Force between a magnetic tip and an inhomogeneous superconductor at zero field

The force exerted by a semi-infinite inhomogeneous superconductor with a planar interface to vacuum on a magnetic tip is studied theoretically in the absence of external magnetic fields. It is shown that the force has a contribution from inhomogeneities due to material defects with unique characteristics. Defects are taken into account in the London limit by allowing the mass parameter to vary spatially. The contribution from defects to the force is calculated analytically to first order in the deviation of the mass parameter from its constant value for the homogeneous superconductor, assuming that the tip is a point dipole perpendicular to the interface, and that it does not spontaneously create vortex matter. Random point defects and linear localized defects are considered phenomenologically. For each defect type the force dependence on the dipole position coordinates is obtained, and the force magnitudes are estimated numerically. The predictions for the dependence of the linear defect force on the dipole lateral position are found to agree qualitatively with experiment.Comment: 4 pages, 3 figures, submitted to Physica C - Proceedings VORTEX VI Conferenc

On the vacuum entropy and the cosmological constant

It is generally accepted that the entropy of an asymptotically de Sitter universe is bounded by the area, in Planck units, of the de Sitter horizon. Based on an analysis of the entropy associated to the vacuum quantum fluctuations, we suggest that the existence of such a holographic bound constitutes a possible explanation for the observed value of the cosmological constant, theoretically justifying a relation proposed 35 years ago by Zel'dovich.Comment: Version to appear in the GRF2003 Special Issue of IJMP

The cosmological dark sector as a scalar σ\sigma-meson field

Previous quantum field estimations of the QCD vacuum in the expanding space-time lead to a dark energy component scaling linearly with the Hubble parameter, which gives the correct figure for the observed cosmological term. Here we show that this behaviour also appears at the classical level, as a result of the chiral symmetry breaking in a low energy, effective $\sigma$-model. The dark sector is described in a unified way by the $\sigma$ condensate and its fluctuations, giving rise to a decaying dark energy and a homogeneous creation of non-relativistic dark particles. The creation rate and the future asymptotic de Sitter horizon are both determined by the $\sigma$ mass scale.Comment: A typo was fixed in Eq. (19)-(20), and a reference adde

Tunable interactions between vortices and a magnetic dipole

The interactions between vortices in a thin superconducting film and one magnetic dipole in the presence of a magnetic field applied parallel to the film surfaces are studied theoretically in the London limit. The dipole magnetic moment is assumed to have constant magnitude and freedom to rotate. The pinning potential for an arbitrary vortex configuration is calculated exactly. It is found that, due to the dipole freedom to rotate, the pinning potential differs significantly from that for a permanent dipole. In particular, its dependence on the applied field is non-trivial and allows for tuning of the pinning potential by the applied field. The critical current for one vortex pinned by the dipole is obtained numerically as a function of the applied field and found to depend strongly on the field. Order of magnitude changes in the critical current resulting from changes in the direction and magnitude of the applied field are reported, with discontinuous changes taking place in some cases. The effect of vortex pinning by random material defects on the critical current is investigated using a simple model. It is found that if random pinning is weak the critical current remains strongly dependent on the applied field. Possible applications to vortices pinned by arrays of magnetic dots are briefly considered.Comment: 9 pages, 9 figures. Submitted to Phys. Rev.

Open cosmologies with rotation

We study a rotating and expanding, Godel type metric, originally considered by Korotkii and Obukhov, showing that, in the limit of large times and nearby distances, it reduces to the open metric of Friedmann. In the epochs when radiation or dust matter dominate the energy density, our solutions are similar to the isotropic ones and, in what concerns processes occurring at small times, the rotation leads only to higher order corrections. At large times, the solution is dominated by a decaying positive cosmological term, with negative pressure, and necessarily describes a quasi-flat universe if the energy conditions have to be satisfied. The absence of closed time-like curves requires a superior limit for the global angular velocity, which appears as a natural explanation for the observed smallness of the present rotation. The conclusion is that the introduction of a global rotation, in addition to be compatible with observation, can enrich the standard model of the Universe, explaining issues like the origin of galaxies rotation and the quasi-flatness problem.Comment: Final version, accepted for publication in General Relativity and Gravitatio