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Vortex Fluctuations in High-Tc Films: Flux Noise Spectrum and Complex Impedance
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science.
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1999 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 83, no 26, 5567-5570 p.Article in journal (Refereed) Published
Abstract [en]

The flux noise spectrum and complex impedance for a 500 Å thick YBCO film are measured and comparedwith predictions for two-dimensional vortex fluctuations. It is verified that the complex impedanceand the flux noise spectra are proportional to each other, that the logarithm of the flux noise spectrafor different temperatures has a common tangent with slope 21, and that the amplitude of the noisedecreases as d23, where d is the height above the film at which the magnetic flux is measured. Acrossover from normal to anomalous vortex diffusion is indicated by the measurements and is discussedin terms of a two-dimensional decoupling.

Place, publisher, year, edition, pages
1999. Vol. 83, no 26, 5567-5570 p.
National Category
Engineering and Technology
URN: urn:nbn:se:uu:diva-91198DOI: 10.1103/PhysRevLett.83.5567OAI: oai:DiVA.org:uu-91198DiVA: diva2:163851
Available from: 2003-12-22 Created: 2003-12-22 Last updated: 2015-09-11Bibliographically approved
In thesis
1. On the Zero and Low Field Vortex Dynamics: An Experimental Study of Type-II Superconductors
Open this publication in new window or tab >>On the Zero and Low Field Vortex Dynamics: An Experimental Study of Type-II Superconductors
2003 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Dynamic properties of type-II superconductors have been experimentally studied in zero and low magnetic fields using SQUID magnetometry and I–V measurements.

In zero magnetic field close to the critical temperature, the physical properties of type-II superconductors are dominated by spontaneously created vortices. In three dimensions (3D) such vortices take the form of vortex loops and in two dimensions (2D) as vortex-antivortex pairs.

The 2D vortex dynamics has been probed using mutual inductance and flux noise measurements on YBa2Cu3O7 (YBCO) and MgB2 thin films in zero and low magnetic fields. In such measurements, information about vortex correlations is obtained through a temperature dependent characteristic frequency, below (above) which the vortex movements are uncorrelated (correlated). The results obtained in zero magnetic field indicate that sample heterogeneities influence the vortex physics and hinder the divergence of the vortex-antivortex correlation length.

In low magnetic fields the vortex dynamics is strongly dependent on the applied magnetic field and a power law dependence of the characteristic frequency with respect to the magnetic field is observed. The results indicate that there is a co-existence of thermally and field generated vortices.

The I–V characteristics of untwinned YBCO single crystals show that only a small broadening of the transition region influences the length scale over which the vortex movements are correlated. The dynamic and static critical exponents therefore exhibit values being larger in magnitude as compared to values predicted by relevant theoretical models. The results also suggest that the copper oxide planes in YBCO decouple slightly below the mean field critical temperature and hence, the system has a crossover from 3D to 2D behaviour as the temperature is increased.

From temperature dependent DC-magnetisation measurements performed on untwinned YBCO single crystals in weak applied fields, detailed information about the critical current density and the irreversibility line is obtained.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2003. 51 p.
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 919
Engineering physics, high temperature superconductors, thermal fluctuations, YBCO, MgB2, anisotropic superconductors, vortex dynamics, Kosterlitz-Thouless transition, current voltage characteristics, complex conductivity, Flux noise, Teknisk fysik
National Category
Other Engineering and Technologies
urn:nbn:se:uu:diva-3907 (URN)91-554-5837-8 (ISBN)
Public defence
2004-01-21, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:00
Available from: 2003-12-22 Created: 2003-12-22Bibliographically approved

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