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Vortex fluctuations in high-Tc films close to the resistive transition
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.
2004 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 70, no 2, 024511Article in journal (Refereed) Published
Abstract [en]

Flux noise and ac susceptibility measurements have been performed on epitaxial YBa2Cu3O7 films to investigate dynamical properties of vortex fluctuations close to the resistive transition. The validity of the fluctuation-dissipation theorem is verified by the proportionality between flux noise and ac susceptibility results. The zero-field flux-noise spectrum [Sϕ(f)] can be characterized as follows. Below a temperature dependent frequency f0(T), the flux noise spectrum is frequency independent. For frequencies f>f0(T), the flux noise spectrum follows f−x, with x≈1.5–1.75. The characteristic frequency f0 exhibits a dramatic decrease close to the resistive transition; f0 decreases by 3–4 orders of magnitude in a temperature interval of ΔT≈1K. Moreover, the flux noise spectrum scales as fSϕ=g(f∕f0(T)), where g(∙) is a scaling function. The influence of a weak perturbing magnetic field (Hdc) on the characteristics of the flux noise spectrum has been investigated. While the general characteristics of the flux noise spectrum remain, except for a change in frequency dependence at f>f0, the characteristic frequency f0 exhibits a dramatic increase with increasing applied field; f0 increases by 3–4 orders of magnitude on increasing the field from zero to a few oersteds. Moreover, it is found that f0 is proportional to Hydc, with y≈1.15. The results are interpreted in terms of thermally generated vortex-antivortex fluctuations in combination with fluctuations of field-generated vortices.

Place, publisher, year, edition, pages
2004. Vol. 70, no 2, 024511
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-91203DOI: 10.1103/PhysRevB.70.024511OAI: oai:DiVA.org:uu-91203DiVA: diva2:163856
Available from: 2003-12-22 Created: 2003-12-22 Last updated: 2017-12-14Bibliographically 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.
Series
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 919
Keyword
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
Identifiers
urn:nbn:se:uu:diva-3907 (URN)91-554-5837-8 (ISBN)
Public defence
2004-01-21, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:00
Opponent
Supervisors
Available from: 2003-12-22 Created: 2003-12-22Bibliographically approved

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Svedlindh, Peter

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