Coulomb singularity effects in tunnelling spectroscopy of individual impurities

Non-equilibrium Coulomb effects in resonant tunnelling processes through deep impurity states are analyzed. It is shown that Coulomb vertex corrections to the tunnelling transfer amplitude lead to a power-law singularity in current- voltage characteristicsComment: 7 pages, 2 figure

Renormalization group analysis of the 2D Hubbard model

Salmhofer [Commun. Math. Phys. 194, 249 (1998)] has recently developed a new renormalization group method for interacting Fermi systems, where the complete flow from the bare action of a microscopic model to the effective low-energy action, as a function of a continuously decreasing infrared cutoff, is given by a differential flow equation which is local in the flow parameter. We apply this approach to the repulsive two-dimensional Hubbard model with nearest and next-nearest neighbor hopping amplitudes. The flow equation for the effective interaction is evaluated numerically on 1-loop level. The effective interactions diverge at a finite energy scale which is exponentially small for small bare interactions. To analyze the nature of the instabilities signalled by the diverging interactions we extend Salmhofers renormalization group for the calculation of susceptibilities. We compute the singlet superconducting susceptibilities for various pairing symmetries and also charge and spin density susceptibilities. Depending on the choice of the model parameters (hopping amplitudes, interaction strength and band-filling) we find commensurate and incommensurate antiferromagnetic instabilities or d-wave superconductivity as leading instability. We present the resulting phase diagram in the vicinity of half-filling and also results for the density dependence of the critical energy scale.Comment: 16 pages, RevTeX, 16 eps figure

Parquet Graph Resummation Method for Vortex Liquids

We present in detail a nonperturbative method for vortex liquid systems. This method is based on the resummation of an infinite subset of Feynman diagrams, the so-called parquet graphs, contributing to the four-point vertex function of the Ginzburg-Landau model for a superconductor in a magnetic field. We derive a set of coupled integral equations, the parquet equations, governing the structure factor of the two-dimensional vortex liquid system with and without random impurities and the three-dimensional system in the absence of disorder. For the pure two-dimensional system, we simplify the parquet equations considerably and obtain one simple equation for the structure factor. In two dimensions, we solve the parquet equations numerically and find growing translational order characterized by a length scale $R_c$ as the temperature is lowered. The temperature dependence of $R_c$ is obtained in both pure and weakly disordered cases. The effect of disorder appears as a smooth decrease of $R_c$ as the strength of disorder increases.Comment: 15 pages, 12 PostScript figures, uses multicols.sty and epsf.st

Parquet solution for a flat Fermi surface

We study instabilities occurring in the electron system whose Fermi surface has flat regions on its opposite sides. Such a Fermi surface resembles Fermi surfaces of some high-$T_c$ superconductors. In the framework of the parquet approximation, we classify possible instabilities and derive renormalization-group equations that determine the evolution of corresponding susceptibilities with decreasing temperature. Numerical solutions of the parquet equations are found to be in qualitative agreement with a ladder approximation. For the repulsive Hubbard interaction, the antiferromagnetic (spin-density-wave) instability dominates, but when the Fermi surface is not perfectly flat, the $d$-wave superconducting instability takes over.Comment: REVTeX, 36 pages, 20 ps figures inserted via psfig. Submitted to Phys. Rev.

Asymmetric and symmetric fission of excited nuclei of 180,190Hg and 184,192,202Pb formed in the reactions with 36Ar and 40,48Ca ions

Background: Observation of asymmetric fission of 180Hg has led to intensive theoretical and experimental studies of fission of neutron-deficient nuclei in the lead region. Purpose: The study of asymmetric and symmetric fission modes of 180,190Hg and 184,192,202Pb nuclei. Methods: Mass-energy distributions of fission fragments of 180,190Hg and 184Pb formed in the 36Ar+144,154Sm and 40Ca+144Sm reactions, respectively, at energies near the Coulomb barrier have been measured using the double-arm time-of-flight spectrometer CORSET and compared with previously measured 192,202Pb isotopes produced in the 48Ca+144,154Sm reactions. The mass distributions for 180,190Hg and 184,192,202Pb together with old data for 187Ir, 195Au, 198Hg, 201Tl, 205,207Bi, 210Po, and 213At [J. Nucl. Phys. 53, 1225 (1991)] have been decomposed into symmetric and asymmetric fission modes. The total kinetic-energy distributions for different fission fragment mass regions have been analyzed for 180,190Hg and 184Pb. Results: The stabilization role of proton numbers at Z≈36, 38, Z≈45, 46, and Z=28/50 in asymmetric fission of excited preactinide nuclei has been observed. The high (≈145−MeV) and the low (≈128−MeV) energy components have been found in the total kinetic-energy distributions of 180,190Hg fission fragments corresponding to the fragments with proton numbers near Z≈46 and Z≈36, respectively. In the case of fission of 184Pb only the low-energy component (≈135MeV) for the fragments with masses corresponding to the proton numbers Z≈36 and 46 has been found. Conclusions: The studied properties of asymmetric fission of 180,190Hg and 184,192,202Pb nuclei point out the existence of well deformed proton shell at Z≈36 and less deformed proton shell at Z≈46.peerReviewe

Redundant BLDC motors control system for scientific uncrewed space missions

Brushless DC (BLDC) motors are widely used in payloads of scientific space missions. To work with this type of motor it’s required to develop a complex control system considering basic technical requirements as well as the demands to reliability linked with the space application of the system. The basic technical requirements and the demands for the reliability of the system were formulated in the article. The block commutation algorithm was selected for the system since it’s simple and it doesn’t require using a high-performance processor and precision rotor position sensors. Based on the selected algorithm and the requirements structural scheme of the control system with redundancy features was designed. Further, schematics for output stages of the system were considered. They allowed connecting one set of motors and Hall sensors to both the main and the redundant control system. To test the structural and schematic solutions a prototype board was developed. The prototype was successfully tested and the solutions were verified

Investigation on Competing Fission Modes in 178Pt* Produced by 36Ar + 142Nd Reaction up to High Excitation Energies

Abstract: The mass–energy distributions of binary fission fragments (FFs) of excited exotic nucleus 178Pt* were measured via the fusion reaction 36Ar + 142Nd at different beam energies within 158–222 MeV range using the double arm time-of-flight spectrometer CORSET. The analysis of mass–energy matrices reveals the presence of both asymmetric and symmetric modes corresponding to respective lower and higher total kinetic energy (TKE) of the fragments within 42–64 MeV excitation energy. The agreement of measured mass width with theoretical systematics and dominance of the associated symmetric TKE component at 93 MeV excitation energy indicates the significant predominance/suppression of symmetric/asymmetric mode, respectively. The most probable light and heavy mass peaks of asymmetric fission mode of 178Pt are spotted at 79 and 99 u, respectively. As inferred from the theory, the mass–TKE events observed at the highest measured excitation energy (93 MeV) endorse the presence of the fast-fission process in 178Pt* at high angular momentum

Investigating Mass–Energy Distributions of Fragments Produced in the 32S + 232Th → 264Sg Reaction at Energies Below and Near the Coulomb Barrier

Abstract: The mass-energy distributions of fragments of the reaction 32S + 232Th leading to the formation of 264Sg (Z = 106) at energies of incident ions 32S of 165, 181 and 200 MeV are measured. The contribution from the quasifission process is found at energies below and above the Coulomb barrier in the region of symmetric masses (АCN/2 ± 20). A high-energy symmetric fission mode is found at an energy of 165 MeV, which corresponds to the excitation energy of the compound 264Sg nucleus 45 MeV

Asymmetric and symmetric fission of excited nuclei of Hg 180,190 and Pb 184,192,202 formed in the reactions with Ar 36 and Ca 40,48 ions

Background: Observation of asymmetric fission of Hg180 has led to intensive theoretical and experimental studies of fission of neutron-deficient nuclei in the lead region. Purpose: The study of asymmetric and symmetric fission modes of Hg180,190 and Pb184,192,202 nuclei. Methods: Mass-energy distributions of fission fragments of Hg180,190 and Pb184 formed in the Ar36+Sm144,154 and Ca40+Sm144 reactions, respectively, at energies near the Coulomb barrier have been measured using the double-arm time-of-flight spectrometer CORSET and compared with previously measured Pb192,202 isotopes produced in the Ca48+Sm144,154 reactions. The mass distributions for Hg180,190 and Pb184,192,202 together with old data for Ir187, Au195, Hg198, Tl201, Bi205,207, Po210, and At213 [J. Nucl. Phys. 53, 1225 (1991)] have been decomposed into symmetric and asymmetric fission modes. The total kinetic-energy distributions for different fission fragment mass regions have been analyzed for Hg180,190 and Pb184. Results: The stabilization role of proton numbers at Z≈36, 38, Z≈45, 46, and Z=28/50 in asymmetric fission of excited preactinide nuclei has been observed. The high (≈145-MeV) and the low (≈128-MeV) energy components have been found in the total kinetic-energy distributions of Hg180,190 fission fragments corresponding to the fragments with proton numbers near Z≈46 and Z≈36, respectively. In the case of fission of Pb184 only the low-energy component (≈135MeV) for the fragments with masses corresponding to the proton numbers Z≈36 and 46 has been found. Conclusions: The studied properties of asymmetric fission of Hg180,190 and Pb184,192,202 nuclei point out the existence of well deformed proton shell at Z≈36 and less deformed proton shell at Z≈46