Ultrafast dynamical properties of multifunctional materials

电磁力是自然界最基本的力之一，它产生于电和磁的耦合。电荷的流动会产生磁场，而变化的磁场会产生电场。光本身就是电磁波，是电磁场中的行进振荡。电磁效应支撑了当今技术的绝大多数，发电机、感应电机和变压器都依赖于电磁。然而，电磁铁通常由线圈组成，因此笨重、笨重，很难生产。一种被称为多铁性的新型多功能材料展示了电和磁之间的强耦合，并可能在原子尺度上制造电磁铁。在多铁性材料中，电流可以产生磁化，反之，磁场可以产生电极化。这一非凡的行为似乎将给自旋电子技术带来革命性的变化，例如磁性数据存储(硬盘)和计算机内存。在过去的几年里，对多铁性的研究出现了复兴，在《自然》和《科学》等期刊上发表了许多引人注目的文章。被称为电磁子的复杂新粒子被认为是在多铁性中通过原子振动和磁性之间的耦合而产生的，并已初步被建议在太赫兹频率范围内吸收低能量的光。然而，科学家们仍然对电磁铁和多铁性的性质知之甚少，也对电极化和磁化之间的动态耦合速度知之甚少。我对这次奖学金的愿景是在多功能材料的超快物理方面创建一个世界级的研究小组，这是一个对英国未来的技术基础至关重要的科学领域。在该联谊会中，我建议在从不到1皮秒到超过1纳秒的超短时间尺度上研究多铁材料的动力学性质。在无机半导体等传统材料中，在这个时间尺度上可以观察到的现象包括晶格的振荡、电荷的散射以及粒子的产生和破坏。所有这些现象都可以用太赫兹频率的超短光脉冲观察到。我的实验方法将是用来自激光的光脉冲扰乱多铁体的平衡状态，然后使用太赫兹频率的同步低能量光脉冲来跟踪电磁铁的动态导电性。这项研究是非常新颖的，因为电磁振子模的折射率和吸收的变化都将在皮秒时间尺度上获得。我的研究的一个关键科学成果将是更好地理解磁化和电极化之间的耦合，我将通过评估电磁在多铁性中的普遍性以及它们在受到电场和磁场扰动时的稳定性来获得这一点。我将通过测量电极化转换的速度，试图发现多铁性存储元件的极限速度。我最近证明了镍酸盐，一种具有强电荷和自旋有序的相关材料，在其平衡状态下显示出低能量的集体振动。在这个研究计划中，我还将研究这些模式的超快动力学。这项研究将极大地促进我们对多铁性材料的认识，并将有利于工业功能器件的发展。这笔奖学金将使我能够在对英国未来技术基础至关重要的科学领域创建一个世界级的研究小组。

The electromagnetic force, one of the fundamental forces of nature, arises from the coupling of electricity and magnetism. The flow of electric charges can generate a magnetic field, while a changing magnetic field induces an electric field. Light itself is an electromagnetic wave, a travelling oscillation in an electromagnetic field. Electromagnetic effects underpin the vast majority of today's technology, with electric power generators, induction motors and transformers all relying upon electromagnetism. However, electromagnets typically consist of coils of wire and are therefore cumbersome, bulky and hard to produce. A novel class of multifunctional materials called multiferroics exhibit strong coupling between electricity and magnetism, and may enable the manufacture of electromagnets on an atomic scale. In a multiferroic material an electric current can produce a magnetisation, and conversely a magnetic field can generate an electric polarisation. This remarkable behaviour looks set to revolutionise spintronic technology, such as magnetic data storage (hard disks) and computer memory. The last few years has seen a renaissance in research into multiferroics, with many high profile articles in journals such as Nature and Science. Complex new particles called electromagnons are thought to be created in multiferroics by the coupling between atomic vibrations and magnetism, and have tentatively been suggested to absorb light at low energies, in the terahertz frequency range. However, the properties of electromagnons and multiferroics are still poorly understood by scientists, and little is known about the speed of the dynamic coupling between electric polarisation and magnetisation. My vision for this Fellowship is to create a world-class research group in the ultrafast physics of multifunctional materials, an area of science vital for the UK's future technology base. In the fellowship I propose to investigate the dynamic properties of multiferroic materials on ultra-short timescales, from less than one picosecond to over one nanosecond. In traditional materials such as inorganic semiconductors the phenomena that are observable on this timescale include oscillations of crystal lattices, the scattering of charges, and particle creation and destruction. All of these occurrences can be observed using ultra-short pulses of light at terahertz frequencies. My experimental approach will be to perturb the equilibrium state of a multiferroic with pulses of light from a laser, and then use a synchronised pulse of low-energy light at terahertz frequencies to track the dynamic conductivity of electromagnons. This investigation is highly novel, as the change in both the refractive index and absorption of electromagnon modes will be obtained on picosecond timescales. A key scientific result of my research will be a better understanding of the coupling between magnetisation and electric polarisation, which I will obtain by assessing the universality of electromagnons in multiferroics, and how stable they are when perturbed by electric and magnetic fields. I will seek to discover the ultimate speed limit of multiferroic memory elements, by measuring how rapidly the electric polarisation can be switched. I recently demonstrated that nickelates, related materials with strong charge and spin ordering, exhibit low energy collective vibrations in their equilibrium state. In this research programme I will also investigate the ultrafast dynamics of these modes. The proposed study will advance substantially our knowledge of multiferroic materials, and will benefit the development of functional devices by industry. This fellowship will allow me to create a world-class research group in an area of science vital for the UK's future technology base.