Unusual Hall effect Anomaly in MnSi under pressure

Recent works in B20 type transition metal compounds have revealed a new topological object in spin systems -- the skyrmion, a particle-like object in which spins point all directions to wrap around the sphere. While neutron scattering and scanning probe experiments\footnote{S. M\"{u}hlbauer {\it et al.,} Science {\bf 323,} 915 (2009).}$^,$\footnote{X. Z. Yu, {\it et al.}, Nature, {\bf 465,} 901 (2010).} confirmed the existence of individual skyrmions and skyrmion lattices in a particular part of the phase diagram, the interaction between skyrmions and electronic degrees of freedom remains to be unveiled. In this talk, we report the observation of a highly unusual Hall current in the helical magnet MnSi under pressure.\footnote{M.Lee {\it et al.,} Phys. Rev. Lett {\bf 102,} 186601 (2009).} In addition to the normal Hall effect and the anomalous part that arises from spontaneous magnetization, the Hall conductivity displays a distinctive stepwise field profile quite unlike any other Hall response observed in solids. This additional contribution was observed in a much larger range of temperature and applied field than the so-called $A$-phase,where the skyrmion lattice was observed in ambient pressure. It suggests that fluctuating, {\it i.e.} non-static, skyrmions might be present over a broad range of the phase diagram under pressure when the magnetic ordering becomes weakened.