Ultrafast dynamical properties of multifunctional materials

电磁力是自然界的基本力之一，是由电和磁的耦合产生的。电荷的流动会产生磁场，而变化的磁场会产生电场。光本身是一种电磁波，在电磁场中振荡。电磁效应是当今绝大多数技术的基础，发电机、感应电动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、笨重且难以生产。一种被称为多铁质的新型多功能材料表现出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。这种引人注目的行为似乎将彻底改变自旋电子技术，如磁性数据存储（硬盘）和计算机内存。在过去的几年里，对多铁性材料的研究出现了复兴，在《自然》和《科学》等杂志上发表了许多引人注目的文章。被称为电子磁子的复杂新粒子被认为是由原子振动和磁性之间的耦合在多铁性中产生的，并且已经初步提出在太赫兹频率范围内吸收低能量的光。然而，科学家们对电磁子和多铁质的性质仍然知之甚少，对电极化和磁化之间动态耦合的速度知之甚少。我对这项奖学金的愿景是在多功能材料的超快物理领域建立一个世界级的研究小组，这是对英国未来技术基础至关重要的科学领域。在奖学金中，我建议研究多铁材料在超短时间尺度上的动态特性，从不到一皮秒到一纳秒以上。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脉冲光来观察。我的实验方法是用激光脉冲扰动多铁体的平衡状态，然后使用太赫兹频率的同步低能量光脉冲来跟踪电子磁子的动态电导率。这项研究是非常新颖的，因为折射率和吸收模式的变化将在皮秒时间尺度上得到。我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，我将通过评估多铁性电磁子的普适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结果。我将通过测量电极化切换的速度，寻求发现多铁性存储元件的最终速度极限。我最近证明了镍酸盐，具有强电荷和自旋有序的相关材料，在其平衡状态下表现出低能量集体振动。在这个研究计划中，我还将研究这些模式的超快动力学。该研究将大大提高我们对多铁性材料的认识，并将有利于工业上功能器件的发展。这项奖学金将使我能够在一个对英国未来技术基础至关重要的科学领域建立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.