A combination of theory and experiment by researchers in the School's Centre for Science at Extreme Conditions (CSEC) has revealed surprising behaviour in high-pressure dense hydrogen-deterium mixtures. Reported this week as Editor's Choice in the prestigious "Physical Review Letters”, the study shows that the concept of a phonon loses its meaning when isotopes have very different masses.
The great unsolved challenge of high pressure physics is to metallize hydrogen. The giant magnetic fields of Jupiter and Saturn prove the existence of such a material, but it has never been synthesized on Earth or studied at low temperature. The School's Ross Howie and Eugene Gregoryanz, and Alex Goncharov from the Carnegie Institution squeezed tiny amounts of solid hydrogen/deuterium mixtures between diamond anvils, to create some of the highest static pressures ever achieved on Earth. (Their unique experimental set-up in the CSEC laboratories in Edinburgh is shown in the figure.)
However, this means that with the mixtures the Raman signal is three times weaker per mode than in pure isotope and that only by using the state-of-the-art red Raman system provided by the School of Physics was it possible to build the apparatus and conduct these technically-demanding experiments.
Some information about the state of the material was extracted by Raman spectroscopy. Detailed theoretical analysis of the material by Ioan Magdau and Graeme Ackland using EPCC's supercomputers shows that the Raman signal corresponds to a phenomenon known as "Anderson Localization", in which vibrations are unable to propagate through the material. The cause of the localization is the disorder of the hydrogen and deuterium atoms in the structure, which scatters travelling waves.
A further consequence is that all vibrations in the crystal have some Raman scattering effect, compared with the normal case where only a handful of so-called "Gamma-modes" contribute to the observable effect.
But is it metallic? A 2011 report by a group from Mainz claimed in a Nature paper that pure hydrogen was indeed conductive at these pressures. Their evidence was equivocal, but our samples show no sign of metallic behaviour (see papers in Physical Review Letters and Physical Review B), and our calculations suggest there should be a sizable band gap regardless of the isotopic composition. The Germans are debunked and the hunt for metallic hydrogen goes on.
"Phonon localization by mass disorder in dense hydrogen-deuterium binary alloy" Ross T. Howie, Ioan B. Magdău, Alexander F. Goncharov, Graeme J. Ackland, and Eugene Gregoryanz: Phys. Rev. Lett. 113, 175501 (published 21 October 2014).
Image gallery
Dr Wood to take up a secondment with the makers of Deep Heat.
Dr Wood will work with Mentholatum in East Kilbride, Scotland, using advanced imaging techniques to look at how the microstructure of the company's major product, Deep Heat, determines its performance. She will also explore new routes to formulation.
"I feel honoured to receive a Royal Society Industrial Fellowship and I am looking forward to working with the company to explore their traditional processes and to developing innovative routes to formulation development," Dr Tiffany Wood
Royal Society Industry Fellowships
The Royal Society has created eight new fellowships aimed at strengthening links between academia and industry. The fellowships are awarded to academic scientists who want to work on a collaborative project with industry and for scientists in industry who want to work on a collaborative project with an academic organisation.
The Royal Society Industry Fellowship scheme provides each scientist's basic salary for the duration of their secondment, which lasts for up to two years full-time or four years part-time.
Edinburgh Complex Fluids Partnership
Dr Tiffany Wood runs the Edinburgh Complex Fluids Partnership (ECFP), drawing on academic expertise within the Institute of Condensed Matter and Complex Systems to help industries solve problems in formulation science and product processing. ECFP has worked with a wide range of industries from food and drink, agrochemicals through to personal care with a focus on dispersions, emulsions, gels and liquid crystal composites. Long-term stability of products, complex behaviour under shear and product innovation are all common themes. ECFP works particularly closely with colleagues in Soft Matter Physics and the Collaborative Optical Spectroscopy, Micromanipulation and Imaging Centre (COSMIC).
ENQUIRE is the new magazine for our alumni.
We have created ENQUIRE to give you a taster of the great variety of world-class research that’s undertaken here in the School. You can also find out about our work with industry, our post-graduate courses and the services we offer our graduates, plus alumni stories.
We'd love to hear what you think of ENQUIRE. Please send your comments to: enquiries [at] ph.ed.ac.uk (subject: ENQUIRE%20magazine)
View ENQUIRE (PDF).
Download Acrobat PDF Reader (free).
Over 300 people visited the School as part of Doors Open Day on Saturday 27th September. This annual event allows visitors to see behind the scenes at venues across Edinburgh.
At the James Clerk Maxwell Building, visitors were able to visit the laboratories of the Soft Matter Physics group of the Institute for Condensed Matter and Complex Systems and talk to some of the researchers.
In the foyer, there were demonstration experiments illustrating the unique properties of complex fluids and the research taking place in the group on the physics of bacteria.
The most popular experiments were a large vat of corn starch which visitors were invited to punch, illustrating the shear thickening nature of the material. Visitors could also make their own complex fluid, slime, which can bounce as a solid but flow like a liquid. Over 3 litres of slime were made during the day.
Image gallery
Scientists at the Royal Observatory Edinburgh will spearhead efforts to probe the distant reaches of the early Universe using a powerful new telescopic tool.
Stargazing power
Edinburgh astronomers will lead on the development of MOONS (Multi Object Optical and Near-infrared Spectrograph), a new instrument that will boost the stargazing powers of the Very Large Telescope (VLT) in northern Chile.
The VLT is the world’s most productive ground-based astronomical facility. MOONS will be used to tackle some of the most compelling astronomical puzzles, such as how stars and galaxies form and evolve, and to probe the structure of the Milky Way. It will allow astronomers to see obscured areas in the Milky Way at a distance of around 40,000 light years away, and enable them to create a 3D map of our galaxy.
Researchers at the UK Astronomy Technology Centre (UK ATC), based at the Observatory, will manage the multinational consortium that will construct MOONS. The instrument is scheduled to become operational by 2019.
“MOONS is a unique instrument able to pioneer a wide range of galactic, extragalactic and cosmological studies and provide crucial follow-up for major facilities such as Gaia, the Visible and Infrared Survey Telescope for Astronomy (VISTA), Euclid and the Large Synoptic Survey Telescope (LSST). I am hugely proud of the dedication and skill demonstrated over the project’s selection stages by our engineers and scientists at the UK ATC and across the consortium. We look forward to building this exceptional piece of technology and paving the way for many discoveries.”
Dr Michele Cirasuolo, School of Physics and Astronomy and Principal Investigator of MOONS project
Like any spectrograph, MOONS will use the colour of light emitted by objects to reveal their chemical composition, mass, speed and other properties. Breaking new ground by simultaneously observing 1000 objects using fibre-optic cables to feed their visible and infrared light into the instrument, it will survey large samples of objects far faster than any existing instrument and conduct surveys that would be virtually impossible using today’s technologies. Not surprisingly, the design will pose extraordinary technical demands. For example, each of the 1000-plus fibres will have to move into position very quickly, with great accuracy and without colliding with each other.
VLT platform
In collaboration with the University of Cambridge and other UK universities, the UK ATC’s world-leading expertise in fields such as miniaturised mechanics and precision optics will be harnessed on key aspects of the project. The UK ATC will develop the most innovative component, the individual motorised systems allowing each fibre to move rapidly into position. It will also develop the cryostat system (used to cool MOONS down to -170°C) vital to enabling the infrared observations needed to penetrate galactic and intergalactic dust clouds. The University of Cambridge will develop complex cameras capable of meeting the instrument’s demanding performance requirements.
Once MOONS is up and running, the international consortium will receive 300 nights of observations using the instrument. In particular, this will benefit two ground-breaking projects: one to produce an unprecedented sophisticated survey of the centre of the Milky Way; the other to look far back in time at ultra-distant galaxies to uncover the secrets of their early evolution. For more information, visit the ROE website.
UK ATC
Based at the Royal Observatory in Edinburgh and operated by STFC, the UK Astronomy Technology Centre (UK ATC) is the national centre for astronomical technology. The UK ATC designs and builds instruments for many of the world’s major telescopes. It also project-manages UK and international collaborations and its scientists carry out observational and theoretical research into questions such as the origins of planets and galaxies. The UK ATC has been at the forefront of previous key initiatives at the VLT, including the construction of KMOS (K-band Multi-Object Spectrograph) which enables 24 objects to be observed simultaneously in infrared light.
Very Large Telescope
The Very Large Telescope (VLT) is the world's most advanced optical instrument, consisting of four Unit Telescopes with main mirrors 8.2m in diameter and four movable 1.8m diameter Auxiliary Telescopes. The telescopes can work together to form a giant ‘interferometer’, the ESO Very Large Telescope Interferometer (VLTI), allowing astronomers to see details up to 25 times finer than with the individual telescopes. The Unit Telescopes can also be used individually. With one such telescope, images of celestial objects as faint as magnitude 30 can be obtained in a one-hour exposure, corresponding to seeing objects four billion times fainter than can be seen with the unaided eye.
Image gallery
The School has been awarded Athena SWAN Silver status in recognition of its ongoing efforts to create a more equitable workplace.
"I am absolutely delighted that we have been awarded Athena SWAN Silver status. This award follows our gaining Juno Champion Status earlier this year.
"Both awards affirm the progress we have made in ensuring all staff and students in the School can flourish because, although they focus on stemming the attrition of women from science, we hope our ongoing efforts benefit everyone. We will continue to work to make our School a place where everyone feels supported and valued for their contribution.""
Prof. Cait MacPhee, Institute for Condensed Matter and Complex Systems
Athena SWAN Charter
The Athena SWAN award recognises commitment to advancing women's careers in science, technology, engineering, maths and medicine (STEMM) employment in higher education and research.
ECU’s Athena SWAN Charter has been developed to encourage and recognise commitment to combating this underrepresentation and advancing the careers of women in STEMM research and academia.
It covers:
- Women in academic roles
- Progression of students into academia
- Working environment for all staff
Paolo Beltrame of the School's Nuclear Physics group explains the background to his co-authored paper which appears in this month's Physical Review D.
Which is better: a dark matter WIMP or the Imp from GoT (Game of Thrones)? I don’t know, but I advise you not to forget the axions from GUT (Grand Unification Theories). Axions, if they exist, could solve several problems in understanding our universe and in the description of the forces that govern the subatomic world.
Axions were first postulated by Roberto Peccei and Helen Quinn in 1977 to explain the discrepancy between theory and observation in Quantum Chromodynamics in relation to the Charge-Parity Violation. They could be an excellent dark matter candidate and could also solve the CPV problem. But what does this mean?
In the Standard Model of particle physics, the fundamental force that regulates the interaction among quarks is called the Strong Force. Quarks are thought to be the fundamental constituent of hadrons, among which we have the nucleons, ie the protons and neutrons which make up atoms. Each type of quark is named after a colour. This doesn’t mean that quarks are literally red, green or blue: the colours are just a naming convention to distinguish the different kinds of quarks. Because of these colours, the quantum theory formalism that describes the quarks gets the name of chromo: Quantum Chromo Dynamics or QCD.
The Weak Force
Now, in the Standard Model we have another force, called the Weak Force. This Weak Force is responsible for the decay of the nuclei; and whenever a neutrino is involved. Why do we care about Weak Interaction if the axions deal with the Strong one? This is because of the CP symmetry violation.
In 1964 it was found that the Weak Interaction violates the CP symmetry. The fundamental particles may come with a charge (C), like the electron, and with a parity (P), which can be seen as a spatial symmetry. Like the human face which is symmetrical (although not perfectly so) between left and right. Before 1964 it was expected that by changing the charge of a particle (performing a so-called charge conjugation) you would get something different from what you had at the beginning. So a positron is not an electron, but is instead its charged-conjugated partner. The same thing was expected to happen with the parity conjugation: imagine putting a particle in front of a mirror, the mirrored particle won’t be the same as the original one.
However, it was believed that if you combine these two transformations (if you make a CP conjugation) you obtain the same situation as the one present at the beginning of the process. Well, in 1964, it was proven that this is not the case for the Weak Interactions, that is to say Weak Interactions violate the CP symmetry. Nowadays we understand this process better and we can precisely describe this violation within the Standard Model of particle physics. This CP symmetry violation, although perfectly fine with the Standard Model, has not been observed in the Strong Interaction. Imagine that you see a leaf that is about to fall from a branch, but never falls. The fall is predicted by gravity but it doesn’t happen, therefore we must be missing something like the leaf being stuck to the branch. So, what is it happening to the Strong Interactions? Why haven’t we yet observed the CP violation in the Strong sector of the Standard Model?
We don’t know… yet.
To solve this problem Peccei and Quinn introduced this new particle, the axion, that takes away the CP violation in the Strong Interaction processes, restoring the symmetry. It is like preventing the leaf from falling, and making the violation invisible.
Why is this important for us?
Simple. Now that the Higgs boson has been discovered and we have a clearer idea of how particles acquire mass, we are still unable to explain why we are living in a matter-dominated universe rather than an antimatter-dominated one. The definition of what is matter and what is antimatter is a purely human convention: the two options, matter or antimatter universes, would be completely indistinguishable in terms of the laws of nature. The only difference you might experience is that instead of electrons flowing when you switch on the light, you would do the same using positrons instead.
So why has Nature chosen matter (electron) instead of antimatter (positron)?
We think the solution lies in understanding the CP violation, and the axion is one of the keystones in the building of this cathedral. There are several experimental groups searching for these particles, and many theoretical physicists are working on various axion models (oscillating between predictions and readjustment, as experimental results are published).
Concerning the experimental searches, it was recently realised that the dark matter detectors (like CDMS, EDELWEISS), especially xenon-based instruments, are particularly suited to this challenge. So the dark matter community has involved itself in this venture.
Supported by several theoretical models (also arising from Grand Unification Theories), we expect the axions to interact with the normal baryonic matter by coupling with photons, nucleons or electrons. By normal baryonic matter we mean the building blocks that constitute the Universe with which we naturally interact. Everything you see, everything you touch, is normal baryonic matter. Also our detector is made of only baryonic matter, because this is the only thing that we have learnt to master. Nevertheless, our standard matter could allow us to detect the axions by using the so-called axio-electric effect, testing the probability that an axion will interact with an electron of our detector target (see Fig 1 below), kicking it out.
The axio-electric effect is very similar to the photo-electric effect (whose discovery won Albert Einstein the Nobel Prize for Physics in 1921), with one crucial difference: in our case instead of a photon we consider an axion, which hits the electron and ionises the xenon target. The axio-electric effect was first introduced and formalised by A. Derevianko and others in the late 1990s. This was understood and initiated by me, while I was a member of the XENON collaboration.
What happens when an axion hits the xenon target?
It generates a small spark, immediately detected by the photomultiplier tubes that continuously monitor the situation inside the detector.
The XENON100 detector has been particularly good at discovering the axions through this effect. The secret lies in the cleanliness of the detector. Dark matter detectors, particularly the ones based on Xe, are among the cleanest places in the Universe. In what way? Everything surrounding us is radioactive, emits radiation which continuously hits us: even when you wash your hands you receive some radiation, particularly if the washbasin is made of ceramic because of the cobalt it contains. This radiation is completely harmless for your body so we never worry about it. But in contrast, if you put the same amount of ceramic inside a dark matter detector, the whole experiment would be spoiled! Hence, every single component has been carefully selected and the detector is operated in such a way that everything that generates a spark in its interior can be considered as a good signal, and not some spurious radiation.
To give you an idea of the cleanliness of the XENON100 detector: imagine you could sit inside the inner part of it (wearing the proper clothes, since the temperature is about -100 degrees). That place is so radiation-clean that you will have to wait for about a day between one low-energy event (visible as a tiny spark) and another. This means that if we see some light there is a good chance that it is coming from something interesting — such as axions.
We ran an experiment in the XENON100 detector for more than a year, then skimmed the data that we had collected during that time. At the end of the skimming procedure we have found no evidences of axions, as shown in Fig 2 below.
We have performed two different searches. One targets the axions which are emitted by the Sun (also called solar axions), which then reach the Earth at high velocity, ie energy, and interact within the detector. The results are shown on the left plot of Fig 2. The other search looks for axions that are slowly moving within our galaxy. Our planet, and the XENON detector, passes through this cloud of axions, possibly causing an interaction with the target. The outcome of this second approach is shown on the right plot of Fig 2.
How to read these graphs?
Consider the plot on the right, the one referring to the galactic axions. The y-axis shows the coupling of the axion with the electron, ie a way to describe the probability that they interact with the electrons. The x-axis shows the hypothetical mass of the axion. Since we don’t know either the coupling nor the mass, we have to plot them in such a graph to check where they like to live (for a given mass the corresponding coupling and vice-versa).
In these so-called exclusion plots, we show different experiments (whose names you can find on the plot) which have excluded certain phase space: each point [coupling, mass] above the line for a particular experiment has been rejected, and if the axion exists, it can be only in the region below these lines. For example, it is highly impossible that an axion in the galaxy can have a mass of 2 keV and a coupling to the electrons 1E-11 (ie one in eight hundredth of millionth), since these characteristics have been excluded by CoGeNT, CDMS, EDELWEISS and more recently by XENON100. An axion with a mass of 2 keV and a coupling of 1E-13 is still possible: we haven’t been able to search for that yet.
You can think of it like fishing: we try to sink our fishing lines deeper and deeper into different parts of the lake. You can immediately see that the XENON100 has reached the deepest level of this search. Similarly you can read the plot on the left, on which you can see different experiments and the XENON100 results. Other non-dashed lines represent constraints based on astronomical observations. In particular, the DFSZ and KSVZ refer to the theoretical models that allow the axions to solve the strong CP problem that I mentioned at the beginning.
For further and more technical information you are invited to read the original paper: 'First axion results from the XENON100 experiment' (Phys. Rev. D90, 062009 (2014), doi: 10.1103/PhysRevD.90.062009).
The effort to obtain the results from the XENON detector, which are presently world-leading, was led by myself in collaboration with the Weizmann Institute of Science. I am now a researcher at the School of Physics & Astronomy within the Dark Matter Direct Search group, along with Alex Murphy, James Dobson, Thomas Davidson and Maria Francesca Marzioni.
The University of Edinburgh has key roles within the LUX and LZ collaborations, which are the leading forces in dark matter research. The greater capability of the LUX detector means we can expect the quality of its results to exceed those of the XENON100 by a factor of five. And... perhaps discover the elusive axions!
It took 40 years to find the Higgs boson. The search for the axion has just begun and Edinburgh is leading the hunt for these fundamental, elusive particles.
Image gallery
School of Physics researchers will be part of an EU-wide extravaganza of discovery, debate and entertainment on Friday 26th September.
- PhD student Natasha Nicholson will be one of the performers at “Steampunk Science”, a sci-fi treatment of some of today’s most innovative research.
- The Soft Matter Group will give visitors a chance to see behind the scenes of JCMB in a Doors Open Day preview,
- EPCC will keep younger audiences entertained as part of Meet the Experts @ Our Dynamic Earth, with activities covering "Supercomputers to Dinosaurs".
- Victoria Martin and other members of the Particle Physics Experiment group will be at Meet the Experts @ The National Museum of Scotland from 1-4.30pm with activities on "The Higgs Boson and Beyond".
Further information
Find out more on the website: www.explorathon.co.uk
EU Researchers’ Night in Scotland is led by the University of Aberdeen, with partners at the universities of Edinburgh, Strathclyde and Glasgow. The programme is funded by the European Commission and will run until the end of 2015.
A collaboration between the School and Edinburgh College of Art has resulted in an award-winning photograph.
School lab managers Sarah Brown and Joe Rendle worked with Product Design student Joshua Notley to produce "Floral attraction: A world without bees".
The photograph won second prize in the Agri-Science category of the ‘Picture This’ competition run by Edinburgh Research and Innovation, which invited Edinburgh staff and students to submit images in a range of categories relating to the University’s research activities.
"The photograph was taken for Joshua Notley's course work for his studies at ECA in collaboration with technical support and editing from Joe and myself, so Josh is the creative one although we all have an interest in photography." Sarah Brown
"Floral attraction: A world without bees"
A Van de Graaf generator produces a strong electrostatic charge. This photograph illustrates the generator attracting a stream of pollen onto its metal dome. We propose an idea utilising similar technology for the collection of pollen from plants. In an environmental landscape where bee populations are dwindling the sustained pollination of plants should be considered. When the dome of the Van De Graaf generator is earthed the pollen is no longer held in place by the electrostatic force and falls from the surface, ready for collection and dissemination as required.
Image gallery
Researchers have found that the Milky Way is approximately half the weight of the neighbouring Andromeda galaxy.
For the first time, scientists have been able to precisely measure the mass of the galaxy that contains our solar system. They have found that the Milky Way is approximately half the weight of the neighbouring Andromeda galaxy, which has a similar structure to our own. The Milky Way and Andromeda are the two largest in a region of galaxies which astronomers call the Local Group.
A team of scientists led by the University of Edinburgh used recently-published data on the known distances between galaxies in the Local Group – as well as their velocities – to calculate the total masses of Andromeda and the Milky Way. Previous studies were only able to measure the mass enclosed within both galaxies’ inner regions. In this new study, researchers were also able to work out the mass of invisible matter found in the outer regions of both galaxies, and reveal their total weights. They say 90 per cent of both galaxies’ matter is invisible.
“We always suspected that Andromeda is more massive than the Milky Way, but weighting both galaxies simultaneously proved to be extremely challenging. Our study combined recent measurements of the relative motion between our galaxy and Andromeda with the largest catalogue of nearby galaxies ever compiled to make this possible.” Dr Jorge Peñarrubia, School of Physics and Astronomy, who led the study.
Scientists say that Andromeda’s extra weight must be present in the form of dark matter, a little-understood invisible substance which makes up most of the outer regions of galaxies. They estimate that Andromeda contains twice as much dark matter as the Milky Way, causing it to be twice as heavy.
Researchers say their work should help them learn more about how the outer regions of galaxies are structured. Although both galaxies appear to be of similar dimensions, until now scientists had been unable to prove which is larger.
The study, published in the journal Monthly Notices of the Royal Astronomical Society, was carried out in collaboration with the University of British Colombia, Carnegie Mellon University and NRC Herzberg Institute of Astrophysics. The work was supported by the UK’s Science and Technology Facilities Council. Findings from the study are supported by research led by Jonathan Diaz at the University of Cambridge, which used different data and methods and produced very similar results.