Seonju You, Marjorie Ladd-Parada, Kyeongmin Nam, Aigerim Karina, Seoyoung Lee, Myeongsik Shin, Cheolhee Yang, Yeseul Han, Sangmin Jeong, Kichan Park, Kyeongwon Kim, Minjeong Ki, Robin Tyburski, Iason Andronis, Keely Ralf, Jae Hyuk Lee, Intae Eom, Minseok Kim, Rory Ma, Dogeun Jang, Fivos Perakis, Peter H. Poole, Katrin Amann-Winkel, Kyung Hwan Kim, Anders Nilsson
The search for the liquid-liquid critical point in supercooled water is challenging owing to rapid crystallization. We studied supercooled water at timescales before ice formation by heating high- and low-density amorphous ices using infrared ultrafast laser pulses, followed by x-ray scattering. By varying the pump laser fluence, we accessed liquid states straddling the predicted critical point. We observed a crossover from a discontinuous to a continuous transition at which broad and slow structural variations occurred, consistent with critical fluctuations and slowing down. We also observed a rapid increase in the heat capacity indicating a critical divergence at 210 ± 8 K coincident with enhanced density fluctuations. These results suggest that our experiments have directly probed the vicinity of a critical point in supercooled water.