Mathieu Le Tacon presented with 2014 Young Scientist Award
Along with fellow physicists Giacomo Ghiringhelli and Lucio Braicovich from the Politecnico di Milano, Le Tacon took some of the key measurements for this research on the ESRF’s ID08 beamline. Here they produced evidence for the long sought after new phase, the charge density wave.
In order to gain some insights about the mechanism leading to the formation of the new phase, the team studied the behaviour of phonons on ESRF’s ID28 beamline. It allowed them to unravel the largest signature of electron-phonon interaction in cuprates and to conclude, among other things, that it cannot account for high temperature superconductivity.
This work earned Le Tacon and his colleagues a publication in Nature Physics.
Since 2012, he has co-authored 8 publications directly related to the charge density waves.
“More than 27 years after the discovery of the high Tc cuprate superconductors, we still don’t have a clear theoretical view of the mechanism”, says Yves Petroff, former Director General of the ESRF. “However, the first step is to understand the charge, spin and orbital correlations in the normal state. Mathieu Le Tacon has made recently two very important contributions. Using hard X-ray inelastic scattering, he has shown clearly that the electron–phonon interaction does not contribute significantly to the superconductivity: this has been a very controversial topic since the discovery of the high Tc. With resonant soft X-ray scattering, he also demonstrates that an incipient charge density wave instability exists and competes with the superconductivity”.
Why are cuprates so interesting?
Cuprate high temperature superconductors (or High Tc superconductors) were discovered more than 20 years ago and figure among the most attractive subjects studied across all fields of research. They are known as the “kings” of the superconductors, however the origin of their superconductivity has not been found yet and remains one of the most important and difficult mysteries of modern physics. The possible applications for these materials are extensive ranging from magnetic resonance imaging (MRI) of soft tissue to magnets for particle accelerators to fast digital circuits.
These materials do not cease to fascinate the scientific community. Very small changes in their chemical composition give rise to completely different physical properties, for example they can switch from being insulators to superconductors. Unlike conventional metals, this is due to the fact that the parameters that favour an insulating or a metallic behaviour are of the same amplitude. Under such conditions, every small detail counts. One of these ‘details’ is the interaction of the electrons with the excitations of the crystal lattice, the phonons (the quantum mechanical analogues of sound waves propagating in solids). This interaction is responsible for superconductivity in most of the superconductors that were known until 1986, but its role in the physics of cuprates, and in particular whether or not it plays a role in superconductivity, remains highly controversial. This is partly due to the chemical complexity of these materials.
Two years ago, using state of the art synchrotron radiation techniques, Mathieu, working with Ghiringhelli and Braicovich, found direct evidence for a new type of electronic phase of cuprates. This phase is known as a charge density wave and it competes directly with superconductivity. This phase had been sought without success for many years; it had been evidenced from NMR studies, but only in a very large magnetic field.
Mathieu Le Tacon (front) and Alexeï Bosak (back) mount a sample on ESRF beamline ID28. Credit: ESRF/Blascha Faust
As the electron-phonon interaction is usually involved in the formation of charge density waves in other compounds, the group immediately thought that studying it would provide useful insight. However, these studies are complicated due to the complex crystal structures of cuprates. The main difficulty is to determine where to search for phonon anomalies. This difficulty was circumvented using a two-step approach developed at the ESRF by Alexey Bosak and Michael Krisch on beamline ID28. Using a super-sensitive detector that had previously been used for protein diffraction, Bosak made a so-called ‘diffuse scattering’ map of the crystal on beamline ID29 that immediately allowed the team to identify the wavelength at which the phonons coupled with the electrons. “This turned out to be in an extremely narrow region of the reciprocal space and can be likened to finding a needle in a haystack!” says Le Tacon.
The behaviour of the phonons was studied across the various phases (charge density wave, superconductivity). This allowed the team to conclude that the electron-phonon cannot account for high temperature superconductivity.
Mathieu Le Tacon is currently working at the Max Planck Institute in Stuttgart, in the team of Bernhard Keimer. On top of cuprates, he studies oxides from other transition metals such as iron, manganese or nickel in search of new superconductors and interesting phenomena.
Captured by physics at an early age
Mathieu grew up in Brittany, France and remembers being fascinated as an 8-year old when a friend showed him two equations on a piece of paper: H2O and E=mc2, then said “these are water and energy”. That was the decisive moment that drew him to science although he admits that ‘exciting’ was rarely an adjective he used for the physics lectures.
Mathieu first came to the ESRF in 2007 originally for the love of his wife who had accepted a job offer in Paris. Although almost 600 kilometres distance Grenoble from Paris, the PostDoc position on the ESRF’s ID28 beamline was tempting enough to justify the extra travelling. “I was really impressed the first time I came to the ESRF. I’d never worked on a synchrotron before and at the ESRF I met fantastic people and really enjoyed the working atmosphere.”
Organisation of the Young Scientist Award
Each year since 1995, the Young Scientist Award (YSA) is presented to a scientist aged 35 or under in recognition of outstanding work carried out at the ESRF. The ESRF Users Organisation chooses a chairperson for the YSA. The chairperson then forms a selection committee composed of distinguished scientists whose expertise covers the most important areas of synchrotron science. The panel calls for nominations from institutes around the world and evaluates nominees on the basis of the following criteria:
• Significant and personal contribution to either a novel method or technique, or to the advancement of a particular field based on ESRF measurements
• Quality and quantity of publications, conference contributions and responsibilities
• Importance of the specific field for synchrotron science research
For the 2014 YSA the selection committee was chaired by Sine Larsen, former ESRF Director of Research and currently senior advisor at the Max laboratory, University of Lund, Sweden. The other two members were Bill Hunter from the College of Life Sciences, University of Dundee, Scotland and Gerhard Gruebel, former ESRF scientist and currently head of the Institute of Laser Physics at DESY, Germany.