7,655 research outputs found
Emergence of District-Heating Networks; Barriers and Enablers in the Development Process
Infrastructure provision business models that promise resource efficiencies and additional benefits, such as job
creation, community cohesion and crime reduction exist at sub-national scales. These local business models,
however, exist only as isolated cases of good practice and their expansion and wider adoption has been limited in
the context of many centralised systems that are currently the norm. In this contribution, we present a conceptual
agent based model for analysing the potential for different actors to implement local infrastructure provision business
models. The model is based on agents’ ability to overcome barriers that occur throughout the development (i.e.
feasibility, business case, procurement, and construction), and operation and maintenance of alternative business
models. This presents a novel approach insofar as previous models have concentrated on the acceptance of
alternative value provision models rather than the emergence of underlying business models. We implement the
model for the case study of district heating networks in the UK, which have the potential to significantly contribute to
carbon emission reductions, but remain under-developed compared with other European countries
Spontaneous Emission in ultra-cold spin-polarised anisotropic Fermi Seas
We examine and explain the spatial emission patterns of ultracold excited
fermions in anisotropic trapping potentials in the presence of a spin polarised
Fermi sea of ground state atoms. Due to the Pauli principle, the Fermi sea
modifies the available phase space for the recoiling atom and thereby modifies
its decay rate and the probability of the emitted photon's direction. We show
that the spatial anisotropies are due to an intricate interplay between Fermi
energies and degeneracy values of specific energy levels and identify a regime
in which the emission will become completely directional. Our results are
relevant for recent advances in trapping and manipulating cold fermionic
samples experimentally and give an example of a conceptually new idea for a
directional photon source.Comment: 7 pages, 7 figure
Thermodynamics of Cu47Ti34Zr11Ni8, Zr52.5Cu17.9Ni14.6Al10Ti5 and Zr57Cu15.4Ni12.6Al10Nb5 bulk metallic glass forming alloys
The differences in the thermodynamic functions between the liquid and the crystalline states of three bulk metallic glass forming alloys, Cu47Ti34Zr11Ni8, Zr52.5Cu17.9Ni14.6Al10Ti5, and Zr57Cu15.4Ni12.6Al10Nb5, were calculated. The heat capacity was measured in the crystalline solid, the amorphous solid, the supercooled liquid, and the equilibrium liquid. Using these heat capacity data and the heats of fusion of the alloys, the differences in the thermodynamic functions between the liquid and the crystalline states were determined. The Gibbs free energy difference between the liquid and the crystalline states gives a qualitative measure of the glass forming ability of these alloys. Using the derived entropy difference, the Kauzmann temperatures for these alloys were determined
Thermal emission from finite photonic crystals
We present a microscopic theory of thermal emission from finite-sized photonic crystals and show that the directional spectral emissivity and related quantities can be evaluated via standard bandstructure computations without any approximation. We then identify the physical mechanisms through which interfaces modify the potentially super-Planckian radiation flow inside infinite photonic crystals, such that thermal emission from finite-sized samples is consistent with the fundamental limits set by Planck's law. As an application, we further demonstrate that a judicious choice of a photonic crystal's surface termination facilitates considerable control over both the spectral and angular thermal emission properties. © 2009 American Institute of Physics
On the complementarity of the quadrature observables
In this paper we investigate the coupling properties of pairs of quadrature
observables, showing that, apart from the Weyl relation, they share the same
coupling properties as the position-momentum pair. In particular, they are
complementary. We determine the marginal observables of a covariant phase space
observable with respect to an arbitrary rotated reference frame, and observe
that these marginal observables are unsharp quadrature observables. The related
distributions constitute the Radon tranform of a phase space distribution of
the covariant phase space observable. Since the quadrature distributions are
the Radon transform of the Wigner function of a state, we also exhibit the
relation between the quadrature observables and the tomography observable, and
show how to construct the phase space observable from the quadrature
observables. Finally, we give a method to measure together with a single
measurement scheme any complementary pair of quadrature observables.Comment: Dedicated to Peter Mittelstaedt in honour of his eightieth birthda
On the atomic structure of cocaine in solution
Cocaine is an amphiphilic drug which has the ability to cross the blood–brain barrier (BBB). Here, a combination of neutron diffraction and computation has been used to investigate the atomic scale structure of cocaine in aqueous solutions. Both the observed conformation and hydration of cocaine appear to contribute to its ability to cross hydrophobic layers afforded by the BBB, as the average conformation yields a structure which might allow cocaine to shield its hydrophilic regions from a lipophilic environment. Specifically, the carbonyl oxygens and amine group on cocaine, on average, form ~5 bonds with the water molecules in the surrounding solvent, and the top 30% of water molecules within 4 Å of cocaine are localized in the cavity formed by an internal hydrogen bond within the cocaine molecule. This water mediated internal hydrogen bonding suggests a mechanism of interaction between cocaine and the BBB that negates the need for deprotonation prior to interaction with the lipophilic portions of this barrier. This finding also has important implications for understanding how neurologically active molecules are able to interact with both the blood stream and BBB and emphasizes the use of structural measurements in solution in order to understand important biological function.Peer ReviewedPostprint (author's final draft
Radiating dipoles in photonic crystals
The radiation dynamics of a dipole antenna embedded in a Photonic Crystal are
modeled by an initially excited harmonic oscillator coupled to a non--Markovian
bath of harmonic oscillators representing the colored electromagnetic vacuum
within the crystal. Realistic coupling constants based on the natural modes of
the Photonic Crystal, i.e., Bloch waves and their associated dispersion
relation, are derived. For simple model systems, well-known results such as
decay times and emission spectra are reproduced. This approach enables direct
incorporation of realistic band structure computations into studies of
radiative emission from atoms and molecules within photonic crystals. We
therefore provide a predictive and interpretative tool for experiments in both
the microwave and optical regimes.Comment: Phys. Rev. E, accepte
Self-adjoint Lyapunov variables, temporal ordering and irreversible representations of Schroedinger evolution
In non relativistic quantum mechanics time enters as a parameter in the
Schroedinger equation. However, there are various situations where the need
arises to view time as a dynamical variable. In this paper we consider the
dynamical role of time through the construction of a Lyapunov variable - i.e.,
a self-adjoint quantum observable whose expectation value varies monotonically
as time increases. It is shown, in a constructive way, that a certain class of
models admit a Lyapunov variable and that the existence of a Lyapunov variable
implies the existence of a transformation mapping the original quantum
mechanical problem to an equivalent irreversible representation. In addition,
it is proved that in the irreversible representation there exists a natural
time ordering observable splitting the Hilbert space at each t>0 into past and
future subspaces.Comment: Accepted for publication in JMP. Supercedes arXiv:0710.3604.
Discussion expanded to include the case of Hamiltonians with an infinitely
degenerate spectru
At what time does a quantum experiment have a result?
This paper provides a general method for defining a generalized quantum
observable (or POVM) that supplies properly normalized conditional
probabilities for the time of occurrence (i.e., of detection). This method
treats the time of occurrence as a probabilistic variable whose value is to be
determined by experiment and predicted by the Born rule. This avoids the
problematic assumption that a question about the time at which an event occurs
must be answered through instantaneous measurements of a projector by an
observer, common to both Rovelli (1998) and Oppenheim et al. (2000). I also
address the interpretation of experiments purporting to demonstrate the quantum
Zeno effect, used by Oppenheim et al. (2000) to justify an inherent uncertainty
for measurements of times.Comment: To appear in proceedings of 2015 ETH Zurich Workshop on Time in
Physic
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