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Random field Ising model : statistical properties of low-energy excitations and of equilibrium avalanches

With respect to usual thermal ferromagnetic transitions, the zero-temperature finite-disorder critical point of the Random-field Ising model (RFIM) has the peculiarity to involve some 'droplet' exponent $θ$ that enters the generalized hyperscaling relation $2-α= ν(d-θ)$. In the present paper, to better understand the meaning of this droplet exponent $θ$ beyond its role in the thermodynamics, we discuss the statistics of low-energy excitations generated by an imposed single spin-flip with respect to the ground state, as well as the statistics of equilibrium avalanches i.e. the magnetization jumps that occur in the sequence of ground-states as a function of the external magnetic field. The droplet scaling theory predicts that the distribution $dl/l^{1+θ}$ of the linear-size $l$ of low-energy excitations transforms into the distribution $ds/s^{1+{θ/d_f}}$ for the size $s$ (number of spins) of excitations of fractal dimension $d_f$ ($s \sim l^{d_f}$). In the non-mean-field region $d<d_c$, droplets are compact $d_f=d$, whereas in the mean-field region $d>d_c$, droplets have a fractal dimension $d_f=2 θ$ leading to the well-known mean-field result $ds/s^{3/2}$. Zero-field equilibrium avalanches are expected to display the same distribution $ds/s^{1+{θ/d_f}}$. We also discuss the statistics of equilibrium avalanches integrated over the external field and finite-size behaviors. These expectations are checked numerically for the Dyson hierarchical version of the RFIM, where the droplet exponent $θ(σ)$ can be varied as a function of the effective long-range interaction $J(r) \sim 1/r^{d+σ}$ in $d=1$.