Improving the thermal stability of 1-3 piezoelectric composite transducers manufactured using thermally conductive polymeric fillers

With a view to improving the thermal stability of ultrasonic transducers prepared using 1-3 piezoelectric composites, the use of front face layers manufactured from thermally insulating and partially thermally conductive polymeric materials has been investigated. Experimentally, heat dissipation was investigated, in air and in water, using different transducer configurations and the advantage of including a front face layer manufactured from thermally conductive polymeric material is demonstrated. The PZFlex finite element modelling package was utilised to assess the thermal diffusivity of each polymer in the different transducer configurations and was found to compare well with experiment

All-electron GW calculation for molecules: Ionization energy and electron affinity of conjugated molecules

An efficient all-electron G$^0$W$^0$ method and a quasiparticle selfconsistent GW (QSGW) method for molecules are proposed in the molecular orbital space with the full random phase approximation. The convergence with basis set is examined. As an application, the ionization energy ($I$) and electron affinity ($A$) of a series of conjugated molecules (up to 32 atoms) are calculated and compared to experiment. The QSGW result improves the G$^0$W$^0$ result and both of them are in significantly better agreement with experimental data than those from Hartree-Fock (HF) and hybrid density functional calculations, especially for $A$. The nearly correct energy gap and suppressed self-interaction error by the HF exchange make our method a good candidate for investigating electronic and transport properties of molecular systems.Comment: 4 pages, 2 figures, 1 tabl

Multi-layered piezoelectric composite transducers

Multilayered piezoelectric materials present themselves as a suitable technology for the development of sub 100kHz transducers. A variety of different configurations have been proposed, including stacked 2-2, 1-3 and 3-1 connectivity configurations. Historically multilayer devices designed for low frequency of operation have comprised uniform layer thickness through the height of the device. The potential for extended bandwidth through the use of non-uniform layers through the thickness dimension has been investigated. In addition commercially available stacked ceramic mechanical actuators have been investigated. A combination of theoretical and experimental assessment has been employed to evaluate each transducer technology. Selection of the passive phase for these multilayer devices is critical. Typically, these devices operate in the high power regime and as such selection of the passive polymer material is crucial - thermal stability coupled with thermal conductivity would be a virtue. To this end a number of polymer materials possessing the appropriate thermal properties have been investigated

An exactly size consistent geminal power via Jastrow factor networks in a local one particle basis

The accurate but expensive product of geminals ansatz may be approximated by a geminal power, but this approach sacrifices size consistency. Here we show both analytically and numerically that a size consistent form very similar to the product of geminals can be recovered using a network of location specific Jastrow factors. Upon variational energy minimization, the network creates particle number projections that remove the charge fluctuations responsible for size inconsistency. This polynomial cost approach captures strong many-electron correlations, giving a maximum error of just 1.8 kcal/mol during the double-bond dissociation of H2O in an STO-3G atomic orbital basis.Comment: Updated the original arXiv submission to include improvements resulting from journal peer review. 5 pages, 4 figures, 1 tabl

Comment on "Modifying the variational principle in the action integral functional derivation of time-dependent density functional theory" by Jochen Schirmer [arXiv:1010.4223]

In a paper recently published in Phys. Rev. A [arXiv:1010.4223], Schirmer has criticized an earlier work of mine [arXiv:0803.2727], as well as the foundations of time-dependent density functional theory. In Ref.[2], I showed that the so-called "causality paradox" - i.e., the failure of the exchange-correlation potential derived from the Runge-Gross time-dependent variational principle to satisfy causality requirements - can be solved by a careful reformulation of that variational principle. Fortunately, the criticism presented in Ref.[1] is based on elementary misunderstandings of the nature of functionals, gauge transformations, and the time-dependent variational principle. In this Comment I wish to point out and clear these misunderstandings.Comment: 4 pages. Accepted for publication in Phys. Rev.

Treatments of the exchange energy in density-functional theory

Following a recent work [Gal, Phys. Rev. A 64, 062503 (2001)], a simple derivation of the density-functional correction of the Hartree-Fock equations, the Hartree-Fock-Kohn-Sham equations, is presented, completing an integrated view of quantum mechanical theories, in which the Kohn-Sham equations, the Hartree-Fock-Kohn-Sham equations and the ground-state Schrodinger equation formally stem from a common ground: density-functional theory, through its Euler equation for the ground-state density. Along similar lines, the Kohn-Sham formulation of the Hartree-Fock approach is also considered. Further, it is pointed out that the exchange energy of density-functional theory built from the Kohn-Sham orbitals can be given by degree-two homogeneous N-particle density functionals (N=1,2,...), forming a sequence of degree-two homogeneous exchange-energy density functionals, the first element of which is minus the classical Coulomb-repulsion energy functional.Comment: 19 pages; original manuscript from 2001 (v1) revised for publication, with presentation substantially improved, some errors corrected, plus an additional summarizing figure (Appendix B) include

First principles investigation of finite-temperature behavior in small sodium clusters

A systematic and detailed investigation of the finite-temperature behavior of small sodium clusters, Na_n, in the size range of n= 8 to 50 are carried out. The simulations are performed using density-functional molecular-dynamics with ultrasoft pseudopotentials. A number of thermodynamic indicators such as specific heat, caloric curve, root-mean-square bond length fluctuation, deviation energy, etc. are calculated for each of the clusters. Size dependence of these indicators reveals several interesting features. The smallest clusters with n= 8 and 10, do not show any signature of melting transition. With the increase in size, broad peak in the specific heat is developed, which alternately for larger clusters evolves into a sharper one, indicating a solidlike to liquidlike transition. The melting temperatures show irregular pattern similar to experimentally observed one for larger clusters [ M. Schmidt et al., Nature (London) 393, 238 (1998) ]. The present calculations also reveal a remarkable size-sensitive effect in the size range of n= 40 to 55. While Na_40 and Na_55 show well developed peaks in the specific heat curve, Na_50 cluster exhibits a rather broad peak, indicating a poorly-defined melting transition. Such a feature has been experimentally observed for gallium and aluminum clusters [ G. A. Breaux et al., J. Am. Chem. Soc. 126, 8628 (2004); G. A.Breaux et al., Phys. Rev. Lett. 94, 173401 (2005) ].Comment: 8 pages, 11 figure

The Huon factor

The Huon factor Catalogue of an exhibition held at the Centre for the Arts Gallery, University of Tasmania, Hobart 19 May-9 June 1989. Exhibition co-curated by Geoff Parr and John Smit

Spin-independent v-representability of Wigner crystal oscillations in one-dimensional Hubbard chains: The role of spin-charge separation

Electrons in one-dimension display the unusual property of separating their spin and charge into two independent entities: The first, which derive from uncharged spin-1/2 electrons, can travel at different velocities when compared with the second, built from charged spinless electrons. Predicted theoretically in the early sixties, the spin-charge separation has attracted renewed attention since the first evidences of experimental observation, with usual mentions as a possible explanation for high-temperature superconductivity. In one-dimensional (1D) model systems, the spin-charge separation leads the frequencies of Friedel oscillations to suffer a 2k_F -- 4k_F crossover, mainly when dealing with strong correlations, where they are referred to as Wigner crystal oscillations. In non-magnetized systems, the current density functionals which are applied to the 1D Hubbard model are not seen to reproduce this crossover, referring to a more fundamental question: Are the Wigner crystal oscillations in 1D systems non-interacting v-representable? Or, is there a spin-independent Kohn-Sham potential which is able to yield spin-charge separation? Finding an appropriate answer to both questions is our main task here. By means of exact and DMRG solutions, as well as, a new approach of exchange-correlation potential, we show the answer to be positive. Specifically, the v-representable 4k_F oscillations emerge from attractive interactions mediated by positively charged spinless holes -- the holons -- as an additional contribution to the repulsive on-site Hubbard interaction

Experimental assessment of periodic piezoelectric composite arrays incorporating an anisotropic passive phase

This paper discusses the experimental assessment of a number of piezoelectric composite array structures incorporating a novel passive phase exhibiting anisotropic elastic properties. The passive polymer phase has been designed to limit inter-element crosstalk by attenuating lateral propagation across the array aperture. A selection of water coupled linear array coupons, operating with a nominal 400 kHz fundamental thickness mode frequency, has been prepared comprising the novel anisotropic passive phase. As a control, comparisons are made to similarly configured devices employing isotropic filler materials. Scanning laser vibrometry and measurements of electrical impedance characteristic on the array substrate demonstrate that the fundamental thickness mode of the devices configured with anisotropic polymer fillers is not contaminated by parasitic modes of vibration. The reasons for this are explained by considering the dispersion characteristics of the substrate. Water coupled hydrophone measurements of array element directivity; transmit voltage response and subsequently efficiency calculations illustrate that the observed reduction in mechanical cross talk has not been achieved at the expense of element sensitivity. Finally, comparisons between the experimental data and the PZFlex derived array responses are made, with good corroboration demonstrate