Biasing crystallization in fused silica: An assessment of optimal metadynamics parameters

Metadynamics (MetaD) is a useful technique to study rare events such as crystallization. It has been only recently applied to study nucleation and crystallization in glass-forming liquids such as silicates, but the optimal set of parameters to drive crystallization and obtain converged free energy surfaces is still unexplored. In this work, we systematically investigated the effects of the simulation conditions to efficiently study the thermodynamics and mechanism of crystallization in highly viscous systems. As a prototype system, we used fused silica, which easily crystallizes to β-cristobalite through MetaD simulations, owing to its simple microstructure. We investigated the influence of the height, width, and bias factor used to define the biasing Gaussian potential, as well as the effects of the temperature and system size on the results. Among these parameters, the bias factor and temperature seem to be most effective in sampling the free energy landscape of melt to crystal transition and reaching convergence more quickly. We also demonstrate that the temperature rescaling from T > Tm is a reliable approach to recover free energy surfaces below Tm, provided that the temperature gap is below 600 K and the configurational space has been properly sampled. Finally, albeit a complete crystallization is hard to achieve with large simulation boxes, these can be reliably and effectively exploited to study the first stages of nucleation

Disclosing crystal nucleation mechanism in lithium disilicate glass through molecular dynamics simulations and free-energy calculations

Unraveling detailed mechanism of crystal nucleation from amorphous materials is challenging for both experimental and theoretical approaches. In this study, we have examined two methods to understand the initial stage of crystal precipitation from lithium disilicate glasses using molecular dynamics simulations. One of the methods is a modified exploring method to find structurally similar crystalline clusters in the glass models, enabling us to find three different embryos, such as Li2Si2O5 (LS2), Li2SiO3 (LS) and Li3PO4 (LP), in the 33Li2O·66SiO2·1P2O5 glass (LS2P1), in which P2O5 is added as a nucleating agent. Interestingly, LS2 and LP crystals were found inside the LS2P1 glass while LS crystal appeared on the glass surface, which agrees with experimental observations. The other method is free energy calculation using a subnano-scale spherical crystal embedded in the glass model. This method, which we called Free-Energy Seeding Method (FESM), allows us to evaluate free energy change as a function of crystal radius and to identify critical size of the crystal precipitation. The free energy profiles for LS and LS2 crystal nuclei in the LS2 glass models possess maximum energy at a critical radius as expected by classical nucleation theory. Furthermore, the critical radius and the energy barrier height agree well with recent experimental investigation, proving the applicability of this method to design glass–ceramics by atomistic modeling

Ubiquitous Superconducting Diode Effect in Superconductor Thin Films

The macroscopic coherence in superconductors supports dissipationless supercurrents which could play a central role in emerging quantum technologies. Accomplishing unequal supercurrents in the forward and backward directions would enable unprecedented functionalities. This nonreciprocity of critical supercurrents is called superconducting (SC) diode effect. We demonstrate strong SC diode effect in conventional SC thin films, such as niobium and vanadium, employing external magnetic fields as small as 1 Oe. Interfacing the SC layer with a ferromagnetic semiconductor EuS, we further accomplish non-volatile SC diode effect reaching a giant efficiency of 65%. By careful control experiments and theoretical modeling, we demonstrate that the critical supercurrent nonreciprocity in SC thin films could be easily accomplished with asymmetrical vortex edge/surface barriers and the universal Meissner screening current governing the critical currents. Our engineering of the SC diode effect in simple systems opens door for novel technologies. Meanwhile, we reveal the ubiquity of Meissner screening effect induced SC diode effect in superconducting films, which should be eliminated with great care in the search of exotic superconducting states harboring finite-momentum Cooper pairing.Comment: 27 pages, 16 figure

Regular dorsal dimples and damaged mites of Varroa destructor in some Iranian honey bees (Apis mellifera)

The frequency of damaged Varroadestructor Anderson and Trueman (Mesostigmata: Varroidae) found on the bottom board of hives of the honey bee, Apis mellifera L. (Hymenoptera: Apidae) has been used as an indicator of the degree of tolerance or resistance of honey bee colonies against mites. However, it is not clear that this measure is adequate. These injuries should be separated from regular dorsal dimples that have a developmental origin. To investigate damage to Varroa mites and regular dorsal dimples, 32 honey bee (A. mellifera) colonies were selected from four Iranian provinces: Isfahan, Markazi, Qazvin, and Tehran. These colonies were part of the National Honey bee Breeding Program that resulted in province-specific races. In April, Varroa mites were collected from heavily infested colonies and used to infest the 32 experimental colonies. In August, 20 of these colonies were selected (five colonies from each province). Adult bees from these colonies were placed in cages and after introducing mites, damaged mites were collected from each cage every day. The average percentage of injured mites ranged from 0.6 to 3.0% in four provinces. The results did not show any statistical differences between the colonies within provinces for injuries to mites, but there were some differences among province-specific lines. Two kinds of injuries to the mites were observed: injuries to legs and pedipalps, and injuries to other parts of the body. There were also some regular dorsal dimples on dorsal idiosoma of the mites that were placed in categories separate from mites damaged by bees. This type of classification helps identifying damage to mites and comparing them with developmental origin symptoms, and may provide criteria for selecting bees tolerant or resistant to this mite

An Electronic Approach for Stochastic Sensing

In recent years, we have assisted to an ever-increasing capability of electronic systems to detect extremely small signals in noisy environments. Following this trend, the capability to electronically detect single molecular binding events could bring to a new, high performance class of biosensors. One of the best transducers coding single molecule event into an electric signal is already existing in nature and widely used by cells for interacting with the external environment: the ligand-gated ion channel. The biological cell is filled with all types of ion channels that control the trafficking of ions and molecules in and out of the cell and among the subcellular structures. However, the signaling derived by ion channels upon molecular binding is intrinsically stochastic, due to the thermal agitation of the physical system at molecular scale. Properties of their gating are strongly influenced by binding between receptive sites located on the channel surface and specific target molecules. In this paper we propose to use signals deriving from ligand-gated ion channels for realizing quantitative sensors, able to detect specific chemical species in fluid mixtures. Following this goal, we have implemented an electronic system, able to record ionic currents derived by single gated ion channels having hundreds of femto-amperes of resolution. Additionally, we propose a statistical approach for processing the electrical information, in order to estimate the concentration value of the target molecules. The proposed algorithm was tested using a Monte Carlo simulator and a simple channel model taken from literature

Aggiornamento della Flora del Modenese, considerazioni sui dati raccolti e sulla conservazione del patrimonio floristico

A nove anni dalla pubblicazione del volume Flora del Modenese si presentano i risultati ottenuti nel corso del primo aggiornamento e alcune considerazioni sugli ambienti particolarmente a rischio nella provincia di Modena: le aree umide di pianura e montagna e la conseguente crisi e/o riduzione delle idrofite. Sono oltre 723 le segnalazioni rilevanti che, assommate alle precedenti ante 2010, portano ad oltre 44.000 le segnalazioni del data base provinciale gestito dall\u2019Istituto Beni Culturali della Regione Emilia-Romagna. I 62 nuovi taxa corrispondono a 39 specie, 6 sottospecie e 17 specie alloctone. L\u2019incremento delle alloctone e delle specie di origine mediterranea, conferma i grandi mutamenti in atto, legati non solo ai cambiamenti climatici in corso, ma anche all\u2019attivit\ue0 umana spesso fonte di pressione a danno della biodiversit\ue0. Oltremodo interessanti sono le conferme di 34 taxa storici, di cui 6 addirittura ritenuti estinti

Possibilità di controllo del parassita intestinale delle api (Apis mellifera) provocata da Nosema ceranae.

La nosemiasi è una malattia delle api adulte provocata dai microsporidi Nosema apis e Nosema ceranae. Quest’ultimo è stato recentemente identificato su Apis mellifera oltre che su Apis ceranae, ed è ritenuto da alcuni ricercatori come un fattore importante nell’incremento delle mortalità delle api registrato negli ultimi anni in molte parti del mondo. L’unico prodotto curativo efficace è l’antibiotico fumagillina, il cui uso però non è consentito in molti paesi, inclusa l’Italia, e pone comunque il problema di residui nei prodotti dell’alveare e di reinfezioni. Il presente lavoro rappresenta il seguito di una indagine circa il potenziale utilizzo nel controllo della nosemiasi di alcuni prodotti di origine naturale, di cui in precedenza erano state controllate in prove di laboratorio l’efficacia nell’inibizione dello sviluppo di Nosema ceranae e la mancanza di tossicità nei confronti delle api. In seguito a risultati promettenti sono stati selezionati alcuni prodotti da utilizzare per prove di campo: il timolo ed un prodotto commerciale contenente acido acetilsalicilico, che sono stati somministrati alle colonie (10 per tesi) insieme a uno sciroppo zuccherino durante le nutrizioni primaverili, per 2 stagioni successive. I livelli d’infezione delle colonie sottoposte alla prova sono stati stimati in campioni di api di età nota, marcate alla nascita e prelevate dall’alveare dopo due settimane. Davanti agli alveari trattati sono state poste le gabbie “underbasket” di raccolta api morte, per valutare eventuali effetti tossici dei prodotti sulle api. Inoltre nel corso della stagione sono stati registrati i parametri relativi allo sviluppo delle famiglie. I risultati non hanno evidenziato nel primo anno differenze significative nei livelli d’infezione, nello sviluppo delle famiglie, nella mortalità delle api, benché gli alveari trattati con il prodotto commerciale avessero la metà del numero di spore rispetto al gruppo di controllo. Nell’anno successivo invece è stata evidenziata una differenza significativa nei decrementi dei livelli d’infezione: nel gruppo trattato con timolo il numero di spore è dimezzato mentre nel gruppo di controllo è aumentato

Evidence of Multiple Crystallization Pathways in Lithium Disilicate: A Metadynamics Investigation

Metadynamics simulations driven by using two X-ray diffraction peaks identified three alternative crystallization pathways of the lithium disilicate crystal from the melt. The most favorable one passes through the formation of disordered layered structures undergoing internal ordering in a second step. The second pathway involves the formation of phase-separated structures composed of nuclei of beta-cristobalite crystals surrounded by lithium -rich phases in which metasilicate chains are formed. The conversion of these structures to the stable lithium disilicate crystal involves an intermediate structure whose silicate layers are connected by silicate rings with the energy barrier of 2.5 kJ/mol per formula unit (f.u.). The third pathway is highly unlikely because of the huge energy barrier involved (20 kJ/mol per f.u.). This path also involves the passage through a phase-separated structure of an indefinite silica region surrounded mainly by amorphous lithium oxide