A. S. Mel'nikov

Mechanisms of inheritance of rift faulting in the western branch of the East African Rift, Tanzania

The western branch of the East African Rift system is commonly cited as a result of Phanerozoic reactivation of the Paleoproterozoic Ubendian belt in western Tanzania. Geological evidence is provided to show that prominent mechanical anisotropies successively appeared during Proterozoic evolution of the Precambrian basement and that their different reactivation behavior contributed to the Phanerozoic rift pattern. The Ubende belt (1950–1850 Ma) is a NW oriented, amphibolite facies ductile lateral shear belt in which older (2100–2025 Ma) and complex granulite facies terranes are included along trend. Retrograde multiphase sinistral strike-slip mylonites developed along the NW oriented ductile shear belt. They reflect persistent Proterozoic wrench fault reactivation of the latter. Shallow level sedimentary basins upon and along the ductile shear belt display deformational structures attributable to the Proterozoic wrench fault reactivation. Neoproterozoic sinistral transpression produced the final geometrical pattern of the wrench fault zone, which appears as an elongate and NW trending positive flower structure, locally enhanced by late Proterozoic contraction. Phanerozoic rifting is demonstrated by others to occur in three distinct episodes, during which the complex rift segment formed upon the multiphase Proterozoic wrench fault zone. The evaluation of the relationship between multiphase rift and multiphase prerift fabrics is reconsidered. The Proterozoic prerift fabrics correspond with a dextral transpressional and ductile deformational pattern, which became selectively reactivated by sinistral transpressional ductile-brittle mylonites. Proterozoic mylonites constitute shallow level mechanical anisotropies and define the general trend of the rift faults. According to the position of these mylonites in the center or in the external parts of their NW oriented Neoproterozoic transpression, they reactivate as complex and multiphase rift faults or as normal and recent faults, respectively. The Paleoproterozoic NW oriented and ductile lateral shear belt constitutes the deep level mechanical anisotropy. Its reactivation in Phanerozoic stress fields is likely dextral oblique transtension, considered as a leading mechanism of the pluriphase and NW oriented deep rift basins.

Vortex states induced by proximity effect in hybrid ferromagnet-superconductor systems

We consider superconductivity nucleation in multiply connected mesoscopic samples such as thinwalled cylinders or rings placed in electrical contact with a ferromagnet. The superconducting critical temperature and order parameter structure are studied on the basis of linearized Usadel equations. We suggest a mechanism of switching between the superconducting states with different vorticities caused by the exchange field and associated with the oscillatory behavior of the Cooper pair wave function in a ferromagnet.

Fulde-Ferrell-Larkin-Ovchinnikov states and quantum oscillations in mesoscopic superconductors and superfluid ultracold Fermi gases

We have studied the distinctive features of the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) instability and phase transitions in two–dimensional (2D) mesoscopic superconductors placed in magnetic field of arbitrary orientation and rotating superfluid Fermi gases with imbalanced state populations. Using a generalized version of the phenomenological Ginzburg-Landau theory we have shown that the FFLO states are strongly modified by the effect of the trapping potential confining the condensate. The phenomenon of the inhomogeneous state formation is determined by the interplay of three length scales: (i) length scale of the FFLO instability; (ii) 2D system size; (iii) length scale associated with the orbital effect caused either by the Fermi condensate rotation or magnetic field component applied perpendicular to the superconducting disc. We have studied this interplay and resulting quantum oscillation effects in both superconducting and superfluid finite – size systems with FFLO instability and described the hallmarks of the FFLO phenomenon in a restricted geometry. The finite size of the system is shown to affect strongly the conditions of the observability of switching between the states with different vorticities.

Vortex clusters and multiquanta flux lattices in thin films of anisotropic superconductors

The distinctive features of equilibrium vortex structures in thin films of anisotropic superconductors in tilted magnetic fields are studied for the limits of moderate and strong anisotropy. The energetically favorable shape of isolated vortex lines is found in the framework of two particular models describing these limiting cases: London theory with an anisotropic mass tensor and London-type model for a stack of Josephson–decoupled superconducting layers. The increase of the field tilting is shown to result in qualitative changes in the vortex–vortex interaction potential: the balance between long–range attractive and repulsive forces occurs to be responsible for a formation of a minimum of the interaction potential vs the intervortex distance. This minimum appears to exist only for a certain restricted range of the vortex tilting angles which shrinks with the decrease of the system anisotropy parameter. Tilted vortices with such unusual interaction potential form clusters with the size depending on the field tilting angle and film thickness or/and can arrange into multiquanta flux lattice. The magnetic flux through the unit cells of the corresponding flux line lattices equals to an integer number M of flux quanta. Thus, the increase in the field tilting should be accompanied by the series of the phase transitions between the vortex lattices with different M .

Quasiparticle spectrum near the gap node directions in the mixed state of d-wave superconductors

Abstract The distinctive features of the local density of states, zero-bias tunneling conductance and scaling behavior of thermodynamic and transport characteristics in the mixed state of d-wave superconductors are studied for various vortex lattice structures on the basis of the Bogolubov–de Gennes theory for the quasiparticle spectrum near the gap nodes.

Magnetic field dependence of the density of states for thin films of d-wave superconductors

Abstract The density of states (DOS) produced by the vortices in thin films of superconductors with the anisotropic pairing is studied in the low temperature limit using the semiclassical approach. For ultrathin films of thickness d ⪡ λ (where λ is the London penetration depth) the magnetic field dependence of the spatially averaged DOS N ( B ) at the Fermi level appears to be qualitatively different from the one calculated previously for bulk superconductors in Ref.[1]. The N ( B ) behavior is shown to depend essentially on the ratio of the intervortex distance to the effective penetration depth λ eff = λ 2 / d . The peculiarities of DOS produced by 2D pancake vortices (far away from the normal cores) in d-wave layered superconductors are also discussed.

Vortex lattice distortions in hexagonal unconventional superconductors

Abstract A noticeable magnetic field dependence of the vortex lattice distortions in hexagonal unconventional superconductors is obtained below the second order phase transition line H ∗ (T) (H ∗ (T) c2 (T)) for H perpendicular to the hexagonal axis c . The range of the Ginzburg-Landau coefficients is shown to exist where our theoretical results are in a qualitative agreement with the neutron diffraction measurements in U Pt3.