We study the low-temperature
$(2+1)$D solid-on-solid model on
with zero boundary conditions and nonnegative heights (a floor at height
$0$). Caputo et al. (2016) established that this random surface typically admits either
$\mathfrak h $ or
$\mathfrak h+1$ many nested macroscopic level line loops
$\{\mathcal L_i\}_{i\geq 0}$ for an explicit
$\mathfrak h\asymp \log L$, and its top loop
$\mathcal L_0$ has cube-root fluctuations: For example, if
$\rho (x)$ is the vertical displacement of
$\mathcal L_0$ from the bottom boundary point
$(x,0)$, then
$\max \rho (x) = L^{1/3+o(1)}$ over
. It is believed that rescaling
$\rho $ by
$L^{1/3}$ and
$I_0$ by
$L^{2/3}$ would yield a limit law of a diffusion on
$[-1,1]$. However, no nontrivial lower bound was known on
$\rho (x)$ for a fixed
$x\in I_0$ (e.g.,
$x=\frac L2$), let alone on
$\min \rho (x)$ in
$I_0$, to complement the bound on
$\max \rho (x)$. Here, we show a lower bound of the predicted order
$L^{1/3}$: For every
$\epsilon>0$, there exists
$\delta>0$ such that
$\min _{x\in I_0} \rho (x) \geq \delta L^{1/3}$ with probability at least
$1-\epsilon $. The proof relies on the Ornstein–Zernike machinery due to Campanino–Ioffe–Velenik and a result of Ioffe, Shlosman and Toninelli (2015) that rules out pinning in Ising polymers with modified interactions along the boundary. En route, we refine the latter result into a Brownian excursion limit law, which may be of independent interest. We further show that in a
$ K L^{2/3}\times K L^{2/3}$ box with boundary conditions
$\mathfrak h-1,\mathfrak h,\mathfrak h,\mathfrak h$ (i.e.,
$\mathfrak h-1$ on the bottom side and
$\mathfrak h$ elsewhere), the limit of
$\rho (x)$ as
$K,L\to \infty $ is a Ferrari–Spohn diffusion.