Quantum Critical Dynamics in Magnetoelectric Multiferroics

在这个计画中，我们将借由宽频介电光谱来研究过渡金属氧化物的磁电性质。工作重点应放在临界端点附近的动力学上，这些端点在低温下由量子涨落决定。 在受尊敬的材料类别中，复杂的磁序与电极化或至少极化率的开始相耦合。因此，外部磁场可以被用作控制参数的基态，而耦合的磁电激发可以通过电场光谱检查。为此，在过去几年中，开发了一种实验装置，以在1 mHz至10 GHz的频率范围内，在高达15 T的外部磁场中，该装置的特殊实验灵敏度允许在所考虑的温度和能量范围内研究动力学过程（1 GHz ~ 50 mK ~ 4 Micro-eV），这是其他方法无法达到的，特别是考虑到可能的频率范围的宽度。要研究的材料基础达到从多铁性量子自旋链LiCuVO 4或铁磁和磁电系统Cu 2 OSeO 3的自旋冰Dy 2 Ti 2 O 7，其中的磁耦合激发电偶极矩的耦合被假定。实验方法和材料类的特定组合可以为理解量子相变附近迄今为止仅很少检查的动力学方面打开无与伦比的前景。

In this project we will examine the magneto-electric properties of transition-metal oxides by means of broadband dielectric spectroscopy. The efforts shall focus on the dynamics in the vicinity of critical end-points which at low temperatures are determined by quantum fluctuations. In the respected class of materials complex magnetic order is coupled to the onset of electric polarization or at least polarizability. Thus the external magnetic field can be utilized as control parameter for the ground state while the coupled magnetoelectric excitations can be examined via electric-field spectroscopy. For this purpose during the last years an experimental setup was developed to perform measurements of the complex permittivity in the frequency range from 1 mHz to 10 GHz, in external magnetic fields up to 15 T, and for temperatures down to 25 mK.The exceptional experimental sensitivity of this setup allows for the investigation of dynamical processes in the regarded temperature and energy range (1 GHz ~ 50 mK ~ 4 Micro-eV) which is not reachable by other methods, especially considering the width of the possible frequency range. The material basis which is to be studied reaches from the multiferroic quantum-spin chain LiCuVO4 or the ferromagnetic and magnetoelectric system Cu2OSeO3 to the spin-ice Dy2Ti2O7 for which a coupling of the magnetic monopole excitations to electric dipole moments was postulated. The particular combination of experimental methodology and material class can open unrivaled prospects for the understanding of the so far only sparely examined dynamical aspects in the vicinity of quantum phase transitions.