Matter Modeling Asked on November 15, 2021

This question is related to other one here in the MatterModelingSE:

Here *J* is the exchange interaction parameters between two nearest-neighbor spins using in Heisenberg and Ising hamiltonians to study magnetic systems:

begin{equation}

tag{Heisenberg}

hat{H}_H=-sum_{langle i jrangle}Jhat{S}_ihat{S}_j

end{equation}

begin{equation}

tag{Ising}

hat{H}_I=-sum_{langle ijrangle}Jhat{S}_i^zhat{S}_j^z

end{equation}

**Is there an experimental (and measurable) observable that can be related to the J parameter?**

Here I'll assume that the material is *already* believed to be roughly described by a model of the Heisenberg or Ising form. In that case, you just want a quantity that is easy to measure in your experiment and easy to extract from numerics (or with a well-established theoretical value).

In practice, the choice of which quantity to use will depend on convenience. If you are already set up to measure the temperature dependence of specific heat, you might do that. You might also measure $T_c$, or the magnetic field required to achieve total saturation. The choice will also be influenced by the parameter regime (temperature, field, pressure) in which the comparison to the theoretical model is the most relevant.

Here are a couple examples:

- In this neutron scattering study, they determined $J$ by comparing the magnon excitation dispersion (measured with neutron scattering) to the theoretical predictions. Dally
*et al.*Phys. Rev. Lett.**124,**197203 (2020) - In this field-polarized study, they used the magnetic susceptibility fit to the (previously documented) Bonner-Fisher result. Blanc, Trinh
*et al.*Nat. Phys.**14,**273 (2018)

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