Thesis (PhD (Physics))--University of Stellenbosch, 2002. / Includes bibliography. / ENGLISH ABSTRACT: High temperature superconductivity is an important topic in contemporary solid state physics,
and an area of very active research. Due to it’s potential for application in low temperature
electronic devices, the material has attracted the attention of researchers in the electronic
engineering and material science fields alike. Moreover, from a fundamental point of view,
several questions remain unanswered, related to the origin of superconductivity of this class
of materials and the nature of quantised magnetic flux present in magnetised samples.
In this work, flux creep phenomena in a thin superconducting YBa₂Cu₃O₇ film deposited by
pulsed laser deposition, is investigated near the critical temperature 0 ≤ Tc – T ≤ 10 K. Creep
activation energy U0 and critical current density jc were determined as a function of
temperature close to Tc, providing important data to a problem of high-Tc superconductivity
which is still a matter of debate. In particular it is still an open question whether restoring the
temperature in a creep freezing experiment in fact restores the film to it's original state before
the freezing. The most important novel results concern the regime of critical fluctuations in
the vicinity Tc - T < 1 K.
We studied the isothermal relaxation of trapped magnetic flux, and determined that the long
time decay follows a power law, where the exponent is inversely proportional to the creep
activation energy. The temperature dependence of the critical current density jc(T) of the
YBa₂Cu₃O₇ film close to Tc was obtained during warming runs. It was determined that jc(T)
follows a square root dependence on T to high accuracy in the range 0.2 ≤ Tc – T ≤ 1.5 K.
During flux creep experiments an interesting phenomenon called creep freezing related to the
strong temperature dependence of the relaxation rate was observed. A pronounced slowing of
relaxation with only a small drop in temperature from a starting temperature close to Tc was
detected. Experiments were conducted by initiating an isothermal flux decay run. At a certain
point the temperature was slightly lowered, and the flux decay stopped within experimental
accuracy. When the temperature was restored to the initial value, the flux decay resumed at
the previous rate before cooling. An argument based on vortex drift velocity was employed to
explain the phenomenon qualitatively.
During the course of this investigation, a pulsed laser deposition (PLD) system was designed
and built from scratch. PLD involves the interaction of a focussed laser pulse with a multielemental
solid target material. Material ablated from the target forms a fast moving plume
consisting of atomic and molecular particles, directed away from the target, and towards a
usually heated substrate on which the particles condense layer by layer to form a thin film.
The substrate temperature and background gas are carefully controlled to be conductive to the
growth of a desired phase of the multi-elemental compound.
The PLD system proved to be quite versatile in the range of materials that could be deposited.
It was used to deposit thin films of different materials, most notable were good quality
superconducting YBa₂Cu₃O₇, thermochromic VO2, and magnetoresistive LaxCa1-xMnO3.
Metallic Au and Ag layers were also successfully deposited on YBa2Cu3O7 thin films, to
serve as protective coatings. The critical temperatures of the best superconducting films were
90 K as determined by resistivity measurement. The optimal deposition conditions to deposit
high quality superconducting YBa₂Cu₃O₇ thin films was found to be: deposition temperature
780°C, laser energy density 2-3 J/cm2, oxygen partial pressure 0.2 mbar, and target-substrate
distance 35 mm. This yields film with Tc ~ 90 K. It was found that deposition temperature
plays the predominant role in determining the quality of YBa₂Cu₃O₇ thin films deposited by
PLD. / AFRIKAANSE OPSOMMING: Hoë temperatuur supergeleiding is tans ’n aktuele onderwerp van vastetoestandfisika en dit is
’n gebied van baie aktiewe navorsing. Weens die potensiaal vir toepassings van hoë
temperatuur supergeleiers in elektronika, het dié klas materiale die aandag van fisici and
elektronici getrek. Verskeie fundamentele vraagstukke bly steeds onbeantwoord, veral met
betrekking tot die oorsprong van supergeleiding in hierdie materiale en die gedrag van
gekwantiseerde magnetiese vloed (“vortekse”) in gemagnetiseerde monsters.
In hierdie werk word diffusie van vortekse in dun supergeleidende YBa₂Cu₃O₇ films
ondersoek naby die kritieke temperatuur (0 ≤ Tc - T ≤ 10 K). Die temperatuur afhanklikheid
van die diffusie aktiveringsenergie U0 en die kritieke stroomdigtheid jc word bepaal naby Tc.
Dit verskaf belangrike inligting tot probleme in hoë temperatuur supergeleiding wat tans nog
onbeantwoord bly. In die besonder is dit steeds ’n ope vraag of die herstel van die
aanvanklike temperatuur in ’n vloedstollings eksperiment waarlik die film tot die
oorspronklike toestand herstel. Die belangrikste nuwe resultate hou verband met die gebied
van kritieke fluktuasies van die orde parameter in die omgewing 0 < Tc - T < 1 K.
Ons het die isotermiese ontspanning van vortekse verstrik in die kristalstruktuur bestudeer, en
bepaal dat die lang tydsverval ’n magsverwantskap handhaaf, waar die eksponent omgekeerd
eweredig is aan U0. Die temperatuur afhanklikheid van die kritieke stroomdigtheid jc(T) van
die YBa₂Cu₃O₇ film naby Tc is bepaal tydens verhittingslopies. Daar is bevind dat naby Tc, jc
’n vierkantswortel verband met T volg, tot hoë noukeurigheid in die gebied 0.2 ≤ Tc –
T ≤ 1.5 K.
Gedurende vorteksdiffusie eksperimente is ’n interessante verskynsel naamlik vloedstolling
(“flux freezing”) waargeneem. Dit hou verband met die sterk temperatuur afhanklikheid van
die vervaltempo van die magnetiese moment van ’n gemagnetiseerde film. ’n Skerp daling
van die vervaltempo, weens slegs ’n klein temperatuurdaling vanaf die begin temperatuur
naby Tc, is waargeneem. Gedurende eksperimente is daar aanvanklik ’n isotermiese
vloedontspanning teweeg gebring. Op ’n sekere tydstip is die temperatuur effens verlaag,
waarby die vloedontspanning tot stilstand gekom het binne grense van waarneming. Wanneer
die temperatuur weer herstel is na die oorspronklike, het die vloedontspanning voortgegaan
teen die tempo voor die temperatuurverlaging. ’n Verklaring wat gebaseer is op vorteks
dryfsnelheid was aan die hand gedoen om hierdie gedrag te verklaar.
’n Groot komponent van die projek was om die dun YBa₂Cu₃O₇ films self te vervaardig.
Tydens hierdie ondersoek, is ’n gepulseerde laser deposisie (“PLD”) sisteem eiehandig
ontwerp en gebou. PLD behels die interaksie van ’n gefokuseerde laser puls met ’n teiken
bestaande uit ’n multi-element vastestofverbinding. Materiaal wat verdamp (“ablate”) word,
vorm ’n snelbewegende pluim bestaande uit atomiese en molekulêre deeltjies. Dit beweeg
vanaf die teiken na ’n verhitte substraat, waarop die deeltjies kondenseer om laag vir laag ’n
dun film te vorm. Die substraat temperatuur en agtergrond gas word sorgvuldig beheer om die
groei van die verlangde fase van die multi-element verbinding teweeg te bring.
Die PLD sisteem is baie veeldoelig ten opsigte van die verskeidenheid materiale wat
suksesvol neergeslaan kan word. Dit was aangewend om verskillende materiale neer te slaan,
onder andere supergeleidende YBa₂Cu₃O₇, termochromiese VO2, en magnetoresistiewe
LaxCa1-xMnO3. Geleidende Au en Ag lagies is ook suksesvol neergeslaan op YBa₂Cu₃O₇ dun
films, om te dien as beskermingslagies. Die kritieke temperatuur van die beste
supergeleidende films was 90 K soos bepaal deur weerstandsmetings. Die optimale neerslaan
toestand vir hoë kwaliteit YBa₂Cu₃O₇ dun films was: substraat temperatuur 780°C, laser
energiedigtheid 2 - 3 J/cm2, suurstofdruk 0.2 mbar, en teiken-substraat afstand 35 mm. Daar is
bevind dat die substraat temperatuur die deurslaggewende rol speel tydens die neerslaan
proses om die kwaliteit van die supergeleidende films te bepaal.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/6394 |
| Date | 03 1900 |
| Creators | Maritz, E. J. (Erasmus Jacobus) |
| Contributors | Krylov, I. P., Pretorius, R., De Kock, P. R., University of Stellenbosch. Faculty of Science. Dept. of Physics |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 187 p. : ill. |
| Rights | University of Stellenbosch |
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