16,989 research outputs found
Coulomb Oscillations in Antidots in the Integer and Fractional Quantum Hall Regimes
We report measurements of resistance oscillations in micron-scale antidots in
both the integer and fractional quantum Hall regimes. In the integer regime, we
conclude that oscillations are of the Coulomb type from the scaling of magnetic
field period with the number of edges bound to the antidot. Based on both
gate-voltage and field periods, we find at filling factor {\nu} = 2 a tunneling
charge of e and two charged edges. Generalizing this picture to the fractional
regime, we find (again, based on field and gate-voltage periods) at {\nu} = 2/3
a tunneling charge of (2/3)e and a single charged edge.Comment: related papers at http://marcuslab.harvard.ed
Spin Amplification for Magnetic Sensors Employing Crystal Defects
Recently there have been several theoretical and experimental studies of the
prospects for magnetic field sensors based on crystal defects, especially
nitrogen vacancy (NV) centres in diamond. Such systems could potentially be
incorporated into an AFM-like apparatus in order to map the magnetic properties
of a surface at the single spin level. In this Letter we propose an augmented
sensor consisting of an NV centre for readout and an `amplifier' spin system
that directly senses the local magnetic field. Our calculations show that this
hybrid structure has the potential to detect magnetic moments with a
sensitivity and spatial resolution far beyond that of a simple NV centre, and
indeed this may be the physical limit for sensors of this class
Signatures of Random Matrix Theory in the Discrete Energy Spectra of Subnanosize Metallic Clusters
Lead clusters deposited on Si(111) substrates have been studied at low
temperatures using scanning tunneling microscopy and spectroscopy. The
current-voltage characteristics exhibit current peaks that are irregularly
spaced and varied in height. The statistics of the distribution of peak heights
and spacings are in agreement with random matrix theory for several clusters.
The distributions have also been studied as a function of cluster shape.Comment: 10 pages, 9 figures, to appear in Phys. Rev.
Quantum Hall conductance of two-terminal graphene devices
Measurement and theory of the two-terminal conductance of monolayer and
bilayer graphene in the quantum Hall regime are compared. We examine features
of conductance as a function of gate voltage that allow monolayer, bilayer, and
gapped samples to be distinguished, including N-shaped distortions of quantum
Hall plateaus and conductance peaks and dips at the charge neutrality point.
Generally good agreement is found between measurement and theory. Possible
origins of discrepancies are discussed
Superconducting, Insulating, and Anomalous Metallic Regimes in a Gated Two-Dimensional Semiconductor-Superconductor Array
The superconductor-insulator transition in two dimensions has been widely
investigated as a paradigmatic quantum phase transition. The topic remains
controversial, however, because many experiments exhibit a metallic regime with
saturating low-temperature resistance, at odds with conventional theory. Here,
we explore this transition in a novel, highly controllable system, a
semiconductor heterostructure with epitaxial Al, patterned to form a regular
array of superconducting islands connected by a gateable quantum well. Spanning
nine orders of magnitude in resistance, the system exhibits regimes of
superconducting, metallic, and insulating behavior, along with signatures of
flux commensurability and vortex penetration. An in-plane magnetic field
eliminates the metallic regime, restoring the direct superconductor-insulator
transition, and improves scaling, while strongly altering the scaling exponent
Photovoltaic and Rectification Currents in Quantum Dots
We investigate theoretically and experimentally the statistical properties of
dc current through an open quantum dot subject to ac excitation of a
shape-defining gate. The symmetries of rectification current and photovoltaic
current with respect to applied magnetic field are examined. Theory and
experiment are found to be in good agreement throughout a broad range of
frequency and ac power, ranging from adiabatic to nonadiabatic regimes.Comment: 4 pages, 3 figures; related articles at http://marcuslab.harvard.ed
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