Magnetic monopoles are hypothesized particles that have a magnetic, rather than electric, charge. Despite a long history of non-detection, monopoles continue to be a topic of study because of their possible role in the unification of the fundamental forces. In new theoretical work, Tanmay Vachaspati has considered whether monopoles could be generated through the scattering of waves. His results suggest monopoles might be detected in collisions between high-intensity, circularly polarized light waves.
Doping a semiconductor induces a change in lattice parameter, which is caused not only by the different sizes of the impurity and host atoms, but also by strain developed when occupation of electronic energy bands is modified. Experimentalists and theoreticians have struggled for decades to separate size and electronic effects. Following up on significant earlier developments, which focused on doped Si, a Physics-Chemistry ASU team collected data for n-type Ge doped with novel precursors and discovered trends as a function of the donor species in both Ge and Si. These results could be key to resolving the doping dependence of the lattice parameter.
Published in Physical Review B (doi:10.1103/PhysRevB.93.041201), and highlighted as an Editors' Suggestion.
Dexin Kong, a recently graduated PhD student in the Drucker group, measured the optical response of epitaxial Ag islands grown on Si(100), which host localized surface plasmon resonances (LSPR) that are oscillations in their charge density. LSPR modes that oscillate parallel to the Si surface have different frequencies than those that oscillate perpendicular to the Si surface. Theoretical models allow identification of these modes. The figures show scanning and transmission electron micrographs of the Ag/Si(100) islands and comparison of the real and imaginary parts of the dielectric functions of Si and the experimentally measured and simulated Ag/Si(100) pseudo-dielectric functions.
See: J. Appl. Phys., 118, 213103 (2015).
Former Physics graduate student Luying Li collaborated with Physics faculty Dave Smith and Molly McCartney to address the possibility of detecting polarization fields across polytype interfaces in InAs nanopillars using electron holography. The investigation included studying the local crystal polarity on an individual column-by-column basis. The false color image shows an aberration-corrected electron micrograph from an InAs nanopillar with irregular stacking disorder, and individual In and As atomic columns can be resolved.
See: Adv. Mater., 26, 1052 (2014)
In a recent collaboration with UT-Austin, physics faculty Dave Smith and Molly McCartney studied a classical semiconductor/ferroelectric interface between germanium and barium titanate. Aberration-corrected electron micrographs convincingly demonstrate that the Ge(001) 2x1 surface reconstruction remained intact during subsequent BaTiO3 growth. This information was key to explaining the otherwise unsolved electronic properties of the interface.
Image shows stepped interface between barium titanate epilayer and germanium substrate, together with overlaid structural model.
See: Appl. Phys, Lett., 104, 242908 (2014)
Materials graduate student Desai Zhang, working with Physics faculty Molly McCartney, used electron holography to resolve a longstanding question about the multi-grain nature of magnetic domains in thin spinel ferrite films: a) Lorentz image; b) phase image; b) map of magnetic domains; d) domain overlay on Lorentz image demonstrating the multigrain domains.
The composite image was featured as a cover illustration in Journal of Applied Physics.
From: J. Appl. Phys., 116, 083901 (2014)
Dr Daniel Martin in the Matyushov group took advantage of one of the fastest high performance supercomputers (Anton, D. E. Shaw) to look at electron tunneling in proteins. Membrane-bound bc1 complex is a key element of biology’s energy production chain in mitochondria of animals and photosynthetic centers of bacteria. More than 10 microseconds of fully atomistic computer simulations allowed the protein's low frequency motions driving electron transfer to be studied. A broad range of fluctuations, spanning from picoseconds to microseconds, affected the transition. Surprisingly, slow motions, in the range 0.1-1.6 microseconds turned out to be particularly important.
See: J. Chem. Phys. 142, 161101 (2015).
Zeolites are important silicate-based materials related to sand (i.e. quartz), which contain pores and tunnels that can absorb water and small hydrocarbon molecules, unlike quartz. This micro-porosity gives zeolites enormous internal surface area, enabling a rich and diverse array of chemical uses. Many millions of low-energy hypothetical topologies have been discovered in the computer by treating zeolites as periodic graphs, and there is theoretically an in¬finite number of hypothetical frameworks. “Why are so few topologies found in nature?” The key appears to be framework flexibility. Known zeolite frameworks are flexible if their SiO4 tetrahedra are ideal and rigid, allowing flexing only at the vertices. Thus, low framework energy is not enough, the framework must be able to “breathe”. This key property allows the smaller subset of realizable hypothetical frameworks to be identified.
See: Z. Krist., 212 768–791 (1997); Microporous & Mesoporous Materials 74 121–132. (2004).
The structures of amorphous materials are hard to determine. They are highly disordered, but atoms cannot get arbitrarily close, so there must be strong short-range order. “What is the length range of this order?” Recent graduate student Aram Rezikyan studied amorphous silicon and carbon using Fluctuation Electron Microscopy (FEM). By studying random di¬ffraction speckle as a function of di¬ffraction vector, he probed the role of electron beam damage in disruption of the amorphous structure. His results showed that atoms moved under the electron beam much more than previously thought.
Combining spectroscopy with high spatial resolution in the electron microscope is a powerful way to investigate chemistry, including local bonding changes, at the atomic scale. Traditionally, the equivalent of UV and soft X-ray spectroscopy could only be performed with energy resolution of just under 1 eV. The Nion electron microscope, with a specially developed monochromator, gives 10 meV energy resolution, allowing the equivalent of optical and infrared spectroscopy, while still at the nm scale. The spectrum on the left comes from a guanine fish scale. All peaks arising from vibration of hydrogen attached to C or N atoms are distinguishable. Since the presence of hydrogen is generally inferred in electron microscopy, but not directly detected, this represents a major advance for characterization of polymers and materials of biological origin.