438 research outputs found
Domain walls in gapped graphene
The electronic properties of a particular class of domain walls in gapped
graphene are investigated. We show that they can support mid-gap states which
are localized in the vicinity of the domain wall and propagate along its
length. With a finite density of domain walls, these states can alter the
electronic properties of gapped graphene significantly. If the mid-gap band is
partially filled,the domain wall can behave like a one-dimensional metal
embedded in a semi-conductor, and could potentially be used as a single-channel
quantum wire.Comment: 4 pgs. revte
Spin Versus Charge Density Wave Order in Graphene-like Systems
A variational technique is used to study sublattice symmetry breaking by
strong on-site and nearest neighbor interactions in graphene. When interactions
are strong enough to break sublattice symmetry, and with relative strengths
characteristic of graphene, a charge density wave Mott insulator is favored
over the spin density wave condensates. In the spin density wave condensate we
find that introduction of a staggered on-site energy (quasiparticle mass) leads
to a splitting of the fermi velocities and mass gaps of the quasiparticle spin
states.Comment: 5 pages, 4 figures; some comments adde
Charge Screening in the Finite Temperature Schwinger Model
We compute the effective action and correlators of the Polyakov loop operator
in the Schwinger model at finite temperature and discuss the realization of the
discrete symmetries that occur there. We show that, due to nonlocal effects of
massless fermions in two spacetime dimensions, the discrete symmetry which
governs the screening of charges is spontaneously broken even in an effective
one-dimensional model, when the volume is infinite. In this limit, the thermal
state of the Schwinger model screens an arbitrary external charge; consequently
the model is in the deconfined phase, with the charge of the deconfined
fermions completely screened. In a finite volume we show that the Schwinger
model is always confining.Comment: 27 pages, latex, no figures. References addded and some misprints
correcte
Unitary matrix integrals in the framework of Generalized Kontsevich Model. I. Brezin-Gross-Witten Model
We advocate a new approach to the study of unitary matrix models in external
fields which emphasizes their relationship to Generalized Kontsevich Models
(GKM) with non-polynomial potentials. For example, we show that the partition
function of the Brezin-Gross-Witten Model (BGWM), which is defined as an
integral over unitary matrices, , can also be considered as a GKM with potential . Moreover, it can be interpreted as the generating functional for
correlators in the Penner model. The strong and weak coupling phases of the
BGWM are identified with the "character" (weak coupling) and "Kontsevich"
(strong coupling) phases of the GKM, respectively. This sort of GKM deserves
classification as one (i.e. or ) when in the Kontsevich
phase. This approach allows us to further identify the
Harish-Chandra-Itzykson-Zuber (IZ) integral with a peculiar GKM, which arises
in the study of theory and, further, with a conventional 2-matrix model
which is rewritten in Miwa coordinates. Inspired by the considered unitary
matrix models, some further extensions of the GKM treatment which are inspired
by the unitary matrix models which we have considered are also developed. In
particular, as a by-product, a new simple method of fixing the Ward identities
for matrix models in an external field is presented.Comment: FIAN/TD-16/93, ITEP-M6/93, UBC/S-93/93 (39 pages
A Holographic Quantum Hall Ferromagnet
A detailed numerical study of a recent proposal for exotic states of the
D3-probe D5 brane system with charge density and an external magnetic field is
presented. The state has a large number of coincident D5 branes blowing up to a
D7 brane in the presence of the worldvolume electric and magnetic fields which
are necessary to construct the holographic state. Numerical solutions have
shown that these states can compete with the the previously known chiral
symmetry breaking and maximally symmetric phases of the D3-D5 system. Moreover,
at integer filling fractions, they are incompressible with integer quantized
Hall conductivities. In the dual superconformal defect field theory, these
solutions correspond to states which break the chiral and global flavor
symmetries spontaneously. The region of the temperature-density plane where the
D7 brane has lower energy than the other known D5 brane solutions is
identified. A hypothesis for the structure of states with filling fraction and
Hall conductivity greater than one is made and tested by numerical computation.
A parallel with the quantum Hall ferromagnetism or magnetic catalysis
phenomenon which is observed in graphene is drawn. As well as demonstrating
that the phenomenon can exist in a strongly coupled system, this work makes a
number of predictions of symmetry breaking patterns and phase transitions for
such systems.Comment: 38 pages, 7 figures, references adde
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