Quantum transport and far from equilibrium response in nano-junctions

目的和计划本项目是关于一种全新的低电压方法，它的电子传输特性将从理论上进行研究。IBM的实验合作伙伴和合作者正在构建第一个纳米级的PET实现，这使得理论和实验进展多少是携手的。研究纳米尺度的薄膜异质结构用于开关操作，包括多体效应和对这种技术在低维的可扩展性进行评估。最终目的是发展一种最先进的理论框架，用于模拟纳米结中的非平衡量子输运。方法和目的该理论项目包括发展和应用原子级电子结构和量子输运计算技术来模拟压阻材料在器件中与金属电极接触的电导。所使用的方法是基于密度泛函理论(DFT)，结合非平衡格林函数(NEGF)技术用于量子输运，如Smeagol代码中所实现的。尽管电子结构计算通常被很好地理解和控制，特别是使用密度泛函理论(DFT)，但正确地描述量子输运需要更高级的数学描述，例如非平衡Keyldysh Green函数。在文献中，通常采用非平衡格林函数方法在密度泛函框架下描述输运过程。它正确地预测了这些材料的带隙随压力减小的定性性质。但DOS、能带结构和f电子电导与实验不符。这是因为，对于高度局域的Smf壳层，没有正确地捕捉到强烈的电子-电子关联。压阻材料，如Sm或SmSe，通常包括与Cor强烈相关的f电子，因此它们的输运性质不能用最先进的DFT方法来捕捉。为了能够描述这样的系统，因此需要新的算法，它将基于嵌入在NEGF框架中的动力学平均场理论(DMFT)。DMFT被认为可以减少f电子的过度离域，从而减少电导高估。到目前为止，这些方法还处于起步阶段，该项目的一个重要方面将是将多体DMFT+NEGF计算算法开发成成熟的软件，该软件也将分发给社区使用。部分计算还将使用CASTEP软件进行，目前正在与伦敦国王学院合作，在CASTEP中实施DMFT项目。然后，压阻剂/金属纳米器件的原子尺度导电特性将被传递给我们的合作者，并嵌入到包含有限元模拟和分析模型的多尺度方法中，这将允许为IBM的实验合作伙伴开发新的器件设计。该项目是国家物理实验室和伦敦大学皇家霍洛威大学作为电子结构方法开发中心的合作项目，Ivan Rungger和Keith Refson分别作为Smeagol和CASTEP软件包的核心开发者。该项目的合作者包括：IBM TJ Watson(美国)、IBM Ruesch-Likon(瑞士)、CASTEP开发者小组(英国)、巴斯克大学纳米生物光谱小组(西班牙)、奥格斯堡大学(德国)、伦敦国王学院(英国)。

Aim and PlanThis project is about an entirely new low-voltage approach whose electron trans- port characteristics will be investigated theoretically. The experimental partners and collaborators at IBM are building the first nanoscale realization of the PET which makes the theoretical and the experimental progress somewhat hand in hand. The objective is the study of thin film heterostructure at nanometre scales for switching operations including the many-body effects and evaluation of scalability of such technology to low dimensions.The ultimate aim is the development of a state-of-the-art theoretical frame- work for modelling non-equilibrium quantum transport in nanojunctions.Methods and ObjectivesThis theoretical project includes the development and application of atomic scale electronic structure and quantum transport computational techniques to model the conductance of piezoresistive materials in contact with metal electrodes in a device setup. The used methods are based on the density functional theory (DFT), combined with the non-equilibrium Greens functions (NEGF) technique for quantum transport, as implemented in the Smeagol code.Although the electronic structure calculations, in general, are well under- stood and controlled especially using the Density Functional Theory(DFT), the proper description of quantum transport requires more advanced mathematical descriptions e.g., non-equilibrium Keyldysh Green's Functions. In the litera- ture usually Non-Equilibrium Green's Function methods are employed in the framework of DFT to describe transport. It predicts correctly the qualitative nature of reduction in bandgap of these materials with pressure. But the DOS, bandstructure and f-electron conductnces do not agree with experiments. This is because, strong electron-electron correlations are not captured correctly for the highly localised Sm f-shells.Piezoresistive materials such as SmS or SmSe typically include strongly cor- related f-electrons, so that their transport properties cannot be captured by state of the art DFT methods. To be able to describe such systems new algo- rithms are therefore required, which will be based on the dynamical mean field theory (DMFT) embedded in the NEGF framework. DMFT is supposed to re- duce the over-delocalistion of f-electrons and may thus, reduce the conductance overestimation.To date such methods are only in their infancy, and an important aspect of the project will be the development of many-body DMFT+NEGF compu- tational algorithms into a mature software, which will also be distributed for use to the community. Part of the calculations will also be performed with the CASTEP software, and a project to implement DMFT in CASTEP is under way in collaboration with King's College London. The atomic scale con- ductive properties of the piezoresist/metal nanodevices will then be passed to our collaborators and embedded in a multiscale approach encompassing finite elements simulations and analytical models, which will allow to develop new device designs for the experimental partners at IBM.This project is a collaboration between the National Physical Laboratory and Royal Holloway, University of London, as centres for the development of electronic structure methods, where Ivan Rungger and Keith Refson are basedas core developers of the Smeagol and CASTEP software packages, respectively. Collaborators in this project include: IBM TJ Watson (USA), IBM Ruesch- likon (Switzerland), CASTEP developers group (UK), Nano-bio spectroscopy group in the University of the Basque Country (Spain), University of Augsburg (Germany), Kings College London (UK).