Effective Hamiltonian for the motion of holes in the Hubbard-Anderson model

The motion of (interacting) holes in the Hubbard-Anderson model for high-Tc superconductivity is translated into the motion of (coupled) spinless fermions. The entities responsible for the coupling are localized spin excitations and are described by bosons. The new description shows resemblance with the BCS description of electrons and photons of the ¿classical¿ superconductors

Symmetry breaking in the Anderson-Hubbard model

The Anderson-Hubbard (A-H) model with one or two holes and with periodic boundary conditions on a 4Mx 4N square lattice is considered. On grounds of an intuitive generalization of Marshall's theorem we split the A-H Hamiltonian (HA−H) into a zeroth order term (H0) and a perturbation term (H'). With H0 we construct unfrustrated states: the zeroth order approximation of the degenerate ground state (GS). The one-hole system has a four-fold symmetry broken H0-GS with k = (π/2, ±π/2), (-π/2, ±π/2). Group theory shows that this symmetry breaking (SB) may be stable if H' is taken into account. For the two-hole system we derive candidates for the H0-GS with the corresponding good quantum numbers k and total spin S. Here we find no SB or a two-fold SB: again, this result may hold for the complete HA−H. Second order perturbation calculation possibly describes an effective coupling of two holes

Sustainable use and production of energy in the 21st century

It is foreseen that oil and gas will continue to be the key energy sources in the 21st century. Therefore, it is important that oil and gas be produced in a sustainable way during the next decades. This requires technology development to ensure that the environmental impact and pollution from these activities are minimal. The following aspects are being highlighted in this paper: • Development of projects with the minimum of impact on the environment and problems for local populations. • Sustainable drilling without the use of oil-based mud, and collection of all drilling waste during offshore drilling operations in the most environmentally sensitive areas. • Treatment of produced water, sand and minerals from the well stream to avoid pollution. • Limitation of flaring to be performed only when required for safety reasons. • Continuous checking of pipelines to ensure that gas pipelines are run within their actual pressure capacity and that oil pipelines are not leaking into rivers and lakes. • Provision of sufficient storage capacity for gas to ensure timely delivery of gas during high demand peaks. • Injection of CO2 into sealed underground formations where large quantities are produced, such as at LNG factories. • Optimization of production from existing fields to avoid huge amounts of oil and gas being left in place, following a ‘hit and run’ recovery plan. Furthermore, all primary energy sources need to be converted into end-user energy services known as mechanical work, electricity, heating and cooling. In the process of conversion, only a portion of the primary energy is transformed into the new form, while the rest remains unaltered and is lost. The various forms of energy services produced represent different values or qualities, e.g. heat holds an energy quality ranging from 0 and upwards, depending on the temperature difference which is utilized, as defined by the second law of thermodynamics. Energy efficiency in this context may also be defined as the ratio between energy quality output and input. Practically, all fossil fuels are converted into energy services via combustion and heat, i.e. the conversion efficiency is solely determined by temperatures, meaning that high-energy efficiency can only be obtained at large temperature differences, such as in power generation, while ordinary domestic heating will yield a very low efficiency. Given that some 30–40 % of all fossil fuels today are used for domestic heating, representing an end-user energy quality of (say) 1/10 of what is obtained in modern power generation, there is a large potential globally for energy efficiency improvements, not to mention the associated emission reductions. The obvious solution is to pay more attention to the second law of thermodynamics, i.e. to shift from direct combustion heating to thermodynamic principles, e.g. by the use of electrical-driven heat pumps and/or combined heat and power as another alternative. The objectives of this paper are to highlight how energy production could become more effective, thus leading to a reduction in pollution to land, sea and atmosphere and also to identify how energy production should be carried out to minimize the polluting effects. The goal is to provide a reminder that much can be gained with respect to the reduction of pollution by focusing on cleaner energy production

The two-hole ground state of the Hubbard-Anderson model, approximated by a variational RVB-type wave function

In this paper the Hubbard-Anderson model on a square lattice with two holes is studied. The ground state (GS) is approximated by a variational RVB-type wave function. The holes interact by exchange of a localized spin excitation (SE), which is created or absorbed if a hole moves to a nearest-neighbour site. An SE can move over the sublattice on which it is created. A variational calculation of the GS and the GS-energy is performed for an open-ended 4 × 4 lattice with two holes with the restriction that the SE is neighbouring both holes and does not move over its sublattice. It is found that the two holes prefer a bound state in which their mutual distance is 1 or V2 (with lattice spacing 1)

Approximations to the two-hole ground state of the Hubbard-Anderson model: a numerical test

Several resonating-valence-bond-type states are being considered as an approximation of the two-hole ground state of the two-dimensional Hubbard-Anderson model. These states have been carefully constructed by Traa and Caspers with such algebraic properties, as to optimise different contributions of the Hubbard-Anderson hamiltonian. In this paper, the different contributions to their energies are calculated for lattices with sizes from 8 × 8 up to 16 × 16 and periodic boundary conditions, using a variational Monte-Carlo method. We show which state is lowest in energy and, more important, why this is so. In accordance with the optimal state from this tested set, we propose a bound state. It will be shown that this state is indeed the most stable state

Primary Health Care in Practice: Is It Effective?

Primary health care is accepted as the model for delivering basic health care to low income populations in developing countries. Using El Salvador as a case study, the paper draws on three data sets and a qualitative survey to assess health care access and utilization across public and private sector options (including NGOs). Multivariate analysis is used to estimate the quantitative determinants of health seeking behavior. Physical and financial access is generally good. Households do not value the community health workers, and prefer high cost private care, even the poorest families, because of the lower waiting times and higher probability of successful treatment. Similarly, higher level public facilities—health centers and hospitals—are preferred because they are less costly in terms of time as they offer "one stop shopping" and do not require multiple visits, and treatment success is higher than among health posts, health units or community health workers. These results combined with the small size of El Salvador suggest that alternative strategies to community health workers may be a more cost effective approach. While prevention is desirable, community health workers do not have the skills or services that the communities value, which makes them less effective in promoting prevention. Alternative modes of reaching the community could reduce costs and raise the effectiveness of public health spending.health care, El Salvador, public health spending

Challenging El Salvador's rural health care strategy

Can a supply-driven network of under-skilled rural health promoters make a difference in rural health care? There are few, if any, signs that the current rural health strategy in El Salvador is working, whether the health promoters are government employees or non-governmental organization (NGO) workers. The authors arrived at this conclusion after conducting interviews and analyzing primary and secondary data. The village-based health promoters lack incentives and supervision, and ultimately have little to offer to local communities. NGO workers are more successful than government workers, but neither group performs satisfactorily. Even the rural poor use private services quite intensively, despite the high cost of the services and of getting access to them. Moreover, people seem to seek the services they need. They select self-treatment in 50 percent of illness episodes, with about the same success rate as when they use health providers. Other options should be considered, as results can be improved without increasing costs.Health Systems Development&Reform,Health Monitoring&Evaluation,Early Child and Children's Health,Public Health Promotion,Housing&Human Habitats,Regional Rural Development,Health Systems Development&Reform,Agricultural Knowledge&Information Systems,Health Monitoring&Evaluation,Housing&Human Habitats

Multichannel source separation and tracking with phase differences by random sample consensus

Blind audio source separation (BASS) is a fascinating problem that has been tackled from many different angles. The use case of interest in this thesis is that of multiple moving and simultaneously-active speakers in a reverberant room. This is a common situation, for example, in social gatherings. We human beings have the remarkable ability to focus attention on a particular speaker while effectively ignoring the rest. This is referred to as the ``cocktail party effect'' and has been the holy grail of source separation for many decades. Replicating this feat in real-time with a machine is the goal of BASS. Single-channel methods attempt to identify the individual speakers from a single recording. However, with the advent of hand-held consumer electronics, techniques based on microphone array processing are becoming increasingly popular. Multichannel methods record a sound field from various locations to incorporate spatial information. If the speakers move over time, we need an algorithm capable of tracking their positions in the room. For compact arrays with 1-10 cm of separation between the microphones, this can be accomplished by applying a temporal filter on estimates of the directions-of-arrival (DOA) of the speakers. In this thesis, we review recent work on BSS with inter-channel phase difference (IPD) features and provide extensions to the case of moving speakers. It is shown that IPD features compose a noisy circular-linear dataset. This data is clustered with the RANdom SAmple Consensus (RANSAC) algorithm in the presence of strong reverberation to simultaneously localize and separate speakers. The remarkable performance of RANSAC is due to its natural tendency to reject outliers. To handle the case of non-stationary speakers, a factorial wrapped Kalman filter (FWKF) and a factorial von Mises-Fisher particle filter (FvMFPF) are proposed that track source DOAs directly on the unit circle and unit sphere, respectively. These algorithms combine directional statistics, Bayesian filtering theory, and probabilistic data association techniques to track the speakers with mixtures of directional distributions