790 research outputs found
A density matrix approach to multiconfiguration calculations
Determination of density matrix components for multiconfiguration wave functions and Hamiltonian interaction matrice
Incorporating episodicity into estimates of Critical Loads for juvenile salmonids in Scottish streams
International audienceCritical Load (CL) methodology is currently used throughout Europe to assess the risks of ecological damage due to sulphur and nitrogen emissions. Critical acid neutralising capacity (ANCCRIT) is used in CL estimates for freshwater systems as a surrogate for biological damage. Although UK CL maps presently use an ANC value of 0 ?eq l-1, this value has been based largely on Norwegian lake studies, in which brown trout is chosen as a representative indicator organism. In this study, an ANC value specific for brown trout in Scottish streams was determined and issues were addressed such as salmon and trout sensitivity in streams, episodicity, afforestation and complicating factors such as dissolved organic carbon (DOC) and labile aluminium (Al-L). Catchments with significant forest cover were selected to provide fishless sites and to provide catchment comparisons in unpolluted areas. Chemical factors were the primary determinant with land use a secondary determinant of the distribution of salmonid populations at the twenty-six study sites. ANC explained more variance in brown trout density than pH. The most significant index of episodicity was percent of time spent below an ANC of 0 ?eq l-1. An ANCCRIT value of 39 ?eq l-1 was obtained based on a 50% probability of brown trout occurrence. The use of this revised ANCCRIT value in the CL equation improved the relationship between trout status and exceedance of CLs. Uncertainties associated with variations in Al-L at any fixed ANCCRIT, particularly within forested catchments, and the role of DOC in modifying the toxicity of Al-L are discussed. Keywords: Critical Load, Critical acid neutralising capacity, brown trout, episodes, stream
Measurements of strongly-anisotropic g-factors for spins in single quantum states
We have measured the full angular dependence, as a function of the direction
of magnetic field, for the Zeeman splitting of individual energy states in
copper nanoparticles. The g-factors for spin splitting are highly anisotropic,
with angular variations as large as a factor of five. The angular dependence
fits well to ellipsoids. Both the principal-axis directions and g-factor
magnitudes vary between different energy levels within one nanoparticle. The
variations agree quantitatively with random-matrix theory predictions which
incorporate spin-orbit coupling.Comment: 4 pages, 3 figures, 2 in colo
Causality re-established
Causality never gained the status of a "law" or "principle" in physics. Some
recent literature even popularized the false idea that causality is a notion
that should be banned from theory. Such misconception relies on an alleged
universality of reversibility of laws of physics, based either on determinism
of classical theory, or on the multiverse interpretation of quantum theory, in
both cases motivated by mere interpretational requirements for realism of the
theory. Here, I will show that a properly defined unambiguous notion of
causality is a theorem of quantum theory, which is also a falsifiable
proposition of the theory. Such causality notion appeared in the literature
within the framework of operational probabilistic theories. It is a genuinely
theoretical notion, corresponding to establish a definite partial order among
events, in the same way as we do by using the future causal cone on Minkowski
space. The causality notion is logically completely independent of the
misidentified concept of "determinism", and, being a consequence of quantum
theory, is ubiquitous in physics. In addition, as classical theory can be
regarded as a restriction of quantum theory, causality holds also in the
classical case, although the determinism of the theory trivializes it. I then
conclude arguing that causality naturally establishes an arrow of time. This
implies that the scenario of the "Block Universe" and the connected "Past
Hypothesis" are incompatible with causality, and thus with quantum theory: they
both are doomed to remain mere interpretations and, as such, not falsifiable,
similar to the hypothesis of "super-determinism". This article is part of a
discussion meeting issue "Foundations of quantum mechanics and their impact on
contemporary society".Comment: Presented at the Royal Society of London, on 11/12/ 2017, at the
conference "Foundations of quantum mechanics and their impact on contemporary
society". To appear on Philosophical Transactions of the Royal Society
Hadronic production and the Gottfried Sum Rule
The difference in production rate between and at hadron colliders
is very sensitive to the the difference between up- and down-quark
distributions in the proton. This sensitivity allows for a variety of useful
measurements. We consider the difference in the sea
distributions and the difference in the
polarized parton distribution functions. In both cases we construct an
asymmetry to reduce systematic uncertainties. Although we discuss measurements
at the Tevatron and future hadron colliders, we find that the Brookhaven
Relativistic Heavy Ion Collider (RHIC) is the most appropriate hadron collider
for these measurements.Comment: 19 pages (20 figures available from the authors), MAD/PH/74
N-representability and stationarity in time-dependent density functional theory
To construct an N-representable time-dependent density-functional theory, a
generalization to the time domain of the Levy-Lieb (LL) constrained search
algorithm is required. That the action is only stationary in the Dirac-Frenkel
variational principle eliminates the possibility of basing the search on the
action itself. Instead, we use the norm of the partial functional derivative of
the action in the Hilbert space of the wave functions in place of the energy of
the LL search. The electron densities entering the formalism are
-representable, and the resulting universal action functional has a unique
stationary point in the density at that corresponding to the solution of the
Schr\"{o}dinger equation. The original Runge-Gross (RG) formulation is subsumed
within the new formalism. Concerns in the literature about the meaning of the
functional derivatives and the internal consistency of the RG formulation are
allayed by clarifying the nature of the functional derivatives entering the
formalism.Comment: 9 pages, 0 figures, Phys. Rev. A accepted. Introduction was expanded,
subsections reorganized, appendix and new references adde
Hadron collider limits on anomalous couplings
A next-to-leading log calculation of the reactions and
is presented including a tri-boson
gauge coupling from non-Standard Model contributions. Two approaches are made
for comparison. The first approach considers the tri-boson coupling
as being uniquely fixed by tree level unitarity at high energies to its
Standard Model form and, consequently, suppresses the non-Standard Model
contributions with form factors. The second approach is to ignore such
considerations and calculate the contributions to non-Standard Model tri-boson
gauge couplings without such suppressions. It is found that at Tevatron
energies, the two approaches do not differ much in quantitative results, while
at Large Hadron Collider (LHC) energies the two approaches give significantly
different predictions for production rates. At the Tevatron and LHC, however,
the sensitivity limits on the anomalous coupling of are too weak to
usefully constrain parameters in effective Lagrangian models.Comment: Revtex 23 pages + 8 figures, UIOWA-94-1
Symmetric Informationally Complete Measurements of Arbitrary Rank
There has been much interest in so-called SIC-POVMs: rank 1 symmetric
informationally complete positive operator valued measures. In this paper we
discuss the larger class of POVMs which are symmetric and informationally
complete but not necessarily rank 1. This class of POVMs is of some independent
interest. In particular it includes a POVM which is closely related to the
discrete Wigner function. However, it is interesting mainly because of the
light it casts on the problem of constructing rank 1 symmetric informationally
complete POVMs. In this connection we derive an extremal condition alternative
to the one derived by Renes et al.Comment: Contribution to proceedings of International Conference on Quantum
Optics, Minsk, 200
Constraint-based, Single-point Approximate Kinetic Energy Functionals
We present a substantial extension of our constraint-based approach for
development of orbital-free (OF) kinetic-energy (KE) density functionals
intended for the calculation of quantum-mechanical forces in multi-scale
molecular dynamics simulations. Suitability for realistic system simulations
requires that the OF-KE functional yield accurate forces on the nuclei yet be
relatively simple. We therefore require that the functionals be based on DFT
constraints, local, dependent upon a small number of parameters fitted to a
training set of limited size, and applicable beyond the scope of the training
set. Our previous "modified conjoint" generalized-gradient-type functionals
were constrained to producing a positive-definite Pauli potential. Though
distinctly better than several published GGA-type functionals in that they gave
semi-quantitative agreement with Born-Oppenheimer forces from full Kohn-Sham
results, those modified conjoint functionals suffer from unphysical
singularities at the nuclei. Here we show how to remove such singularities by
introducing higher-order density derivatives. We give a simple illustration of
such a functional used for the dissociation energy as a function of bond length
for selected molecules.Comment: 16 pages, 9 figures, 2 tables, submitted to Phys. Rev.
Biofortification of UK food crops with selenium
Se is an essential element for animals. In man low dietary Se intakes are associated with health disorders including oxidative stress-related conditions, reduced fertility and immune functions and an increased risk of cancers. Although the reference nutrient intakes for adult females and males in the UK are 60 and 75 μg Se/d respectively, dietary Se intakes in the UK have declined from >60 μg Se/d in the 1970s to 35 μg Se/d in the 1990s, with a concomitant decline in human Se status. This decline in Se intake and status has been attributed primarily to the replacement of milling wheat having high levels of grain Se and grown on high-Se soils in North America with UK-sourced wheat having low levels of grain Se and grown on low-Se soils. An immediate solution to low dietary Se intake and status is to enrich UK-grown food crops using Se fertilisers (agronomic biofortification). Such a strategy has been adopted with success in Finland. It may also be possible to enrich food crops in the longer term by selecting or breeding crop varieties with enhanced Se-accumulation characteristics (genetic biofortification). The present paper will review the potential for biofortification of UK food crops with Se
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