118 research outputs found
Density Functional Model for Nondynamic and Strong Correlation
A single-term density functional model for nondynamic and strong correlation
is presented, based on single-determinant Kohn-Sham density functional theory.
It is derived from modeling the adiabatic connection and contains only two
nonlinear empirical parameters. Preliminary tests show that the model recovers
majority of nondynamic correlation during a molecular dissociation and at the
same time performs reasonably for atomization energies. It demonstrates the
feasibility of developing DFT functionals for nondynamic and strong correlation
within the single-determinant KS scheme.Comment: Journal of Chemical Theory and Computation, 201
Low-power methods of power sensing and frequency detection for wideband vibration energy harvesting
Power maximisation techniques in wideband vibration energy harvesting typically require the periodic sensing of input power or excitation frequency. This paper presents low- power circuits and sensing methods to obtain this information. First, an excitation frequency measurement circuit is presented that permits a reduced timer run-time compared to reported methods. Second, a power sensing method is presented, which extends the measurement range of reported techniques by adapting to the levels of the available power. Experimental results for the frequency measurement circuit tested in the range 35-51 Hz show a power consumption of 3.7 μW. The power-sensing technique is experimentally validated over a power range of 370690 μW, and its power consumption is 7.5 μW
Inductive power transfer for on-body sensors defining a design space for safe, wirelessly powered on-body health sensors
Pervasive Health: 9th International Conference on Pervasive Computing Technologies for Healthcare, 20-23 May 2015, Istanbul, TurkeyDesigners of on-body health sensing devices face a difficult choice. They must either minimise the power consumption of devices, which in reality means reducing the sensing capabilities, or build devices that require regular battery changes or recharging. Both options limit the effectiveness of devices. Here we investigate an alternative. This paper presents a method of designing safe, wireless, inductive power transfer into on-body sensor products. This approach can produce sensing devices that can be worn for longer durations without the need for human intervention, whilst also having greater sensing and data capture capabilities. The paper addresses significant challenges in achieving this aim, in particular: device safety, sufficient power transfer, and human factors regarding device geometry. We show how to develop a device that meets stringent international safety guidelines for electromagnetic energy on the body and describe a design space that allows designers to make trade-offs that balance power transfer with other constraints, e.g. size and bulk, that affect the wearability of devices. Finally we describe a rapid experimental method to investigate the optimal placement of on-body devices and the actual versus theoretical power transfer for on-body, inductively powered devices. EPSR
A Flexible 2.45-GHz Power Harvesting Wristband with Net System Output from -24.3 dBm of RF Power
This is the final version. Available from IEEE via the DOI in this recordThis paper presents a flexible 2.45-GHz wireless power harvesting wristband that generates a net dc output from a -24.3-dBm RF input. This is the lowest reported system sensitivity for systems comprising a rectenna and impedance-matching power management. A complete system has been implemented comprising: a fabric antenna, a rectifier on rigid substrate, a contactless electrical connection between rigid and flexible subsystems, and power electronics impedance matching. Various fabric and flexible materials are electrically characterized at 2.45 GHz using the two-line and the T-resonator methods. Selected materials are used to design an all-textile antenna, which demonstrates a radiation efficiency above 62% on a phantom irrespective of location, and a stable radiation pattern. The rectifier, designed on a rigid substrate, shows a best-in-class efficiency of 33.6% at -20 dBm. A reliable, efficient, and wideband contactless connection between the fabric antenna and the rectifier is created using broadside-coupled microstrip lines, with an insertion loss below 1 dB from 1.8 to over 10 GHz. A self-powered boost converter with a quiescent current of 150 nA matches the rectenna output with a matching efficiency above 95%. The maximum end-to-end efficiency is 28.7% at -7 dBm. The wristband harvester demonstrates net positive energy harvesting from -24.3 dBm, a 7.3-dB improvement on the state of the art.Engineering and Physical Sciences Research Council (EPSRC
A Flexible 2.45-GHz Power Harvesting Wristband with Net System Output from -24.3 dBm of RF Power
This paper presents a flexible 2.45-GHz wireless power harvesting wristband that generates a net dc output from a -24.3-dBm RF input. This is the lowest reported system sensitivity for systems comprising a rectenna and impedance-matching power management. A complete system has been implemented comprising: a fabric antenna, a rectifier on rigid substrate, a contactless electrical connection between rigid and flexible subsystems, and power electronics impedance matching. Various fabric and flexible materials are electrically characterized at 2.45 GHz using the two-line and the T-resonator methods. Selected materials are used to design an all-textile antenna, which demonstrates a radiation efficiency above 62% on a phantom irrespective of location, and a stable radiation pattern. The rectifier, designed on a rigid substrate, shows a best-in-class efficiency of 33.6% at -20 dBm. A reliable, efficient, and wideband contactless connection between the fabric antenna and the rectifier is created using broadside-coupled microstrip lines, with an insertion loss below 1 dB from 1.8 to over 10 GHz. A self-powered boost converter with a quiescent current of 150 nA matches the rectenna output with a matching efficiency above 95%. The maximum end-to-end efficiency is 28.7% at -7 dBm. The wristband harvester demonstrates net positive energy harvesting from -24.3 dBm, a 7.3-dB improvement on the state of the art.</p
Spin Resolution of the Electron-Gas Correlation Energy: Positive same-spin contribution
The negative correlation energy per particle of a uniform electron gas of
density parameter and spin polarization is well known, but its
spin resolution into up-down, up-up, and down-down contributions is not.
Widely-used estimates are incorrect, and hamper the development of reliable
density functionals and pair distribution functions. For the spin resolution,
we present interpolations between high- and low-density limits that agree with
available Quantum Monte Carlo data. In the low-density limit for ,
we find that the same-spin correlation energy is unexpectedly positive, and we
explain why. We also estimate the up and down contributions to the kinetic
energy of correlation.Comment: new version, to appear in PRB Rapid Communicatio
Evaluation of Exchange-Correlation Energy, Potential, and Stress
We describe a method for calculating the exchange and correlation (XC)
contributions to the total energy, effective potential, and stress tensor in
the generalized gradient approximation. We avoid using the analytical
expressions for the functional derivatives of E_xc*rho, which depend on
discontinuous second-order derivatives of the electron density rho. Instead, we
first approximate E_xc by its integral in a real space grid, and then we
evaluate its partial derivatives with respect to the density at the grid
points. This ensures the exact consistency between the calculated total energy,
potential, and stress, and it avoids the need of second-order derivatives. We
show a few applications of the method, which requires only the value of the
(spin) electron density in a grid (possibly nonuniform) and returns a
conventional (local) XC potential.Comment: 7 pages, 3 figure
Libxc: a library of exchange and correlation functionals for density functional theory
The central quantity of density functional theory is the so-called
exchange-correlation functional. This quantity encompasses all non-trivial
many-body effects of the ground-state and has to be approximated in any
practical application of the theory. For the past 50 years, hundreds of such
approximations have appeared, with many successfully persisting in the
electronic structure community and literature. Here, we present a library that
contains routines to evaluate many of these functionals (around 180) and their
derivatives.Comment: 15 page
Analytic structure factors and pair-correlation functions for the unpolarized homogeneous electron gas
We propose a simple and accurate model for the electron static structure
factors (and corresponding pair-correlation functions) of the 3D unpolarized
homogeneous electron gas. Our spin-resolved pair-correlation function is built
up with a combination of analytic constraints and fitting procedures to quantum
Monte Carlo data, and, in comparison to previous attempts (i) fulfills more
known integral and differential properties of the exact pair-correlation
function, (ii) is analytic both in real and in reciprocal space, and (iii)
accurately interpolates the newest, extensive diffusion-Monte Carlo data of
Ortiz, Harris and Ballone [Phys. Rev. Lett. 82, 5317 (1999)]. This can be of
interest for the study of electron correlations of real materials and for the
construction of new exchange and correlation energy density functionals.Comment: 14 pages, 5 figures, submitted to Phys. Rev.
- …