The elimination of hierarchy in a completely cyclic competition system

Interactions among competing units are crucial to maintaining biodiversity, and non-hierarchical interactions can promote biodiversity in cyclic competing systems. In the present study, we explore the role of hierarchical interactions, existing ubiquitously in reality, in the co-evolution of a cyclic competing system. In systems composed of cyclic competing species with hierarchy interactions in which one predator species has more than one prey, we find that hierarchy disappears in a rather short evolving time. In the process of co-evolution, a hierarchical competing system tends to transit to a cyclic non-hierarchical competing system described by the rock–paper–scissors game. In other words, the cyclic competing interactions appear to eradicate hierarchy. This conclusion is analyzed by a mean-field approach and is tested by stochastic simulations.     

Periodic Minimizers in 1D Local Mean Field Theory

There are not many physical systems where it is possible to demonstate rigorously that energy minimizers are periodic. Using reflection positivity techniques we prove, for a class of mesoscopic free-energies representing 1D systems with competing interactions, that all minimizers are either periodic, with zero average, or of constant sign. Examples of both phenomena are given. This extends our previous work where such results were proved for the ground states of lattice systems with ferromagnetic nearest neighbor interactions and dipolar type antiferromagnetic long range interactions.     

Magnetism in systems with reduced dimensionality and chemical disorder: The local environment effects

We study influence of the local chemical environment, the so-called local environment effects, on the electronic structure and properties of magnetic systems with reduced dimensionality and chemical disorder, and show that they play a crucial role in a vicinity of magnetic instability. As a model, we consider Fe–Ni Invar. We present results obtained from ab initio calculations of the electronic structure, magnetic moments, and exchange interactions in random fcc Fe–Ni alloy, for a single monolayer alloy film on a Cu (0 0 1) substrate as well as in the bulk. We analyze the difference between the film and the bulk magnetization, which is found to be most pronounced for dilute alloys. We also analyze a sensitivity of the individual magnetic moments and effective exchange parameters to the local chemical environment of the atoms.     

Structure and fragmentation in colloidal artificial molecules and nuclei

Motivated by recent experiments on colloidal systems with competing attractive and repulsive interactions, we simulate a two-dimensional system of colloids with competing interactions that can undergo fragmentation. In the absence of any other confining potential, the colloids can form stable clusters depending on the strength of the short range attractive term. By suddenly changing the strength of one of the interaction terms we find a rich variety of fragmentation behavior which is affected by the existence of “magic” cluster numbers. Such soft matter systems can be used to construct artificial nuclei.     

Order and disorder lines in systems with competing interactions: I. Quantum spins atT=0

In the parameter space of systems with competing interactions there are specific trajectories called order (disorder) lines. Along these trajectories the competition between the different interactions effectively reduces the dimensionality of the system and the model can be exactly solved. It is shown that the order (disorder) trajectories end up at a multicritical point. The method of Peschel and Emery is used to determine the (anisotropic) critical behavior of the spin-spin correlation functions near the multicritical point. The quantum spin systems discussed here include theXYZ chain in a field, the straggeredXYZ chain in a field, and a Hamiltonian version of a three-dimensional Ising model with biaxial competing interactions.On leave from and address after September 1, 1982: Institute for Theoretical Physics, Eötvös University, Puskin U. 5-7, 1088 Budapest, Hungary.     

Universal aspects of coulomb-frustrated phase separation

We study the consequences of Coulomb interactions on a system undergoing a putative first order phase transition. In two dimensions (2D), near the critical density, the system is universally unstable to the formation of new intermediate phases, which we call "electronic microemulsion phases," which consist of an intermediate scale mixture of regions of the two competing phases. A corollary is that there can be no direct transition as a function of density from a 2D Wigner crystal to a uniform electron liquid. In 3D, if the strength of the Coulomb interactions exceeds a critical value, no phase separation occurs, while for the weaker Coulomb strength electronic microemulsions are inevitable. This tendency is considerably more pronounced in anisotropic (quasi-2D or quasi-1D) systems, where a devil's staircase of transitions is possible.     

Lifshitz points in ising systems

Phase diagrams of Ising systems with competing interactions are calculated using (a) the method of Müller-Hartmann and Zittartz to determine the transition temperature via the vanishing of an interface free energy (b) a Migdal-Kadanoff bond-moving scheme and (c) Monte Carlo simulations. It is shown that in two-dimensional Ising systems a uniaxial Lifshitz point can exist at non-zero temperatures, whereas the lower critical dimensiond _l for a Lifshitz point in a system with identical competing interactions along each of its cartesian axis isd _l 2.     

Theory of electronic structure of non-dilute transition metal alloys : Ni-Cu

The density of states for the Ni-Cu alloy system has been calculated as a function of concentration by the supercell method. This method takes into account inter-site interactions in contrast with current effective medium or single site theories. We show that the method is practical for the study of disordered systems. Comparison is made with previous work as well as with photoemission data.     

Spin waves in FeAlSi system

Results of the neutron measurements of magnon dispersion relation in Fe_2.937Al_0.381Si_0.682 (Sendust) alloy are presented. The iron-iron interaction in the FeAlSi system is considered and it is shown that the values of nearest-neighbour exchange integrals in the ordered alloys are linearly dependent on the interatomic distance, the rate of the decrease being about 40 meV/Å. This strongly supports a model of competing ferro- and antiferromagnetic interactions usually assumed for concentrated Fe-Al alloys exhibiting spin-glass behaviour.     

Magnetic frustration in ferric fluorides investigated by Mössbauer spectrometry

The origin of magnetic frustration in antiferromagnetic ionic systems is discussed in terms first of cationic topology and then of competing superexchange and supersuperexchange interactions. Some selected examples based on ferric fluorides and oxyfluorides are presented to illustrate both the origins of magnetic frustration and the contribution of zero-field and in-field Mössbauer experiments.     

Corrections to scaling in disordered systems with competing interactions

Using a mean-field effective-field model for disordered systems with competing interactions we have (a) obtained explicit expressions for the corrections to scaling for both ferromagnets and spin glasses up to order h³ in the magnetization, and (b) performed numerical calculations of the non-linear susceptibility of both ferromagnets and spin glasses as a function of reduced field and temperature. Scaling plots constructed from the numerical data show that the departures from universality are much more significant for spin glasses than they are for ferromagnets and that, while the discrepancy can be reduced through a redefinition of the scaling variables, a knowledge of the proper non-universal scaling correction is essential for spin glasses if a reasonable degree of universality is to be achieved.     

Hubert model for modulated states in systems with competing exchange interactions

Within a continuum model for magnetic systems with competing interactions introduced by Hubert the classification of all possible modulated structures for axial easy-plane systems has been carried out, and the phase portraits for these solutions are constructed. Analytic solutions for the structure of isolated domain walls between helicoids of different chirality are presented. The equilibrium structures of the modulated phases in an applied magnetic field and the critical field for transitions from helical to fan phase are in qualitative agreement with results obtained within the corresponding discretized model and various experimental data.     

二元合金多层膜中的固态反应非晶化

本文报道在Fe-Ti和Fe-Si系中发生的固态反应非晶化,肯定了Ni-Ti系中非晶相的形成,认为这类新型非晶化主要是一种动力学相竞争的结果,并初步讨论了在固态反应中形成非晶相的动力学判据。 关键词：     

非晶态合金与氢相互作用的研究进展

非晶态合金在力学性能、耐磨耐蚀性、磁性等方面比传统晶态合金具有显著优势,是一类有优良应用前景的新型结构与功能材料.非晶态合金与氢相互作用可以产生很多有趣的物理化学现象和应用.本文从物理基础和材料应用两个方面评述非晶态合金和氢相互作用的研究进展,在物理基础研究方面,从氢在非晶态合金中的存在状态出发,讨论氢在非晶态合金中的溶解、分布、占位和扩散等相关物理问题,进而分析氢对非晶态合金的热稳定性、磁性、内耗、氢脆等的影响.在材料应用研究方面,对非晶态储氢合金、非晶态合金氢功能膜、吸氢改善非晶态合金的塑性和玻璃形成能力、氢致非晶化、利用非晶态合金制备纳米储氢材料等方面的研究进展进行评述.最后总结并展望有关非晶态合金与氢相互作用的研究和应用.     

Correlation Functions by Cluster Variation Method for Ising Model with NN, NNN, and Plaquette Interactions

We consider a procedure for calculating the pair correlation function in the context of cluster variation methods (CVM). As specific cases, we study the pair correlation function in the paramagnetic phase of the Ising model with nearest neighbors, next to nearest neighbors, and plaquette interactions in two and three dimensions. In presence of competing interactions, the so-called disorder line separates in the paramagnetic phase a region where the correlation function has the usual exponential behavior from a region where the correlation has an oscillating, exponentially damped behavior. In two dimensions, using the plaquette as the maximal cluster of the CVM approximation, we calculate the phase diagram and the disorder line for a case where a comparison is possible with known results for the eight-vertex model. In three dimensions, in the CVM cube approximation, we calculate the phase diagram and the disorder line in some cases of particular interest. The relevance of our results for experimental systems like mixtures of oil, water, and surfactant is also discussed.     

Bulk and surface properties of liquid X-Zr (X=Ag, Cu) compound forming alloys

The phase diagrams of the Ag-Zr and Cu-Zr systems exhibit the existence of different intermetallic compounds in the solid state, and since the structure of a liquid alloy is in some respects similar to that of a crystal, the compound formation phenomenon in these liquid alloy systems has been analysed through the study of surface properties (surface tension and surface composition), dynamic properties (chemical diffusion and viscosity) and microscopic functions (concentration fluctuations in the long-wavelength limit and chemical short-range order parameter) in the frame of the compound formation model (CFM). Moreover, the associative tendency between unlike constituent elements qualitatively expressed by the microscopic functions indicates the glass-forming ability of both systems at higher Zr-concentrations. These results are in agreement with reported experimental data and confirm the applicability of a statistical mechanical theory in conjunction with the CFM to describe the mixing behaviour of compound forming alloys.