Chiral mixed phase in disordered 3d Heisenberg models

Using Monte Carlo simulations, we compute the spin stiffness of a site-random 3d Heisenberg model with competing ferromagnetic and antiferromagnetic interactions. Our results for the pure limit yield values of the the critical temperature and the critical exponent $\nu$ in excellent agreement with previous high precision studies. In the disordered case, a mixed "chiral" phase is found which may be in the same universality class as 3d Heisenberg spin glasses.Comment: 5 pages, 4 figures, accepted in PRB Rapid Communication

Two spin liquid phases in the spatially anisotropic triangular Heisenberg model

The quantum spin-1/2 antiferromagnetic Heisenberg model on a two dimensional triangular lattice geometry with spatial anisotropy is relevant to describe materials like ${\rm Cs_2 Cu Cl_4}$ and organic compounds like {$\kappa$-(ET)$_2$Cu$_2$(CN)$_3$}. The strength of the spatial anisotropy can increase quantum fluctuations and can destabilize the magnetically ordered state leading to non conventional spin liquid phases. In order to understand these intriguing phenomena, quantum Monte Carlo methods are used to study this model system as a function of the anisotropic strength, represented by the ratio $J'/J$ between the intra-chain nearest neighbor coupling $J$ and the inter-chain one $J'$. We have found evidence of two spin liquid regions. The first one is stable for small values of the coupling J'/J \alt 0.65, and appears gapless and fractionalized, whereas the second one is a more conventional spin liquid with a small spin gap and is energetically favored in the region 0.65\alt J'/J \alt 0.8. We have also shown that in both spin liquid phases there is no evidence of broken translation symmetry with dimer or spin-Peirls order or any broken spatial reflection symmetry of the lattice. The various phases are in good agreement with the experimental findings, thus supporting the existence of spin liquid phases in two dimensional quantum spin-1/2 systems.Comment: 35 pages, 24 figures, 3 table

Organizational learning and emotion: constructing collective meaning in support of strategic themes

Missing in the organizational learning literature is an integrative framework that reflects the emotional as well as the cognitive dynamics involved. Here, we take a step in this direction by focusing in depth over time (five years) on a selected organization which manufactures electronic equipment for the office industry. Drawing on personal construct theory, we define organizational learning as the collective re-construal of meaning in the direction of strategically significant themes. We suggest that emotions arise as members reflect on progress or lack of progress in achieving organizational learning. Our evidence suggests that invalidation – where organizational learning fails to correspond with expectations – gives rise to anxiety and frustration, while validation – where organizational learning is aligned with or exceeds expectations – evokes comfort or excitement. Our work aims to capture the key emotions involved as organizational learning proceeds

Phase Transition in the Two-Dimensional Gauge Glass

The two-dimensional XY gauge glass, which describes disordered superconducting grains in strong magnetic fields, is investigated, with regard to the possibility of a glass transition. We compute the glass susceptibility and the correlation function of the system via extensive numerical simulations and perform the finite-size scaling analysis. This gives strong evidence for a finite-temperature transition, which is expected to be of a novel type.Comment: 5pages, 3 figures, revtex, to appear in Phys. Rev.

Development and Evaluation of TiAl Sheet Structures for Hypersonic Applications

A cooperative program between the National Aeronautics and Space Administration (NASA), the Austrian Space Agency (ASA), Pratt & Whitney, Engineering Evaluation and Design, and Plansee AG was undertaken to determine the feasibility of achieving significant weight reduction of hypersonic propulsion system structures through the utilization of TiAl. A trade study defined the weight reduction potential of TiAl technologies as 25 to 35 percent compared to the baseline Ni-base superalloy for a stiffener structure in an inlet, combustor, and nozzle section of a hypersonic scramjet engine (ref. 1). A scramjet engine inlet cowl flap was designed, along with a representative subelement, using design practices unique to TiAl. A sub-element was fabricated and tested to assess fabricability and structural performance and validate the design system. The TiAl alloy selected was Plansee's third generation alloy Gamma Met PX (Plansee AG ), a high temperature, high strength gamma-TiAl alloy with high Nb content (refs. 2 and 3). Characterization of Gamma Met PX sheet, including tensile, creep, and fatigue testing was performed. Additionally, design-specific coupons were fabricated and tested in order to improve subelement test predictions. Based on the sheet characterization and results of the coupon tests, the subelement failure location and failure load were accurately predicted

Numerical studies of the 2 and 3D gauge glass at low temperature

We report results from Monte Carlo simulations of the two- and three-dimensional gauge glass at low temperature using parallel tempering Monte Carlo. In two dimensions, we find strong evidence for a zero-temperature transition. By means of finite-size scaling, we determine the stiffness exponent theta = -0.39 +/- 0.03. In three dimensions, where a finite-temperature transition is well established, we find theta = 0.27 +/- 0.01, compatible with recent results from domain-wall renormalization group studies.Comment: 3 pages, 3 figures. Proceedings of "2002 MMM Conference", Tampa, F

Bosonic Mean Field Theory of the Spiral Phases of Heisenberg Antiferromagnets on a Chain

We develop a novel bosonic mean field theory to describe the spiral phases of a Heisenberg antiferromagnet on a one-dimensional chain, in terms of three bosons at each site. The ground state is disordered and for large values of the spin $S$, two different and exponentially small energy gaps are found. The spin-spin correlation function is computed and is shown to decay exponentially at large distances. Our mean field theory is also shown to be exact in a large-$N$ generalization.Comment: plain.tex, TIFR/TH/92-66, IISc/CTS/92-1

Spin Glass Ordering in Diluted Magnetic Semiconductors: a Monte Carlo Study

We study the temperature-dilution phase diagram of a site-diluted Heisenberg antiferromagnet on a fcc lattice, with and without the Dzyaloshinskii-Moriya anisotropic term, fixed to realistic microscopic parameters for $IIB_{1-x} Mn_x Te$ (IIB=Cd, Hg, Zn). We show that the dipolar Dzyaloshinskii-Moriya anisotropy induces a finite-temperature phase transition to a spin glass phase, at dilutions larger than 80%. The resulting probability distribution of the order parameter P(q) is similar to the one found in the cubic lattice Edwards-Anderson Ising model. The critical exponents undergo large finite size corrections, but tend to values similar to the ones of the Edwards-Anderson-Ising model.Comment: 4 pages plus 3 postscript figure

Systematic 1/S study of the 2D Hubbard model at half-filling

The 2D Hubbard model is extended by placing 2S orbitals at each lattice site and studied in a systematic 1/S expansion. The 1/S results for the magnetic susceptibility and the spectra of spin-wave excitations at half-filling are consistent with the large S calculations for the Heisenberg antiferromagnet. The 1/S corrections to the fermionic spectrum lift the degeneracy along the edge of the magnetic Brillouin zone yielding minima at (+- pi/2, +- pi/2). Relation to previous papers on the subject is discussed.Comment: 18 pages, emTex version 3.

Modified Spin Wave Thoery of the Bilayer Square Lattice Frustrated Quantum Heisenberg Antiferromagnet

The ground state of the square lattice bilayer quantum antiferromagnet with nearest and next-nearest neighbour intralayer interaction is studied by means of the modified spin wave method. For weak interlayer coupling, the ground state is found to be always magnetically ordered while the quantum disordered phase appear for large enough interlayer coupling. The properties of the disordered phase vary according to the strength of the frustration. In the regime of weak frustration, the disordered ground state is an almost uncorrelated assembly of interlayer dimers, while in the strongly frustrated regime the quantum spin liquid phase which has considerable N\'eel type short range order appears. The behavior of the sublattice magnetization and spin-spin correlation length in each phase is discussed.Comment: 15 pages, revtex, figures upon reques