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

VIDEO SURVEILLANCE OF NESTING WHOOPING CRANES

The primary objective of the whooping crane (Grus americana) recovery plan is to establish and maintain 3 selfsustaining wild populations, 1 being a non-migratory Florida population. From 1993 to 2005, we released 289 captive cranes in central Florida, with 31 surviving as of 1 June 2008. From 1995 to 2008 we monitored 68 nests of the Florida population; from those, only 32 chicks hatched and 9 fledged. It often was not apparent why nests failed, and it was not practical to conduct labor-intensive observations at nests; therefore, we collected behavioral data using 12-volt VHS video surveillance cameras at 13 nests from 2000 to 2008. We positioned cameras and time-lapse video recorders 200-500 m from each nest. We programmed 1-3 days/tape of recording to occur from 1 hour before sunrise to 1 hour after sunset. Seven of 13 nests were successful in hatching chicks, while the remainder failed during the incubation period. Relevant events recorded on the videotapes included: incubation durations, frequency of adults switching off during incubation, frequency of standing and egg turning, duration eggs were not incubated while adults were away, and possible causes of nest failure. Summary statistics relating to these behaviors and also to their possible implications regarding nest survival will be discussed. Video surveillance was a valuable tool for the efficient gathering of behavioral data at whooping crane nests