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[Fizinfo] BME Elmeleti Fizika Tanszek szeminariuma

Chronological Thread 
  • From: Varga Imre <varga AT>
  • To: fizinfo AT
  • Subject: [Fizinfo] BME Elmeleti Fizika Tanszek szeminariuma
  • Date: Mon, 21 Feb 2005 13:48:50 +0100 (CET)
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  • List-id: ELFT HÍRADÓ <>


a BME Fizikai Intezet, Elmeleti Fizika Tanszek

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.

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

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