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P53 family members regulate the Otx1 gene expression in differentiation of breast cancer stem cells and in mammary gland development.
No full textTp53, Tp63 and Tp73 family members encode for transcription factors which play a key role in
control of the genome integrity inducing cell-cycle arrest, senescence, apoptosis or cell
differentiation. They take a part in cell stress response and in tumor suppression (De Young MP
and Ellisen LW, 2007).
Wild type p53 protein is a growth modulator and its inactivation is a critical event in malignant
transformation (Gasco M, 2002).
It has been recently demonstrated that p53 has developmental and differentiation functions (Hu
W, 2008). Indeed an over-expression of p53 in tumor cells induces asymmetrical division
avoiding a self-renewal of cancer stem cells (CSCs) and promoting their differentiation (Cicalese
A, 2009).
In this study 43 human ductal and lobular invasive breast carcinomas have been analyzed for the
expression of p53, p63, p73 and a pool of non-clustered homeobox genes. The homeogenes play
a crucial role in embryogenesis, regulating cell differentiation and proliferation (Pagani IS,
2010). They are expressed in adult mammary gland and when deregulated, are involved in
breast cancer (Lewis MT, 2000).
We demonstrated that the Otx1 homeogene is transcribed in breast cancer, in CSCs
differentiation and in adult mammary gland development.
We established that the p53 and p73 proteins directly induce the Otx1 expression by acting on its
promoter. Otx1 has been described as a critical molecule for axon refinement in corticogenesis
(Zhang YA, 2002), and its activity in breast cancer suggests a synergistic function with p53 and
p73 in CSCs differentiation.
In adult mammary gland development the Otx1 expression is not regulated by p53, but is
correlated with the expression of Tp73 in lactation and in regression. This suggests that in
physiological conditions Otx1 is regulated by p73, while in the tumors p53 regulates its
expression
Molecular and functional characterization of the Odorant Receptor2 (OR2) in the tiger mosquito Aedes albopictus.
No full textIn mosquitoes, olfactory system plays a crucial role in many behaviors, including nectar feeding, host preference selection, searching for the right place to lay eggs. A.albopicus, known also as tiger mosquito, is an anthropophilic species which in the last years, due to a strong ecological plasticity, has spread throughout the world and all over Italy with a high abundance in man-made environments. Although long considered a secondary vector of viruses, the potentiality of its vectorial capacity is very dangerous and may constitute the foundation for a public health alert. Nevertheless, to date, for this mosquito nothing is known at molecular level. Based on the idea that an improved understanding of the olfactory system of mosquitoes may help in developing control methods that interfere with its behavior, recently we have undertaken a study aimed to characterize the A. albopictus Odorant Receptors. During my PhD work, I focused my attention on the identification, cloning and functional characterization of the A. albopictus OR2 ortholog. My data indicate that A. albopictus OR2 (AalOR2) shares a high degree of identity with the other mosquito OR2 orthologs characterized to date, confirming that OR2 is one of the most conserved mosquito ORs; furthermore, AalOR2 is expressed in the olfactory appendages of larvae and adults and its expression increases after a blood meal, as determined by a semi-quantitative RT-PCR.
Interestingly, this is the first report of an up-regulation of an OR in response to a blood meal; this increase could suggest a role of AalOR2 in searching oviposition right places. AalOR2, such as the other orthologs, is narrowly tuned to indole, a ubiquitous volatile compound that has been linked to host seeking, and oviposition. The de-orphaning of AalOR2 has been obtained, with same results, through Ca2+ imaging assay in HEK293 cells, and “in vivo” experiments using the Single Sensillum Recording (SSR) in an engineered neuron of the fruitfly Drosophila melanogaster that express AalOR2.
Furthermore, by using this technique, I was able to identify also a molecule, (-)Menthone, that produced an inhibitory effect on this Odorant Receptor. In summary, this work led to the cloning and de-orphaning of the first Odorant Receptor in A. albopictus, that may be used as potential molecular target for developing environmentally friendly strategies to control mosquito populations
Gamma-ray pulsars: a multi-band view
No full textThis Thesis is based mainly on the results of the timing analysis applied to the gamma-ray pulsars observed principally by the AGILE satellite and also extended, in some cases, to observations with the Fermi-LAT satellite and the MAGIC Cherenkov telescope. Aim of this extended study of pulsars at the high energies was to characterize their properties based, now, on a more statistically relevant sample, and be able to disentangle useful informations that can be key to explain the emission mechanisms in pulsars.
THE SCIENTIFIC CONTEXT Pulsars are highly-magnetized, rapidly-rotating neutron stars. As explained in Chapter 1, they have been observed in the radio band over the past forty years, due to their highly anisotropic emission, which, when combined with the misalignment between the rotation and the magnetic axis, produces the pulsed emission we observed, also called the ”lighthouse effect”. Pulsars have been observed as gamma-ray emitters as well, but it is only in the past three years that their number hit the double digits and they started to yield their potential as keys to explain the neutron stars mechanisms.
In Chapter 1, the basics of pulsar theory are given. While the phenomenological aspects have been widely studied thanks to extensive radio observations through the years, their electrodynamics represents an articulated field that is difficult to probe. The ”classical” models of magnetosphere predict the presence of regions of particles’ under-density (a ”gap”), inside an overall force-free magnetized area that surrounds the pulsar, where the particles can be accelerated and can produce the observed radiation, after a number of cascade processes. More ”modern” models of magnetosphere, with their roots in old predictions, discuss the hypothesis of a totally force-free magnetosphere. The discussed theories search for a confirmation in our gamma-ray observations, as the gamma-rays are the ones carrying away a good fraction of the rotational energy loss.
THE NEW PERSPECTIVES OPENED BY THE MULTI-BAND OBSERVATIONS
In April 2007 the Italian Space Agency launched the AGILE satellite for gamma-ray astronomy. About one year later, AGILE was joined in the observation of gammarays by the 16 times bigger Fermi-LAT satellite launched by NASA. AGILE and Fermi-LAT, with their wide field of view and large collective area, are particularly suited for the study of pulsars at high energies. Most recently, the window of veryhigh energy observations has opened up to pulsar studies and, in particular, by the MAGIC telescope, with the lowest up to now threshold for ground-based telescopes, at 25 GeV. Its observations are briefly described, together with AGILE and Fermi-LAT’s, in Chapter 2.
The techniques for studying pulsars in the gamma-rays are also explained in Chapter 2, with the fundamental premise about the radio observations which were part of my analysis work, as they constitute the primary basis for the gammaray observations. The advances with respect to the observations of the previous generation gamma-ray instruments are highlighted. In particular, AGILE was able to take into account, for the first time in gamma-ray observations, the timing noise that affects young pulsars. In this way, the observations can be carried out for longer time spans without being affected by sensible light curve smearing. Thus, we could take advantage of the long time span, up to now the longest for gamma-ray observations, to increase the resolution of our light curves and see structures at the sub-millisecond level.
GAMMA-RAY PULSARS AGILE and Fermi-LAT pulsar observations first concentrated on the known gamma-ray pulsars. As shown in Chapter 3, the apparently ”familiar” pulsars actually hid thriving new prospects for pulsar studies, as well as the new pulsars subsequently detected, described in Chapter 4.
In these two Chapters, the properties of the gamma-ray emission ars analyzed for a number of pulsars, mainly using AGILE data, but also with Fermi-LAT and MAGIC observations. The light curves are investigated with increased resolution from previous observations and the spectral properties are addressed. The availability of a statistically significant sample of gamma-ray pulsars led us to draw some lines on the models. The classical polar cap model seems to be failing the test of gamma-ray observations for most of the present sample, and a simple explanation of which can be found in conservation laws arguments discussed in Chapter 4. At the same time it starts getting clear that a model that contemplates a single gap zone does not seem to be feasible to explain the observed pulse profiles. And, possibly, the entire gap theory should be combined with the more physical force-free models.
Episodes of variability in pulsars have been observed and studied in this context. The Vela glitch of August 2007 was observed by AGILE in search for gamma-ray emission. The Crab pulsar could have a contribution to the emission from a newly observed third pulsar peak, that is less significant and much weaker than the canonical two, and could be due to giant pulses. AGILE observed the first gamma-ray millisecond pulsar but its emission only appeared in a restricted time interval, leading to the interesting possibility that pulsar emission might have some intrinsic variability.
HIGH MAGNETIC FIELD PULSARS After the advent of Fermi-LAT, AGILE found its collocation in the gamma-ray astronomy in the characterization of the low-energy gamma-rays (from 30 to 100 MeV), where the collective areas of the two instruments is equivalent, but AGILE deals with much lower background. For this reason, we concentrated on those pulsars that show a low-energy cutoff, which were theorized to emit gamma-ray radiation through the exotic QED process of photon splitting.
A detailed analysis of the two most significant cases is given in Chapter 5. We have found that the concurrence of a high magnetic field and an aligned geometry, could overcome the objections from Chapter 4 against inner magnetosphere emission and be, indeed, dominated by polar cap emission. Interestingly, this phenomenology, that is observed in pulsars that are similar to magnetars, may be observed in objects that are transitioning from pulsar to magnetar.
THE ENVIRONMENT OF PULSARS Young pulsars are known to power a relativistic wind of particles that surrounds the pulsar and is best known as its Pulsar Wind Nebula (PWN). Important phenomena take place in the PWN and they are powered by the pulsar inside it. As discussed in Chapter 6, very high energy emission was already observed from PWN, but high energy emission was missing, in a spectral region where important constraints on the emission processes could be given.
AGILE was the first satellite to detect GeV emission from a PWN apart from Crab, Vela X, and it was also the first to claim the unexpected flux variation in the Crab Nebula which underwent two intense flares in 2010 and 2011. In Chapter 6 we give a description of the events and a possible trail for an interpretation, although no clear picture can yet emerge from the observed events.
CONCLUSIONS AND FUTURE PROSPECTS The multi-band approach that has been used for the observations described in this Thesis has proven valid for the exploitation of new science and the most useful approach for the comprehensive analysis of pulsar phenomena across the electromagnetic spectrum. As a completion to this work, the more comprehensive AGILE Pulsar Catalog is in preparation. It will comprise all the pulsars observed by AGILE and particularly focus on the low-energy tail of them, which present interesting properties that bridge pulsars and magnetars
Exploiting supramolecular interactions for asymmetric catalysis.
No full textSupramolecular chemistry refers to the area of chemistry beyond the molecules and focuses on the chemical systems made up of a discrete number of assembled molecular subunits or components. While traditional chemistry concentrates on the covalent bond, supramolecular chemistry examines the weaker and reversible non-covalent interactions between molecules. These forces include hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, pi-pi interactions and electrostatic effects. The aim of this thesis is the use of this type of interactions, in particular hydrogen bonding, for the development of new catalytic systems.
In the first part of the work the synthesis of a new class of supramolecular bis(oxazolines) named SupraBox using urea moiety as self-assembly inducer is reported. A library of 16 ligands, with different degree of structural diversity, were prepared by a 3-step modular synthesis starting from readily available starting materials. Palladium- and Copper-complexes were characterized by NMR spectroscopy, UV spectroscopy and mass spectrometry to investigate their structures and properties and used as catalysts in kinetic resolutions of racemic diols and in the desymmetrization of meso diols with good results.
In the second part, a new class of peptidomimetic organocatalysts starting from two bifunctional diketopiperazines was investigated. Four different organocatalysts were prepared and tested in the conjugate addition reaction of several aldehydes to beta-nitrostyrene and (E)-2-(furan-2-yl)nitroethene with good to excellent diastereo- and enantioselectivities. A rationale for the mechanism is also proposed: a Monte Carlo/Energy Minimization (MC/EM) conformational search by molecular mechanics methods was undertaken on the four catalysts proving the importance of hydrogen bonding between catalyst and substrate
Rilevanza dei determinanti ambientali nella definizione degli scenari espositivi in ambito REACH.
No full textIl progetto di Dottorato di Ricerca verte sulle tematiche di studio inerenti il rischio chimico e sull’analisi applicativa dei determinanti ambientali nella definizione degli scenari espositivi a contaminanti industriali.
La prima parte del lavoro è stata dedicata all’analisi della normativa europea vigente, con una particolare attenzione alle linee guida proposte dal neo regolamento REACH sulla valutazione della sicurezza chimica. Successivamente l’attenzione è stata concentrata su un progetto di indagine a carattere sperimentale, rispettivamente focalizzato su scenari espositivi ad inquinanti per la categoria dei consumatori.
Il recente testo normativo REACH n.1907/2006 (acronimo di Registration, Evaluation, Authorisation and Restriction of Chemicals) entrato in vigore lo scorso giugno 2007, concernente l’autorizzazione e valutazione delle sostanze chimiche, sottolinea l’importanza delle tecniche predittive nel campo della valutazione del rischio chimico e ne consiglia l’utilizzo come primo approccio valutativo nella caratterizzazione degli scenari espositivi.
L’utilizzo di strumenti informatici per la modellazione dell’esposizione risulta particolarmente importante nell’approccio valutativo della sicurezza chimica. Permette di creare di fatto scenari prossimi alla realtà, per caratterizzare in maniera preventiva il rischio
per la salute umana e per l’ambiente prendendo in considerazione i processi coinvolti, le condizioni d’uso e le misure di gestione del rischio relative all’utilizzo di una sostanza nei suoi usi identificati.
Il progetto sperimentale è stata focalizzato sulla valutazione della possibile esposizione ad alcuni agenti inquinanti derivati dall’utilizzo di differenti tipologie di candele in alcuni ambienti domestici ritenuti maggiormente rappresentativi.
Le candele, classificate secondo il regolamento REACH come preparati, devono essere sottoposte ad una valutazione della sicurezza chimica. Per ottemperare agli obblighi di informazione a valle previsti dagli articoli 31 e 32 del regolamento è raccomandata l’analisi dettagliata dei potenziali scenari espositivi delle sostanze emesse durante il loro uso.
Sono state sottoposte ad indagine tre tipologie di materie prime (paraffina, intermedio e ‘slack’), caratterizzate da un diverso contenuto percentuale di sostanze alto bollenti, e differenti candele commerciali colorate caratterizzate dalla presenza di fragranze (corteccia, frangipane, rabarbaro e aloe).
La determinazione dei fattori di emissione dei differenti inquinanti atmosferici ha riguardato i seguenti inquinanti: sostanze organiche volatili apolari quali BTEX, sostanze organiche volatili polari quali aldeidi leggere, alcuni dei principali IPA, CO, ossidi di azoto (NOx), SO2 e particolato atmosferico.
La misura dei fattori di emissione è stata effettuata in una camera di combustione steadyburning appositamente progettata e realizzata per standardizzare le variabili sperimentali e rendere riproducibili i risultati.
La valutazione del rischio è stata in seguito sviluppata solo sulle sostanze con caratteristiche rilevanti di tossicità acuta e/o cancerogenicità, in combinazione con l’entità dei livelli di emissione determinati sperimentalmente in camera di combustione. E’ stata quindi condotta un’indagine tossicologica approfondita sui seguenti agenti inquinanti: formaldeide, acroleina, acetaldeide, benzene, naftalene, benzo(a)pirene, CO, NOx, SO2 e PM; al fine di selezionare i rispettivi valori di soglia utili ad una valutazione del rischio specifica per gli ambienti di vita. Tale selezione è avvenuta tramite una raccolta sistematica di tutte le informazioni rilevanti nella letteratura scientifica e nella documentazione prodotta da organismi internazionali e nazionali utilizzando i seguenti criteri: i) privilegiando i riferimenti internazionali; ii) privilegiando valori di soglia specifici per gli ambienti indoor; iii) privilegiando limiti short-term (< 24 ore), data la sporadicità dell’utilizzo di candele da parte dei consumatori; iv) scegliendo i limiti più restrittivi, in ottemperanza al principio di precauzione.
Tramite la costruzione di scenari espositivi con il modello matematico validato singlecompartment mass balance ConsExpo 4.1, sono state stimate successivamente le concentrazioni indoor dei contaminanti sotto indagine sulla base dei rispettivi fattori di emissione e parametri chimico-fisici. Il modello scelto è progettato per la caratterizzazione dell’esposizione inalatoria dei consumatori a sostanze rilasciate dall’utilizzo di articoli, e rientra nell’elenco dei modelli certificati dalla comunità europea e consigliati per l’attuazione delle indicazioni del regolamento europeo REACH.
Per soddisfare il principio cautelativo è stato ipotizzato uno scenario reasonable worst-case, per la valutazione delle concentrazioni raggiunte anche nelle condizioni operative più improbabili. Lo scenario peggiorativo è stato strutturato con le seguenti variabili: i) combustione contemporanea di 4 candele per la durata di 4 ore; ii) volumetria minima di un ambiente indoor (25 m3, equivalente alla volumetria di una stanza da bagno); iii) ventilazione minima (0,1 ricambi/ora).
La simulazione modellistica per i composti gassosi è stata condotta considerando un tasso di rilascio dell’inquinante costante, che approssima meglio la situazione reale. La stima delle concentrazioni di PM è stata invece realizzata ipotizzando un rilascio costante assimilabile alla vaporizzazione di un composto non volatile, sottoposto ad una velocità di sedimentazione governata dalla legge di Stokes.
Uno dei limiti dell’utilizzo del modello ConsExpo 4.1 risiede nell’impossibilità di stimare la variabilità spaziale (specie in verticale) delle concentrazioni predette. Difatti le equazioni alla base del modello effettuano un’approssimazione della dispersione dell’inquinante in atmosfera come omogenea, trascurando l’eventuale stratificazione in funzione dei fattori di diffusione, delle proprietà chimico-fisiche della sostanza, dei fattori vettoriali di ventilazione e delle caratteristiche strutturali dei locali considerati. Il modello utilizzato è stato tuttavia un utile strumento per ottenere stime dell’ordine di grandezza delle concentrazioni di esposizione degli inquinanti in indagine.
La valutazione dei risultati che ne consegue, sulla base della revisione critica delle attuali conoscenze tossicologiche e dei limiti di esposizione che ne derivano, è stata utile per asserire quanto segue. Per la valutazione delle emissioni derivate dalla combustione di candele con fragranze, è stato determinato che:
- Le concentrazioni stimate per formaldeide, acetaldeide, benzene, naftalene, benzo(a)pirene in tutti gli scenari ipotizzati non hanno superato i limiti di riferimento da letteratura, anche considerando lo scenario peggiore per quantità di candele utilizzate, volumetrie e ventilazione degli ambienti.
- Le concentrazioni stimate per l’acroleina, nelle diverse condizioni caratterizzanti gli scenari, hanno superato in taluni casi il valore di riferimento considerato più cautelativo (pari a 0,2 μg/m3). Nella simulazione dello scenario la concentrazione media di acroleina mediata su tutte le fragranze è risultata essere pari a 2,57 μg/m3. Si evidenzia tuttavia che, per l’acroleina, l’effetto critico prevedibile per un’esposizione short-term derivato dall’uso di candele è di tipo irritativo, anche se è stato scelto di utilizzare nella valutazione del rischio un limite di esposizione cautelativo stabilito per esposizione cronica life-time. Inoltre la concentrazione predetta di acroleina raggiungerebbe lo stesso ordine di grandezza del valore dei livelli di fondo presenti negli ambienti di vita indoor (compresi tra 2,1 e 12,2 μg/m3).
- La stima delle concentrazioni di PM non ha portato al superamento del limite di esposizione consigliato per il PM2.5 (pari a 25 μg/m3) in nessuno degli scenari presi in considerazione.
La valutazione delle emissioni derivate dalla combustione delle tre tipologie di materie prime utilizzate per la produzione di candele ha permesso di affermare quanto segue:
- La valutazione effettuata per il CO non ha portato al superamento del limite pari a 10 mg/m3 richiesto dal DM 02/04/2002, n° 60.
- Le concentrazioni di NOX dello scenario worst-case sono state superiori (stesso ordine di grandezza), per tutte e tre le matrici paraffiniche, rispetto al limite di 200 μg/m3 consigliato dalla WHO per il solo NO2 e imposto dal DM 60 del 02/04/2002 come limite mediato su un’esposizione massima di un’ora. Poiché precedenti dati di letteratura, hanno evidenziato un rapporto di emissione per NOX /NO2 compreso tra 3 e 4, le concentrazioni indoor di NO2, nel caso peggiore, dovrebbero essere prossime al limite fissato per esposizioni acute. Va inoltre rilevato che le concentrazioni medie di NO2 riportate in letteratura all’interno di nuclei abitativi in zone urbane italiane sono pari a circa 20 ÷ 60 μg/m3.
- Le concentrazioni ottenute dalla stima modellistica dei fattori di emissione per l’SO2 hanno portato al superamento del limite orario pari a 350 μg/m3 consigliato dal D.M. n° 60, 2002, solamente per la matrice ‘slack’ (circa di un ordine di grandezza nello scenario worst-case).
- Nel caso del benzene, gli scenari ipotizzati non hanno portato al superamento della concentrazione di riferimento di 5 μg/m3.
- La stima delle concentrazioni indoor di naftalene ha determinato una concentrazione massima evidenziata nello scenario worst-case per la matrice intermedio pari a 0,11 μg/m3.
Tale concentrazione non ha superato il valore limite di 3 μg/m3 consigliato dall’EPA per l’esposizione a naftalene.
- Per quanto riguarda il benzo(a)pirene, le concentrazioni predette per la sola matrice ‘slack’, nella valutazione peggiore hanno superato il limite di esposizione consigliato per la popolazione generale di 1 ng/m3. La massima concentrazione stimata per lo scenario worstcase è stata pari a 4,76 ng/m3.
Occorre tuttavia considerare che la tossicità del benzo(a)pirene, non è tale da far presupporre effetti acuti o sub-acuti in seguito ad esposizioni a basse dosi. Il limite preso in considerazione è stato infatti sviluppato per garantire un livello di rischio cancerogeno ritenuto accettabile, per esposizioni croniche life-time.
Inoltre il benzo(a)pirene presenta un intervallo di concentrazioni negli ambienti outdoor delle aree urbane dei paesi industrializzati compreso tra 0,5 e 3 ng/m3 (zone limitrofe a strade con intenso traffico autoveicolare). Le concentrazioni ambientali sono quindi già prossime o superiori al limite di legge.
- La stima delle concentrazioni di PM ha portato al superamento del limite di esposizione consigliato dall’Organizzazione Mondiale della Sanità per il PM2,5 (25 μg/m3) per i fumi di candele prodotti con matrici ‘slack’ e intermedio. Le concentrazioni di PM predette in seguito a combustione della matrice intermedio hanno superato il limite consigliato solamente nel worst-case scenario. Al contrario, la matrice ‘slack’ è risultata costantemente superiore al valore soglia consigliato, anche in scenari di utilizzo meno conservativi
Effective interactions and phase behavior of colloidal dispersions.
No full textThe thesis deals with a number topics concerning the equilibrium properties and the phase behavior of colloidal suspensions, a class of physical systems that have attracted a broad interest during the last few decades, serving both as a valuable test bench for ideas in condensed matter physics and as building blocks for novel materials. An emphasis is placed on the notion of effective interaction, which enables the construction of simple models that are studied by use of theory and computer simulation. The questions addressed include the thermodynamics of particles interacting through a short-ranged attraction, fluctuation-induced (critical Casimir) forces, and the effect of polydispersity on the freezing transition of charged colloids
Multiloop amplitudes in superstring theory.
No full textString theory is a candidate for a unified theory of all forces in nature.
String theory is a quantum theory, and, because it includes a state that can be identified with the graviton, it is a quantum theory of gravity. So it could be a framework for describing the four fundamental interactions. The main idea is to replace the description of particles as zero dimensional object with one dimensional object, a string. Mimicking the procedure followed in QFT one can formulate the theory using the Feynmann path integral. In this context one would be able to compute the amplitudes for a process of string scattering, as, for example, in QED. It was shown that correct definition of the amplitudes is strictly connected with the construction of a suitable measure on the moduli space of certain Riemann surfaces of genus g. Thus an essential element in the computation of string amplitudes is the definition of this chiral measure.
During the last fifteen years the correct expression for the superstring measure was found just up to genus two. In this thesis I will present a general method to compute the measure up to five loop which as a byproduct also improves the known results for genus two and shows that the previous formulated ansatze for the genus three amplitude was wrong. In the first part I present the well known construction of the partition function in the bosonic theory. Actually, this is the starting point for the the generalization to the supersymmetric case. Then, I introduce the mathematical background we need to obtain the superstring measure. Finally, I discuss in great details the construction of the measure up to genus five and some open problems are emphasized. Moreover, I will also point out that the path integral procedure leads to some ambiguities related to the infinite dimension of the functional measure appearing in the path integral and I will explain the main difficulties related to its mathematically rigorous definition
Multiple-welled tunnelling systems in glasses at low temperatures.
No full textThe thesis examines anomalous effects in the physical properties of some multicomponent glasses at low temperatures in the absence and in the presence of a magnetic field.
These anomalous effects can be explained well in terms of a new (anomalous) tunnelling systems (ATS) model. ATS in glasses are provided by the process of partial devitrification of the glassy network due to the presence of network-modifying ions in real glasses. A new probability distribution, inversely proportional to the energy asymmetry, takes into account partial devitrification. The effect of the magnetic field is explained by introducing the Aharonov-Bohm coupling into the Hamiltonian of a single ATS.
The multi-welled ATS together with the standard two-level tunneling systems explain qualitatively, as well as quantitatively and with reasonable parameters, the relative change of the dielectric permittivity and loss at zero and weak magnetic fields for multicomponent glasses such as AlBaSiO (or BAS) and BK7. They also explain the relative change of the dielectric permittivity and heat capacity with temperature T and with alkali concentration x in the mixed (SiO2)1-x(K2O)x glass.
The polarization echo’s theory in glasses has been improved and extended to the case of the independent ATS model describing glasses in a magnetic field. The agreement between theory and experiment is highly satisfactory, given the simplifications used in the theory. The isotope substitution effect on the dipole-echo amplitude also can be explained in a simple way in our model.
The interpretation of the extracted material parameters brings us to confirm the existence of coherent tunneling clusters of N true particles, with a value of N ranging from about 25 at the lowest temperatures, to about 600 at the higher temperatures
