2,569 research outputs found
Interaction-induced charge and spin pumping through a quantum dot at finite bias
We investigate charge and spin transport through an adiabatically driven,
strongly interacting quantum dot weakly coupled to two metallic contacts with
finite bias voltage. Within a kinetic equation approach, we identify
coefficients of response to the time-dependent external driving and relate
these to the concepts of charge and spin emissivities previously discussed
within the time-dependent scattering matrix approach. Expressed in terms of
auxiliary vector fields, the response coefficients allow for a straightforward
analysis of recently predicted interaction-induced pumping under periodic
modulation of the gate and bias voltage [Phys. Rev. Lett. 104, 226803 (2010)].
We perform a detailed study of this effect and the related adiabatic Coulomb
blockade spectroscopy, and, in particular, extend it to spin pumping. Analytic
formulas for the pumped charge and spin in the regimes of small and large
driving amplitude are provided for arbitrary bias. In the absence of a magnetic
field, we obtain a striking, simple relation between the pumped charge at zero
bias and at bias equal to the Coulomb charging energy. At finite magnetic
field, there is a possibility to have interaction-induced pure spin pumping at
this finite bias value, and generally, additional features appear in the pumped
charge. For large-amplitude adiabatic driving, the magnitude of both the pumped
charge and spin at the various resonances saturate at values which are
independent of the specific shape of the pumping cycle. Each of these values
provide an independent, quantitative measurement of the junction asymmetry.Comment: 17 pages, 8 figure
The subdwarf B star SB 290 - A fast rotator on the extreme horizontal branch
Hot subdwarf B stars (sdBs) are evolved core helium-burning stars with very
thin hydrogen envelopes. In order to form an sdB, the progenitor has to lose
almost all of its hydrogen envelope right at the tip of the red giant branch.
In close binary systems, mass transfer to the companion provides the
extraordinary mass loss required for their formation. However, apparently
single sdBs exist as well and their formation is unclear since decades. The
merger of helium white dwarfs leading to an ignition of core helium-burning or
the merger of a helium core and a low mass star during the common envelope
phase have been proposed. Here we report the discovery of SB 290 as the first
apparently single fast rotating sdB star located on the extreme horizontal
branch indicating that those stars may form from mergers.Comment: 5 pages, 4 figures, A&A letters, accepte
We are not raised by wolves: Decentering human exceptionalism in nature
Some of the earliest writings to which we have access introduce the myth of a human child raised by wolves. Enkidu is the âwildâ friend of Gilgamesh in the eponymous Sumerian epic; Romulus and Remus of Rome are the infants who suckle from the she-wolf Lupa; and Mowgli's story has been told ever since he was conceived by Rudyard Kipling in The Jungle Book. While this wolf story might seem to imagine a friendly way of living with other-than-human beings, its contemporary uptake in media also serves as a prop for white supremacist orientations to the myth that reassert the primacy of âhumanâ life, while always determining who counts as human. Nature, on this stage, is a savage, dangerous backdrop against which human cruelties and violence are portrayed as the âsurvival of the fittest.
Two candidate brown dwarf companions around core helium-burning stars
Hot subdwarf stars of spectral type B (sdBs) are evolved, core helium-burning
objects. The formation of those objects is puzzling, because the progenitor
star has to lose almost its entire hydrogen envelope in the red-giant phase.
Binary interactions have been invoked, but single sdBs exist as well. We report
the discovery of two close hot subdwarf binaries with small radial velocity
amplitudes. Follow-up photometry revealed reflection effects originating from
cool irradiated companions, but no eclipses. The lower mass limits for the
companions of CPD-64481 () and PHL\,457
() are significantly below the stellar mass limit. Hence
they could be brown dwarfs unless the inclination is unfavourable. Two very
similar systems have already been reported. The probability that none of them
is a brown dwarf is very small, 0.02%. Hence we provide further evidence that
substellar companions with masses that low are able to eject a common envelope
and form an sdB star. Furthermore, we find that the properties of the observed
sample of hot subdwarfs in reflection effect binaries is consistent with a
scenario where single sdBs can still be formed via common envelope events, but
their low-mass substellar companions do not survive.Comment: accepted to A&
Chiral superconductivity with enhanced quantized Hall responses in moiré transition metal dichalcogenides
Experimental demonstrations of tunable correlation effects in magic-angle twisted bilayer graphene have put two-dimensional moiré quantum materials at the forefront of condensed-matter research. Other twisted few-layer graphitic structures, boron-nitride, and homo- or hetero-stacks of transition metal dichalcogenides (TMDs) have further enriched the opportunities for analysis and utilization of correlations in these systems. Recent experiments within the latter material class confirmed the relevance of many-body interactions and demonstrated the importance of their extended range. Since the interaction, its range, and the filling can be tuned experimentally by twist angle, substrate engineering and gating, we here explore Fermi surface instabilities and resulting phases of matter of hetero-bilayer TMDs. Using an unbiased renormalization group approach, we establish in particular that hetero-bilayer TMDs are platforms to realize topological superconductivity with winding number |N|=4. We show that this state reflects in pronounced experimental signatures, such as distinct quantum Hall features
Coherent theta oscillations in the cerebellum and supplementary motor area mediate visuomotor adaptation
The cerebellum and its interaction with cortical areas play a key role in our ability to flexibly adapt a motor program in response to sensory input. Current knowledge about specific neural mechanisms underlying the process of visuomotor adaptation is however lacking. Using a novel placement of EEG electrodes to record electric activity from the cerebellum, we studied local cerebellar activity, as well as its coupling with neocortical activity to obtain direct neurophysiological markers of visuomotor adaptation in humans. We found increased theta (4-8 Hz) power in "cerebellar" as well as cortical electrodes, when subjects first encountered a visual manipulation. Theta power decreased as subjects adapted to the perturbation, and rebounded when the manipulation was suddenly removed. This effect was observed in two distinct locations: a cerebellar cluster and a central cluster, which were localized in left cerebellar crus I (lCB) and right supplementary motor area (rSMA) using linear constrained minimum variance beamforming. Importantly, we found that better adaptation was associated with increased theta power in left cerebellar electrodes and a right sensorimotor cortex electrode. Finally, increased rSMA -> lCB connectivity was significantly decreased with adaptation. These results demonstrate that: (1) cerebellar theta power is markedly modulated over the course of visuomotor adaptation and (2) theta oscillations could serve as a key mechanism for communication within a cortico-cerebellar loop
Competition of Density Waves and Superconductivity in Twisted Tungsten Diselenide
Evidence for correlated insulating and superconducting phases around regions of high density of states was reported in the strongly spin-orbit coupled van-der Waals material twisted tungsten diselenide (tWSe2). We investigate their origin and interplay by using a functional renormalization group approach that allows to describe superconducting and spin/charge instabilities in an unbiased way. We map out the phase diagram as function of filling and perpendicular electric field, and find that the moiré Hubbard model for tWSe2 features mixed-parity superconducting order parameters with s/f-wave and topological d/p-wave symmetry next to (incommensurate) density wave states. Our work systematically characterizes competing interaction-driven phases in tWSe2 beyond mean-field approximations and provides guidance for experimental measurements by outlining the fingerprint of correlated states in interacting susceptibilities
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