Exploring the phase diagram of the two-impurity Kondo problem

A system of two exchange-coupled Kondo impurities in a magnetic field gives rise to a rich phase space hosting a multitude of correlated phenomena. Magnetic atoms on surfaces probed through scanning tunnelling microscopy provide an excellent platform to investigate coupled impurities, but typical high Kondo temperatures prevent field-dependent studies from being performed, rendering large parts of the phase space inaccessible. We present an integral study of pairs of Co atoms on insulating Cu2N/Cu(100), which each have a Kondo temperature of only 2.6 K. In order to cover the different regions of the phase space, the pairs are designed to have interaction strengths similar to the Kondo temperature. By applying a sufficiently strong magnetic field, we are able to access a new phase in which the two coupled impurities are simultaneously screened. Comparison of differential conductance spectra taken on the atoms to simulated curves, calculated using a third order transport model, allows us to independently determine the degree of Kondo screening in each phase.Comment: paper: 14 pages, 4 figures; supplementary: 3 pages, 1 figure, 1 tabl

Magneto-quantum oscillations of the conductance of a tunnel point-contact in the presence of a single defect

The influence of a quantizing magnetic field $H$ to the conductance of a tunnel point contact in the presence of the single defect has been considered. We demonstrate that the conductance exhibits specific magneto-quantum oscillations, the amplitude and period of which depend on the distance between the contact and the defect. We show that a non-monotonic dependence of the point-contact conductance results from a superposition of two types of oscillations: A short period oscillation arising from electron focusing by the field $H$ and a long period oscillation of Aharonov-Bohm-type originated from the magnetic flux passing through the closed trajectories of electrons moving from the contact to the defect and returning back to the contact.Comment: 13 pages, 3 figure

Searching for additional heating - [OII] emission in the diffuse ionized gas of NGC891, NGC4631 and NGC3079

We present spectroscopic data of ionized gas in the disk--halo regions of three edge-on galaxies, NGC 891, NGC 4631 and NGC 3079, covering a wavelength range from [\ion{O}{2}] $\lambda$3727\AA to [\ion{S}{2}] $\lambda$6716.4\AA. The inclusion of the [\ion{O}{2}] emission provides new constraints on the properties of the diffuse ionized gas (DIG), in particular, the origin of the observed spatial variations in the line intensity ratios. We used three different methods to derive electron temperatures, abundances and ionization fractions along the slit. The increase in the [\ion{O}{2}]/H$\alpha$ line ratio towards the halo in all three galaxies requires an increase either in electron temperature or in oxygen abundance. Keeping the oxygen abundance constant yields the most reasonable results for temperature, abundances, and ionization fractions. Since a constant oxygen abundance seems to require an increase in temperature towards the halo, we conclude that gradients in the electron temperature play a significant role in the observed variations in the optical line ratios from extraplanar DIG in these three spiral galaxies.Comment: 43 pages, 29 figure

First Detection of the Crab Pulsar above 100 GeV

We present the detection of pulsed gamma-ray emission from the Crab pulsar above 100 GeV with the VERITAS array of atmospheric Cherenkov telescopes. Gamma-ray emission at theses energies was not expected in pulsar models. The detection of pulsed emission above 100 GeV and the absence of an exponential cutoff makes it unlikely that curvature radiation is the primary production mechanism of gamma rays at these energies.Comment: 5 pages, proceedings of the TAUP 2011 conference in Munich, German

Vibrationally Induced Two-Level Systems in Single-Molecule Junctions

Single-molecule junctions are found to show anomalous spikes in dI/dV spectra. The position in energy of the spikes are related to local vibration mode energies. A model of vibrationally induced two-level systems reproduces the data very well. This mechanism is expected to be quite general for single-molecule junctions. It acts as an intrinsic amplification mechanism for local vibration mode features and may be exploited as a new spectroscopic tool.Comment: 4 pages, 4 figure

Observation of electronic and atomic shell effects in gold nanowires

The formation of gold nanowires in vacuum at room temperature reveals a periodic spectrum of exceptionally stable diameters. This is identified as shell structure similar to that which was recently discovered for alkali metals at low temperatures. The gold nanowires present two competing `magic' series of stable diameters, one governed by electronic structure and the other by the atomic packing.Comment: 4 pages, 4 figure