Thin-Film Trilayer Manganate Junctions

Spin-dependent conductance across a manganate-barrier-manganate junction has recently been demonstrated. The junction is a La$_{0.67}$Sr$_{0.33}$MnO$_3$% -SrTiO$_3$-La$_{0.67}$ Sr$_{0.33}$MnO$_3$ trilayer device supporting current-perpendicular transport. Large magnetoresistance of up to a factor of five change was observed in these junctions at 4.2K in a relatively low field of the order of 100 Oe. Temperature and bias dependent studies revealed a complex junction interface structure whose materials physics has yet to be understood.Comment: 20 pages, 14 figures. To appear in Phil. Trans. R. Soc. Lond. A vol.356 (1998

In search of space: Fourier spectroscopy, 1950-1970

In the large grey area between science and technology, specialisms emerge with associated specialists. But some specialisms remain ‘peripheral sciences’, never attaining the status of ‘disciplines’ ensconced in universities, and their specialists do not become recognised professionals. A major social component of such side-lined sciences – one important grouping of technoscientific workers – is the ‘research-technology community’. An important question concerning research-technology is to explain how the grouping survives without specialised disciplinary and professional affiliations. The case discussed illustrates the dynamics of one such community

In search of space: Fourier spectroscopy, 1950-1970

In the large grey area between science and technology, specialisms emerge with associated specialists. But some specialisms remain ‘peripheral sciences’, never attaining the status of disciplines ensconced in universities, and their specialists do not become recognised professionals. A major social component of such side-lined sciences – one important grouping of techno-scientific workers – is the research-technology community. An important question concerning research-technology is to explain how the grouping survives without specialised disciplinary and professional affiliations. The case discussed illustrates the dynamics of one such community

Electron-Phonon Interactions in C28_{28}-derived Molecular Solids

We present {\it ab initio} density-functional calculations of molecular solids formed from C$_{28}$-derived closed-shell fullerenes. Solid C$_{28}$H$_4$ is found to bind weakly and exhibits many of the electronic structure features of solid C$_{60}$ with an enhanced electron-phonon interaction potential. We show that chemical doping of this structure is feasible, albeit more restrictive than its C$_{60}$ counterpart, with an estimated superconducting transition temperature exceeding those of the alkali-doped C$_{60}$ solids.Comment: Lower quality postscript file for Figure 1 is used in the manuscript in order to meet submission quota for pre-print server. Higher quality postscript file available from author: [email protected] This article has been updated to reflect changes incorporated during the peer review process. It is published in PRB 70, 140504(R) 200

Tetrahedrally coordinated carbonates in Earth's lower mantle

Carbonates are the main species that bring carbon deep into our planet through subduction. They are an important rock-forming mineral group, fundamentally distinct from silicates in Earth's crust in that carbon binds to three oxygen atoms, while silicon is bonded to four oxygens. Here, we present experimental evidence that under the sufficiently high pressures and high temperatures existing in the lower mantle, ferromagnesian carbonates transform to a phase with tetrahedrally coordinated carbons. Above 80 GPa, in situ synchrotron infrared experiments show the unequivocal spectroscopic signature of the high-pressure phase of (Mg,Fe)CO$_3$. Using ab-initio calculations, we assign the new IR signature to C-O bands associated with tetrahedrally coordinated carbon with asymmetric C-O bonds. Tetrahedrally coordinated carbonates are expected to exhibit substantially different reactivity than low pressure three-fold coordinated carbonates, as well as different chemical properties in the liquid state. Hence this may have significant implications on carbon reservoirs and fluxes and the global geodynamic carbon cycle

Quantum Information Processing with Ferroelectrically Coupled Quantum Dots

I describe a proposal to construct a quantum information processor using ferroelectrically coupled Ge/Si quantum dots. The spin of single electrons form the fundamental qubits. Small (<10 nm diameter) Ge quantum dots are optically excited to create spin polarized electrons in Si. The static polarization of an epitaxial ferroelectric thin film confines electrons laterally in the semiconductor; spin interactions between nearest neighbor electrons are mediated by the nonlinear process of optical rectification. Single qubit operations are achieved through "g-factor engineering" in the Ge/Si structures; spin-spin interactions occur through Heisenberg exchange, controlled by ferroelectric gates. A method for reading out the final state, while required for quantum computing, is not described; electronic approaches involving single electron transistors may prove fruitful in satisfying this requirement.Comment: 10 pages, 3 figure

Germanium quantum dots: Optical properties and synthesis

Three different size distributions of Ge quantum dots (>~200, 110, and 60 Å) have been synthesized via the ultrasonic mediated reduction of mixtures of chlorogermanes and organochlorogermanes (or organochlorosilanes) by a colloidal sodium/potassium alloy in heptane, followed by annealing in a sealed pressure vessel at 270 °C. The quantum dots are characterized by transmission electron microscopy, x-ray powder diffraction, x-ray photoemission, infrared spectroscopy, and Raman spectroscopy. Colloidal suspensions of these quantum dots were prepared and their extinction spectra are measured with ultraviolet/visible (UV/Vis) and near infrared (IR) spectroscopy, in the regime from 0.6 to 5 eV. The optical spectra are correlated with a Mie theory extinction calculation utilizing bulk optical constants. This leads to an assignment of three optical features to the E(1), E(0'), and E(2) direct band gap transitions. The E(0') transitions exhibit a strong size dependence. The near IR spectra of the largest dots is dominated by E(0) direct gap absorptions. For the smallest dots the near IR spectrum is dominated by the Gamma25-->L indirect transitions

Electrical writing, deleting, reading, and moving of magnetic skyrmioniums in a racetrack device

A magnetic skyrmionium (also called 2$\pi$-skyrmion) can be understood as a skyrmion - a topologically non-trivial magnetic whirl - which is situated in the center of a second skyrmion with reversed magnetization. Here, we propose a new optoelectrical writing and deleting mechanism for skyrmioniums in thin films, as well as a reading mechanism based on the topological Hall voltage. Furthermore, we point out advantages for utilizing skyrmioniums as carriers of information in comparison to skyrmions with respect to the current-driven motion. We simulate all four constituents of an operating skyrmionium-based racetrack storage device: creation, motion, detection and deletion of bits. The existence of a skyrmionium is thereby interpreted as a '1' and its absence as a '0' bit.Comment: This is a post-peer-review, pre-copyedit version of an article published in Scientific Reports. The final authenticated version is available online at [DOI