8,426 research outputs found
Semiclassical transport in nearly symmetric quantum dots II: symmetry-breaking due to asymmetric leads
In this work - the second of a pair of articles - we consider transport
through spatially symmetric quantum dots with leads whose widths or positions
do not obey the spatial symmetry. We use the semiclassical theory of transport
to find the symmetry-induced contributions to weak localization corrections and
universal conductance fluctuations for dots with left-right, up-down, inversion
and four-fold symmetries. We show that all these contributions are suppressed
by asymmetric leads, however they remain finite whenever leads intersect with
their images under the symmetry operation. For an up-down symmetric dot, this
means that the contributions can be finite even if one of the leads is
completely asymmetric. We find that the suppression of the contributions to
universal conductance fluctuations is the square of the suppression of
contributions to weak localization. Finally, we develop a random-matrix theory
model which enables us to numerically confirm these results.Comment: (18pages - 9figures) This is the second of a pair of articles (v3
typos corrected - including in equations
Does GD 356 have a Terrestrial Planetary Companion?
GD 356 is unique among magnetic white dwarfs because it shows Zeeman-split
Balmer lines in pure emission. The lines originate from a region of nearly
uniform field strength (delta B/B is approximately 0.1) that covers 10 per cent
of the stellar surface in which there is a temperature inversion. The energy
source that heats the photosphere remains a mystery but it is likely to be
associated with the presence of a companion. Based on current models we use
archival Spitzer IRAC observations to place a new and stringent upper limit of
12 Jupiter masses for the mass of such a companion. In the light of this result
and the recent discovery of a 115 min photometric period for GD 356, we exclude
previous models that invoke accretion and revisit the unipolar inductor model
that has been proposed for this system. In this model a highly conducting
planet with a metallic core orbits the magnetic white dwarf and, as it cuts
through field lines, a current is set flowing between the two bodies. This
current dissipates in the photosphere of the white dwarf and causes a
temperature inversion. Such a planet is unlikely to have survived the RGB/AGB
phases of evolution so we argue that it may have formed from the circumstellar
disc of a disrupted He or CO core during a rare merger of two white dwarfs. GD
356 would then be a white dwarf counterpart of the millisecond binary pulsar
PSR 1257+12 which is known to host a planetary system.Comment: 9 pages, 4 figures, accepted by MNRA
How to Knit Your Own Markov Blanket
Hohwy (Hohwy 2016, Hohwy 2017) argues there is a tension between the free energy principle and leading depictions of mind as embodied, enactive, and extended (so-called ‘EEE1 cognition’). The tension is traced to the importance, in free energy formulations, of a conception of mind and agency that depends upon the presence of a ‘Markov blanket’ demarcating the agent from the surrounding world. In what follows I show that the Markov blanket considerations do not, in fact, lead to the kinds of tension that Hohwy depicts. On the contrary, they actively favour the EEE story. This is because the Markov property, as exemplified in biological agents, picks out neither a unique nor a stationary boundary. It is this multiplicity and mutability– rather than the absence of agent-environment boundaries as such - that EEE cognition celebrates
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