1,534 research outputs found

### Gauge fields, quantized fluxes and monopole confinement of the honeycomb lattice

Electron hopping models on the honeycomb lattice are studied. The lattice
consists of two triangular sublattices, and it is non-Bravais. The dual space
has non-trivial topology. The gauge fields of Bloch electrons have the U(1)
symmetry and thus represent superconducting states in the dual space. Two
quantized Abrikosov fluxes exist at the Dirac points and have fluxes $2pi$ and
$-2pi$, respectively. We define the non-Abelian SO(3) gauge theory in the
extended 3$d$ dual space and it is shown that a monopole and anti-monoplole
solution is stable. The SO(3) gauge group is broken down to U(1) at the 2$d$
boundary.The Abrikosov fluxes are related to quantized Hall conductance by the
topological expression. Based on this, monopole confinement and deconfinement
are discussed in relation to time reversal symmetry and QHE.
The Jahn-Teller effect is briefly discussed.Comment: 10 pages, 11 figure

### Variational wave functions of a vortex in cyclotron motion

In two dimensions the microscopic theory, which provides a basis for the
naive analogy between a quantized vortex in a superfluid and an electron in an
uniform magnetic field, is presented. A one-to-one correspondence between the
rotational states of a vortex in a cylinder and the cyclotron states of an
electron in the central gauge is found. Like the Landau levels of an electron,
the energy levels of a vortex are highly degenerate. However, the gap between
two adjacent energy levels does not only depend on the quantized circulation,
but also increases with the energy, and scales with the size of the vortex.Comment: LaTeX, 4 pages, 2 EPS figures, To appear in ``Series on Advances in
Quantum Many-Body Theory'' ed. by R.F. Bishop, C.E. Campbell, J.W. Clark and
S. Fantoni (World Scientific, 2000

### Crystalline ground states for classical particles

Pair interactions whose Fourier transform is nonnegative and vanishes above a
wave number K_0 are shown to give rise to periodic and aperiodic infinite
volume ground state configurations (GSCs) in any dimension d. A typical three
dimensional example is an interaction of asymptotic form cos(K_0 r)/r^4. The
result is obtained for densities rho >= rho_d where rho_1=K_0/2pi,
rho_2=(sqrt{3}/8)(K_0/pi)^2 and rho_3=(1/8sqrt{2})(K_0/pi)^3. At rho_d there is
a unique periodic GSC which is the uniform chain, the triangular lattice and
the bcc lattice for d=1,2,3, respectively. For rho>rho_d the GSC is nonunique
and the degeneracy is continuous: Any periodic configuration of density rho
with all reciprocal lattice vectors not smaller than K_0, and any union of such
configurations, is a GSC. The fcc lattice is a GSC only for rho>=(1/6
sqrt{3})(K_0/pi)^3.Comment: final versio

### Resonant atom-field interaction in large-size coupled-cavity arrays

We consider an array of coupled cavities with staggered inter-cavity
couplings, where each cavity mode interacts with an atom. In contrast to
large-size arrays with uniform-hopping rates where the atomic dynamics is known
to be frozen in the strong-hopping regime, we show that resonant atom-field
dynamics with significant energy exchange can occur in the case of staggered
hopping rates even in the thermodynamic limit. This effect arises from the
joint emergence of an energy gap in the free photonic dispersion relation and a
discrete frequency at the gap's center. The latter corresponds to a bound
normal mode stemming solely from the finiteness of the array length. Depending
on which cavity is excited, either the atomic dynamics is frozen or a
Jaynes-Cummings-like energy exchange is triggered between the bound photonic
mode and its atomic analogue. As these phenomena are effective with any number
of cavities, they are prone to be experimentally observed even in small-size
arrays.Comment: 12 pages, 4 figures. Added 5 mathematical appendice

### Particle-hole symmetry breaking in the pseudogap state of Pb0.55Bi1.5Sr1.6La0.4CuO6+d: A quantum-chemical perspective

Two Bi2201 model systems are employed to demonstrate how, beside the Cu-O
\sigma-band, a second band of purely O2p\pi character can be made to cross the
Fermi level owing to its sensitivity to the local crystal field. This result is
employed to explain the particle-hole symmetry breaking across the pseudo-gap
recently reported by Shen and co-workers, see M. Hashimoto et al., Nature
Physics 6, (2010) 414. Support for a two-bands-on-a-checkerboard candidate
mechanism for High-Tc superconductivity is claimed.Comment: 25 pages, 8 figure

### Magnetoconductance of carbon nanotube p-n junctions

The magnetoconductance of p-n junctions formed in clean single wall carbon
nanotubes is studied in the noninteracting electron approximation and
perturbatively in electron-electron interaction, in the geometry where a
magnetic field is along the tube axis. For long junctions the low temperature
magnetoconductance is anomalously large: the relative change in the conductance
becomes of order unity even when the flux through the tube is much smaller than
the flux quantum. The magnetoconductance is negative for metallic tubes. For
semiconducting and small gap tubes the magnetoconductance is nonmonotonic;
positive at small and negative at large fields.Comment: 5 pages, 2 figure

### Holstein model and Peierls instability in 1D boson-fermion lattice gases

We study an ultracold bose-fermi mixture in a one dimensional optical
lattice. When boson atoms are heavier then fermion atoms the system is
described by an adiabatic Holstein model, exhibiting a Peierls instability for
commensurate fermion filling factors. A Bosonic density wave with a wavenumber
of twice the Fermi wavenumber will appear in the quasi one-dimensional system.Comment: 5 pages, 4 figure

### Possible Lattice Distortions in the Hubbard Model for Graphene

The Hubbard model on the honeycomb lattice is a well known model for
graphene. Equally well known is the Peierls type of instability of the lattice
bond lengths. In the context of these two approximations we ask and answer the
question of the possible lattice distortions for graphene in zero magnetic
field. The answer is that in the thermodynamic limit only periodic,
reflection-symmetric distortions are allowed and these have at most six atoms
per unit cell as compared to two atoms for the undistorted lattice.Comment: 5 pages, 3 figure

### Spontaneous parity breaking of graphene in the quantum Hall regime

We propose that the inversion symmetry of the graphene honeycomb lattice is
spontaneously broken via a magnetic field dependent Peierls distortion. This
leads to valley splitting of the $n=0$ Landau level but not of the other Landau
levels. Compared to quantum Hall valley ferromagnetism recently discussed in
the literature, lattice distortion provides an alternative explanation to all
the currently observed quantum Hall plateaus in graphene.Comment: 4 pages, to appear in Phys. Rev. Let

### Dark-field transmission electron microscopy and the Debye-Waller factor of graphene

Graphene's structure bears on both the material's electronic properties and
fundamental questions about long range order in two-dimensional crystals. We
present an analytic calculation of selected area electron diffraction from
multi-layer graphene and compare it with data from samples prepared by chemical
vapor deposition and mechanical exfoliation. A single layer scatters only 0.5%
of the incident electrons, so this kinematical calculation can be considered
reliable for five or fewer layers. Dark-field transmission electron micrographs
of multi-layer graphene illustrate how knowledge of the diffraction peak
intensities can be applied for rapid mapping of thickness, stacking, and grain
boundaries. The diffraction peak intensities also depend on the mean-square
displacement of atoms from their ideal lattice locations, which is
parameterized by a Debye-Waller factor. We measure the Debye-Waller factor of a
suspended monolayer of exfoliated graphene and find a result consistent with an
estimate based on the Debye model. For laboratory-scale graphene samples,
finite size effects are sufficient to stabilize the graphene lattice against
melting, indicating that ripples in the third dimension are not necessary.Comment: 10 pages, 4 figure

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