磁单极子

Observation of Magnetic Monopoles in Spin Ice Authors: Hiroaki Kadowaki, Naohiro Doi, Yuji Aoki, Yoshikazu Tabata, Taku J. Sato, Jeffrey W. Lynn, Kazuyuki Matsuhira, Zenji Hiroi (Submitted on 25 Aug 2009 (v1), revised 27 Aug 2009 (this version, v2), latest version 11 Sep 2009 (v4)) Abstract: From the symmetry of Maxwell's equations of electromagnetism as well as field theoretical arguments, magnetic charges or monopoles would be expected to exist. But magnetic monopoles have never been observed despite longstanding experimental searches. Recently, attention has turned to condensed matter systems where tractable analogs of magnetic monopoles might be found, and one prediction is for an emergent elementary excitation in the spin ice compound, where the strongly competing magnetic interactions exhibit the same type of frustration as water ice. We directly probe the monopoles in spin ice using neutrons, and show that they interact via the magnetic Coulomb force. Specific heat measurements show that the density of monopoles can be controlled by temperature and magnetic field, with the density following the expected Arrhenius law. 附件(277.3KB) Observation_of_Magnetic_Monopoles_in_Spin_Ice.pdf Magnetic Charge Transport Authors: S. T. Bramwell, S. R. Giblin, S. Calder, R. Aldus, D. Prabhakaran, T. Fennell (Submitted on 6 Jul 2009) Abstract: It has recently been predicted that certain magnetic materials contain mobile magnetic charges or `monopoles'. Here we address the question of whether these magnetic charges and their associated currents (`magnetricity') can be directly measured in experiment, without recourse to any material-specific theory. By mapping the problem onto Onsager's theory of weak electrolytes, we show that this is possible, and devise an appropriate method. Then, using muon spin rotation as a convenient local probe, we apply the method to a real material: the spin ice Dy2Ti2O7. Our experimental measurements prove that magnetic charges exist in this material, interact via a Coulomb interaction, and have measurable currents. We further characterise deviations from Ohm's Law, and determine the elementary unit of magnetic charge to be 5 muB per Angstrom, which is equal to that predicted by Castelnovo, Moessner and Sondhi using the microscopic theory of spin ice. Our demonstration of magnetic charge transport has both conceptual and technological implications. 附件(466.9KB) Magnetic_Charge_Transport.pdf

其实我不懂…… 【 在 jc168 (Doraemon) 的大作中提到: 】 : 磁单极子是理论物理学弦理论中指一些仅带有北极或南极单一磁极的磁性物质，它们的磁感线分布类似于点电荷的电场线分布。这种物质的存在性在科学界时有纷争，截至2008年尚未发现这种物体。可以说是21世纪物理学界重要的研究主题之一。在2009年9月3日的《科学》杂志上 : 英国物理学家保罗·狄拉克（Paul Dirac）早在1931年利用数学公式预言了磁单极子的存在。当时他认为既然带有基本电荷的电子在宇宙中存在，那么理应带有基本“磁荷”的粒子存在。从而启发了许多物理学家开始了他们寻找磁单极子的工作。通过种种方式寻找磁单极子包括使 : 如果我们将带有磁性的金属棒截断为二，新得到的两根磁棒则会“自动地”产生新的磁场，重新编排磁场的北极、南极，原先的北极南极两极在截断磁棒后会转换成四极各磁棒一南一北。如果继续截下去，磁场也同时会继续改变磁场的分布，每段磁棒总是会有相应的南北两极。不 : ...................