909 research outputs found
The emergence of classical behavior in magnetic adatoms
A wide class of nanomagnets shows striking quantum behavior, known as quantum
spin tunneling (QST): instead of two degenerate ground states with opposite
magnetizations, a bonding-antibonding pair forms, resulting in a splitting of
the ground state doublet with wave functions linear combination of two
classically opposite magnetic states, leading to the quenching of their
magnetic moment. Here we study how QST is destroyed and classical behavior
emerges in the case of magnetic adatoms, as the strength of their coupling,
either to the substrate or to each other, is increased. Both spin-substrate and
spin-spin coupling renormalize the QST splitting to zero allowing the
environmental decoherence to eliminate superpositions between classical states,
leading to the emergence of spontaneous magnetization.Comment: 5 pages, 4 figure
Trap Barrier System (TBS) as a New Tool for Rodent Pest Management in Irrigated Rice in Africa
Rodent infestation poses a serious threat to smallholder farmers in both developed and developing countries where a large proportion of potential crop yield is lost. In Tanzania, the average annual yield loss of rice is estimated to be around 5–12%. Management of rodent pests in Africa relies mostly on the use of rodenticides which, however, are often applied only when damage has already occurred rather than routinely. Rodenticides used in this way are rarely economically and ecologically sustainable for managing rodents in irrigated rice. A “community-based Trap-Barrier-System (cTBS)” provides an alternative novel rodent control approach for controlling rodents in rice fields. This is basically a system where rodents are trapped in a rice field that is planted a short period earlier than the surrounding fields and therefore attracting rodents from a much wider area than the field itself. The system has proved very successful in irrigated rice fields in Tanzania, increasing rice yields in the intervention fields by 40.91%. A single cTBS can confer protection in up to 16 ha of irrigated rice field. Therefore, if scaled up and used widely, TBS has a great potential for managing rodent pests and improving yield in irrigated rice fields
On the Connection of Anisotropic Conductivity to Tip Induced Space Charge Layers in Scanning Tunneling Spectroscopy of p-doped GaAs
The electronic properties of shallow acceptors in p-doped GaAs{110} are
investigated with scanning tunneling microscopy at low temperature. Shallow
acceptors are known to exhibit distinct triangular contrasts in STM images for
certain bias voltages. Spatially resolved I(V)-spectroscopy is performed to
identify their energetic origin and behavior. A crucial parameter - the STM
tip's work function - is determined experimentally. The voltage dependent
potential configuration and band bending situation is derived. Ways to validate
the calculations with the experiment are discussed. Differential conductivity
maps reveal that the triangular contrasts are only observed with a depletion
layer present under the STM tip. The tunnel process leading to the anisotropic
contrasts calls for electrons to tunnel through vacuum gap and a finite region
in the semiconductor.Comment: 11 pages, 8 figure
Spin Excitations of a Kondo-Screened Atom Coupled to a Second Magnetic Atom
Screening the electron spin of a magnetic atom via spin coupling to conduction electrons results in a strong resonant peak in the density of states at the Fermi energy, the Kondo resonance. We show that magnetic coupling of a Kondo atom to another unscreened magnetic atom can split the Kondo resonance into two peaks. Inelastic spin excitation spectroscopy with scanning tunneling microscopy is used to probe the Kondo effect of a Co atom, supported on a thin insulating layer on a Cu substrate, that is weakly coupled to a nearby Fe atom to form an inhomogeneous dimer. The Kondo peak is split by interaction with the non-Kondo atom, but can be reconstituted with a magnetic field of suitable magnitude and direction. Quantitative modeling shows that this magnetic field results in a spin-level degeneracy in the dimer, which enables the Kondo effect to occur
Variable Repetition Rate THz Source for Ultrafast Scanning Tunneling Microscopy
Broadband THz pulses enable ultrafast electronic transport experiments on the nanoscale by coupling THz electric fields into the devices with antennas, asperities, or scanning probe tips. Here, we design a versatile THz source optimized for driving the highly resistive tunnel junction of a scanning tunneling microscope. The source uses optical rectification in lithium niobate to generate arbitrary THz pulse trains with freely adjustable repetition rates between 0.5 and 41 MHz. These induce subpicosecond voltage transients in the tunnel junction with peak amplitudes between 0.1 and 12 V, achieving a conversion efficiency of 0.4 V/(kV/cm) from far-field THz peak electric field strength to peak junction voltage in the STM. Tunnel currents in the quantum limit of less than one electron per THz pulse are readily detected at multi-MHz repetition rates. The ability to tune between high pulse energy and high signal fidelity makes this THz source design effective for exploration of ultrafast and atomic-scale electron dynamics
Controlling Silver Nanoparticle Size and Morphology with Photostimulated Synthesis
Photo-induced synthesis and control over the size and shape of colloidal
silver nanoparticles is investigated in contrast to photo-stimulated
aggregation of small nanoparticles into large fractal-type structures. The
feasibility of light-driven nanoengineering which enables manipulation of the
sizes and shapes of the isolated nanoparticles is studied by varying the amount
and type of the stabilizing agent and the type of optical irradiation.Comment: 10 pages, 7 figures, 11 image
Ionic conductivity on a wetting surface
Recent experiments measuring the electrical conductivity of DNA molecules
highlight the need for a theoretical model of ion transport along a charged
surface. Here we present a simple theory based on the idea of unbinding of ion
pairs. The strong humidity dependence of conductivity is explained by the
decrease in the electrostatic self-energy of a separated pair when a layer of
water (with high dielectric constant) is adsorbed to the surface. We compare
our prediction for conductivity to experiment, and discuss the limits of its
applicability.Comment: 5 pages, 3 figures; one section and two illustrations added; figures
updated and discussion added; typo fixe
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