A. Marshall Stoneham

Radiation effects in glasses used for immobilization of high-level waste and plutonium disposition

This paper is a comprehensive review of the state-of-knowledge in the field of radiation effects in glasses that are to be used for the immobilization of high-level nuclear waste and plutonium disposition. The current status and issues in the area of radiation damage processes, defect generation, microstructure development, theoretical methods and experimental methods are reviewed. Questions of fundamental and technological interest that offer opportunities for research are identified.

Potential for spin-based information processing in a thin-film molecular semiconductor.

Organic semiconductors are studied intensively for applications in electronics and optics, and even spin-based information technology, or spintronics. Fundamental quantities in spintronics are the population relaxation time (T1) and the phase memory time (T2): T1 measures the lifetime of a classical bit, in this case embodied by a spin oriented either parallel or antiparallel to an external magnetic field, and T2 measures the corresponding lifetime of a quantum bit, encoded in the phase of the quantum state. Here we establish that these times are surprisingly long for a common, low-cost and chemically modifiable organic semiconductor, the blue pigment copper phthalocyanine, in easily processed thin-film form of the type used for device fabrication. At 5 K, a temperature reachable using inexpensive closed-cycle refrigerators, T1 and T2 are respectively 59 ms and 2.6 μs, and at 80 K, which is just above the boiling point of liquid nitrogen, they are respectively 10 μs and 1 μs, demonstrating that the performance of thin-film copper phthalocyanine is superior to that of single-molecule magnets over the same temperature range. T2 is more than two orders of magnitude greater than the duration of the spin manipulation pulses, which suggests that copper phthalocyanine holds promise for quantum information processing, and the long T1 indicates possibilities for medium-term storage of classical bits in all-organic devices on plastic substrates.

Experimental and computational study of the gas-sensor behaviour and surface chemistry of the solid-solution Cr2−xTixO3(x≤ 0.5)

The solid solution Cr2−xTixO3 is an excellent gas sensor material, with stability of performance over the short and long-term and minor influences of variations of humidity. It is the first new material to be successfully commercialised in large-volume manufacture for sensing of hydrocarbons, volatile organic compounds (VOC), hydrogen and carbon monoxide at low (ppm) concentrations in air since the introduction of SnO2 for this purpose in the 1960s. The phase limit is at x ≃ 0.3–0.4, above which a 2-phase mixture with CrTiO3 is found. Substitution of Ti strongly decreases the electrical conductivity of the porous bodies studied. Surface high-valency Cr, assumed to be CrVI, whose proportion is decreased by Ti substitution, is detected by XPS. This effect, and the surface segregation of Ti, control the gas sensor behaviour. Defect models of the (0001) and (102) surfaces have been assessed by computational modelling: in the absence of Ti, one stable defect is a CrVI–VCr‴ pair, which is surface segregated at (0001) and contributes to the relatively high p-type conductivity shown by finely porous bodies of Cr2O3 at elevated temperature; with Ti addition, a stable defect, also surface segregated, is the complex (Ti˙Cr)3VCr‴. Distortion of the arrangement of surface oxygen above the Cr vacancy creates a possible binding site; the high-valency surface cation creates another. It is suggested that the two sites act in concert to promote the dissociation of oxygen and the surface reaction needed for gas sensing.

The initialization and manipulation of quantum information stored in silicon by bismuth dopants

A prerequisite for exploiting spins for quantum data storage and processing is long spin coherence times. Phosphorus dopants in silicon (Si:P) have been favoured as hosts for such spins because of measured electron spin coherence times (T2) longer than any other electron spin in the solid state: 14 ms at 7 K with isotopically purified silicon. Heavier impurities such as bismuth in silicon (Si:Bi) could be used in conjunction with Si:P for quantum information proposals that require two separately addressable spin species. However, the question of whether the incorporation of the much less soluble Bi into Si leads to defect species that destroy coherence has not been addressed. Here we show that schemes involving Si:Bi are indeed feasible as the electron spin coherence time T2 is at least as long as for Si:P with non-isotopically purified silicon. We polarized the Si:Bi electrons and hyperpolarized the I=9/2 nuclear spin of (209)Bi, manipulating both with pulsed magnetic resonance. The larger nuclear spin means that a Si:Bi dopant provides a 20-dimensional Hilbert space rather than the four-dimensional Hilbert space of an I=1/2 Si:P dopant.

Theory of the growth mode for a thin metallic film on an insulating substrate

We have developed a novel theory predicting the growth mode of a thin metallic film on an insulating substrate. This combines ab initio electronic structure calculations for several ordered metal/insulator interfaces (varying both coverage and substrate lattice constant), with a thermodynamic approach based on microscopic calculations. We illustrate this approach for Ag film deposited on MgO(0 0 1) substrate. Ab initio calculations predict high mobility of adsorbed silver atoms on the perfect magnesia surface even at low temperatures. Our theoretical analysis clearly demonstrates that the growth of metallic islands is predominant at the initial stage of silver deposition, which agrees with the experimental data. 2001 Published by Elsevier Science B.V.

Excitonic model of track registration of energetic heavy ions in insulators

Abstract The consequence of generation of dense electronic excitation along the paths of energetic heavy ions is discussed, emphasizing the fates of electron–hole pairs. It is pointed out that a substantial part of the energy imparted to electron–hole pairs in the materials in which excitons are self-trapped is converted directly to defect formation energy but do not contribute to heating. However, the thermal spike model can be an appropriate macroscopic model of the track registration of the materials in which excitons are self-trapped, because energy deposited to the material remains along the ion paths. The energy imparted to electron–hole pairs is diffused away from the ion paths in the materials in which excitons are not self-trapped. This explains the reason why the critical stopping power for track registration is higher in these materials. The difficulty for application of the thermal spike model to these materials is pointed out and it is suggested that nominal defects in densely excited region nucleate fragmental tracks.

A New Type of Radical-Pair-Based Model for Magnetoreception

Certain migratory birds can sense the Earths magnetic field. The nature of this process is not yet properly understood. Here we offer a simple explanation according to which birds literally see the local magnetic field through the impact of a physical rather than a chemical signature of the radical pair: a transient, long-lived electric dipole moment. Based on this premise, our picture can explain recent surprising experimental data indicating long lifetimes for the radical pair. Moreover, there is a clear evolutionary path toward this field-sensing mechanism: it is an enhancement of a weak effect that may be present in many species.

Ultralong copper phthalocyanine nanowires with new crystal structure and broad optical absorption.

The development of molecular nanostructures plays a major role in emerging organic electronic applications, as it leads to improved performance and is compatible with our increasing need for miniaturization. In particular, nanowires have been obtained from solution or vapor phase and have displayed high conductivity or large interfacial areas in solar cells. In all cases however, the crystal structure remains as in films or bulk, and the exploitation of wires requires extensive postgrowth manipulation as their orientations are random. Here we report copper phthalocyanine (CuPc) nanowires with diameters of 10-100 nm, high directionality, and unprecedented aspect ratios. We demonstrate that they adopt a new crystal phase, designated eta-CuPc, where the molecules stack along the long axis. The resulting high electronic overlap along the centimeter length stacks achieved in our wires mediates antiferromagnetic couplings and broadens the optical absorption spectrum. The ability to fabricate ultralong, flexible metal phthalocyanine nanowires opens new possibilities for applications of these simple molecules.

Could one make a diamond-based quantum computer?

We assess routes to a diamond-based quantum computer, where we specifically look towards scalable devices, with at least 10 linked quantum gates. Such a computer should satisfy the deVincenzo rules and might be used at convenient temperatures. The specific examples that we examine are based on the optical control of electron spins. For some such devices, nuclear spins give additional advantages. Since there have already been demonstrations of basic initialization and readout, our emphasis is on routes to two-qubit quantum gate operations and the linking of perhaps 10-20 such gates. We analyse the dopant properties necessary, especially centres containing N and P, and give results using simple scoping calculations for the key interactions determining gate performance. Our conclusions are cautiously optimistic: it may be possible to develop a useful quantum information processor that works above cryogenic temperatures.

Adhesion trends and growth mode of ultra-thin copper films on MgO

Ab initio simulations are performed for Cu atoms adsorbed on the perfect MgO(001) substrate, with an ordered metal coverage varied from 1 monolayer (ML), i.e. almost single atoms, up t o1M L. As trong dependence of the adhesion energy and the sub-monolayer film distance from the substrate on the surface coverage and adsorbate positions (Mg 2+ or O 2− )i s discussed. The nature of interfacial bonding at all coverages is physisorption .W hen increasing Cu atomic fraction, a decrease of the substrate-induced polarization of adatoms accompanied by an increase of both in-plane metallic bonding and the interfacial distance has been found. Combining results of ab initio calculations with thermodynamic theory (taking into account the lattice mismatch), we show that the metal cluster formation becomes the predominant growth mode even at low Cu coverages, in agreement with experiment. (Some figures in this article are in colour only in the electronic version)