Implementation of elastic-plastic structural analysis into NASTRAN

Elastic-plastic analytic capabilities were incorporated into the NASTRAN program. The present implementation includes a general rigid format and additional bulk data cards as well as to two new modules. The modules are specialized to include only perfect plasticity of the CTRMEN and CROD elements but can easily be expanded to include other plasticity theories and elements. The practical problem of an elastic-plastic analysis of a ship's bracket connection is demonstrated and compared to an equivalent analysis using Grumman's PLANS program. The present work demonstrates the feasibility of incorporating general elastic-plastic capabilities into NASTRAN

Simultaneous electric and magnetic field induced nonvolatile memory

We investigate the electric field induced resistive switching effect and magnetic field induced fraction enlargement on a polycrystalline sample of a colossal magnetoresistive compound displaying intrinsic phase coexistence. Our data show that the electric effect (presumably related to the presence of inhomogeinities) is present in a broad temperature range(300 to 20 K), being observable even in a mostly homogeneous ferromagnetic state. In the temperature range in which low magnetic field determines the phase coexistence fraction, both effects, though related to different mechanisms, are found to determine multilevel nonvolatile memory capabilities simultaneously.Comment: Submited to AP

The electrical current effect in phase separated La5/8-yPryCa3/8MnO3: Charge order melting vs. Joule heating

We have studied the effect of electric field on transport properties of the prototypical phase separated manganite La5/8-yPryCa3/8MnO3 with y=0.34. Our results show that the suggested image in which the charge ordered state is melted by the appliance of an electric current and/or voltage has to be revised. We were able to explain the observed resistivity drop in terms of an artifact related to Joule heating and the particular hysteresis that the system under study display, common to many other phase separated manganites.Comment: 2 figures. Accepted in J. Appl. Phy

Microscopic theory of Cooper pair beam splitters based on carbon nanotubes

We analyze microscopically a Cooper pair splitting device in which a central superconducting lead is connected to two weakly coupled normal leads through a carbon nanotube. We determine the splitting efficiency at resonance in terms of geometrical and material parameters, including the effect of spin-orbit scattering. While the efficiency in the linear regime is limited to 50% and decay exponentially as a function of the width of the superconducting region we show that it can rise up to $\sim 100$ in the non-linear regime for certain regions of the stability diagram.Comment: 5 pages, 5 figure

Transport in superlattices on single layer graphene

We study transport in undoped graphene in the presence of a superlattice potential both within a simple continuum model and using numerical tight-binding calculations. The continuum model demonstrates that the conductivity of the system is primarily impacted by the velocity anisotropy that the Dirac points of graphene develop due to the potential. For one-dimensional superlattice potentials, new Dirac points may be generated, and the resulting conductivities can be approximately described by the anisotropic conductivities associated with each Dirac point. Tight-binding calculations demonstrate that this simple model is quantitatively correct for a single Dirac point, and that it works qualitatively when there are multiple Dirac points. Remarkably, for a two dimensional potential which may be very strong but introduces no anisotropy in the Dirac point, the conductivity of the system remains essentially the same as when no external potential is present.Comment: 8 pages, 7 figures, submitted to Phys. Rev.

Development of technology for modeling of a 1/8-scale dynamic model of the shuttle Solid Rocket Booster (SRB)

A NASTRAN analysis of the solid rocket booster (SRB) substructure of the space shuttle 1/8-scale structural dynamics model. The NASTRAN finite element modeling capability was first used to formulate a model of a cylinder 10 in. radius by a 200 in. length to investigate the accuracy and adequacy of the proposed grid point spacing. Results were compared with a shell analysis and demonstrated relatively accurate results for NASTRAN for the lower modes, which were of primary interest. A finite element model of the full SRB was then formed using CQUAD2 plate elements containing membrane and bending stiffness and CBAR offset bar elements to represent the longerons and frames. Three layers of three-dimensional CHEXAI elements were used to model the propellant. This model, consisting of 4000 degrees of freedom (DOF) initially, was reduced to 176 DOF using Guyan reduction. The model was then submitted for complex Eigenvalue analysis. After experiencing considerable difficulty with attempts to run the complete model, it was split into two substructres. These were run separately and combined into a single 116 degree of freedom A set which was successfully run. Results are reported

Electronic inhomogeneity at magnetic domain walls in strongly-correlated systems

We show that nano-scale variations of the order parameter in strongly-correlated systems can induce local spatial regions such as domain walls that exhibit electronic properties representative of a different, but nearby, part of the phase diagram. This is done by means of a Landau-Ginzburg analysis of a metallic ferromagnetic system near an antiferromagnetic phase boundary. The strong spin gradients at a wall between domains of different spin orientation drive the formation of a new type of domain wall, where the central core is an insulating antiferromagnet, and connects two metallic ferromagnetic domains. We calculate the charge transport properties of this wall, and find that its resistance is large enough to account for recent experimental results in colossal magnetoresistance materials. The technological implications of this finding for switchable magnetic media are discussed.Comment: Version submitted to Physical Review Letters, except for minor revisions to reference

Quantum refrigerator driven by current noise

We proposed a scheme to implement a self-contained quantum refrigerator system composed of three rf-SQUID qubits, or rather, flux-biased phase qubits. The three qubits play the roles of the target, the refrigerator and the heat engine respectively. We provide different effective temperatures for the three qubits, by imposing external current noises of different strengths. The differences of effective temperatures give rise to the flow of free energy and that drives the refrigerator system to cool down the target. We also show that the efficiency of the system approaches the Carnot efficiency.Comment: 5 pages, 1 figur