6,562 research outputs found
Influence of Mg, Ag and Al substitutions on the magnetic excitations in the triangular-lattice antiferromagnet CuCrO2
Magnetic excitations in CuCrO, CuCrMgO,
CuAgCrO, and CuCrAlO have been
studied by powder inelastic neutron scattering to elucidate the element
substitution effects on the spin dynamics in the Heisenberg triangular-lattice
antiferromagnet CuCrO. The magnetic excitations in
CuCrMgO consist of a dispersive component and a flat
component. Though this feature is apparently similar to CuCrO, the energy
structure of the excitation spectrum shows some difference from that in
CuCrO. On the other hand, in CuAgCrO and
CuCrAlO the flat components are much reduced, the
low-energy parts of the excitation spectra become intense, and additional
low-energy diffusive spin fluctuations are induced. We argued the origins of
these changes in the magnetic excitations are ascribed to effects of the doped
holes or change of the dimensionality in the magnetic correlations.Comment: 7 pages, 5 figure
Bubbling Calabi-Yau geometry from matrix models
We study bubbling geometry in topological string theory. Specifically, we
analyse Chern-Simons theory on both the 3-sphere and lens spaces in the
presence of a Wilson loop insertion of an arbitrary representation. For each of
these three manifolds we formulate a multi-matrix model whose partition
function is the vev of the Wilson loop and compute the spectral curve. This
spectral curve is the reduction to two dimensions of the mirror to a Calabi-Yau
threefold which is the gravitational dual of the Wilson loop insertion. For
lens spaces the dual geometries are new. We comment on a similar matrix model
which appears in the context of Wilson loops in AdS/CFT.Comment: 30 pages; v.2 reference added, minor correction
Performance Measurements of 8-Gsps 1-bit ADCs Developed for Wideband Radio Astronomical Observations
8-Gsps 1-bit Analog-to-Digital Converters (ADCs) were newly developed toward
the realization of the wideband observation. The development of the wideband
ADCs is one of the most essential developments for the radio interferometer. To
evaluate its performance in interferometric observations, the time (phase)
stability and frequency response were measured with a noise source and a signal
generator. The results of these measurements show that the developed ADCs can
achieve the jitter time less than 0.05 psec at the time interval of 1 sec, the
passband frequency response with the slope less than 0.73 dB/GHz and the ripple
less than 1.8 dB, and the aperture time less than 20 psec. The details of the
developed ADC design, the measurement methods, and the results of these
measurements are presented in this paper.Comment: Accepted for publication in PASJ. 19 pages, 15 figure
Why Is Supercritical Disk Accretion Feasible?
Although the occurrence of steady supercritical disk accretion onto a black
hole has been speculated about since the 1970s, it has not been accurately
verified so far. For the first time, we previously demonstrated it through
two-dimensional, long-term radiation-hydrodynamic simulations. To clarify why
this accretion is possible, we quantitatively investigate the dynamics of a
simulated supercritical accretion flow with a mass accretion rate of ~10^2
L_E/c^2 (with L_E and c being, respectively, the Eddington luminosity and the
speed of light). We confirm two important mechanisms underlying supercritical
disk accretion flow, as previously claimed, one of which is the radiation
anisotropy arising from the anisotropic density distribution of very optically
thick material. We qualitatively show that despite a very large radiation
energy density, E_0>10^2L_E/(4 pi r^2 c) (with r being the distance from the
black hole), the radiative flux F_0 cE_0/tau could be small due to a large
optical depth, typically tau 10^3, in the disk. Another mechanism is photon
trapping, quantified by vE_0, where v is the flow velocity. With a large |v|
and E_0, this term significantly reduces the radiative flux and even makes it
negative (inward) at r<70r_S, where r_S is the Schwarzschild radius. Due to the
combination of these effects, the radiative force in the direction along the
disk plane is largely attenuated so that the gravitational force barely exceeds
the sum of the radiative force and the centrifugal force. As a result, matter
can slowly fall onto the central black hole mainly along the disk plane with
velocity much less than the free-fall velocity, even though the disk luminosity
exceeds the Eddington luminosity. Along the disk rotation axis, in contrast,
the strong radiative force drives strong gas outflows.Comment: 8 pages, 7 figures, accepted for publication in Ap
Large transconductance oscillations in a single-well vertical Aharonov-Bohm interferometer
Aharonov-Bohm (AB) interference is reported for the first time in the
conductance of a vertical nanostructure based on a single GaAs/AlGaAs quantum
well (QW). The two lowest subbands of the well are spatially separated by the
Hartree barrier originating from electronic repulsion in the modulation-doped
QW and provide AB two-path geometry. Split-gates control the in-plane
electronic momentum dispersion. In our system, we have clearly demonstrated AB
interference in both electrostatic and magnetic modes. In the latter case the
magnetic field was applied parallel to the QW plane, and perpendicular to the
0.02 um^2 AB loop. In the electrostatic mode of operation the single-QW scheme
adopted led to large transconductance oscillations with relative amplitudes
exceeding 30 %. The relevance of the present design strategy for the
implementation of coherent nanoelectronic devices is underlined.Comment: Accepted for publication on Physical Review B Rapid Communication
Wilson Loops, Geometric Transitions and Bubbling Calabi-Yau's
Motivated by recent developments in the AdS/CFT correspondence, we provide
several alternative bulk descriptions of an arbitrary Wilson loop operator in
Chern-Simons theory. Wilson loop operators in Chern-Simons theory can be given
a description in terms of a configuration of branes or alternatively
anti-branes in the resolved conifold geometry. The representation of the Wilson
loop is encoded in the holonomy of the gauge field living on the dual brane
configuration. By letting the branes undergo a new type of geometric
transition, we argue that each Wilson loop operator can also be described by a
bubbling Calabi-Yau geometry, whose topology encodes the representation of the
Wilson loop. These Calabi-Yau manifolds provide a novel representation of knot
invariants. For the unknot we confirm these identifications to all orders in
the genus expansion.Comment: 26 pages; v.2 typos corrected, explanations clarified; v.3 typos
corrected, reference adde
D-branes as a Bubbling Calabi-Yau
We prove that the open topological string partition function on a D-brane
configuration in a Calabi-Yau manifold X takes the form of a closed topological
string partition function on a different Calabi-Yau manifold X_b. This
identification shows that the physics of D-branes in an arbitrary background X
of topological string theory can be described either by open+closed string
theory in X or by closed string theory in X_b. The physical interpretation of
the ''bubbling'' Calabi-Yau X_b is as the space obtained by letting the
D-branes in X undergo a geometric transition. This implies, in particular, that
the partition function of closed topological string theory on certain bubbling
Calabi-Yau manifolds are invariants of knots in the three-sphere.Comment: 32 pages; v.2 reference adde
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