Quantum Low Dimensional Materials and their Topological and Collective Properties

量子材料使我们能够探索一个复杂过程的迷人世界。材料充当各种粒子或准粒子的主体，并定义粒子的属性及其相互作用规则。例如，在单层石墨烯中，电子和声子除了被限制在两个维度之外，还具有特殊的性质，其中低能电子具有特殊的线性色散(狄拉克费米子)，并且磁场中的电子激发可以具有分数电荷和非常规统计。石墨烯中的声子也非常特殊。它们是石墨烯导热系数非常高的原因，一些研究人员认为这与声子的流体动力学传输有关(电子也表现出类似的行为)。对于声子来说，这与集体激发的第二声有关。其中许多有趣的性质是主体材料二维性质的结果，其中石墨烯通常被认为是平面的，但它可以使石墨烯弯曲，甚至可以改变其几何拓扑，比如通过在单个石墨烯层中形成一个Möbius环。这种局部宿主环境的新几何形状将如何影响粒子的性质，如电子和声子，甚至自旋？在这个研究计划中，我们将重点研究这些低维系统中的不同相互作用以及它们与拓扑和无序的相互作用。我们还将特别详细地探索石墨烯中的声子工程领域。可以说，通过最近在石墨烯中合成第一个同位素超晶格，我们在实验上打开了这一新的研究领域，现在我们处于有利地位，可以探索这个新的宇宙。其基本思想是基于在合成石墨烯晶体的过程中通过控制碳13和碳12原子的位置来创建人造同位素结构。这是通过在石墨烯的化学气相沉积中使用相应的同位素纯甲烷气体来实现的。利用这种技术，我们能够合成周期小于6 nm的同位素超晶格，它显示了新的石墨烯声子色散，在拉曼光谱中具有显著的新特征。由于热导率的降低，这具有有趣的潜在应用，这应该会导致非常高的Seebeck系数和热电应用的高品质因数。这项研究计划的一部分，是测量作为同位素周期函数的这些传输特性。在仪器方面，我们将应用超分辨率技术通过拉曼光谱对化学物种进行高空间分辨率成像，这将把这项技术推向新的未知领域，为先进材料的表征开辟新的途径。

Quantum Materials allow us to explore a fascinating world of complex processes. The material acts as a host to various particles or quasi particles and defines the properties of the particles and their rules of interactions. For instance, in single layer graphene, in addition to being confined in two dimensions, electrons and phonons have special properties, where the low energy electrons have a peculiar linear dispersion (Dirac fermions) and where electronic excitations in a magnetic field can have fractional charges and unconventional statistics. Phonons in graphene are also very special. They are responsible for the very high thermal conductivity of graphene and some researchers have argued that this is related to the hydrodynamic transport of phonons (electrons show similar behavior too). For phonons this is related to the second sound of collective excitations. Many of these interesting properties are a consequence of the two-dimensional nature of the host material, where graphene is usually thought of as planar, but it is possible to curve graphene and even change its geometric topology like by forming a Möbius ring out of a single graphene layer. How would this new geometry of the local host environment impact the properties of particles like electrons and phonons or even spins? In this research program we will focus on the different interactions in these low-dimensional systems and their interplay with topology and disorder.  In particular, we will also explore in more detail the field of phonon engineering in graphene. Having arguably opened experimentally this new field of research by recently synthesizing the first isotope superlattices in graphene, we are now well positioned to explore this new Universe. The underlying idea is based on creating artificial isotope structures by controlling the position of the Carbon 13 and Carbon 12 atoms during the synthesis of graphene crystals. This is achieved by using the corresponding isotopic pure methane gases in the chemical vapor deposition of graphene. Using this technique, we were able to synthesize isotope superlattices with periods down to 6nm, which show a new graphene phonon dispersion with striking new features in the Raman spectrum. This has interesting potential applications due to the reduced thermal conductivity, which should lead to a very high Seebeck coefficient and a high figure of merit for thermoelectric applications. Part of this research program, is to measure these transport properties as a function of isotope periodicity. On the instrumentation side, we will apply super-resolution techniques to high spatial resolution imaging of chemical species via Raman spectroscopy, which will push this technique to new unexplored regime to open new pathways for advanced material characterizations.