111 research outputs found
On a generalization of distance sets
A subset in the -dimensional Euclidean space is called a -distance
set if there are exactly distinct distances between two distinct points in
and a subset is called a locally -distance set if for any point
in , there are at most distinct distances between and other points
in .
Delsarte, Goethals, and Seidel gave the Fisher type upper bound for the
cardinalities of -distance sets on a sphere in 1977. In the same way, we are
able to give the same bound for locally -distance sets on a sphere. In the
first part of this paper, we prove that if is a locally -distance set
attaining the Fisher type upper bound, then determining a weight function ,
is a tight weighted spherical -design. This result implies that
locally -distance sets attaining the Fisher type upper bound are
-distance sets. In the second part, we give a new absolute bound for the
cardinalities of -distance sets on a sphere. This upper bound is useful for
-distance sets for which the linear programming bound is not applicable. In
the third part, we discuss about locally two-distance sets in Euclidean spaces.
We give an upper bound for the cardinalities of locally two-distance sets in
Euclidean spaces. Moreover, we prove that the existence of a spherical
two-distance set in -space which attains the Fisher type upper bound is
equivalent to the existence of a locally two-distance set but not a
two-distance set in -space with more than points. We also
classify optimal (largest possible) locally two-distance sets for dimensions
less than eight. In addition, we determine the maximum cardinalities of locally
two-distance sets on a sphere for dimensions less than forty.Comment: 17 pages, 1 figur
Stochastic simulation of thermally assisted magnetization reversal in sub-100 nm dots with perpendicular anisotropy
Thermally assisted magnetization reversal of sub-100 nm dots with perpendicular anisotropy has been
investigated using a micromagnetic Langevin model. The performance of the two different reversal
modes of (i) a reduced barrier writing scheme and (ii) a Curie point writing scheme are compared. For
the reduced barrier writing scheme, the switching field Hswt decreases with an increase in writing
temperature but is still larger than that of the Curie point writing scheme. For the Curie point writing
scheme, the required threshold field Hth, evaluated from 50 simulation results, saturates at a value,
which is not simply related to the energy barrier height. The value of Hth increases with a decrease in
cooling time owing to the dynamic aspects of the magnetic ordering process. Dependence of Hth on
material parameters and dot sizes has been systematically studied
A Nuclear Magnetic Resonance Study on Rubrene-cobalt Nano-composites
We implemented a nuclear magnetic resonance (NMR) study on rubrene(C42H28)-Co
nano-composites that exhibit an enhanced magnetoresistance (MR) ratio of 80%.
The 59Co NMR spin echo experiment enabled clarification of the hyperfine field
of Co at the interface between the ferromagnet and the molecules, which has not
been investigated for molecular spintronics. An enhanced hyperfine field of the
Co was observed in the rubrene-Co nano-composites, which may be related to the
enhancement of the MR ratio. This study demonstrates the importance of
microscopic investigation of the interface between molecules and ferromagnets
that governs spin-dependent transport in molecular spin devices.Comment: 12pages, 3 figures, 1 tabl
Spin Injection into a Graphene Thin Film at Room Temperature
We demonstrate spin injection into a graphene thin film with high reliability
by using non-local magnetoresistance (MR) measurements, in which the electric
current path is completely separated from the spin current path. Using these
non-local measurements, an obvious MR effect was observed at room temperature;
and the MR effect was ascribed to magnetization reversal of ferromagnetic
electrodes. This result is a direct demonstration of spin injection into a
graphene thin film. Furthermore, this is the first report of spin injection
into molecules at room temperature.Comment: 12 pages, 3 figure
Ferroelectricity driven-resistive switching and Schottky barrier modulation at CoPt/MgZnO interface for non-volatile memories
Ferroelectric memristors have attracted much attention as a type of
nonvolatile resistance switching memories in neuromorphic computing, image
recognition, and information storage. Their resistance switching mechanisms
have been studied several times in perovskite and complicated materials
systems. It was interpreted as the modulation of carrier transport by
polarization control over Schottky barriers. Here, we experimentally report the
isothermal resistive switching across a CoPt/MgZnO Schottky barrier using a
simple binary semiconductor. The crystal and texture properties showed
high-quality and single-crystal CoPt/MgZnO
hetero-junctions. The resistive switching was examined by an electric-field
cooling method that exhibited a ferroelectric T of MgZnO close to the bulk
value. The resistive switching across CoPt/MgZnO Schottky barrier was
accompanied by a change in the Schottky barrier height of 26.5 meV due to an
interfacial charge increase and/or orbital hybridization induced reversal of
MgZnO polarization. The magnitude of the reversed polarization was estimated to
be a reasonable value of 3.0 (8.25) C/cm at 300 K (2 K). These
findings demonstrated the utilities of CoPt/MgZnO interface as a potential
candidate for ferroelectric memristors and can be extended to probe the
resistive switching of other hexagonal ferroelectric materials
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