The Isotope Effect in Superconductors

We review some aspects of the isotope effect (IE) in superconductors. Our focus is on the influence of factors not related to the pairing mechanism. After summarizing the main results obtained for conventional superconductors, we review the effect of magnetic impurities, the proximity effect and non-adiabaticity on the value of the isotope coefficient (IC). We discuss the isotope effect of $T_c$ and of the penetration depth $\delta$. The theory is applied to conventional and high-$T_c$ superconductors. Experimental results obtained for YBa$_2$Cu$_3$O$_{7-\delta}$ related materials (Zn and Pr-substituted as well as oxygen-depleted systems) and for La$_{2-x}$Sr$_x$CuO$_4$ are discussed.Comment: 31 pages, 10 figures. Review article to appear in "Pair Correlation in Many Fermions Systems", Plenum Press 199

A batch-service queueing model with a discrete batch Markovian arrival process

Queueing systems with batch service have been investigated extensively during the past decades. However, nearly all the studied models share the common feature that an uncorrelated arrival process is considered, which is unrealistic in several real-life situations. In this paper, we study a discrete-time queueing model, with a server that only initiates service when the amount of customers in system (system content) reaches or exceeds a threshold. Correlation is taken into account by assuming a discrete batch Markovian arrival process (D-BMAP), i.e. the distribution of the number of customer arrivals per slot depends on a background state which is determined by a first-order Markov chain. We deduce the probability generating function of the system content at random slot marks and we examine the influence of correlation in the arrival process on the behavior of the system. We show that correlation merely has a small impact on the threshold that minimizes the mean system content. In addition, we demonstrate that correlation might have a significant influence on the system content and therefore has to be included in the model

Construction and Verification of the Constitutive Model of Pure Copper Deformation at Elevated Temperatures

The deformation behavior of pure copper was studied in hot compression tests in the temperature range of 773–1173 K and strain rate interval of 0.001–1.0 s⁻¹, the corresponding flow stress curves were plotted. The new method to calculate critical and saturation stresses was devised, quantitative analysis of strain hardening and dynamic softening was presented, a three-stage constitutive model was constructed to predict the flow stress of pure copper. As predicted and measured flow stress comparison indicate, the physical constitutive model can accurately characterize hot deformation of pure copper. With dynamic recovery and/or recrystallization. Numerical simulation of an upsetting process is carried out by implementing the constitutive model into commercial software. This model can be put to practical use and be quite promising for improving efficiency of a hot forging process for pure copper components.Изучено деформационное поведение чистой меди при испытании горячих проб на сжатие в диапазоне температур 773 1173 К и скорости деформации 0.001–1.0 с⁻¹, построены соответствующие кривые напряжения пластического течения. Разработан новый метод расчета критического напряжения и напряжения насыщения, представлен количественный анализ деформационного упрочнения и динамического разупрочнения, построена трехступенчатая определяющая модель прогнозирования напряжения пластического течения чистой меди. Как показывает сравнение прогнозируемого и измеренного напряжений пластического течения, с помощью такой модели можно точно описать горячее деформирование чистой меди при динамическом возврате и/или рекристаллизации. Выполнено численное моделирование процесса высадки путем реализации данной модели в рамках коммерческого программного обеспечения. Модель весьма перспективна и может использоваться для повышения эффективности процесса горячей ковки деталей из чистой меди.Вивчено деформаційне поведінку чистої міді при випробуванні гарячих проб на стиск в діапазоні температур 773-1173 К і швидкості деформації 0.001–1.0 с⁻¹, побудовані відповідні криві напружень пластичної течії. Розроблено новий метод розрахунку критичної напруги і напруги насичення, представлений кількісний аналіз деформаційного зміцнення і динамічного знеміцнення, побудована триступенева визначає модель прогнозування напружень пластичної течії чистої міді. Як показує порівняння прогнозованого і виміряного напружень пластичної течії, за допомогою такої моделі можна точно описати гаряче деформування чистої міді при динамічному поверненні і / або рекристалізації. Виконано чисельне моделювання процесу висадки шляхом реалізації даної моделі в рамках комерційного програмного забезпечення. Модель вельми перспективна і може використовуватися для підвищення ефективності процесу гарячого кування деталей з чистої міді

A machine learning pipeline for discriminant pathways identification

Motivation: Identifying the molecular pathways more prone to disruption during a pathological process is a key task in network medicine and, more in general, in systems biology. Results: In this work we propose a pipeline that couples a machine learning solution for molecular profiling with a recent network comparison method. The pipeline can identify changes occurring between specific sub-modules of networks built in a case-control biomarker study, discriminating key groups of genes whose interactions are modified by an underlying condition. The proposal is independent from the classification algorithm used. Three applications on genomewide data are presented regarding children susceptibility to air pollution and two neurodegenerative diseases: Parkinson's and Alzheimer's. Availability: Details about the software used for the experiments discussed in this paper are provided in the Appendix

Imaging and manipulating the structural machinery of living cells on the micro- and nanoscale

The structure, physiology, and fate of living cells are all highly sensitive to mechanical forces in the cellular microenvironment, including stresses and strains that originate from encounters with the extracellular matrix (ECM), blood and other flowing materials, and neighbouring cells. This relationship between context and physiology bears tremendous implications for the design of cellular micro-or nanotechnologies, since any attempt to control cell behavior in a device must provide the appropriate physical microenvironment for the desired cell behavior. Cells sense, process, and respond to biophysical cues in their environment through a set of integrated, multi-scale structural complexes that span length scales from single molecules to tens of microns, including small clusters of force-sensing molecules at the cell surface, micron-sized cell-ECM focal adhesion complexes, and the cytoskeleton that permeates and defines the entire cell. This review focuses on several key technologies that have recently been developed or adapted for the study of the dynamics of structural micro-and nanosystems in living cells and how these systems contribute to spatially-and temporally-controlled changes in cellular structure and mechanics. We begin by discussing subcellular laser ablation, which permits the precise incision of nanoscale structural elements in living cells in order to discern their mechanical properties and contributions to cell structure. We then discuss fluorescence recovery after photobleaching and fluorescent speckle microscopy, two live-cell fluorescence imaging methods that enable quantitative measurement of the binding and transport properties of specific proteins in the cell. Finally, we discuss methods to manipulate cellular structural networks by engineering the extracellular environment, including microfabrication of ECM distributions of defined geometry and microdevices designed to measure cellular traction forces at micron-scale resolution. Together, these methods form a powerful arsenal that is already adding significantly to our understanding of the nanoscale architecture and mechanics of living cells and may contribute to the rational design of new cellular micro-and nanotechnologies

Density functional study of Aun_n (n=2-20) clusters: lowest-energy structures and electronic properties

We have investigated the lowest-energy structures and electronic properties of the Au$_n$(n=2-20) clusters based on density functional theory (DFT) with local density approximation. The small Au$_n$ clusters adopt planar structures up to n=6. Tabular cage structures are preferred in the range of n=10-14 and a structural transition from tabular cage-like structure to compact near-spherical structure is found around n=15. The most stable configurations obtained for Au$_{13}$ and Au$_{19}$ clusters are amorphous instead of icosahedral or fcc-like, while the electronic density of states sensitively depend on the cluster geometry. Dramatic odd-even alternative behaviors are obtained in the relative stability, HOMO-LUMO gaps and ionization potentials of gold clusters. The size evolution of electronic properties is discussed and the theoretical ionization potentials of Au$_n$ clusters compare well with experiments.Comment: 6 pages, 7 figure

pi and rho loop corrections to omega photoproduction in the resonance region

One-loop corrections due to the intermediate $\pi N$ and $\rho N$ states are studied in $\omega$ photoproduction near threshold. Our results show that the coupled-channel effects should be taken into account in extracting reliable nucleon resonance parameters from the forthcoming vector meson photoproduction data in the resonance region.Comment: 4 pages, to be published in the proceedings of XVI International Conference on Particles and Nuclei (PANIC'02), Osaka, Japan, Sep. 30 - Oct. 4, 200

Absence of the zero bias peak in vortex tunneling spectra of high temperature superconductors?

The c-axis tunneling matrix of high-Tc superconductors is shown to depend strongly on the in-plane momentum of electrons and vanish along the four nodal lines of the d(x^2-y^2)-wave energy gap. This anisotropic tunneling matrix suppresses completely the contribution of the most extended quasiparticles in the vortex core to the c-axis tunneling current and leads to a spectrum similar to that of a nodeless superconductor. Our results give a natural explanation of the absence of the zero bias peak as well as other features observed in the vortex tunneling spectra of high-Tc cuprates.Comment: 4 pages 3 figures, minor corrections, to appear in Phys Rev