Theory of Magnetic Short--Range Order for High-T_c Superconductors

A theory of magnetic short--range order for high--$T_c$ cuprates is presented on the basis of the one--band $t$--$t^{\prime}$--Hubbard model combining the four--field slave--boson functional integral technique with the Bethe cluster method. The ground--state phase diagram evaluated self--consistently at the saddle--point and pair--approximation levels shows the experimentally observed suppression of magnetic long--range order in the favour of a paraphase with antiferromagnetic short--range order. In this phase the uniform static spin susceptibility consists of interrelated itinerant and local parts and increases upon doping up to the transition to the Pauli paraphase. Using realistic values of the Hubbard interaction we obtain the cusp position and the doping dependence of the zero--temperature susceptibility in reasonable agreement with experiments on $\rm La_{2-\delta}Sr_{\delta}CuO_4$.Comment: 3 pages, 2 Postscript figure, Proc. Int. Conf. SCES Zuerich Switzerland Aug. 96, to appear in Physica

Formality and informality in the summative assessment of motor vehicle apprentices: a case study

This article explores the interaction of formal and informal attributes of competence‐based assessment. Specifically, it presents evidence from a small qualitative case study of summative assessment practices for competence‐based qualifications within apprenticeships in the motor industry in England. The data are analysed through applying an adaptation of a framework for exploring the interplay of formality and informality in learning. This analysis reveals informal mentoring as a significant element which influences not only the process of assessment, but also its outcomes. We offer different possible interpretations of the data and their analysis, and conclude that, whichever interpretation is adopted, there appears to be a need for greater capacity‐building for assessors at a local level. This could acknowledge a more holistic role for assessors; recognise the importance of assessors’ informal practices in the formal retention and achievement of apprentices; and enhance awareness of inequalities that may be reinforced by both informal and formal attributes of assessment practices

Antiferromagnetic Phases of One-Dimensional Quarter-Filled Organic Conductors

The magnetic structure of antiferromagnetically ordered phases of quasi-one-dimensional organic conductors is studied theoretically at absolute zero based on the mean field approximation to the quarter-filled band with on-site and nearest-neighbor Coulomb interaction. The differences in magnetic properties between the antiferromagnetic phase of (TMTTF)$_2$X and the spin density wave phase in (TMTSF)$_2$X are seen to be due to a varying degrees of roles played by the on-site Coulomb interaction. The nearest-neighbor Coulomb interaction introduces charge disproportionation, which has the same spatial periodicity as the Wigner crystal, accompanied by a modified antiferromagnetic phase. This is in accordance with the results of experiments on (TMTTF)$_2$Br and (TMTTF)$_2$SCN. Moreover, the antiferromagnetic phase of (DI-DCNQI)$_2$Ag is predicted to have a similar antiferromagnetic spin structure.Comment: 8 pages, LaTeX, 4 figures, uses jpsj.sty, to be published in J. Phys. Soc. Jpn. 66 No. 5 (1997

Teachers' classroom feedback: still trying to get it right

This article examines feedback traditionally given by teachers in schools. Such feedback tends to focus on children's acquisition and retrieval of externally prescribed knowledge which is then assessed against mandated tests. It suggests that, from a sociocultural learning perspective, feedback directed towards such objectives may limit children's social development. In this article, I draw on observation and interview data gathered from a group of 27 9- to 10-year olds in a UK primary school. These data illustrate the children's perceived need to conform to, rather than negotiate, the teacher's feedback comments. They highlight the children's sense that the teacher's feedback relates to school learning but not to their own interests. The article also includes alternative examples of feedback which draw on children's own inquiries and which relate to the social contexts within which, and for whom, they act. It concludes by suggesting that instead of looking for the right answer to the question of what makes teachers' feedback effective in our current classrooms, a more productive question might be how a negotiation can be opened up among teachers and learners themselves, about how teachers' feedback could support children's learning most appropriately

Ordering phenomena in quasi one-dimensional organic conductors

Low-dimensional organic conductors could establish themselves as model systems for the investigation of the physics in reduced dimensions. In the metallic state of a one-dimensional solid, Fermi-liquid theory breaks down and spin and charge degrees of freedom become separated. But the metallic phase is not stable in one dimension: as the temperature is reduced, the electronic charge and spin tend to arrange themselves in an ordered fashion due to strong correlations. The competition of the different interactions is responsible for which broken-symmetry ground state is eventually realized in a specific compound and which drives the system towards an insulating state. Here we review the various ordering phenomena and how they can be identified by optic and magnetic measurements. While the final results might look very similar in the case of a charge density wave and a charge-ordered metal, for instance, the physical cause is completely different. When density waves form, a gap opens in the density of states at the Fermi energy due to nesting of the one-dimension Fermi surface sheets. When a one-dimensional metal becomes a charge-ordered Mott insulator, on the other hand, the short-range Coulomb repulsion localizes the charge on the lattice sites and even causes certain charge patterns. We try to point out the similarities and conceptional differences of these phenomena and give an example for each of them. Particular emphasis will be put on collective phenomena which are inherently present as soon as ordering breaks the symmetry of the system.Comment: Review article Naturwissenschaften 200

The barocaloric effect: A Spin-off of the Discovery of High-Temperature Superconductivity

Some key results obtained in joint research projects with Alex M\"uller are summarized, concentrating on the invention of the barocaloric effect and its application for cooling as well as on important findings in the field of high-temperature superconductivity resulting from neutron scattering experiments.Comment: 26 pages, 9 figure

Thermopower in the strongly overdoped region of single-layer Bi2Sr2CuO6+d superconductor

The evolution of the thermoelectric power S(T) with doping, p, of single-layer Bi2Sr2CuO6+d ceramics in the strongly overdoped region is studied in detail. Analysis in term of drag and diffusion contributions indicates a departure of the diffusion from the T-linear metallic behavior. This effect is increased in the strongly overdoped range (p~0.2-0.28) and should reflect the proximity of some topological change.Comment: 4 pages, 4 figure

T-helper cell polarisation following severe polytrauma

Introduction Severe polytrauma induces an immunosuppressive response and is associated with a very high incidence of nosocomial infections. Previous studies have inferred that this detrimental immune response results from polarisation of the T helper (Th) response towards an anti-inflammatory, TH2 dominated, response at the expense of a bactericidal, Th1 response [1]. Objectives 1) To define alterations in TH cell subsets following severe blunt polytrauma. Methods Patients presenting to the emergency department within 2 hours of severe polytrauma were eligible if intubated either at the scene or in ED. Isolated head injuries and those not expected to survive 24 hours were excluded. EDTA anti-coagulated blood was drawn at 0hr (within 2 hours of injury), at 24 and 72hrs. Samples were immediately lysed, washed, stained and analysed using a standardised human 8-colour TH 1, 2 & 17 panel [2] on an LSR II flow cytometer. A paired white cell count differential was obtained at each sampling point. Patients were followed until discharge or death. Data were analysed using non-parametric statistics, with results presented as median and IQR. Results 15 consecutive severe polytrauma patients requiring Intensive Care Unit (ICU) admission were recruited. Demographic and clinical data are outlined in Figure 1. Twelve (80%) lymphocytosis (3.3x109/L, 2.5 - 4.4x109/L) (Figyre 2A). At 72 hours leukocytes had fallen (P < 0.01, figure 2A) such that 6 (54%) of those surviving were lymphopenic (0.9x109/L, 0.6 - 1.2x109/L). Circulating CD4+ (P = 0.01; Figure 2B) and CD4+CD25+ (P < 0.05) lymphocytes increased over 72 hours. When expressed as a percentage of total circulating lymphocytes no significant change in the proportions of the TH 1, 2 & 17 subpopulations was detected (Figure 2C-E). Conclusions Severe polytrauma patients swiftly become lymphopenic. Although a failure to normalise this during the ICU stay correlates with higher mortality [3] our study of TH cell subtypes demonstrates no evidence of a switch to a detrimental anti-inflammatory TH2 subtype at the expense of the potentially protective bactericidal TH1 subtype

Silicon intercalation into the graphene-SiC interface

In this work we use LEEM, XPEEM and XPS to study how the excess Si at the graphene-vacuum interface reorders itself at high temperatures. We show that silicon deposited at room temperature onto multilayer graphene films grown on the SiC(000[`1]) rapidly diffuses to the graphene-SiC interface when heated to temperatures above 1020. In a sequence of depositions, we have been able to intercalate ~ 6 ML of Si into the graphene-SiC interface.Comment: 6 pages, 8 figures, submitted to PR