Proučavanje promjena Fermijeve plohe različitim klasama Diracovih materijala

doctoral thesis

• Title (english): Exploring perturbations of the Fermi surface in different classes of Dirac-based materials
• Author: Bruno Gudac
• Mentor: Mario Novak (mentor)
• Committee member: Mihael S. Grbić (predsjednik povjerenstva)
• Committee member: Mario Novak (član povjerenstva)
• Committee member: Dino Novko (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Physics); Zagreb
• Defense date and country: 2024-03-22, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics
• Universal decimal classification (UDC): 53 - Physics

Abstract

Diracovim materijalima nazivamo materijale u kojima postoje fermioni koje efektivno opisuje Diracova jednadžba. Zajednička karakteristika im je formiranje simetrijom zaštićene Diracove točke u impulsnom prostoru dodirom valentne i vodljive vrpce. Vrpce su linearne u blizini Diracove točke, što povlači mnoga zanimljiva i poneka zajednička im svojstva. Postavljanjem materijala u magnetsko polje dolazi do kvantiziranja elektronu dostupnih energijskih razina u vrpce zvane Landauovi nivoi. Udaljenost nivoa proporcionalna je korijenu jakosti magnetskog polja kod Diracovih materijala pa povećanjem istog dolazi do oscilacija u mnogim fizičkim veličinama, pri čemu je frekvencija oscilacija proporcionalna presjeku Fermijeve plohe. S pomoću kutne ovisnosti kvantnih oscilacija u otpornosti i magnetskoj susceptibilnosti ispitana je izotropnost odabranih Diracovih materijala. Optimizirana je sinteza visokokvalitetnih monokristala topoloških izolatora Bi1.1Sb0.9TeSe2 i Bi1.1Sb0.9Te2S, Diracovih polumetala Zr1−yHfySiS i kvazi-1D sustava TaNiTe5. Uočena je izražena temperaturna ovisnost frekvencije kvantnih oscilacija u Bi1.1Sb0.9Te2S i ponuđeno je objašnjenje fenomena. Napravljena je usporedba frekvencija de Haas-van Alphen oscilacija u ZrSiS i HfSiS s teorijskim izračunima Fermijeve plohe uz slaganje u trendu. Objašnjena je anomalna temperaturna ovisnost susceptibilnosti u ZrSiS eksperimentom uvođenja kemijskog tlaka supstitucijom cirkonija hafnijem u Zr1−yHfySiS. Povećanjem udjela Hf dolazi do Lifshitzovog prijelaza koji se manifestira nestajanjem frekvencije kvantnih oscilacija. Uočen je efekt magnetskog proboja u Shubnikov-de Haas oscilacijama na dilucijskim temperaturama u sustavima ZrSiS i Zr0.95Hf0.05SiS. Značajna 3D anizotropija TaNiTe5 utvrđena je magnetotransportnom i optičkom karakterizacijom, a složenost Fermijeve plohe nazire se iz mjerenja kvantnih oscilacija. Diracova točka uočena je direktnim mjerenjem vrpčaste strukture kutno razlučivom fotoemisijskom spektroskopijom kod uzoraka Bi1.1Sb0.9Te2S i TaNiTe5.

Abstract (english)

The Fermi surface is the surface in reciprocal space that separates occupied from unoccupied electron states. Its shape is derived from the periodicity and symmetry of the crystalline lattice and from the occupation of electronic energy bands. Various experimental techniques are used to determine the Fermi surface in a given material. Angle-resolved photoemission spectroscopy (ARPES) is the most direct and the leading experimental probe for obtaining the band structure of a material. Another viable option is exploiting the quantum oscillation phenomena. When the magnetic field is applied to a system of free-charged fermions, their energy states quantize into Landau levels. In a quantum oscillation experiment, the external magnetic field is varied, which causes Landau levels to pass over the Fermi level. This in turn results in oscillations of the electronic density of states at the Fermi level, and with it, oscillations in various properties, such as resistivity and magnetic susceptibility. The oscillation frequency is proportional to the extremal area of Fermi surface elements in a plane perpendicular to the applied magnetic field. Using different angles of external field, one can reconstruct elements of the Fermi surface contributing to oscillations. Dirac materials are a class of materials where the low-energy excitation spectrum can be effectively described by the Dirac equation. They share a property of forming the symmetryprotected Dirac nodes (or lines) on valence and conductive band crossings, with bands linear in close vicinity of nodes. These symmetries are different for different materials, but their net effect is to preserve Dirac nodes. Seemingly diverse materials such as high-temperature dwave superconductors, superfluid phases of 3He, graphene, and topological insulators possess properties that are a direct consequence of the Dirac spectrum of the quasiparticles and are universal. Universal properties include the response to impurities and magnetic fields, suppressed backscattering, transport properties, and optical conductivity. Dirac materials we focused our research on are topological insulators Bi1.1Sb0.9TeSe2 and Bi1.1Sb0.9Te2S, nodal-line semimetals Zr1−xHfxSiS and quasi-1D TaNiTe5 with topologicaly protected states.

• Keywords: Diracovi materijali; Fermijeva ploha; kvantne oscilacije; topološki izolator; topološki linijski polumetal
• Keywords (english): Dirac materials; Fermi surface; quantum oscillations; topological insulator; topological nodal semimetal
• Language: croatian
• URN:NBN: urn:nbn:hr:217:235365
• Promotion: 2024
• Project: Number: IP-2018-01-8912; Title: Temeljna elektronska svojstva novih kvantnih materijala: bezmaseni i korelirani fermion; Title: Fundamental electronic properties of new quantum materials: massless and correlated fermion; Acronym: FEPNQMMCF; Leader: Ivan Kokanović; Jurisdiction: Croatia; Funder: Hrvatska zaklada za znanost; Funding stream: Research Projects
• Study programme: Title: Doctoral study; Study programme type: university; Study level: postgraduate; Academic / professional title: doktor/doktorica znanosti u području prirodnih znanosti (doktor/doktorica znanosti u području prirodnih znanosti)
• Type of resource: Text
• Extent: 121 str.
• File origin: Born digital
• Access conditions: Open access
• Created on: 2024-05-23 10:01:08