99,368 research outputs found
Spin relaxation and decoherence of two-level systems
We revisit the concepts of spin relaxation and spin decoherence of two level
(spin-1/2) systems. From two toy-models, we clarify two issues related to the
spin relaxation and decoherence: 1) For an ensemble of two-level particles each
subjected to a different environmental field, there exists an ensemble
relaxation time which is fundamentally different from . When the
off-diagonal coupling of each particle is in a single mode with the same
frequency but a random coupling strength, we show that is finite while
the spin relaxation time of a single spin and the usual ensemble
decoherence time are infinite. 2) For a two-level particle under only a
random diagonal coupling, its relaxation time shall be infinite but its
decoherence time is finite.Comment: 5 pages, 2 figure
Probing and modelling the localized self-mixing in a GaN/AlGaN field-effect terahertz detector
In a GaN/AlGaN field-effect terahertz detector, the directional photocurrent
is mapped in the two-dimensional space of the gate voltage and the drain/source
bias. It is found that not only the magnitude, but also the polarity, of the
photocurrent can be tuned. A quasistatic self-mixing model taking into account
the localized terahertz field provides a quantitative description of the
detector characteristics. Strongly localized self-mixing is confirmed. It is
therefore important to engineer the spatial distribution of the terahertz field
and its coupling to the field-effect channel on the sub-micron scale.Comment: 12 pages, 4 figures, submitted to AP
Pitfalls in the analysis of low-temperature thermal conductivity of high-Tc cuprates
Recently, it was proposed that phonons are specularly reflected below about
0.5 K in ordinary single-crystal samples of high-T_c cuprates, and that the
low-temperature thermal conductivity should be analyzed by fitting the data up
to 0.5 K using an arbitrary power law. Such an analysis yields a result
different from that obtained from the conventional analysis, in which the
fitting is usually restricted to a region below 0.15 K. Here we show that the
proposed new analysis is most likely flawed, because the specular phonon
reflection means that the phonon mean free path \ell gets LONGER than the mean
sample width, while the estimated \ell is actually much SHORTER than the mean
sample width above 0.15 K.Comment: 4 pages, 1 figure; manuscript for the Proceedings of LEHTSC2007 to be
published in Journal of Physics: Conference Serie
Phantom Accretion by Five Dimensional Charged Black Hole
This paper deals with the dynamical behavior of phantom field near five
dimensional charged black hole. We formulate equations of motion for
steady-state spherically symmetric flow of phantom fluids. It is found that
phantom energy accretes onto black holes for . Further, the location of
critical point of accretion are evaluated that leads to mass to charge ratio
for 5D charged black hole. This ratio implies that accretion cannot transform a
black hole into a naked singularity. We would like to mention here that this
work is an irreducible extension of 4D charged black hole.Comment: 8 pages, accepted for publication in Mod. Phys. Lett.
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Life cycle assessment of white roof and sedum-tray garden roof for office buildings in China
White roof (WR) and Sedum lineare tray garden roof (STGR) have been convinced to improve the energy-efficiency and provide various benefits for conventional impervious grey roofs. Some national and local standards have standardized and recommended these technologies in existing building retrofits, however, they do not include assessment and choice of a particular roof retrofit in different climates. This paper presents a 40-year life-cycle cost analysis (LCCA) of an office building roof retrofitted by adding either WR or STGR over an existing grey roof in five cities, located in four Chinese climate zones. The LCCA find that the WR retrofits exhibit positive life-cycle net savings (NS) in warm winter zones, ranging 5.7–35.1 CNY/m 2 , and STGR retrofits have negative NS of -81.3– -16.7 CNY/m 2 in all climate zones. The NS of both WR and STGR generally tend to improve as one moves from the coldest cities to the warmest cities. LCCA results suggest that adding new building codes concerning crediting or prescribing WR and STGR retrofits into office buildings with grey roofs in hot summer climate zones and warm winter zone in China, respectively. And featured by more specific requirements, the localized Technical Norms help promote the implementation of new building codes
The Suppression and Recovery of the Ferroelectric Phase in Multiferroic
We report the discovery of a complete suppression of ferroelectricity in
by 10 % iron substitution and its restoration in external magnetic
fields. The spontaneous polarization in arises below 12
K in external fields above 4 T. The magnetic/ferroelectric phase diagram is
constructed from the anomalies of the dielectric constant, polarization,
magnetization, and heat capacity. The observations are qualitatively described
by a mean field model with competing interactions and strong anisotropy. We
propose that the magnetic field induces a non-collinear inversion symmetry
breaking magnetic structure in
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