arXiv subjects

T. Amemiya

Publications and source records attributed to T. Amemiya.

5 recordsLinked to original sources

arXiv2009

Partial ferromagnetic ordering and indirect exchange interaction in spatially anisotropic kagome antiferromagnet Cs_2Cu_3CeF_{12}

We report the crystal structure and unconventional magnetic ordering of Cs_2Cu_3CeF_{12}, which is composed of buckled kagome lattice of Cu^{2+} ions. The exchange network in the buckled kagome lattice is fairly anisotropic, so that the present spin system can be divided into two subsystems: alternating Heisenberg chains with strong antiferromagnetic exchange interactions and dangling spins. Although the direct exchange interactions between neighboring spins were found to be all antiferromagnetic, ferromagnetic magnetic ordering of the dangling spins was observed. Magnetization exhibits a plateau at one-third of the saturation magnetization. These observations can be understood in terms of the indirect interaction between dangling spins mediated by the chain spin.

cond-mat.str-el↗

arXiv2005

Food Webs: Experts Consuming Families of Experts

The question what determines the structure of natural food webs has been listed among the nine most important unanswered questions in ecology. It arises naturally from many problems related to ecosystem stability and resilience. The traditional view is that population-dynamical stability is crucial for understanding the observed structures. But phylogeny (evolutionary history) has also been suggested as the dominant mechanism. Here we show that observed topological features of predatory food webs can be reproduced to unprecedented accuracy by a mechanism taking into account only phylogeny, size constraints, and the heredity of the trophically relevant traits of prey and predators. The analysis reveals a tendency to avoid resource competition rather than apparent competition. In food webs with many parasites this pattern is reversed.

q-bio.PE↗

arXiv2005

Some Properties of the Speciation Model for Food-Web Structure - Mechanisms for Degree Distributions and Intervality

We present a mathematical analysis of the speciation model for food-web structure, which had in previous work been shown to yield a good description of empirical data of food-web topology. The degree distributions of the network are derived. Properties of the speciation model are compared to those of other models that successfully describe empirical data. It is argued that the speciation model unifies the underlying ideas of previous theories. In particular, it offers a mechanistic explanation for the success of the niche model of Williams and Martinez and the frequent observation of intervality in empirical food webs.

q-bio.PE↗

arXiv2005

An explanatory model for food-web structure and evolution

Food webs are networks describing who is eating whom in an ecological community. By now it is clear that many aspects of food-web structure are reproducible across diverse habitats, yet little is known about the driving force behind this structure. Evolutionary and population dynamical mechanisms have been considered. We propose a model for the evolutionary dynamics of food-web topology and show that it accurately reproduces observed food-web characteristic in the steady state. It is based on the observation that most consumers are larger than their resource species and the hypothesis that speciation and extinction rates decrease with increasing body mass. Results give strong support to the evolutionary hypothesis.

q-bio.PE↗

arXiv2005

High Temperature Ferromagnetism in GaAs-based Heterostructures with Mn Delta Doping

We show that suitably-designed magnetic semiconductor heterostructures consisting of Mn delta-doped GaAs and p-type AlGaAs layers, in which the locally high concentration of magnetic moments of Mn atoms are controllably overlapped with the 2-dimensional hole gas wavefunction, realized remarkably high ferromagnetic transition temperatures (TC). Significant reduction of compensative Mn interstitials by varying the growth sequence of the structures followed by low temperature annealing led to high TC up to 250 K. The heterostructure with high TC exhibited peculiar anomalous Hall effect behavior, whose sign depends on temperature.

cond-mat.mtrl-sci↗