Super-normal Transition Research Articles

Information cascade on networks and phase transitions

Herein, we consider a voting model for information cascades on several types of networks – a random graph, the Barabási–Albert(BA) model, and lattice networks – by using one parameter ω; ω=1,0,−1 respectively correspond to these networks. Our objective is to study the relation between the phase transitions and networks using the parameter, ω which is related to the size of hubs. We discuss the differences between the phases in which the networks depend. In ω≠−1, without a lattice, the following two types of phase transitions can be observed: information cascade transition and super-normal transition. The first is the transition between a state where most voters make correct choices and a state where most of them are wrong. This is an absorption transition that belongs to the non-equilibrium transition. In the symmetric case, the phase transition is continuous and the universality class is the same as nonlinear Pólya model. In contrast, in the asymmetric case, there is a discontinuous phase transition, where the gap depends on the network. As ω increases, the size of the hub and the gap increase. Therefore, a network that has hubs has a greater effect through this phase transition. The critical point of information cascade transition does not depend on ω. The super-normal transition is the transition of the convergence speed, and the critical point of the convergence speed transition depends on ω. At ω=1, in the BA model, this transition disappears, and the range where we can observe the phase transition is the same as that in the percolation model on the network. Both phase transitions disappear at ω=−1 in the lattice case. In conclusion, as the performance near the lattice case, ω∼−1 exhibits the best performance of the voting in all networks. As the hub size decreases, the performance improves. Finally, we show the relation between the voting model and the elephant walk model.

Information cascade on networks

In this paper, we discuss a voting model by considering three different kinds of networks: a random graph, the Barabási–Albert (BA) model, and a fitness model. A voting model represents the way in which public perceptions are conveyed to voters. Our voting model is constructed by using two types of voters–herders and independents–and two candidates. Independents conduct voting based on their fundamental values; on the other hand, herders base their voting on the number of previous votes. Hence, herders vote for the majority candidates and obtain information relating to previous votes from their networks. We discuss the difference between the phases on which the networks depend. Two kinds of phase transitions, an information cascade transition and a super-normal transition, were identified. The first of these is a transition between a state in which most voters make the correct choices and a state in which most of them are wrong. The second is a transition of convergence speed. The information cascade transition prevails when herder effects are stronger than the super-normal transition. In the BA and fitness models, the critical point of the information cascade transition is the same as that of the random network model. However, the critical point of the super-normal transition disappears when these two models are used.In conclusion, the influence of networks is shown to only affect the convergence speed and not the information cascade transition. We are therefore able to conclude that the influence of hubs on voters’ perceptions is limited.

REBCO コイルにおける熱暴走のメカニズムと対策

REBCO conductors were commercialized only recently in 2009 and therefore the magnet technology for REBCO conductors remains undeveloped. The major technological problems for REBCO coils encountered thus far are (a) degradation in the superconducting performance for mechanical reasons, (b) difficulty in protecting the magnet in the case of a super-normal transition of the REBCO coil and (c) the enormous effect of a screening-current-induced magnetic field. In this paper, the basic mechanism of premature thermal runaway in the REBCO superconductor is investigated through experiments and numerical simulations on small model coils. Effective remedies for suppressing the premature thermal runaway are discussed.

REBCO コイルにおける遮蔽電流磁場のメカニズムと抑制方法

REBCO conductors were commercialized only recently in 2009 and therefore the magnet technology for REBCO conductors remains undeveloped. The major technological problems for REBCO coils encountered thus far are (a) degradation in the superconducting performance for mechanical reasons, (b) difficulty in protecting the magnet in the case of a super-normal transition of the REBCO coil and (c) the enormous effect of a screening-current-induced magnetic field. In this paper, the basic mechanism of the screening-current-induced magnetic field and effective remedies for suppressing the screening field is investigated through experiments and numerical simulations.

R&D of HTS power cable and fault current limiter in Super-ACE project

Super-ACE (superconducting AC equipment) project started as a national project in 2000 fiscal year. This project is R&D of the basic technologies of high temperature superconducting (HTS) cable, HTS fault current limiter (FCL), HTS magnet for reactor and transformer. Main subjects are to study HTS cable, HTS fault current limiter (FCL), HTS AC magnet, and the analysis of HTS power system integrated these AC equipment. This paper describes the background, the target, the experimental results from 2000 till now and the future activities from 2002 to 2004. The major items are design philosophy, manufacturing method, and testing method of 500 m HTS cable with single core, 3 kA HTS conductor for power cable, 6.6 kV FCL using super–normal transition, and 66 kV FCL using reactor and thyristor bridge circuit. This paper describes the power system analysis and introducing effects of urban area network to introduce HTS Cable and FCL. We concluded that HTS cable and FCL are expected as supreme technology to innovate future power transmission system if these basic technologies are established, and it takes at least 10 years to realize these commercial stage.

Apparatus for deformation tests of solids in liquid 3He

An apparatus for deformation of solids in liquid 3He is constructed. Either tensile deformation or compression of a specimen can be performed by exchanging the assemblies in the 3He pot which has a capacity of about 30 cm3. The pulling rod for transmitting load from the tensile testing machine to the specimen runs inside the outlet tube of 3He, being isolated from 4He bath and almost free from mechanical friction. To measure the change in flow stress with the supernormal transition of superconducting metals, a superconducting magnet is mounted outside of the vacuum chamber which separates the 3He pot and the 4He bath. Under an applied load for plastic deformation the system is stably operative down to 0.6 K, while the lowest temperature achieved is 0.5 K. Some results on Ta and NaCl are presented.

Interpretation of Enhanced Plasticity in Superconducting Tantalum in Terms of Enhanced Quantum Tunneling of Dislocation Through the Peierls Potential

The super-normal effect of the flow stress, i.e. a reversible increase of the flow stress accompanying the transition from the superconducting to the normal state by application of a magnetic field, observed in ultra-high purity tantalum single crystals has been analyzed according to the recent theory by Petukhov et al . on dissipative dislocation tunneling [Philos. Mag. A 73 (1998) 1041]. Experimentally obtained temperature dependence of the super-normal effect and that of the strain rate sensitivity of the flow stress can well be fitted by the theory. Electronic viscosity coefficient of the dislocation motion in the normal state obtained in this fitting is 0.016 N·s/m 2 . According to this analysis, the super-normal effect in tantalum is due to the change of the rate of tunneling through the Peierls potential accompanying the change of electron viscosity by the super-normal transition.

Flux pinning strength of different superconducting artificial pins in Nb-Ti multifilamentary composites

To improve the critical current density (J/sub c/) of multifilamentary Nb-Ti superconductors, the pinning characteristics of wires having different superconducting artificial pinning materials and pin sizes were studied. Nb, Nb-7.5wt.%Ta and Ta pins were introduced into each Nb-Ti filament as an artificial pinning center (APC). Magnetization was measured to obtain the magnetic field dependence of the pinning force density (F/sub P/) at various temperatures. A significant enhancement of F/sub P/ has been observed for APC wires in comparison to conventional wires. F/sub P/ increases in ascending order of Ta, Nb-Ta and Nb pins. This result reflects the elementary pinning force (f/sub P/) calculated by the Ginzburg-Landau (G-L) theory. The slope of the F/sub P/ curve for Ta pins changes at 4.4 K, where a super-normal transition occurs. We also investigated the F/sub P/ properties in a magnetic field parallel to the wire axis. In this case, the peak of F/sub P/ shifts to a higher magnetic field compared with that of a perpendicular magnetic field. This result suggests that the number of effective pinning sites in a parallel magnetic field is larger than in a perpendicular magnetic field due to the anisotropic microstructure of the pins. Furthermore, we report on their flux pinning scaling behaviors.

DC-transport properties of QMG current limiting elements

V-I properties of YBCO bulk superconductor (QMG: YBa/sub 2/Cu/sub 3/O/sub x/ bulk single crystal containing Y/sub 2/BaCuO/sub x/ particles) were measured using pulse current at 77 K and 87 K to obtain basic data for super-normal transition. Because of its high Jc property QMG is a promising bulk material for HTC-FCL application. I-shaped QMG rods having cross section of 0.5 mm/spl times/1.0 mm were prepared to measure voltage due to pulse current. The plots of InV vs. InI yielded a straight line with a slope ranging from 5.8 to 7.9 at 77 K. By changing the amplitude and duration of pulses the time from the start of feeding current till the occurrence of quench were obtained for various amplitudes of pulse current. In this experiment, it is considered that the quench did not occur in the whole rod but only in a small part.

高温超伝導体ＹＢａ２Ｃｕ４Ｏ８の超伝導転移温度近傍における仕事関数 電気抵抗との同時測定の場合

高温超伝導体ＹＢａ２Ｃｕ４Ｏ８の超伝導転移温度近傍における仕事関数 電気抵抗との同時測定の場合