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Condensed Matter Physics Group
                     Sub-Atomic Movements of Magnetic Domain Walls  

June 2013

Controlling magnetic clouds in graphene 

Graphene Spin file name: GrapheneSpin_200x200.jpg  

Graphene can be made magnetic and its magnetism switched on and off at the press of a button, as reported in Nature Communications by Manchester researchers.
In this work, a team led by Drs Irina Grigorieva and Rahul Nair have introduced adatoms and vacancies in graphene. These defects are found to force electrons to condense into microscopic clouds around them. Each of the clouds behaves like a magnet carrying one unit of magnetism, spin. Graphene starts behaving as a paramagnet. By using chemical doping, the magnetic clouds can be controllably dissipated and then condensed back. This is the first time magnetism itself has been toggled, rather than the magnetization direction being reversed. This work opens a path towards active spintronics devices.

“Dual origin of defect magnetism in graphene and its reversible switching by molecular doping” DOI: 10.1038/ncomms3010 

The full story on the University of Manchester website can be found here.

 

 

 

May 2013

Cloning of Dirac fermions in graphene superlattices 

Hofstadter butterfly, Moire BN graphene file name: butterfly2_300x200.jpg  

Superlattices have attracted great interest because their use may make it possible to modify the spectra of two-dimensional electron systems and, ultimately, create materials with tailored electronic properties. In previous studies, it proved difficult to realize superlattices with short periodicities and weak disorder, and most of their observed features could be explained in terms of cyclotron orbits commensurate with the superlattice. Evidence for the formation of superlattice minibands (forming a fractal spectrumknownas Hofstadter’s butterfly) has been limited to the observation of new low-field oscillations and an internal structure within Landau levels.

Here we report transport properties of graphene placed on a boron nitride substrate and accurately aligned along its crystallographic directions. The substrate’s moire´ potential acts as a superlattice and leads to profound changes in the graphene’s electronic spectrum. Secondgeneration Dirac points appear as pronounced peaks in resistivity, accompanied by reversal of the Hall effect. The latter indicates that the effective sign of the charge carriers changeswithin graphene’s conduction and valence bands. Strong magnetic fields lead to Zaktype cloning of the third generation of Dirac points, which are observed as numerous neutrality points in fieldswhere a unit fraction of the flux quantum pierces the superlattice unit cell. Graphene superlattices such as this one provide a way of studying the rich physics expected in incommensurable quantum systems and illustrate the possibility of controllably modifying the electronic spectra of two-dimensional atomic crystals by varying their crystallographic alignment within van derWaals heterostuctures.

 

The full story on the University of Manchester website can be found here.

 

 

May 2013

Heterostructures based on 2D atomic crystals for flexible photovoltaic applications 

Heterostructures Photovoltaic Bendable    

The development of graphene technology led to the discovery of the whole new family of one-atom-thick materials. Collectively, such materials cover an extremely large parameter space of properties: from the most conductive to insulating, from strongest to softest, from opaque to optically transparent, etc.

Recently, researchers came with a new paradigm in material science: heterostructures based on such 2D materials. Combining several of such one atom thick layers in a three-dimensional stuck, researchers have been able to observe extremely exciting physical phenomena and come with a range of new electronic devices. Furthermore, every new layer of 2D materials in such stack add new functionality, so this paradigm is very natural for creation of novel, multifunctional devices. It is exactly the case when one plus one is greater than two – the combinations of 2D crystals allows one to achieve functionality not available from any of the individual monolayers.

In this paper, researchers from Manchester and Singapore expanded the functionality of such heterostructures to optoelectronics and photonics. By combining graphene with monolayers of transition metal dichalcogenides (TMDC) the researchers were able to created extremely sensitive and efficient photovoltaic devices. Such devices could potentially be used as ultrasensitive photodetectors or very efficient solar cells. In these devices, layers of TMDC were sandwiched between two layers of graphene, combining the exciting properties of both 2D crystals. TMDC layers act as very efficient light absorbers and graphene as a transparent conductive layer. This will allow for further integration of such photovoltaic devices into more complex, more multifunctional heterostructures. DOI: 10.1126/science.1235547 

 

The full story on the University of Manchester website can be found here.

 

 

April 2013

Graphene-based transistor with bistable characteristics

Vertical Transistor (small)      

Writing in Nature Communications, researchers from Manchester and Nottingham report the first graphene-based transistor with bistable characteristics, which means that the device can spontaneously switch between two electronic states. Such devices are in great demand as emitters of electromagnetic waves in the high-frequency range between radar and infra-red, relevant for applications such as security systems and medical imaging.

Bistability is a common phenomenon – a seesaw-like system has two equivalent states and small perturbations can trigger spontaneous switching between them. The way in which charge-carrying electrons in graphene transistors move makes this switching incredibly fast – trillions of switches per second. 

The device consists of two layers of graphene separated by an insulating layer of boron nitride just a few atomic layers thick. The electron clouds in each graphene layer can be tuned by applying a small voltage. This can induce the electrons into a state where they move spontaneously at high speed between the layers.  

Because the insulating layer separating the two graphene sheets is ultra-thin, electrons are able to move through this barrier by ‘quantum tunnelling’. This process induces a rapid motion of electrical charge which can lead to the emission of high-frequency electromagnetic waves. 

These new transistors exhibit the essential signature of a quantum seesaw, called negative differential conductance, whereby the same electrical current flows at two different applied voltages. The next step for researchers is to learn how to optimise the transistor as a detector and emitter.

 

The full story on the University of Manchester website can be found here.

 

 

February 2013

Interaction phenomena in graphene seen through quantum capacitance    

Graphene Quantum Capacitance       

Capacitance measurements provide a powerful means of probing the density of states. The technique has proved particularly successful in studying 2D electron systems, revealing a number of interesting many-body effects. Here, we use large-area high-quality graphene capacitors to study behaviour of the density of states in this material in zero and high magnetic fields. Clear renormalization of the linear spectrum due to electron–electron interactions is observed in zero field. Quantizing fields lead to splitting of the spin- and valley-degenerate Landau levels into quartets separated by interaction-enhanced energy gaps. These many-body states exhibit negative compressibility but the compressibility returns to positive in ultrahigh magnetic fields. The reentrant behaviour is attributed to a competition between field-enhanced interactions and nascent fractional states.

 

The full story on the University of Manchester website can be found here.

 

 

Graphene has been touted as the next silicon. There is one major problem though - it is too conductive to be used in computer chips. Now scientists from The University of Manchester have given its prospects a new lifeline.

In a paper published in Science, a Manchester team lead by Professor Sir Andre Geim and Professor Sir Kostya Novoselov has literally opened a third dimension in graphene research. One of many potential applications of graphene is its use as the basic material for computer chips instead of silicon. This potential has alerted the attention of major chip manufactures, including IBM, Samsung, Texas Instruments and Intel to name but a few. Individual transistors with very high frequencies (up to 300 GHz) have already been demonstrated by several groups worldwide. Unfortunately, those transistors cannot be packed densely in a computer chip because they leak too much current, even in the most insulating state of graphene. This electric current would cause chips to melt within a fraction of a second.

The University of Manchester scientists now suggest using graphene not  laterally (in plane) – as all the previous studies did – but in the vertical direction. Here graphene gets help from other one atom thick materials. Sandwiched together they acquire extra functionality.  Particularly graphene can be used as an electrode from which electrons tunnelled through a dielectric into another metal. This is called a tunnelling diode.
Even better, such “Layer Cake” superstructure can be used as a transistor. Manchester scientists exploited a truly unique feature of graphene – that an external voltage can strongly change the energy of tunnelling electrons. As a result they got a new type of the device – vertical field-effect tunnelling transistor in which graphene is a critical ingredient.

Dr Leonid Ponomarenko, who spearheaded the experimental effort, said:  “We have proved a conceptually new approach to graphene electronics.  Our transistors already work pretty well. I believe they can be improved much further, scaled down to nanometre sizes and work at sub-THz frequencies.” 

 

    

Graphene has revealed another of its extraordinary properties - University of Manchester researchers have found that it is superpermeable with respect to water.
Graphene is one of the wonders of the science world, with the potential to create foldaway mobile phones, wallpaper-thin lighting panels and the next generation of aircraft. The new finding at The University of Manchester gives graphene’s potential a most surprising dimension – graphene can also be used for distilling alcohol.

In a report published in Science, a team led by Professor Sir Andre Geim shows that graphene-based membranes are impermeable to all gases and liquids (vacuum-tight). However, water evaporates through them as quickly as if the membranes were not there at all.

Now the University of Manchester scientists have studied membranes from a chemical derivative of graphene called graphene oxide. Graphene oxide is the same graphene sheet but it is randomly covered with other molecules such as hydroxyl groups OH-. Graphene oxide sheets stack on top of each other and form a laminate.

The researchers prepared such laminates that were hundreds times thinner than a human hair but remained strong, flexible and were easy to handle.

When a metal container was sealed with such a film, even the most sensitive equipment was unable to detect air or any other gas, including helium, to leak through.

It came as a complete surprise that, when the researchers tried the same with ordinary water, they found that it evaporates without noticing the graphene seal. Water molecules diffused through the graphene-oxide membranes with such a great speed that the evaporation rate was the same independently whether the container was sealed or completely open.

Dr Rahul Nair, who was leading the experimental work, offers the following explanation: “Graphene oxide sheets arrange in such a way that between them there is room for exactly one layer of water molecules. They arrange themselves in one molecule thick sheets of ice which slide along the graphene surface with practically no friction.

“If another atom or molecule tries the same trick, it finds that graphene capillaries either shrink in low humidity or get clogged with water molecules.”

“Helium gas is hard to stop. It slowly leaks even through a millimetre -thick window glass but our ultra-thin films completely block it. At the same time, water evaporates through them unimpeded. Materials cannot behave any stranger,” comments Professor Geim. “You cannot help wondering what else graphene has in store for us”.

“This unique property can be used in situations where one needs to remove water from a mixture or a container, while keeping in all the other ingredients”, says Dr Irina Grigorieva who also participated in the research.

“Just for a laugh, we sealed a bottle of vodka with our membranes and found that the distilled solution became stronger and stronger with time. Neither of us drinks vodka but it was great fun to do the experiment”, adds Dr Nair.

The Manchester researchers report this experiment in their Science paper, too, but they say they do not envisage use of graphene in distilleries, nor offer any immediate ideas for applications.

However, Professor Geim adds ‘The properties are so unusual that it is hard to imagine that they cannot find some use in the design of filtration, separation or barrier membranes and for selective removal of water’.

 

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