1,956 research outputs found
Quantum Creation of the Randall-Sundrum Bubble
We investigate the semiclassical instability of the Randall-Sundrum brane
world. We carefully analyze the bubble solution with the Randall-Sundrum
background, which expresses the decay of the brane world. We evaluate the decay
probability following the Euclidean path integral approach to quantum gravity.
Since a bubble rapidly expands after the nucleation, the entire spacetime will
be occupied by such bubbles.Comment: 13 pages, 6 figures, To appear in Prog. Theor. Phy
Plus-minus construction leads to perfect invisibility
Recent theoretical advances applied to metamaterials have opened new avenues
to design a coating that hides objects from electromagnetic radiation and even
the sight. Here, we propose a new design of cloaking devices that creates
perfect invisibility in isotropic media. A combination of positive and negative
refractive indices, called plus-minus construction, is essential to achieve
perfect invisibility (i.e., no time delay and total absence of reflection).
Contrary to the common understanding that between two isotropic materials
having different refractive indices the electromagnetic reflection is
unavoidable, our method shows that surprisingly the reflection phenomena can be
completely eliminated. The invented method, different from the classical
impedance matching, may also find electromagnetic applications outside of
cloaking devices, wherever distortions are present arising from reflections.Comment: 24 pages, 10 figure
High-Temperature Transport Properties of Yb4−xSmxSb3
Polycrystalline L4Sb3 (L = La, Ce, Sm, and Yb) and Yb4−x Sm x Sb3, which crystallizes in the anti-Th3P4 structure type (I-43d no. 220), were synthesized via high-temperature reaction. Structural and chemical characterization were performed by x-ray diffraction and electronic microscopy with energy-dispersive x-ray analysis. Pucks were densified by spark plasma sintering. Transport property measurements showed that these compounds are n-type with low Seebeck coefficients, except for Yb4Sb3, which shows semimetallic behavior with hole conduction above 523 K. By partially substituting Yb by a trivalent rare earth we successfully improved the thermoelectric figure of merit of Yb4Sb3 up to 0.7 at 1273 K
Investigation of environmental change pattern in Japan
The author has identified the following significant results. A detailed land use classification for a large urban area of Tokyo was made using MSS digital data. It was found that residential, commercial, industrial, and wooded areas and grasslands can be successfully classified. A mesoscale vortex associated with large ocean current, Kuroshio, which is a rare phenomenon, was recognized visually through the analysis of MSS data. It was found that this vortex affects the effluent patterns of rivers. Lava flowing from Sakurajima Volcano was clearly classified for three major erruptions (1779, 1914, and 1946) using MSS data
Photonic analog of graphene model and its extension -- Dirac cone, symmetry, and edge states --
This paper presents a theoretical analysis on bulk and edge states in
honeycomb lattice photonic crystals with and without time-reversal and/or
space-inversion symmetries. Multiple Dirac cones are found in the photonic band
structure and the mass gaps are controllable via symmetry breaking. The zigzag
and armchair edges of the photonic crystals can support novel edge states that
reflect the symmetries of the photonic crystals. The dispersion relation and
the field configuration of the edge states are analyzed in detail in comparison
to electronic edge states. Leakage of the edge states to free space is inherent
in photonic systems and is fully taken into account in the analysis. A
topological relation between bulk and edge, which is analogous to that found in
quantum Hall systems, is also verified.Comment: 9 pages, 7 figure
Influence of Magnetic Moment Formation on the Conductance of Coupled Quantum Wires
In this report, we develop a model for the resonant interaction between a
pair of coupled quantum wires, under conditions where self-consistent effects
lead to the formation of a local magnetic moment in one of the wires. Our
analysis is motivated by the experimental results of Morimoto et al. [Appl.
Phys. Lett. \bf{82}, 3952 (2003)], who showed that the conductance of one of
the quantum wires exhibits a resonant peak at low temperatures, whenever the
other wire is swept into the regime where local-moment formation is expected.
In order to account for these observations, we develop a theoretical model for
the inter-wire interaction that calculated the transmission properties of one
(the fixed) wire when the device potential is modified by the presence of an
extra scattering term, arising from the presence of the local moment in the
swept wire. To determine the transmission coefficients in this system, we
derive equations describing the dynamics of electrons in the swept and fixed
wires of the coupled-wire geometry. Our analysis clearly shows that the
observation of a resonant peak in the conductance of the fixed wire is
correlated to the appearance of additional structure (near or
) in the conductance of the swept wire, in agreement with the
experimental results of Morimoto et al
EVM and Achievable Data Rate Analysis of Clipped OFDM Signals in Visible Light Communication
Orthogonal frequency division multiplexing (OFDM) has been considered for
visible light communication (VLC) thanks to its ability to boost data rates as
well as its robustness against frequency-selective fading channels. A major
disadvantage of OFDM is the large dynamic range of its time-domain waveforms,
making OFDM vulnerable to nonlinearity of light emitting diodes (LEDs). DC
biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM
(ACO-OFDM) are two popular OFDM techniques developed for the VLC. In this
paper, we will analyze the performance of the DCO-OFDM and ACO-OFDM signals in
terms of error vector magnitude (EVM), signal-to-distortion ratio (SDR), and
achievable data rates under both average optical power and dynamic optical
power constraints. EVM is a commonly used metric to characterize distortions.
We will describe an approach to numerically calculate the EVM for DCO-OFDM and
ACO-OFDM. We will derive the optimum biasing ratio in the sense of minimizing
EVM for DCO-OFDM. Additionally, we will formulate the EVM minimization problem
as a convex linear optimization problem and obtain an EVM lower bound against
which to compare the DCO-OFDM and ACO-OFDM techniques. We will prove that the
ACO-OFDM can achieve the lower bound. Average optical power and dynamic optical
power are two main constraints in VLC. We will derive the achievable data rates
under these two constraints for both additive white Gaussian noise (AWGN)
channel and frequency-selective channel. We will compare the performance of
DCO-OFDM and ACO-OFDM under different power constraint scenarios
Overview of Hybrid Ventilation Control System and Full Scale Monitoring
Mix-mode or hybrid ventilation systems have been previously shown to reduce cooling and ventilation energy consumption. This paper presents a case study of a new ten story 83,700 ft² (7,780 m²) office building in downtown Tokyo with a hybrid ventilation system that uses only 1.7% of the building footprint for ventilation shafts. The control system design is presented as an example of balancing the comfort expectations of multiple tenants in a mix-mode system. On-site measurements are presented from a three week commissioning project to show: modest temperature differences within the occupied zone with a maximum difference of 1.5 °F (0.83 °C), large differences of up to 6.1 °F (3.4 °C) between the measured outdoor temperature in the control system and the actual inlet temperature for the natural ventilation system, the importance of smooth integration of various design teams, and the use of low-power fans, 1.2-0.60 hp (900-450 W), that leverage the low pressure drop through the building when natural driving forces are insufficient for pure natural ventilation. Practical lessons learned from the design and commissioning of the building are also shared.Hulic Co., Ltd
Duality Relation among Periodic Potential Problems in the Lowest Landau Level
Using a momentum representation of a magnetic von Neumann lattice, we study a
two-dimensional electron in a uniform magnetic field and obtain one-particle
spectra of various periodic short-range potential problems in the lowest Landau
level.We find that the energy spectra satisfy a duality relation between a
period of the potential and a magnetic length. The energy spectra consist of
the Hofstadter-type bands and flat bands. We also study the connection between
a periodic short-range potential problem and a tight-binding model.Comment: 6 pages, 3 figures, final version to appear in PR
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