Disordered two-dimensional superconductors: roles of temperature and interaction strength

We have considered the half-filled disordered attractive Hubbard model on a square lattice, in which the on-site attraction is switched off on a fraction $f$ of sites, while keeping a finite $U$ on the remaining ones. Through Quantum Monte Carlo (QMC) simulations for several values of $f$ and $U$, and for system sizes ranging from $8\times 8$ to $16\times 16$, we have calculated the configurational averages of the equal-time pair structure factor $P_s$, and, for a more restricted set of variables, the helicity modulus, $\rho_s$, as functions of temperature. Two finite-size scaling {\it ansatze} for $P_s$ have been used, one for zero-temperature and the other for finite temperatures. We have found that the system sustains superconductivity in the ground state up to a critical impurity concentration, $f_c$, which increases with $U$, at least up to U=4 (in units of the hopping energy). Also, the normalized zero-temperature gap as a function of $f$ shows a maximum near $f\sim 0.07$, for $2\lesssim U\lesssim 6$. Analyses of the helicity modulus and of the pair structure factor led to the determination of the critical temperature as a function of $f$, for $U=3,$ 4 and 6: they also show maxima near $f\sim 0.07$, with the highest $T_c$ increasing with $U$ in this range. We argue that, overall, the observed behavior results from both the breakdown of CDW-superconductivity degeneracy and the fact that free sites tend to "push" electrons towards attractive sites, the latter effect being more drastic at weak couplings.Comment: 9 two-column pages, 14 figures, RevTe

Impurities near an Antiferromagnetic-Singlet Quantum Critical Point

Heavy fermion systems, and other strongly correlated electron materials, often exhibit a competition between antiferromagnetic (AF) and singlet ground states. Using exact Quantum Monte Carlo (QMC) simulations, we examine the effect of impurities in the vicinity of such AF- singlet quantum critical points, through an appropriately defined impurity susceptibility, $\chi_{imp}$. Our key finding is a connection, within a single calculational framework, between AF domains induced on the singlet side of the transition, and the behavior of the nuclear magnetic resonance (NMR) relaxation rate $1/T_1$. We show that local NMR measurements provide a diagnostic for the location of the QCP which agrees remarkably well with the vanishing of the AF order parameter and large values of $\chi_{imp}$. We connect our results with experiments on Cd-doped CeCoIn$_5$

Immunization and Aging: a Learning Process in the Immune Network

The immune system can be thought as a complex network of different interacting elements. A cellular automaton, defined in shape-space, was recently shown to exhibit self-regulation and complex behavior and is, therefore, a good candidate to model the immune system. Using this model to simulate a real immune system we find good agreement with recent experiments on mice. The model exhibits the experimentally observed refractory behavior of the immune system under multiple antigen presentations as well as loss of its plasticity caused by aging.Comment: 4 latex pages, 3 postscript figures attached. To be published in Physical Review Letters (Tentatively scheduled for 5th Oct. issue

Understanding the coach-coachee-client relationship: a conceptual framework for executive coaching

Objectives: There is a need for a more comprehensive understanding of how coaching processes psychologically operate. This paper presents the findings from a study aimed to characterise the coaching process experience and to identify how specific experiences contribute to coaching outcomes. Design: A qualitative design was adopted. Data was analysed by Interpretative Phenomenological Analysis (Smith, 2008). Method: Data was collected from 10 participants, this included coaches (N = 4), coachees (N = 5) and one commissioner, three times along the coaching process. A total of 30 interviews were undertaken. Findings: Coaching outcomes can be generated by three essential mechanisms: Projection of Future Self; Perspectivation of Present Self; and Confirmation of Past/Present Self. Each mechanism’s name represents a particular effect on coachee’s self and may evolve diverse coaching behaviours. Although they all can be actively managed to generate sustainability of outcomes, each mechanism tends to contribute differently to that sustainability. Conclusion: The study provides a comprehensive understanding of the different methodological and experiential ingredients of the coaching process and its implications. While most coaching research is focused on identifying coaching results based on a retrospective analysis, this is one of the first studies accompanying longitudinally the coaching process and capturing an integrative understanding of its dynamics. Moreover, the study provides evidence of how coaching can differently deliver sustainable outcomes and be used as a valuable developmental tool in organisations. The study contributes to our understanding of theory building and raises questions for further research on the uniqueness of coaching interventions