181 research outputs found
An Electronic Mach-Zehnder Interferometer
Double-slit electron interferometers, fabricated in high mobility
two-dimensional electron gas (2DEG), proved to be very powerful tools in
studying coherent wave-like phenomena in mesoscopic systems. However, they
suffer from small fringe visibility due to the many channels in each slit and
poor sensitivity to small currents due to their open geometry. Moreover, the
interferometers do not function in a high magnetic field, namely, in the
quantum Hall effect (QHE) regime, since it destroys the symmetry between left
and right slits. Here, we report on the fabrication and operation of a novel,
single channel, two-path electron interferometer that functions in a high
magnetic field. It is the first electronic analog of the well-known optical
Mach-Zehnder (MZ) interferometer. Based on single edge state and closed
geometry transport in the QHE regime the interferometer is highly sensitive and
exhibits very high visibility (62%). However, the interference pattern decays
precipitously with increasing electron temperature or energy. While we do not
understand the reason for the dephasing we show, via shot noise measurement,
that it is not a decoherence process that results from inelastic scattering
events.Comment: to appear in Natur
Optimization principles of dendritic structure
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licens
Local Thermometry of Neutral Modes on the Quantum Hall Edge
A system of electrons in two dimensions and strong magnetic fields can be
tuned to create a gapped 2D system with one dimensional channels along the
edge. Interactions among these edge modes can lead to independent transport of
charge and heat, even in opposite directions. Measuring the chirality and
transport properties of these charge and heat modes can reveal otherwise hidden
structure in the edge. Here, we heat the outer edge of such a quantum Hall
system using a quantum point contact. By placing quantum dots upstream and
downstream along the edge of the heater, we can measure both the chemical
potential and temperature of that edge to study charge and heat transport,
respectively. We find that charge is transported exclusively downstream, but
heat can be transported upstream when the edge has additional structure related
to fractional quantum Hall physics.Comment: 24 pages, 18 figure
The Wiring Economy Principle: Connectivity Determines Anatomy in the Human Brain
Minimization of the wiring cost of white matter fibers in the human brain appears to be an organizational principle. We investigate this aspect in the human brain using whole brain connectivity networks extracted from high resolution diffusion MRI data of 14 normal volunteers. We specifically address the question of whether brain anatomy determines its connectivity or vice versa. Unlike previous studies we use weighted networks, where connections between cortical nodes are real-valued rather than binary off-on connections. In one set of analyses we found that the connectivity structure of the brain has near optimal wiring cost compared to random networks with the same number of edges, degree distribution and edge weight distribution. A specifically designed minimization routine could not find cheaper wiring without significantly degrading network performance. In another set of analyses we kept the observed brain network topology and connectivity but allowed nodes to freely move on a 3D manifold topologically identical to the brain. An efficient minimization routine was written to find the lowest wiring cost configuration. We found that beginning from any random configuration, the nodes invariably arrange themselves in a configuration with a striking resemblance to the brain. This confirms the widely held but poorly tested claim that wiring economy is a driving principle of the brain. Intriguingly, our results also suggest that the brain mainly optimizes for the most desirable network connectivity, and the observed brain anatomy is merely a result of this optimization
The Morphological Identity of Insect Dendrites
Dendrite morphology, a neuron's anatomical fingerprint, is a
neuroscientist's asset in unveiling organizational principles in the
brain. However, the genetic program encoding the morphological identity of a
single dendrite remains a mystery. In order to obtain a formal understanding of
dendritic branching, we studied distributions of morphological parameters in a
group of four individually identifiable neurons of the fly visual system. We
found that parameters relating to the branching topology were similar throughout
all cells. Only parameters relating to the area covered by the dendrite were
cell type specific. With these areas, artificial dendrites were grown based on
optimization principles minimizing the amount of wiring and maximizing synaptic
democracy. Although the same branching rule was used for all cells, this yielded
dendritic structures virtually indistinguishable from their real counterparts.
From these principles we derived a fully-automated model-based neuron
reconstruction procedure validating the artificial branching rule. In
conclusion, we suggest that the genetic program implementing neuronal branching
could be constant in all cells whereas the one responsible for the dendrite
spanning field should be cell specific
Imaging Coulomb Islands in a Quantum Hall Interferometer
In the Quantum Hall regime, near integer filling factors, electrons should
only be transmitted through spatially-separated edge states. However, in
mesoscopic systems, electronic transmission turns out to be more complex,
giving rise to a large spectrum of magnetoresistance oscillations. To explain
these observations, recent models put forward that, as edge states come close
to each other, electrons can hop between counterpropagating edge channels, or
tunnel through Coulomb islands. Here, we use scanning gate microscopy to
demonstrate the presence of quantum Hall Coulomb islands, and reveal the
spatial structure of transport inside a quantum Hall interferometer. Electron
islands locations are found by modulating the tunneling between edge states and
confined electron orbits. Tuning the magnetic field, we unveil a continuous
evolution of active electron islands. This allows to decrypt the complexity of
high magnetic field magnetoresistance oscillations, and opens the way to
further local scale manipulations of quantum Hall localized states
Computational exploration of molecular receptive fields in the olfactory bulb reveals a glomerulus-centric chemical map
© The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.Progress in olfactory research is currently hampered by incomplete knowledge about chemical receptive ranges of primary receptors. Moreover, the chemical logic underlying the arrangement of computational units in the olfactory bulb has still not been resolved. We undertook a large-scale approach at characterising molecular receptive ranges (MRRs) of glomeruli in the dorsal olfactory bulb (dOB) innervated by the MOR18-2 olfactory receptor, also known as Olfr78, with human ortholog OR51E2. Guided by an iterative approach that combined biological screening and machine learning, we selected 214 odorants to characterise the response of MOR18-2 and its neighbouring glomeruli. We found that a combination of conventional physico-chemical and vibrational molecular descriptors performed best in predicting glomerular responses using nonlinear Support-Vector Regression. We also discovered several previously unknown odorants activating MOR18-2 glomeruli, and obtained detailed MRRs of MOR18-2 glomeruli and their neighbours. Our results confirm earlier findings that demonstrated tunotopy, that is, glomeruli with similar tuning curves tend to be located in spatial proximity in the dOB. In addition, our results indicate chemotopy, that is, a preference for glomeruli with similar physico-chemical MRR descriptions being located in spatial proximity. Together, these findings suggest the existence of a partial chemical map underlying glomerular arrangement in the dOB. Our methodology that combines machine learning and physiological measurements lights the way towards future high-throughput studies to deorphanise and characterise structure-activity relationships in olfaction.Peer reviewe
Evolution of Landau Levels into Edge States at an Atomically Sharp Edge in Graphene
The quantum-Hall-effect (QHE) occurs in topologically-ordered states of
two-dimensional (2d) electron-systems in which an insulating bulk-state
coexists with protected 1d conducting edge-states. Owing to a unique
topologically imposed edge-bulk correspondence these edge-states are endowed
with universal properties such as fractionally-charged quasiparticles and
interference-patterns, which make them indispensable components for QH-based
quantum-computation and other applications. The precise edge-bulk
correspondence, conjectured theoretically in the limit of sharp edges, is
difficult to realize in conventional semiconductor-based electron systems where
soft boundaries lead to edge-state reconstruction. Using scanning-tunneling
microscopy and spectroscopy to follow the spatial evolution of bulk
Landau-levels towards a zigzag edge of graphene supported above a graphite
substrate we demonstrate that in this system it is possible to realize
atomically sharp edges with no edge-state reconstruction. Our results single
out graphene as a system where the edge-state structure can be controlled and
the universal properties directly probed.Comment: 16 pages, 4 figure
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