Morris D J P, Tennant D A, Grigera S A, Klemke B, Castelnovo C, Moessner R, Czternasty C, Meissner M, Rule K C, Hoffmann J-U, Kiefer K, Gerischer S, Slobinsky D, Perry R S
Helmholtz-Zentrum Berlin für Materialien und Energie, Glienicker Str. 100, D-14109 Berlin, Germany.
Science. 2009 Oct 16;326(5951):411-4. doi: 10.1126/science.1178868. Epub 2009 Sep 3.
Sources of magnetic fields-magnetic monopoles-have so far proven elusive as elementary particles. Condensed-matter physicists have recently proposed several scenarios of emergent quasiparticles resembling monopoles. A particularly simple proposition pertains to spin ice on the highly frustrated pyrochlore lattice. The spin-ice state is argued to be well described by networks of aligned dipoles resembling solenoidal tubes-classical, and observable, versions of a Dirac string. Where these tubes end, the resulting defects look like magnetic monopoles. We demonstrated, by diffuse neutron scattering, the presence of such strings in the spin ice dysprosium titanate (Dy2Ti2O7). This is achieved by applying a symmetry-breaking magnetic field with which we can manipulate the density and orientation of the strings. In turn, heat capacity is described by a gas of magnetic monopoles interacting via a magnetic Coulomb interaction.
磁场源——磁单极子——作为基本粒子，至今仍难以捉摸。凝聚态物理学家最近提出了几种类似于磁单极子的涌现准粒子的情形。一个特别简单的提议涉及高度受挫的焦绿石晶格上的自旋冰。自旋冰态被认为可以用排列成类似螺线管的偶极子网络很好地描述——这是狄拉克弦的经典且可观测的版本。在这些“管子”的末端，产生的缺陷看起来就像磁单极子。我们通过非弹性中子散射证明了在自旋冰钛酸镝（Dy2Ti2O7）中存在这样的弦。这是通过施加一个破缺对称性的磁场来实现的，利用这个磁场我们可以操纵弦的密度和取向。反过来，热容量由通过磁库仑相互作用相互作用的磁单极子气体来描述。
