Bramwell S T, Giblin S R, Calder S, Aldus R, Prabhakaran D, Fennell T
London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, 17-19 Gordon Street, London WC1H 0AH, UK.
Nature. 2009 Oct 15;461(7266):956-9. doi: 10.1038/nature08500.
The transport of electrically charged quasiparticles (based on electrons or ions) plays a pivotal role in modern technology as well as in determining the essential functions of biological organisms. In contrast, the transport of magnetic charges has barely been explored experimentally, mainly because magnetic charges, in contrast to electric ones, are generally considered at best to be convenient macroscopic parameters, rather than well-defined quasiparticles. However, it was recently proposed that magnetic charges can exist in certain materials in the form of emergent excitations that manifest like point charges, or magnetic monopoles. Here we address the question of whether such magnetic charges and their associated currents-'magnetricity'-can be measured directly in experiment, without recourse to any material-specific theory. By mapping the problem onto Onsager's theory of electrolytes, we show that this is indeed possible, and devise an appropriate method for the measurement of magnetic charges and their dynamics. Using muon spin rotation as a suitable local probe, we apply the method to a real material, the 'spin ice' Dy(2)Ti(2)O(7) (refs 5-8). Our experimental measurements prove that magnetic charges exist in this material, interact via a Coulomb potential, and have measurable currents. We further characterize deviations from Ohm's law, and determine the elementary unit of magnetic charge to be 5 mu(B) A(-1), which is equal to that recently predicted using the microscopic theory of spin ice. Our measurement of magnetic charge and magnetic current establishes an instance of a perfect symmetry between electricity and magnetism.
带电准粒子（基于电子或离子）的输运在现代技术以及决定生物有机体的基本功能方面都起着关键作用。相比之下，磁荷的输运几乎尚未得到实验探索，主要是因为与电荷不同，磁荷通常充其量被认为是方便的宏观参数，而非明确界定的准粒子。然而，最近有人提出磁荷可以以涌现激发的形式存在于某些材料中，这些激发表现得像点电荷，即磁单极子。在此，我们探讨这样一个问题：在不借助任何特定材料理论的情况下，能否在实验中直接测量这种磁荷及其相关电流——“磁流”。通过将该问题映射到昂萨格的电解质理论上，我们表明这确实是可行的，并设计出一种测量磁荷及其动力学的合适方法。利用μ子自旋旋转作为一种合适的局部探针，我们将该方法应用于一种实际材料——“自旋冰”Dy₂Ti₂O₇（参考文献5 - 8）。我们的实验测量证明这种材料中存在磁荷，它们通过库仑势相互作用，并且有可测量的电流。我们进一步刻画了与欧姆定律的偏差，并确定磁荷的基本单位为5μ₈A⁻¹，这与最近利用自旋冰微观理论预测的结果相等。我们对磁荷和磁流的测量建立了电与磁之间完美对称的一个实例。
