Laboratoire de Physique des Solides, 91400 Orsay, France.
Synchrotron SOLEIL, L'Orme des Merisiers, BP 48 Saint-Aubin, 91192 Gif-sur-Yvette Cedex, France.
Phys Rev Lett. 2015 Mar 20;114(11):117601. doi: 10.1103/PhysRevLett.114.117601. Epub 2015 Mar 16.
It is established that the multiferroics RMn(2)O(5) crystallize in the centrosymmetric Pbam space group and that the magnetically induced electric polarization appearing at low temperature is accompanied by a symmetry breaking. However, both our present x-ray study-performed on compounds with R=Pr,Nd,Gd,Tb, and Dy-and first-principles calculations unambiguously rule out this picture. Based on structural refinements, geometry optimization, and physical arguments, we demonstrate in this Letter that the actual space group is likely to be Pm. This turns out to be of crucial importance for RMn(2)O(5) multiferroics since Pm is not centrosymmetric. Ferroelectricity is thus already present at room temperature, and its enhancement at low temperature is a spin-enhanced process. This result is also supported by direct observation of optical second harmonic generation. This fundamental result calls into question the actual theoretical approaches that describe the magnetoelectric coupling in this multiferroic family.
已确定多铁性 RMn(2)O(5) 晶体属于中心对称的 Pbam 空间群，低温下出现的磁致电极化伴随着对称破缺。然而，我们目前对 R=Pr、Nd、Gd、Tb 和 Dy 化合物进行的 X 射线研究和第一性原理计算都明确排除了这种情况。基于结构精修、几何优化和物理论证，我们在这封信中证明，实际的空间群可能是 Pm。这对于 RMn(2)O(5)多铁性材料来说至关重要，因为 Pm 不是中心对称的。因此，铁电性在室温下就已经存在，低温下的增强是一个自旋增强的过程。这一结果也得到了光学二次谐波产生的直接观察的支持。这一基本结果质疑了实际描述该多铁性家族中磁电耦合的理论方法。
