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Heat-capacity study of nematic-isotropic and nematic–smectic- A transitions for octylcyanobiphenyl in silica aerogels

1995; American Physical Society; Volume: 51; Issue: 3 Linguagem: Inglês

10.1103/physreve.51.2157

1538-4519

Lei Wu, Bo Zhou, C. W. Garland, Tommaso Bellini, Dale W. Schaefer,

Theoretical and Computational Physics

Quenched randomness and finite size can both have substantial effects on critical behavior at phase transitions. A high-resolution ac calorimeter study has been carried out on octylcyanobiphenyl (8CB) in four silica aerogels of different porosities (mass densities \ensuremath{\rho}=0.08--0.60 g ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$). The weakly-first-order nematic-isotropic (N-I) and second-order nematic--smectic-A (N--Sm-A) transitions in bulk liquid crystals belong to different universality classes and have been very well characterized in bulk 8CB. The excess heat capacity peaks \ensuremath{\Delta}${\mathit{C}}_{\mathit{p}}$(N-I) and \ensuremath{\Delta}${\mathit{C}}_{\mathit{p}}$(N--Sm-A) are observed to undergo distinctly different changes as a function of aerogel density. The changes in peak height h\ensuremath{\equiv}\ensuremath{\Delta}${\mathit{C}}_{\mathit{p}}$(max) and peak position ${\mathit{T}}_{\mathrm{peak}}$ relative to the bulk values are not well represented by finite-size scaling for either transition, and the underlying influence of quenched randomness is discussed as the major cause of the observed effects.