Quantum Phase Transitions of Magnetic Rotons

Because of weak spin-orbit coupling and broken inversion symmetry the paramagnons of an itinerant, almost ferromagnetic system become magnetic rotons. Using self-consistent Hartree and renormalization group calculations, we study weak fluctuation-driven first-order quantum phase transitions, a quantum tricritical point controlled by anisotropy, and non-Fermi liquid behavior associated with the large phase volume of magnetic rotons. We propose that magnetic rotons are essential for the description of the anomalous high-pressure behavior of the itinerant helical ferromagnet MnSi.

Figure 1

The lowest paramagnon mode of a helical magnet becomes, for weak anisotropy, a magnetic roton with minima for all $\left|\mathbf{q}\right|=q_0>0$. The large phase space of magnetic roton fluctuations is responsible for the unconventional behavior.

Figure 2

(a) Flow diagram for the two cou­pling constants ${\lambda }_{\parallel }$ and $\overline{\lambda }$ in units of ${\overline{\lambda }}^{*}$. (b) Resulting $\overline{\lambda }\mathrm{\text{−}}{r}_0$ phase diagram for fixed ${\lambda }_{\parallel }$ with quantum tricritical point at $\overline{\lambda }=2{\lambda }_{\parallel }$ separating the regimes with second and first-order phase transition.