Event

Programme and Abstracts

The programme will include 12 lectures, a series of hands-on activities, short presentations by the students, as well as plenty of discussion time.

 

Lecture Abtracts

Basic Notions of Graphene Physics
N. M. R. Peres (U. Minho, Portugal)

In this lecture we shall discuss the tight-binding model of graphene and we will obtain the low-energy Hamiltonian. We then use this latter model to discuss several topics: the opacity of graphene, graphene nanoribbons, Landau levels in graphene, the effect of strain, and, time permitting, some basic transport problems.  

Introduction to graphene spintronics
P. Seneor (CNRS, Paris)

In this lecture, we will introduce key concepts of spintronics. This will start with an introduction to spin currents in ferromagnets and a subsequent description of the main effects of giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR). From these basic concepts, we will briefly review the spin resistance and spin accumulation, key parameters for spin transport in devices. We will describe the impedance mismatch issue and highlight the stringent conditions that apply for spin transport in lateral devices. We will give example of efficient spin transport for a graphene channel.

 

Intrinsic magnetism in graphene
I.V. Grigorieva (School of Physics and Astronomy, University of Manchester, UK)

Graphene – a sheet of carbon atoms densely packed in a honeycomb lattice – has been dubbed a miracle material due to a unique combination of superior properties. This strictly two-dimensional material exhibits an exceptionally high crystal and electronic quality and already revealed a cornucopia of new physical phenomena. Among them is the possibility to induce and control a magnetic response – a subject of long-standing interest in a pure carbon-based system, in the absence of d- or f- electrons normally associated with magnetic behavior. I will discuss recent experiments on inducing and controlling magnetic response in graphene via introduction of point defects such as vacancies and adatoms. Graphene is hailed as potentially an ideal material for spintronics due to its weak spin-orbit interaction and the ability to control its electronic properties by the electric field effect. As the defect magnetism in graphene is itinerant (i.e. due to localisation of conduction electrons), it can be easily controlled by doping, so that the induced magnetic moments can be switched on and off. This has already been demonstrated experimentally and adds unique functionality to potential graphene devices.

 

Experimental  graphene spintronics
B. Van Wees (U. Groningen, NL)

I will give an introduction into experimental graphene spintronics. I will show how the non-local technique can be applied  to study spin transport and spin precession (Hanle effect) in graphene. Various experimental platforms for graphene spintronics will be discussed, including exfoliated graphene on Si/SiO2, suspended graphene, graphene on SiC, transfer of CVD grown graphene, and graphene on and encapsulated by boron nitride (BN). The outstanding questions about the mechanisms for spin relaxation in graphene will be discussed.

 

Thermoelectrics and spin caloritronics in graphene and other systems 
B. Van Wees (U. Groningen, NL)

I will give an introduction into the Seebeck and Peltier effects en show how they can be applied to graphene. Next I will introduce the field of spin caloritronics, where heat, charge and spin transport are coupled. Experimental  examples  will be given how these concepts can be applied to metallic and insulating systems as well as graphene.

 

Spintronics in Graphene and in Topological Insulators 
A. H. MacDonald (UT Austin, US)

Graphene and the surface states of three-dimensional topological insulator are both  described approximately by two-dimensional Dirac equations.  There is however an important distinction between the materials from a spintronics point of view.  In the topological insulator surface state case the Dirac equation describes the coupling between surface state momentum and spin.  In contrast, graphene's Dirac equation  describes coupling between momentum and a pseudo spin degree of freedom constructed from  the two honeycomb sub lattices.  I will describe some of the consequences of this distinction for spintronics phenomenon, explain why spin-orbit coupling in extremely weak in a perfect graphene sheets and much stronger when external perturbations break its mirror symmetry, and point out that  ideal graphene sheets are  two-dimensional topological insulators with extremely small energy gaps.   Finally I will also comment briefly on the quantized anomalous Hall effect which occurs  in thin films of diluted magnetic topological insulators. 

 

Magnetism and hyperfine interactions in graphene nanostructures 
O. Yazyev (EPFL,  Switzerland)

In the first part of my lecture, I will focus on our theory efforts directed towards creating magnetic graphene nanostructures with potential applications in spintronics [1]. In particular, I will discuss several scenarios leading to the emergence local magnetic moments in graphene nanostructures of various dimensionalities: finite-size fragments [2], nanoribbons [3,4], disordered graphene and graphite [5,6]. Special attention will be devoted to our joint experiment/theory investigation of chiral graphene nanoribbons with atomically smooth edges [4,7]. In the second part of my lecture I will discuss the mechanism of hyperfine interactions in graphene involving both intrinsic (C-13) and extrinsic (e.g. protons due to hydrogen adsorption and edge termination) nuclear spins [8]. Recent muon spin rotation experiments will be covered too [9,10].   

 

References:
[1] O. V. Yazyev, Emergence of magnetism in graphene materials and nanostructures. Rep. Prog. Phys. 73, 056501 (2010).
[2] W. L. Wang, O. V. Yazyev, S. Meng, and E. Kaxiras, Topological Frustration in Graphene Nanoflakes: Magnetic Order and Spin Logic Devices.Phys. Rev. Lett. 102, 157201 (2009).
[3] O. V. Yazyev, and M. I.  Katsnelson, Magnetic Correlations at Graphene Edges: Basis for Novel Spintronics Devices. Phys. Rev. Lett. 100, 047209 (2008).
[4] O. V. Yazyev, R. B. Capaz, and S. G. Louie, Theory of magnetic edge states in chiral graphene nanoribbons. Phys. Rev. B 84, 115406 (2011).
[5] O. V. Yazyev and L. Helm, Defect-induced magnetism in graphene. Phys. Rev. B 75, 125408 (2007).
[6] O. V. Yazyev, Magnetism in Disordered Graphene and Irradiated Graphite. Phys. Rev. Lett. 101, 037203 (2008).
[7] C. Tao, et al. Spatially resolving edge states of chiral graphene nanoribbons. Nature Phys. 7, 616 (2011).
[8] Yazyev, O. V. Hyperfine interactions in graphene and related carbon nanostructures. Nano Lett. 8, 1011 (2008).
[9] Riccò, M. et al. Muons Probe Strong Hydrogen Interactions with Defective Graphene. Nano Lett. 11, 4919 (2011).
[10] Riccò, M. et al. Muons probe magnetism and hydrogen interaction in graphene. Physica Scripta 88, 068508 (2013).

 

The quest for ferromagnetism in graphene-based systems
Juan José Palacios (Universidad Autónoma de Madrid, Spain)

Hydrogenation of carbon nanostructures is recently attracting a lot of  interest as a methodology that allows for the tuning of their  mechanical, electronic, and magnetic properties. In contrast to direct  manipulation of the carbon atoms, e.g., creating vacancies or reshaping  edges  hydrogenation can effectively affect the electronic properties in  a similar manner with the advantage that is a reversible process. I will  present results of the dynamics, electronic structure, and magnetic  properties of hydrogenated graphene multilayers using density functional  theory (DFT), model Hamiltonians, and Monte Carlo simulations.
 
I will show, as previously reported, that the interaction between  hydrogen atoms on graphene favors adsorption on different sublattices  along with an antiferromagnetic coupling of the induced magnetic  moments. On the contrary, when hydrogenation takes place on the surface  of graphite or graphene multilayers (in Bernal stacking), the  interaction between hydrogen atoms competes with the different  adsorption energies for different sublattices. I will show how kinetic  Monte Carlo simulations help us in the study of the time evolution of an  initial random distribution of H atoms on the surface. The calculated 
desorption and migration barriers along with the energetics of H  clusters on the surface are used as the input parameters in the simulations. These reveal that, at room temperature and low  concentrations, the atoms redistribute themselves randomly among the  sites of one sublattice only. This occurs in a time scale  of the order  of minutes which is much shorter than typical desorption rates, as  recently observed experimentally. At higher concentrations the
probability of cluster formation increases to the point of  counterbalancing this selective sublattice adsorption.
 
These simulations show that H atoms adsorb on the same sublattice and,  thereby,  form a ferromagnetic state. Based on the exchange couplings 
obtained from the DFT calculations, one can evaluate the Curie temperature by mapping this system onto an Ising-like model with  randomly located spins. Remarkably, the long-range nature of the  magnetic coupling in these systems makes the Curie temperature size  dependent and larger than room temperature for typical concentrations  and sizes.
 

Spin relaxation mechanism in graphene
Jaroslav Fabian (U. Regensburg, Germany)

Spin relaxation limits the spin lifetime in metals and semiconductors. There are two major mechanisms of spin relaxation: Elliott-Yafet (EY) and Dyakonov-Perel (DP). Both mechanisms are based on spin-orbit coupling and momentum relaxation. However, the two mechanisms are qualitatively different. In the EY mechanism the electron flips its spin at the collision with impurities or phonons. The more the electron scatters, the more likely its spin flips and relaxes. In the DP mechanism, the electron flips the spin in between collisions. The more the electron scatters, the less likely its spin flips.  In principle one can distinguish the two mechanisms by this idiosyncratic behavior with respect to momentum relaxation (resistivity). However, such a distinction is not possible in graphene, due to various factors. In fact, the problem of the spin relaxation in graphene has been the most outstanding problem in graphene spintronics. Neither EY nor DP mechanism seems capable of explaining the observed short spin lifetime (100 ps). Instead, it is likely that the spin in graphene is flipped by the presence of magnetic impurities sitting on resonant sites. In this lecture I will give the basics of spin relaxation theory in general, and consider the intricacies in graphene, as well as describe the new spin relaxation mechanism due to resonant scattering off magnetic impurities.

2D materials beyond graphene: synthesis, characterization and properties
Andrés Castellanos-Gómez, Delft University of Technology, the Netherlands

Mechanical exfoliation has proved to be an eective technique to cleave bulk layered materials down to the single- and few-layer limits. In fact, the isolation of single-layer graphene by mechanical exfoliation has unleashed the interest in a whole family of atomically thin materials which exhibit a variety of interesting properties ranging from wide-bandgap insulator behavior to superconductivity. The aim of this lecture is to introduce the most studied 2D materials, beyond graphene. The synthesis and characterization methods to study this emerging family of materials as well as their electrical, optical and mechanical properties will be discussed in detail.

 

Simulation workshop
Joaquín Fernández-Rossier, INL, Portugal 

In this 2 hour activity students will be able to run a home made simulation code that permits with a graphical interface that permits to  calculate a variety of spin effects in graphene ribbons, including the effect of spin-orbit coupling, edge magnetism, Landau levels, for different geometries. 

         

Twisted Bilayer Systems
Joao Lopes Dos Santos (U. Porto, Portugal)

Shortly after the isolation of a single graphene layer,  the Manchester Group showed that graphene was just one realization of many possible single layer systems [1]. Also, it was clear that the very same property that allowed the isolation of a single layer raised the possibility of fabricating weakly coupled multilayers [2]. This weak coupling  allows consecutive layers to have slightly different orientations forming twisted multilayer systems [3].  In this lecture, I will discuss some general features  of twisted bilayer systems, which can be considered a new form of 2D crystal,  when the structure is commensurate.  I will focus on some general ideas for  the characterization of the electronic structure, and discuss in particular the properties of the best studied bilayer, that of graphene [4].
 
References
[1]   Novoselov, K. S. et.al , "Two-dimensional atomic crystals", PNAS 102, 30 (2005), pp. 10451 -- 10453.
[2]   Geim, A. K. and Grigorieva, I. V., "Van der Waals heterostructures", Nature 499, 7459 (2013), pp. 419--425.
[3]   A. Reina et. al., "Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition", Nano Letters 9, 1 (2009), pp. 30-35.
[4]  Lopes dos Santos, J. M. B. and Peres, N. M. R. and Castro Neto, A. H., "Continuum model of the twisted graphene bilayer", Phys. Rev. B 86 (2012), pp. 155449.
 
 

Detection of the Spin Hall Effect
Sergio O. Valenzuela,  ICN,  Barcelona.

 
The spin Hall effect is a phenomenon stemming from spin-orbit interaction that can be used to electrically generate spin currents in nonmagnetic systems. Together with its reciprocal effect, the so-called inverse spin Hall effect, constitutes a powerful tool to gain valuable insights into spin dynamics and, quite possible, for applications. In this lecture, I will review basic concepts of these effects and the experimental procedures that have been proposed and used to detect them, in particular, in the context of graphene.

 

Student Talks

Cristian Alvino: Hybrid graphene-molecular magnet devices for spintronic
José Luis Lado:  Tunable edge states in graphene Quantum hall bars
Susane Irmer:  Spin orbit coupling in fluorinated graphene
Josep Ingla:  Encapsulated graphene on boron nitride
Mallikarjuna Gurram:  Magnetoelectronics in graphene with self-assembled molecular layer
Alexey  Kaverzin:  Graphene hydrogenation via HSQ dissociation
Siddhartha Omar:  Spin dependent noise in graphene