Effects of disorder on lattice Ginzburg-Landau model of d-wave superconductors and superfluids

We study the effects of quenched disorder on the two-dimensional d-wave superconductors (SC's) at zero temperature by Monte-Carlo simulations. The model is defined on the three-dimesional (3D) lattice and the SC pair field is put on each spatial link as motivated in the resonating-valence-bond theory of the high-$T_{\rm c}$ SC's. For the nonrandom case, the model exhibits a second-order phase transition to a SC state as density of charge carriers is increased. It belongs to the universality class {\it different from} that of the 3D XY model. Quenched disorders (impurities) are introduced both in the hopping amplitude and the plaquette term of pair fields. Then the second-order transition disappears at a critical concentration of quenched disorder, $p_c\simeq 15$. Implication of the results to cold atomic systems in optical lattices is also discussed.Comment: 4 pages, 8 figure

An Analysis of Traveling-wave Amplification of Surface Magnetostatic Wave

The amplification of a surface magnetostatic wave is analyzed which propagates on the contacting surface of ferrite and semiconductor layers magnetized transversely. A hydrodynamical and collision dominant model is adopted for carriers in the semiconductor. It is explained that neither diffusion effect nor surface charges induced by the Hall effect have to be considered in this case. Maxwell's equation under adequate boundary conditions is solved to give a complex characteristic equation of real ω and complex k. The following results have been obtained from the equation. 1) When the carrier density or the thickness of the semiconductor is small, the amplification factor is proportional to both of them. 2) There is an optimum value of the carrier density or the thickness to obtain the maximum amplification factor. 3) The amplification factor depends on the direction of static magnetic field

Contextual attributes to promote positive social interdependence in problem-based learning:a focus group study

Background Problem-based learning (PBL) is classified as a collaborative learning approach, wherein students learn while contributing meaning to experiences and interactions with others. An important theoretical fundament of PBL is social interdependence theory (SIT) because positive social interdependence within a group has been found to be key to better learning performance and future attitudes towards team practice. However, most previous studies in health professions education focused on cognitive outcomes, and few studies have focused on collaborative behaviors in PBL groups. The lack of this empirical insight makes implementation of PBL difficult, especially in contexts where there is limited experience with collaborative learning. Therefore, the aim of this study was to elucidate what promotes or hinders positive social interdependence and how the attributes work during PBL. Methods We conducted four focus groups among clinical year medical students (n = 26) who participated in PBL tutorials in the formal curriculum. We asked semi-structured questions that corresponded with the overall concept of SIT. We analyzed the transcript using constructivist grounded theory and developed a model to explain contextual attributes that promote or hinder positive social interdependence in PBL. Results Two contextual attributes of "academic inquisition" and "desire for efficiency" affect social interdependence among a student group in PBL. Academic inquisition is students' desire to engage in their academic learning, and desire for efficiency is students' attitude toward learning as an imposed duty and desire to complete it as quickly as possible. These attributes are initially mutually conflicting and constructing social interdependence through multiple steps including inquisition from a case, seeking efficient work, sharing interest in problem solving, expecting mutual contributions, and complementing learning objectives. Conclusion These findings will contribute to understanding collaborative learning environments in PBL and may help explain contexts where PBL is less successful. The model can also be used as a tool to support innovation of PBL as collaborative learning

Phase structure of Z2Z_2 gauge theories for frustrated antiferromagnets in two dimensions

In this paper, we study phase structure of $Z_2$ lattice gauge theories that appear as an effective field theory describing low-energy properties of frustrated antiferromagnets in two dimensions. Spin operators are expressed in terms of Schwinger bosons, and an emergent U(1) gauge symmetry reduces to a $Z_2$ gauge symmetry as a result of condensation of a bilinear operator of the Schwinger boson describing a short-range spiral order. We investigated the phase structure of the gauge theories by means of the Monte-Carlo simulations, and found that there exist three phases, phase with a long-range spiral order, a dimer state, and a spin liquid with deconfined spinons. Detailed phase structure and properties of phase transitions depend on details of the models.Comment: 11 pages, 27 figures, Version to be published in Phys.Rev.