2,100 research outputs found
Real-Time Imaging of K atoms on Graphite: Interactions and Diffusion
Scanning tunneling microscopy (STM) at liquid helium temperature is used to
image potassium adsorbed on graphite at low coverage (~0.02 monolayer). Single
atoms appear as protrusions on STM topographs. A statistical analysis of the
position of the atoms demonstrates repulsion between adsorbates, which is
quantified by comparison with molecular dynamics simulations. This gives access
to the dipole moment of a single adsorbate, found to be 10.5 Debye. Time lapse
imaging shows that long range order is broken by thermally activated diffusion,
with a 32 meV barrier to hopping between graphite lattice sites
Ground state spin and Coulomb blockade peak motion in chaotic quantum dots
We investigate experimentally and theoretically the behavior of Coulomb
blockade (CB) peaks in a magnetic field that couples principally to the
ground-state spin (rather than the orbital moment) of a chaotic quantum dot. In
the first part, we discuss numerically observed features in the magnetic field
dependence of CB peak and spacings that unambiguously identify changes in spin
S of each ground state for successive numbers of electrons on the dot, N. We
next evaluate the probability that the ground state of the dot has a particular
spin S, as a function of the exchange strength, J, and external magnetic field,
B. In the second part, we describe recent experiments on gate-defined GaAs
quantum dots in which Coulomb peak motion and spacing are measured as a
function of in-plane magnetic field, allowing changes in spin between N and N+1
electron ground states to be inferred.Comment: To appear in Proceedings of the Nobel Symposium 2000 (Physica
Scripta
Conductance fluctuations in quasi-two-dimensional systems: a practical view
The universal conductance fluctuations of quasi-two-dimensional systems are
analyzed with experimental considerations in mind. The traditional statistical
metrics of these fluctuations (such as variance) are shown to have large
statistical errors in such systems. An alternative characteristic is
identified, the inflection point of the correlation function in magnetic field,
which is shown to be significantly more useful as an experimental metric and to
give a more robust measure of phase coherence.Comment: 9 pages, 7 figure
Large spin-orbit effects in small quantum dots
We consider small ballistic quantum dots weakly coupled to the leads in the
chaotic regime and look for significant spin-orbit effects. We find that these
effects can become quite prominent in the vicinity of degeneracies of many-body
energies. We illustrate the idea by considering a case where the intrinsic
exchange term -JS^2 brings singlet and triplet many-body states near each
other, while an externally tunable Zeeman term then closes the gap between the
singlet and the one of the triplet states (with spin projection parallel the
external field). Near this degeneracy, the spin-orbit coupling leads to a
striking temperature dependence of the conductance, with observable effects of
order unity at temperatures lower than the strength of the spin-orbit coupling.
Under favorable circumstances, spelled out in the paper, these order unity
effects in the conductance persist to temperatures much higher than the
spin-orbit coupling strength. Our conclusions are unaffected by the presence of
non-universal perturbations. We suggest a class of experiments to explore this
regime.Comment: 13 pages, 8 figure
Nuclear Polarization in Quantum Point Contacts in an In-Plane Magnetic Field
Nuclear spin polarization is typically generated in GaAs quantum point
contacts (QPCs) when an out-of-plane magnetic field gives rise to
spin-polarized quantum Hall edge states, and a voltage bias drives transitions
between the edge states via electron-nuclear flip-flop scattering. Here, we
report a similar effect for QPCs in an in-plane magnetic field, where currents
are spin polarized but edge states are not formed. The nuclear polarization
gives rise to hysteresis in the d.c. transport characteristics, with relaxation
timescales around 100 seconds. The dependence of anomalous QPC conductance
features on nuclear polarization provides a useful test of their
spin-sensitivity.Comment: 5 page
Previous attentional set can induce an attentional blink with task-irrelevant initial targets
Identification of a second target is often impaired by the requirement to process a prior target in a rapid serial visual presentation (RSVP). This is termed the attentional blink. Even when the first target is task-irrelevant an attentional blink may occur providing this first target shares similar features with the second target (contingent capture). An RSVP experiment was undertaken to assess whether this first target can still cause an attentional blink when it did not require a response and did not share any features with the following target. The results revealed that such task-irrelevant targets can induce an attentional blink providing that they were task-relevant on a previous block of trials. This suggests that irrelevant focal stimuli can distract attention on the basis of a previous attentional set
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