| Summary: | Conformal field theories (CFTs) with MN and tetragonal global symmetry in
$d=2+1$ dimensions are relevant for structural, antiferromagnetic and
helimagnetic phase transitions. As a result, they have been studied in great
detail with the $\varepsilon=4-d$ expansion and other field theory methods. The
study of these theories with the nonperturbative numerical conformal bootstrap
is initiated in this work. Bounds for operator dimensions are obtained and they
are found to possess sharp kinks in the MN case, suggesting the existence of
full-fledged CFTs. Based on the existence of a certain large-$N$ expansion in
theories with MN symmetry, these are argued to be the CFTs predicted by the
$\varepsilon$ expansion. In the tetragonal case no new kinks are found,
consistently with the absence of such CFTs in the $\varepsilon$ expansion.
Estimates for critical exponents are provided for a few cases describing phase
transitions in actual physical systems. In two particular MN cases,
corresponding to theories with global symmetry groups $O(2)^2\rtimes S_2$ and
$O(2)^3\rtimes S_3$, a second kink is found. In the $O(2)^2\rtimes S_2$ case it
is argued to be saturated by a CFT that belongs to a new universality class
relevant for the structural phase transition of NbO$_2$ and
paramagnetic-helimagnetic transitions of the rare-earth metals Ho and Dy. In
the $O(2)^3\rtimes S_3$ case it is suggested that the CFT that saturates the
second kink belongs to a new universality class relevant for the
paramagnetic-antiferromagnetic phase transition of the rare-earth metal Nd.
|
|---|
