arXiv2024
We study the dynamics of critical spin fluctuations and hot electrons at the metallic antiferromagnetic quantum critical points with $Z_2$ and $O(2)$ spin symmetries, building upon earlier works on the $O(3)$ symmetric theory. The interacting theories in $2+1$ dimensions are approached from $3+1$-dimensional theories in the $\epsilon$-expansion that tunes the co-dimension of Fermi surface as a control parameter. The low-energy physics of the $Z_2$ and $O(2)$ theories qualitatively differ from each other and also from that of the $O(3)$ theory. The difference is caused by higher-order quantum corrections beyond the one-loop order that are important even to the leading order in $\epsilon$. The naive loop-expansion breaks down due to dynamical quenching of kinetic energy: the speed of the collective mode ($c$) and the Fermi velocity perpendicular to the magnetic ordering vector ($v$) become vanishingly small at low energies. What sets the three theories apart is the hierarchy that emerges between the quenched kinetic terms. At the infrared fixed point, $c/v$ becomes $0$, $1$ and $\infty$ in the $Z_2$, $O(2)$ and $O(3)$ theories, respectively. At intermediate energy scales, the slow renormalization group (RG) flows of $c$ and $v$ toward their fixed point values create approximate scale invariance controlled by approximate marginal parameters. The manifold of those quasi-fixed points and the RG flow therein determines crossovers from scaling behaviours with transient critical exponents at intermediate energy scales to the universal scaling in the low-energy limit. If the symmetry group is viewed as a tuning parameter, the $O(2)$ theory corresponds to a multi-critical point which has one additional quasi-marginal parameter than the other two theories.