6,273 research outputs found
Symmetry causes a huge conductance peak in double quantum dots
We predict a huge interference effect contributing to the conductance through
large ultra-clean quantum dots of chaotic shape. When a double-dot structure is
made such that the dots are the mirror-image of each other, constructive
interference can make a tunnel barrier located on the symmetry axis effectively
transparent. We show (via theoretical analysis and numerical simulation) that
this effect can be orders of magnitude larger than the well-known universal
conductance fluctuations and weak-localization (both less than a conductance
quantum). A small magnetic field destroys the effect, massively reducing the
double-dot conductance; thus a magnetic field detector is obtained, with a
similar sensitivity to a SQUID, but requiring no superconductors.Comment: 5pages 3 figures and an appendix ONLY in arXiv versio
Berry phase in a non-isolated system
We investigate the effect of the environment on a Berry phase measurement
involving a spin-half. We model the spin+environment using a biased spin-boson
Hamiltonian with a time-dependent magnetic field. We find that, contrary to
naive expectations, the Berry phase acquired by the spin can be observed, but
only on timescales which are neither too short nor very long. However this
Berry phase is not the same as for the isolated spin-half. It does not have a
simple geometric interpretation in terms of the adiabatic evolution of either
bare spin-states or the dressed spin-resonances that remain once we have traced
out the environment. This result is crucial for proposed Berry phase
measurements in superconducting nanocircuits as dissipation there is known to
be significant.Comment: 4 pages (revTeX4) 2 fig. This version has MAJOR changes to equation
Nowhere minimal CR submanifolds and Levi-flat hypersurfaces
A local uniqueness property of holomorphic functions on real-analytic nowhere
minimal CR submanifolds of higher codimension is investigated. A sufficient
condition called almost minimality is given and studied. A weaker necessary
condition, being contained a possibly singular real-analytic Levi-flat
hypersurface is studied and characterized. This question is completely resolved
for algebraic submanifolds of codimension 2 and a sufficient condition for
noncontainment is given for non algebraic submanifolds. As a consequence, an
example of a submanifold of codimension 2, not biholomorphically equivalent to
an algebraic one, is given. We also investigate the structure of singularities
of Levi-flat hypersurfaces.Comment: 21 pages; conjecture 2.8 was removed in proof; to appear in J. Geom.
Ana
Comment on "Inconsistency of the conventional theory of superconductivity" by J.E. Hirsch
J. E. Hirsch [EPL 130 (2020) 17006] claimed an inconsistency between
thermodynamics and the theory of superconductivity. We argue that he overlooked
a crucial term which determines the supercurrent dynamics and ensures energy
conservation by providing an internal energy source for the Joule heating.
Thermodynamic consistency is restored by restoring energy conservation. The
correct dynamics is given by Maxwell's equations in the superconductor.Comment: Comment (2 pages) and Supplementary Material (3 pages) in a single
file. Version 2 has corrected a confusing typo above Eq. (1
Herschel Observations of a Newly Discovered UX Ori Star in the Large Magellanic Cloud
The LMC star, SSTISAGE1C J050756.44-703453.9, was first noticed during a
survey of EROS-2 lightcurves for stars with large irregular brightness
variations typical of the R Coronae Borealis (RCB) class. However, the visible
spectrum showing emission lines including the Balmer and Paschen series as well
as many Fe II lines is emphatically not that of an RCB star. This star has all
of the characteristics of a typical UX Ori star. It has a spectral type of
approximately A2 and has excited an H II region in its vicinity. However, if it
is an LMC member, then it is very luminous for a Herbig Ae/Be star. It shows
irregular drops in brightness of up to 2 mag, and displays the reddening and
"blueing" typical of this class of stars. Its spectrum, showing a combination
of emission and absorption lines, is typical of a UX Ori star that is in a
decline caused by obscuration from the circumstellar dust. SSTISAGE1C
J050756.44-703453.9 has a strong IR excess and significant emission is present
out to 500 micron. Monte Carlo radiative transfer modeling of the SED requires
that SSTISAGE1C J050756.44-703453.9 has both a dusty disk as well as a large
extended diffuse envelope to fit both the mid- and far-IR dust emission. This
star is a new member of the UX Ori subclass of the Herbig Ae/Be stars and only
the second such star to be discovered in the LMC.Comment: ApJ, in press. 9 pages, 5 figure
Semiclassical Theory of Quantum Chaotic Transport: Phase-Space Splitting, Coherent Backscattering and Weak Localization
We investigate transport properties of quantized chaotic systems in the short
wavelength limit. We focus on non-coherent quantities such as the Drude
conductance, its sample-to-sample fluctuations, shot-noise and the transmission
spectrum, as well as coherent effects such as weak localization. We show how
these properties are influenced by the emergence of the Ehrenfest time scale
\tE. Expressed in an optimal phase-space basis, the scattering matrix
acquires a block-diagonal form as \tE increases, reflecting the splitting of
the system into two cavities in parallel, a classical deterministic cavity
(with all transmission eigenvalues either 0 or 1) and a quantum mechanical
stochastic cavity. This results in the suppression of the Fano factor for
shot-noise and the deviation of sample-to-sample conductance fluctuations from
their universal value. We further present a semiclassical theory for weak
localization which captures non-ergodic phase-space structures and preserves
the unitarity of the theory. Contrarily to our previous claim [Phys. Rev. Lett.
94, 116801 (2005)], we find that the leading off-diagonal contribution to the
conductance leads to the exponential suppression of the coherent backscattering
peak and of weak localization at finite \tE. This latter finding is
substantiated by numerical magnetoconductance calculations.Comment: Typos in central eqns corrected (this paper supersedes
cond-mat/0509186) 20page
Employable knowledge: benchmarking education about standardization in the UK
For academics and students in the United Kingdom the main source of standards is via British Standards Institution (BSI). The research demonstrates that British, European and International Standards play a key role in many areas of education. In some disciplines their inclusion in the course is mandatory, e.g. in building construction and performing risk assessments of equipment. Where not a requirement, other courses successfully encourage students to understand and apply specific standards and principles from them, to their design and project work, in topic areas such as quality management and user-computer interface design. Assessment practice is a key part of learning and academics have indicated how this fitted into the learning activity, e.g. by expecting students to develop an understanding of standards and reference them in all assessed work.
Likewise, students taking part in the survey also suggest that they were actively engaged with one or more standards and that their understanding was measured through an assessed activity. However, students also emphasised the importance of additional support, e.g. introduction to standards in the workplace or as part of work placements, by library staff as an information resource, and by presentations from BSI experts.
Employers have also indicated the importance of students’ knowledge, understanding and appreciation of relevant standards in the right context, emphasising their desire for standards to be included more widely in the curriculum.
The draft recommendations from this study were subject to International review and comment, the results from this review served to strengthen the recommendations of this work.
This research was commissioned by British Standards Institution (BSI).
This research was produced in association with the Department for Business, Innovation and Skills as part of its ongoing programme of support for standardization.
Grateful thanks are due to all those who filled in the questionnaire or who attended the workshops
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