Sinteza i karakterizacija uzoraka topoloških izolatora mjerenjem transportnih i termoelektričnih svojstava

master's thesis

• Title (english): Syntheses and characterisation of topological insulators by electrical transport and thermoelectric measurements
• Author: Bruno Gudac
• Mentor: Mario Novak (mentor)
• Committee member: Mario Novak (predsjednik povjerenstva)
• Committee member: Vito Despoja (član povjerenstva)
• Committee member: Emil Tafra (član povjerenstva)
• Committee member: Ivan Kokanović (član povjerenstva)
• Committee member: Ivo Batistić (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Physics); Zagreb
• Defense date and country: 2017-07-14, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics

Abstract

Topološki izolatori predstavljaju novo kvantno stanje tvari karakterizirano specijalnim, vodljivim rubnim ili površinskim elektronskim stanjima. Otvaraju nove kvalitativne aspekte u kvantnoj mehanici. Topološki izolatori su ’topološki’ jer su valne funkcije koje opisuju elektronska stanja definirane na Hilbertovom prostoru netrivijalne topologije povezane sa rubnim stanjima bez procjepa. Opisuje se navedena netrivijalna topologija 2D rubnih stanja uz poopćenije na površinska stanja 3D topoloških izolatora. Tema ovog rada je električna i termoelektrična karakterizacija izabranih sintetiziranih monokristala topoloških izolatora BiSbTeSe2 i BiSbTe2S. Navedeni materijali su odabrani jer pokazuju izolatorsko ponašanje s padom temperature, što upućuje da se Fermijeva energija nalazi u energijskom procjepu i to nije slučaj za mnoge druge materijale topoloških izolatora. U radu je detaljno prezentirana sinteza monokristala navedenih materijala te postupak kako se izdvajaju i pripremaju uzorci za sintezu debljine 10µm. Prikazana je strukturna analiza dobivenog materijala koja je potvrdila da imamo vrlo čisti materijal bez prisustva drugih faza. Izvršena su mjerenja električnog transporta bez prisustva magnetskog polja i u jakom magnetskom polju do 16 T te je izmjeren magnetootpor i Hallova otpornost materijala. Uz električna transportna mjerenja mjeren je i Seebeck koeficijent te je izvršena komparacija rezultata transportnih i termoelektričnih mjerenja. Pokazalo se da temperature na kojima pada Seebeckov koeficijent i počinje linearno prilaziti nuli odgovaraju temperaturama na kojim otpornost materijala prestaje rasti. Iz mjerenja Hallove otpornosti u magnetskom polju je procjenjena efektivna gustoća dominantnih nosioca naboja šupljina neff ≈ 1014 cm−3. Dominantni nosioci iz Hallove otpornosti se ne slažu sa dobivenima mjerenjem Seebeckovog koeficijenta BiSbTe2S, no za konkretne zaključke treba napraviti više mjerenja na više uzoraka.

Abstract (english)

Topological insulators represent a new quantum state of matter which is characterized by special edge or surface states. They are ”topological” because the wave functions describing their electronic states span the Hilbert space that has a nontrivial topology. Nontrivial topology is explained on the 2D quantum Hall state and generalization is made for surface states of 3D materials. We have grown single crystals of topological insulator material BiSbTeSe2 and BiSbTe2S. Materials are chosen for their good insulator behavior and are characterized by electric and thermoelectric measurements. Resistivity grows with lower temperature which shows that the Fermi level is inside an energy gap and that is not characteristic of many known topological insulators. It is shown that temperatures where Seebeck coefficient starts with linear trend are the same as the temperatures where resistivity stops growing. Procedure of growing crystals is shown with details, as is procedure for making 10µm thick samples. Structural analysis of grown material has been made and materials don’t show signs of unwanted phases. Electrical measurements are made with and without strong magnetic field up to 16 T. Effective density of charge carriers is approximated from the Hall resistivity measurements and is of order 1014 cm−3. Hall resistivity shows holes as dominant charge carriers like Seebeck coefficient for BiSbTeSe2, but for BiSbTe2S we get different results.

• Keywords: topološki izolatori; Seebeck koeficijent; sinteza kristala; magnetootpor; Hallova otpornost
• Keywords (english): topological insulators; Seebeck coefficient; single-crystal synthesis; magnetoresistance; Hall resistivity
• Language: croatian
• URN:NBN: urn:nbn:hr:217:026691
• Study programme: Title: Physics; specializations in: Research; Course: Research; Study programme type: university; Study level: integrated undergraduate and graduate; Academic / professional title: magistar/magistra fizike (magistar/magistra fizike)
• Type of resource: Text
• File origin: Born digital
• Access conditions: Open access
• Created on: 2017-07-31 11:53:47