70 research outputs found
Effective low-energy Hamiltonians for interacting nanostructures
We present a functional renormalization group (fRG) treatment of trigonal
graphene nanodiscs and composites thereof, modeled by finite-size Hubbard-like
Hamiltonians with honeycomb lattice structure. At half filling, the
noninteracting spectrum of these structures contains a certain number of
half-filled states at the Fermi level. For the case of trigonal nanodiscs,
including interactions between these degenerate states was argued to lead to a
large ground state spin with potential spintronics applications. Here we
perform a systematic fRG flow where the excited single-particle states are
integrated out with a decreasing energy cutoff, yielding a renormalized
low-energy Hamiltonian for the zero-energy states that includes effects of the
excited levels. The numerical implementation corroborates the results obtained
with a simpler Hartree-Fock treatment of the interaction effects within the
zero-energy states only. In particular, for trigonal nanodiscs the degeneracy
of the one-particle-states with zero-energy turns out to be very robust against
influences of the higher levels. As an explanation, we give a general argument
that within this fRG scheme the zero-energy degeneracy remains unsplit under
quite general conditions and for any size of the trigonal nanodisc. We
furthermore discuss the differences in the effective Hamiltonian and their
ground states of single nanodiscs and composite bow-tie-shaped systems.Comment: 13 page
Mathematical applications of inductive logic programming
Accepted versio
Few simple rules governing hydrogenation of graphene dots
We investigated binding of hydrogen atoms to small Polycyclic Aromatic
Hydrocarbons (PAHs) - i.e. graphene dots with hydrogen-terminated edges - using
density functional theory and correlated wavefunction techniques. We considered
a number of PAHs with 3 to 7 hexagonal rings and computed binding energies for
most of the symmetry unique sites, along with the minimum energy paths for
significant cases. The chosen PAHs are small enough to not present radical
character at their edges, yet show a clear preference for adsorption at the
edge sites which can be attributed to electronic effects. We show how the
results, as obtained at different level of theory, can be rationalized in
detail with the help of few simple concepts derivable from a tight-binding
model of the electrons
Topological Frustration in Graphene Nanoflakes: Magnetic Order and Spin Logic Devices
Magnetic order in graphene-related structures can arise from size effects or
from topological frustration. We introduce a rigorous classification scheme for
the types of finite graphene structures (nano-flakes) which lead to large net
spin or to antiferromagnetic coupling between groups of electron spins. Based
on this scheme, we propose specific examples of structures that can serve as
the fundamental (NOR and NAND) logic gates for the design of high-density
ultra-fast spintronic devices. We demonstrate, using ab initio electronic
structure calculations, that these gates can in principle operate at room
temperature with very low and correctable error rates.Comment: Typo in title fixe
Emergence of magnetism in graphene materials and nanostructures
Magnetic materials and nanostructures based on carbon offer unique
opportunities for future technological applications such as spintronics. This
article reviews graphene-derived systems in which magnetic correlations emerge
as a result of reduced dimensions, disorder and other possible scenarios. In
particular, zero-dimensional graphene nanofragments, one-dimensional graphene
nanoribbons, and defect-induced magnetism in graphene and graphite are covered.
Possible physical mechanisms of the emergence of magnetism in these systems are
illustrated with the help of computational examples based on simple model
Hamiltonians. In addition, this review covers spin transport properties,
proposed designs of graphene-based spintronic devices, magnetic ordering at
finite temperatures as well as the most recent experimental achievements.Comment: tutorial-style review article -- 18 pages, 19 figure
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