Compact Spacelike Surfaces in the 3-Dimensional de Sitter Space

We establish several sufficient conditions for a compact spacelike surface in the 3-dimensional de Sitter space to be totally geodesic or spherical

Measuring the gap in ARPES experiments

Angle-resolved photoemission spectroscopy (ARPES) is considered as the only experimental tool from which the momentum distribution of both the superconducting and pseudo-gap can be quantitatively derived. The binding energy of the leading edge of the photoemission spectrum, usually called the leading edge gap (LEG), is the model-independent quantity which can be measured in the modern ARPES experiments with the very high accuracy--better than 1 meV. This, however, may be useless as long as the relation between the LEG and the real gap is unknown. We present a systematic study of the LEG as a function of a number of physical and experimental parameters. The absolute gap values which have been derived from the numerical simulation prove, for example that the nodal direction in the underdoped Bi-2212 in superconducting state is really the node--the gap is zero. The other consequences of the simulations are discussed.Comment: revtex4, 9 pages, 6 figure

An ARPES view on the high-Tc problem: phonons vs spin-fluctuations

We review the search for a mediator of high-Tc superconductivity focusing on ARPES experiment. In case of HTSC cuprates, we summarize and discuss a consistent view of electronic interactions that provides natural explanation of both the origin of the pseudogap state and the mechanism for high temperature superconductivity. Within this scenario, the spin-fluctuations play a decisive role in formation of the fermionic excitation spectrum in the normal state and are sufficient to explain the high transition temperatures to the superconducting state while the pseudogap phenomenon is a consequence of a Peierls-type intrinsic instability of electronic system to formation of an incommensurate density wave. On the other hand, a similar analysis being applied to the iron pnictides reveals especially strong electron-phonon coupling that suggests important role of phonons for high-Tc superconductivity in pnictides.Comment: A summary of the ARPES part of the Research Unit FOR538, http://for538.wmi.badw.d

Asymptotic Properties of Hilbert Geometry

It is shown that the spheres in Hilbert geometry have the same volume growth entropy as those in the Lobachevsky space. Asymptotic estimates for the ratio of the volume of metric ball to the area of the metric sphere in Hilbert geometry are given. Derived estimates agree with the well-known fact in the Lobachevsky space

Evaluation of information systems

У статті розглядається узагальнений критерій оцінки ефективності інформаційних систем як засіб порівняння між собою різних систем, що вирішують одне і те саме завдання. Це дозволяє вибрати серед них найбільш ефективне. Такий критерій повинен поєднувати в собі окремі критерії таким чином, щоб урівноважити вплив кожного з них на об'єднувальний узагальнений критерій. Вдало вибраний узагальнений критерій дає можливість знаходити ефективні нестандартні рішення під час побудови і роботи складних систем. При цитуванні документа, використовуйте посилання http://essuir.sumdu.edu.ua/handle/123456789/24412В статье рассматривается обобщенный критерий оценки эффективности информационных систем как способ сравнения между собой различных систем, решающих одну и ту же задачу. Это позволяет выбрать среди них наиболее эффективную. Такой критерий должен объединять в себе частные критерии таким образом, чтобы уравновесить влияние каждого из них на объединяющий обобщенный критерий. Удачно выбранный обобщенный критерий дает возможность находить эффективные нестандартные решения при построении и работе сложных систем. При цитировании документа, используйте ссылку http://essuir.sumdu.edu.ua/handle/123456789/24412The article deals with the generalized criterion evaluating the efficiency of information systems as a way to compare different systems that solve the same problem with each other. This allows to choose the most effective among them. Such a criterion must combine the partial criteria in order to counterbalance the influence of each of them on the combined generalized criterion. Well chosen generalized criterion allows to find effective innovative solutions in the construction and operation of complex systems. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/2441

ARPES on high-temperature superconductors: simplicity vs. complexity

A notable role in understanding of microscopic electronic properties of high temperature superconductors (HTSC) belongs to angle resolved photoemission spectroscopy (ARPES). This technique supplies a direct window into the reciprocal space of solids: the momentum–energy space where quasiparticles (electrons dressed in clouds of interactions) dwell. Any interaction in the electronic system, e.g., superconducting pairing, leads to modification of the quasiparticle spectrum—to redistribution of the spectral weight over the momentum–energy space probed by ARPES. Continued development of the technique had the effect that the picture seen through the ARPES window became clearer and sharper until the complexity of the electronic band structure of the cuprates had been resolved. Now, in the doping range optimal for superconductivity, the cuprates much resemble a normal metal with well-predicted electronic structure, though with rather strong electron–electron interaction. This principal disentanglement of the complex physics from complex structure reduced the mystery of HTSC to the tangible problem of the interaction responsible for quasiparticle formation. Here we present a short overview of resent ARPES results, which, we believe, suggest a way to resolve the HTSC puzzle

Amorphization kinetics under electron irradiation

A model is brought forward of the kinetics of amorphization occurring under electron irradiation. The model is based on consideration of the phase transformations controlled by the structural relaxation of small-scale unstable atomic configurations excited by electrons.Наведено модель кінетики аморфізації під електронним випромінюванням. Модель грунтується на розгляді фазових перетворень, які контролюються структурной релаксацієй дрібномасштабних нестабільних атомних конфігурацій, які збуджуються електронами.Приведена модель кинетики аморфизации под электронным облучением. Модель основывается на рассмотрении фазовых превращений, которые контролируются структурной релаксацией мелкомасштабных нестабильных атомных конфигураций, которые возбуждаются электронами

About the relation between the quasiparticle Green's function in cuprates obtained from ARPES data and the magnetic susceptibility

Angle resolved photoemission spectroscopy (ARPES) provides a detailed view of the renormalized band structure in cuprates and, consequently, is a key to the self-energy and the quasiparticle Green's function. Such information gives a clue to the comparison of ARPES with scanning tunneling microscopy, inelastic neutron scattering (INS), and Raman scattering data. Here we touch on a potential possibility of such a comparison with the dynamical magnetic susceptibility measured in INS experiments. Calculations based on the experimentally measured quasiparticle self-energies in cuprates lead to the estimated magnetic susceptibility response with many-body effects taken into account.Comment: Will be presented at the M2S-HTSC-VIII conference in Dresde

From tunneling to photoemission: correlating two spaces

Correlating the data measured by tunneling and photoemission spectroscopies is a long-standing problem in condensed matter physics. The quasiparticle interference, recently discovered in high-Tc cuprates, reveals a possibility to solve this problem. Application of modern phase retrieval algorithms to Fourier transformed tunneling data allows to recover the distribution of the quasiparticle spectral weight in the reciprocal space of solids measured directly by photoemission. This opens a direct way to unify these two powerful techniques and may help to solve a number of problems related with space/time inhomogeneities predicted in strongly correlated electron systems.Comment: more info at http://www.imp.kiev.ua/~kord/AC-ARPES/index.htm

Origin of the shadow Fermi surface in Bi-based cuprates

We used angle-resolved photoemission spectroscopy to study the shadow Fermi surface in one layer Bi2Sr1.6La0.4CuO6+delta and two layer (Bi,Pb)2Sr2CaCu2O8+delta. We find the shadow band to have the same peakwidth and dispersion as the main band. In addition, the shadow band/main band intensity ratio is found to be binding energy independent. Consequently, it is concluded that the shadow bands in Bi-based HTSC do not originate from antiferromagnetic interactions but have a structural origin.Comment: 10 pages, 2 figure