arXiv subjects

F. J. Himpsel

Publications and source records attributed to F. J. Himpsel.

16 recordsLinked to original sources

arXiv2018

Dynamical Symmetry Breaking by SU(2) Gauge Bosons

This work explores the possibility of obtaining a mass gap in Yang-Mills theories via the intrinsic gauge bosons, without invoking a separate Higgs boson or fermion-antifermion pairs. Instead, pairs of gauge bosons in the spin and isospin singlet state form a pair of composite Higgs bosons which can be viewed as the simplest possible glueball of Yang-Mills gauge theories. Quadratic and quartic gauge boson self-interactions form a potential that leads to a finite expectation value of the gauge boson amplitude. Transverse polarization ensures Lorentz invariance of the vacuum after averaging over all possible polarization vectors. But the scalar pair products exhibit a finite vacuum expectation value which breaks the gauge symmetry dynamically. Compatibility with the standard Higgs potential determines the quadratic and quartic coupling constants.

hep-ph↗

arXiv2017

The Exchange Hole in the Dirac Sea

This work is motivated by the long-standing question about the internal stability of the electron. While one cannot investigate internal properties of a point-like particle, it is fair to analyze the response of the Dirac sea to an electron. For this purpose the concept of an exchange hole is generalized from the Fermi sea of condensed matter physics to the Dirac sea of quantum electrodynamics. In order to conserve charge and angular momentum, an exchange electron is added to the exchange hole, forming a neutral exchange exciton. Consequently, the pair correlation defining the exchange hole is generalized to a three-fermion correlation. An approximation by sequential pair correlations provides analytic results for charge densities, average distances, and force densities of the exchange hole and electron. The analysis is supported by analogies to the negative ion of positronium.

quant-ph↗

arXiv2017

Hyperfine Wave Functions and Force Densities for the Hydrogen Atom

This study addresses the effect of the magnetic hyperfine interaction on the relativistic H1s wave functions. These are used to calculate the electric, magnetic, and confinement force densities acting on the 1s electron. The magnetic field couples Dirac equations for different angular momenta. These are solved numerically for the hyperfine singlet and triplet, as well as for a classical magnetic dipole. In the singlet ground state the hyperfine interaction shifts the electron density toward the proton. A similar shift is found for the classical dipole, and an opposite shift for the triplet. The cross-over between charge accumulation and depletion occurs at 1.325 times the Bohr radius. The behavior of the wave functions is investigated down to distances smaller than the proton radius, including the incorporation of virtual positrons. The force densities are determined and balanced against each other.

physics.atom-ph↗

arXiv2016

Force Density Balance inside the Hydrogen Atom

Motivated by the long-debated question about the internal stability of the electron, the force densities acting on the charge density of the 1s electron in the H atom are investigated. The problem is mapped onto the canonical formalism for a classical Dirac field coupled to the electric field of an external point charge. An explicit calculation shows that the attractive Coulomb force density is balanced exactly at every point in space by the repulsive confinement force density. The latter requires evaluating the divergence of the stress tensor for the 1s solution of the Dirac equation. Such a local force balance goes beyond the global stability criteria that are usually given for the H atom. This concept is extended to the internal stability of any charged particle by investigating the force densities acting on its surrounding vacuum polarization. At large distances one has to consider only the charge density of virtual electrons and positrons, induced by a point charge in the vacuum of quantum electrodynamics.

physics.atom-ph↗

arXiv2016

The Stability of the Vacuum Polarization Surrounding a Charged Particle

The internal stability of the electron has been debated for a century at both the classical and the quantum level. Recently, a local force density balance was established for the 1s electron in the H atom, based on the energy-momentum tensor of the classical Dirac field. This methodology is now extended to quantum fields by considering the force densities acting on the vacuum polarization induced by a point charge. Such a model is applicable to any charged particle at large distances, since the only vestige of its internal structure is the electric Coulomb field together with the vacuum polarization induced by it. While the polarization charge density is attracted to the point charge, it is kept from collapsing by repulsive forces due to confinement and degeneracy. It is shown analytically that the corresponding force densities are balanced for every filled shell of mj states at a given angular momentum j. The force densities are then summed over all single-electron states in the Dirac sea and renormalized by subtracting singular terms. In leading order of alpha, the force densities remain balanced. This result establishes a local force balance for a prototypical manybody system.

quant-ph↗

arXiv2015

A Higgs Boson Composed of Gauge Bosons

It is proposed to replace the Higgs boson of the standard model by a Lorentz- and gauge-invariant combination of SU(2) gauge bosons. A pair of Higgs bosons is identified with pairs of gauge bosons by setting their mass Lagrangians equal to each other. That immediately determines the mass of the composite Higgs boson. It becomes simply half of the vacuum expectation value of the standard Higgs boson, which matches the observed mass with tree-level accuracy (2%). The two parameters of the standard Higgs potential are replaced by five one-loop self-interactions of the SU(2) gauge bosons, derived from the fundamental gauge couplings. The Brout-Englert-Higgs mechanism of spontaneous symmetry breaking is generalized from scalars to vectors. Their transverse components acquire finite vacuum expectation values which generate masses for both gauge bosons and the Higgs boson. This concept leads beyond the standard model by enabling calculations of the Higgs mass and its potential without adjustable parameters. It can be applied to non-abelian gauge theories in general, such as grand unified models and supersymmetry.

hep-ph↗

arXiv2014

Spectroscopic evidence for spin-polarized edge states in graphitic Si nanowires

The step edges on the Si(553)-Au surface undergo a 1 x 3 reconstruction at low temperature which has recently been interpreted theoretically as the x3 ordering of spin-polarized silicon atoms at the edges of the graphitic Si nanowires on this vicinal surface. This predicted magnetic ground state has a clear spectroscopic signature - a silicon step-edge state at 0.5 eV above the Fermi level - that arises from strong exchange splitting and hence would not occur without spin polarization. Here we report spatially resolved scanning tunneling spectroscopy data for these nanowires. At low temperature we find an unoccupied state at 0.5 eV above every third step edge silicon atom, in excellent agreement with the spin-polarized ground state predicted theoretically. This spin-polarized state survives up to room temperature where the position of the spins rapidly fluctuates among all Si step-edge sites.

cond-mat.mes-hall↗

arXiv2011

Anharmonic Waves in Field Theory

This work starts from the premise that sinusoidal plane waves cease to be solutions of field theories when turning on an interaction. A nonlinear interaction term generates harmonics analogous to those observed in nonlinear optical media. This calls for a generalization to anharmonic waves in both classical and quantum field theory. Three simple requirements make anharmonic waves compatible with relativistic field theory and quantum physics. Some non-essential concepts have to be abandoned, such as orthogonality, the superposition principle, and the existence of single-particle energy eigenstates. The most general class of anharmonic waves allows for a zero frequency term in the Fourier series, which corresponds to a quantum field with a non-zero vacuum expectation value. Anharmonic quantum fields are defined by generalizing the expansion of a field operator into creation and annihilation operators. This method provides a framework for handling exact quantum fields, which define exact single particle states.

hep-th↗

arXiv2011

Quantum Electrodynamics with Anharmonic Waves

This is the second step of a program to use anharmonic plane waves as basis set in non-perturbative quantum field theory. The general framework developed previously is applied to quantum electrodynamics. To test the compatibility with standard quantum electrodynamics, the Feynman rules are generalized to anharmonic waves by expanding the field operators into anharmonic plane waves. A sum rule for the Fourier coefficients of anharmonic waves ensures that the generalized Feynman rules are equivalent to the standard rules after summing over all harmonics. It is possible to construct diagrams for the generation of harmonics. They are of O(alpha^2) for photons and of O(alpha) for electrons. To tackle intrinsically non-perturbative phenomena it is proposed to insert anharmonic field operators into the Dyson-Schwinger equations while retaining only the lowest harmonics.

hep-th↗

arXiv2009

Structure and energetics of Si(111)-(5x2)-Au

We propose a new structural model for the Si(111)-(5x2)-Au reconstruction. The model incorporates a new experimental value of 0.6 monolayer for the coverage of gold atoms, equivalent to six gold atoms per 5x2 cell. Five main theoretical results, obtained from first-principles total-energy calculations, support the model. (1) In the presence of silicon adatoms the periodicity of the gold rows spontaneously doubles, in agreement with experiment. (2) The dependence of the surface energy on the adatom coverage indicates that a uniformly covered phase is unstable and will phase-separate into empty and covered regions, as observed experimentally. (3) Theoretical scanning tunneling microscopy images are in excellent agreement with experiment. (4) The calculated band structure is consistent with angle-resolved photoemission spectra; analysis of their correspondence allows the straightforward assignment of observed surface states to specific atoms. (5) The calculated activation barrier for diffusion of silicon adatoms along the row direction is in excellent agreement with the experimentally measured barrier.

cond-mat.mtrl-sci↗

arXiv2002

Measurement of electron wave functions and confining potentials via photoemission

Wave functions and electron potentials of laterally-confined surface states are determined experimentally by means of photoemission from stepped Au(111) surfaces. Using an iterative formalism borrowed from x-ray diffraction, we retrieve the real-space wave functions from the Fourier transform of their momentum representations, whose absolute values in turn are directly measured by angle-resolved photoemission. The effective confining potential is then obtained by introducing the wave functions into Schroedinger's equation.

cond-mat.mtrl-sci↗

arXiv2002

Gd disilicide nanowires attached to Si(111) steps

Self-assembled electronic devices, such as quantum dots or switchable molecules, need self-assembled nanowires as connections. We explore the growth of conducting Gd disilicide nanowires at step arrays on Si(111). Atomically smooth wires with large aspect ratios are formed at low coverage and high growth rate (length >1 micron, width 10nm, height 0.6nm). They grow parallel to the steps in the [-1 1 0 ] direction, which is consistent with a lattice match of 0.8% with the a-axis of the hexagonal silicide, together with a large mismatch in all other directions. This mechanism is similar to that observed previously on Si(100). In contrast to Si(100), the wires are always attached to step edges on Si(111) and can thus be grown selectively on regular step arrays.

cond-mat↗

arXiv2002

Atomic Scale Memory at a Silicon Surface

The limits of pushing storage density to the atomic scale are explored with a memory that stores a bit by the presence or absence of one silicon atom. These atoms are positioned at lattice sites along self-assembled tracks with a pitch of 5 atom rows. The writing process involves removal of Si atoms with the tip of a scanning tunneling microscope. The memory can be reformatted by controlled deposition of silicon. The constraints on speed and reliability are compared with data storage in magnetic hard disks and DNA.

cond-mat↗

arXiv2002

Fermi Surfaces of Surface States on Si(111) + Ag, Au

Metallic surface states on semiconducting substrates provide an opportunity to study low-dimensional electrons decoupled from the bulk. Angle resolved photoemission is used to determine the Fermi surface, group velocity, and effective mass for surface states on Si(111)sqrt3xsqrt3-Ag, Si(111)sqrt3x sqrt3-Au, and Si(111)sqrt21xsqrt21-(Ag+Au). For Si(111)sqrt3xsqrt3-Ag the Fermi surface consists of small electron pockets populated by electrons from a few percent excess Ag. For Si(111)sqrt21xsqrt21-(Ag+Au) the added Au forms a new, metallic band. The sqrt21xsqrt21 superlattice leads to an intricate surface umklapp pattern and to minigaps of 110 meV, giving an interaction potential of 55 meV for the sqrt21xsqrt21 superlattice.

cond-mat↗

arXiv2000

Enhanced Spin Polarization of Conduction Electrons in Ni, explained by comparison with Cu

The spin-split Fermi level crossings of the conduction band in Ni are mapped out by high-resolution photoemission and compared to the equivalent crossing in Cu. The area of the quasiparticle peak decreases rapidly below Ef in Ni, but not in Cu. Majority spins have larger spectral weight at Ef than minority spins, thereby enhancing the spin-polarization beyond that expected from the density of states. A large part of the effect can be traced to a rapid variation of the matrix element with {\bf k} at the point where the s,p-band begins to hybridize with the $dz^2$ state. However, it is quite possible that the intensity drop in Ni is reinforced by a transfer of spectral weight from single-particle to many-electron excitations. The results suggest that the matrix element should be considered for explaining the enhanced spin polarization observed for Ni in spin-polarized tunneling.

cond-mat↗

arXiv2000

Orbital character of O 2p unoccupied states near the Fermi level in CrO2

The orbital character, orientation, and magnetic polarization of the O 2$p$ unoccupied states near the Fermi level ($E_F$) in CrO$_2$ was determined using polarization-dependent X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) from high-quality, single-crystal films. A sharp peak observed just above $E_F$ is excited only by the electric field vector ($\bf E$) normal to the tetragonal $c$-axis, characteristic of a narrow band ($\approx$ 0.7 eV bandwidth) constituted from O 2$p$ orbitals perpendicular to $c$ (O 2$p_y$) hybridized with Cr 3$d_{xz-yz}$ $t_{2g}$ states. By comparison with band-structure and configuration-interaction (CI) cluster calculations our results support a model of CrO$_2$ as a half-metallic ferromagnet with large exchange-splitting energy ($Δ_{exch-split}$ $\approx$ 3.0 eV) and substantial correlation effects.

cond-mat.mtrl-sci↗