A conjecture on the lower bound of the length-scale critical exponent $\nu$ at continuous phase transitions

TL;DR

The paper proposes a lower bound for the critical exponent  in continuous phase transitions within LGW  theories, supported by various theoretical and numerical evidence, constraining the possible values of  and related exponents.

Contribution

It introduces a conjectured inequality   (2-ta)^{-1} for a broad class of LGW  theories, providing a new theoretical lower bound for critical exponents.

Findings

The inequality   (2-ta)^{-1} holds for many universality classes.

Unitarity implies   1/2 for these theories.

The bound is more restrictive than the  > 1/d bound at first-order transitions.

Abstract

A fundamental issue in the renormalization-group (RG) theory of critical phenomena concerns the allowed values of critical exponents that are consistent with the continuous nature of a phase transition. Here we conjecture a lower bound for the length-scale exponent $\nu$, which should hold for the large class of continuous transitions associated with $d$-dimensional Landau-Ginzburg-Wilson (LGW) $\Phi^4$ theories with a multicomponent scalar field ${\varphi}$ and a unique ${\varphi}\cdot {\varphi}$ quadratic term (including some extensions with fermionic and gauge fields), describing many universality classes of critical phenomena. If $\Delta_\varphi=(d-2+\eta)/2$ is the dimension of the order-parameter field ${\varphi}$, and $\Delta_\varepsilon=d-1/\nu$ is the RG dimension of the energy operator $\varepsilon$, which can be identified with $[{\varphi}\cdot {\varphi}]$ (the squared field with a proper subtraction of the mixing with the identity), we conjecture the inequality $\Delta_\varepsilon \ge 2 \Delta_\varphi$, which implies $\nu \ge (2-\eta)^{-1}$ and $\gamma = (2-\eta)\nu\ge 1$. These inequalities are supported by general arguments for ferromagnetic lattice models, by $\epsilon$-expansion results for generic LGW $\Phi^4$ theories close to four dimensions, exact relations for two-dimensional minimal conformal field theories, and are consistent with all further known (numerical, perturbative, and exact) results for LGW $\Phi^4$ theories. In particular, since unitarity requires $\eta\ge 0$, the above inequality implies $\nu\ge 1/2$ for unitary theories. This lower bound is more restrictive than $\nu > 1/d$, derived by noting that $\nu=1/d$ characterizes the singular finite-size behavior at first-order transitions.