Ispitivanje niskotemperaturne molekulske dinamike u sustavu neuređene rešetke elektronskom spinskom rezonancijom

doctoral thesis

• Title (english): Low-temperature molecular dynamics of systems exhibiting lattice disorder probed by electron spin resonance
• Author: Jurica Jurec
• Mentor: Marina Ilakovac-Kveder (mentor)
• Committee member: Ivo Batistić (predsjednik povjerenstva)
• Committee member: Marina Ilakovac-Kveder (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Physics); Zagreb
• Defense date and country: 2019-11-04, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics
• Universal decimal classification (UDC): 53 - Physics

Abstract

Staklasto stanje tvari je amorfno stanje materijala koje najčešće nastaje brzim hlađenjem kapljevine ispod temperature staklišta 𝑇𝑔. Takav neuređeni sustav nije u stanju termodinamičke ravnoteže i s obzirom na minimum energije svi stupnjevi slobode nisu optimizirani. Izostanak dugodosežne translacijske i rotacijske simetrije dovodi do pojave mnogih interesantnih fizičkih fenomena poput bozonskog vrška. Ako su takovi sustavi paramagnetski ili ih dopiramo paramagnetskim centrima, moguće ih je proučavati metodom elektronske spinske rezonancije (ESR) pri čemu utjecaj dinamike spinova jezgara promatranog materijala na spin elektrona možemo detektirati kao gubitak fazne koherencije elektronskih spinova. Koristeći specifične impulsne sljedove u ESR eksperimentima proučavali smo razlike spinske dinamike između staklastih i kristalastih realizacija istog materijala. Cilj našeg rada je doprinijeti razumijevanju utjecaja neuređenosti na dinamičke efekte opažene u staklima na niskim temperaturama u odnosu na kristalasto stanje, a za koje još ne postoji usuglašeni teorijski opis. Koristeći različite impulsne tehnike, a posebice inačicu konstantnog vremena Carr-Purcell-Meiboom-Gill (CPMG) impulsnih sljedova proučavali smo dva modelna sustava: etanol dopiran paramagnetskim nitroksilnim radikalom TEMPO i trehalozu podvrgnutu 𝛾-zračenju. Oba materijala proučavali smo u staklastom i polikristalastom stanju na temperatu-rama 𝑇≤20 K što odgovara temperaturama 𝑇≤0.2 𝑇𝑔. U tom temperaturnom intervalu molekulska dinamika odgovorna za relaksaciju spinskog sustava u termodinamičku ravnotežu je libracija molekulskih segmenata i kvantno-mehaničko tuneliranje. Dinamičkim rasprezanjem hiperfinog međudjelovanja spina elektrona sa spinovima protona matrice proučavali smo utjecaj neuređenosti molekulskog pakiranja na spinsku dinamiku. U slučaju etanola nuklearna spektralna difuzija (NSD) pokazala se dominantnim mehanizmom gubitka fazne koherencije spinova elektrona. Međutim, u slučaju trehaloze uz NSD uočili smo i dodatni relaksacijski mehanizam, trenutnu difuziju, koji onemogućuje direktan zaključak o utjecaju neuređenosti na spinsku dinamiku jer interferira s utjecajem NSD u gubitku fazne koherencije elektronskih spinova. Dobiveni eksperimentalni podaci predstavljaju osnovu za daljnja teorijska razmatranja modela molekulske dinamike u neuređenom materijalu jer poznavanje dinamičkih svojstava nuklearnih spinova (NSD), koja detektiramo putem hiperfinog međudjelovanja, direktno odražavaju dinamička svojstva promatranog materijala.

Abstract (english)

The glassy state of matter is the amorphous state of the material most commonly formed by rapid cooling of the liquid below the glass transition temperature 𝑇𝑔. Such disordered system is not in a state of thermodynamic equilibrium, and with respect to the minimum energy, all degrees of freedom are not optimized. The absence of long-range translational and rotational symmetry leads to the appearance of many interesting physical phenomena such as the boson peak. If these systems are paramagnetic or if they are doped with paramagnetic centres, it is possible to study them using the electron spin resonance (ESR) method in which the influence of the spin dynamics of the nuclei of the observed material on the electron spin can be detected as a phase decoherence of electron spins. Using specific pulse sequences in ESR experiments, we have studied the differences between spin dynamics of glassy and crystalline realization of the same material. The aim of our work is to contribute to the understanding of the influence of the disorder on the dynamic effects observed in glass at low temperatures but not in the crystalline state, for which a theoretical description has not been agreed upon yet. Using various pulse techniques and focusing on a constant-time version of the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences, we have studied two model systems: ethanol doped with nitroxyl radical TEMPO and 𝛾-irradiated trehalose. Both materials were studied in glassy and polycrystalline state at temperatures 𝑇≤20 K which correspond to temperatures 𝑇≤0.2 𝑇𝑔. In this temperature interval, molecular dynamics responsible for the relaxation of the spin system into thermodynamic equilibrium is the libration of molecular segments and quantum-mechanical tunnelling. Using dynamic decoupling of hyperfine interaction of electron spin with matrix proton spins, we studied the influence of disorder of molecular packing on spin dynamics. In the case of ethanol, nuclear spectral diffusion (NSD) proved to be a dominant mechanism of the phase decoherence of electron spins. However, in the case of trehalose, along with NSD, we have detected additional relaxation mechanisms, instantaneous diffusion, which prevents a direct conclusion about the influence of disorder on spin dynamics because it interferes with NSD in the phase decoherence of electron spins. The obtained experimental data are the basis for further theoretical studies of the molecular dynamics models in a disordered material since dynamic properties of nuclear spins (NSD), which are detected through the hyperfine interaction, directly reflect dynamic properties of the observed material.

• Keywords: niskotemperaturna molekulska dinamika; sustav neuređene rešetke; elektronska spinska rezonancija
• Keywords (english): Rad ne sadrži ključne riječi na drugom jeziku.
• Language: croatian
• URN:NBN: urn:nbn:hr:217:442368
• Study programme: Title: Physics; Study programme type: university; Study level: postgraduate; Academic / professional title: doktor/doktorica znanosti, područje prirodnih znanosti, polje fizika (doktor/doktorica znanosti, područje prirodnih znanosti, polje fizika)
• Type of resource: Text
• Extent: 139 str.
• File origin: Born digital
• Access conditions: Open access
• Created on: 2019-11-08 16:36:08