39 research outputs found
Flat edge modes of graphene and of Z2 topological insulator
A graphene nano-ribbon in the zigzag edge geometry exhibits a specific type of gapless edge modes with a partly flat band dispersion. We argue that the appearance of such edge modes are naturally understood by regarding graphene as the gapless limit of a Z2 topological insulator. To illustrate this idea, we consider both Kane-Mele (graphene-based) and Bernevig-Hughes-Zhang models: the latter is proposed for HgTe/CdTe 2D quantum well. Much focus is on the role of valley degrees of freedom, especially, on how they are projected onto and determine the 1D edge spectrum in different edge geometries
Energy gaps, topological insulator state and zero-field quantum Hall effect in graphene by strain engineering
Among many remarkable qualities of graphene, its electronic properties
attract particular interest due to a massless chiral character of charge
carriers, which leads to such unusual phenomena as metallic conductivity in the
limit of no carriers and the half-integer quantum Hall effect (QHE) observable
even at room temperature [1-3]. Because graphene is only one atom thick, it is
also amenable to external influences including mechanical deformation. The
latter offers a tempting prospect of controlling graphene's properties by
strain and, recently, several reports have examined graphene under uniaxial
deformation [4-8]. Although the strain can induce additional Raman features
[7,8], no significant changes in graphene's band structure have been either
observed or expected for realistic strains of approx. 10% [9-11]. Here we show
that a designed strain aligned along three main crystallographic directions
induces strong gauge fields [12-14] that effectively act as a uniform magnetic
field exceeding 10 T. For a finite doping, the quantizing field results in an
insulating bulk and a pair of countercirculating edge states, similar to the
case of a topological insulator [15-20]. We suggest realistic ways of creating
this quantum state and observing the pseudo-magnetic QHE. We also show that
strained superlattices can be used to open significant energy gaps in
graphene's electronic spectrum
Properties of Graphene: A Theoretical Perspective
In this review, we provide an in-depth description of the physics of
monolayer and bilayer graphene from a theorist's perspective. We discuss the
physical properties of graphene in an external magnetic field, reflecting the
chiral nature of the quasiparticles near the Dirac point with a Landau level at
zero energy. We address the unique integer quantum Hall effects, the role of
electron correlations, and the recent observation of the fractional quantum
Hall effect in the monolayer graphene. The quantum Hall effect in bilayer
graphene is fundamentally different from that of a monolayer, reflecting the
unique band structure of this system. The theory of transport in the absence of
an external magnetic field is discussed in detail, along with the role of
disorder studied in various theoretical models. We highlight the differences
and similarities between monolayer and bilayer graphene, and focus on
thermodynamic properties such as the compressibility, the plasmon spectra, the
weak localization correction, quantum Hall effect, and optical properties.
Confinement of electrons in graphene is nontrivial due to Klein tunneling. We
review various theoretical and experimental studies of quantum confined
structures made from graphene. The band structure of graphene nanoribbons and
the role of the sublattice symmetry, edge geometry and the size of the
nanoribbon on the electronic and magnetic properties are very active areas of
research, and a detailed review of these topics is presented. Also, the effects
of substrate interactions, adsorbed atoms, lattice defects and doping on the
band structure of finite-sized graphene systems are discussed. We also include
a brief description of graphane -- gapped material obtained from graphene by
attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic
Holographic Fermionic Fixed Points in d=3
We present a top-down string theory holographic model of strongly interacting
relativistic 2+1-dimensional fermions, paying careful attention to the discrete
symmetries of parity and time reversal invariance. Our construction is based on
probe -branes in , stabilized by internal fluxes. We find
three solutions, a parity and time reversal invariant conformal field theory
which can be viewed as a particular deformation of Coulomb interacting
graphene, a parity and time reversal violating but gapless field theory and a
system with a parity and time reversal violating charge gap. We show that the
Chern-Simons-like electric response function, which is generated perturbatively
at one-loop order by parity violating fermions and which is protected by a
no-renormalization theorem at orders beyond one loop, indeed appears with the
correctly quantized coefficient in the charge gapped theory. In the gapless
parity violating solution, the Chern-Simons response function obtains quantum
corrections which we compute in the holographic theory.Comment: 25 pages, six figure
The Immune System in Stroke
Stroke represents an unresolved challenge for both developed and developing countries and has a huge socio-economic impact. Although considerable effort has been made to limit stroke incidence and improve outcome, strategies aimed at protecting injured neurons in the brain have all failed. This failure is likely to be due to both the incompleteness of modelling the disease and its causes in experimental research, and also the lack of understanding of how systemic mechanisms lead to an acute cerebrovascular event or contribute to outcome. Inflammation has been implicated in all forms of brain injury and it is now clear that immune mechanisms profoundly influence (and are responsible for the development of) risk and causation of stroke, and the outcome following the onset of cerebral ischemia. Until very recently, systemic inflammatory mechanisms, with respect to common comorbidities in stroke, have largely been ignored in experimental studies. The main aim is therefore to understand interactions between the immune system and brain injury in order to develop novel therapeutic approaches. Recent data from clinical and experimental research clearly show that systemic inflammatory diseases -such as atherosclerosis, obesity, diabetes or infection - similar to stress and advanced age, are associated with dysregulated immune responses which can profoundly contribute to cerebrovascular inflammation and injury in the central nervous system. In this review, we summarize recent advances in the field of inflammation and stroke, focusing on the challenges of translation between pre-clinical and clinical studies, and potential anti-inflammatory/immunomodulatory therapeutic approaches
Unconventional mass enhancement around the Dirac nodal loop in ZrSiS
The topological properties of fermions arise from their low-energy Dirac-like
band dispersion and associated chiralities. Initially confined to points,
extensions of the Dirac dispersion to lines and even loops have now been
uncovered and semimetals hosting such features have been identified. However,
experimental evidence for the enhanced correlation effects predicted to occur
in these topological semimetals has been lacking. Here, we report a quantum
oscillation study of the nodal loop semimetal ZrSiS in high magnetic fields
that reveals significant enhancement in the effective mass of the
quasiparticles residing near the nodal loop. Above a threshold field, magnetic
breakdown occurs across gaps in the loop structure with orbits that enclose
different windings around its vertices, each winding accompanied by an
additional \pi-Berry phase. The amplitudes of these breakdown orbits exhibit an
anomalous temperature dependence. These findings demonstrate the emergence of
novel, correlation-driven physics in ZrSiS associated with the Dirac-like
quasiparticles.Comment: 20 pages, 4 figure
Characterisation of the British honey bee metagenome
Numerous microbial symbionts, both commensal and pathogenic, are associated with honey bees. Here, the authors genomically characterize this ‘metagenome’ of the British honey bee, identifying a diversity of commensal microbes as well as known and putative pathogen