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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 2 (1983), S. 965-992 
    ISSN: 0392-6737
    Keywords: Amorphous and polymeric materials
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Description / Table of Contents: Riassunto Si propone un modello chimico-fisico per i diffetti di tipo IVAP nella silice vetrosa. Si dimostra che tali difetti a due centri sono diamagnetici e, perT〈10 K, si comportano come sistemi a due livelli. Inoltre essi risultano associati a momenti di dipolo elettrico, in accordo con recenti dati sperimentali. Si pone in evidenza una stretta analogia con le ipotesi di base della teoria dei «tunnelling atom», il che comprova la connessione tra le eccitazioni di bassa energia nei materiali amorfi e la doppia degenerazione delle posizioni di equilibrio relative a particolari centri atomici del reticolo. Diversamente dalle teorie precedenti, questo modello prevede un grande contributo di sistemi a due livelli «veloci» (con brevi tempi di rilassamento) a temperature molto basse (T〈0.1 K); al contrario, vi sono forti indicazioni che i sistemi a due livelli «lenti» (con lunghi tempi di rilassamento) siano termicamente attivati solo a temperature piú elevate. Questo risultato trova una conferma indiretta nella deviazione dalla linearità del calore specifico a bassa temperatura, e sembra tener conto in modo soddi-sfacente di altri risultati sperimentali che apparivano contraddittori nello schema della teoria dei «tunnelling atom», cosí come viene solitamente formulata.
    Abstract: Резюме Предлагается физико-химическая модель дефектов, типа IVAP, в стеклообразном кремнеземе. Показывается, что эти двухцентровые дефекты являются диамагнитными и дляT〈10 K обнаруживают двухуровневый энергетический спектр. Они также обнаруживают дипольную природу в соответствии с недавними экспериментальными данными. По аналогии с основной идеей, лежащей в основе теории «туннелирующего атома», отмечается связь между низкоэнергетическими возбуждениями в аморфных материалах и двухкратно вырожденными равновесными положениями узлов решетки. В противоположность предыдущим теориям, предложенная модель предсказывает большой вклад быстро релаксирующих двухуровневых систем при очень низких температурах (T〈0.1 K), тогда как медленно релаксирующие возбуждения активируются только при высоких температурах. Этот результат косвенно подтверждается отклонениями от линейности удельной теплоемкости при низких температурах и, по-видимому, позволяет удовлетворительно обьяснить экспериментальные результаты, которые противоречат теории «туннелирующего атома».
    Notes: Summary A first-principle chemical model for IVAP-like defects in vitreous silica is proposed. It is shown that these two-centre defects are diamagnetic and, forT〈10 K, display a two-level-like energy spectrum. They also exhibit a dipolar nature in agreement with recent experimental data. A close analogy with the basic idea underlying the tunnelling-atom theory is shown, supporting the connection between low-energy excitations in amorphous materials and twofold degenerate equilibrium positions of certain lattice sites. In contrast to previous theories, the present model predicts a large contribution of fast relaxing two-level systems at very low temperatures (T〈0.1 K), while slowly relaxing excitations achieve thermal activation only at higher temperature. This result is indirectly confirmed by deviations from linearity of the low-temperature specific heat and seems to bridge in a satisfactory way experimental results which appeared contradictory in the framework of the tunnelling-atom theory as it is usually formulated.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 12 (1990), S. 1519-1530 
    ISSN: 0392-6737
    Keywords: Effective mass andg factors ; Collective modes (e.g. in one-dimensional conductors.)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary In the thight-binding approximation, we study a linear periodic chain with complex transfer integrals between first neighbours. When their real part vanishes, a dispersionless quantum transport is shown to occur for certain quasistationary solutions. The resulting effect of a self-sustaining current is discussed.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1685-1688 
    ISSN: 0392-6737
    Keywords: Superconducting materials ; Material effects onT c,K, critical currents (including impurities, ion implantation etc.) ; Decomposition reactions (pyrolisis dissociation and group ejection) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Polycrystalline YBCO was prepared by a pyrolytic process starting from citrate and tartrate precursors. The effect of the precursor on the superconducting properties was investigated by means of magnetic measurements using a SQUID magnetometer, a Vibrating-Sample Magnetometer and an a.c. susceptometer. The critical temperature is not affected by the type of precursor (T c=85 K for both). On the other hand, the precursor plays an important role on the critical current density, which is found to be an order of magnitude higher in the sample obtained from citrates (J c(77K,H=0)=2.3·104 A/cm2 and 103 A/cm2 for the from-citrate and from-tartrate sample, respectively).
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 3 (1984), S. 945-955 
    ISSN: 0392-6737
    Keywords: Amorphous and polymeric materials
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Description / Table of Contents: Riassunto La dinamica dei difetti tipo IVAP è stata studiata in riferimento alla diffusione dei fononi e ai processi attivati termicamente. Si è dimostrato che la presenza di un termine anomalo βδ nell'accoppiamento fra fononi e difetti tipo IVAP è dovuta alla polarità ξ caratteristica di tali difetti. Si sono inoltre ottenute analiticamente le espressioni formali del «rate» di diffusione risonante fononica e della densità degli stati dipendente dal tempo. Infine si è fatto un confronto dettagliato con il formalismo standard dei sistemi a due livelli previsti dalla teoria dei «tunnelling atoms», in quanto la presente formulazione generale contiene tale teoria come caso particolare (β=0).
    Notes: Summary The first-principle dynamics of IVAP-originated two-level systems is developed with reference to phonon-scattering and thermally activated processes. It is shown that the anomalous coupling term βδ, appearing in the phonon-IVAP interaction, originates from the elastic polarity ξ characteristic of the IVAP-like defects. The formal expressions for the quantities of interest, such as the resonant-scattering rate of phonons and the time-dependent density of TLS states, are given analytically. Since the tunnelling-atom theory is a limit case (β=0) of the present general approach, a point-by-point comparison with the standard formalism of tunnelling-originated TLS is performed.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Four novel antiviral WIN compounds, that contain a methyl tetrazole ring as well as isoxazole, pyridazine or acetylfuran rings, have had their structures determined in human rhinovirus serotype 14 at 2.9 Å resolution. These compounds bind in the VP1 hydrophobic pocket, but are shifted significantly towards the pocket pore when compared to previously examined WIN compounds. A putative water network at the pocket pore is positioned to hydrogen bond with these four WIN compounds, and this network can account for potency differences seen in structurally similar WIN compounds.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Annals of the New York Academy of Sciences 612 (1990), S. 0 
    ISSN: 1749-6632
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Natural Sciences in General
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Annals of the New York Academy of Sciences 612 (1990), S. 0 
    ISSN: 1749-6632
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Natural Sciences in General
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 0375-9474
    Keywords: Nuclear Reactions
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Carbohydrate Research 124 (1983), S. C5-C7 
    ISSN: 0008-6215
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Carbohydrate Research 131 (1984), S. 180-184 
    ISSN: 0008-6215
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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