Kendziora C A, Sergienko I A, Jin R, He J, Keppens V, Sales B C, Mandrus D
Naval Research Laboratory, Washington, DC 20375-6365, USA.
Phys Rev Lett. 2005 Sep 16;95(12):125503. doi: 10.1103/PhysRevLett.95.125503.
We have measured the polarized Raman scattering spectra of Cd2Re2O7, the first superconducting pyrochlore, as a function of temperature. For temperatures below the cubic-to-tetragonal structural phase transition (SPT) at 200 K, a peak with B1 symmetry develops at zero frequency with divergent intensity. We identify this peak as the first observation of the Goldstone phonon in a crystalline solid. The Goldstone phonon is a collective excitation that exists due to the breaking of the continuous symmetry with the SPT. Its emergence coincides with that of a Raman-active soft mode. The order parameter for both features derives from an unstable doubly degenerate vibration (with Eu symmetry) of the O1 atoms which drives the SPT.
我们测量了首个超导烧绿石Cd2Re2O7的偏振拉曼散射光谱随温度的变化。对于温度低于200 K时的立方相向四方相结构相变（SPT），在零频率处出现了一个具有B1对称性且强度发散的峰。我们将此峰确定为在晶体固体中首次观测到的戈德斯通声子。戈德斯通声子是一种由于与SPT相关的连续对称性破缺而存在的集体激发。它的出现与拉曼活性软模的出现同时发生。这两个特征的序参量都源自驱动SPT的O1原子的不稳定双重简并振动（具有Eu对称性）。
