Role of tip size, orientation, and structural relaxations in first-principles studies of magnetic exchange force microscopy and spin-polarized scanning tunneling microscopy

Using first-principles calculations based on density functional theory (DFT), we investigate the exchange interaction between a magnetic tip and a magnetic sample which is detected in magnetic exchange force microscopy (MExFM) and also occurs in spin-polarized scanning tunneling microscopy (SP-STM) experiments. As a model tip-sample system, we choose Fe tips and one monolayer Fe on W(001) which exhibits a checkerboard antiferromagnetic structure and has been previously studied with both SP-STM and MExFM. We calculate the exchange forces and energies as a function of tip-sample distance using different tip models ranging from single Fe atoms to Fe pyramids consisting of up to 14 atoms. We find that modelling the tip by a single Fe atom leads to qualitatively different tip-sample interactions than using clusters consisting of several atoms. Increasing the cluster size changes the calculated forces quantitatively enhancing the detectable exchange forces. Rotating the tip with respect to the surface unit cell has only a small influence on the tip-sample forces. Interestingly, the exchange forces on the tip atoms in the nearest and next-nearest layers from the apex atom are non-negligible and can be opposite to that on the apex atom for a small tip. In addition, the apex atom interacts not only with the surface atoms underneath but also with nearest-neighbors in the surface. We find that structural relaxations of tip and sample due to their interaction depend sensitively on the magnetic alignment of the two systems. As a result the onset of significant exchange forces is shifted towards larger tip-sample separations which facilitates their measurement in MExFM. At small tip-sample separations, structural relaxations of tip apex and surface atoms can either enhance or reduce the magnetic contrast measured in SP-STMComment: 14 pages, 13 figure

Local spectroscopy and atomic imaging of tunneling current, forces and dissipation on graphite

Theory predicts that the currents in scanning tunneling microscopy (STM) and the attractive forces measured in atomic force microscopy (AFM) are directly related. Atomic images obtained in an attractive AFM mode should therefore be redundant because they should be \emph{similar} to STM. Here, we show that while the distance dependence of current and force is similar for graphite, constant-height AFM- and STM images differ substantially depending on distance and bias voltage. We perform spectroscopy of the tunneling current, the frequency shift and the damping signal at high-symmetry lattice sites of the graphite (0001) surface. The dissipation signal is about twice as sensitive to distance as the frequency shift, explained by the Prandtl-Tomlinson model of atomic friction.Comment: 4 pages, 4 figures, accepted at Physical Review Letter

Event-based prospective remembering in a virtual world

Most laboratory-based prospective memory (PM) paradigms pose problems that are very different from those encountered in the real world. Several PM studies have reported conflicting results when comparing laboratory with naturalistic based studies (e.g., Bailey,Henry, Rendell, Phillips & Kliegel, 2010). One key contrast is that for the former, how and when the PM cue is encountered typically is determined by the experimenter, whereas in the latter case, cue availability is determined by participant actions. However, participant-driven access to the cue has not been examined in laboratory studies focused on healthy young adults, and its relationship with planned intentions is poorly understood. Here we report a study of PM performance in a controlled, laboratory setting, but with participant-driven actions leading to the availability of the PM cue. This uses a novel PM methodology based upon analysis of participant movements as they attempted a series of errands in a large virtual building on the computer screen. A PM failure was identified as a situation in which a participant entered and exited the “cue” area outside an errand related room without performing the required errand whilst still successfully remembering that errand post-test. Additional individual difference measures assessed retrospective and working memory capacity, planning ability and PM. Multiple regression analysis showed that the independent measures of verbal working memory span, planning ability and PM were significant predictors of PM failure. Correlational analyses with measures of planning suggest that sticking with an original plan (good or bad) is related to better overall PM performance

Bio-inspired Highly Scattering Networks via Polymer Phase Separation

A common strategy to optimize whiteness in living organisms consists in using three-dimensional random networks with dense and polydisperse scattering elements constituted by relatively low-refractive index materials. Inspired by these natural architectures, we developed a fast and scalable method to produce highly scattering porous polymer films via phase separation. By varying the molecular weight of the polymer, we modified the morphology of the porous films and therefore tuned their scattering properties. The achieved transport mean free paths are in the micrometer range, improving the scattering strength of analogous low-refractive index systems, e.g. standard white paper, by an order of magnitude. The produced porous films show a broadband reflectivity of approximately 75 % whilst only 4 m thick. In addition, the films are flexible and can be readily index-matched with water (i.e. they become transparent when wet), allowing for various applications such as coatings with tunable transmittance and responsive paints

Shape-memory polymers as flexible resonator substrates for continuously tunable organic DFB lasers

We introduce shape-memory polymers (SMP) as substrate material for active optical devices. As an exemplary application we build a tunable organic semiconductor distributed feedback (DFB) laser. Hence, we transfer a second order Bragg grating with a period of 400 nm into SMP foils by hot embossing. The composite organic gain medium Alq3:DCM evaporated on the SMP substrate serves as laser active material. Mechanical stretching of the substrate increases the grating period temporarily and triggering the shape-memory effect afterwards reduces the period on demand. In this way, we can adjust the grating period to achieve a broad continuously tuning of the laser emission wavelength by 30 nm

Electromagnetic Simulation and Design of a Novel Waveguide RF Wien Filter for Electric Dipole Moment Measurements of Protons and Deuterons

The conventional Wien filter is a device with orthogonal static magnetic and electric fields, often used for velocity separation of charged particles. Here we describe the electromagnetic design calculations for a novel waveguide RF Wien filter that will be employed to solely manipulate the spins of protons or deuterons at frequencies of about 0.1 to 2 MHz at the COoler SYnchrotron COSY at J\"ulich. The device will be used in a future experiment that aims at measuring the proton and deuteron electric dipole moments, which are expected to be very small. Their determination, however, would have a huge impact on our understanding of the universe.Comment: 10 pages, 10 figures, 4 table

Thermal effects on atomic friction

We model friction acting on the tip of an atomic force microscope as it is dragged across a surface at non-zero temperatures. We find that stick-slip motion occurs and that the average frictional force follows $|\ln v|^{2/3}$, where $v$ is the tip velocity. This compares well to recent experimental work (Gnecco et al, PRL 84, 1172), permitting the quantitative extraction of all microscopic parameters. We calculate the scaled form of the average frictional force's dependence on both temperature and tip speed as well as the form of the friction-force distribution function.Comment: Accepted for publication, Physical Review Letter