917 research outputs found
Bistability in superconducting rings containing an inhomogeneous Josephson junction
We investigate the magnetic response of a superconducting Nb ring containing
a ferromagnetic PdNi Josephson junction and a tunnel junction in parallel. A
doubling of the switching frequency is observed within certain intervals of the
external magnetic field. Assuming sinusoidal current-phase relations of both
junctions our model of a dc-SQUID embedded within a superconducting ring
explains this feature by a sequence of current reversals in the ferromagnetic
section of the junction in these field intervals. The switching anomalies are
induced by the coupling between the magnetic fluxes in the two superconducting
loops.Comment: 5 pages, 4 figure
Balancing Surface Energy Terms for Stable Growth of Planar Surfaces
We investigate the driving forces that determine the growth mode of heteroepitaxial Ge layers grown from solution on Si substrates with orientations (001), (011) and (111) by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Using liquid phase epitaxy, we can study the influences of strain and surface energy terms independently on effects due to limited surface diffusion. In (001) and (011) orientated layers, {111} faceted islands form (Stranski-Krastanov growth). In contrast, (111) orientated layers grow in a two-dimensional step flow growth mode (Frank-van der Merwe growth).
We model these investigations in terms of energy minimisation considering surface energy reduction by formation of faceted islands and elastic strain energy relaxation by island formation. The strain energy relaxation by island formation is calculated by the finite element method. According to our considerations, two-dimensional growth is obtained by selective increase of the free surface energy of the low indices facet planes to a value higher than that of the substrate surface. Formation of faceted islands thus would increase the total surface energy; as a consequence, island formation is suppressed. By choosing the appropriate solvent and temperature in solution growth, we can provide for thermodynamically stable two-dimensional growth
Finite-temperature magnetism of FePd and CoPt alloys
The finite-temperature magnetic properties of FePd and
CoPt alloys have been investigated. It is shown that the
temperature-dependent magnetic behaviour of alloys, composed of originally
magnetic and non-magnetic elements, cannot be described properly unless the
coupling between magnetic moments at magnetic atoms (Fe,Co) mediated through
the interactions with induced magnetic moments of non-magnetic atoms (Pd,Pt) is
included. A scheme for the calculation of the Curie temperature () for
this type of systems is presented which is based on the extended Heisenberg
Hamiltonian with the appropriate exchange parameters obtained from
{\em ab-initio} electronic structure calculations. Within the present study the
KKR Green's function method has been used to calculate the parameters.
A comparison of the obtained Curie temperatures for FePd and
CoPt alloys with experimental data shows rather good agreement.Comment: 10 pages, 12 figure
Engineering hydrophobin DewA to generate surfaces that enhance adhesion of human but not bacterial cells
Selective dispersion of single walled carbon nanotubes with specific chiral indices by poly(N-decyl-2,7-carbazole)
Critical temperature oscillations in magnetically coupled superconducting mesoscopic loops
We study the magnetic interaction between two superconducting concentric
mesoscopic Al loops, close to the superconducting/normal phase transition. The
phase boundary is measured resistively for the two-loop structure as well as
for a reference single loop. In both systems Little-Parks oscillations,
periodic in field are observed in the critical temperature Tc versus applied
magnetic field H. In the Fourier spectrum of the Tc(H) oscillations, a weak
'low frequency' response shows up, which can be attributed to the inner loop
supercurrent magnetic coupling to the flux of the outer loop. The amplitude of
this effect can be tuned by varying the applied transport current.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.
Mesoscopic Ferromagnet/Superconductor Junctions and the Proximity Effect
We have measured the electrical transport of submicron ferromagnets (Ni) in
contact with a mesoscopic superconductor (Al) for a range of interface
resistances. In the geometry measured, the interface and the ferromagnet are
measured separately. The ferromagnet itself shows no appreciable
superconducting proximity effect, but the ferromagnet/superconductor interface
exhibits strong temperature, field and current bias dependences. These effects
are dependent on the local magnetic field distribution near the interface
arising from the ferromagnet. We find that the temperature dependences may be
fit to a modified version of the Blonder-Tinkham-Klapwijk theory for
normal-superconductor transport.Comment: 4 eps fig
The structural distortion in antiferromagnetic LaFeAsO investigated by a group-theoretical approach
As experimentally well established, undoped LaFeAsO is antiferromagnetic
below 137K with the magnetic moments lying on the Fe sites. We determine the
orthorhombic body-centered group Imma (74) as the space group of the
experimentally observed magnetic structure in the undistorted lattice, i.e., in
a lattice possessing no structural distortions in addition to the
magnetostriction. We show that LaFeAsO possesses a partly filled "magnetic
band" with Bloch functions that can be unitarily transformed into optimally
localized Wannier functions adapted to the space group Imma. This finding is
interpreted in the framework of a nonadiabatic extension of the Heisenberg
model of magnetism, the nonadiabatic Heisenberg model. Within this model,
however, the magnetic structure with the space group Imma is not stable but can
be stabilized by a (slight) distortion of the crystal turning the space group
Imma into the space group Pnn2 (34). This group-theoretical result is in
accordance with the experimentally observed displacements of the Fe and O atoms
in LaFeAsO as reported by Clarina de la Cruz et al. [nature 453, 899 (2008)]
Influence of the confinement geometry on surface superconductivity
The nucleation field for surface superconductivity, , depends on the
geometrical shape of the mesoscopic superconducting sample and is substantially
enhanced with decreasing sample size. As an example we studied circular,
square, triangular and wedge shaped disks. For the wedge the nucleation field
diverges as with decreasing angle () of
the wedge, where is the bulk upper critical field.Comment: 4 pages, 3 figures. Accepted for publication in Phys. Rev.
- …