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Resistive studies of vortices and fluctuations in single crystal YBa2Cu3O7-
KTH, Superseded Departments, Physics.
2004 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

High-temperature superconductors have been intensely studiedsince the discovery, almost 20 years ago. Their layeredstructure, extreme type-II behavior, large anisotropy, andstrong fluctuations have led to a large number of new andinteresting problems. In this work the resistive transitionhave been studied in YBa2Cu3O7-δsingle crystals, from the superconductingfluctuations above Tcdown to the vortex matter near the onset of resistivity.Superconducting fluctuations above Tcare suitably studied by measuring themagnetoconductivity Δσ = 1/ρ(B)-1/ρ(0).Such experiments were performed on untwinned, optimally dopedsamples, forI||ab. ForH||c, fluctuations in the CuO-chains was not important, andfluctuations in the CuO2 planes seemed isotropic. The in-planecoherence length anisotropy was determined to be close tounity. ForH||ab, Δσbdepended on the field direction, indicating an effect ofthe chains on the magnetoconductivity in thisconfiguration.

The nature of the vortex phase below Tccan be examined through vortex-correlation measurements.Using a modified pseudo-flux transformer (PFT) geometry forin-plane experiments, the magnetic field could be appliedparallel to both the a, b, and c axis. A strong Lorentz forcewas detected, and observed to be a requirement for the meltingstep feature. The resistive anisotropy close to Tcwas strongly field and temperature dependent, which wasexplained by an almost isotropic fluctuation contribution.

The effect of anisotropy on vortex-liquid correlation infieldsH||ab, was studied byc-axis transport measurements in oxygen-deficientsamples, using a PFT configuration. Increasing disordersuppressed both longitudinal and transverse correlation. Inheavily underdoped samples, vortex correlation was enhanced formagnetic fields exactly aligned with theabplane because of the strong anisotropy. Also, thesolid-to-liquid transition temperature became nearly fieldindependent.

Attempts were also made to connect the fluctuation regime ofthe resistive transition with the vortex region,by accountingfor critical fluctuations close to the transition.

Keywords:high-temperature superconductors, YBa2Cu3O7-δ, untwinned YBCO, oxygen deficiency,superconducting fluctuations, magnetoconductivity, in-planeanisotropy, vortex liquid, vortex solid, vortex correlation

Place, publisher, year, edition, pages
Stockholm: Fysik , 2004. , xi, 87 p.
Trita-FYS, ISSN 0280-316X ; 5294
Keyword [en]
condensed matter physics, high temperature superconductors, YB2Cu3O7-d, untwinned YBCO, oxygen deficiency, superconducting fluctuations, magnetoconductivity, in-plane anisotropy, vortex liquid, vortex solid, vortex correlation
URN: urn:nbn:se:kth:diva-3723ISBN: 91-7283-715-2OAI: diva2:9564
Public defence
2004-04-16, 00:00
Available from: 2004-03-23 Created: 2004-03-23 Last updated: 2012-03-19

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