Deutschländer Sven, Puertas Antonio M, Maret Georg, Keim Peter
Physics Department, University of Konstanz, 78464 Konstanz, Germany.
Department of Applied Physics, University of Almeria, 04120 Almeria, Spain.
Phys Rev Lett. 2014 Sep 19;113(12):127801. doi: 10.1103/PhysRevLett.113.127801. Epub 2014 Sep 18.
We report the specific heat cN around the melting transition(s) of micrometer-sized superparamagnetic particles confined in two dimensions, calculated from fluctuations of positions and internal energy, and corresponding Monte Carlo simulations. Since colloidal systems provide single particle resolution, they offer the unique possibility to compare the experimental temperatures of the peak position of cN(T) and symmetry breaking, respectively. While order parameter correlation functions confirm the Kosterlitz-Thouless-Halperin-Nelson-Young melting scenario where translational and orientational order symmetries are broken at different temperatures with an intermediate so called hexatic phase, we observe a single peak of the specific heat within the hexatic phase, with excellent agreement between experiment and simulation. Thus, the peak is not associated with broken symmetries but can be explained with the total defect density, which correlates with the maximum increase of isolated dislocations. The absence of a latent heat strongly supports the continuous character of both transitions.
我们报告了二维受限微米级超顺磁性颗粒在熔化转变附近的比热(c_N)，它是根据位置和内能的涨落计算得出的，并给出了相应的蒙特卡罗模拟结果。由于胶体系统提供单粒子分辨率，它们分别提供了比较(c_N(T))峰值位置的实验温度和对称性破缺的独特可能性。虽然序参量关联函数证实了科斯特利茨 - 索利斯 - 哈珀林 - 尼尔森 - 杨熔化情景，即平移和取向序对称性在不同温度下被打破，中间存在所谓的六重相，但我们在六重相内观察到比热的单峰，实验与模拟结果高度吻合。因此，该峰与对称性破缺无关，而是可以用总缺陷密度来解释，总缺陷密度与孤立位错的最大增加量相关。潜热的缺失有力地支持了这两种转变的连续特性。
