Observation of Magnetic Monopoles in Spin Ice

Observation of Magnetic Monopoles in Spin Ice -...

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arXiv:0908.3568v2 [cond-mat.dis-nn] 27 Aug 2009 Observation of Magnetic Monopoles in Spin Ice Hiroaki Kadowaki, 1 Naohiro Doi, 1 Yuji Aoki, 1 Yoshikazu Tabata, 2 Taku J. Sato, 3 Jeffrey W. Lynn, 4 Kazuyuki Matsuhira, 5 Zenji Hiroi 6 1 Department of Physics, Tokyo Metropolitan University, Hachioji-shi, Tokyo 192-0397, Japan. 2 Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan. 3 NSL, Institute for Solid State Physics, University of Tokyo, Tokai, Ibaraki 319-1106, Japan. 4 NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, USA. 5 Department of Electronics, Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan. 6 Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan. To whom correspondence should be addressed. E-mail: kadowaki@phys.metro-u.ac.jp International Conference on Neutron Scattering 2009 (May 7, 2009; ICNS2009 C9.2) 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 longstand- ing 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 com- pound, where the strongly competing magnetic interactions exhibit the same 1
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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 con- trolled by temperature and magnetic field, with the density following the ex- pected Arrhenius law. From the symmetry of Maxwell’s equations of electromagnetism ( 1 ), electric charges pro- duce electric fields and magnetic charges or monopoles would produce magnetic fields; moving electric and magnetic charges produce magnetic and electric fields, respectively. At present, magnetic monopoles are absent in the classical equations, because they appear only as pairs of + and charges, i.e., magnets. About 80 years ago, a quantum mechanical hypothesis of the existence of magnetic monopoles was proposed by Dirac ( 2 ). Since then, many experimental searches have been performed, ranging from a monopole search in rocks of the moon to exper- iments using high energy accelerators ( 3 ). But none of them was successful, and the monopole is an open question in experimental physics. Theoretically, monopoles are predicted in grand unified theories, beyond the standard model of particle physics, as topological defects in the energy range of the order 10 16 GeV ( 3 ). However these enormous energies preclude all hope of creating them in any present-day laboratory experiments. Alternatively, recent theories predict that analogs of the magnetic monopole can be observed
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Observation of Magnetic Monopoles in Spin Ice -...

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