Temperature-induced pair correlations in clusters and nuclei

The pair correlations in mesoscopic systems such as $nm$-size superconducting clusters and nuclei are studied at finite temperature for the canonical ensemble of fermions in model spaces with a fixed particle number: i) a degenerate spherical shell (strong coupling limit), ii) an equidistantly spaced deformed shell (weak coupling limit). It is shown that after the destruction of the pair correlations at T=0 by a strong magnetic field or rapid rotation, heating can bring them back. This phenomenon is a consequence of the fixed number of fermions in the canonical ensemble

Magnetoplasmon resonances in polycrystalline bismuth as seen via terahertz spectroscopy

We report the magnetic field-dependent far-infrared reflectivity of polycrystalline bismuth. We observe four distinct absorptions that we attribute to magnetoplasmon resonances, which are collective modes of an electron-hole liquid in magnetic field and become optical and acoustic resonances of the electron-hole system in the small-field limit. The acoustic mode is expected only when the masses of distinct components are very different, which is the case in bismuth. In a polycrystal, where the translational symmetry is broken, a big shift of spectral weight to acoustic plasmon is possible. This enables us to detect an associated plasma edge. Although the polycrystal sample has grains of randomly distributed orientations, our reflectivity results can be explained by invoking only two, clearly distinct, series of resonances. In the limit of zero field, the optical modes of these two series converge onto plasma frequencies measured in monocrystal along the main optical axes.Comment: Accepted in PR

Infrared spectroscopy of hole doped ABA-stacked trilayer graphene

Using infrared spectroscopy, we investigate bottom gated ABA-stacked trilayer graphene subject to an additional environment-induced p-type doping. We find that the Slonczewski-Weiss-McClure tight-binding model and the Kubo formula reproduce the gate voltage-modulated reflectivity spectra very accurately. This allows us to determine the charge densities and the potentials of the {\pi}-band electrons on all graphene layers separately and to extract the interlayer permittivity due to higher energy bands.Comment: 6 pages, 6 figures Corrected sign of fig 3 and visibilty of fig

Intraband Optical Spectral Weight in the presence of a van Hove singularity: application to Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta}

The Kubo single band sum rule is used to determine the optical spectral weight of a tight binding band with further than nearest neighbour hopping. We find for a wide range of parameters and doping concentrations that the change due to superconductivity at low temperature can be either negative or positive. In contrast, the kinetic energy change is always negative. We use an ARPES determined tight binding parametrization of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ to investigate whether this can account for recent observations of a positive change in the spectral weight due to the onset of superconductivity. With this band structure we find that in the relevant doping regime a straightforward BCS calculation of the optical spectral weight cannot account for the experimental observations.Comment: 10 page 9 figure

The two colors of MgB2

We present the anisotropic optical conductivity of MgB$_{2}$ between 0.1 and 3.7 eV at room temperature obtained on single crystals of different purity by the spectroscopic ellipsometry and reflectance measurements. The bare (unscreened) plasma frequency $\omega_{p}$ is almost isotropic and equal to 6.3 eV, which contrasts some earlier reports of a very small value of $\omega_{p}$. The data suggests that the $\sigma$-bands are characterized by a stronger electron-phonon coupling $\lambda_ {tr}$ but smaller impurity scattering $\gamma_{imp}$, compared to the $\pi$-bands. The optical response along the boron planes is marked by an intense interband transition at 2.6 eV, due to which the reflectivity plasma edges along the a- and c-axes are shifted with respect to each other. As a result, the sample spectacularly changes color from a blueish-silver to the yellow as the polarization is rotated from the in-plane direction towards the c-axis. The optical spectra are in good agreement with the published {\it ab initio} calculations. The remaining discrepancies can be explained by the relative shift of $\sigma$-bands and $\pi$-bands by about 0.2 eV compared to the theoretical band structure, in agreement with the de Haas-van Alphen experiments. The widths of the Drude and the interband peaks are both very sensitive to the sample purity.Comment: 11 pages, 13 figure

Coupled quantum wires

We study a set of crossed 1D systems, which are coupled with each other via tunnelling at the crossings. We begin with the simplest case with no electron-electron interactions and find that besides the expected level splitting, bound states can emerge. Next, we include an external potential and electron-electron interactions, which are treated within the Hartree approximation. Then, we write down a formal general solution to the problem, giving additional details for the case of a symmetric external potential. Concentrating on the case of a single crossing, we were able to explain recent experinents on crossed metallic and semiconducting nanotubes [J. W. Janssen, S. G. Lemay, L. P. Kouwenhoven, and C. Dekker, Phys. Rev. B 65, 115423 (2002)], which showed the presence of localized states in the region of crossing.Comment: 11 pages, 10 figure

Collimator-size effect on the parametric-radiation spectrum

It is shown that, by varying the angular size of a collimator, the spectral distribution of parametric x-ray radiation can be split into two isolated peaks and that the center of the spectral distribution of this radiation is shifted in frequency in response to an increase in the collimator sizeyesBelgorod State Universit

Formation of Spherical Nanoparticles BaTiO3 by Peroxide Method

Structure of barium titanate particles obtained by peroxide method was studied by SEM, X-ray phase analysis, IR spectroscopy and Raman scattering. Found, that particles have preferably a spherical shape with a diameter of 20-200 nm and contain both cubic and tetragonal phases. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3365

University as an unique subject of social cognition (philosophical discourse)

У статті наведено філософський аналіз університету як унікального суб'єкту соціального пізнання. Розглянуто особливості когнітивної взаємодії університету із суспільством, що трансформується, та сучасний стан утисків соціально-гуманітарної освіти у вищому навчальному закладі; досліджується зміна місії університету в суспільному дискурсі; розглянуто вплив цінностей комунікативної університетської спільноти на процес соціального пізнання; проаналізовано роль університету як інтегратора знання; визначено вплив автономії університету на його культуро-породжуючу функцію; позначено зв'язок соціальної відповідальності університету з його роллю як суб'єкта соціального пізнання. Встановлено, що особливістю когнітивної взаємодії університету з суспільством є наявність суб'єкт-об'єктного протиріччя в процесі філософської рефлексії, аналогічного суб'єкт-об'єктній взаємодії у процесах соціального пізнання. Зміна місії університету в суспільному дискурсі призводить до появи його нової специфіки як соціального суб'єкту соціального пізнання. Процес соціального пізнання відбувається, головним чином, у вигляді соціально-історичного процесу розвитку знання, і результати його доводяться до студентів у вигляді рефлексії, що апріорі звернена у минуле, в крайньому випадку – у сьогодення. Однак швидкий розвиток суспільства вимагає від цього суспільства зверненості у майбутнє, а відтак потребує від університетів не тільки досліджень, пов'язаних із можливими соціальними сценаріями, тобто виконання прогнозної функції як осередку соціального пізнання, а і відповідної трансформації соціально-гуманітарної підготовки студентів. Процес філософської рефлексії університету має важливе методологічне значення для розкриття його унікальної ролі в процесі соціального пізнання. Філософська рефлексія університету у якості ідеї, як соціального інституту і як організації в дискурсі соціального пізнання має бути предметом поглиблених досліджень сучасної гуманістики.The article deals with the philosophical analysis of the university as a unique subject of social cognition. The article considers the features of cognitive interaction of the university with the society that is transforming, and the modern condition of oppressions of education of social sciences and humanities in higher educational institution. It investigates the change of the university mission in the public discussion, it is represented the influence of values of communicative university community on the process of social cognition; it is analyzed the role of university as an integrator of knowledge. It is defined the influence of university autonomy on its culture-creating function. The connection of social responsibility of university with its role as a subject of social cognition is illustrated. It is determined that the special feature of cognitive interaction of the university with the society is a presence of subject and object contradiction in the process of philosophical reflection that is analogous to the subject and object interaction in the processes of social cognition. The change of the university mission in the public discussion leads to the appearing of its new specifics as the social subject of the social cognition. Essentially, the process of the social cognition happens as a social and historical process of the knowledge development and its results are forwarded to the students in the form of reflection that is a priori deepened in the past, in extreme case – in the contemporaneity. However, the fast development of society requests from this society deepening in the future and thus, needs from universities not only research that is connected with the possible social scenarios, it means the fulfilling of the forecast function as a core of the social cognition and the corresponding transformation of students’ training in social sciences and humanities. The process of philosophical reflection of the university has an important methodological meaning for understanding its unique role in the process of the social cognition. Philosophical reflection of the university as an idea of the social institute and as organization in discussion of the social cognition must be a subject of profound research of modern humanistics