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

Сравнительное исследование регенерации у губок (Porifera)

International audiencet is well known that sponges have unusually high regenerative abilities, which are often associated with their low tissue organization and high dynamics of cell differentiation. In order to identify the diversity of morphogenesis and cellular mechanisms involved in the restoration processes in Porifera, we carried out a detailed comparative study of the regeneration of a number of sponge species from phylogenetically deleted taxa that differ in their anatomical and histological organization. The objects of this project were sponges from the classes Demospongiae, Homoscleromorpha, and Calcarea, which have the leukonoid, siconoid, and asconoid types of the aquiferous system e-mines, as well as different degrees of epithelial development. The main mechanism for the regeneration of leukonoid Demospongiae is the epithelio-mesenchymal transformation involving polypotent archaeocyte cells. Epithelial morphogenesis and cell transdifferentiation are the basis for the regeneration of leukoid Homoscleromorpha and siconoid and asconoid Calcarea. Asconoid Calcarea regeneration is a rare example of “pure” morphallaxis. We have shown that the morphogenetic processes detected in sponges are as complex and diverse as in other higher animals. Thus, sponges can be a source of important information that will allow us to better understand the early evolution of molecular and cellular mechanisms of morphogenesis in animals.2 СПбГУ, биологический факультет 3 МГУ им. Ломоносова, биологический факультет, Беломорская биологическая станция им. Н.А. Перцова Хорошо известно, что губки обладают необычайно высокими регенерационными сп о-собностями что часто связывают с их низкой тканевой организацией и высокой динамикой клеточной дифференцировки. Для того, чтобы выявить разнообразие морфогенезов и клеточ-ных механизмов вовлеченных в восстановительные процессы у Porifera, нами было предпр и-нято детальное сравнительное исследование регенерации ряда видов губок из филогенетич е-ски удаленных таксонов, отличающиеся анатомической и гистологической организацией. Объектами данного проекта послужили губки из классов Demospongiae, Homoscleromorpha и Calcarea, обладающие лейконоидной, сиконоидной и асконоидной типами водоносной сист е-мы, а также разной степенью развития эпителия. Основным механизмом регенерации лейко-ноидных Demospongiae является эпителио-мезенхимная трансформация с вовлечением поли-потентных клеток-археоцитов. Основой регенерации лейконоидных Homoscleromorpha и сиконоидных и асконоидных Calcarea служат эпителиальные морфогенезы и трансдифферен-цировка клеток. Регенерация асконоидных Calcarea представляет собой редкий пример «чи-стого» морфаллаксиса. Мы показали, что морфогенетические процессы, выявленные у губок, являются столь же сложными и разнообразными, как и у других высших животных. Таким образом, губки могут быть источником важной информации, которая позволит нам глубже понять раннюю эволюцию молекулярных и клеточных механизмов морфогенеза у животных. Благодарности Данная работа осуществлена при финансовой поддержке грантов РФФИ № 16-04-00084, и РНФ № 17-14-01089.-8

Nanotube-based source of charges for experiments with solid helium at low temperatures

Methods of preparation of the field-emission sources of charges from carbon nanotubes suitable for study of injected charges in solid helium at low temperatures T < 1 K are presented. The sources have been prepared by arc discharge deposition of nanotubes onto a flat copper substrate or by mechanical rubbing of nanotubes into porous metal surface. The test study of the voltage-current characteristics of a diode cell with the nanotube source in superfluid He II have shown that at voltages above 120 V one can observe a relatively large current I ≥ 10⁻¹³ A of negative charges in liquid helium. The field and temperature dependences of positive and negative currents in solid ⁴He were studied in samples grown by the blocked capillary technique. Usage of the nanotube based source of injected charges had permitted us for the first time to observe motion of the positive charges in solid helium at temperatures below 0.1 К. The current-voltage dependence could be described by a power law I ~ Uα, with the value of the exponent α >> 2, much higher than what one would expect for the regime of space charge limited currents

Single-photon emitters in GaSe

Single-photon sources are important building blocks for quantum information technology. Emitters based on solid-state systems provide a viable route to integration in photonic devices. Here, we report on single-photon emitters in the layered semiconductor GaSe. We identify the exciton and biexciton transition of the quantum emitters with power-dependent photoluminescence and photon statistics measurements. We find evidence that the localization of the excitons is related to deformations of the GaSe crystal, caused by nanoscale selenium inclusions, which are incorporated in the crystal. These deformations give rise to local strain fields, which induce confinement potentials for the excitons. This mechanism lights the way for the controlled positioning of single-photon emitters in GaSe on the nanoscale

Photoluminescence of two-dimensional GaTe and GaSe films

Gallium chalcogenides are promising building blocks for novel van der Waals heterostructures. We report on the low-temperature micro-photoluminescence (PL) of GaTe and GaSe films with thicknesses ranging from 200 nm to a single unit cell. In both materials, PL shows a dramatic decrease by 10e4–10e5 when film thickness is reduced from 200 to 10 nm. Based on evidence from continuous-wave (cw) and time-resolved PL, we propose a model explaining the PL decrease as a result of non-radiative carrier escape via surface states. Our results emphasize the need for special passivation of two-dimensional films for optoelectronic applications

ARPES Spectra of the Hubbard model

We discuss spectra calculated for the 2D Hubbard model in the intermediate coupling regime with the dynamical cluster approximation, which is a non-perturbative approach. We find a crossover from a normal Fermi liquid with a Fermi surface closed around the Brillouin zone center at large doping to a non-Fermi liquid for small doping. The crossover is signalled by a splitting of the Fermi surface around the $X$ point of the 2D Brillouin zone, which eventually leads to a hole-like Fermi surface closed around the point M. The topology of the Fermi surface at low doping indicates a violation of Luttinger's theorem. We discuss different ways of presenting the spectral data to extract information about the Fermi surface. A comparison to recent experiments will be presented.Comment: 8 pages, 7 color figures, uses RevTeX

Effect of electron irradiation on vortex dynamics in YBa_2Cu_3O_{7-x} single crystals

We report on drastic change of vortex dynamics with increase of quenched disorder: for rather weak disorder we found a single vortex creep regime, which we attribute to a Bragg-glass phase, while for enhanced disorder we found an increase of both the depinning current and activation energy with magnetic field, which we attribute to entangled vortex phase. We also found that introduction of additional defects always increases the depinning current, but it increases activation energy only for elastic vortex creep, while it decreases activation energy for plastic vortex creep.Comment: 4 pages, 3 figures, submited to Phys. Rev.

Сравнительное исследование регенерации у губок (Porifera)

International audiencet is well known that sponges have unusually high regenerative abilities, which are often associated with their low tissue organization and high dynamics of cell differentiation. In order to identify the diversity of morphogenesis and cellular mechanisms involved in the restoration processes in Porifera, we carried out a detailed comparative study of the regeneration of a number of sponge species from phylogenetically deleted taxa that differ in their anatomical and histological organization. The objects of this project were sponges from the classes Demospongiae, Homoscleromorpha, and Calcarea, which have the leukonoid, siconoid, and asconoid types of the aquiferous system e-mines, as well as different degrees of epithelial development. The main mechanism for the regeneration of leukonoid Demospongiae is the epithelio-mesenchymal transformation involving polypotent archaeocyte cells. Epithelial morphogenesis and cell transdifferentiation are the basis for the regeneration of leukoid Homoscleromorpha and siconoid and asconoid Calcarea. Asconoid Calcarea regeneration is a rare example of “pure” morphallaxis. We have shown that the morphogenetic processes detected in sponges are as complex and diverse as in other higher animals. Thus, sponges can be a source of important information that will allow us to better understand the early evolution of molecular and cellular mechanisms of morphogenesis in animals.2 СПбГУ, биологический факультет 3 МГУ им. Ломоносова, биологический факультет, Беломорская биологическая станция им. Н.А. Перцова Хорошо известно, что губки обладают необычайно высокими регенерационными сп о-собностями что часто связывают с их низкой тканевой организацией и высокой динамикой клеточной дифференцировки. Для того, чтобы выявить разнообразие морфогенезов и клеточ-ных механизмов вовлеченных в восстановительные процессы у Porifera, нами было предпр и-нято детальное сравнительное исследование регенерации ряда видов губок из филогенетич е-ски удаленных таксонов, отличающиеся анатомической и гистологической организацией. Объектами данного проекта послужили губки из классов Demospongiae, Homoscleromorpha и Calcarea, обладающие лейконоидной, сиконоидной и асконоидной типами водоносной сист е-мы, а также разной степенью развития эпителия. Основным механизмом регенерации лейко-ноидных Demospongiae является эпителио-мезенхимная трансформация с вовлечением поли-потентных клеток-археоцитов. Основой регенерации лейконоидных Homoscleromorpha и сиконоидных и асконоидных Calcarea служат эпителиальные морфогенезы и трансдифферен-цировка клеток. Регенерация асконоидных Calcarea представляет собой редкий пример «чи-стого» морфаллаксиса. Мы показали, что морфогенетические процессы, выявленные у губок, являются столь же сложными и разнообразными, как и у других высших животных. Таким образом, губки могут быть источником важной информации, которая позволит нам глубже понять раннюю эволюцию молекулярных и клеточных механизмов морфогенеза у животных. Благодарности Данная работа осуществлена при финансовой поддержке грантов РФФИ № 16-04-00084, и РНФ № 17-14-01089.-8