Current-Driven Nonequilibrium Electrodynamics and Thermodynamics in Quantum Materials at the Nanoscale

非技术摘要：许多材料可以通过施加压力、温度或光激发从不导电（绝缘体）转变为导电（金属）。在某些量子材料系统中，这种绝缘体到金属的转变可以使用非常适度的电流或电压来实现，这使得快速电气开关、节能存储器或晶体管器件的潜在应用成为可能。然而，了解电流或电压感应开关的机制并非易事。在许多实际障碍中，一个主要的挫折是无法区分电流驱动的非热态和“微不足道的”热诱导高温相。此外，不均匀应变、缺陷和化学掺杂增加了复杂性，特别是当样品接近亚微米尺度时，这在现代纳米电子器件中很常见。本研究项目旨在开发新技术，建立由电流诱导的电导率开关过程中量子材料的光学和热性质的协调微观映射。更具体地说，本研究使用最先进的扫描探针显微镜和光谱学，在纳米到微米的特征长度尺度上，对电流驱动的光学介电函数、温度和导热系数的局部变化进行了定量描述。基于对具有代表性的量子材料的系统研究，如具有3d或4d轨道的相关过渡金属氧化物，首席研究员将建立一个综合研究和教育计划，以探索在红外到太赫兹频率范围内的纳米尺度非平衡态。这项研究还为年轻研究人员和未来一代科学家提供了广泛知识方面的复杂培训。这包括各种类型的显微镜，光谱学和纳米制造技术。技术摘要：当量子材料的电子或晶格受到各种刺激的扰动时，它们会产生令人惊讶的多种相变。本研究将散射型扫描近场光学显微镜和扫描热显微镜结合在一个集成仪器中，研究了具有代表性的量子材料（如Ca2-xSrxRuO4）中电流诱导的非平衡绝缘子到金属的相变。研究小组将创建一个实验程序来区分电流驱动相和热相或应变诱导相在单晶或外延薄膜中。介电常数、临界电流密度、纳米尺度的电和热流输运可以在小于50 nm的空间分辨率下系统地研究。近场实验可以借助理论努力（玻尔兹曼理论）来研究纳米尺度电极附近的非局部现象。这项工作建立了一个严格的程序来仔细检查量子材料中的热和非热电流驱动的相变。受控实验环境下的纳米尺度光学和热成像研究为系统研究电流诱导的介观相变、分离、电子-晶格相关和非局部效应提供了一个独特的平台。该方法具有广泛的适用性，其介电常数提取程序可为光学学界带来极大的益处。这一领域的研究不仅将深刻拓宽过渡金属氧化物中的莫特物理和纳米器件中的非局域热输运等主题的基础知识，而且还将开辟在纳米尺度上用电学手段控制固有电子性质的新途径。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-Technical Abstract:Many materials can make the transition from nonconducting (insulator) to conducting (metal) through applied pressure, temperature, or photoexcitation. In certain quantum material systems, this insulator to metal transition can be achieved using very modest electric current or voltage, making potential applications to fast electrical switch, energy-efficient memory or transistor devices possible. To understand the mechanism of current or voltage induced switching, however, is nontrivial. Among many practical obstacles, one major setback is the inability to distinguish between the current-driven nonthermal state and a 'trivial' heat-induced high-temperature phase. Moreover, inhomogeneous strain, defects, and chemical doping add complications, especially when samples approach submicron scale, which is common in modern nano-electronic devices. This research project aims to develop new techniques and establish a coordinated microscopic mapping of the optical and thermal properties of quantum materials during conductivity switching induced by electric current. More specifically, this research provides a quantitative description of the current-driven local variations in optical dielectric function, temperature, and thermal conductivity at characteristic length scales ranging from nanometers to microns, using state-of-the-art scanning probe microscopy and spectroscopy. Based on systematic studies of representative quantum materials such as correlated transition metal oxides with 3d or 4d orbitals, the Principle Investigator will establish an integrated research and education program to explore nonequilibrium states at the nanoscale over a broad infrared to terahertz frequency range. This research also provides sophisticated training to young researchers and a future generation of scientists in a broad range of knowledge. This includes various types of microscopy, spectroscopy, and nanofabrication techniques.Technical Abstract: Quantum materials host a surprisingly diverse set of phase changes when their electrons or lattice are perturbed by various stimuli. By combining the scattering-type scanning near-field optical microscope and scanning thermal microscope into one integrated apparatus, this research studies the current-induced nonequilibrium insulator-to-metal phase transitions in representative quantum materials (e.g. Ca2-xSrxRuO4). The research team will create an experimental routine to distinguish between current-driven and thermal or strain induced phases in bulk single crystals or epitaxial thin films. Dielectric constant, critical current density, nanoscale electrical and heat current transport can be systematically investigated with a spatial resolution smaller than 50 nm. Near-field experiments can be aided with theoretical effort (Boltzman theory) to study nonlocal phenoemon in the proximity of the electrodes at the nanoscale. This work establishes a rigorous procedure to scrutinize the thermal and nonthermal current-driven phase transition in quantum materials. The nanoscale optical and thermal imaging investigation in controlled experimental environments provides a unique platform to systematically study current induced mesoscopic phase transition, separation, electron-lattice correlations and nonlocal effects. The methodology is widely applicable, and the dielectric constant extraction program can be extremely beneficial for the optics community. Research in this area will not only profoundly broaden the fundamental knowledge of topics including Mott physics in transition metal oxides and nonlocal heat transport in nanodevices but will also open new routes to control the intrinsic electronic properties with electric means at the nanoscale.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.