Quantum Critical Dynamics in Magnetoelectric Multiferroics

在这个项目中，我们将用宽带介电光谱的方法来研究过渡金属氧化物的磁电性质。努力的重点应放在关键终点附近的动力学上，在低温下，这是由量子涨落决定的。在备受推崇的一类材料中，复数磁序与电极化或至少是极化率的开始有关。因此，外加磁场可以作为基态的控制参数，而耦合的磁电激发可以通过电场光谱来检测。为此，在过去的几年中发展了一套实验装置，用于在1 MHz到10 GHz的频率范围内，在高达15T的外部磁场中，在25mK以下的温度范围内进行复介电常数的测量。该装置具有极高的实验灵敏度，允许在所考虑的温度和能量范围(1 GHz~50mk~4 Micro-eV)内研究动力学过程，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。所要研究的物质基础从多铁量子自旋链LiCuVO4或铁磁和磁电系统Cu2OSeO_3到自旋冰Dy2Ti2O7，对其提出了磁单极激发与电偶极矩耦合的假设。实验方法论和材料类的特殊结合可以为理解量子相变附近迄今为止仅被稀少研究的动力学方面开辟无与伦比的前景。

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.