Instabilities, thermal fluctuations, defects and dislocations in the crystal-$R_I$-$R_{II}$ rotator phase transitions of n-alkanes

The theoretical study of instabilities, thermal fluctuations, and topological defects in the crystal-rotator-I-rotator-II ($X-R_I-R_{II}$) phase transitions of n-alkanes has been conducted. First, we examine the nature of the $R_I-R_{II}$ phase transition in nanoconfined alkanes. We propose that under confined conditions, the presence of quenched random orientational disorder makes the $R_I$ phase unstable. This disorder-mediated transition falls within the Imry-Ma universality class. Next, we discuss the role of thermal fluctuations in certain rotator phases, as well as the influence of dislocations on the $X-R_I$ phase transition. Our findings indicate that the Herringbone order in the $X$-phase and the Hexatic order in the $R_{II}$-phase exhibit quasi-long-range characteristics. Furthermore, we find that in two dimensions, the unbinding of dislocations does not result in a disordered liquid state.