Beyond the Boson Graphene's 3D Counterparts Superconductivity

Autumn 2014, Number 5 Department of Physics Newsletter Beyond the boson The next steps for particle physics Superconductivity: Strike while the iron is hot Oxford researchers synthesise new iron based high temperature superconductor Graphene’s 3D counterparts Oxford researchers have discovered a new series of materials that are a 3D version of graphene ALUMNI STORIES EVENTS MOSELEY & X-RAYS PEOPLE Jean Chu (Holmes) reflects Alumni day at the Clarendon Part II of Prof Derek Stacey’s Five minutes with David Lloyd; on life as a female physics Laboratory; Bonn in Oxford; remarkable story celebrating Comings, Goings and Awards student at St Hugh’s, 1956–59 Alumni experience the LHC Henry Moseley’s work www.physics.ox.ac.uk SCIENCE NEWS SCIENCE NEWS www.physics.ox.ac.uk/research www.physics.ox.ac.uk/research in research labs, to steering oxides were shown to exhibit superconductivity the beams around particle at temperatures well above 100 Kelvin (a accelerators (such as the phenomenon which is still far from understood). SUPERCONDUCTIVITY: Large Hadron Collider at CERN) and confining the EXPLORING NEW PHYSICS STRIKE WHILE THE IRON IS HOT plasma in a fusion reactor. But perhaps their most Here in Oxford our current focus is on the newly important application has A little over a century after the discovery of quantum coherent state, a concept also applicable to discovered materials that contain iron. Some been in magnetic resonance superconductivity we are still missing some vital superfluids and Bose-Einstein condensates of cold of these have transition temperatures above imaging (MRI), the clues in understanding how and why the effect atoms. According to the BCS theory, lattice vibrations 50 Kelvin, respectable but not yet record breakers. extraordinary technique that comes about. Over the years, the most important induce an attractive interaction between electrons However, they are more ductile than oxides allows clinicians to examine and interesting superconducting materials have that then form ‘Cooper pairs’, and these pairs in turn and so may be easier to fashion into wires and salami slices through a living all been found by chance, and so the discovery of condense at low temperature into a superconducting tapes. Moreover, they seem to be more amenable © STEPHEN BLUNDELL & ANDREW BOOTHROYD & BLUNDELL STEPHEN © human body with very a new family of superconductors always generates state. The pair condensate is a coherent state of to theoretical analysis than the copper oxides Prof Stephen Blundell and Figure 1: Levitation by the Meissner high resolution and without harming the patient great excitement. That is exactly what has happened matter since, as for singers in a well-trained choir while veering away from the basic paradigm of Prof Andrew Boothroyd effect: a block of copper oxide in any way. This is made possible by placing the in the last few years following the unexpected performing an oratorio, the individual components BCS that describes the ‘traditional’ materials. superconductor floats above an patient inside a very homogeneous magnetic field discovery of superconductivity in a series of iron- are performing ‘in phase’ and lose their individual array of permanent magnets. of several Tesla, which would be impossible inside A consensus is emerging that the magnetic behaviour containing compounds. Iron is magnetic and, so it identity in the service of the joint enterprise. an electromagnet wound with copper wire because of iron in the new superconductors, rather than had been thought, was the last element you would the electrical current needed would be sufficient being detrimental, is actually a key component want in a superconductor because magnetism Oxford has been a major centre for superconductivity to melt the copper due to its electrical resistance. of the pairing interaction required to form the tends to break apart the electron pairs, which are research ever since Frederick Lindemann (later With a superconducting magnet there is no heating superconducting state. This contrasts with simple BCS the building blocks of the superconducting state. Lord Cherwell) attracted a number of brilliant émigré scientists from Germany in the 1930s. because the resistance is zero, and generation of superconductors, in which the pairing force derives Before describing these latest developments, let’s Fritz and Heinz London made the most important the field straightforward. If you ever have an MRI from vibrations of the crystal lattice, but bears some step back a bit. What is superconductivity? When theoretical contribution to the field in the first half scan, it’s intriguing to contemplate that your whole resemblance to a type of fluctuating magnetism found Kammerlingh Onnes discovered superconductivity of the twentieth century while working here, and the body will be encircled by a coherent quantum wave in the copper-oxide superconductors. However, there in mercury in 1911 the defining feature was the low-temperature physics of Kurt Mendelssohn and function, every bit as perfect and unchanging as that are differences too. In the copper oxides, the electrons vanishing of all electrical resistance below a critical Nicholas Kurti yielded valuable experimental data. describing an electron in an atomic shell! The MRI form a so-called ‘d-wave’ pairing state, which has temperature which, in the case of mercury, is It was Kurti’s technician, Martin Wood, who in the magnet business is a billion-dollar industry, and non-zero angular momentum and is analogous to four degrees above absolute zero (4 Kelvin). This 1960s commercialised the then emerging technology much of it is based in Oxfordshire as a direct result an atomic d orbital, whereas the pairs in the iron- lossless conductivity gave rise to the name: ‘super- of superconducting magnets and, together with his of work done in the Clarendon half a century ago. based superconductors seem to be predominantly in a type of s-wave state with zero angular momentum. conductor’. It means that an electrical current set up wife Audrey, founded Oxford Instruments, one of Below: Molecular structure Moreover, superconductivity is thought to derive in a loop of superconducting wire will continue to the world’s leading companies making cryogenic and of newly discovered iron- MATERIALS DISCOVERY – A HOT TOPIC from multiple iron orbitals. The theoretical challenge flow round and round forever, without need of any superconducting magnet systems. These magnets based superconductor If you ever have is to develop a multi-orbital model incorporating electrical power to keep it going. What Onnes had have had many uses, from leading-edge experiments Lix(NH2)y(NH3)1−yFe2Se2. an MRI scan, it’s So what is the current challenge? Well, the problem the electronic correlations needed to account found was a quantum mechanical ‘stationary state’, with superconductors is that they only work at intriguing to contemplate both for the superconducting pairing interaction like an electron orbiting the nucleus of an atom, low temperatures (usually less than 100 Kelvin), that your whole body will and the observed magnetic fluctuations. that does not need to be sustained by an external be encircled by a coherent and this is why they have not so far been used to driving force, but just exists in isolation, its internal quantum wave function, every transmit power along the national grid or why On the experimental side, scientists are searching for motion simply part of its own intrinsic nature. bit as perfect and unchanging levitating trains have not become all the rage for new iron-containing superconductors and exploring as that describing an HS2. However, there are a vast number of ways the link between their atomic arrangements and their PERFECT DIAMAGNETISM electron in an atomic to combine atoms to form different alloys and physical properties. Together with Simon Clarke shell. compounds, and imaginative chemical synthesis in the Inorganic Chemistry Laboratory in Oxford, We now think of the zero resistance property as a has led to all sorts of new surprises, which is why we have been attempting to make and study new secondary consequence of the eternal ‘coherent’ ‘materials discovery’ is a hot topic at the moment. iron-containing superconductors that incorporate nature of the superconducting state, which is more molecules and have recently found that this approach directly manifested through the complete expulsion of The physicist Wolfgang Pauli (famous for his turns iron selenide (FeSe, an 8 K superconductor) magnetic flux when a superconductor is cooled below Exclusion Principle) was famously sceptical of this into a new molecular iron-based superconductor its transition temperature, a phenomenon known approach and derided what he called ‘dirt physics’, with a critical temperature of 43 K (a picture of as the Meissner–Ochsenfeld effect. This ‘perfect maintaining that physicists should concentrate the molecular structure of this newly discovered diamagnetism’ is responsible for the counterintuitive on pure elements (in his day, impurities in material is shown opposite). This molecular route has ability of superconductors to levitate on the top of semiconductors were being shown to determine the potential to open up new avenues in materials permanent magnets and, even more remarkably, to properties; he didn’t like this, but of course ‘n’ and discovery and provides an alternative strategy hang beneath a magnet without any physical means ‘p’ doped semiconductors are fundamental to the for optimising superconducting

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