Charge density wave ground state in the intercalated graphite CaC6

doctoral thesis

• Title (english): Osnovno stanje vala gustoće naboja u interkaliranom grafitu CaC6
• Author: Petra Grozić
• Mentor: Danko Radić (mentor)
• Committee member: Miroslav Požek (predsjednik povjerenstva)
• Committee member: Dino Novko (član povjerenstva)
• Committee member: Danko Radić (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Physics); Zagreb
• Defense date and country: 2024-06-28, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics
• Universal decimal classification (UDC): 53 - Physics

Abstract

The physics of graphite intercalated compounds (GIC), in spite of the fact that these materials were synthesised quite long ago, is still the focus of modern theoretical and experimental research. The interest of the scientific community is focused on GIC, mostly due to the possibility of forming unusual superconducting (SC) and charge density wave (CDW) ground states. We focus on one of these materials, CaC6, in which experiments clearly demonstrate the existence of CDWs. The very existence of the CDW is quite unusual due to the fact that the graphene Fermi surface, chemically doped by calcium intercalates, shows no so-called "nesting property," i.e., no Peierls mechanism, which has been the paradigmatic mechanism behind the CDW instability since the 1950s and was experimentally realised during the 1980s in highly anisotropic materials with open Fermi surfaces. We show that the CaC6 Fermi surface undergoes a different CDW stabilisation mechanism based on the topological reconstruction (of the Fermi Surface) and critical strength of the electron-phonon interaction. Such topological reconstruction, which transforms the set of closed Fermi pockets into open Fermi sheets, has profound effects on transport and magnetotransport properties. In this thesis, we show how the Fermi surface reconstruction stabilises the CDW ground state. Furthermore, we present the magnetotransport properties, taking into account magnetic breakdown.

Abstract (english)

Cilj ovog rada je opisati i objasniti pojavu nastanka valova gustoće naboja u interkaliranom grafitu CaC6 tzv. minimalnim modelom, koji sadrži samo fizikalne odlike nužne za samu egzistenciju pojave vala gustoće naboja. Inicijalni problem sustava vezanih elektrona i fonona opisujemo dvodimenzionalnim Fröhlichovim hamiltonijanom. Njega tretiramo u aproksimaciji srednjeg polja i uvodimo kompleksni parametar uređenja. Nakon tretmana aproksimacijom srednjeg polja dobivamo izraz za elektronske vrpce. Kako bismo dobili oblik rekonstruiranih elektronskih vrpci, posežemo za posebno razvijenim metodama razvoja podintegralnih funkcija kako bismo dobili analitički rezultat za kondenzacijsku energiju sustava. Da bi novonastalo stanje s valom gustoće naboja bilo stabilno, kondenzacijska energija mora biti pozitivna, a njezina maksimizacija vrši se optimizacijom izraza po samosuglasnom valnom vektoru vala gustoće i po njegovom parametru uređenja. Nadalje, određujemo koji je fononski mod kristalne rešetke u CaC6 strukturi zaslužan za pojavu vala gustoće. Pokazuje se da je to TA mod titranja kalcijeve superrešetke u njezinoj ravnini. Ovaj mod je "najmekši", odnosno ima najmanju energiju, ali i najjače vezan je vezan na grafenske elektrone čime su eliminirani svi grafenski vibracijski modovi, kao i kalcijevi fleksuralni modovi titranja.

• Keywords: Charge density wave (CDW); Fermi surface reconstruction; intercalated graphite; magnetoconductivity; magnetic breakdown
• Keywords (english): val gustoće naboja; topološka rekonstrukcija Fermijeve površine; interkalirani grafit; magnetovodljivost; magnetski proboj
• Language: english
• URN:NBN: urn:nbn:hr:217:942206
• Promotion: 2024
• Project: Number: IP-2016-06-2289; Title: Kolektivni efekti, tunelirajući i topološki transport u novim nanospojevima; Title: Collective effects, tunneling and topological transport in novel nanojunctions; Acronym: C3TiNN; Leader: Danko Radić; 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
• File origin: Born digital
• Access conditions: Open access; Embargo expiration date: 2025-01-03
• Created on: 2024-07-18 10:43:02