14 research outputs found
Quantum Nature of Edge Magnetism in Graphene
It is argued that the subtle crossover from decoherence-dominated classical
magnetism to fluctuation-dominated quantum magnetism is experimentally
accessible in graphene nanoribbons. We show that the width of a nanoribbon
determines whether the edge magnetism is on the classical side, on the quantum
side, or in between. In the classical regime, decoherence is dominant and leads
to static spin polarizations at the ribbon edges, which are well described by
mean-field theories. The quantum Zeno effect is identified as the basic
mechanism which is responsible for the spin polarization and thereby enables
the application of graphene in spintronics. On the quantum side, however, the
spin polarization is destroyed by dynamical processes. The great tunability of
graphene magnetism thus offers a viable route for the study of the
quantum-classical crossover.Comment: 5 pages, 3 figure
Ground state phase diagram of the half-filled bilayer Hubbard model
Employing a combination of functional renormalization group calculations and
projective determinantal quantum Monte Carlo simulations, we examine the
Hubbard model on the square lattice bilayer at half filling. From this combined
analysis, we obtain a comprehensive account on the ground state phase diagram
with respect to the extent of the system's metallic and (antiferromagnetically
ordered) Mott-insulating as well as band-insulating regions. By means of an
unbiased functional renormalization group approach, we exhibit the
antiferromagnetic Mott-insulating state as the relevant instability of the free
metallic state, induced by any weak finite onsite repulsion. Upon performing a
careful analysis of the quantum Monte Carlo data, we resolve the difficulty of
identifying this antiferromagnetic ground state for finite interlayer hopping
in the weak-coupling regime, where nonmonotonous finite-size corrections are
shown to relate to the two-sheeted Fermi surface structure of the metallic
phase. On the other hand, quantum Monte Carlo simulations are well suited to
identify the transition between the Mott-insulating phase and the band
insulator in the intermediate-to-strong coupling regime. Here, we compare our
numerical findings to indications for the transition region obtained from the
functional renormalization group procedure.Comment: 12 pages, 15 figure
Effective models for strong electronic correlations at graphene edges
We describe a method for deriving effective low-energy theories of electronic
interactions at graphene edges. Our method is applicable to general edges of
honeycomb lattices (zigzag, chiral, and even disordered) as long as localized
low-energy states (edge states) are present. The central characteristic of the
effective theories is a dramatically reduced number of degrees of freedom. As a
consequence, the solution of the effective theory by exact diagonalization is
feasible for reasonably large ribbon sizes. The quality of the involved
approximations is critically assessed by comparing the correlation functions
obtained from the effective theory with numerically exact quantum Monte-Carlo
calculations. We discuss effective theories of two levels: a relatively
complicated fermionic edge state theory and a further reduced Heisenberg spin
model. The latter theory paves the way to an efficient description of the
magnetic features in long and structurally disordered graphene edges beyond the
mean-field approximation.Comment: 13 pages, 9 figure
Quantum Monte Carlo studies of edge magnetism in chiral graphene nanoribbons
We investigate chiral graphene nanoribbons using projective quantum Monte
Carlo simulations within the local Hubbard model description and study the
effects of electron-electron interactions on the electronic and magnetic
properties at the ribbon edges. Static and dynamical properties are analyzed
for nanoribbons of varying width and edge chirality, and compared to a
self-consistent Hartee-Fock mean-field approximation. Our results show that for
chiral ribbons of sufficient width, the spin correlations exhibit exceedingly
long correlation lengths, even between zigzag segments that are well separated
by periodic armchair regions. Characteristic enhancements in the magnetic
correlations for distinct ribbon widths and chiralities are associated with
energy gaps in the tight-binding limit of such ribbons. We identify specific
signatures in the local density of states and low- energy modes in the local
spectral function which directly relate to enhanced electronic correlations
along graphene nanoribbons and which can be accessed scanning tunneling
spectroscopy.Comment: 11 pages, 15 figure
Basic science232. Certolizumab pegol prevents pro-inflammatory alterations in endothelial cell function
Background: Cardiovascular disease is a major comorbidity of rheumatoid arthritis (RA) and a leading cause of death. Chronic systemic inflammation involving tumour necrosis factor alpha (TNF) could contribute to endothelial activation and atherogenesis. A number of anti-TNF therapies are in current use for the treatment of RA, including certolizumab pegol (CZP), (Cimzia ®; UCB, Belgium). Anti-TNF therapy has been associated with reduced clinical cardiovascular disease risk and ameliorated vascular function in RA patients. However, the specific effects of TNF inhibitors on endothelial cell function are largely unknown. Our aim was to investigate the mechanisms underpinning CZP effects on TNF-activated human endothelial cells. Methods: Human aortic endothelial cells (HAoECs) were cultured in vitro and exposed to a) TNF alone, b) TNF plus CZP, or c) neither agent. Microarray analysis was used to examine the transcriptional profile of cells treated for 6 hrs and quantitative polymerase chain reaction (qPCR) analysed gene expression at 1, 3, 6 and 24 hrs. NF-κB localization and IκB degradation were investigated using immunocytochemistry, high content analysis and western blotting. Flow cytometry was conducted to detect microparticle release from HAoECs. Results: Transcriptional profiling revealed that while TNF alone had strong effects on endothelial gene expression, TNF and CZP in combination produced a global gene expression pattern similar to untreated control. The two most highly up-regulated genes in response to TNF treatment were adhesion molecules E-selectin and VCAM-1 (q 0.2 compared to control; p > 0.05 compared to TNF alone). The NF-κB pathway was confirmed as a downstream target of TNF-induced HAoEC activation, via nuclear translocation of NF-κB and degradation of IκB, effects which were abolished by treatment with CZP. In addition, flow cytometry detected an increased production of endothelial microparticles in TNF-activated HAoECs, which was prevented by treatment with CZP. Conclusions: We have found at a cellular level that a clinically available TNF inhibitor, CZP reduces the expression of adhesion molecule expression, and prevents TNF-induced activation of the NF-κB pathway. Furthermore, CZP prevents the production of microparticles by activated endothelial cells. This could be central to the prevention of inflammatory environments underlying these conditions and measurement of microparticles has potential as a novel prognostic marker for future cardiovascular events in this patient group. Disclosure statement: Y.A. received a research grant from UCB. I.B. received a research grant from UCB. S.H. received a research grant from UCB. All other authors have declared no conflicts of interes