Intrinsic adaptation in autonomous recurrent neural networks

A massively recurrent neural network responds on one side to input stimuli and is autonomously active, on the other side, in the absence of sensory inputs. Stimuli and information processing depends crucially on the qualia of the autonomous-state dynamics of the ongoing neural activity. This default neural activity may be dynamically structured in time and space, showing regular, synchronized, bursting or chaotic activity patterns. We study the influence of non-synaptic plasticity on the default dynamical state of recurrent neural networks. The non-synaptic adaption considered acts on intrinsic neural parameters, such as the threshold and the gain, and is driven by the optimization of the information entropy. We observe, in the presence of the intrinsic adaptation processes, three distinct and globally attracting dynamical regimes, a regular synchronized, an overall chaotic and an intermittent bursting regime. The intermittent bursting regime is characterized by intervals of regular flows, which are quite insensitive to external stimuli, interseeded by chaotic bursts which respond sensitively to input signals. We discuss these finding in the context of self-organized information processing and critical brain dynamics.Comment: 24 pages, 8 figure

Spontaneous breaking of the Fermi surface symmetry in the t-J model: a numerical study

We present a variational Monte Carlo (VMC) study of spontaneous Fermi surface symmetry breaking in the t-J model. We find that the variational energy of a Gutzwiller projected Fermi sea is lowered by allowing for a finite asymmetry between the x- and the y-directions. However, the best variational state remains a pure superconducting state with d-wave symmetry, as long as the underlying lattice is isotropic. Our VMC results are in good overall agreement with slave boson mean field theory (SBMFT) and renormalized mean field theory (RMFT), although apparent discrepancies do show up in the half-filled limit, revealing some limitations of mean field theories. VMC and complementary RMFT calculations also confirm the SBMFT predictions that many-body interactions can enhance any anisotropy in the underlying crystal lattice. Thus, our results may be of consequence for the description of strongly correlated superconductors with an anisotropic lattice structure.Comment: 6 pages, 7 figures; final versio

Quantum phases and topological properties of interacting fermions in one-dimensional superlattices

The realization of artificial gauge fields in ultracold atomic gases has opened up a path towards experimental studies of topological insulators and, as an ultimate goal, topological quantum matter in many-body systems. As an alternative to the direct implementation of two-dimensional lattice Hamiltonians that host the quantum Hall effect and its variants, topological charge-pumping experiments provide an additional avenue towards studying many-body systems. Here, we consider an interacting two-component gas of fermions realizing a family of one-dimensional superlattice Hamiltonians with onsite interactions and a unit cell of three sites, whose groundstates would be visited in an appropriately defined charge pump. First, we investigate the grandcanonical quantum phase diagram of individual Hamiltonians, focusing on insulating phases. For a certain commensurate filling, there is a sequence of phase transitions from a band insulator to other insulating phases (related to the physics of ionic Hubbard models) for some members of the manifold of Hamiltonians. Second, we compute the Chern numbers for the whole manifold in a many-body formulation and show that, related to the aforementioned quantum phase transitions, a topological transition results in a change of the value and sign of the Chern number. We provide both an intuitive and conceptual explanation and argue that these properties could be observed in quantum-gas experiments

Control of the finite size corrections in exact diagonalization studies

We study the possibility of controlling the finite size corrections in exact diagonalization studies quantitatively. We consider the one- and two dimensional Hubbard model. We show that the finite-size corrections can be be reduced systematically by a grand-canonical integration over boundary conditions. We find, in general, an improvement of one order of magnitude with respect to studies with periodic boundary conditions only. We present results for ground-state properties of the 2D Hubbard model and an evaluation of the specific heat for the 1D and 2D Hubbard model.Comment: Phys. Rev. B (Brief Report), in pres

Electronic structure of strongly correlated d-wave superconductors

We study the electronic structure of a strongly correlated d-wave superconducting state. Combining a renormalized mean field theory with direct calculation of matrix elements, we obtain explicit analytical results for the nodal Fermi velocity, v_F, the Fermi wave vector, k_F, and the momentum distribution, n_k, as a function of hole doping in a Gutzwiller projected d-wave superconductor. We calculate the energy dispersion, E_k, and spectral weight of the Gutzwiller-Bogoliubov quasiparticles, and find that the spectral weight associated with the quasiparticle excitation at the antinodal point shows a non monotonic behavior as a function of doping. Results are compared to angle resolved photoemission spectroscopy (ARPES) of the high temperature superconductors.Comment: final version, comparison to experiments added, 4+ pages, 4 figure

The J_1-J_2 model revisited : Phenomenology of CuGeO_3

We present a mean field solution of the antiferromagnetic Heisenberg chain with nearest (J_1) and next to nearest neighbor (J_2) interactions. This solution provides a way to estimate the effects of frustration. We calculate the temperature-dependent spin-wave velocity, v_s(T) and discuss the possibility to determine the magnitude of frustration J_2/J_1 present in quasi 1D compounds from measurements of v_s(T). We compute the thermodynamic susceptibility at finite temperatures and compare it with the observed susceptibility of the spin-Peierls compound CuGeO_3. We also use the method to study the two-magnon Raman continuum observed in CuGeO_3 above the spin-Peierls transition.Comment: Phys. Rev.

Die Bedeutung der Hüllschichten der Xenopus-Oozyte für den Effekt der Antiarrhythmika Propafenon, Flecainid und Verapamil auf den klonierten Kaliumkanal Kv2.1

Es war Ziel dieser Arbeit, den Einfluss der Hüllschichten der Xenopus-Oozyte auf die Wirkung der Antiarrhythmika Propafenon, Flecainid und Verapamil zu untersuchen. Zudem sollte geprüft werden, ob Ergebnisse aus Versuchen an Oozyten geeignet sind, Angaben zu therapeutisch wirksamen Substanzkonzentrationen zu erarbeiten. Aus den Ergebnissen der Arbeit lässt sich ableiten, dass 1. die Entfernung der Hüllschichten zu einer Zunahme der Empfindlichkeit gegenüber bestimmten Antiarrhythmika führt, 2. im Vergleich zu anderen Expressionssystemen selbst in Oozyten ohne Hüllen höhere Substanzkonzentrationen nötig sind, um denselben blockierenden Effekt zu erreichen, 3. die therapeutischen Plasmakonzentrationen im Bereich der Werte anderer Expressionssysteme liegen, jedoch von denen der Xenopus-Oozyte abweichen. Die Arbeit zeigt, dass es nicht möglich ist, auf der Basis von Ergebnissen an Xenopus-Oozyten Angaben zu therapeutisch wirksamen Substanzkonzentrationen von Antiarrhythmika zu erarbeiten

Spin-charge separation at small lengthscales in the 2D t-J model

We consider projected wavefunctions for the 2D $t-J$ model. For various wavefunctions, including correlated Fermi-liquid and Luttinger-type wavefunctions we present the static charge-charge and spin-spin structure factors. Comparison with recent results from a high-temperature expansion by Putikka {\it et al.} indicates spin-charge separation at small lengthscales.Comment: REVTEX, 5 pages, 5 figures hardcopies availabl

Variasi Genetik Troides Helena (Lepidoptera: Papilionidae) Berdasarkan Gen COI (Cytochrome C Oxydase I)

Kupu-kupu Troides helena (Kupu-kupu Raja) merupakan salah satu spesies langka yang dilindungi. Eksploitasi yang berlebihan, serta alih fungsi hutan menjadi ancaman bagi kehidupan kupu-kupu ini. Penelitian ini dilakukan untuk mengetahui variasi pada gen cytochrome C oxidase I Troides helena yang diperoleh dari dua lokasi yang berbeda, yaitu Gunung Tumpa dan Gunung Dua-sudara. Analisis sekuens menunjukkan adanya perbedaan satu pasang basa nukleotida dari kedua spesimen tersebut. Selain itu, variasi juga ditunjukan pada sampel yang diperoleh dari basis data GenBank dengan adanya perbedaan 7-8 pasang basa nukleotida dengan spesimen pada penelitian ini. Hasil perhitungan jarak genetik menunjukkan bahwa meskipun secara geografis spesimen-spesimen uji ini berasal dari lokasi yang berjauhan, variasi genetik masih berada dalam kisaran variasi intraspesies.Troides helena (Common Birdwing) is listed as one of endangered and protected butterfly species. Excessive exploitation and forest conversion have become threat to the life of this butterfly. This study was conducted to determine the genetic variation of Troides helena obtained from Mt. Tumpa and Mt. Dua-sudara based on cytochrome C oxidase I gene. Sequence analysis shows one nucleotide difference between these two specimens. Moreover, genetic variation also has been shown by comparing these two specimen with other Troides helena obtained from database in GenBank. There are 7-8 nucleotides differences among tested specimens. The result of genetic distance calculations indicates that although geographically these test specimens derived from remote locations, the genetic variation is still within the range of intraspecific variation

The repeated evolution of stripe patterns is correlated with body morphology in the adaptive radiations of East African cichlid fishes

Abstract Color patterns are often linked to the behavioral and morphological characteristics of an animal, contributing to the effectiveness of such patterns as antipredatory strategies. Species-rich adaptive radiations, such as the freshwater fish family Cichlidae, provide an exciting opportunity to study trait correlations at a macroevolutionary scale. Cichlids are also well known for their diversity and repeated evolution of color patterns and body morphology. To study the evolutionary dynamics between color patterns and body morphology, we used an extensive dataset of 461 species. A phylogenetic supertree of these species shows that stripe patterns evolved ~70 times independently and were lost again ~30 times. Moreover, stripe patterns show strong signs of correlated evolution with body elongation, suggesting that the stripes? effectiveness as antipredatory strategy might differ depending on the body shape. Using pedigree-based analyses, we show that stripes and body elongation segregate independently, indicating that the two traits are not genetically linked. Their correlation in nature is therefore likely maintained by correlational selection. Lastly, by performing a mate preference assay using a striped CRISPR-Cas9 mutant of a nonstriped species, we show that females do not differentiate between striped CRISPR mutant males and nonstriped wild-type males, suggesting that these patterns might be less important for species recognition and mate choice. In summary, our study suggests that the massive rates of repeated evolution of stripe patterns are shaped by correlational selection with body elongation, but not by sexual selection.Peer reviewe