5,911 research outputs found
On the variation of leaf border in Rhizomnium (Cinclidiaceae)
In the genus Rhizomnium T. Kop. most taxa have a strong leaf border several cells broad and bi- to tristratose. Rhizomnium striatulum (Mitt.) T. Kop. belongs to that group. In Northeast China one population of R. striatulum was found with a very weak leaf border, and also the costa of these plants is weaker than is characteristic for the species. The deviating population is figured and its significance discussed. The distribution of R. striatulum is mapped
Robust one-dimensional wires in lattice mismatched bilayer graphene
We show that lattice mismatched bilayer graphene can realize robust
one-dimensional wires. By considering a single domain wall where the masses of
the Dirac electrons change their sign, we establish a general projection
principle. This determines how the existence of topological zero-energy domain
wall states depends on the direction of the domain wall and locations of the
massive Dirac cones inside the bulk Brillouin zone. We generalize this idea for
arbitrary patterns of domain walls, showing that the topologically protected
states exist only in the presence of an odd number of topological domain walls.Comment: 8 preprint pages, 3 figure
Zeeman field induced topological phase transitions in triplet superconductors
We develop a general Ginzburg-Landau theory which describes the effect of a
Zeeman field on the superconducting order parameter in triplet superconductors.
Starting from Ginzburg-Landau theories that describe fully gapped time-reversal
symmetric triplet superconductors, we show that the Zeeman field has dramatic
effects on the topological properties of the superconductors. In particular, in
the vicinity of a critical chemical potential separating two topologically
distinct phases, it is possible to induce a phase transition to a topologically
nontrivial phase which supports chiral edge modes. Moreover, for specific
directions of the Zeeman field, we obtain nodal superconducting phases with an
emerging chiral symmetry, and with Majorana flat bands at the edge. The
Ginzburg-Landau theory is microscopically supported by a self-consistent
mean-field theory of the doped Kitaev-Heisenberg model
Angular momentum of a strongly focussed Gaussian beam
A circularly polarized rotationally symmetric paraxial laser beams carries
hbar angular momentum per photon as spin. Focussing the beam with a
rotationally symmetric lens cannot change this angular momentum flux, yet the
focussed beam must have spin less than hbar per photon. The remainder of the
original spin is converted to orbital angular momentum, manifesting itself as a
longitudinal optical vortex at the focus. This demonstrates that optical
orbital angular momentum can be generated by a rotationally symmetric optical
system which preserves the total angular momentum of the beam.Comment: 4 pages, 3 figure
Competition between d-wave and topological p-wave superconductivity in the doped Kitaev-Heisenberg model
The competition between Kitaev and Heisenberg interactions away from half
filling is studied for the the hole-doped Kitaev-Heisenberg --
model on a honeycomb lattice. While the isotropic Heisenberg coupling supports
a time-reversal violating d-wave singlet state, we find that the Kitaev
interaction favors a time-reversal invariant p-wave superconducting phase,
which obeys the rotational symmetries of the microscopic model, and is robust
for . Within the p-wave superconducting phase, a critical chemical
potential separates a topologically trivial phase for from a topologically non-trivial time-reversal invariant
spin-triplet phase for .Comment: published version, 4.5 pages, 5 figure
Mordell-Weil Torsion and the Global Structure of Gauge Groups in F-theory
We study the global structure of the gauge group of F-theory compactified
on an elliptic fibration . The global properties of are encoded in the
torsion subgroup of the Mordell-Weil group of rational sections of .
Generalising the Shioda map to torsional sections we construct a specific
integer divisor class on as a fractional linear combination of the
resolution divisors associated with the Cartan subalgebra of . This divisor
class can be interpreted as an element of the refined coweight lattice of the
gauge group. As a result, the spectrum of admissible matter representations is
strongly constrained and the gauge group is non-simply connected. We exemplify
our results by a detailed analysis of the general elliptic fibration with
Mordell-Weil group and as well as a further
specialization to . Our analysis exploits the
representation of these fibrations as hypersurfaces in toric geometry.Comment: 42 pages, 10 figures; v2: references adde
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