308,761 research outputs found

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

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

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 LPSUB$m$ Approximation Scheme for Lattice Models

A new approximation hierarchy, called the LPSUB$m$ 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
LSUB$m$ and DSUB$m$ schemes. Each of the three CCM schemes (LSUB$m$, DSUB$m$
and LPSUB$m$) 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-$\frac{1}{2}$ $J_{1}$--$J_{2}$--$J_{1}^{\perp}$ Heisenberg Antiferromagnet on a Bilayer Honeycomb Lattice

We use the coupled cluster method (CCM) to study a frustrated
spin-$\frac{1}{2}$ $J_{1}$--$J_{2}$--$J_{1}^{\perp}$ Heisenberg antiferromagnet
on a bilayer honeycomb lattice with $AA$ stacking. Both nearest-neighbor (NN)
and frustrating next-nearest-neighbor antiferromagnetic (AFM) exchange
interactions are present in each layer, with respective exchange coupling
constants $J_{1}>0$ and $J_{2} \equiv \kappa J_{1} > 0$. The two layers are
coupled with NN AFM exchanges with coupling strength $J_{1}^{\perp}\equiv
\delta J_{1}>0$. We calculate to high orders of approximation within the CCM
the zero-field transverse magnetic susceptibility $\chi$ in the N\'eel phase.
We thus obtain an accurate estimate of the full boundary of the N\'eel phase in
the $\kappa\delta$ plane for the zero-temperature quantum phase diagram. We
demonstrate explicitly that the phase boundary derived from $\chi$ 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-$\frac{1}{2}$ $J_{1}$--$J_{2}$--$J_{1}^{\perp}$ Heisenberg model on an $AA$-stacked bilayer honeycomb lattice

The zero-temperature quantum phase diagram of the spin-$\frac{1}{2}$
$J_{1}$--$J_{2}$--$J_{1}^{\perp}$ model on an $AA$-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 $J_{1} > 0$ and $J_{2} \equiv
\kappa J_{1}>0$. The two layers are coupled via a comparable Heisenberg
exchange interaction between NN interlayer pairs, with a strength
$J_{1}^{\perp} \equiv \delta J_{1}$. 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 $\kappa \delta$ half-plane with $\kappa > 0$.
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 $M$
and the excitation energy $\Delta$ from the $s^{z}_{T}=0$ ground state to the
lowest-lying $|s^{z}_{T}|=1$ excited state (where $s^{z}_{T}$ is the total $z$
component of spin for the system as a whole, and where the collinear ordering
lies along the $z$ 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 ($n \leq 10$) in a systematic series of
approximations involving $n$-spin clusters. The sole approximation made is then
to extrapolate the sequences of $n$th-order results for $M$ and $\Delta$ to the
exact limit, $n \to \infty$

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

We study a frustrated spin-$\frac{1}{2}$
$J_{1}$--$J_{2}$--$J_{3}$--$J_{1}^{\perp}$ Heisenberg antiferromagnet on an
$AA$-stacked bilayer honeycomb lattice. In each layer we consider
nearest-neighbor (NN), next-nearest-neighbor, and next-next-nearest-neighbor
antiferromagnetic (AFM) exchange couplings $J_{1}$, $J_{2}$, and $J_{3}$,
respectively. The two layers are coupled with an AFM NN exchange coupling
$J_{1}^{\perp}\equiv\delta J_{1}$. The model is studied for arbitrary values of
$\delta$ along the line $J_{3}=J_{2}\equiv\alpha J_{1}$ that includes the most
highly frustrated point at $\alpha=\frac{1}{2}$, 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 $\alpha\delta$ plane in the window $0
\leq \alpha \leq 1$, $0 \leq \delta \leq 1$. 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, $\delta = \delta_{c_{i}}(\alpha)$, or
equivalently, $\alpha = \alpha_{c_{i}}(\delta)$, with $i=1$ (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 $J_{1}$--$J_{2}$ Heisenberg antiferromagnet on the honeycomb lattice

We study the zero-temperature quantum phase diagram of a spin-1 Heisenberg
antiferromagnet on the honeycomb lattice with both nearest-neighbor exchange
coupling $J_{1}>0$ and frustrating next-nearest-neighbor coupling $J_{2} \equiv
\kappa J_{1} > 0$, 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 $0 < \kappa < 1$
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 $\kappa < \kappa_{c_{1}} = 0.250(5)$, and another with 4-sublattice
N\'{e}el-II order for $\kappa > \kappa_{c_{2}} = 0.340(5)$. Two different
paramagnetic phases are found to exist in the intermediate region. Over the
range $\kappa_{c_{1}} < \kappa < \kappa^{i}_{c} = 0.305(5)$ 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 $\kappa^{i}_{c} < \kappa <
\kappa_{c_{2}}$ 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-$\frac{1}{2}$ $J_{1}$--$J_{2}$ model on the triangular lattice

We use the coupled cluster method implemented at high orders of approximation
to study the spin-$\frac{1}{2}$ $J_{1}$--$J_{2}$ model on the triangular
lattice with Heisenberg interactions between nearest-neighbour and
next-nearest-neighbour pairs of spins, with coupling strengths $J_{1}>0$ and
$J_{2} \equiv \kappa J_{1} >0$, respectively. In the window $0 \leq \kappa \leq
1$ 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 $\kappa < \kappa^{c}_{1} = 0.060(10)$
and $\kappa > \kappa^{c}_{2} = 0.165(5)$, respectively. The spin-triplet gap is
found to vanish over essentially the entire region $\kappa^{c}_{1} < \kappa <
\kappa^{c}_{2}$ of the intermediate phase

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