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Agreeing to disagree at the next Convergence conference

The closing panel. From left to right: Patrick Brady, Stefania Gori, Immanuel Bloch, Sara Seager and Ian O’Neill.

I have just returned from the Perimeter Institute (PI) in Waterloo, Canada where I enjoyed a fantastic few days immersed in discussions involving some of the sharpest minds in physics. The great and good were at the PI for the first Convergence conference and from what I have heard, the participants are calling it a great success.

But could it be even better next time?

At the panel discussion that closed the conference on Wednesday, several people suggested that “challenge” should be the theme of the next meeting. In particular, the structure of the meeting should facilitate questioning the views of individual researchers as well as more general critiques of accepted wisdom – cosmic inflation was one topic suggested from the audience. Indeed, one person in the audience suggested that participants in a forum could be asked to argue on behalf of an idea that they don’t accept.

In my experience, most physicists are extremely pleasant and polite people who are interested in understanding and developing the ideas of their colleagues rather than challenging them in a public forum. As a result, a “Challenge” conference could be difficult to pull off but it would certainly be an event I would want to attend.

The talk that came closest to fitting the bill for “Challenge” was Kendrick Smith’s lecture “Planck results and future prospects in cosmology”, which you will shortly be able to watch for yourself here.

First image of a black hole expected a year from now

Chalk art of a black hole at the Perimeter Institute for Theoretical Physics (PI).
Chalk art of a black hole at the Perimeter Institute for Theoretical Physics (PI).

By Louise Mayor in Waterloo, Canada

According to Avery Broderick, a physicist at the University of Waterloo and the Perimeter Institute for Theoretical Physics (PI) in Canada, the iconic picture of a black hole from the film Interstellar “really only presages astronomical reality by about a year”. That’s because, as Broderick explains, “as soon as next spring the Event Horizon Telescope is gonna produce images of the black hole at the centre of the Milky Way”.

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The ins and outs of black holes and a new way of thinking about general relativity

 

By Hamish Johnston

While at the Convergence conference at the Perimeter Institute (PI), Physics World’s Louise Mayor and I had dinner with Sean Gryb. He did his PhD at the PI and is now doing a postdoc at Radboud University Nijmegen in the Netherlands. In the above video he shares some of his highlights of the conference.

Gryb is working on “shape dynamics”, which is a new idea for re-evaluating Albert Einstein’s general theory of relativity (GR). The idea was initiated by Julian Barbour and Gryb became involved in the development of shape dynamics while he was at PI. He now belongs to a small international band of physicists who are developing the concept. While shape dynamics is an alternative treatment of GR, the ultimate goal of their work seems to be the creation of a new framework for a theory of quantum gravity – an important goal of theoretical physics.

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Thin semiconductors go through the Mott transition

The optical response of atomically thin materials has been successfully controlled on very short timescales by a group of researchers at Columbia University in New York and Stanford University in California, US. The finding advances our understanding of many-body phenomena in low-dimensional systems. They could also help in the development of photonic devices, such as light emitters and lasers made from novel 2D transition metals.

The team, led by Tony Heinz, studied the 2D material tungsten selenide (WS2). This semiconductor belongs to the family of the transition metal dichalcogenides (TMDCs) – these have the chemical formula MX2, where M is a transition metal (such as Mo or W) and X is a chalcogen (such as S, Se or Te).

The materials go from being indirect band-gap semiconductors in the bulk, to direct band-gap semiconductors when scaled down to monolayer thickness. These monolayers efficiently absorb and emit light, and so might find use in a variety of optoelectronics device applications, such as light-emitting diodes, lasers, photodetectors and solar cells. TMDCs might also be used to make circuits for low-power electronics, low-cost or flexible displays, sensors and even flexible electronics that can be coated onto a variety of surfaces.

Strong photoexcitations

To explore the optical response of these materials in more detail, especially when they are strongly photoexcited, Heinz and colleagues subjected monolayers and bilayers of WS2 to very short and intense laser pulses, which lasted just 250 fs. They probed the resulting photoresponse over a range of wavelengths, using a technique known as “spectrally resolved ultrafast pump–probe spectroscopy”. When the sample absorbs an ultrafast laser pulse, an extremely high density of excited charge carriers (electrons and holes) is injected into the material (up to roughly one electron per square nanometre).

“In this so-called electron–hole plasma regime, the semiconductor material starts to behave somewhat like a metal,” explains team-member Alexey Chernikov, “although the comparison should not be taken too literally, since the band gap in the material is still present.”

Transition time

The presence of these carriers strongly modifies the character of the optically excited states in the material, he says. “In the unexcited material, when a photon is absorbed, it creates an exciton (a bound electron–hole pair), formed by the Coulomb attraction between the oppositely charged carriers,” explains Chernikov. He adds that “at the limit of high excitation density, however, the photogenerated charges mutually screen one another, and a plasma of free electrons and holes is produced. Going from a regime of excitons to one of free carriers is known as a Mott transition, and is of fundamental interest in many-body physics. Understanding the Mott transition in these materials is also important for applications involving high excitation densities of charge carriers”.

That these monolayer materials can sustain very intense light pulses is a property that would benefit devices operating at high intensity, such as lasers, concentrator solar cells and intra-cavity saturable absorbers and modulators, adds team-member Claudia Ruppert, who is currently at the Technical University of Dortmund in Germany.

Electron behaviour

“From a more fundamental point of view, the Mott threshold of the transition from semiconducting to metal-like behaviour identified in our study roughly defines the limit of the regime where phenomena associated with stable exciton particles can be observed,” says Chernikov. “Excitons have attracted a lot of attention in our field, thanks to their large binding energies and their peculiar ‘spin-valley’ and related properties. It is thus important to determine when these particles exist and when they are unstable and ionized.”

The Columbia–Stanford team says that it is now trying to better understand how interacting electrons behave in this class of material. “We will do this by both mapping the ‘phase-space’ diagram for the electronic many-body states in 2D films such as WS2 and by finding out how to efficiently manipulate them,” says Heinz.

The research is published in Nature Photonics 10.1038/nphoton.2015.104.

Women in graphene

 By James Dacey in Manchester

Women in Graphene posterToday is the third day of Graphene Week, a conference at the University of Manchester devoted to the fundamental science and applications of 2D materials. While many of the talks require a PhD in materials science to even understand the title (I for one am struggling), one session taking place this evening has the refreshingly simple title: Women in Graphene. Intrigued, I caught up with the session organizer Katarina Boustedt from Chalmers University of Technology in Sweden.

Graphene Week is an annual event organized by the Graphene Flagship, the EU’s biggest ever research initiative with a budget of €1 billion. As promoting equality is a key part of the Flagship’s mission, Boustedt has launched this initiative to support women working in 2D materials research. Tonight’s two-hour session is designed to start the conversation and find out the types of support that women researchers would like.

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Going beyond ‘shut up and calculate’

As a teenager, the science journalist Amanda Gefter had a “conscientious objection” to mathematics. She often slept through her high school class on meteorology – a class that, incidentally, she only took because she wanted to avoid physics – and when she went to university, she studied creative writing and philosophy rather than science. At the same time, though, Gefter was also reading pretty much every popular-physics book she could find, as part of a private quest in which she and her father sought to understand what science tells us about the nature of reality.

One of the most important figures in Gefter’s quest was the late John Wheeler, who popularized the term “black hole” and also wrote extensively about physics and philosophy. Wheeler’s ideas included the “participatory universe”, which he represented with cartoons like the one shown above. In the cartoon, an observer looks out upon the universe, but its perspective can never be totally independent because it is, itself, a part of the universe it is observing.

In this podcast, you’ll hear Gefter talking about Wheeler, the role of observers and the complex relationship between mathematics and meaning.

Nergis Mavalvala on the upcoming Advanced LIGO run

 

By Louise Mayor in Waterloo, Canada

The search for ripples in space–time known as gravitational waves is one of my favourite scientific endeavours. So here at the Perimeter Institute’s Convergence conference, I couldn’t miss the opportunity to talk to Nergis Mavalvala, one of the speakers here.

A physicist at MIT, Mavalvala works on the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US. LIGO’s first six observing runs took place from 2002 to 2010 and yielded no detection of a gravitational wave. Since then, LIGO physicists have been working on increasing the instrument’s sensitivity – they needed to make it even better at measuring the stretching and compressing of the interferometers’ 4 km-long arms thought to occur if a gravitational wave passes through them.

Five years on, LIGO’s $200m upgrade is now complete.

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Taking a peek inside the UK's National Graphene Institute

Photo of Tony Ling
Look, no dust – National Graphene Institute architect Tony Ling showing off the PVC blackboards (Courtesy: Physics World/James Dacey)

 

By Matin Durrani in Manchester

Do an Internet image search of the word “physicist” and you’ll come across countless pictures of physicists posing in front of blackboards covered with bewildering looking equations. That’s because blackboards are traditionally a common sight in physics labs and research centres – in fact, they’re everywhere at the Perimeter Institute for Theoretical Physics, where my Physics World colleagues Hamish Johnston and Louise Mayor are right now.

But over at the UK’s new £61m National Graphene Insitute (NGI), which I toured earlier today, blackboards are very much verboten. It’s the chalk dust you see, which is a no-no for health-and-safety bosses at the University of Manchester, where the NGI is located. Incidentally, Manchester is also currently home to Andre Geim and Kostya Novoselov, who shared the 2010 Nobel Prize for Physics for isolating graphene for the first time.

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What do strange metals and black holes have in common?

Subir Sachdev.

By Hamish Johnston in Waterloo, Canada

Harvard’s Subir Sachdev has just taken the audience here at the Convergence conference on a delightful romp through the phase diagram of the cuprate high-temperature superconductors. What I found most interesting was not the superconducting phase, but rather Sachdev’s description of the “strange metal” phase.

This phase occurs when the cuprate copper-oxide layer is highly doped with holes and has perplexed physicists for some time – hence its strange moniker. It has no quasiparticles and lots of low-energy excitations so there is no easy way to describe the collective behaviour of the electrons.

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Kirigami patterns make composite materials more stretchy, while staying strong

The Japanese art of “kirigami”, or paper cutting, has been used by scientists in the US to make electrically conductive composite sheets more elastic, increasing their strain from 4% to 370%, without significantly affecting their conductivity. The team has so far demonstrated its new technique by making stretchable plasma electrodes, but adds that its work could have a variety of applications, from reconfigurable structures to optoelectronic devices. The principles could also be used to design other composite materials that retain a specific property under mechanical strain.

Composite materials allow engineers to combine multiple materials with different properties to achieve a combination of properties not found in nature. One common tactic is to combine a strong elastic material with another that has a desired property, such as high electrical conductivity, but which is brittle. Unfortunately, microcracks can form in brittle regions, and stress then concentrates around their edges, allowing the material to fail. Using a composite with only a small proportion of brittle material can allow composites to stretch to many times their original length, but their functional properties are often drastically altered as they do so. “There is always a trade-off there,” explains Nicholas Kotov of the University of Michigan, Ann Arbor. “We want to have the cake and we want to eat it too.”

Cuts and notches

Kotov, together with Sharon Glotzer and colleagues at the University of Michigan, stressed carbon-nanotube/polymer composites designed to be electrically conductive, finding that they primarily deformed by the stretching of their internal fibres, before rupturing at around 5% strain. They then used photolithography to make a series of strategically placed cuts in the materials, according to the rules of kirigami. When they stressed the cut materials, the researchers found that they initially deformed in the same way. However, as the stress rose further, they began to absorb the extra strain energy by opening up the network of cuts, deforming out of the plane of the material and forming a “secondary elastic plateau” as the cuts gradually rotated with increasing load to align themselves with the applied stress.

As the stress increased, the cut regions were gradually pulled back to the centre, concentrating the strain onto the corners of the cuts. The materials finally ruptured when the strain on these corner regions grew too large, but not before they stretched by up to 370%. Crucially, the material’s electrical conductivity remained virtually unchanged as they stretched. The team found that it could manipulate the strength and elasticity of the materials in more detail by altering the length and spacing of the cuts.

Strained electrodes

The researchers used their kirigami system to produce stretchable plasma electrodes able to generate electric fields that could ionize argon gas, and yet still withstand strains of more than 200%. This would typically destroy the plasma by either physically destroying the electrode or reducing its conductivity. “In our case it was opposite,” says Kotov, “We actually saw an increase in the intensity of the plasma spots when we strained the electrode.” This has a direct application to plasma displays, he says, which use a similar process to generate spots of light. “There are no flexible or stretchable plasma devices right now,” he says. Further applications might be found in solar cells, prosthetics or the electrodes of lithium ion batteries, which need to expand and contract repeatedly without damage or loss of conductivity during the charge/discharge cycle.

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“It’s very interesting, although it’s not the only example of this kind of thing – I’ve also seen something like this in graphene,” says Christian Santangelo of the University of Massachusetts, Amherst, in the US. Santangelo is particularly interested in the “pop-up book” aspect in which, when pulled, the material buckles out of the plane. “I can imagine using this as a way to make 3D electronic devices – taking advantage of the third dimension to pack more stuff into an electronic device.” But the more immediate task, he says, is to look in detail at how the materials respond to different, more complex cuts – something that Kotov’s group is already working on.

The research is published in Nature Materials.

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