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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 10 (1988), S. 915-932 
    ISSN: 0392-6737
    Keywords: Quantum effects on the structure and dynamics of nondegenerate fluids
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Description / Table of Contents: Riassunto In questo lavoro si applica la teoria della matriceU per dedurre un'espressione esplicita per lo spettro d'energia di eccitazione dell'4He liquido. Usando un modello per il potenziale chimico efficace per l'4He si è in grado di produrre uno spettro di eccitazione molto simile a quello osservato. L'inverso della massa efficace, cioè 1/m *, è ottenuto in funzione del momentok. Si trova che il rapporto tra massa efficace e la massa dell'atomo di elio ak=2.0 Å−1, cioè vicino alla profondità del rotore, èm * (k=2.0 Å−1)/m He= =0.18298, mentre quello ak=1.0 Å−1 èm * (k=1.0 Å−1)/m he=−0.17103. Il risultato teorico dello spettro di eccitazione è coerente con i dati sperimentali.
    Abstract: Резюме В этой статье мы применяем теориюU-матрицы для вывода явного выражения для энергетического спектра возбуждений Зидкого4He. Используя модель для эффективного химического потенциала для4He, мы воспроизводим спектр возбуждений который оказывается очень близким к зкспериментально наблюдаемому. Вычисляется обратная величина эффективной массы, т.е. 1/m *, как функция импульсаk. Получено отношение эффективной массы к массе атома гелия приk=2.0 Å−1, которое составляетm * (k=2.0 Å−1)/m He=0.18298, тогда как это отношение приk=1.0 Å−1 равноm * (k=1.0 Å−1)/m He=−0.17103. Теоретический результат для спектра возбужений согласуется с имеющимися экспериментальными данными.
    Notes: Summary In this paper, we apply theU-matrix theory to derive an explicit expression for the excitation energy spectrum of liquid4He. Using a model for the effective chemical potential for4He, we are able to produce an excitation spectrum which is very close to the observed one. The inverse of the effective mass,i.e. 1/m *, is obtained as a function of momentumk. The ratio between the effective mass and the mass of helium atom atk=2.0 Å−1,i.e. near the roton depth, is found to bem *(k=2.0 Å−1)/m He=0.18298, while the ratio atk=1.0 Å−1 ism *(k=1.0 Å−1)/m He=−0.17103. The theoretical result of the excitation spectrum is consistent with observational data.
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 10 (1988), S. 247-276 
    ISSN: 0392-6737
    Keywords: Boson degeneracy and superfluidity of helium
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Description / Table of Contents: Riassunto Impiegando la teoria della matriceU nella forma presente, abbiamo presentato una teoria microscopica di un sistema interagente diN bosoni a temperatura non nulla. L'hamiltoniana include un termine che descrive l'influenza esterna, un termine per un'interazione a 2 particelle e une che riguarda l'interazione di tipo dipolare. Per studiare lo stato fondamentale dell'4He liquido s'include solo l'interazione a 2 particelle. Basandoci su dati di scattering, abbiamo dedotto la funzione potenziale a 2 particelle. Con quella, calcoliamo numericamente la distribuzione dell'impulso aT=1.1K per diversi gruppi di parametri. Si mostra che le curve teoriche sono in buon accordo con i risultati osservati. Le corrispondenti curve del fattore di struttura del liquido si adattano benissimo ai dati misurati e il risultato migliore per l'energia dello stato fondamentale è − 7.12K.
    Abstract: Резюме Используь теориюU-матрицы, мы предлагаем микроскопическую теорию взаимодеюствующейN-бозонной системы при ненулевых температурах. Гамильтониан включает член, описывающий внешнее влияние, член двух-частичного взаимодействия и член, относящийся к дипольному типу взаимодействия. При исследовании основного состояния шидкого4He учитяватся только двух-частичное взаимодействие. Основываясь на данных по рассеянию, мы выводим двух-частичную потенциальную функцию. Мы численно определяем распределение по импульсам приT=1.1K для некоторых систем параметров. Показывается, что теоретические кривые согласуются с наблюденными результатами. Соответст вующий структурный фактор зидкости хорошо согласуется с экспериментальными данными. Полученная величина энергии основного состояния составляет −7.12 K.
    Notes: Summary Employing theU-matrix theory in the present form we have presented a microscopic theory of an interactingN-boson system at nonzero temperature. The Hamiltonian includes a term describing the external influence, a term for a two-particle interaction and one pertaining to the dipole-type interaction. To study the ground state of liquid4He, only the two-particle interaction is included. Based on scattering data, we have deduced the two-particle potential function. With that we calculate numerically the momentum atT=1.1 K for several sets of parameters. It is shown that the theoretical curves agree well with the observed results. The corresponding liquid-structure-factor curves fit the measured data very well and the best result for the ground-state energy is −7.12K.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 4634-4639 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: In the past, when thermal diffusivity measurement of materials were carried out by photoacoustic signal detection using transducers, only the piezoelectric or the pyroelectric property of the transducers was considered. In case the transducer exhibits both piezoelectric and pyroelectric properties, one of these properties had been suppressed during the experimentation, obviously more errors are introduced this way. We use polyvinylidene difluoride (PVDF) as the detector for thermal waves. Since PVDF has both piezoelectric and pyroelectric properties, we show in this article that the signal detected by the transducer is a sum of both the piezoelectric and pyroelectric effects. Silicon semiconductor samples are considered in this article to compare the theory with experimental results. Although both the piezoelectric and pyroelectric properties are found in the resultant signal at all the frequency ranges considered, we find that when the samples are thermally thick, the piezoelectric contribution to the detected signal is slightly more than the pyroelectric contribution and vice versa when the sample is thermally thin. This behavior of the combined signal can be explained by the fact that in an optically opaque solid heat is generated very close to the surface, following absorption. This heat is communicated to the PVDF as long as the thermal diffusion length is larger than the thickness (i.e., the sample is thermally thin). At high frequencies the solid becomes thermally thick and the pyroelectric nature decreases as both the optical and thermal contact of the sample with the detector diminishes. Since both the properties are considered in our theory, we can measure the thermal diffusivity of a general sample without "artificial suppression." Moreover, from our analysis we can deduce the physical thickness of the sample from the critical frequency, which is the frequency at which the sample changes from thermally thin to thermally thick. This transition is clearly evident in the amplitude curve as a change in slope is detected at the critical frequency. © 1999 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 67 (1996), S. 1564-1569 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The two-beam photoacoustic phase measurement was applied to measure quantitatively the thermal diffusivity (αs) of a ceramic bulk high-Tc superconductor. Neglecting the effects of thermal dilation, and thermoelastic bending was proved valid in accordance with our composite piston model for the chosen experimental conditions. It was found that αs shows different features at the onset and offset temperatures corresponding to the normal–superconducting (NS) transition. A dip was seen at the resistivity transition onset temperature and a cusp at the offset temperature where the electrical resistance disappears. The presence of the cusp at the offset temperature is proposed to be related to weak coupling between superconducting grains. Our studies indicate that the two-beam phase measurement is a very sensitive method for superconductor characterization and NS transition detection. The experimental results also confirm the presence of a large energy gap and strong electron–phonon coupling mechanism in the YBCO superconductor. © 1996 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 84 (1998), S. 6623-6627 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Mirage technique is proved to be powerful in measurements of thermal diffusivity. Its contactless nature makes it suitable for delicate samples such as thin films and single crystals. However, as the damping of the thermal wave profile increases progressively upon the decrease in thermal diffusivity of the medium, mirage technique becomes more difficult to be applied to low thermal diffusivity measurements. Moreover influences from substrate signals make analysis difficult when the samples are thermally thin. Recently a thermal-wave-coupling method for mirage signal analysis [P. K. Wong, P. C. W. Fung, H. L. Tam, and J. Gao, Phys. Rev. B 51, 523 (1995)] was reported for thermal diffusivity measurements of thin film down to 60 nm thick. In this article we apply the thermal-wave-coupling method to thin films of low thermal diffusivity, especially polymer films. A new lower limit of thermal diffusivity measurable by mirage technique has been reached. © 1998 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 18 (1971), S. 90-99 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Statistical tests, based on the maximum-likelihood method, have been performed on flare series extending over several years. If all flares in each plage region are taken into account, a rich spectrum is obtained. If one carries out similar analysis of a ‘reduced’ flare series, which includes at most one flare from each plage region, the spectrum is almost devoid of structure, and what structure does remain is not statistically significant. The inference is that solar activity does not display genuine rigid-rotation modulation, but that repeated events from individual centers of activity yield modulation which may be mistaken for rigid-rotation modulation. A test for correlation between reduced flare series for the northern and southern hemispheres gives no significant correlation. This test therefore yieds no support to the hypothesis that solar activity is modulated by planetary effects.
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 96 (1994), S. 25-29 
    ISSN: 1434-6036
    Keywords: 74.60.Ec
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Based on the Ginzburg-Landau theory and using the numerical relaxation approach, we have carried out a numerical analysis of an isolated vortex structure for a type-II superconductor in the presence of a twin plane (being a ‘normal region’) situated parallel to the external magnetic field. Meanwhile, the lower critical field is also calculated and is found to decrease as compared to the twin-free case, which is consistent with the intuitive theoretical understanding and the fact that twins are easy channels for the flux penetration.
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Astrophysics and space science 50 (1977), S. 361-381 
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Considering mode coupling as a consequence of the matching of boundary conditions at an infinitesimal discontinuity, a concept introduced by the same authors earlier, we derive explicit expressions for the coupling coefficients for electromagnetic waves propagating in a rather general direction in an inhomogeneous magnetized plasma. Some special cases of the theory are discussed.
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Astrophysics and space science 52 (1977), S. 105-135 
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We use the semi-quantum approach to study the Compton scattering and non-linear scattering processes in a magnetoative plasma under the weak turbulence regime. Analytical expressions for these two types of scattering probabilities are derived from first principles forl-l waves. Transiting to limiting cases, the results obtained in this investigation are shown to be identical to previous derivations by other workers.
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Astrophysics and space science 66 (1979), S. 235-254 
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have deduced the intensity spectral function for the spin-flip synchrotron radiation in the presence of a plasma. Using parameters appropriate to astrophysical conditions, we have attempted to compare the characteristics of the spin-flip synchrotron radiation and the normal synchrotron radiation in a magnetized plasma arising from an electron or positron rotating around the magnetic field. A rotating charge gives the maximum possible synchrotron radiative power as compared to a charge of the same energy but moving in a helical path. Since the spin-flip radiational does not depend on the form of the orbital path, whether circular or helical or along a straight line, the analysis presented here gives the lower limit of the relative importance of the spin-flip radiation to the normal synchrotron radiation emitted by the same radiator.
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