1,879 research outputs found
Magnetoresistance of atomic-scale electromigrated nickel nanocontacts
We report measurements of the electron transport through atomic-scale
constrictions and tunnel junctions between ferromagnetic electrodes. Structures
are fabricated using a combination of e-beam lithography and controlled
electromigration. Sample geometries are chosen to allow independent control of
electrode bulk magnetizations. As junction size is decreased to the single
channel limit, conventional anisotropic magnetoresistance (AMR) increases in
magnitude, approaching the size expected for tunneling magnetoresistance (TMR)
upon tunnel junction formation. Significant mesoscopic variations are seen in
the magnitude and sign of the magnetoresistance, and no evidence is found of
large ballistic magnetoresistance effects.Comment: 3 pages, 3 figure
Three-terminal devices to examine single molecule conductance switching
We report electronic transport measurements of single-molecule transistor
devices incorporating bipyridyl-dinitro oligophenylene-ethynylene dithiol
(BPDN-DT), a molecule known to exhibit conductance switching in other
measurement configurations. We observe hysteretic conductance switching in 8%
of devices with measurable currents, and find that dependence of the switching
properties on gate voltage is rare when compared to other single-molecule
transistor devices. This suggests that polaron formation is unlikely to be
responsible for switching in these devices. We discuss this and alternative
switching mechanisms.Comment: 5 pages, 4 figures. Supporting material available upon reques
Radio-frequency reflectometry on an undoped AlGaAs/GaAs single electron transistor
Radio frequency reflectometry is demonstrated in a sub-micron undoped
AlGaAs/GaAs device. Undoped single electron transistors (SETs) are attractive
candidates to study single electron phenomena due to their charge stability and
robust electronic properties after thermal cycling. However these devices
require a large top-gate which is unsuitable for the fast and sensitive radio
frequency reflectometry technique. Here we demonstrate rf reflectometry is
possible in an undoped SET.Comment: Four pages, three figures, one supplementary fil
Kondo resonances and anomalous gate dependence of electronic conduction in single-molecule transistors
We report Kondo resonances in the conduction of single-molecule transistors
based on transition metal coordination complexes. We find Kondo temperatures in
excess of 50 K, comparable to those in purely metallic systems. The observed
gate dependence of the Kondo temperature is inconsistent with observations in
semiconductor quantum dots and a simple single-dot-level model. We discuss
possible explanations of this effect, in light of electronic structure
calculations.Comment: 5 pages, four figures. Supplementary material at
http://www.ruf.rice.edu/~natelson/publications.htm
Universal Scaling of Nonequilibrium Transport in the Kondo Regime of Single Molecule Devices
Scaling laws and universality are often associated with systems exhibiting
emergent phenomena possessing a characteristic energy scale. We report
nonequilibrium transport measurements on two different types of single-molecule
transistor (SMT) devices in the Kondo regime. The conductance at low bias and
temperature adheres to a scaling function characterized by two parameters. This
result, analogous to that reported recently in semiconductor dots with Kondo
temperatures two orders of magnitude lower, demonstrates the universality of
this scaling form. We compare the extracted values of the scaling coefficients
to previous experimental and theoretical results.Comment: 4.5 pages, 3 figure
Inelastic electron tunneling via molecular vibrations in single-molecule transistors
In single-molecule transistors, we observe inelastic cotunneling features
that correspond energetically to vibrational excitations of the molecule, as
determined by Raman and infrared spectroscopy. This is a form of inelastic
electron tunneling spectroscopy of single molecules, with the transistor
geometry allowing in-situ tuning of the electronic states via a gate electrode.
The vibrational features shift and change shape as the electronic levels are
tuned near resonance, indicating significant modification of the vibrational
states. When the molecule contains an unpaired electron, we also observe
vibrational satellite features around the Kondo resonance.Comment: 5 pages, 4 figures. Supplementary information available upon reques
Propagation effects at low frequencies seen in the LOFAR long-term monitoring of the periodically active FRB 20180916B
LOFAR (LOw Frequency ARray) has previously detected bursts from the periodically active, repeating fast radio burst (FRB) source FRB 20180916B down to unprecedentedly low radio frequencies of 110 MHz. Here, we present 11 new bursts in 223 more hours of continued monitoring of FRB 20180916B in the 110–188 MHz band with LOFAR. We place new constraints on the source’s activity window w = 4.3+0.7-0.2 d and phase centre φ LOFARc  = 0.67+0.03-0.02 in its 16.33-d activity cycle, strengthening evidence for its frequency-dependent activity cycle. Propagation effects like Faraday rotation and scattering are especially pronounced at low frequencies and constrain properties of FRB 20180916B’s local environment. We track variations in scattering and time–frequency drift rates, and find no evidence for trends in time or activity phase. Faraday rotation measure (RM) variations seen between June 2021 and August 2022 show a fractional change >50 per cent with hints of flattening of the gradient of the previously reported secular trend seen at 600 MHz. The frequency-dependent window of activity at LOFAR appears stable despite the significant changes in RM, leading us to deduce that these two effects have different causes. Depolarization of and within individual bursts towards lower radio frequencies is quantified using LOFAR’s large fractional bandwidth, with some bursts showing no detectable polarization. However, the degree of depolarization seems uncorrelated to the scattering time-scales, allowing us to evaluate different depolarization models. We discuss these results in the context of models that invoke rotation, precession, or binary orbital motion to explain the periodic activity of FRB 20180916B
Electronic and optical properties of electromigrated molecular junctions
Electromigrated nanoscale junctions have proven very useful for studying
electronic transport at the single-molecule scale. However, confirming that
conduction is through precisely the molecule of interest and not some
contaminant or metal nanoparticle has remained a persistent challenge,
typically requiring a statistical analysis of many devices. We review how
transport mechanisms in both purely electronic and optical measurements can be
used to infer information about the nanoscale junction configuration. The
electronic response to optical excitation is particularly revealing. We briefly
discuss surface-enhanced Raman spectroscopy on such junctions, and present new
results showing that currents due to optical rectification can provide a means
of estimating the local electric field at the junction due to illumination.Comment: 19 pages, 8 figures, invited paper for forthcoming special issue of
Journal of Physics: Condensed Matter. For other related papers, see
http://www.ruf.rice.edu/~natelson/publications.htm
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