Electronic Structures of Antiperovskite Superconductor MgCNi3_3 and Related Compounds

Electronic structure of a newly discovered antiperovskite superconductor MgCNi$_3$ is investigated by using the LMTO band method. The main contribution to the density of states (DOS) at the Fermi energy $E_{\rm F}$ comes from Ni 3$d$ states which are hybridized with C 2$p$ states. The DOS at $E_{\rm F}$ is varied substantially by the hole or electron doping due to the very high and narrow DOS peak located just below $E_{\rm F}$. We have also explored electronic structures of C-site and Mg-site doped MgCNi$_3$ systems, and described the superconductivity in terms of the conventional phonon mechanism.Comment: 3 pages, presented at ORBITAL2001 September 11-14, 2001 (Sendai, JAPAN

Electronic structures of antiperovskite superconductors: MgXNi3_3 (X=B,C,N)

We have investigated electronic structures of a newly discovered antiperovskite superconductor MgCNi$_3$ and related compounds MgBNi$_3$ and MgNNi$_3$. In MgCNi$_3$, a peak of very narrow and high density of states is located just below $\rm E_F$, which corresponds to the $\pi^*$ antibonding state of Ni-3d and C-$2p$ but with the predominant Ni-3d character. The prominent nesting feature is observed in the $\Gamma$-centered electron Fermi surface of an octahedron-cage-like shape that originates from the 19th band. The estimated superconducting parameters based on the simple rigid-ion approximation are in reasonable agreement with experiment, suggesting that the superconductivity in MgCNi$_3$ is described well by the conventional phonon mechanism.Comment: 5 pages, 5 figure

Electronic structure of metallic antiperovskite compound GaCMn3_3

We have investigated electronic structures of antiperovskite GaCMn$_3$ and related Mn compounds SnCMn$_3$, ZnCMn$_3$, and ZnNMn$_3$. In the paramagnetic state of GaCMn$_3$, the Fermi surface nesting feature along the $\Gamma{\rm R}$ direction is observed, which induces the antiferromagnetic (AFM) spin ordering with the nesting vector {\bf Q} $\sim \Gamma{\rm R}$. Calculated susceptibilities confirm the nesting scenario for GaCMn$_3$ and also explain various magnetic structures of other antiperovskite compounds. Through the band folding effect, the AFM phase of GaCMn$_3$ is stabilized. Nearly equal densities of states at the Fermi level in the ferromagnetic and AFM phases of GaCMn$_3$ indicate that two phases are competing in the ground state.Comment: 4 pages, 5 figure

Modelling the Localized to Itinerant Electronic Transition in the Heavy Fermion System CeIrIn5

We address the fundamental question of crossover from localized to itinerant state of a paradigmatic heavy fermionmaterial CeIrIn5. The temperature evolution of the one electron spectra and the optical conductivity is predicted from first principles calculation. The buildup of coherence in the form of a dispersive many body feature is followed in detail and its effects on the conduction electrons and optical conductivity of the material is revealed. We find multiple hybridization gaps and link them to the crystal structure of the material. Our theoretical approach explains the multiple peak structures observed in optical experiments and the sensitivity of CeIrIn5 to substitutions of the transition metal element and may provide a microscopic basis for the more phenomenological descriptions currently used to interpret experiments in heavy fermion systems.Comment: 12 pages, 3 figure

Chaotic exploration and learning of locomotion behaviours

We present a general and fully dynamic neural system, which exploits intrinsic chaotic dynamics, for the real-time goal-directed exploration and learning of the possible locomotion patterns of an articulated robot of an arbitrary morphology in an unknown environment. The controller is modeled as a network of neural oscillators that are initially coupled only through physical embodiment, and goal-directed exploration of coordinated motor patterns is achieved by chaotic search using adaptive bifurcation. The phase space of the indirectly coupled neural-body-environment system contains multiple transient or permanent self-organized dynamics, each of which is a candidate for a locomotion behavior. The adaptive bifurcation enables the system orbit to wander through various phase-coordinated states, using its intrinsic chaotic dynamics as a driving force, and stabilizes on to one of the states matching the given goal criteria. In order to improve the sustainability of useful transient patterns, sensory homeostasis has been introduced, which results in an increased diversity of motor outputs, thus achieving multiscale exploration. A rhythmic pattern discovered by this process is memorized and sustained by changing the wiring between initially disconnected oscillators using an adaptive synchronization method. Our results show that the novel neurorobotic system is able to create and learn multiple locomotion behaviors for a wide range of body configurations and physical environments and can readapt in realtime after sustaining damage