In one of the most celebrated examples of the theory of universal critical phenomena, the phase transition to the superfluid state of 4He belongs to the same three-dimensional (3D) O(2) universality class as the onset of ferromagnetism in a lattice of classical spins with XY symmetry. Below the transition, the superfluid density ρs and superfluid velocity vs increase as a power law of temperature described by a universal critical exponent that is constrained to be identical by scale invariance. As the dimensionality is reduced toward 1D, it is expected that enhanced thermal and quantum fluctuations preclude long-range order, thereby inhibiting superfluidity. We have measured the flow rate of liquid helium and deduced its superfluid velocity in a capillary flow experiment occurring in single 30-nm-long nanopores with radii ranging down from 20 to 3 nm. As the pore size is reduced toward the 1D limit, we observe the following: (i) a suppression of the pressure dependence of the superfluid velocity; (ii) a temperature dependence of vs that surprisingly can be well-fitted by a power law with a single exponent over a broad range of temperatures; and (iii) decreasing critical velocities as a function of decreasing radius for channel sizes below R ? 20 nm, in stark contrast with what is observed in micrometer-sized channels. We interpret these deviations from bulk behavior as signaling the crossover to a quasi-1D state, whereby the size of a critical topological defect is cut off by the channel radius.
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机译:在普遍临界现象理论中最著名的例子之一中,相变到 4 sup> He的超流体状态与He属于同一三维(3D)O(2)普遍性类。具有XY对称性的经典自旋晶格中的铁磁性的发生。在过渡以下,超流体密度ρ s sub>和超流体速度v s sub>随着温度的幂律而增加,该幂律由一个通用临界指数描述,该通用临界指数受尺度不变性约束。随着尺寸朝1D方向减小,可以预期,增强的热和量子涨落会阻止长程有序化,从而抑制超流动性。我们已经在毛细管流实验中测量了液态氦的流速,并推导出了其超流体速度,该实验发生在半径为20至3 nm的单个30 nm长纳米孔中。随着孔径减小到一维极限,我们观察到以下情况:(i)抑制了超流体速度对压力的依赖性; (ii)v s sub>的温度依赖性令人惊讶地可以通过幂定律很好地拟合,并且在很宽的温度范围内只有一个指数; (iii)对于小于R′的通道,减小临界速度是减小半径的函数。与在微米大小的通道中观察到的形成鲜明对比的是20 nm。我们将这些与整体行为的偏离解释为向交叉信号传递至准1D状态,从而临界拓扑缺陷的大小被通道半径所截断。
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