Dynamics of a Pair of Interacting Spins Coupled to an Environmental Sea

We solve for the dynamics of a pair of spins, coupled to each other and also to an environmental sea of oscillators. The environment mediates an indirect interaction between the spins, causing both mutual coherence effects and dissipation. This model describes a wide variety of physical systems, ranging from 2 coupled microscopic systems (eg., magnetic impurities, bromophores, etc), to 2 coupled macroscopic quantum systems. We obtain analytic results for 3 regimes, viz., (i) The locked regime, where the 2 spins lock together; (ii) The correlated relaxation regime (mutually correlated incoherent relaxation); and (iii) The mutual coherence regime, with correlated damped oscillations. These results cover most of the parameter space of the system.Comment: 49 pages, To appear in Int J. Mod. Phys.

Reply to the Comment on the 'Hole-digging' in ensembles of tunneling molecular magnets

Reply to the Comment of J.J. Alonso and J.F. Fernandez on the paper "'Hole-digging' in ensembles of tunneling molecular magnets" of I.S. Tupitsyn, P.C.E. Stamp and N.V. Prokof'ev (Phys. Rev. B 69, 132406, (2004)).Comment: 1 LaTeX page, 1 PS figure; submitted to PR

Effective Hamiltonian in the Problem of a "Central Spin" Coupled to a Spin Environment

We consider here the problem of a "giant spin", with spin quantum number S>>1, interacting with a set of microscopic spins. Interactions between the microscopic spins are ignored. This model describes the low-energy properties of magnetic grains or magnetic macromolecules interacting with a surrounding spin environment, such as nuclear spins. We describe a general method for truncating the model to another one, valid at low energies, in which a two-level system interacts with the environmental spins, and higher energy terms are absorbed into a new set of couplings. This is done using an instanton technique. We then verify the accuracy of this technique, by comparing the results for the low energy effective Hamiltonian, with results derived for the original giant spin, coupled to a microscopic spin, using exact diagonalisation techniques.Comment: 15 pages, Latex, with 9 ps figure

Trade-offs between ventilation rates and formaldehyde concentrations in new-build dwellings in the UK

The current policies and regulatory frameworks in the construction sector aim to improve energy efficiency of new buildings whilst maintaining acceptable level of indoor environmental quality (IEQ) including indoor air quality (IAQ). In practice, however, there are often important trade-offs between these objectives. The aim of this paper is to investigate the concentrations of volatile organic compounds (VOCs) in a recently built residential block in the UK and the potential trade-offs between ventilation rates and VOCs. Concentration levels of VOCs that are likely to have concentrations higher than their respective exposure limit values (ELVs) in low energy dwellings were measured in five sample apartments in this block during typical weeks in winter and summer using diffusive sampling methods. Whilst most target VOCs had concentrations lower than ELVs, benzene and formaldehyde levels were regularly higher than the limits. Measurement of outdoor concentrations showed that benzene levels were predominantly driven by outdoor sources whilst formaldehyde concentrations were driven by internal sources including construction material and furniture. To investigate how formaldehyde levels can be reduced in a given context determined by typical material used in the industry, two models were developed to calculate the effect of enhanced ventilation on formaldehyde levels and energy efficiency of the apartment with highest formaldehyde. Lack of clear definition of VOC characteristics of building material and ever-increasing use of material with high formaldehyde emission factors such as medium-density fibreboard (MDF) in indoor furniture may contribute to high formaldehyde concentrations in indoor air. The study found that to offset the effect of the existing internal sources in the case study apartment and comply with the best practice ELV for formaldehyde, the ventilation rate should be more than three times the existing rate required in the current Building Regulations, and this can significantly increase energy use. Formaldehyde is currently not regulated in the UK Building Regulations. Given the potential health impact of high formaldehyde concentrations and the empirical evidence, it is necessary to cover formaldehyde in the next edition of the Building Regulations. This study points to the significance of improving the existing regulations and standards to clearly define maximum permissible emission factors for various VOCs in building material and indoor furniture. It is also important to improve source control measures to reduce the concentration of formaldehyde. These measures may be complemented by enhanced ventilation. It is, however, necessary to investigate the implications of enhanced ventilation for energy efficiency

Quantum spin glass in anisotropic dipolar systems

The spin-glass phase in the \LHx compound is considered. At zero transverse field this system is well described by the classical Ising model. At finite transverse field deviations from the transverse field quantum Ising model are significant, and one must take properly into account the hyperfine interactions, the off-diagonal terms in the dipolar interactions, and details of the full J=8 spin Hamiltonian to obtain the correct physical picture. In particular, the system is not a spin glass at finite transverse fields and does not show quantum criticality.Comment: 6 pages, 2 figures, to appear in J. Phys. Condens. Matter (proceedings of the HFM2006 conference

Pair-wise decoherence in coupled spin qubit networks

Experiments involving phase coherent dynamics of networks of spins, such as echo experiments, will only work if decoherence can be suppressed. We show here, by analyzing the particular example of a crystalline network of Fe8 molecules, that most decoherence typically comes from pairwise interactions (particularly dipolar interactions) between the spins, which cause `correlated errors'. However at very low T these are strongly suppressed. These results have important implications for the design of quantum information processing systems using electronic spins.Comment: 4 pages, 4 figures. Final PRL versio

Growth of axile and lateral roots of maize: I development of a phenotying platform

The objective of this study was to develop a phenotyping platform for the non-destructive, digital measurement of early root growth of axile and lateral roots and to evaluate its suitability for identifying maize (Zea mays L.) genotypes with contrasting root development. The system was designed to capture images of the root system within minutes and to batch process them automatically. For system establishment, roots of the inbred line Ac7729/TZSRW were grown until nine days after germination on the surface of a blotting paper in pouches. An A4 scanner was used for image acquisition followed by digital image analysis. Image processing was optimized to enhance the separation between the roots and the background and to remove image noise. Based on the root length in diameter-class distribution (RLDD), small-diameter lateral roots and large-diameter axile roots were separated. Root systems were scanned daily to model the growth dynamics of these root types. While the axile roots exhibited an almost linear growth, total lateral root length increased exponentially. Given the determined exponential growth, it was demonstrated that two plants, germinated one day apart but with the same growth rates differed in root length by 100%. From the growth rates we were able to identify contrasting genotypes from 236 recombinant inbred lines (RILs) of the CML444 x SC-Malawi cross. Differences in the growth of lateral roots of two selected RILs were due to differences in the final length and linear density of the primary lateral roots, as proven by the manual reanalysis of the digital images. The high throughput makes the phenotyping platform attractive for routine genetic studies and other screening purpose

Using simulated co-heating tests to understand weather driven sources of uncertainty within the co-heating test method

The so-called performance gap between designed and as-built building performance threatens to undermine carbon reduction strategies in the built environment. Field measurements to date have indicated that the measured as-built fabric heat loss of tested UK buildings is consistently higher than design values, often considerably so. Currently, our lack of knowledge over the extent of this gap, and the processes that cause it, is compounded by a lack of robust post-construction evaluation tools. Much of this post-construction evaluation work is based, in part, on the use of co-heating tests: a method utilising an energy balance to determine the heat loss across the entire building envelope, defined by the heat loss coefficient (W/K). However, the errors associated with co-heating are not well understood or typically addressed in the literature. Furthermore, the test procedure requires a building to be unoccupied for two to three weeks and is therefore often cited as costly and unsuitable both for developers and as a policy tool. In order to improve the application of this test method it is crucial firstly to understand the sources of uncertainty in co-heating tests and the ‘steady-state’ energy balance they are based upon. However, with a small database of tests performed to date it is difficult to discern these sources of error. This paper presents the results of a method using simulated co-heating tests to show how key weather variables influence the co-heating result and generate uncertainty and bias. In particular the effects of short-wave solar and long-wave sky radiation are presented. Improvements to the co-heating method can be derived from this; in particular the need to consider when dwellings should be tested to avoid large solar-generated errors and the importance of a accurately calculated solar aperture. Recommendations also include the local measurement of sky radiation to avoid outlying data points, bias in the measurement and discrepancies when comparing design and as-built heat loss

Development of food photographs for use with children aged 18 months to 16 years:comparison against weighed food diaries – The Young Person’s Food Atlas (UK)

Traditional dietary assessment methods, used in the UK, such as weighed food diaries impose a large participant burden, often resulting in difficulty recruiting representative samples and underreporting of energy intakes. One approach to reducing the burden placed on the participant is to use portion size assessment tools to obtain an estimate of the amount of food consumed, removing the need to weigh all foods. An age range specific food atlas was developed for use in assessing children’s dietary intakes. The foods selected and portion sizes depicted were derived from intakes recorded during the UK National Diet and Nutrition Surveys of children aged 1.5 to 16 years. Estimates of food portion sizes using the food atlas were compared against 4-day weighed intakes along with in-school / nursery observations, by the research team. Interviews were conducted with parents the day after completion of the diary, and for children aged 4 to 16 years, also with the child. Mean estimates of portion size consumed were within 7% of the weight of food recorded in the weighed food diary. The limits of agreement were wide indicating high variability of estimates at the individual level but the precision increased with increasing age. For children 11 years and over, agreement with weighed food diaries, was as good as that of their parents in terms of total weight of food consumed and of intake of energy and key nutrients. The age appropriate food photographs offer an alternative to weighed intakes for dietary assessment with children