Dynamics accross the liquid-liquid transition in Si and Ga: The fragile-to-strong transition

The existence of liquid-liquid phase transitions (LLPT) has been proposed to explain the anomalies in thermodynamic properties of substances such as, water, Si, Ga, etc. Recently, there has been an increasing interest in the liquid dynamics near LLPTs. It has been also suggested the occurrence of a fragile-to-strong transition (FTST) in the dynamics of water and Si accompanying the LLPT. Using computer simulations we study the dynamics of two atomic liquids that display a LLPT: Si and Ga. In particular, our goal is to investigate to what extent the presence of a dip in the self-intermediate scattering function (SISF) is caused by an excess of vibrational states at low frequencies, which may be associated with a FTST across the LLPT, as suggested recently. Our results do not provide support to these suggestions. In the specific case of Ga, we observe the appearance of an excess of vibrational states at low frequencies, even though the development of a dip in the SISF across the LLPT is not observed. Moreover, our study of the shear viscosities traversing the LLPTs indicates that, despite the appearance of a dip in the SISF for the case of Si and its absence in Ga, both substances exhibit a fragile character above and below the LLPT temperatures.