Manifestations of intrinsic and induced magnetic properties of graphene nanostructures
Испољавање сопствених и индукованих магнетских својстава графенских наноструктура.
Author
Grujić, Marko M.Mentor
Tadić, MilanCommittee members
Peeters, François M.Raković, Dejan
Partoens, Bart
Arsoski, Vlada
Metadata
Show full item recordAbstract
The dissertation can be roughly divided into two parts. In the first part it deals
with magnetic properties of quasi-zero dimensional graphene structures, such as
nanodots and nanorings. In particular, a circular graphene quantum dot is analyzed
in Chapter 3 using the Dirac-Weyl equation. The energy and the optical absorption
spectra are computed for the case of the present external magnetic field. The results are obtained for two distinct boundary conditions, namely infinite-mass and zigzag boundary conditions, which model different physics in the structure with different edges. It is found that the energy spectrum of a dot with zigzag boundary condition exhibits a zero energy band regardless of the value of the magnetic field, while for the infinite mass boundary condition the zero energy states appear only for high magnetic fields in the form of the zeroth Landau level. The analytical results are compared to those obtained from the tight-binding model in order to show the validity r...ange of the continuum model. It is found that the continuum model with infinite mass boundary condition describes rather well its tight binding counterpart, which can partially be attributed to blurring of the mixed edges by the staggered potential.
The mean-field Hubbard model is subsequently used to investigate the formation
of the antiferromagnetic phase in hexagonal graphene quantum rings with inner
zigzag edges. The outer edge of the ring is taken to be either zigzag or armchair,
and it is found that both types of structures exhibit a larger antiferromagnetic
interaction than hexagonal quantum dots. This difference could be partially ascribed to the larger number of zigzag edges per unit area in rings than in dots. Furthermore, edge states localized on the inner ring edge are found to hybridize differently than the edge states of dots, which results in important differences in the magnetism of graphene rings and dots. The largest staggered magnetization is found when the outer edge has a zigzag shape. However, narrow rings with armchair outer edge are found to have larger staggered magnetization than zigzag hexagons. The edge defects are shown to have the least effect on magnetization when the outer ring edge is armchair shaped.
Ova doktorska disertacija se može grubo podeliti u dva dela. U prvom
delu, ona se bavi magnetskim svojstvima kvazi-nulto dimenzionih grafen-
skih struktura, kao što su nanotačke i nanoprstenovi. Konkretno, kružna
grafenska kvantna tačka je analizirana u poglav u 3 koristeći Dirak-Vejlovu
jednačinu. Energetska stanja i optički apsorpcioni spektar su izračunati
za slučaj primenjenog spoljašnjeg magnetnog polja. Rezultati su dobijeni za
dva različita slučaja, primenom graničnog uslova beskonačne mase i takoz-
vanog cikcak graničnog uslova, koji opisuju strukture sa različitim vrstama
ivica. Pronađeno je da energetski spektar tačke za cikcak granični uslov
podržava nultoenergetski nivo, bez obzira na vrednost magnetskog polja, dok
se za granični uslov beskonačane mase nultoenergetska stanja pojavljuju samo
za visoke vrednosti magnetskog polja i to u obliku nultog Landauovog nivoa.
Analitički dobijeni rezultati su zatim upoređeni sa rezultatima dobijenim
pomoću metoda jake veze, da bi se od...redila oblast važenja kontinualnog mod-
ela. Pronađeno je da se kontinualni model sa graničnim uslovom beskonačne
mase dobro slaže sa metodom jake veze, što se donekle može pripisati mini-
mizovanju uticaja ivica, što je posledica naizmeničnog potencijala.
Nakon ovoga disertacija se bavi analizom formiranja antiferomagnetizma
u heksagonalnim grafenskim kvantnim prstenovima sa cikcak unutrašnjom
ivicom koristeći Habardov model u aproksimaciji srednjeg polja. Spoljna
ivica prstena je ili cikcak ili foteljastog tipa. Pronađeno je da obe vrste
struktura pokazuju veću sklonost ka antiferomagnetizmu nego heksagonalne
kvantne tačke. Ova razlika se delimično može pripisati većoj dužini cik-
cak ivica u prstenovima nego u tačkama. Pored toga, hibridizacija ivič-
nih stanja lokalizovanih na unutrašnjoj strani prstena je drugačije prirode
nego hibridizacija stanja na ivici tačaka, što dovodi do bitnih razlika mag-
netizma prstenova u odnosu na tačke. Najveća magnetizacija je dobijena za
slučaj kada je i spoljašnja ivica cikcak tipa. Ipak, vrlo uski prstenovi sa...
Faculty:
Универзитет у Београду, Електротехнички факултетDate:
26-05-2015Projects:
- Optoelectronics nanodimension systems - the rout towards applications (RS-MESTD-Integrated and Interdisciplinary Research (IIR or III)-45003)