A gas cell apparatus for measuring charge exchange cross sections with multicharged ions.

A gas cell apparatus to measure charge exchange cross sections for charge state- and energy-resolved ion beams with neutrals is described. The design features a short well-defined interaction region required for beams of multicharged ions with high cross sections. Our method includes measuring the beam transmission at four different neutral pressures and extracting the cross section from the slope of a beam loss vs pressure plot. The design and procedure were tested for Ar+ interacting with neutral Ar gas over the incident ion energy range of 1.0-5.0 keV. The charge exchange cross sections agree well with previous complementary measurement techniques.

Triarylmethyl-based 2D covalent networks: virtual screening of chemical functionalisation for optimising strain-induced property control.

Two-dimensional covalent networks based on triarylmethyl (TAM) radical monomers have been proposed as versatile materials whose unpaired electrons may be externally localised/delocalised through the application of external uniaxial strain. This phenomenon arises through the strain-induced variance of the dihedral twist angles of the aryl rings within the network, and allows the control of important physico-chemical properties (e.g. magnetic interactions, electronic band gap). In order to experimentally realise such materials, one must find a compromise between the kinetic stability of the TAM monomers (through sterically protecting the radical centre with the appropriate aryl ring functionalisation) and the structural flexibility of the resulting material (provided by low intra-ring steric hindrance). In this work, through an efficient search procedure based on force field-based screening, employing ∼1750 calculations, followed by selected accurate electronic structure calculations, we provide support for the experimental viability of TAM-based 2D networks with highly controllable properties.

Spin-orbit-coupled fermions in an optical lattice clock.

Engineered spin-orbit coupling (SOC) in cold-atom systems can enable the study of new synthetic materials and complex condensed matter phenomena. However, spontaneous emission in alkali-atom spin-orbit-coupled systems is hindered by heating, limiting the observation of many-body effects and motivating research into potential alternatives. Here we demonstrate that spin-orbit-coupled fermions can be engineered to occur naturally in a one-dimensional optical lattice clock. In contrast to previous SOC experiments, here the SOC is both generated and probed using a direct ultra-narrow optical clock transition between two electronic orbital states in 87Sr atoms. We use clock spectroscopy to prepare lattice band populations, internal electronic states and quasi-momenta, and to produce spin-orbit-coupled dynamics. The exceptionally long lifetime of the excited clock state (160 seconds) eliminates decoherence and atom loss from spontaneous emission at all relevant experimental timescales, allowing subsequent momentum- and spin-resolved in situ probing of the SOC band structure and eigenstates. We use these capabilities to study Bloch oscillations, spin-momentum locking and Van Hove singularities in the transition density of states. Our results lay the groundwork for using fermionic optical lattice clocks to probe new phases of matter.

Collective atomic scattering and motional effects in a dense coherent medium.

We investigate collective emission from coherently driven ultracold (88)Sr atoms. We perform two sets of experiments using a strong and weak transition that are insensitive and sensitive, respectively, to atomic motion at 1 µK. We observe highly directional forward emission with a peak intensity that is enhanced, for the strong transition, by >10(3) compared with that in the transverse direction. This is accompanied by substantial broadening of spectral lines. For the weak transition, the forward enhancement is substantially reduced due to motion. Meanwhile, a density-dependent frequency shift of the weak transition (∼10% of the natural linewidth) is observed. In contrast, this shift is suppressed to <1% of the natural linewidth for the strong transition. Along the transverse direction, we observe strong polarization dependences of the fluorescence intensity and line broadening for both transitions. The measurements are reproduced with a theoretical model treating the atoms as coherent, interacting radiating dipoles.

Exchange Coupling Inversion in a High-Spin Organic Triradical Molecule.

The magnetic properties of a nanoscale system are inextricably linked to its local environment. In adatoms on surfaces and inorganic layered structures, the exchange interactions result from the relative lattice positions, layer thicknesses, and other environmental parameters. Here, we report on a sample-dependent sign inversion of the magnetic exchange coupling between the three unpaired spins of an organic triradical molecule embedded in a three-terminal device. This ferro-to-antiferromagnetic transition is due to structural distortions and results in a high-to-low spin ground-state change in a molecule traditionally considered to be a robust high-spin quartet. Moreover, the flexibility of the molecule yields an in situ electric tunability of the exchange coupling via the gate electrode. These findings open a route to the controlled reversal of the magnetic states in organic molecule-based nanodevices by mechanical means, electrical gating, or chemical tailoring.

Quantum simulation. Spectroscopic observation of SU(N)-symmetric interactions in Sr orbital magnetism.

SU(N) symmetry can emerge in a quantum system with N single-particle spin states when spin is decoupled from interparticle interactions. Taking advantage of the high measurement precision offered by an ultrastable laser, we report a spectroscopic observation of SU(N ≤ 10) symmetry in (87)Sr. By encoding the electronic orbital degree of freedom in two clock states while keeping the system open to as many as 10 nuclear spin sublevels, we probed the non-equilibrium two-orbital SU(N) magnetism via Ramsey spectroscopy of atoms confined in an array of two-dimensional optical traps; we studied the spin-orbital quantum dynamics and determined the relevant interaction parameters. This study lays the groundwork for using alkaline-earth atoms as testbeds for important orbital models.

Challenges in modelling the reaction chemistry of interstellar dust.

Studies aiming to understand the physicochemical properties of interstellar dust and the chemical reactions that occur on and in it have traditionally been the preserve of astronomical observation and experimental attempts to mimic astronomically relevant conditions in the laboratory. Increasingly, computational modelling in its various guises is establishing a complementary third pillar of support to this endeavour by providing detailed insights into the complexities of interstellar dust chemistry. Inherently, the basis of computational modelling is to be found in the details (e.g. atomic structure/composition, reaction barriers) that are difficult to probe accurately from observation and experiment. This bottom-up atom-based theoretical approach, often itself based on deeper quantum mechanical principles, although extremely powerful, also has limitations when systems become too large or complex. In this Perspective, after first providing a general background to the current state of observational-based knowledge, we introduce a number of computational modelling methods with reference to recent state-of-the-art studies, in order to highlight the capabilities of such approaches in this field. Specifically, we first outline the use of computational chemistry methods for dust nucleation, structure, and individual reactions on bare and icy dust surfaces. Later, we review kinetic modelling of networks of reactions relevant to dust chemistry and how to take into account quantum tunnelling effects in the low temperature reactions in the interstellar medium. Finally, we point to the future challenges that need to be overcome for computational modelling to provide even more detailed and encompassing perspectives on the nature and reaction chemistry of interstellar dust.

An optical lattice clock with accuracy and stability at the 10(-18) level.

Progress in atomic, optical and quantum science has led to rapid improvements in atomic clocks. At the same time, atomic clock research has helped to advance the frontiers of science, affecting both fundamental and applied research. The ability to control quantum states of individual atoms and photons is central to quantum information science and precision measurement, and optical clocks based on single ions have achieved the lowest systematic uncertainty of any frequency standard. Although many-atom lattice clocks have shown advantages in measurement precision over trapped-ion clocks, their accuracy has remained 16 times worse. Here we demonstrate a many-atom system that achieves an accuracy of 6.4 × 10(-18), which is not only better than a single-ion-based clock, but also reduces the required measurement time by two orders of magnitude. By systematically evaluating all known sources of uncertainty, including in situ monitoring of the blackbody radiation environment, we improve the accuracy of optical lattice clocks by a factor of 22. This single clock has simultaneously achieved the best known performance in the key characteristics necessary for consideration as a primary standard-stability and accuracy. More stable and accurate atomic clocks will benefit a wide range of fields, such as the realization and distribution of SI units, the search for time variation of fundamental constants, clock-based geodesy and other precision tests of the fundamental laws of nature. This work also connects to the development of quantum sensors and many-body quantum state engineering (such as spin squeezing) to advance measurement precision beyond the standard quantum limit.

One-dimensional embedded cluster approach to modeling CdS nanowires.

We present an embedded cluster model to treat one-dimensional nanostructures, using a hybrid quantum mechanical/molecular mechanical (QM/MM) approach. A segment of the nanowire (circa 50 atoms) is treated at a QM level of theory, using density functional theory (DFT) with a hybrid exchange-correlation functional. This segment is then embedded in a further length of wire, treated at an MM level of theory. The interaction between the QM and MM regions is provided by an embedding potential located at the interface. Point charges are placed beyond the ends of the wire segment in order to reproduce the Madelung potential of the infinite system. We test our model on the ideal system of a CdS linear chain, benchmarking our results against calculations performed on a periodic system using a plane-wave DFT approach, with electron exchange and correlation treated at the same level of approximation in both methods. We perform our tests on pure CdS and, importantly, the system containing a single In or Cu impurity. We find excellent agreement in the determined electronic structure using the two approaches, validating our embedded cluster model. As the hybrid QM/MM model avoids spurious interactions between charged defects, it will be of benefit to the analysis of the role of defects in nanowire materials, which is currently a major challenge using a plane-wave DFT approach. Other advantages of the hybrid QM/MM approach over plane-wave DFT include the ability to calculate ionization energies with an absolute reference and access to high levels of theory for the QM region which are not incorporated in most plane-wave codes. Our results concur with available experimental data.

Long range coupling between defect centres in inorganic nanostructures: valence alternation pairs in nanoscale silica.

Valence alternation pair (VAP) states are formed by a closed-shell combination of two space- and charge-separated topological defect centres. These pairs of defects, although historically invoked to explain the electronic properties of bulk inorganic glassy materials (e.g., amorphous silicon dioxide) via the concept of negative-U defects, have more recently been found in a number of theoretical studies of silica surfaces and nanoscale silica clusters. Using density functional theory we systematically probe the structure and internal stability of VAPs in a number of silica nanoclusters with respect to the separation of the two constituent defect centres. We find that VAP states in nanosilica are strongly stabilised by the attractive electrostatic interaction between their separated oppositely charged component defects such that VAPs can persist up to an internal separation of a least 1.5 nanometres. Beyond this distance VAPs become unstable with respect to an open-shell combination of topological defects, virtually indistinguishable from two isolated open-shell defect centres. Finally, we theoretically analyse the possibility of experimental observation of VAP states through their infra-red vibrational spectra.

Olfaction in dentistry.

Practitioners of oral medicine frequently encounter patients with complaints of taste disturbance. While some such complaints represent pathological processes specific to the gustatory system, per se, this is rarely the case. Unless taste-bud mediated qualities such as sweet, sour, bitter, salty, umami, chalky, or metallic are involved, 'taste' dysfunction inevitably reflects damage to the sense of smell. Such 'taste' sensations as chicken, chocolate, coffee, raspberry, steak sauce, pizza, and hamburger are dependent upon stimulation of the olfactory receptors via the nasopharynx during deglutition. In this paper, we briefly review the anatomy, physiology, and pathophysiology of the olfactory system, along with means for clinically assessing its function. The prevalence, etiology, and nature of olfactory disorders commonly encountered in the dental clinic are addressed, along with approaches to therapy and patient management.

The effect of local environment on photoluminescence: a time-dependent density functional theory study of silanone groups on the surface of silica nanostructures.

The optical absorption spectrum and lowest photoluminescence (PL) signal for silanone terminated silica nanostructures are studied using time-dependent density functional theory calculations on a range of realistic low energy silica nanocluster models. We show that the broad experimental absorption spectrum for silanone centers [V. A. Radtsig and I. M. Senchenya Russ. Chem. Bull. 45, 1849 (1996)] is most likely the result of a synergetic combination of inhomogeneous broadening, thermal broadening and the small energy differences between different excitations. We further demonstrate that upon relaxation of the excited state the excited electron and hole localize on only one silanone center, and that there is a clear and distinct link between the local environment of a silanone center and its absorption and PL spectra. Finally, we provide strong evidence that the silanone center does not have a double bond between the constituent silicon and oxygen atoms but rather can be probably more aptly described as the =Si(+)-O(-) charge-transfer species.

Optical excitations of defects in realistic nanoscale silica clusters: comparing the performance of density functional theory using hybrid functionals with correlated wavefunction methods.

Optical excitations of low energy silica (SiO(2))(4) clusters obtained by global optimization, as opposed to constructed by hand, are studied using a range of theoretical methods. By focusing on the lowest energy silica clusters we hope to capture at least some of the characteristic ways by which the dry surfaces of silica nanosystems preferentially terminate. Employing the six lowest energy (SiO(2))(4) cluster isomers, we show that they exhibit a surprisingly wide range of geometries, defects, and associated optical excitations. Some of the clusters show excitations localized on isolated defects, which are known from previous studies using hydrogen-terminated versions of the defect in question. Other clusters, however, exhibit novel charge-transfer excitations in which an electron transfers between two spatially separated defects. In these cases, because of the inherent proximity of the constituent defects due to the small cluster dimensions, the excitation spectrum is found to be very different from that of the same defects in isolation. Excitation spectra of all clusters were calculated using time-dependent density functional theory (TD-DFT) and delta-SCF DFT (DeltaDFT) methods employing two different hybrid density functionals (B3LYP and BB1K) differing essentially in the amount of incorporated Hartree-Fock-like exchange (HFLE). In all cases the results were compared with CASPT2 calculated values which are taken as a benchmark standard. In line with previous work, the spatially localized excitations are found to be well described by TD-DFT/B3LYP but which gives excitation energies that are significantly underestimated in the case of the charge-transfer excitations. The TD-DFT/BB1K combination in contrast is found to give generally good excitation energies for the lowest excited states of both localized and charge-transfer excitations. Finally, our calculations suggest that the increased quality of the predicted excitation spectra by adding larger amounts of HFLE is mainly due to an increased localization of the excited state associated with the elimination of spurious self-interaction inherent to (semi-)local DFT functionals.

Myocardial infarction risk and hormone replacement: differences between products.

OBJECTIVES: To establish the risk of myocardial infarction (MI) in users of hormone replacement therapy (HRT) compared with non-users and to compare the risk between different HRT regimens. METHODS: A population-based cohort and case-control study, and a case-control study nested within a cohort of HRT users, using the UK General Practice Research Database. Differences between HRT regimen, mode of administration and duration and recency of use were examined whilst adjusting for confounding. RESULTS: In the cohort and case-control study, 4537 cases of MI were identified in 2.62 million observed women years, cases were age-matched to 27,220 controls. In both studies, current and past HRT use were associated with reduced risk estimates for MI compared with no prior use. MIs were less likely to be fatal amongst women who had used HRT than amongst never users (OR(adj) 0.58; 95% CI 0.45-0.75). No difference in risk was seen between current and past use, oral and transdermal HRT or between different regimens (p>0.44). In the nested study, no difference was found in the association with MI risk between different oestrogen-progestogen combinations or between different combinations and tibolone. Unopposed oestrogen use was not associated with a decrease in risk compared with combined HRT. CONCLUSIONS: These results are consistent with previous observational studies in supporting the hypothesis that use of postmenopausal HRT is associated with a decrease in risk of acute myocardial infarction (AMI). Case fatality differed between HRT users and non-users, suggesting a protective effect of HRT. This study does not demonstrate a difference between regimens.

Self-diffusion of molecular hydrogen in clathrasils compared: dodecasil 3C versus sodalite.

The self-diffusion coefficient of molecular hydrogen through the all-silica microporous dodecasil 3C structure is calculated by means of molecular-dynamics (MD) calculations, allowing for full framework flexibility, in order to assess the material's feasibility as a hydrogen storage medium. The hydrogen uptake rate into dodecasil 3C is compared to that previously calculated for sodalite and it is found that the latter performs significantly better. The reason for this variation in performance is found to lie in intrinsic topological differences between each framework type. This is explicitly demonstrated by means of a simplified version of transition state theory helping to succinctly rationalize the MD data.

Novel structures and energy spectra of hydroxylated (SiO2)8-based clusters: searching for the magic (SiO2)8O2H3- cluster.

The prominent (SiO(2))(8)O(2)H(3) (-) mass peak resulting from the laser ablation of hydroxylated silica, attributed to magic cluster formation, is investigated employing global optimization with a dedicated interatomic potential and density functional calculations. The low-energy spectra of cluster isomers are calculated for the closed shell clusters: (SiO(2))(8)OH(-) and (SiO(2))(8)O(2)H(3) (-) giving the likely global minima in each case. Based upon our calculated cluster structures and energetics, and further on the known experimental details, it is proposed that the abundant formation of (SiO(2))(8)O(2)H(3) (-) clusters is largely dependent on the high stability of the (SiO(2))(8)OH(-) ground state cluster. Both the (SiO(2))(8)O(2)H(3) (-) and (SiO(2))(8)OH(-) ground state clusters are found to exhibit cagelike structures with the latter containing a particularly unusual tetrahedrally four-coordinated oxygen center not observed before in either bulk silica or silica clusters. The bare ground state (SiO(2))(8)O(2-) cluster ion core is also found to have four tetrahedrally symmetric Si==O terminations making it a possible candidate, when combined with suitable cations, for extended cluster-based structures/materials.

Effect of cation distribution on self-diffusion of molecular hydrogen in Na3Al3Si3O12 sodalite: a molecular dynamics study.

The diffusion of hydrogen in sodium aluminum sodalite (NaAlSi-SOD) is modeled using classical molecular dynamics, allowing for full flexibility of the host framework, in the temperature range 800-1200 K. From these simulations, the self-diffusion coefficient is determined as a function of temperature and the hydrogen uptake at low equilibrium hydrogen concentration is estimated at 573 K. The influence of the cation distribution over the framework on the hydrogen self-diffusion is investigated by comparing results employing a low energy fully ordered cation distribution with those obtained using a less ordered distribution. The cation distribution is found to have a surprisingly large influence on the diffusion, which appears to be due to the difference in framework flexibility for different cation distributions, the occurrence of correlated hopping in case of the ordered distribution, and the different nature of the diffusion processes in both systems. Compared to our previously reported calculations on all silica sodalite (all-Si-SOD), the hydrogen diffusion coefficient of sodium aluminum sodalite is higher in the case of the ordered distribution and lower in case of the disordered distribution. The hydrogen uptake rates of all-Si-SOD and NaSiAl-SOD are comparable at high temperatures (approximately 1000 K) and lower for all-Si-SOD at lower temperatures (approximately 400 K).

Molecular-dynamics analysis of the diffusion of molecular hydrogen in all-silica sodalite.

In order to investigate the technical feasibility of crystalline porous silicates as hydrogen storage materials, the self-diffusion of molecular hydrogen in all-silica sodalite is modeled using large-scale classical molecular-dynamics simulations employing full lattice flexibility. In the temperature range of 700-1200 K, the diffusion coefficient is found to range from 1.610(-10) to 1.810(-9) m(2)/s. The energy barrier for hydrogen diffusion is determined from the simulations allowing the application of transition state theory, which, together with the finding that the pre-exponential factor in the Arrhenius-type equation for the hopping rate is temperature-independent, enables extrapolation of our results to lower temperatures. Estimates based on mass penetration theory calculations indicate a promising hydrogen uptake rate at 573 K.

Utilisation of hormone replacement therapy in the United Kingdom. A descriptive study using the general practice research database.

OBJECTIVE: To determine prevalence and patterns of hormone replacement therapy (HRT) utilisation in women in the UK. DESIGN: Prospective observational study. SETTING: UK general practice. POPULATION: Women from general practices throughout the UK. METHODS: The study period was 1 January 1992 to 31 December 1998. Age-specific prevalence was calculated for each year. Trends in prescribing patterns were described over time. A sub-cohort of 'new starters' on HRT was identified to establish patterns of use, including duration of use and switching of preparations. Characteristics of the sub-cohort were compared with a reference group of non-HRT users. MAIN OUTCOME MEASURES: Prevalence, prescribing patterns and differences in characteristics between HRT users and non-HRT users. RESULTS: Among women aged 45-64, prevalence of HRT use increased from 18.6% in 1992 to 27.7% in 1998. Secular trends were observed away from prescribing combined-sequential preparations and towards use of combined-continuous preparations. Among the prescriptions for combined HRT products, only 4.3% contained medroxyprogesterone acetate (MPA). A total of 45.7% of women without a record of a hysterectomy and 53.8% of women with a record of a hysterectomy used HRT for at least three years. When women were partitioned by year of starting HRT, there was a trend of increasing duration of use across the seven years. Some women without a record of a hysterectomy were receiving unopposed oestrogen without progestogen supplementation. CONCLUSIONS: Within the past decade, use of HRT has increased among women in the UK with large numbers of women using HRT for long periods and treatment often tailored to the individual.

Fully coordinated silica nanoclusters: (SiO2)N molecular rings.

A new form of finite silica with edge-sharing SiO2 units connected in a ring is proposed. High-level density-functional calculations for (SiO2)(N), N=4-14, show the rings to be energetically more stable than the corresponding (SiO2)(N) linear chains for N>11. The rings display frequency modes in remarkable agreement with infrared bands measured on dehydrated silica surfaces indicating their potential as models of strained extended silica systems. Silica rings, if synthesized, may also be useful precursors for new bulk-silica polymorphs with tubular or porous morphologies.