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On the Zero and Low Field Vortex Dynamics: An Experimental Study of Type-II Superconductors
Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Technology, Department of Engineering Sciences.
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. , p. 51
Series
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 919
Keywords [en]
Engineering physics, high temperature superconductors, thermal fluctuations, YBCO, MgB2, anisotropic superconductors, vortex dynamics, Kosterlitz-Thouless transition, current voltage characteristics, complex conductivity, Flux noise
Keywords [sv]
Teknisk fysik
National Category
Other Engineering and Technologies
Identifiers
URN: urn:nbn:se:uu:diva-3907ISBN: 91-554-5837-8 (print)OAI: oai:DiVA.org:uu-3907DiVA, id: diva2:163859
Public defence
2004-01-21, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:00
Opponent
Supervisors
Available from: 2003-12-22 Created: 2003-12-22Bibliographically approved
List of papers
1. Vortex Fluctuations in High-Tc Films: Flux Noise Spectrum and Complex Impedance
Open this publication in new window or tab >>Vortex Fluctuations in High-Tc Films: Flux Noise Spectrum and Complex Impedance
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1999 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 83, no 26, p. 5567-5570Article 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.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-91198 (URN)10.1103/PhysRevLett.83.5567 (DOI)
Available from: 2003-12-22 Created: 2003-12-22 Last updated: 2017-12-14Bibliographically approved
2. Zero-field vortex dynamics in YBCO thin films
Open this publication in new window or tab >>Zero-field vortex dynamics in YBCO thin films
2000 (English)In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 284-288, no Pt. 1, p. 963-964Article in journal (Refereed) Published
Abstract [en]

The dynamics of spontaneously generated vortices in epitaxial YBCO films have been investigated in the superconducting transition region by means of flux noise measurements. The investigated samples differ with respect to film thickness and sample quality and the influence of these parameters on the flux noise spectra is discussed.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-91199 (URN)10.1016/S0921-4526(99)02274-7 (DOI)
Available from: 2003-12-22 Created: 2003-12-22 Last updated: 2017-12-14Bibliographically approved
3. "Double superconducting transition" in YBCO thin films
Open this publication in new window or tab >>"Double superconducting transition" in YBCO thin films
2002 (English)In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 369, no 1-4, p. 295-299Article in journal (Refereed) Published
Abstract [en]

Two 150 nm thick YBCO films have been investigated using flux noise and complex conductance measurements. In both experiments thermally excited vortices are studied. The first film shows a single transition, with Tc≈88.2 K and ΔTc≈0.3 K, while the second film shows a two-step transition with Tc1≈90.1 K and ΔTc≈0.1 K, and Tc2≈89.8 K and ΔTc≈0.15 K. The experimental results obtained for the double transition sample in combination with computer simulation data in Medvedyeva et al. (in this volume) indicate that this sample can be described as two weakly coupled homogeneous superconducting layers, one on top of the other, with different critical temperatures.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-91200 (URN)10.1016/S0921-4534(01)01262-X (DOI)
Available from: 2003-12-22 Created: 2003-12-22 Last updated: 2017-12-14Bibliographically approved
4. Magnetic Flux noise in MgB2 thin films
Open this publication in new window or tab >>Magnetic Flux noise in MgB2 thin films
2003 (English)In: Noise as a Tool for Studying Materials: Proceedings of SPIE, 2003, Vol. 5112, p. 338-345Conference paper, Published paper (Refereed)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-91201 (URN)9780819449726 (ISBN)
Conference
Conference The International Society for Optical Engineering (SPIE), 2003, 1 June, Santa Fe, USA
Available from: 2003-12-22 Created: 2003-12-22 Last updated: 2015-09-11Bibliographically approved
5. Zero field dynamic universality class of high-Tc superconductors
Open this publication in new window or tab >>Zero field dynamic universality class of high-Tc superconductors
In: Physical Review B, ISSN 0163-1829Article in journal (Refereed) In press
Identifiers
urn:nbn:se:uu:diva-91202 (URN)
Available from: 2003-12-22 Created: 2003-12-22 Last updated: 2010-01-14Bibliographically approved
6. Vortex fluctuations in high-Tc films close to the resistive transition
Open this publication in new window or tab >>Vortex fluctuations in high-Tc films close to the resistive transition
2004 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 70, no 2, article id 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.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-91203 (URN)10.1103/PhysRevB.70.024511 (DOI)
Available from: 2003-12-22 Created: 2003-12-22 Last updated: 2017-12-14Bibliographically approved
7. High temperature SQUID magnetometer for zero and low field dynamic measurements
Open this publication in new window or tab >>High temperature SQUID magnetometer for zero and low field dynamic measurements
In: Review of Scientific Instruments, ISSN 0034-6748Article in journal (Refereed) Submitted
Identifiers
urn:nbn:se:uu:diva-91204 (URN)
Available from: 2003-12-22 Created: 2003-12-22Bibliographically approved
8. Low field Magnetization of untwinned YBCO single crystals
Open this publication in new window or tab >>Low field Magnetization of untwinned YBCO single crystals
Show others...
Manuscript (Other academic)
Identifiers
urn:nbn:se:uu:diva-91205 (URN)
Available from: 2003-12-22 Created: 2003-12-22 Last updated: 2010-01-13Bibliographically approved

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