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تسجيل مستخدم جديدCalorimetric study of the nematic to smectic-textit{A} and smectic-textit{A} to smectic-textit{C} phase transitions in liquid-crystal+aerosil dispersions
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A high-resolution calorimetric study has been carried out on nano-colloidal dispersions of aerosils in the liquid crystal 4-textit{n}-pentylphenylthiol-4-textit{n}-octyloxybenzoate ($bar{8}$S5) as a function of aerosil concentration and temperature spanning the smectic-textit{C} to nematic phases. Over this temperature range, this liquid crystal possesses two continuous XY phase transitions: a fluctuation dominated nematic to smectic-textit{A} transition with $alpha approx alpha_{XY} = -0.013$ and a mean-field smectic-textit{A} to smectic-textit{C} transition. The effective critical character of the textit{N}-Smtextit{A} transition remains unchanged over the entire range of introduced quenched random disorder while the peak height and enthalpy can be well described by considering a cut-off length scale to the quasi-critical fluctuations. The robust nature of the textit{N}-Smtextit{A} transition in this system contrasts with cyanobiphenyl-aerosil systems and may be due to the mesogens being non-polar and having a long nematic range. The character of the Smtextit{A}-Smtextit{C} transition changes gradually with increasing disorder but remains mean-field-like. The heat capacity maximum at the Smtextit{A}-Smtextit{C} transition scales as $rho_S^{-0.5}$ with an apparent evolution from tricritical to a simple mean-field step behavior. These results may be generally understood as a stiffening of the liquid crystal (both the nematic elasticity as well as the smectic layer compression modulus $B$) with silica density.
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Liquid crystals offer many unique opportunities to study various phase transitions with continuous symmetry in the presence of quenched random disorder (QRD). The QRD arises from the presence of porous solids in the form of a random gel network. Expe
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