<p>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 YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub>single crystals, from the superconductingfluctuations above T<i>c</i>down to the vortex matter near the onset of resistivity.Superconducting fluctuations above T<i>c</i>are suitably studied by measuring themagnetoconductivity Δσ = 1/ρ(B)-1/ρ(0).Such experiments were performed on untwinned, optimally dopedsamples, for<b>I</b>||<b>ab</b>. For<b>H</b>||<b>c</b>, 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. For<b>H</b>||<b>ab</b>, Δσ<i>b</i>depended on the field direction, indicating an effect ofthe chains on the magnetoconductivity in thisconfiguration.</p><p>The nature of the vortex phase below T<i>c</i>can 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 T<i>c</i>was strongly field and temperature dependent, which wasexplained by an almost isotropic fluctuation contribution.</p><p>The effect of anisotropy on vortex-liquid correlation infields<b>H</b>||<b>ab</b>, was studied by<i>c</i>-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 the<i>ab</i>plane because of the strong anisotropy. Also, thesolid-to-liquid transition temperature became nearly fieldindependent.</p><p>Attempts were also made to connect the fluctuation regime ofthe resistive transition with the vortex region,by accountingfor critical fluctuations close to the transition.</p><p><b>Keywords:</b>high-temperature superconductors, YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub>, untwinned YBCO, oxygen deficiency,superconducting fluctuations, magnetoconductivity, in-planeanisotropy, vortex liquid, vortex solid, vortex correlation</p>
| Identifer | oai:union.ndltd.org:UPSALLA/oai:DiVA.org:kth-3723 |
| Date | January 2004 |
| Creators | Björnängen, Therese |
| Publisher | KTH, Physics, Stockholm : Fysik |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Doctoral thesis, comprehensive summary, text |
| Relation | Trita-FYS, 0280-316X ; 5294 |
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