Clarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, UK.
Science. 2010 Jan 8;327(5962):177-80. doi: 10.1126/science.1180085.
Quantum phase transitions take place between distinct phases of matter at zero temperature. Near the transition point, exotic quantum symmetries can emerge that govern the excitation spectrum of the system. A symmetry described by the E8 Lie group with a spectrum of eight particles was long predicted to appear near the critical point of an Ising chain. We realize this system experimentally by using strong transverse magnetic fields to tune the quasi-one-dimensional Ising ferromagnet CoNb2O6 (cobalt niobate) through its critical point. Spin excitations are observed to change character from pairs of kinks in the ordered phase to spin-flips in the paramagnetic phase. Just below the critical field, the spin dynamics shows a fine structure with two sharp modes at low energies, in a ratio that approaches the golden mean predicted for the first two meson particles of the E8 spectrum. Our results demonstrate the power of symmetry to describe complex quantum behaviors.
量子相变发生在零温度下物质的不同相之间。在相变点附近，可能会出现控制系统激发谱的奇异量子对称性。一个由 E8 Lie 群描述的具有八个粒子谱的对称性，长期以来一直被预测会出现在伊辛链的临界点附近。我们通过使用强横向磁场来调节准一维伊辛铁磁体 CoNb2O6（铌酸钴），使其通过临界点，从而在实验上实现了这个系统。自旋激发从有序相中对扭结的特征变化为顺磁相中自旋翻转。就在临界场以下，自旋动力学显示出精细结构，在低能下有两个尖锐模式，其比值接近 E8 谱的前两个介子粒子的黄金比预测值。我们的结果证明了对称性在描述复杂量子行为方面的强大功能。
