Evaluation of the BCS Approximation for the Attractive Hubbard Model in One Dimension

The ground state energy and energy gap to the first excited state are calculated for the attractive Hubbard model in one dimension using both the Bethe Ansatz equations and the variational BCS wavefunction. Comparisons are provided as a function of coupling strength and electron density. While the ground state energies are always in very good agreement, the BCS energy gap is sometimes incorrect by an order of magnitude, particularly at half-filling. Finite size effects are also briefly discussed for cases where an exact solution in the thermodynamic limit is not possible. In general, the BCS result for the energy gap is poor compared to the exact result.Comment: 25 pages, 5 Postscript figure

A pilot randomised controlled trial of the Peer Tree digital intervention targeting loneliness in young people: a study protocol

Background Young people are vulnerable to experiencing problematic levels of loneliness which can lead to poor mental health outcomes. Loneliness is a malleable treatment target and preliminary evidence has shown that it can be addressed with digital platforms. Peer Tree is a strength-based digital smartphone application aimed at reducing loneliness. The study aim is to reduce loneliness and assess the acceptability, usability, and feasibility of Peer Tree in young people enrolled at university. Methods This will be a pilot randomised controlled trial (RCT) comparing a strength-based digital smartphone application (Peer Tree) with a control condition. Forty-two young people enrolled at university will be recruited for this pilot RCT. Participants with suicidal ideation or behaviours, acute psychiatric symptoms in the past month, or a current diagnosis of a mood or social anxiety disorder will be excluded. Allocation will be made on a 1:1 ratio and will occur after the initial baseline assessment. Assessments are completed at baseline, at post-intervention, and at follow-up. Participants in the control condition complete the same three assessment sessions. The primary outcome of the study will be loneliness. Depression, social anxiety, quality of life, acceptability, usability, feasibility, and safety of Peer Tree will also be measured as secondary outcomes. Discussion This trial will report the findings of implementing Peer Tree, a smartphone application aimed at reducing loneliness in university students. Findings from this trial will highlight the initial efficacy, acceptability, and feasibility of using digital positive psychology interventions to reduce subthreshold mental health concerns. Findings from this trial will also describe the safety of Peer Tree as a digital tool. Results will contribute evidence for positive psychology interventions to address mental ill-health. Trial registration Australian New Zealand Clinical Trial Registry ACTRN12619000350123. Registered on 6 March 202

Recent advances in pulsed-laser deposition of complex-oxides

Pulsed-laser deposition (PLD) is one of the most promising techniques for the formation of complex-oxide heterostructures, superlattices, and well-controlled interfaces. The first part of this paper presents a review of several useful modifications of the process, including methods inspired by combinatorial approaches. We then discuss detailed growth kinetics results, which illustrate that 'true' layer-by-layer (LBL) growth can only be approached, but not fully met, even though many characterization techniques reveal interfaces with unexpected sharpness. Time-resolved surface x-ray diffraction measurements show that crystallization and the majority of interlayer mass transport occur on time scales that are comparable to those of the plume/substrate interaction, providing direct experimental evidence that a growth regime exists in which non-thermal processes dominate PLD. This understanding shows how kinetic growth manipulation can bring PLD closer to ideal LBL than any other growth method available today.Comment: 37 pages, 9 figures. Revie

Extended bound states and resonances of two fermions on a periodic lattice

The high-$T_c$ cuprates are possible candidates for d-wave superconductivity, with the Cooper pair wave function belonging to a non-trivial irreducible representation of the lattice point group. We argue that this d-wave symmetry is related to a special form of the fermionic kinetic energy and does not require any novel pairing mechanism. In this context, we present a detailed study of the bound states and resonances formed by two lattice fermions interacting via a non-retarded potential that is attractive for nearest neighbors but repulsive for other relative positions. In the case of strong binding, a pair formed by fermions on adjacent lattice sites can have a small effective mass, thereby implying a high condensation temperature. For a weakly bound state, a pair with non-trivial symmetry tends to be smaller in size than an s-wave pair. These and other findings are discussed in connection with the properties of high-$T_c$ cuprate superconductors.Comment: 21 pages, RevTeX, 4 Postscript figures, arithmetic errors corrected. An abbreviated version (no appendix) appeared in PRB on March 1, 199

Exclusion statistics for fractional quantum Hall states on a sphere

We discuss exclusion statistics parameters for quasiholes and quasielectrons excited above the fractional quantum Hall states near $\nu=p/(2np+1)$. We derive the diagonal statistics parameters from the (``unprojected'') composite fermion (CF) picture. We propose values for the off-diagonal (mutual) statistics parameters as a simple modification of those obtained from the unprojected CF picture, by analyzing finite system numerical spectra in the spherical geometry.Comment: 9 pages, Revtex, 4 Postscript figures. Universality of the statistics parameters is stressed, 2 figs adde

Two-Channel Kondo Physics from Tunnelling Impurities with Triangular Symmetry

Tunnelling impurities in metals have been known for some time to have the potential for exhibiting Kondo-like physics. However previous models based on an impurity hopping between two equivalent positions have run into trouble due to the existence of relevant operators that drive the system away from the non-Fermi-liquid Kondo fixed point. In the case of an impurity hopping among positions with higher symmetry, such as triangular symmetry, it is shown here that the non-Fermi-liquid behavior at low temperatures can be generic. Using various bosonization techniques, the fixed point is shown to be {\em stable}. However, unlike the conventional two-channel Kondo (2CK) model, it has {\em four} leading irrelevant operators, implying that while the form of the singular temperature dependence of physical quantities is similar to the 2CK model, there will not be simple universal amplitude ratios. The phase diagram of this system is analyzed and a critical manifold is found to separate the non-Fermi-liquid from a conventional Fermi liquid fixed point. Generalization to higher symmetries, such as cubic, and the possibility of physical realizations with dynamic Jahn-Teller impurities is discussed.Comment: 20 pages, 4 figures, RevTex format, submitted to Phys. Rev.

Performance of Concrete Pavements, Volume I: Field Investigation

DTFH61-91-C-00053With the goal of improving future concrete pavement design and construction practices, this project evaluated the performance of 303 inservice concrete pavement sections located throughout North America. An extensive field testing program, consisting of pavement condition surveys, drainage surveys, falling weight deflectometer (FWD) testing, coring/boring operations, and roughness testing, was conducted in order to collect the information needed for analysis. Because many of these pavement sections are part of State-level studies on concrete pavements, a range of design variables (e.g., load transfer, slab thickness, joint spacing, drainage) thought to affect concrete pavement performance are present. Over one-third of the sections was evaluated under a preceding Federal Highway Administration study, meaning that 5-year performance trends are available for some of the sections. Additional pavement performance data are also available for 96 European concrete pavement sections and for 21 Chilean Concrete pavement sections. The average age and average cumulative equivalent single axle loads (ESALs) for the North American sections are 16 years and 7.1 million, respectively, compared to 21 years and 21.8 million for the European sections and 9 years and 5.9 million for the Chilean sections. This volume describes the field data collection activities that were conducted under the study and also summarizes the individual performance of the 303 concrete pavement sections evaluated. Pertinent design and construction data are given for each project and key performance data are presented. Where available, 5-year performance trends are also summarized

Miniatures from domestic contexts in Iron age Iberia

This article reviews a set of miniatures from domestic contexts in Iron Age eastern Iberia, and interprets them in terms of their role in forging social personae. After an introduction to the historical case under consideration, the miniatures are described in terms of their typology and their contexts of provenance are outlined. Though not abundant, they tend to occur in central places in the landscape; specifically, they are often found in houses of the powerful. The vast majority are miniatures of pottery and tools, though some miniature weapons are recorded. We contend that these objects were used as a means of enculturation and for the learning of values and norms. It is no coincidence that they emerge in the archaeological record of Iron Age Iberia at the same time as the rise of a social structure based on hereditary power