Professor Ashvin Vishwanath
UC Berkeley Physics Department


"Spin Crystals and Fluctuating Spirals in the Helimagnet MnSi"

MnSi is an itinerant magnet which at low temperatures develops a helical spin density wave. Under pressure it undergoes a transition into an unusual partially ordered state whose nature is debated. Here we propose that the helical spin crystal (the magnetic analog of a solid) is a useful starting point to understand partial order in MnSi. We consider helical spin crystals with BCC structure (composed of superpositions of six different helical spin-density waves) and determine conditions under which they may be energetically favored. We introduce a Landau theory to study the properties of these states, in particular the effect of crystal anisotropy, magnetic field and disorder and compare the results with existing data on MnSi from neutron scattering and magnetic susceptibility measurements. In addition, MnSi under pressure displays a wide region marked by non-Fermi liquid signatures in resistivity. A scenario involving fluctuating quantum spirals is proposed to account for this unusual phenomenology.