4,586 research outputs found
Classification of extremal elliptic K3 surfaces and fundamental groups of open K3 surfaces
We present a complete list of extremal elliptic K3 surfaces. There are
altogether 325 of them. The first 112 coincides with Miranda-Persson's list for
semi-stable ones. The data include the transcendental lattice which determines
uniquely the K3 surface by a result of Shioda and Inose, the singular fibre
type and the Mordell Weil group. As an application, we give a sufficient
condition for the topological fundamental group of complement to an
ADE-configuration of smooth rational curves on a K3 surface to be trivial.Comment: 25 pages, Nagoya Math. J. to appea
Computing Lyapunov spectra with continuous Gram-Schmidt orthonormalization
We present a straightforward and reliable continuous method for computing the
full or a partial Lyapunov spectrum associated with a dynamical system
specified by a set of differential equations. We do this by introducing a
stability parameter beta>0 and augmenting the dynamical system with an
orthonormal k-dimensional frame and a Lyapunov vector such that the frame is
continuously Gram-Schmidt orthonormalized and at most linear growth of the
dynamical variables is involved. We prove that the method is strongly stable
when beta > -lambda_k where lambda_k is the k'th Lyapunov exponent in
descending order and we show through examples how the method is implemented. It
extends many previous results.Comment: 14 pages, 10 PS figures, ioplppt.sty, iopl12.sty, epsfig.sty 44 k
Time-reversal focusing of an expanding soliton gas in disordered replicas
We investigate the properties of time reversibility of a soliton gas,
originating from a dispersive regularization of a shock wave, as it propagates
in a strongly disordered environment. An original approach combining
information measures and spin glass theory shows that time reversal focusing
occurs for different replicas of the disorder in forward and backward
propagation, provided the disorder varies on a length scale much shorter than
the width of the soliton constituents. The analysis is performed by starting
from a new class of reflectionless potentials, which describe the most general
form of an expanding soliton gas of the defocusing nonlinear Schroedinger
equation.Comment: 7 Pages, 6 Figure
Orbital selectivity of the kink in the dispersion of Sr2RuO4
We present detailed energy dispersions near the Fermi level on the monolayer
perovskite ruthenate Sr2RuO4, determined by high-resolution angle-resolved
photoemission spectroscopy. An orbital selectivity of the kink in the
dispersion of Sr2RuO4 has been found: A kink for the Ru 4d_xy orbital is
clearly observed, but not for the Ru 4d_yz and 4d_zx ones. The result provides
insight into the origin of the kink.Comment: 5 pages, 4 figures. Accepted for publication in Phys. Rev.
MnAs dots grown on GaN(0001)-(1x1) surface
MnAs has been grown by means of MBE on the GaN(0001)-(1x1) surface. Two
options of initiating the crystal growth were applied: (a) a regular MBE
procedure (manganese and arsenic were delivered simultaneously) and (b)
subsequent deposition of manganese and arsenic layers. It was shown that
spontaneous formation of MnAs dots with the surface density of 1
cm and cm, respectively (as observed by AFM),
occurred for the layer thickness higher than 5 ML. Electronic structure of the
MnAs/GaN systems was studied by resonant photoemission spectroscopy. That led
to determination of the Mn 3d - related contribution to the total density of
states (DOS) distribution of MnAs. It has been proven that the electronic
structures of the MnAs dots grown by the two procedures differ markedly. One
corresponds to metallic, ferromagnetic NiAs-type MnAs, the other is similar to
that reported for half-metallic zinc-blende MnAs. Both system behave
superparamagnetically (as revealed by magnetization measurements), but with
both the blocking temperatures and the intra-dot Curie temperatures
substantially different. The intra-dot Curie temperature is about 260 K for the
former system while markedly higher than room temperature for the latter one.
Relations between growth process, electronic structure and other properties of
the studied systems are discussed. Possible mechanisms of half-metallic MnAs
formation on GaN are considered.Comment: 20+ pages, 8 figure
Negative differential conductance and magnetoresistance oscillations due to spin accumulation in ferromagnetic double-island devices
Spin-dependent electronic transport in magnetic double-island devices is
considered theoretically in the sequential tunneling regime. Electric current
and tunnel magnetoresistance are analyzed as a function of the bias voltage and
spin relaxation time in the islands. It is shown that the interplay of spin
accumulation on the islands and charging effects leads to periodic modification
of the differential conductance and tunnel magnetoresistance. For a
sufficiently long spin relaxation time, the modulations are associated with
periodic oscillations of the sign of both the tunnel magnetoresistance and
differential conductance
N=2 Gauge Theories: Congruence Subgroups, Coset Graphs and Modular Surfaces
We establish a correspondence between generalized quiver gauge theories in
four dimensions and congruence subgroups of the modular group, hinging upon the
trivalent graphs which arise in both. The gauge theories and the graphs are
enumerated and their numbers are compared. The correspondence is particularly
striking for genus zero torsion-free congruence subgroups as exemplified by
those which arise in Moonshine. We analyze in detail the case of index 24,
where modular elliptic K3 surfaces emerge: here, the elliptic j-invariants can
be recast as dessins d'enfant which dictate the Seiberg-Witten curves.Comment: 42+1 pages, 5 figures; various helpful comments incorporate
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