Underscreened Kondo Necklace

It has been suggested recently by Gan, Coleman, and Andrei that studying the underscreened Kondo problem may help to understand the nature of magnetism in heavy fermion systems. Motivated by Doniach's work on the S=1/2 Kondo necklace, we introduce the underscreened Kondo necklace models with S>1/2. The underscreened Kondo necklace is the simplest lattice model on which the competition between Kondo spin compensation, and magnetic ordering due to an RKKY-type interaction can be examined. We used the mean-field approximation to determine the phase diagram, and found that the low-temperature phase is always an x-y antiferromagnet. This contention is further supported by the derivation of the exact form of the effective hamiltonian in the limit of very large Kondo coupling: it is found to be an antiferromagnetic x-y model for the residual (S-1/2)-spins. In general, the degree of moment compensation depends on both the Kondo coupling, and on S.Comment: 15 pages (2 figures upon request from [email protected]), LATEX, to appear in Modern Physics Letters

A microscopic model for spiral ordering along (110) on the MnSi lattice

We study an extended Heisenberg model on the MnSi lattice. In the cubic B20 crystal structure of MnSi, Mn atoms form lattices of of corner-shared equilateral triangles. We find an ubiquitous spiral ordering along (110) for J1 0, where J1, J2, and J3 are 1st, 2nd and 3rd nearest neighbor Heisenberg interactions, respectively. While the ordering direction of (110) is reasonably robust to the presence of the Dzyaloshinskii-Moriya interaction, it can be shifted to the (111) direction with the introduction of a magnetic anisotropy term for small J2/|J1|. We discuss the possible relevance of these results to the partially ordered state recently reported in MnSi.Comment: 5 pages, 4 figure

Theory of magnetism and metal-insulator transition in layered perovskite iridates

We investigate the metal-insulator transition in the layered Ruddelsden Popper series of strontium iridates Srn+1IrnO3n+1. Tight-binding models of t2g orbitals for n = 1, 2, and infinity are constructed, and changes in band dispersion due to dimensionality and spin-orbit coupling are presented. Identifying the states near the Fermi level to be predominantly Jeff = 1/2, we use an effective Hubbard model to study the effect of correlations. Transitions from a metallic state to various magnetically ordered states at different critical interactions are obtained. A canted antiferromagnetic insulator is found for Sr2IrO4, a c-axis collinear antiferromagnetic insulator for Sr3Ir2O7, and non-coplanar canted antiferromagnetic insulator via magnetic metal for SrIrO3. We derive the strong-coupling spin-model and compare the magnetic ordering patterns obtained in the weak and strong coupling limits. We find that they are identical, indicating that magnetic ordering is not sufficient to justify Mott physics in this series of iridates.Comment: 11 pages, 18 figure

Quantitative Description of V2O3V_2O_3 by the Hubbard Model in Infinite Dimensions

We show that the analytic single-particle density of states and the optical conductivity for the half-filled Hubbard model on the Bethe lattice in infinite dimensions describe quantitatively the behavior of the gap and the kinetic energy ratio of the correlated insulator $V_2O_3$. The form of the optical conductivity shows $\omega^{3/2}$ rising and is quite similar to the experimental data, and the density of states shows $\omega^{1/2}$ behavior near the band edges.Comment: 9 pages, revtex, 4 figures upon reques

Nematicity as a route to a magnetic field-induced spin density wave order; application to the high temperature cuprates

The electronic nematic order characterized by broken rotational symmetry has been suggested to play an important role in the phase diagram of the high temperature cuprates. We study the interplay between the electronic nematic order and a spin density wave order in the presence of a magnetic field. We show that a cooperation of the nematicity and the magnetic field induces a finite coupling between the spin density wave and spin-triplet staggered flux orders. As a consequence of such a coupling, the magnon gap decreases as the magnetic field increases, and it eventually condenses beyond a critical magnetic field leading to a field-induced spin density wave order. Both commensurate and incommensurate orders are studied, and the experimental implications of our findings are discussed.Comment: 5 pages, 3 figure

Similarity and Practice Schedules: Contextual Interference Variables in Speech Production

Purpose: This study evaluated the influence of phonemic similarity as a variable that facilitates contextual interference (CI), a motor learning phenomenon where poor performance during training results in enhanced performance in transfer conditions. A CI effect was hypothesized: speech performance would be enhanced for nonwords with similar phonemes during the retention phase of motor learning, but only enhanced by nonwords with dissimilar phonemes in the transfer phase.;Method: Twenty-nine young adults with typical speech and hearing participated in a motor-learning study comprised of nonword repetition training followed by an immediate retention and transfer task for nonwords with similar and dissimilar phonemes. Training was counterbalanced by stimuli and participants using a within-subject repeated-measures design. Percent consonants correct was calculated to examine the effects of the different stimuli on stage of skill acquisition.;Results: A CI effect was observed in this study using nonwords that varied in phonemic similarity. Participants accuracy was greatest when producing nonwords with similar phonemes during the retention task; however, during the transfer task, accuracy was greatest when producing nonwords with dissimilar phonemes.;Conclusions: The proposed hypothesis for this study was met: practicing nonwords with dissimilar phonemes lead to greater accuracy in the transfer phase of this experiment. Results indicate phonemic dissimilarity produces a contextual interference and influencing speech motor learning. Future research should determine if these results generalize to other populations, including children with typically developing language and speech skills