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9. Disordered system with KPM

This example uses the kernel polynomial method (KPM) to efficiently calculate the neutron scattering spectrum of a disordered triangular antiferromagnet. The model is inspired by YbMgGaO4, as studied in Paddison et al, Nature Phys., 13, 117–122 (2017) and Zhu et al, Phys. Rev. Lett. 119, 157201 (2017). Disordered occupancy of non-magnetic Mg/Ga sites can be modeled as a stochastic distribution of exchange constants and $g$-factors. Including this disorder introduces broadening of the spin wave spectrum.

using Sunny, GLMakie

Set up minimal triangular lattice system. Include antiferromagnetic exchange interactions between nearest neighbor bonds. Energy minimization yields the magnetic ground state with 120° angles between spins in triangular plaquettes.

latvecs = lattice_vectors(1, 1, 10, 90, 90, 120)
cryst = Crystal(latvecs, [[0, 0, 0]])
sys = System(cryst, [1 => Moment(s=1/2, g=1)], :dipole; dims=(3, 3, 1))
set_exchange!(sys, +1.0, Bond(1, 1, [1,0,0]))

randomize_spins!(sys)
minimize_energy!(sys)
plot_spins(sys; color=[S[3] for S in sys.dipoles], ndims=2)
Example block output

Select a $𝐪$-space path for the spin wave calculations.

qs = [[0, 0, 0], [1/3, 1/3, 0], [1/2, 0, 0], [0, 0, 0]]
labels = ["Γ", "K", "M", "Γ"]
path = q_space_path(cryst, qs, 150; labels)
kernel = lorentzian(fwhm=0.4);

Perform a traditional spin wave calculation. The spectrum shows sharp modes associated with coherent excitations about the K-point ordering wavevector, $𝐪 = [1/3, 1/3, 0]$.

energies = range(0.0, 3.0, 150)
swt = SpinWaveTheory(sys; measure=ssf_perp(sys))
res = intensities(swt, path; energies, kernel)
plot_intensities(res)
Example block output

Use repeat_periodically to enlarge the system by a factor of 10 in each dimension. Use to_inhomogeneous to disable symmetry constraints, and allow for the addition of disordered interactions.

sys_inhom = to_inhomogeneous(repeat_periodically(sys, (10, 10, 1)))
System [Dipole mode]
Supercell (30×30×1)×1
Energy per site -3/8

Use symmetry_equivalent_bonds to iterate over all nearest neighbor bonds of the inhomogeneous system. Modify each AFM exchange with a noise term that has variance of 1/3. The newly minimized energy configuration allows for long wavelength modulations on top of the original 120° order.

for (site1, site2, offset) in symmetry_equivalent_bonds(sys_inhom, Bond(1,1,[1,0,0]))
    noise = randn()/3
    set_exchange_at!(sys_inhom, 1.0 + noise, site1, site2; offset)
end

minimize_energy!(sys_inhom, maxiters=5_000)
plot_spins(sys_inhom; color=[S[3] for S in sys_inhom.dipoles], ndims=2)
Example block output

Traditional spin wave theory calculations become impractical for large system sizes. Significant acceleration is possible with the kernel polynomial method. Enable it by selecting SpinWaveTheoryKPM in place of the traditional SpinWaveTheory. Using KPM, the cost of an intensities calculation becomes linear in system size and scales inversely with the width of the line broadening kernel. Error tolerance is controlled through the dimensionless tol parameter. A relatively small value, tol = 0.01, helps to resolve the large intensities near the ordering wavevector. The alternative choice tol = 0.1 would be twice faster, but would introduce significant numerical artifacts.

Observe from the KPM calculation that disorder in the nearest-neighbor exchange serves to broaden the discrete excitation bands into a continuum.

swt = SpinWaveTheoryKPM(sys_inhom; measure=ssf_perp(sys_inhom), tol=0.01)
res = intensities(swt, path; energies, kernel)
plot_intensities(res)
Example block output

Now apply a magnetic field of magnitude 7.5 (energy units) along the global $ẑ$ axis. This field fully polarizes the spins. Because gap opens, a larger tolerance of tol = 0.1 can be used to accelerate the KPM calculation without sacrificing much accuracy. The resulting spin wave spectrum shows a sharp mode at the Γ-point (zone center) that broadens into a continuum along the K and M points (zone boundary).

set_field!(sys_inhom, [0, 0, 7.5])
randomize_spins!(sys_inhom)
minimize_energy!(sys_inhom)

energies = range(0.0, 9.0, 150)
swt = SpinWaveTheoryKPM(sys_inhom; measure=ssf_perp(sys_inhom), tol=0.1)
res = intensities(swt, path; energies, kernel)
plot_intensities(res)
Example block output

Add disorder to the $z$-component of each magnetic moment $g$-tensor. This further broadens intensities, now across the entire path. Some intensity modulation within the continuum is also apparent. This modulation is a finite-size effect, and would be mitigated by enlarging the system beyond 30×30 chemical cells.

for site in eachsite(sys_inhom)
    noise = randn()/6
    sys_inhom.gs[site] = [1 0 0; 0 1 0; 0 0 1+noise]
end

swt = SpinWaveTheoryKPM(sys_inhom; measure=ssf_perp(sys_inhom), tol=0.1)
res = intensities(swt, path; energies, kernel)
plot_intensities(res)
Example block output

For reference, the equivalent non-disordered system shows a single coherent mode.

set_field!(sys, [0, 0, 7.5])
randomize_spins!(sys)
minimize_energy!(sys)
swt = SpinWaveTheory(sys; measure=ssf_perp(sys))
res = intensities(swt, path; energies, kernel)
plot_intensities(res)
Example block output