Yethiraj Anand, Mukhopadhyay Ranjan, Bechhoefer John
Department of Physics, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6.
Phys Rev E Stat Nonlin Soft Matter Phys. 2002 Feb;65(2 Pt 1):021702. doi: 10.1103/PhysRevE.65.021702. Epub 2002 Jan 16.
We have developed a new, extremely sensitive real-space technique (intensity fluctuation microscopy) to probe the order of the nematic-smectic-A (NA) transition. Using this technique, we show that the NA transition in 4'-n-octyl-4-cyanobiphenyl (8CB) is clearly first order, contrary to calorimetric studies but in agreement with conclusions drawn from front-velocity measurements. We characterize the strength of the discontinuity at the first-order transition by the dimensionless quantity t(0)=(T(NA)-T*)/T(*). By precisely measuring t(0), we have made the first detailed tests of predictions based on the Halperin-Lubensky-Ma (HLM) theory of fluctuation-induced, first-order phase transitions. First, we explore the effect of an external magnetic field on the NA transition. Although modest fields (of order 10 T) are predicted to drive the weakly first-order transition in pure 8CB second order, we observe no such effect; we establish instead that the lower bound on this critical field is approximately 30 T. Likewise, we observe no effect in mixtures of 8CB with its longer chemical homolog 4'-n-decyl-4-cyanobiphenyl (10CB). Second, we examine the dependence of t(0) as a function of 8CB-10CB mixture concentration and find that the data in mixtures with small nematic temperature range are well-fit by the parameters derived by Anisimov et al. based on calorimetric measurements. As we increase the nematic range (by using concentrations closer to pure 8CB), the measured t(0) deviates more and more from the HLM predictions. Smectic fluctuations, which are neglected in the HLM calculation, are an obvious candidate to explain such a discrepancy, but one's naive expectation is that they would reduce t(0) below the HLM levels, whereas the observed values are too large. However, a recent renormalization-group calculation concludes that smectic fluctuations, surprisingly, should indeed increase t(0), explaining the observations presented here.
我们开发了一种全新的、极其灵敏的实空间技术（强度涨落显微镜术）来探测向列相 - 近晶相A（NA）转变的有序性。利用该技术，我们发现4'-正辛基 - 4 - 氰基联苯（8CB）中的NA转变明显是一级相变，这与量热学研究结果相反，但与前沿速度测量得出的结论一致。我们用无量纲量t(0) = (T(NA) - T*)/T(*)来表征一级相变处不连续性的强度。通过精确测量t(0)，我们首次对基于哈尔珀林 - 卢本斯基 - 马（HLM）涨落诱导一级相变理论的预测进行了详细测试。首先，我们探究了外磁场对NA转变的影响。尽管预测适度的磁场（约10 T）会使纯8CB中的弱一级相变转变为二级相变，但我们并未观察到这种效应；相反，我们确定该临界磁场的下限约为30 T。同样，我们在8CB与其更长的同系物4'-正癸基 - 4 - 氰基联苯（10CB）的混合物中也未观察到任何效应。其次，我们研究了t(0)作为8CB - 10CB混合物浓度的函数的依赖性，发现向列相温度范围较小的混合物中的数据能很好地用阿尼西莫夫等人基于量热测量得出的参数拟合。当我们增大向列相范围（通过使用更接近纯8CB的浓度）时，测得的t(0)越来越偏离HLM预测值。HLM计算中忽略的近晶相涨落显然是解释这种差异的一个可能因素，但人们的直观预期是它们会使t(0)低于HLM水平，而观察到的值却太大。然而，最近的重整化群计算得出结论，令人惊讶的是，近晶相涨落确实应该会增大t(0)，这就解释了此处呈现的观测结果。
