42 research outputs found
Charge-4e supercurrent in an InAs-Al superconductor-semiconductor heterostructure
Superconducting qubits with intrinsic noise protection offer a promising
approach to improve the coherence of quantum information. Crucial to such
protected qubits is the encoding of the logical quantum states into
wavefunctions with disjoint support. Such encoding can be achieved by a
Josephson element with an unusual charge-4e supercurrent emerging from the
coherent transfer of pairs of Cooper-pairs. In this work, we demonstrate the
controlled conversion of a conventional charge-2e dominated to a charge-4e
dominated supercurrent in a superconducting quantum interference device (SQUID)
consisting of gate-tunable planar Josephson junctions (JJs). We investigate the
ac Josephson effect of the SQUID and measure a dominant photon emission at
twice the fundamental Josephson frequency together with a doubling of the
number of Shapiro steps, both consistent with the appearance of charge-4e
supercurrent. Our results present a step towards novel protected
superconducting qubits based on superconductor-semiconductor hybrid materials
Gate Tunable Josephson Diode in Proximitized InAs Supercurrent Interferometers
The Josephson diode (JD) is a non-reciprocal circuit element that supports a
larger critical current in one direction compared to the other. This effect has
gained a growing interest because of promising applications in superconducting
electronic circuits with low power consumption. Some implementations of a JD
rely on breaking the inversion symmetry in the material used to realize
Josephson junctions (JJs), but a recent theoretical proposal has suggested that
the effect can also be engineered by combining two JJs hosting highly
transmitting Andreev bound states in a Superconducting Quantum Interference
Device (SQUID) at a small, but finite flux bias [1]. We realized a SQUID with
two JJs fabricated in a proximitized InAs two-dimensional electron gas (2DEG).
We demonstrate gate control of the diode efficiency from zero up to around
\% for different flux biases which comes close to the maximum of \% predicated in Ref. [1]. The key ingredient to the JD effect in the SQUID
arrangement is the presence of an asymmetry between the two SQUID arms.Comment: 9+8 pages, 3+6 figures (main text + supplementary
Nonlocal conductance spectroscopy of Andreev bound states in gate-defined InAs/Al nanowires
The charge character of Andreev bound states (ABSs) in a three-terminal
semiconductor-superconductor hybrid nanowire was measured using local and
nonlocal tunneling spectroscopy. The device is fabricated using an epitaxial
InAs/Al two-dimensional heterostructure with several gate-defined side probes.
ABSs are found to oscillate around zero as a function of gate voltage, with
modifications of their charge consistent with theoretical expectations for the
total Bardeen-Cooper- Schrieffer (BCS) charge of ABSs
Link between supercurrent diode and anomalous Josephson effect revealed by gate-controlled interferometry
In Josephson diodes the asymmetry between positive and negative current
branch of the current-phase relation leads to a polarity-dependent critical
current and Josephson inductance. The supercurrent nonreciprocity can be
described as a consequence of the anomalous Josephson effect -- a
-shift of the current-phase relation -- in multichannel ballistic
junctions with strong spin-orbit interaction. In this work, we simultaneously
investigate -shift and supercurrent diode efficiency on the same
Josephson junction by means of a superconducting quantum interferometer. By
electrostatic gating, we reveal a direct link between -shift and
diode effect. Our findings show that the supercurrent diode effect mainly
results from magnetochiral anisotropy induced by spin-orbit interaction in
combination with a Zeeman field.Comment: 15 pages, 8 figure
Sign reversal of the AC and DC supercurrent diode effect and 0--like transitions in ballistic Josephson junctions
The recent discovery of intrinsic supercurrent diode effect, and its prompt
observation in a rich variety of systems, has shown that nonreciprocal
supercurrents naturally emerge when both space- and time-inversion symmetries
are broken. In Josephson junctions, nonreciprocal supercurrent can be
conveniently described in terms of spin-split Andreev states. Here, we
demonstrate a sign reversal of the supercurrent diode effect, in both its AC
and DC manifestations. In particular, the AC diode effect -- i.e., the
asymmetry of the Josephson inductance as a function of the supercurrent --
allows us to probe the current-phase relation near equilibrium. Using a minimal
theoretical model, we can then link the sign reversal of the AC diode effect to
the so-called 0--like transition, a predicted, but still elusive feature
of multi-channel junctions. Our results demonstrate the potential of inductance
measurements as sensitive probes of the fundamental properties of
unconventional Josephson junctions.Comment: 13 pages, 6 figure
Effects of dietary Na+ deprivation on epithelial Na+ channel (ENaC), BDNF, and TrkB mRNA expression in the rat tongue
<p>Abstract</p> <p>Background</p> <p>In rodents, dietary Na<sup>+ </sup>deprivation reduces gustatory responses of primary taste fibers and central taste neurons to lingual Na<sup>+ </sup>stimulation. However, in the rat taste bud cells Na<sup>+ </sup>deprivation increases the number of amiloride sensitive epithelial Na<sup>+ </sup>channels (ENaC), which are considered as the "receptor" of the Na<sup>+ </sup>component of salt taste. To explore the mechanisms, the expression of the three ENaC subunits (α, β and γ) in taste buds were observed from rats fed with diets containing either 0.03% (Na<sup>+ </sup>deprivation) or 1% (control) NaCl for 15 days, by using <it>in situ </it>hybridization and real-time quantitative RT-PCR (qRT-PCR). Since BDNF/TrkB signaling is involved in the neural innervation of taste buds, the effects of Na<sup>+ </sup>deprivation on BDNF and its receptor TrkB expression in the rat taste buds were also examined.</p> <p>Results</p> <p><it>In situ </it>hybridization analysis showed that all three ENaC subunit mRNAs were found in the rat fungiform taste buds and lingual epithelia, but in the vallate and foliate taste buds, only α ENaC mRNA was easily detected, while β and γ ENaC mRNAs were much less than those in the fungiform taste buds. Between control and low Na<sup>+ </sup>fed animals, the numbers of taste bud cells expressing α, β and γ ENaC subunits were not significantly different in the fungiform, vallate and foliate taste buds, respectively. Similarly, qRT-PCR also indicated that Na<sup>+ </sup>deprivation had no effect on any ENaC subunit expression in the three types of taste buds. However, Na<sup>+ </sup>deprivation reduced BDNF mRNA expression by 50% in the fungiform taste buds, but not in the vallate and foliate taste buds. The expression of TrkB was not different between control and Na<sup>+ </sup>deprived rats, irrespective of the taste papillae type.</p> <p>Conclusion</p> <p>The findings demonstrate that dietary Na<sup>+ </sup>deprivation does not change ENaC mRNA expression in rat taste buds, but reduces BDNF mRNA expression in the fungiform taste buds. Given the roles of BDNF in survival of cells and target innervation, our results suggest that dietary Na<sup>+ </sup>deprivation might lead to a loss of gustatory innervation in the mouse fungiform taste buds.</p
Effect of Rashba and Dresselhaus spin-orbit coupling on supercurrent rectification and magnetochiral anisotropy of ballistic Josephson junctions
Simultaneous breaking of inversion- and time-reversal symmetry in Josephson junction leads to a possible violation of the I(ϕ) = −I(−ϕ) equality for the current-phase relation. This is known as anomalous Josephson effect and it produces a phase shift ϕ0 in sinusoidal current-phase relations. In ballistic Josephson junctions with non-sinusoidal current phase relation the observed phenomenology is much richer, including the supercurrent diode effect and the magnetochiral anisotropy of Josephson inductance. In this work, we present measurements of both effects on arrays of Josephson junctions defined on epitaxial Al/InAs heterostructures. We show that the orientation of the current with respect to the lattice affects the magnetochiral anisotropy, possibly as the result of a finite Dresselhaus component. In addition, we show that the two-fold symmetry of the Josephson inductance reflects in the activation energy for phase slips
Picturing words? Sensorimotor cortex activation for printed words in child and adult readers
Learning to read involves associating abstract visual shapes with familiar meanings. Embodiment theories suggest that word meaning is at least partially represented in distributed sensorimotor networks in the brain (Barsalou, 2008; Pulvermueller, 2013). We explored how reading comprehension develops by tracking when and how printed words start activating these “semantic” sensorimotor representations as children learn to read. Adults and children aged 7–10 years showed clear category-specific cortical specialization for tool versus animal pictures during a one-back categorisation task. Thus, sensorimotor representations for these categories were in place at all ages. However, co-activation of these same brain regions by the visual objects’ written names was only present in adults, even though all children could read and comprehend all presented words, showed adult-like task performance, and older children were proficient readers. It thus takes years of training and expert reading skill before spontaneous processing of printed words’ sensorimotor meanings develops in childhood
On the Role of Object Information in Action Observation: An fMRI Study
Observing other people’s actions activates a network of brain regions that is also activated during the execution of these actions. Here, we used functional magnetic resonance imaging to test whether these “mirror” regions in frontal and parietal cortices primarily encode the spatiomotor aspects or the functional goal-related aspects of observed tool actions. Participants viewed static depictions of actions consisting of a tool object (e.g., key) and a target object (e.g., keyhole). They judged the actions either with regard to whether the objects were oriented correctly for the action to succeed (spatiomotor task) or whether an action goal could be achieved with the objects (function task). Compared with a control condition, both tasks activated regions in left frontoparietal cortex previously implicated in action observation and execution. Of these regions, the premotor cortex and supramarginal gyrus were primarily activated during the spatiomotor task, whereas the middle frontal gyrus was primarily activated during the function task. Regions along the intraparietal sulcus were more strongly activated during the spatiomotor task but only when the spatiomotor properties of the tool object were unknown in advance. These results suggest a division of labor within the action observation network that maps onto a similar division previously proposed for action execution