Istraživanje kvantnih oscilacija u monokristalu topološkog Weylovog semimetala TaP

master's thesis

• Title (english): Study of quantum oscillations in monocrystals of topological Weyl semimetal TaP
• Author: Toni Marković
• Mentor: Mario Novak (mentor)
• Committee member: Mario Novak (predsjednik povjerenstva)
• Committee member: Damir Pajić (član povjerenstva)
• Committee member: Vito Despoja (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: 2016-07-08, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics

Abstract

Weylovi semimetali su nova klasa kvantnih materijala koji posjeduju Weylove fermione kao niskoenergijska pobuđenja. Oni su 3D analogoni grafenu i imaju Weylove točke koje su magnetski monopoli sa zadanom kiralnosti u impulsnom prostoru. Zbog toga što ovi materijali imaju razna egzotična svojstva kao npr. kiralna anomalija koja inducira negativni magnetootpor zanimljivi su i u primjeni, npr. u spintronici. Uspješno su sintetizirani veliki monokristali TaP CVT (eng Chemical Vapour Transport) metodom. Raspršenjem x-zraka na prahu potvrđena je struktura TaP u sintetiziranim kristalima. Oblikovani uzorci su zatim proučavani magnetotransportnim tehnikama u rasponu temperatura od 1:6 do 300 K i magnetskih polja od - 15 do 15 T. U slučaju kada je vanjsko magnetsko polje okomito na smjer struje u uzorku opažen je velik pozitivan magnetootpor. Opaženo je i smanjivanje relativnog magnetootpora u slučaju kad je smjer vanjskog magnetskog polja paralelan sa smjerom struje što potvrduje postojanje kiralne anomalije. Uočene su kvantne oscilacije čijom su se analizom ustanovile dominantne frekvencije, efektivna masa elektrona i vrijeme raspršenja u kristalu. Analizom faze oscilacija potvrdeno je postojanje Berryeve faze π što potvrđuje postojanje Weylovih točaka u sistemu. Iz relativno visoke Dingleove temperature reda veličine 10-ak K ustanovljeno je i da su Weylove točke otporne na perturbacije koje dolaze od nečistoća što je i bilo očekivano s obzirom na njihovu topološku zaštićenost. Eksperimentalni rezultati ovog rada su u vrlo dobrom poklapanju s drugim eksperimentalnim i teorijskim radovima iz literature.

Abstract (english)

Weyl semimetals are a new class of quantum materials that have Weyl fermions as low energy excitations. They are a 3D analogue to graphene and posess Weyl points which are magnetic monopoles with distinct chirality in momentum space. Because these materials have a number of different exotic properties such as the chiral anomaly which induces negative magnetoresistance they are extremely interesting for application purposes for example in spintronics. Big monocrystals of TaP were successfully synthesized with the CVT (Chemical Vapour Transport) method. Powder x-ray diffraction experiment confirmed the TaP structure in the synthesized monocrystals. Samples shaped by grinding were studied with magnetotransport techniques in the temperature range from 1:6 to 300 K and in the magnetic field range from -15 to 15 T. When the magnetic field was perpendicular to the direction of current a large postive magnetoresistance was observed. A deacrease in relative magnetoresistance was observed when the magnetic field was parallel to the direction of current which confirmed the existence of the chiral anomaly effect. Quantum oscillations were isolated and their dominant frequencies as well as the effective mass and scattering time were determined. Phase analysis of the oscillations showed an existence of a π Berry phase consistent with the existence of Weyl cones in the system. A relatively high Dingle temperature around 10 K was determined showing the robustness of Weyl points to perturbation which was expected due to their topological protection. The experimental results in this work are in good correspondence with other experimental as well as theoretical work on this material.

• Keywords: kvantne oscilacije; monokristal; topološki Weylov semimetal
• Keywords (english): quantum oscillations; monocrystals; topological Weyl semimetal
• Language: croatian
• URN:NBN: urn:nbn:hr:217:201182
• 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: 2017-03-04 16:44:58