Enhancing superconductivity: Magnetic impurities and their quenching by magnetic fields

Magnetic fields and magnetic impurities are each known to suppress superconductivity. However, as the field quenches (i.e. polarizes) the impurities, rich consequences, including field-enhanced superconductivity, can emerge when both effects are present. For the case of superconducting wires and thin films, this field-spin interplay is investigated via the Eilenberger-Usadel scheme. Non-monotonic dependence of the critical current on the field (and therefore field-enhanced superconductivity) is found to be possible, even in parameter regimes in which the critical temperature decreases monotonically with increasing field. The present work complements that of Kharitonov and Feigel'man, which predicts non-monotonic behavior of the critical temperature.Comment: 8 pages, 2 figures, EPL forma

Structure of 8B from elastic and inelastic 7Be+p scattering

Motivation: Detailed experimental knowledge of the level structure of light weakly bound nuclei is necessary to guide the development of new theoretical approaches that combine nuclear structure with reaction dynamics. Purpose: The resonant structure of 8B is studied in this work. Method: Excitation functions for elastic and inelastic 7Be+p scattering were measured using a 7Be rare isotope beam. Excitation energies ranging between 1.6 and 3.4 MeV were investigated. An R-matrix analysis of the excitation functions was performed. Results: New low-lying resonances at 1.9, 2.5, and 3.3 MeV in 8B are reported with spin-parity assignment 0+, 2+, and 1+, respectively. Comparison to the Time Dependent Continuum Shell (TDCSM) model and ab initio no-core shell model/resonating-group method (NCSM/RGM) calculations is performed. This work is a more detailed analysis of the data first published as a Rapid Communication. [J.P. Mitchell, et al, Phys. Rev. C 82, 011601(R) (2010)] Conclusions: Identification of the 0+, 2+, 1+ states that were predicted by some models at relatively low energy but never observed experimentally is an important step toward understanding the structure of 8B. Their identification was aided by having both elastic and inelastic scattering data. Direct comparison of the cross sections and phase shifts predicted by the TDCSM and ab initio No Core Shell Model coupled with the resonating group method is of particular interest and provides a good test for these theoretical approaches.Comment: 15 pages, 19 figures, 3 tables, submitted to PR

Low-lying states in 8B

Excitation functions of elastic and inelastic 7Be+p scattering were measured in the energy range between 1.6 and 2.8 MeV in the c.m. An R-matrix analysis of the excitation functions provides strong evidence for new positive parity states in 8B. A new 2+ state at an excitation energy of 2.55 MeV was observed and a new 0+ state at 1.9 MeV is tentatively suggested. The R-matrix and Time Dependent Continuum Shell Model were used in the analysis of the excitation functions. The new results are compared to the calculations of contemporary theoretical models.Comment: 6 pages, 5 figures, accepted as Rapid Communication in Phys. Rev.

Evaluation of the Efficiency of Detection and Capture of Manganese in Aqueous Solutions of FeCeOx Nanocomposites Doped with Nb2O5

: The main purpose of this work is to study the effectiveness of using FeCeOx nanocomposites doped with Nb2O5 for the purification of aqueous solutions from manganese. X-ray diffraction, energy-dispersive analysis, scanning electron microscopy, vibrational magnetic spectroscopy, and mössbauer spectroscopy were used as research methods. It is shown that an increase in the dopant concentration leads to the transformation of the shape of nanoparticles from spherical to cubic and rhombic, followed by an increase in the size of the nanoparticles. The spherical shape of the nanoparticles is characteristic of a structure consisting of a mixture of two phases of hematite (Fe2O3) and cerium oxide CeO2. The cubic shape of nanoparticles is typical for spinel-type FeNbO4 structures, the phase contribution of which increases with increasing dopant concentration. It is shown that doping leads not only to a decrease in the concentration of manganese in model solutions, but also to an increase in the efficiency of adsorption from 11% to 75%

Relaxation processes in silicon heterojunction solar cells probed via noise spectroscopy

We have employed state of the art cross correlation noise spectroscopy CCNS to study carrier dynamics in silicon heterojunction solar cells SHJ SCs . These cells were composed of a light absorbing n doped monocrystalline silicon wafer contacted by passivating layers of i a Si H and doped a Si H selective contact layers. Using CCNS, we are able to resolve and characterize four separate noise contributions 1 shot noise with Fano factor close to unity due to holes tunneling through the np junction, 2 a 1 f term connected to local potential fluctuations of charges trapped in a Si H defects, 3 generation recombination noise with a time constant between 30 and 50 amp; 956;s and attributed to recombination of holes at the interface between the ITO and n a Si H window layer, and 4 a low frequency generation recombination term observed below 100 K which we assign to thermal emission over the ITO ni a Si H interface barrier. These results not only indicate that CCNS is capable of reveling otherwise undetectable relaxation process in SHJ SCs and other multi layer devices, but also that the technique has a spatial selectivity allowing for the identification of the layer or interface where these processes are taking plac

Pair-breaking quantum phase transition in superconducting nanowires

A quantum phase transition (QPT) between distinct ground states of matter is a wide-spread phenomenon in nature, yet there are only a few experimentally accessible systems where the microscopic mechanism of the transition can be tested and understood. These cases are unique and form the experimentally established foundation for our understanding of quantum critical phenomena. Here we report the discovery that a magnetic-field-driven QPT in superconducting nanowires - a prototypical 1d-system - can be fully explained by the critical theory of pair-breaking transitions characterized by a correlation length exponent $\nu \approx 1$ and dynamic critical exponent $z \approx 2$. We find that in the quantum critical regime, the electrical conductivity is in agreement with a theoretically predicted scaling function and, moreover, that the theory quantitatively describes the dependence of conductivity on the critical temperature, field magnitude and orientation, nanowire cross sectional area, and microscopic parameters of the nanowire material. At the critical field, the conductivity follows a $T^{(d-2)/z}$ dependence predicted by phenomenological scaling theories and more recently obtained within a holographic framework. Our work uncovers the microscopic processes governing the transition: The pair-breaking effect of the magnetic field on interacting Cooper pairs overdamped by their coupling to electronic degrees of freedom. It also reveals the universal character of continuous quantum phase transitions.Comment: 22 pages, 5 figure

Extreme alpha-clustering in the 18O nucleus

The structure of the 18O nucleus at excitation energies above the alpha decay threshold was studied using 14C+alpha resonance elastic scattering. A number of states with large alpha reduced widths have been observed, indicating that the alpha-cluster degree of freedom plays an important role in this N not equal Z nucleus. However, the alpha-cluster structure of this nucleus is very different from the relatively simple pattern of strong alpha-cluster quasi-rotational bands in the neighboring 16O and 20Ne nuclei. A 0+ state with an alpha reduced width exceeding the single particle limit was identified at an excitation energy of 9.9+/-0.3 MeV. We discuss evidence that states of this kind are common in light nuclei and give possible explanations of this feature.Comment: 4 pages, 2 figures, 1 table. Resubmission with minor changes for clarity, including removal of one figur