308,761 research outputs found

    Parametric finite-element studies on the effect of tool shape in friction stir welding

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    The success of the Friction Stir Welding (FSW) process, and the weld quality produced, depends significantly on the design of the welding tool. In this paper the effect of variation in various tool geometry parameters on FSW process outcomes, during the plunge stage, were investigated. Specifically the tool shoulder surface angle and the ratio of the shoulder radius to pin radius on tool reaction force, tool torque, heat generation, temperature distribution and size of the weld zone were investigated. The studies were carried out numerically using the finite element method. The welding process used AA2024 aluminium alloy plates with a thickness of 3 mm. It was found that, in plunge stage, the larger the pin radius the higher force and torque the tool experiences and the greater heat generated. It is also found that the shoulder angle has very little effect on energy dissipation as well as little effect on temperature distribution

    Absence of superconductivity in the half-filled band Hubbard model on the anisotropic triangular lattice

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    We report exact calculations of magnetic and superconducting pair-pair correlations for the half-filled band Hubbard model on an anisotropic triangular lattice. Our results for the magnetic phases are similar to those obtained with other techniques. The superconducting pair-pair correlations at distances beyond nearest neighbor decrease monotonically with increasing Hubbard interaction U for all anisotropy, indicating the absence of frustration-driven superconductivity within the model.Comment: 4 pages, 4 EPS figure

    The Coupled Cluster Method Applied to Quantum Magnets: A New LPSUBmm Approximation Scheme for Lattice Models

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    A new approximation hierarchy, called the LPSUBmm scheme, is described for the coupled cluster method (CCM). It is applicable to systems defined on a regular spatial lattice. We then apply it to two well-studied prototypical (spin-1/2 Heisenberg antiferromagnetic) spin-lattice models, namely: the XXZ and the XY models on the square lattice in two dimensions. Results are obtained in each case for the ground-state energy, the ground-state sublattice magnetization and the quantum critical point. They are all in good agreement with those from such alternative methods as spin-wave theory, series expansions, quantum Monte Carlo methods and the CCM using the alternative LSUBmm and DSUBmm schemes. Each of the three CCM schemes (LSUBmm, DSUBmm and LPSUBmm) for use with systems defined on a regular spatial lattice is shown to have its own advantages in particular applications

    Transverse Magnetic Susceptibility of a Frustrated Spin-12\frac{1}{2} J1J_{1}--J2J_{2}--J1J_{1}^{\perp} Heisenberg Antiferromagnet on a Bilayer Honeycomb Lattice

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    We use the coupled cluster method (CCM) to study a frustrated spin-12\frac{1}{2} J1J_{1}--J2J_{2}--J1J_{1}^{\perp} Heisenberg antiferromagnet on a bilayer honeycomb lattice with AAAA stacking. Both nearest-neighbor (NN) and frustrating next-nearest-neighbor antiferromagnetic (AFM) exchange interactions are present in each layer, with respective exchange coupling constants J1>0J_{1}>0 and J2κJ1>0J_{2} \equiv \kappa J_{1} > 0. The two layers are coupled with NN AFM exchanges with coupling strength J1δJ1>0J_{1}^{\perp}\equiv \delta J_{1}>0. We calculate to high orders of approximation within the CCM the zero-field transverse magnetic susceptibility χ\chi in the N\'eel phase. We thus obtain an accurate estimate of the full boundary of the N\'eel phase in the κδ\kappa\delta plane for the zero-temperature quantum phase diagram. We demonstrate explicitly that the phase boundary derived from χ\chi is fully consistent with that obtained from the vanishing of the N\'eel magnetic order parameter. We thus conclude that at all points along the N\'eel phase boundary quasiclassical magnetic order gives way to a nonclassical paramagnetic phase with a nonzero energy gap. The N\'eel phase boundary exhibits a marked reentrant behavior, which we discuss in detail

    Collinear antiferromagnetic phases of a frustrated spin-12\frac{1}{2} J1J_{1}--J2J_{2}--J1J_{1}^{\perp} Heisenberg model on an AAAA-stacked bilayer honeycomb lattice

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    The zero-temperature quantum phase diagram of the spin-12\frac{1}{2} J1J_{1}--J2J_{2}--J1J_{1}^{\perp} model on an AAAA-stacked bilayer honeycomb lattice is investigated using the coupled cluster method (CCM). The model comprises two monolayers in each of which the spins, residing on honeycomb-lattice sites, interact via both nearest-neighbor (NN) and frustrating next-nearest-neighbor isotropic antiferromagnetic (AFM) Heisenberg exchange iteractions, with respective strengths J1>0J_{1} > 0 and J2κJ1>0J_{2} \equiv \kappa J_{1}>0. The two layers are coupled via a comparable Heisenberg exchange interaction between NN interlayer pairs, with a strength J1δJ1J_{1}^{\perp} \equiv \delta J_{1}. The complete phase boundaries of two quasiclassical collinear AFM phases, namely the N\'{e}el and N\'{e}el-II phases, are calculated in the κδ\kappa \delta half-plane with κ>0\kappa > 0. Whereas on each monolayer in the N\'{e}el state all NN pairs of spins are antiparallel, in the N\'{e}el-II state NN pairs of spins on zigzag chains along one of the three equivalent honeycomb-lattice directions are antiparallel, while NN interchain spins are parallel. We calculate directly in the thermodynamic (infinite-lattice) limit both the magnetic order parameter MM and the excitation energy Δ\Delta from the sTz=0s^{z}_{T}=0 ground state to the lowest-lying sTz=1|s^{z}_{T}|=1 excited state (where sTzs^{z}_{T} is the total zz component of spin for the system as a whole, and where the collinear ordering lies along the zz direction) for both quasiclassical states used (separately) as the CCM model state, on top of which the multispin quantum correlations are then calculated to high orders (n10n \leq 10) in a systematic series of approximations involving nn-spin clusters. The sole approximation made is then to extrapolate the sequences of nnth-order results for MM and Δ\Delta to the exact limit, nn \to \infty

    A high-order study of the quantum critical behavior of a frustrated spin-12\frac{1}{2} antiferromagnet on a stacked honeycomb bilayer

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    We study a frustrated spin-12\frac{1}{2} J1J_{1}--J2J_{2}--J3J_{3}--J1J_{1}^{\perp} Heisenberg antiferromagnet on an AAAA-stacked bilayer honeycomb lattice. In each layer we consider nearest-neighbor (NN), next-nearest-neighbor, and next-next-nearest-neighbor antiferromagnetic (AFM) exchange couplings J1J_{1}, J2J_{2}, and J3J_{3}, respectively. The two layers are coupled with an AFM NN exchange coupling J1δJ1J_{1}^{\perp}\equiv\delta J_{1}. The model is studied for arbitrary values of δ\delta along the line J3=J2αJ1J_{3}=J_{2}\equiv\alpha J_{1} that includes the most highly frustrated point at α=12\alpha=\frac{1}{2}, where the classical ground state is macroscopically degenerate. The coupled cluster method is used at high orders of approximation to calculate the magnetic order parameter and the triplet spin gap. We are thereby able to give an accurate description of the quantum phase diagram of the model in the αδ\alpha\delta plane in the window 0α10 \leq \alpha \leq 1, 0δ10 \leq \delta \leq 1. This includes two AFM phases with N\'eel and striped order, and an intermediate gapped paramagnetic phase that exhibits various forms of valence-bond crystalline order. We obtain accurate estimations of the two phase boundaries, δ=δci(α)\delta = \delta_{c_{i}}(\alpha), or equivalently, α=αci(δ)\alpha = \alpha_{c_{i}}(\delta), with i=1i=1 (N\'eel) and 2 (striped). The two boundaries exhibit an "avoided crossing" behavior with both curves being reentrant

    Ground-state phases of the spin-1 J1J_{1}--J2J_{2} Heisenberg antiferromagnet on the honeycomb lattice

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    We study the zero-temperature quantum phase diagram of a spin-1 Heisenberg antiferromagnet on the honeycomb lattice with both nearest-neighbor exchange coupling J1>0J_{1}>0 and frustrating next-nearest-neighbor coupling J2κJ1>0J_{2} \equiv \kappa J_{1} > 0, using the coupled cluster method implemented to high orders of approximation, and based on model states with different forms of classical magnetic order. For each we calculate directly in the bulk thermodynamic limit both ground-state low-energy parameters (including the energy per spin, magnetic order parameter, spin stiffness coefficient, and zero-field uniform transverse magnetic susceptibility) and their generalized susceptibilities to various forms of valence-bond crystalline (VBC) order, as well as the energy gap to the lowest-lying spin-triplet excitation. In the range 0<κ<10 < \kappa < 1 we find evidence for four distinct phases. Two of these are quasiclassical phases with antiferromagnetic long-range order, one with 2-sublattice N\'{e}el order for κ<κc1=0.250(5)\kappa < \kappa_{c_{1}} = 0.250(5), and another with 4-sublattice N\'{e}el-II order for κ>κc2=0.340(5)\kappa > \kappa_{c_{2}} = 0.340(5). Two different paramagnetic phases are found to exist in the intermediate region. Over the range κc1<κ<κci=0.305(5)\kappa_{c_{1}} < \kappa < \kappa^{i}_{c} = 0.305(5) we find a gapless phase with no discernible magnetic order, which is a strong candidate for being a quantum spin liquid, while over the range κci<κ<κc2\kappa^{i}_{c} < \kappa < \kappa_{c_{2}} we find a gapped phase, which is most likely a lattice nematic with staggered dimer VBC order that breaks the lattice rotational symmetry

    Spin-gap study of the spin-12\frac{1}{2} J1J_{1}--J2J_{2} model on the triangular lattice

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    We use the coupled cluster method implemented at high orders of approximation to study the spin-12\frac{1}{2} J1J_{1}--J2J_{2} model on the triangular lattice with Heisenberg interactions between nearest-neighbour and next-nearest-neighbour pairs of spins, with coupling strengths J1>0J_{1}>0 and J2κJ1>0J_{2} \equiv \kappa J_{1} >0, respectively. In the window 0κ10 \leq \kappa \leq 1 we find that the 3-sublattice 120^{\circ} N\'{e}el-ordered and 2-sublattice 180^{\circ} stripe-ordered antiferromagnetic states form the stable ground-state phases in the regions κ<κ1c=0.060(10)\kappa < \kappa^{c}_{1} = 0.060(10) and κ>κ2c=0.165(5)\kappa > \kappa^{c}_{2} = 0.165(5), respectively. The spin-triplet gap is found to vanish over essentially the entire region κ1c<κ<κ2c\kappa^{c}_{1} < \kappa < \kappa^{c}_{2} of the intermediate phase
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