Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Low-dimensional materials are known to undergo phase transitions to differently or-
dered states, when cooled to lower temperatures. These phases often show a periodic
modulation of the charge density (called a charge density wave – CDW) coupled with
a periodic perturbation of the crystal lattice (called a periodic lattice distortion – PLD).
Although many experiments have been performed and much has been learnt about
CDW phases in low-dimensional materials, the reasons for their existence are still
not fully understood yet. Many processes, involving either strong electron–electron or
electron–lattice coupling, have been observed which all might play a role in explaining
the formation of different phases under different conditions.
With the availability of femtosecond lasers it has become possible to study materials
under highly nonequilibrium conditions. By suddenly introducing a known amount
of energy into the system, the equilibrium state is disturbed and the subsequent relax-
ation processes are then observed on timescales of structural and electronic responses.
These experiments can deliver valuable information about the complex interactions
between the different constituents of condensed matter, which would be inaccessible
under equilibrium conditions.
We use time resolved electron diffraction to investigate the behaviour of a CDW
system perturbed by a short laser pulse. From the observed changes in the diffraction
patterns we can directly deduce changes in the lattice structure of our sample.
A femtosecond electron diffraction setup was developed at the Laser Research In-
stitute in Stellenbosch, South Africa. Short laser pulses produce photo electrons which
are accelerated to an energy of 30 keV. Despite space charge broadening effects, elec-
tron pulses shorter than 500 fs at sample position can be achieved. Technical details
of this system and its characterisation as well as sample preparation techniques and
analysis methods are described in detail in this work.
Measurements on two members of the quasi-two-dimensional transition metal di-
chalcogenides, namely 4Hb-TaSe2 and 1T-TaS2, are shown and discussed. Both show
fast (subpicosecond) changes due to the suppression of the PLD and a rapid heating
of the lattice. When the induced temperature rise heats the sample above a phase tran-
sition temperature, a complete transformation into the new phase was observed. For
4Hb-TaSe2 we found that the recovery to the original state is significantly slower if
the PLD was completely suppressed compared to only disturbing it. On 1T-TaS2 we
could not only study the suppression of the original phase but also the formation of
the higher energetic CDW phase. Long (100 ps) time constants were found for the tran-
sition between the two phases. These suggest the presence of an energy barrier which
has to be overcome in order to change the CDW phase. Pinning of the CDW by de-
fects in the crystal structure result in such an energy barrier and consequently lead to
a phase of domain growth which is considerably slower than pure electron or lattice
dynamics. / AFRIKAANSE OPSOMMING: Dit is bekend dat lae-dimensionele materie fase oorgange ondergaan na anders ge-
ori¨enteerde toestande wanneer afgekoel word tot laer temperature. Hierdie fases toon
dikwels ’n periodiese modulasie van die elektron digtheid (genoem ’n “charge density
wave” – CDW), tesame met ’n periodiese effek op die kristalrooster (genoem ’n “peri-
odic lattice distortion” – PLD). Alhoewel baie eksperimente al uitgevoer is en al baie
geleer is oor hierdie CDW fase, is die redes vir hul bestaan nog steeds nie ten volle
verstaan nie. Baie prosesse, wat of sterk elektron–elektron of elektron–fonon interaksie
toon, is al waargeneem en kan ’n rol speel in die verduideliking van die vorming van
die verskillende fases onder verskillende omstandighede.
Met die beskikbaarheid van femtosekonde lasers is dit nou moontlik om materie
onder hoogs nie-ewewig voorwaardes te bestudeer. Deur skielik ’n bekende hoeveel-
heid energie in die stelsel in te voer, word die ewewigstaat versteur en word die daar-
opvolgende ontspanning prosesse waargeneem op die tydskaal van atomies struktu-
rele en elektroniese bewiging. Hierdie eksperimente kan waardevolle inligting lewer
oor die komplekse interaksies tussen die verskillende atomiese komponente van ge-
kondenseerde materie, wat ontoeganklik sou wees onder ewewig voorwaardes.
Ons gebruik elektrondiffraksie met tyd resolusie van onder ’n pikosekonde om die
gedrag van ’n CDW stelsel te ondersoek nadat dit versteur is deur ’n kort laser puls.
Van die waargenome veranderinge in die diffraksie patrone kan ons direk aflei watse
veranderinge die kristalstruktuur van ons monster ondergaan.
’n Femtosekonde elektronendiffraksie opstelling is ontwikkel by die Lasernavors-
ingsinstituut in Stellenbosch, Suid-Afrika. Kort laser pulse produseer foto-elektrone
wat dan na ’n energie van 30 keV versnel word. Ten spyte van Coulomb afstoting ef-
fekte, kan elektron pulse korter as 500 fs by die monster posisie bereik word. Tegniese
besonderhede van hierdie opstelling, tegnieke van die voorbereiding van monsters
asook analise metodes word volledig in hierdie tesis beskryf.
Metings op twee voorbeelde van kwasi-tweedimensionele semi-metale, naamlik
4Hb-TaSe2 en 1T-TaS2, word gewys en bespreek. Beide wys ’n vinnige (subpikosekon-
de) verandering as gevolg van die versteuring van die PLD en ’n vinnige verhitting van
die kristalrooster. Wanneer die ge¨ınduseerde temperatuur bo die fase oorgang tempe-
ratuur styg, is ’n volledige transformasie na die nuwe fase waargeneem. Vir 4Hb-TaSe2
het ons gevind dat die herstelling na die oorspronklike toestand aansienlik stadiger is
as die PLD heeltemal viernietig is in vergelyking met as die PLD net versteur is. Met
1T-TaS2 kon ons nie net alleenlik die vernietiging van die oorspronklike fase sien nie,
maar ook die vorming van ’n ho¨er energie CDW fase. Lang (100 ps) tydkonstante is
gevind vir die oorgang tussen die twee fases. Hierdie dui op die teenwoordigheid van
’n energie-versperring wat eers oorkom moet word om die CDW fase voledig te ver-
ander. Vaspenning van die CDW deur defekte in die kristalstruktuur veroorsaak so’n
energie versperring en gevolglik lei dit tot ’n fase van groeiende CDW gebiede wat
heelwat stadiger as pure elektron of kritalrooster dinamika is.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/85608 |
| Date | 12 1900 |
| Creators | Haupt, Kerstin Anna |
| Contributors | Schwoerer, Heinrich, Rohwer, Erich G., Stellenbosch University. Faculty of Science. Dept. of Physics. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | vi, 64 p. : col. ill. |
| Rights | Stellenbosch University |
Page generated in 0.0025 seconds