Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: In this thesis the structural dynamics, upon photo-excitation, of the charge-densitywave
(CDW) material 4Hb-TaSe2 is investigated on the time-scale of atomic motion
and simultaneously on the spatial-scale of atomic dimensions.
CDW materials have been of interest since their discovery in the 1970’s because of their
remarkable non-linear and anisotropic electrical properties, gigantic dielectric constants,
unusual elastic properties and rich dynamical behaviour. Some of these exotic
properties were extensively investigated in thermal equilibrium soon after their discovery
but only recently have ultrafast techniques like femtosecond spectroscopy become
available to study their out-of-equilibrium behaviour on the time-scale of atomic
motion. By studying their behaviour on this time-scale a more in-depth understanding
of their macroscopic properties can be gained. However, to do investigations on the
atomic time-scale and simultaneously directly observe the evolution of the atomic arrangements
is another challenge. One approach is through the previously mentioned
technique of femtosecond pump-probe spectroscopy but converting the usual ultrashort
optical probing source to an ultrashort electron or x-ray source that can diffract
off the sample and reveal structural detail on the atomic level. Here, the femto-to-picosecond out-of-equilibrium behaviour upon photo-excitation in
4Hb-TaSe2 is investigated using an ultrashort electron probe source. Two variations
of using an electron probe source are used: conventional scanning Femtosecond Electron
Diffraction (FED) and a new approach namely Femtosecond Streaked Electron
Diffraction (FSED). The more established FED technique, based on femtosecond pumpprobe
spectroscopy, is used as the major investigating tool while the FSED technique,
based on ultrafast streak camera technology, is an attempt at broadening the scope of
available techniques to study structural dynamics in crystalline material on the subpicosecond
time-scale.
With these two techniques, the structural dynamics during the phase transition from
the commensurate- to incommensurate-CDW phase in 4Hb-TaSe2 is observed through
diffraction patterns with a temporal resolution of under 500 fs. The study reveals
strong coupling between the electronic and lattice systems of the material and several
time-constants of under and above a picosecond are extracted from the data. Using
these time-constants, the structural evolution during the phase transition is better understood
and with the newly gained knowledge, a model of all the processes involved
after photo-excitation is proposed. / AFRIKAANSE OPSOMMING: In hierdie tesis word die strukturele dinamika van die lading-digtheid-golf (LDG) materiaal
4Hb-TaSe2 ondersoek op die tydskaal van atomiese bewegings en gelyktydig op
die ruimtelikeskaal van atomiese dimensies.
LDG materie is al van belang sedert hul ontdekking in die 1970’s as gevolg van hul
merkwaardige nie-lineêre en anisotrope elektriese eienskappe, reuse diëlektriese konstantes,
ongewone elastiese eienskappe en ryk dinamiese gedrag. Sommige van hierdie
eksotiese eienskappe is omvattend ondersoek in termiese ewewig kort na hul ontdekking,
maar eers onlangs is dit moontlik deur middle van ultravinnige tegnieke
soos femtosekonde spektroskopie om hulle uit-ewewigs gedrag te bestudeer op die
tydskaal van atomiese beweging. Deur die gedrag op hierdie tydskaal te bestudeer
kan ’n meer insiggewende begrip van hul makroskopiese eienskappe verkry word.
Om ondersoeke in te stel op die atomiese tydskaal en gelyktydig direk die evolusie
van die atoom posisie te waarneem is egter ’n moeilike taak. Een benadering is deur
middle van femtosekonde “pump-probe” spektroskopie maar dan die gewone optiese
“probe” puls om te skakel na ’n electron of x-straal puls wat van die materiaal kan
diffrak en dus strukturele inligting op die atomiese vlak kan onthul. Hier word die femto-tot-pico sekonde uit-ewewig gedrag in 4Hb-TaSe2 ondersoek met
behulp van elektron pulse. Twee variasies van die gebruik van ’n elektron bron word
gebruik: konvensionele “Femtosecond Electron Diffraction” (FED) en ’n nuwe benadering,
naamlik, “Femtosecond Streaked Electron Diffraction” (FSED). Die meer gevestigde
FED tegniek, wat gebaseer is op femtosekonde “pump-probe” spektroskopie,
word gebruik as die hoof ondersoek metode terwyl die FSED tegniek, wat gebaseer is
op die ultra vinnige “streak camera” tegnologie, ’n poging is om beskikbare tegnieke
uit te brei wat gebruik kan word om strukturele dinamika in materie te bestudeer op
die sub-picosekonde tydskaal.
Met behulp van hierdie twee tegnieke, word die strukturele dinamika tydens die fase
oorgang van die ooreenkomstige tot nie-ooreenkomstige LDG fase in 4Hb-TaSe2 deur
diffraksie patrone met ’n tydresolusie van minder as 500 fs waargeneem. Die studie
toon ’n sterk korrelasie tussen die elektroniese sisteem en kristalrooster. Verskeie
tydkonstantes van onder en bo ’n picosekonde kon ook uit die data onttrek word en
gebruik word om die strukturele veranderinge beter te verstaan. Hierdie nuwe kennis
het ons in staat gestel om ’n model van al die betrokke prosesse voor te stel.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/79934 |
| Date | 03 1900 |
| Creators | Erasmus, Nicolas |
| Contributors | Schwoerer, Heinrich, Rohwer, Erich, Stellenbosch University. Faculty of Science. Dept. of Physics. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | xiv, 76 p. : ill. (some col.) |
| Rights | Stellenbosch University |
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