One-dimensional Ising spin-glass with power-law interaction : real-space renormalization at zero temperature

For the one-dimensional long-ranged Ising spin-glass with random couplings decaying with the distance $r$ as $J(r) \sim r^{-\sigma}$ and distributed with the L\'evy symmetric stable distribution of index $1 <\mu \leq 2$ (including the usual Gaussian case $\mu=2$), we consider the region $\sigma>1/\mu$ where the energy is extensive. We study two real space renormalization procedures at zero temperature, namely a simple box decimation that leads to explicit calculations, and a strong disorder decimation that can be studied numerically on large sizes. The droplet exponent governing the scaling of the renormalized couplings $J_L \propto L^{\theta_{\mu}(\sigma)}$ is found to be $\theta_{\mu}(\sigma)=\frac{2}{\mu}-\sigma$ whenever the long-ranged couplings are relevant $\theta_{\mu}(\sigma)=\frac{2}{\mu}-\sigma \geq -1$. For the statistics of the ground state energy $E_L^{GS}$ over disordered samples, we obtain that the droplet exponent $\theta_{\mu}(\sigma)$ governs the leading correction to extensivity of the averaged value $\overline{E_L^{GS}} \simeq L e_0 +L^{\theta_{\mu}(\sigma)} e_1$. The characteristic scale of the fluctuations around this average is of order $L^{\frac{1}{\mu}}$, and the rescaled variable $u=(E_L^{GS}-\overline{E_L^{GS}})/L^{\frac{1}{\mu}}$ is Gaussian distributed for $\mu=2$, or displays the negative power-law tail in $1/(-u)^{1+\mu}$ for $u \to -\infty$ in the L\'evy case $1<\mu<2$.