Transportna i optička svojstva Diracovih polumetala

master's thesis

• Title (english): Transport and optical properties of Dirac semimetals
• Author: Josip Kordić
• Mentor: Ivan Kupčić (mentor)
• Committee member: Ivan Kupčić (predsjednik povjerenstva)
• Committee member: Zoran Rukelj (član povjerenstva)
• Committee member: Mario Novak (član povjerenstva)
• Committee member: Danko Radić (član povjerenstva)
• Committee member: Ivan Kokanović (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Physics); Zagreb
• Defense date and country: 2022-09-27, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics

Abstract

2D i 3D Diracovi materijali klase su materijala koje se intenzivno istražuju posljednjih 15 godina, a koje karakterizira činjenica da disperzije vodljivih elektrona nalikuju disperziji relativističkih Diracovih elektrona. Od otkrića prvih Diracovih materijala, velika je pažnja posvećena objašnjenju njihovih transportnih i optičkih svojstava. U ovom diplomskom radu uspoređuju se ta svojstva u 2D i 3D Diracovim sustavima. Heksagonalni borov nitrid primjer je 2D sustava s konačnom Diracovom masom, a Bi_2Se_3 tipični je 3D Diracov polumetal. Za ta dva primjera načinjen je račun elemenata tenzora vodljivosti u aproksimaciji relaksacijskih vremena. Određene su ovisnosti tenzora vodljivosti o raznim parametrima, o dopiranju, o međuvrpčanom procjepu i o relaksacijskim vremenima. U tipičnim Diracovim polumetalima niskoenergetska optička vodljivost sastoji se od unutarvrpčanih i međuvrpčanih doprinosa. Unutarvrpčani doprinosi izračunati u ovom radu imaju očekivano jednostavne ovisnosti o dopiranju i o međuvrpčanom procjepu. Izračunati međuvrpčani doprinosi, s druge strane, imaju ovisnosti o parametrima modela koje se dobro slažu s rezultatima mjerenja na različitim 3D Diracovim materijalima. Pokazano je da se i temperaturna ovisnost optičke vodljivosti iz eksperimenata može lako razumjeti, ili koristeći račun na konačnim temperaturama, ili pomoću efektivnog modela u kojem međuvrpčana mjera relaksacija ovisi o temperaturi. Primijećena je velika razlika u strukturi transportnih koeficijenata između 2D i 3D Diracovih sustava. U 2D sustavima primijećena je značajna komponenta efektivnog broja nosioca naboja međuvrpčanog porijekla. U 3D sustavima ta je komponenta zanemariva. Porijeklo te razlike jest u dodatnom faktoru k u dk u 3D integralima s obzirom na dk u 2D integralima.

Abstract (english)

2D and 3D Dirac materials have been intensively studied for the last 15 years. They are characterized by the fact that the dispersions of conduction electrons resemble the dispersion of relativistic Dirac electrons. Since the discovery of the first Dirac materials, great attention has been devoted to the explanation of their transport and optical properties. In this thesis, the optical properties of 2D and 3D Dirac systems are studied in detail. Hexagonal boron nitride is an example of a 2D system with finite Dirac mass, and Bi_2Se_3 is a typical 3D Dirac semimetal. For those two examples, the calculation of the elements of the conductivity tensor is performed by using the relaxation time approximation. The dependences of the conductivity tensor on various parameters, on doping, on the interband gap and on relaxation times, are determined. In typical Dirac semimetals, the low-energy optical conductivity consists of intraband and interband contributions. The intraband contributions are found to have simple dependences on the doping level and on the value of the interband gap. The interband contributions, on the other hand, have dependences on the model parameters that agree well with the results of measurements on different 3D Dirac materials. In this thesis, it is shown that the temperature dependence of the interband optical conductivity found in experiments can be easily understood, either by using the finite temperature approach, or by using an effective model in which the interband relaxation rate is temperature dependent. A large difference between transport coefficients in 2D and 3D Dirac systems is found. In 2D systems, there is a significant interband contribution to the effective number of charge carriers. In 3D systems, this component is negligible. The origin of this difference is in the additional factor k in dk in 3D integrals with respect to dk in 2D integrals.

• Keywords: Diracovi polumetali; grafen; heksagonalni borov nitrid; optička svojstva; transportna svojstva; dinamička vodljivost
• Keywords (english): Dirac semimetals; graphene; hexagonal boron nitride; optical properties; transport properties; dynamical conductivity
• Language: croatian
• URN:NBN: urn:nbn:hr:217:643516
• 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: 2022-10-20 09:32:28