuCo. The calculations show that (i) the overall geometric structure of the nanoalloy and the shape and placement of its inner core are strictly correlated; (ii) centered cores can be obtained in icosahedral nanoparticles but not in crystalline or decahedral ones, in which asym- metric quasi-Janus morphologies form; (iii) in icosahedral nanoparticles, when the core exceeds a critical size, a new type of morphological instability develops, making the core asymmetric and extending it to- wards the nanoparticle surface; (iv) multi-center patterns can be obtained in polyicosahedral nanoalloys. Analogies and differences between the instability of the core in icosahedral nanoalloys and the Stranski- Krastanov instability occurring in thin-film growth are discussed. All these issues are crucial for designing strategies to achieve effective coatings of the cores.Comment: 30 pages, 15 figures. Accepted for publication in Physical Review

Structural transformations in Cu, Ag, and Au metal nanoclusters

Finite-temperature structures of Cu, Ag, and Au metal nanoclusters are calculated in the entire temperature range from 0 K to melting using a computational methodology that we proposed recently [Settem \emph{et al.}, Nanoscale, 2022, 14, 939]. In this method, Harmonic Superposition Approximation (HSA) and Parallel Tempering Molecular Dynamics (PTMD) are combined in a complementary manner. HSA is accurate at low temperatures and fails at higher temperatures. PTMD, on the other hand, effectively samples the high temperature region and melting. This method is used to study the size- and system-dependent competition between various structural motifs of Cu, Ag, and Au nanoclusters in the size range 1 to 2 nm. Results show that there are mainly three types of structural changes in metal nanoclusters depending on whether a solid-solid transformation occurs. In the first type, global minimum is the dominant motif in the entire temperature range. In contrast, when a solid-solid transformation occurs, the global minimum transforms either completely to a different motif or partially resulting in a co-existence of multiple motifs. Finally, nanocluster structures are analyzed to highlight the system-specific differences across the three metals.Comment: The following article has been accepted by Journal of Chemical Physics. After it is published, it will be found at https://doi.org/10.1063/5.0159257. Accepted version of the manuscript (along with supplementary material) consists of 32 pages, 11 figure

Local Enhancement of Lipid Membrane Permeability Induced by Irradiated Gold Nanoparticles

Photothermal therapies are based on the optical excitation of plasmonic nanoparticles in the biological environment. The effects of the irradiation on the biological medium depend critically on the heat transfer process at the nanoparticle interface, on the temperature reached by the tissues, as well as on the spatial extent of temperature gradients. Unfortunately, both the temperature and its biological effects are difficult to be probed experimentally at the molecular scale. Here, we approach this problem using nonequilibrium molecular dynamics simulations. We focus on photoporation, a photothermal application based on the irradiation of gold nanoparticles by single, short-duration laser pulses. The nanoparticles, stably bound to cell membranes, convert the radiation into heat, inducing transient changes of membrane permeability. We make a quantitative prediction of the temperature gradient around the nanoparticle upon irradiation by typical experimental laser fluences. Water permeability is locally enhanced around the nanoparticle, in an annular region that extends only a few nanometers from the nanoparticle interface. We correlate the local enhancement of permeability at the nanoparticle-lipid interface to the temperature inhomogeneities of the membrane and to the consequent availability of free volume pockets within the membrane core

Au Nanoparticles in Lipid Bilayers: a Comparison between Atomistic and Coarse Grained Models

The computational study of the interaction between charged, ligand-protected metal nanoparticles and model lipid membranes has been recently addressed both at atomistic and coarse grained level. Here we compare the performance of three versions of the coarse grained Martini force field at describing the nanoparticle-membrane interaction. The three coarse-grained models differ in terms of treatment of long-range electrostatic interactions and water polarizability. The NP-membrane interaction consists in the transition from a metastable NP- membrane complex, in which the NP is only partially embedded in the membrane, to a configuration in which the NP is anchored to both membrane leaflets. All the three coarse grained models provide a description of the metastable NP-membrane complex that is consistent with that obtained using an atomistic force field. As for the anchoring transition, the polarizable- water Martini correctly describes the molecular mechanisms and the energetics of the transition. The standard version of the Martini model, instead, underestimates the free energy barriers for anchoring and does not completely capture the membrane deformations involved in the transition process

Evidence of thermal transport anisotropy in stable glasses of vapour deposited organic molecules

Vapour-deposited organic glasses are currently in use in many optoelectronic devices. Their operation temperature is limited by the glass transition temperature of the organic layers and thermal management strategies become increasingly important to improve the lifetime of the device. Here we report the unusual finding that molecular orientation heavily influences heat flow propagation in glassy films of small molecule organic semiconductors. The thermal conductivity of vapour-deposited thin-film semiconductor glasses is anisotropic and controlled by the deposition temperature. We compare our data with extensive molecular dynamics simulations to disentangle the role of density and molecular orientation on heat propagation. Simulations do support the view that thermal transport along the backbone of the organic molecule is strongly preferred with respect to the perpendicular direction. This is due to the anisotropy of the molecular interaction strength that limit the transport of atomic vibrations. This approach could be used in future developments to implement small molecule glassy films in thermoelectric or other organic electronic devices.Comment: main manuscript: 17 pages and 7 figures; supplementary material: 6 pages and 7 figure

Charting nanocluster structures via convolutional neural networks

A general method to obtain a representation of the structural landscape of nanoparticles in terms of a limited number of variables is proposed. The method is applied to a large dataset of parallel tempering molecular dynamics simulations of gold clusters of 90 and 147 atoms, silver clusters of 147 atoms, and copper clusters of 147 atoms, covering a plethora of structures and temperatures. The method leverages convolutional neural networks to learn the radial distribution functions of the nanoclusters and to distill a low-dimensional chart of the structural landscape. This strategy is found to give rise to a physically meaningful and differentiable mapping of the atom positions to a low-dimensional manifold, in which the main structural motifs are clearly discriminated and meaningfully ordered. Furthermore, unsupervised clustering on the low-dimensional data proved effective at further splitting the motifs into structural subfamilies characterized by very fine and physically relevant differences, such as the presence of specific punctual or planar defects or of atoms with particular coordination features. Owing to these peculiarities, the chart also enabled tracking of the complex structural evolution in a reactive trajectory. In addition to visualization and analysis of complex structural landscapes, the presented approach offers a general, low-dimensional set of differentiable variables which has the potential to be used for exploration and enhanced sampling purposes.Comment: 28 pages, 13 figure

Tuning the coalescence degree in the growth of Pt–Pd nanoalloys

Coalescence is a phenomenon in which two or more nanoparticles merge to form a single larger aggregate. By means of gas-phase magnetron-sputtering aggregation experiments on Pt-Pd nanoalloys, it is shown that the degree of coalescence can be tuned from a growth regime in which coalescence is negligible to a regime where the growth outcome is dominated by coalescence events. This transition is achieved by varying both the length of the aggregation zone and the pressure difference between the aggregation and the deposition chamber. In the coalescence-dominated regime, a wide variety of coalescing aggregates is produced and analyzed by TEM. The experimental results are interpreted with the aid of molecular-dynamics simulations. This allows to distinguish four different steps through which coalescence proceeds towards equilibrium. These steps, occurring on a hierarchy of well-separated time scales, consist in: (i) alignment of atomic columns; (ii) alignment of close-packed atomic planes; (iii) equilibration of shape; (iv) equilibration of chemical ordering

Site-Specific Wetting of Iron Nanocubes by Gold Atoms in Gas-Phase Synthesis

A key challenge in nanotechnology is the rational design of multicomponent materials that beat the properties of their elemental counterparts. At the same time, when considering the material composition of such hybrid nanostructures and the fabrication process to obtain them, one should favor the use of nontoxic, abundant elements in view of the limited availability of critical metals and sustainability. Cluster beam deposition offers a solvent- and, therefore, effluent-free physical synthesis method to achieve nanomaterials with tailored characteristics. However, the simultaneous control of size, shape, and elemental distribution within a single nanoparticle in a small-size regime (sub-10 nm) is still a major challenge, equally limiting physical and chemical approaches. Here, a single-step nanoparticle fabrication method based on magnetron-sputtering inert-gas condensation is reported, which relies on selective wetting of specific surface sites on precondensed iron nanocubes by gold atoms. Using a newly developed Fe-Au interatomic potential, the growth mechanism is decomposed into a multistage model implemented in a molecular dynamics simulation framework. The importance of growth kinetics is emphasized through differences between structures obtained either experimentally or computationally, and thermodynamically favorable configurations determined via global optimization techniques. These results provide a roadmap for engineering complex nanoalloys toward targeted applications.Peer reviewe

Brown amphibole as tracer of tectono-magmatic evolution of the Atlantis Bank Oceanic Core Complex (IODP Hole U1473A)

Brown amphibole is a minor but common mineral component in lower oceanic crust. It is generally interpreted as products of migrating SiO2 and H2O-rich fluids or melts, which can be either residual melts from advanced magmatic differentiation of Mid-Ocean Ridge Basalt (MORB), or hydrothermal fluids including a seawater component. Within the lower oceanic crust exhumed at the Atlantis Bank Oceanic Core Complex (OCC), along the ultraslow Southwest Indian Ridge, brown amphibole is ubiquitous in all lithologies from olivine- to oxide-gabbros and diorites, including both undeformed and plastically deformed varieties. We here show the results of a systematic petrological study conceived to unravel the nature of the H2O-rich component recorded in brown amphiboles and document: (i) the evolution of migrating melts during the magmatic stage and (ii) different extents of melt-bearing deformation events recorded throughout the entire crustal transect. The low Cl contents and the light over heavy Rare Earth Elements (LREE/HREE) ratios and high Ti contents in brown amphiboles, indicate they crystallized from melts with a magmatic hydrous component. Consistently, their δ18O values are in equilibrium with MORB composition, except for diorite amphiboles that possibly record the local assimilation of altered minerals. In undeformed olivine gabbros, interstitial pargasite crystallized at hypersolidus conditions (~1000°C) from the melt residual after late stages of MORB differentiation. We speculate that before the olivine gabbro crystal mush reached fully solid state, some aliquots of residual melts were extracted and accumulated within discrete intervals. There, ferrobasaltic melts differentiated through the early crystallization of Fe-Ti oxides and clinopyroxene as liquidus phases, ultimately forming the oxide gabbros. This process promoted rapid Si enrichment and depletion in Fe, Ti, V in the residual melt, later extracted to form the crosscutting diorite veins. The mylonitic olivine gabbros record high-temperature plastic deformation (~900°C ± 50°C) under hypersolidus conditions, involving melts residual from previous crystallization of the gabbroic rock. Further solid-state plastic deformation led to substantial grain-size reduction and, consequently, to an increase in porosity. This created pathways for subsequent melt focussing, which likely represent late-stage differentiated melts migrating throughout the lower crustal section. This study shows that brown amphibole in the Atlantis Bank lower oceanic crust is the crystallization product of melts residual from advanced magmatic differentiation, which are also locally involved in the plastic deformation events during crustal accretion