Influence of surface-related strain and electric field on acceptor wave functions in Zincblende semiconductors

The spatial distribution of the local density of states (LDOS) at Mn acceptors near the (110) surface of p-doped InAs is investigated by Scanning Tunneling Microscopy (STM). The shapes of the acceptor contrasts for different dopant depths under the surface are analyzed. Acceptors located within the first ten subsurface layers of the semiconductor show a lower symmetry than expected from theoretical predictions of the bulk acceptor wave function. They exhibit a (001) mirror asymmetry. The degree of asymmetry depends on the acceptor atoms' depths. The measured contrasts for acceptors buried below the 10th subsurface layer closely match the theoretically derived shape. Two effects are able to explain the symmetry reduction: the strain field of the surface relaxation and the tip-induced electric field.Comment: 8 pages, 4 figure

Anomalously large capacitance of an ionic liquid described by the restricted primitive model

We use Monte Carlo simulations to examine the simplest model of an ionic liquid, called the restricted primitive model, at a metal surface. We find that at moderately low temperatures the capacitance of the metal/ionic liquid interface is so large that the effective thickness of the electrostatic double-layer is up to 3 times smaller than the ion radius. To interpret these results we suggest an approach which is based on the interaction between discrete ions and their image charges in the metal surface and which therefore goes beyond the mean-field approximation. When a voltage is applied across the interface, the strong image attraction causes counterions to condense onto the metal surface to form compact ion-image dipoles. These dipoles repel each other to form a correlated liquid. When the surface density of these dipoles is low, the insertion of an additional dipole does not require much energy. This leads to a large capacitance $C$ that decreases monotonically with voltage $V$, producing a "bell-shaped" curve $C(V)$. We also consider what happens when the electrode is made from a semi-metal rather than a perfect metal. In this case, the finite screening radius of the electrode shifts the reflection plane for image charges to the interior of the electrode and we arrive at a "camel-shaped" $C(V)$. These predictions seem to be in qualitative agreement with experiment.Comment: 7 pages, 5 figures; some numerical comparisons added; schematic figure added, additional discussion of effect of electrode material, section added with comparison to semiconductor devices; plotting error fixed in Fig.

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

Large capacitance enhancement and negative compressibility of two-dimensional electronic systems at LaAlO3_3/SrTiO3_3 interfaces

Novel electronic systems forming at oxide interfaces comprise a class of new materials with a wide array of potential applications. A high mobility electron system forms at the LaAlO$_3$/SrTiO$_3$ interface and, strikingly, both superconducts and displays indications of hysteretic magnetoresistance. An essential step for device applications is establishing the ability to vary the electronic conductivity of the electron system by means of a gate. We have fabricated metallic top gates above a conductive interface to vary the electron density at the interface. By monitoring capacitance and electric field penetration, we are able to tune the charge carrier density and establish that we can completely deplete the metallic interface with small voltages. Moreover, at low carrier densities, the capacitance is significantly enhanced beyond the geometric capacitance for the structure. In the same low density region, the metallic interface overscreens an external electric field. We attribute these observations to a negative compressibility of the electronic system at the interface. Similar phenomena have been observed previously in semiconducting two-dimensional electronic systems. The observed compressibility result is consistent with the interface containing a system of mobile electrons in two dimensions.Comment: 4 figures in main text; 4 figures in the supplemen

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

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

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

Spatio-temporal patterns in the distribution of the multi-mammate mouse, Mastomys natalensis, in rice crop and fallow land habitats in Tanzania

An understanding of the dispersion patterns of a pest is an important pre-requisite for developing an effective management programme for the pest. In this study, rodents were trapped in two rice fields and two fallow fields for three consecutive nights each month from June 2010 to May 2012. Mastomys natalensis was the most abundant rodent pest species in the study area, accounting for > 95% of the trapped rodent community. Rattus rattus, Dasymys incomtus, Acomys spinosissimus and Grammomys dolichurus comprised relatively small proportions of the trapped population. Morisita’s index of dispersion was used to measure the relative dispersal pattern aggregate, random, uniform) of individuals across each trapping grid as a means of comparing rodent distribution in rice and fallow fields over time. This analysis revealed that the rodents in rice fields generally exhibited an aggregated spatio-temporal distribution. However, the rodents in fallow fields were generally less aggregated, approaching a random distribution in some habitats and seasons. Heat maps of trapping grids visually confirmed these dispersal patterns, indicating the clumped or random nature of captured rodents. ANOVA showed that the parameters of habitat (rice, fallow), crop stage (transplanting, vegetative, booting, maturity) and cropping season (wet, dry) all significantly impacted the number of rodents captured, with the vegetative, dry season, fallow habitat having the highest number of rodents; and the transplanting, wet season, rice habitat with the least number of rodents. Therefore, such spatio-temporal patterns can serve as a tool for developing stratified biodiversity sampling plans for small mammals and decision making for rodent pest management strategies