arXiv subjects

W. A. Hofer

Publications and source records attributed to W. A. Hofer.

At least 19 recordsLinked to original sources

arXiv2008

Hydroxyl vacancies in single-walled aluminosilicate and aluminogermanate nanotubes

We report the first theoretical study of hydroxyl vacancies in aluminosilicate and aluminogermanate single-walled metal-oxide nanotubes. The defects are modeled on both sides of the tube walls and lead to occupied and empty states in the band gap which are highly localized both in energy and in real space. We find different magnetization states depending on both the chemical composition and the specific side with respect to the tube cavity. The defect-induced perturbations to the pristine electronic structure are related to the electrostatic polarization across the tube walls and the ensuing change in Brønsted acid-base reactivity. Finally, the capacity to counterbalance local charge accumulations, a characteristic feature of these systems, is discussed in view of their potential application as insulating coatings for one-dimensional conducting nanodevices.

cond-mat.mtrl-sci↗

arXiv2006

Conductance and Kondo effect of a controlled single atom contact

The tip of a low-temperature scanning tunneling microscope is brought into contact with individual Kondo impurities (cobalt atoms) adsorbed on a Cu(100) surface. A smooth transition from the tunneling regime to a point contact with a conductance of $G\approx\text{G}_0$ occurs. Spectroscopy in the contact regime, {\it i. e.}, at currents in a $μ\text{A}$ range was achieved. A modified line shape is observed indicating a significant change of the Kondo temperature $T_{\text{K}}$ at contact. Model calculations indicate that the proximity of the tip shifts the cobalt $d$-band and thus affects $T_{\text{K}}$.

cond-mat.str-el↗

arXiv2006

An orbital-free density functional method based on inertial fields

In this paper we revisit the Levy-Perdew-Sahni equation. We establish that the relation implicitly contains the conservation of energy density at every point of the system. The separate contributions to the total energy density are described in detail, and it is shown that the key difference to standard density functional methods is the existence of a general exchange-correlation potential, which does not explicitly depend on electron charge. We derive solutions for the hydrogen-like atoms and analyse local properties. It is found that these systems are stable due to the existence of a vector potential ${\bf A}$, related to electron motion, which leads to two general effects: (i) The root of the charge density acquires an additional complex phase; and (ii) for single electrons, the vector potential cancels the effect of electrostatic repulsions. We determine the density of states of a free electron gas based on this model and find that the vectorpotential also accounts for the Pauli exclusion principle. Implications of these results for direct methods in density functional theory are discussed. It seems that the omission of vector potentials in formulating the kinetic energy density functionals may be the main reason that direct methods so far are not generally applicable. Finally, we provide an orbital free self-consistent formulation for determining the groundstate charge density in a local density approximation.

cond-mat.mtrl-sci↗

arXiv2004

Atom Transfer and Single-Adatom Contacts

The point contact of a tunnel tip approaching towards Ag(111) and Cu(111) surfaces is investigated with a low temperature scanning tunneling microscope. A sharp jump-to-contact, random in nature, is observed in the conductance. After point contact, the tip-apex atom is transferred to the surface, indicating that a one-atom contact is formed during the approach. In sharp contrast, the conductance over single silver and copper adatoms exhibits a smooth and reproducible transition from tunneling to contact regime. Numerical simulations show that this is a consequence of the additional dipolar bonding between the homoepitaxial adatom and the surface atoms.

cond-mat.str-el↗

arXiv2002

Comment on ''Matter-Wave Interferometer for Large Molecules'' (quant-ph/0202158)

Diffraction experiments have moved to ever heavier objects in recent years, now standing at the level of large molecules. Experiments in materials science on the other hand have come down to ever smaller sizes largely due to the success of the scanning tunneling microscope (STM), which allows studying single molecules on surfaces. Since both fields coming from opposite side of the size spectrum are now meeting, there is a need of consistent theories. The simulations presented in the article, which we are referring to, are not consistent with molecular dynamics simulations which are routinely used to interpret STM results. Our comment points this out in detail.

quant-ph↗

arXiv2002

Determining surface magnetization and local magnetic moments with atomic scale resolution

We propose a method to determine the direction of surface magnetization and local magnetic moments on the atomic scale. The method comprises high resolution scanning tunneling microscope experiments in conjunction with first principles simulations of the tunneling current. The potential of the method is demonstrated on a model system, antiferromagnetic Mn overlayers on W(110). We expect that it will ultimately allow to study the detailed changes of magnetic surface structures in the vicinity of dopants or impurities.

cond-mat.mtrl-sci↗

arXiv2001

Tip effects in scanning tunnelling microscopy of atomic-scale magnetic structures

The spin-polarized scanning tunnelling microscope (STM) can in principle resolve not only the electronic, but also the magnetic surface structure. We model recent STM measurements achieving magnetic resolution on the atomic scale by a first-principles method. It is shown that the signature of a specific magnetic or non-magnetic STM tip can unambiguously be identified. It is also established that the model of Tersoff and Hamann would yield an electronic as well as magnetic contrast of the surface which is well below STM resolution.

cond-mat.mtrl-sci↗

arXiv2001

Surface Relaxations, Current Enhancements, and Absolute Distances in High Resolution Scanning Tunneling Microscopy

We have performed the most realistic simulation to date of the operation of a scanning tunneling microscope. Probe-sample distances from beyond tunneling to actual surface contact are covered. We simultaneously calculate forces, atomic displacements, and tunneling currents, allowing quantitative comparison with experimental values. A distance regime below which the probe becomes unstable is identified. It is shown that the real distance differs substantially from previous estimates because of large atomic displacements on the surface and at the probe-tip.

cond-mat.mtrl-sci↗

arXiv2001

Numerical simulation of interference experiments in a local hidden variables model

We present a theoretical model which allows to keep track of all photons in an interferometer. The model is implemented in a numerical scheme, and we simulate photon interference measurements on one, two, four, and eight slits. Measurements are simulated for the high intensity regime, where we show that our simulations describe all experimental results so far. With a slightly modified concept we can also model interference experiments in the low intensity regime, these experiments have recently been performed with single molecules. Finally, we predict the result of polarization measurements, which allow to check the model experimentally.

quant-ph↗

arXiv2001

Numerical simulation of Einstein-Podolsky-Rosen experiments in a local hidden variables model

We simulate correlation measurements of entangled photons numerically. The model employed is strictly local. In our model correlations arise from a phase, connecting the electromagnetic fields of the two photons at their separate points of measurement. We sum up coincidences for each pair individually and model the operation of a polarizer beam splitter numerically. The results thus obtained differ substantially from the classical results. In addition, we analyze the effects of decoherence and non-ideal beam splitters. It is shown that under realistic experimental conditions the Bell inequalities are violated by more than 30 standard deviations.

quant-ph↗

arXiv2001

Visibility, efficiency, and Bell violations in real Einstein-Podolsky-Rosen experiments

The violation of Bell's inequalities in Einstein-Podolsky-Rosen experiments has been demonstrated for photons and ions. In all experiments of this kind the relation between visibility, efficiency, and Bell violation is generally unknown. In this paper we show that simulations based on a local hidden variables models for entangled photons provide this information. It is established that these properties are closely related by the way, in which photons are detected after a polarizer beam splitter. On this basis we suggest controlled experiments which, for the first time, subject the superposition principle to experimental tests.

quant-ph↗

arXiv2001

Simulation of Einstein-Podolsky-Rosen experiments in a local hidden variables model with limited efficiency and coherence

We simulate correlation measurements of entangled photons numerically. The model employed is strictly local. The correlation is determined by its classical expression with one decisive difference: we sum up coincidences for each pair individually. We analyze the effects of decoherence, detector efficiency and polarizer thresholds in detail. The Bell inequalities are violated in these simulations. The violation depends crucially on the threshold of the polarizer switches and can reach a value of 2.0 in the limiting case. Existing experiments can be fully accounted for by limited coherence and non-ideal detector switches. It seems thus safe to conclude that the Bell inequalities are no suitable criterium to decide on the nonlocality issue.

quant-ph↗

arXiv2001

Information transfer via the phase: A local model of Einstein-Podolksy-Rosen experiments

Conventionally, one interprets the correlations observed in Einstein-Podolsky-Rosen experiments by Bell's inequalities and quantum nonlocality. We show, in this paper, that identical correlations arise, if the phase relations of electromagnetic fields are considered. In particular, we proceed from an analysis of a one-photon model. The correlation probability in this case contains a phase relation cos(b - a) between the two settings. In the two photon model the phases of the photon's electromagnetic fields are related at the origin. It is shown that this relation can be translated into a linearity requirement for electromagnetic fields between the two polarizers. Along these lines we compute the correlation integral with an expression conserving linearity. This expression, as shown, correctly describes the measured values. It seems thus that quantum nonlocality can be seen as a combination of boundary conditions on possible electromagnetic fields between the polarizers and a relation of the electromagnetic fields of the two photons via a phase. We expect the same feature to arise in every experiment, where joint probabilities of separate polarization measurements are determined.

quant-ph↗

arXiv2001

Masses and fields in Microdynamics: a possible foundation for dynamic gravity

The quest for a complete theory of microphysics is probably near the top of the agenda in fundamental physics today. We survey existing modifications of quantum mechanics to assess their potential. In the following we present recent results in microdynamics, focussing on dynamic charge. The theoretical model, valid at all length scales, relates mass oscillations to static electric and gravity fields. The same concept is used, with substantially lower frequencies, to compute the intensity of gravity waves within the solar system. These waves, in the range of kilohertz, can in principle be detected.

quant-ph↗

arXiv2000

Understanding Spin: the field theory of magnetic interactions

Spin is commonly thought to reflect the true quantum nature of microphysics. We show that spin is related to intrinsic and field-like properties of single particles. These properties change continuously in external magnetic fields. Interactions of massive particles with homogeneous and inhomogeneous fields result in two discrete particle states, symmetric to the original one. We analyze the difficulties in quantum mechanics to give a precise spacetime account of the experiments and find that they arise from unsuitable analogies for spin. In particular from the analogy of an angular momentum. Several experiments are suggested to check the model against the standard model in quantum mechanics.

quant-ph↗

arXiv2000

A dynamic model of atoms: structure, internal interactions and photon emissions of hydrogen

The standard solution of the Schroedinger equation for the hydrogen atom is analyzed. Comparing with the recently established internal properties of electrons it is found, that these solutions cannot be seen as physically valid states of the electron wave. The paper therefore proposes a new model of hydrogen based on internal properties of electrons. The ground state of the hydrogen system (T=0) is an inertial aggregation within the atomic shell, the calculation yields an atomic radius of 0.330 nm. Electron proton interaction within the atom are treated with a causal and deterministic model, the resonance frequency of the hydrogen system of $ 6.57 \times 10^{15} Hz $ is referred to elastic deformations of its nucleus, resonance levels are a result of boundary conditions for radial electron waves and photon interactions due to nuclear oscillations. Spectral emissions of excited atoms can be referred to a decay of the state of motion of the coupled electron-proton system. The framework developed is essentially deterministic, microphysical processes analyzed are referred to material characteristics of particles involved. Statistical effects are referred to interactions with the atomic environment, the results derived are compatible with the second and third principle of thermodynamics.

quant-ph↗

arXiv2000

Measurements in quantum physics: towards a physical picture of relevant processes

We propose a new structure of ensembles in quantum theory, based on the recently introduced intrinsic properties of electrons and photons. On this statistical basis the spreading of a wave-packet, collapse of the wave function, the quantum eraser, and interaction-free measurements are re-analyzed and the usual conceptual problems removed.

quant-ph↗

arXiv2000

Evidence for a dynamic origin of charge

The fundamental equations of particle motion lead to a modified Poisson equation including dynamic charge. This charge derives from density oscillations of a particle; it is not discrete, but continuous. Within the dynamic model of hydrogen it accounts for all features of electron proton interactions, its origin are density oscillations of the proton. We propose a new system of electromagnetic units, based on meter, kilogram, and second, bearing on these findings. The system has none of the disadvantages of traditional three-unit systems. On the basis of our theoretical model we can genuinely derive the scaling factor between electromagnetic and mechanic variables, which is equal, within a few percent, to Planck's constant h. The implications of the results in view of unifying gravity and quantum theory are discussed. It seems that the hypothetical solar gravity waves, in the low frequency range of the electromagnetic spectrum, are open to experimental detection.

quant-ph↗