RS Fellow - EPSRC grant (2014): Mitigating spin-current relaxation in spin-orbit coupled graphene: towards spin current routing in 2D carbon

Lead Research Organisation: University of York
Department Name: Physics

Abstract

Graphene has opened a new chapter in solid-state physics due its fascinating electronic properties and high potential for innovative applications in domains as diverse as flexible electronics and solar cells. One aspect of graphene research that is currently attracting much attention is in the control of a fundamental property of electrons known as spin. This dictates that an electron is always in one of two possible states, either 'up' or 'down', meaning that the electron itself could provide the basis for a binary logic bit ('0' or '1'). Devices based on such a concept promise faster processing speeds with less energy consumption than current charge-based technologies. However, the practical realisation of spin-based transistors awaits emergent nanomaterials enabling efficient manipulation of the spin state of charge currents. Graphene is one of the most promising candidates due to the fact that it entirely consists of a 2D network of carbon that is only one atom thick. This feature leads to extreme surface sensitivity and the possibility to very precisely tailor electronic, chemical, and magnetic properties through, for example, adsorption of atomic species (adatoms). We have recently predicted that adatoms leading to local enhancement of the spin-orbit coupling in graphene can drive the formation of macroscopic spin currents in the absence of magnetic fields, a phenomenon known as the spin Hall effect. An outstanding question is concerned with the main spin relaxation mechanisms that limit the lifetimes of spin signals generated through the spin Hall effect in adatom-decorated graphene. This theoretical project will employ a broad scope approach, combining analytical tools and a novel method for fully quantum transport simulations with billions of atoms, to uncover adatom decorations incorporating both robust spin current generation and spin coherence over relevant time scales. The latter is a crucial step towards the implementation of spin logic functions necessary to explore the recently discovered spin Hall effect.

Planned Impact

Please refer to attached Royal Society application
 
Description The project has developed a rigorous pen-and-paper theory' to understand the behavior of the electron's spin - its tiny compass needle - in atomically-thin materials. The research focuses on how spin-orbital effects 'inherited' from close atomic contact to another crystal affect the spin dynamics. Such a "transfer" between material interfaces at close vicinity is dubbed "proximity effect" and is allowed by a unique property of electrons: the capability to "tunnel" through large energy barriers. We discovered that spin-related processes (including spin relaxation) are extremely sensitive to proximity effects - particularly to another atomically thin material. The main findings are as follows: (1) we predicted that graphene placed on transition metal dichalcogenide (TMDC) enables a fine electrical control over the electron's spin: when a small current is passed through the graphene layer, the spin polarises in plane due to 'spin-orbital' forces brought about by the proximity to the TMDC base. The calculations also show that the efficiency of charge-to-spin conversion can be quite high at room temperature (around 20-30%); (2) we discovered a microscopic mechanism for generation of dissipation-less spin currents (spin Hall effect). The novel mechanism - ubiquitous in graphene samples with broken sub-lattice symmetry - has its origin in asymmetric (skew) scattering of electrons from impurities activated by the out-of-plane spin texture of the band structure; and (3) we developed a major extension of the standard quantum diagrammatic technique for disordered systems to incorporate self-consistently the effects of spin-orbital coupling and impurity scattering. The detailed theory - as well as the computer codes used to carry out the complex calculations - are publicly available on Figshare online digital repository. The technique developed through this grant may lead to the discovery of novel spin-dependent phenomena in a wide class of materials currently attracting much interest, including Weyl semi-metals and topological insulators.
Exploitation Route The electrical control over the electron's tiny "compass needle" in atomically-thin materials discovered through this grant can inspire the development of ultra-low-power transistors and novel digital technologies, contributing to a greener society. The pen-and-paper theory may be put to use by other scientists interested in the interplay between spin-orbit-coupled phenomena and disorder.
Sectors Digital/Communication/Information Technologies (including Software),Electronics
URL https://figshare.com/collections/Optimal_Charge-to-Spin_Conversion_in_Graphene_on_Transition-Metal_Dichalcogenides/3904732
 
Description Advanced Newton Fellowship
Amount £70,020 (GBP)
Funding ID NA150043 
Organisation The Royal Society 
Sector Charity/Non Profit
Country United Kingdom
Start 09/2015 
End 09/2017
 
Description Research Fellows Enhancement Award 2017
Amount £82,354 (GBP)
Funding ID RGF\EA\180276 
Organisation The Royal Society 
Sector Charity/Non Profit
Country United Kingdom
Start 10/2018 
End 03/2021
 
Description Research Priming Fund
Amount £3,212 (GBP)
Funding ID 50094501 
Organisation University of York 
Sector Academic/University
Country United Kingdom
Start 02/2015 
End 07/2015
 
Title Diagrammatic Technique for Spin-Orbit-Coupled Dirac Fermions 
Description An accurate diagrammatic technique developed in our group enables the evaluation of response functions of spin-orbit-coupled Dirac materials with multiple internal degrees of freedom, incorporating disorder corrections beyond previous approaches based on Gaussian/Born approximations. A Mathematica notebook illustrating the use of the technique in a Dirac model system is available to researchers and public via figshare. 
Type Of Material Improvements to research infrastructure 
Year Produced 2017 
Provided To Others? Yes  
Impact Our research tool was recently used by our group to determine the microscopic mechanisms underlying the anomalous Hall effect in magnetised graphene (https://arxiv.org/abs/1801.07713). 
URL https://figshare.com/collections/Optimal_Charge-to-Spin_Conversion_in_Graphene_on_Transition-Metal_D...
 
Title Data Collection - Inverse Spin Galvanic Effect (2017) 
Description Datasets for "Optimal Charge-to-Spin Conversion in Graphene on Transition-Metal Dichalcogenides" published in Physical Review Letters 119, 196801 (2017). 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
Impact N.A. 
URL https://figshare.com/articles/Datasets/5510440
 
Title Quantum KITE - Open-Source Computer Code 
Description KITE is a general purpose open-source tight-binding software for accurate real-space simulations of electronic structure and quantum transport properties of large-scale molecular and condensed systems with tens of billions of atomic orbitals (N ~ 10^10). KITE's core is written in C++, with a versatile Python-based interface, and is fully optimized for shared memory multi-node CPU architectures, thus scalable, efficient and fast. At the core of KITE is a seamless spectral expansion of lattice Green's functions, which enables large-scale calculations of generic target functions with uniform convergence and fine control over energy resolution. Several functionalities are demonstrated, ranging from simulations of local density of states and photo-emission spectroscopy of disordered materials to large-scale computations of optical conductivity tensors and real-space wave-packet propagation in the presence of magneto-static fields and spin-orbit coupling. On-the-fly calculations of real-space Green's functions are carried out with an efficient domain decomposition technique, allowing KITE to achieve nearly ideal linear scaling in its multi-threading performance. Crystalline defects and disorder, including vacancies, adsorbates and charged impurity centres, can be easily set up with KITE's intuitive interface, paving the way to user-friendly large-scale quantum simulations of equilibrium and non-equilibrium properties of molecules, disordered crystals and heterostructures subject to a variety of perturbations and external conditions. 
Type Of Material Computer model/algorithm 
Year Produced 2020 
Provided To Others? Yes  
Impact No identified impacts as yet. 
URL https://zenodo.org/record/3485089#.Xl5GvS2cb6g
 
Description Collaboration York-Rome Tres 
Organisation Roma Tre University
Country Italy 
Sector Academic/University 
PI Contribution Our research team has developed a self-consistent diagrammatic technique for the evaluation of response functions in spin-orbit-coupled transport materials with multiple degrees of freedom, including (vertex-) corrections from impurity scattering beyond the commonly employed Born approximation. Armed with this technique, we proved the vanishing of the spin Hall effect in minimal Dirac-Rashba models, discovered a novel skew scattering mechanism in generalized Dirac-Rashba models and discovered nearly optimal charge-to-spin conversion (Edelstein effect) in graphene with proximity-induced spin-orbit coupling.
Collaborator Contribution Our partner has developed systematic analysis of the perturbative structure in the diagrammatic theory, developed a picture of spin relaxation in the minimal Dirac-Rashba model based on the diagrammatic results, and assisted on the demonstration of a vanishing spin Hall effect in minimal Dirac-Rashba models.
Impact The collaboration has published two articles in Physical Review Letters reporting a set of conservation laws in Dirac-Rashba systems (e.g., realised in graphene with proximity effects) and their consequences.
Start Year 2017
 
Title KITE quantum transport software 
Description KITE is an open source software for accurate modelling of electronic structure and response functions of large molecules, crystals and hybrid heterostructures. 
Type Of Technology Software 
Year Produced 2018 
Open Source License? Yes  
Impact N.A. 
URL https://quantum-kite.com/
 
Description CCP9 Young Researchers Event 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact Around 30 PhD students from various UK Universities attended my talk on quantum criticality in graphene at the Young Researchers Event, University of York, March 25-26, which sparked questions afterwards.
Year(s) Of Engagement Activity 2015
URL https://eventbooking.stfc.ac.uk/news-events/ccp9-young-researchers-event-262
 
Description Department Visit (Instituto Superior Tecnico, Lisbon) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Around 40 post-graduate and undergraduate students attended my presentation on "2D spintronics", which sparked questions and discussion afterwards.
Year(s) Of Engagement Activity 2016
URL https://fenix.tecnico.ulisboa.pt/departamentos/df/coloquios-do-departamento
 
Description Department Visit (Trinity College Dublin) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Around 15 post-graduate students and academics attended my presentation on "2D Spintronics", which sparked questions and discussion afterward.
Year(s) Of Engagement Activity 2016
 
Description Department Visit and Lecture (Universidade Federal Rio de Janeiro, Brazil) 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Postgraduate students
Results and Impact Around 20 postgraduate students and their supervisors based at the Department of Physics, Universidade Federal do Rio de Janeiro, attended my lecture entitled "Spectral Expansions" on the use of state-of-the-art computational methodologies to investigate the electronic properties of materials, which resulted in an increased interest in the use of numerical approaches to simulate condensed matter systems.
Year(s) Of Engagement Activity 2017
 
Description Institute Visit (AIST, Tsukuba, Japan) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Around 20 post-graduate students and academics attended my presentation on "2D Spintronics", which sparked questions and discussion afterward.
Year(s) Of Engagement Activity 2017
 
Description Institute Visit (La Sapienza University, Rome, Italy) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Around 20 postgraduates and academics attended my presentation on "Inverse Spin Galvanic Effect in Graphene-based Heterostructures", which sparked questions and discussion afterward.
Year(s) Of Engagement Activity 2018
 
Description Institute Visit (NIMS, Tsukuba, Japan) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Around 20 post-graduate students and researchers attended my presentation on "2D Spintronics", which sparked questions and discussion afterward.
Year(s) Of Engagement Activity 2017
 
Description Institute Visit (NTHU, Hsinchu, Taiwan) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Around 30 postgraduates, undergraduate students and senior researchers attended my presentation on "Spin Hall effect in graphene", which sparked questions and discussion afterward.
Year(s) Of Engagement Activity 2017
 
Description Institute Visit (National Graphene Institute, Manchester, UK) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact 100+ postgraduates, undergraduate students and academics attended my presentation on "Optimal Spin-to-Charge Conversion in Graphene-based Heterostructures", which sparked questions and discussion afterward.
Year(s) Of Engagement Activity 2017
 
Description Physics Outreach Talk at York Physics Day 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Around 30 school teachers attended my popular science talk to the York Physics day at the University of York, June 17th, which sparked questions afterwards.
Year(s) Of Engagement Activity 2016
 
Description Poster presented at Postgraduate Poster Event York 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Around 10 postgraduates, undergraduates and their academic supervisors attended my poster presentation on my PhD project at the Postgraduate Poster Event, University of York on October 31st.
Year(s) Of Engagement Activity 2016
 
Description Press Release 2015 - Quantum Criticality in Graphene 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Media (as a channel to the public)
Results and Impact "Critical Delocalization of Chiral Zero Energy Modes in Graphene" published in Physical Review Letters was highlighted by several on-line media outlets and by Partner News magazine, which sparked interest from general public and scientists.
Year(s) Of Engagement Activity 2015
URL https://www.york.ac.uk/news-and-events/news/2015/research/graphene-electrons-physics/
 
Description Press Release 2017 - Two-dimensional Materials Unlock the Path to Ultra-Low-Power Transistors 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact My work "Optimal Charge-to-Spin Conversion in Graphene on Transition-Metal Dichalcogenides" published in Physical Review Letters was highlighted by several international media outlets, which sparked questions and discussion afterwards.
Year(s) Of Engagement Activity 2017
URL https://www.york.ac.uk/news-and-events/news/2017/research/two-dimensional-materials/
 
Description SIESTA Postgraduate Talk Series 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Around 15 postgraduates attended my talk on "Spin relaxation in graphene" to the SIESTA Postgraduate 2016 Series, which sparked questions and discussion afterward.
Year(s) Of Engagement Activity 2016
 
Description UCAS Mini Lecture 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Around 60 pupils and parents attended my mini lecture entitled "Graphene and Beyond: 2D materials of the future" on a UCAS open day, which sparked questions and discussion afterwards.
Year(s) Of Engagement Activity 2015