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Lee, Y.-W.,Jang, T.-H.,Dissanayake, S. E.,Lee, Seunghun,Jeong, Yoon H. IOP Publishing - Europhysics Letters 2016 Europhysics letters Vol.113 No.2
<P>Single crystals of the orthorhombic polar oxide alpha-Cu2V2O7 with space group Fdd2 are synthesized and their physical properties are measured. Neutron powder diffraction is also performed on a polycrystal sample to extract the magnetic structure. The ground state is shown to be weakly ferromagnetic, that is, collinearly antiferromagnetic in the a-direction with a small remanent magnetization in the c-direction. When an external magnetic field is applied in the c-direction, further spin canting, accompanied by the induced electric polarization, occurs. It is demonstrated that the magnetoelectric effect in alpha-Cu2V2O7 is adequately described if spin-dependent p-d hybridization due to spin-orbit coupling as well as magnetic domain effects are simultaneously taken into account. We discuss the implication of the present result in the search for materials with multiferroicity and/or magnetoelectricity. Copyright (C) EPLA, 2016</P>
Ranking influential spreaders is an ill-defined problem
Gu, Jain,Lee, Sungmin,Saramä,ki, Jari,Holme, Petter IOP Publishing - Europhysics Letters 2017 Europhysics letters Vol.118 No.6
<P>Finding influential spreaders of information and disease in networks is an important theoretical problem, and one of considerable recent interest. It has been almost exclusively formulated as a node-ranking problem - methods for identifying influential spreaders output a ranking of the nodes. In this work, we show that such a greedy heuristic does not necessarily work: the set of most influential nodes depends on the number of nodes in the set. Therefore, the set of n most important nodes to vaccinate does not need to have any node in common with the set of n + 1 most important nodes. We propose a method for quantifying the extent and impact of this phenomenon. By this method, we show that it is a common phenomenon in both empirical and model networks. Copyright (C) EPLA, 2017</P>
Kamble, Bhaskar,Akbari, Alireza,Eremin, Ilya IOP Publishing - Europhysics Letters 2016 Europhysics letters Vol.114 No.1
<P>We employ a five-orbital tight-binding model to develop the mean-field solution for various possible spin density wave states in the iron-chalcogenides. The quasiparticle interference (QPI) technique is applied to detect signatures of these states due to scatterings arising from non-magnetic impurities. Apart from the experimentally observed double-striped structure with ordering vector (pi/2, pi/2), the QPI method is investigated for the extended-stripe as well as the orthogonal double-stripe phase. We discuss QPI as a possible tool to detect and classify various magnetic structures with different electronic structure reconstruction within the framework of the Fe1+yTe compound. Copyright (C) EPLA, 2016</P>
Han, Jihoon,Ryu, Seunghwa,Kim, Dong-Kyu,Woo, Wanchuck,Sohn, Dongwoo IOP Publishing - Europhysics Letters 2016 Europhysics letters Vol.114 No.6
<P>Nanoindentation experiments and simulations are carried out to estimate the elastic modulus of freely-suspended-multilayer graphene. However, due to the difficulty of clamping all layers of multilayer graphene in experiments, and to the ambiguity of imposing the clamped boundary conditions in numerical simulations, the estimated values of elastic modulus exhibit large variation. In particular, interlayer sliding can affect the estimation of elastic modulus. From a series of molecular dynamics simulations, we demonstrate that the estimated elastic modulus of multilayer graphene depends on the level of interlayer sliding involved in boundary conditions. Under fully clamped boundary conditions that prevent interlayer sliding, the elastic modulus is constant regardless of the number of layers. In contrast, under weakly clamped boundary conditions that involve interlayer sliding, the elastic modulus decreases with increasing number of layers. In the case of weakly clamped conditions, a few wrinkles form in the interlayer and then coalesce into a single large wrinkle due to interlayer sliding. Our findings provide an understanding of the variation of elastic modulus observed in other experimental and numerical studies. Copyright (C) EPLA, 2016</P>
Origin of the spike-timing-dependent plasticity rule
Cho, Myoung Won,Choi, M. Y. IOP Publishing - Europhysics Letters 2016 Europhysics letters Vol.115 No.3
<P>A biological synapse changes its efficacy depending on the difference between pre- and post-synaptic spike timings. Formulating spike-timing-dependent interactions in terms of the path integral, we establish a neural-network model, which makes it possible to predict relevant quantities rigorously by means of standard methods in statistical mechanics and field theory. In particular, the biological synaptic plasticity rule is shown to emerge as the optimal form for minimizing the free energy. It is further revealed that maximization of the entropy of neural activities gives rise to the competitive behavior of biological learning. This demonstrates that statistical mechanics helps to understand rigorously key characteristic behaviors of a neural network, thus providing the possibility of physics serving as a useful and relevant framework for probing life. Copyright (C) EPLA, 2016</P>
Kim, Yong-Jin,Lee, Jin Hong,Kim, Sang-Woo,Koo, Tae Yeong,Yang, Chan-Ho IOP Publishing - Europhysics Letters 2016 Europhysics letters Vol.116 No.2
<P>We investigate the effects of misfit strain on the orbital order of epitaxial lanthanum manganite thin films grown on (LaAlO3)(0.3)-(Sr2AlTaO6) (0.7) (LSAT) and GdScO3 (GSO) substrates. Resonant X-ray scattering near the Mn K-edge is employed to identify the cooperative Jahn-Teller distortion at room temperature and determine the orientation of the orbital-ordered plane (OOP). We find that coherent growth on LSAT (GSO) makes the OOPs be vertical (parallel) to the film surface. This finding not only offers useful insight into the interplay between misfit strain and orbital order, but also holds promise for strain control of orbital-dependent physical properties. Copyright (C) EPLA, 2016</P>