SciPost Phys. 15, 090 (2023) ·
published 12 September 2023

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It was recently found that the classical 3d O$(N)$ model in the semiinfinite geometry can exhibit an "extraordinarylog" boundary universality class, where the spinspin correlation function on the boundary falls off as $\langle \vec{S}(x) \cdot \vec{S}(0)\rangle \sim \frac{1}{(\log x)^q}$. This universality class exists for a range $2 ≤ N < N_c$ {and MonteCarlo simulations and conformal bootstrap} indicate $N_c > 3$. In this work, we extend this {result} to the 3d O$(N)$ model in an infinite geometry with a plane defect. We use renormalization group (RG) to show that in this case the extraordinarylog universality class is present for any finite $N ≥ 2$. We additionally show, {in agreement with our RG analysis}, that the line of defect fixed points which is present at $N = ∞$ is lifted to the ordinary, special (no defect) and extraordinarylog universality classes by $1/N$ corrections. We study the "central charge" $a$ for the $O(N)$ model in the boundary and interface geometries and provide a nontrivial detailed check of an $a$theorem by Jensen and O'Bannon. Finally, we revisit the problem of the O$(N)$ model in the semiinfinite geometry. We find evidence that at $N = N_c$ the extraordinary and special fixed points annihilate and only the ordinary fixed point is left for $N > N_c$.
Jaychandran Padayasi, Abijith Krishnan, Max A. Metlitski, Ilya A. Gruzberg, Marco Meineri
SciPost Phys. 12, 190 (2022) ·
published 9 June 2022

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This paper studies the critical behavior of the 3d classical $\mathrm{O}(N)$ model with a boundary. Recently, one of us established that upon treating $N$ as a continuous variable, there exists a critical value $N_c > 2$ such that for $2 \leq N < N_c$ the model exhibits a new extraordinarylog boundary universality class, if the symmetry preserving interactions on the boundary are enhanced. $N_c$ is determined by a ratio of universal amplitudes in the normal universality class, where instead a symmetry breaking field is applied on the boundary. We study the normal universality class using the numerical conformal bootstrap. We find truncated solutions to the crossing equation that indicate $N_c \approx 5$. Additionally, we use semidefinite programming to place rigorous bounds on the boundary CFT data of interest to conclude that $N_c > 3$, under a certain positivity assumption which we check in various perturbative limits.