Physical review. B, Condensed matter
Critical behavior at a Lifshitz point: Calculation of a universal amplitude ratio
David Mukamel, Marshall Luban
Abstract
The phase diagram in the vicinity of a Lifshitz point is analyzed using scaling arguments and renormalization-group calculations. Feynman-graph techniques in $d=4+\frac{1}{2}m\ensuremath{-}\ensuremath{\epsilon}$ dimensions are used to calculate the universal amplitude ratio governing the shape of the two branches of the critical line which intersect at an $m$-fold Lifshitz point. For $m<6$ the amplitude ratio is found to be $\frac{{A}_{H}(m)}{{A}_{F}(m)}=1\ensuremath{-}\frac{2}{m}+O(\ensuremath{\epsilon})$, where ${A}_{H}({A}_{F})$ is the amplitude associated with the helical (ferromagnetic) branch, so that to zeroth order in $\ensuremath{\epsilon}$, this ratio is independent of the number $n$ of components of the order parameter. The calculation indicates that along the remainder ($6<~m<~8$) of the critical dimensionality line, $\ensuremath{\epsilon}=0$, no helical long-range order is expected. We also calculate $\frac{{A}_{H}}{{A}_{F}}$ for a spherical model of a $d$-dimensional system exhibiting an $m$-fold Lifshitz point. Along the critical dimensionality line $\ensuremath{\epsilon}=0$, the spherical model result reduces to that of the renormalization-group calculation.