Modelling spin correlations in graphene and chiral molecules
Entity
UAM. Departamento de Física de la Materia CondensadaDate
2018-05-11Subjects
Grafenos - Tesis doctorales; Magnetismo - Tesis doctorales; Elasticidad - Tesis doctorales; FísicaNote
Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 11-05-2018
Esta obra está bajo una licencia de Creative Commons Reconocimiento-NoComercial-SinObraDerivada 4.0 Internacional.
Abstract
We first develop an analytical model to explain the spin-selectivity in experiments
that measure conductance through DNA molecules attached to
a Ni substrate and a gold electrode. Our model involves an electron con-
ned to a helix potential; the spin-orbit due to the carbon atomic cores is
modeled by a Rashba term. We calculate the eigenstates of the electron in
the SO-active helix and by calculating the expectation value of the currents
for eigenstates of di erent spins, we nd that electrons of di erent spins
propagate with di erent velocities, thus generating the spin- ltering seen in
the experiments.
Moving on to graphene, we begin by studying superlattices of periodically
hydrogenated graphene in a dilute regime. We include in our model the
adatom-induced magnetism and spin-orbit couplings, and we investigate the
topological properties of the band structure via a Berry curvature analysis.
A direct visualization of the edge states is also carried out by calculating the
spatial distribution of midgap states in the hydrogenated nanoribbon structure,
and by looking at the DOS at the edge of semi-in nite structure. We
also investigate the magnetic anisotropy induced by the spin-orbit coupling
within a Hubbard model at the mean- eld approximation.
Next, we consider pairwise interactions between adatoms in graphene.
For distances in which their orbitals do not overlap, the adatoms may yet
have indirect interactions mediated by the electrons of graphene. We calculate
the total interaction energy via a two-impurity Anderson model. In unstrained
graphene the interactions oscillate according to cos2( K
2 :r) a type
of periodicity that is referred to in the literature as Hidden Kekul e ordering.
We investigate how elastic strains in graphene modulate the pair-wise
interactions between adatoms. We include in our description the e ects of
adatom magnetization and consider also the interactions between adatoms
in the hollow position and benzene-like adsorbates.
Lastly, the e ect of electron-electron interactions in twisted bilayer graphene
are investigated. The Fermi velocity is reduced for small twisting angles,
leading to nearly
at bands (strongly localized in the regions of AA-stacking)
around the Fermi level for some twisting angles. We calculate the magnetic
order within one unit cell using a collinear mean- eld approximation for the
Hubbard term and we obtain that the semimetal-Mott insulator transition
is facilitated by the reduction of the Fermi velocity. Unlike the antiferromagnetic
phase in the monolayer honeycomb, this antiferromagnetism is
strongly localized in the AA regions. We also take into account the e ect of
an applied interlayer bias, which in the non-interacting limit enhances the
electron-con nement. This enhanced con nement turns the moir e pattern
of TBLG into a triangular superlattice of electrons con ned in AA-regions,
and we nd that under interlayer bias the ground state becomes a 120 N eel
state
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Texto de la Tesis Doctoral
Google Scholar:González Árraga, Luis Alberto
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