Phase diagram of the itinerant helical magnet MnSi at high pressures and strong magnetic fields

We performed a series of resistivity, heat capacity and ultrasound speed measurements of a MnSi single crystal at high pressures and strong magnetic fields [1-3]. Two notable features of the phase transition in MnSi that disappear on pressure increasin are a sharp peak marking the first order phase transition and a shallow maximum, situated slightly above the critical temperature and pointing to the domain of prominent helical fluctuations. The longitudinal and transverse ultrasound speeds and attenuation were measured in a MnSi single crystal in the temperature range of 2-40 K and magnetic fields to 7 Tesla. The magnetic phase transition in MnSi in zero magnetic field is signified by a quasi-discontinuity in the c11 elastic constant, which almost vanishes at the skyrmion - paramagnetic transition at high magnetic fields. The powerful fluctuations at the minima of c11 make the mentioned crossing point of the minima and the phase transition lines similar to a critical end point, where a second order phase transition meets a first order one. \begin{enumerate} \item Alla E. Petrova and Sergei M. Stishov, Phys. Rev. B 86, 174407 (2012) \item V. A. Sidorov, et al., Phys. Rev. B 89, 100403(R) (2014) \item A. E. Petrova and S. M. Stishov, Phys. Rev. B 91, 214402 (2015) \end{enumerate}