ELFT HÍRADÓ

[Varga Imre <varga AT neumann.phy.bme.hu>]

                                     MEGHIVO

                  a BME Fizikai Intezet, Elmeleti Fizika Tanszek
                                  szeminariumara:

                               Francis I.B. Williams
                         CEA/SPEC, Saclay, Gif-sur-Yvette
                                KFKI-SZFKI, Budapest


                "Two Dimensional Periodic Systems in Random Field:
        Experiments on 2-D magnetically induced Wigner solid and quasi
                           2-D vortex solid in BSCCO"

        Helye: BME Fizikai Intezet,
               Elmeleti Fizika Tanszek
               Budafoki ut 8.
               F-epulet, III. lepcsohaz, magasfoldszint
               szeminariumi szoba

        Ideje: 2005. februar 25. pentek, 10:15.

        Abstract:
        Periodic systems are unstable to application of a random field in
        dimension less than 4. Some of the best known examples are quasi
        1-D Charge and Spin Density Wave systems. Another long standing
        example is the vortex network in 3-D type II superconductors.
        Recently it has become possible to do experiments on real 2-D and
        quasi 2-D systems cited in the title.

        A Wigner solid can be induced from a quantum liquid of electrons
        or holes at high quality GaAs/GaAlAs heterojunctions by
        neutralising the quantum fluctuations with the help of a strong
        magnetic field. The system then becomes insulating because of
        pinning to roughness of the interface and manifests a pinning
        frequency for small perturbation and a threshold field for
        conduction on perturbation by larger applied electric fields. The
        two measurements are shown to be simply related and they give
        information about the form of the random field. Experiments on the
        electric field transverse to the current show channelling guided
        by the random field but no real Hall effect (which should show
        electric field reversal on reversal of magnetic field) before
        reaching a threshold transverse Lorentz force. This is taken to be
        a manifestation of a threshold force to (transverse)
        dechannelling, an idea proposed by Giamarchi and LeDoussal and
        supported by simulation studies of Moon, Scalleter and Zimanyi.

        BSCCO is a very anisotropic type II high Tc superconductor
        modelled by 2-D superconducting planes coupled by Josephson
        junctions. The vortices induced by a magnetic field parallel to
        the c-axis are only weakly coupled from plane to plane with the
        result that the vortex network at low temperature consists of
        quasi 2-D solids pinned to defects (random field) with little
        c-axis correlation.  Vortex dynamics are investigated by applying
        a force by means of a transport supercurrent and detecting the
        motion by the electric field set up by the phase variation
        resulting from the vortex velocity. In what is expected to be the
        free flux flow regime, there are signs of guided motion and
        absence of Hall effect (which again would show electric field
        reversal on reversal of magnetic field for same transport
        current). This may have a similar origin to the depinned Wigner
        solid behaviour.

        Minden erdeklodot szivesen latunk.


                                                        Varga Imre