Resolving the open controversies on the anomalous #superconducting trends in metastable phases of #Phosphorus

Among elemental compounds, the high-pressure superconducting phase diagram of phosphorus is one of the most complex. In this work, we measured electrical resistivity and performed ab initio superconductivity calculations in order to solve, for the first time the open controversies on the anomalous superconducting trends. Our work forms on a single picture a consistent scenario of multiple metastable structures which coexist beyond their thermodynamical stability range.

These metastable structures exhibit critical temperatures, which are  distinctively higher than the putative ground-state structures, suggesting that the selective stabilization of metastable phases represents a viable strategy to improve superconductivity properties on conventional superconductors.

This work is just highlighted this month ad an Editor’s Suggestion and published in Phy. Rev. Materials. (see my Publications)

 

Layered binaries as candidates for hard-magnets

For the most recent work on hard-magnetic systems we focused in binaries stacked layers of FePt, MnAl and MnGa. In this work an enhancement of the  mangetocrystalline  anisotropy was calculated for specially stacked structures. After a long search and great effort of the wonderful team of collaborators (special thanks and all the credit goes to my  friend Yu Ichiro Matsuchita) you can read now this research published in Annalen der Physik (link).

Piz Daint 3rd fastest supercomputer

The supercomputer ranking published on 19 June 2017, places Switzerland’s 19.6 petaflop Piz Daint supercomputer third in the world after Sunway TaihuLight and Tianhe 2, two Chinese supercomputers. Piz Daint’s recent upgrades allowed it to climb five positions up the ranking.

With a performance of 93 petaflops, China’s TaihuLight is by far the most powerful number-cruncher on the planet. Tianhe-2, which translates to Milky Way-2, comes in second at 33.9 petaflops, losing its number one spot in June 2016.

The Piz Daint computer, run by the Swiss National Supercomputing Centre (CSCS) is located the commune of Manno near Lugano. Named after a peak in the Alps, it is the most powerful computer in Europe. The monster computer is used by Switzerland’s weather service for climate modelling, the Swiss Institute of Particle Physics, the Human Brain Project and numerous others.

CSCS was created in 1985 (what a coincidence! )  after the Swiss government decided the country needed to invest in computing. The CSCS computing centre uses as much electricity every day as a small town. About a third of this electricity is used for cooling – computers get hot and must be cooled otherwise they melt. Piz Daint is cooled with up to 760 litres of water per second from nearby Lake Lugano. Using cool water from the lake significantly reduces overall electricity consumption. The water, taken 45m down is around 6 degrees. For ecological reasons, the water returning to the lake must never be over 25 degrees.

This is the first time since 1996, when three Japanese supercomputers captured the top three spots, that the United States has failed to secure a top-three position. The US still claims five of the top ten supercomputers, more than any other nation.

Thanks to this computer and the grant for our project, in following months we will have interesting results on different classes of materials.

The record of running cores for my calculations in Daint is 32,000 computing cores.

Cornell 2017: Hoffmann, Ashcroft, Mermin

During my days in Ithaca (Cornell University), I had the great opportunity to show part of my research work. I decided to present the research I had just finished at that time about the possibility to induce a metallic state in ice under pressure and upon doping; thinking that I could attract more attention and get more input for my research.

Lucky me, that week Prof. Hemley was visiting Cornell and could attend my talk. Expert in high-pressure research. The talk was in a very informal and illustrative way, conveying, I believe the main message successfully. Next day, I received an email from Prof. Hoffmann, suggesting that “would be good to meet”. After some email exchanges, we agreed on a date.

Next day, nervous and with shaking hands, I reach his office punctual within a second precision. After briefly introduce myself, soon I realized that he knew everything and every single point from my project, he was inquiring questions, precise points that at the time weren’t clear to me. He is a very sharp person that have distilled knowledge over the years that simply with hand-gestures can explain complex concepts, in this case, bonding and high-pressure chemistry at the current state of the art.

I do not know really to whom I should thank, maybe destiny or the serendipity path (a complex set of taken decisions) that put me there, that day in that office. I had a great and unique chance to meet a Nobel Laureate. I received first-hand criticism of my work and suggestions to improve, more ideas and the motivation to keep what I am doing. More than 60 minutes of scientific discussion and interchange of ideas. In the end, the discussion started to slightly shift towards a more general topic, Science, times to be scientific, old times, and at the very last elegantly he asked me from where it comes to my accent. He has been several times in Mexico. Indeed I could verify that: beautiful pieces of art (distinguishable for the colours) hangs in his office (perhaps the biggest office I know to date). I would like to share the picture of this nice event that definitive scratch a mark in my scientific path.

 

This text was written as a post-analysis when cruising from Labadee to Puerto Rico, SMEC conference 2017.

Computing allocation in CSCS granted for my project !

Our first computing grant was just accepted by the CSCS!

Last year I submitted a project requesting for 700 thousand computing nodes hours to run an exploration over hundreds of molecules. These systems are candidates to test under pressure, and upon doping for the potential to become high temperature superconductors. Last march 24th (2017) our request was accepted and the entire allocation was granted for a period of two years.

The computer is Piz Daint, the fastest supercomputer in Europe and 8 ranked world wide. This supercomputer is hosted by the Swiss National Supercomputing Centre(Italian: Centro Svizzero di Calcolo Scientifico; CSCS) which is the national high-performance computing centre of Switzerland  (Lugano-Cornaredo).

The interest of this project is double-fold goal, not only we aim to elucidate which systems are the best candidates to be synthesized and achieve room temperature superconductivity, but the throughout of the investigation will generate data that will be used to train machines to learn the “physics behind”. The second part of  this research is conducted by the expertise of our collaborators from the Chemical department, next door here in the University of Basel.

Stay tuned! coming months surely we will have exciting news !

The elephant in the room of density-functional theory calculations

While basis set convergence sounds straightforward (though time-consuming) it is hard to rule out that underlying assumptions in  the design of the basis set influences the results.  However, converged basis set DFT results are needed to separate basis set errors from errors due to the functional. Multiwavelets, a systematic and adaptive multiresolution numerical solution of the one-electron problem, is now the basis set that can reach the highest precision.
Together with our collaborators in Norway and U.S.A, we show in our recent paper that LDA, PBE and PBE0 total energies, atomization energies, and dipoles moments for more than 211 molecules that are converged with respect to basis set to μHartree accuracy. Furthermore these numbers were compared to other basis set such as Gaussian-type orbitals (GTOs), all-electron numeric atom-centered orbitals (NAOs) and full-potential augmented plane wave (APW) calculations. In the case of atomization energies, a quintuple GTO basis set (aug-cc-pV5Z) is needed to reach a 1 kcal/mol accuracy in both MAE and RMSE. For aug-cc-pVQZ the MAE is below 1 kcal/mol, but the RMSE is about 1.5 kcal/mol.  Perhaps more importantly, the maxAE goes from ca 10 to 2-5 kcal/mol on going from quadruple to pentuple basis set.  So even aug-cc-pV5Z cannot consistently reach the basis set limit for atomization energies!  This research has been just published in J. Phys. Chem. Lett. (link).

H2O ice as superconductor? yes, superconducting water !

We have recently investigated the possibility of achieve high-temperature superconductivity in hydrides under pressure by inducing metallization of otherwise insulating phases through doping, a path previously used to render standard semiconductors superconducting at ambient pressure. Following this idea, we study H_2O, which is one of the most abundant and well-studied substances in the universe! We identify nitrogen as the most likely and promising substitution/dopant. We show that for realistic levels of doping of a few percent, the phase X of ice becomes superconducting with a critical temperature of about 60 K at 150 GPa. In view of the vast number of hydrides that are strongly covalent bonded, but that remain insulating until rather large pressures.  This could open new search paths in the quest towards the room-temperature superconductivity. Link to acces to the arXiv on-line version.

structures

The image shows the water molecule at ambient condition of pressure, at high pressure and low temperature. The crystal of water transform to a symmetric proton phase above 70 GPa. We use this phase, which is covalently bonded with to hole dope it with nitrogen at different concentrations.

Lindau Nobel Laureate Physics Meeting 2016

Probably one of the most excited week of my life. For many reasons and in many contexts that week in Lindau was simply: magical. After the semester ends, and with new projects in hands, with two nice papers ready to submit, summer that knocks the door, and personally ready to turn on the page.  All these ingredients were put in a big pot: Lindau together with nice weather, superb small Bavarian town and hosted by a wonderful local family. I did good friends and met super bad-ass smart people. I had keratosis pilaris (or chicken skin) when I saw for the first time 29 Nobels just there in front (couple of meters). Ensemble of the Vienna Philharmonic Orchestra play intro and a repertoire of nice melodies. An <speechless> speech by the President of Austria who shared his memories with Austrian scientists who were close friends.

All these memories I will take it with me, as much as I can and as a long as I can. Those feelings are special, not only provide further understanding of how I work, but also how humans do. We are such “complex machines” with intricate thoughts, outburst of anger, rush of tenderness, spams of fears yet banal, at the time sophisticated.

For over a week I scout patiently and listen to each of the Nobels. Carefully selected my questions and privately or publicly asked. I would like to share some of the pictures I took either with Nobel laurates or with good friend I had the pleasure to met.

 

Panel Discussion: Glimpses Beyond the Standard Model with Professors Chu, Gross, Kajita, Rubbia, CERN experts (live video) and moderator F. Pauss.

 

Keynote of Prof. Roy J. Glauber

 

Personal archive (photos) Prof. Glauber. Left: Pauli (Switzerland playing football) Right: Heisenberg and Pauli looking at top spinning.

 

Left: Prof. Kajita (we had dinner together).  Prof. Josephson,  and bottom: Prof. Deisenhofer (we also had dinner together).

 

Left: Marie-Curie delegation preselected by the European Union. Right: young scientist from India delegation.

First Superhydrides meeting in Rome

In past May (2016) I attended the first edition of the International Workshop in “Superhydrides  Towards Room Temperature Superconductivity: Hydrides and More” . This was a very important meeting because it gathers the world-wide experts (experimentalists and theoreticians) in this matter. The urgent of the meeting was motivated by the 200 K superconductivity discovered only few months ago.

The venue was the CNR headquarters in Rome. Needless to says that I have been never disappointed by the quality of the food, and the Italian lifestyle, besides that I could  have the chance to practice my Italian.   I have had great discussions with many good friends also attending the workshop and I also had the opportunity to show my latest results. The great outcome is that now I have a nice collaboration with a world-leading experimentalist group in superhydrides. Stay tuned for, hopefully nice and exciting results !

Superhydrides meeting in Rome

 

International collaboration demonstrates reliability of quantum simulations for materials design

An international collaboration with colleagues from over 30 universities and institutes to investigated to what extent quantum simulations of material properties agree when they are performed by different researchers and with different software. Thanks to an online collaboration, they successfully demonstrated that the most recent generations of codes agree well, in contrast to earlier generations. Their study appeared in week’s edition of Science.

Reproducibility does not come easily

It’s a corner stone of science: independent yet identical experiments should produce identical results. Only in this way can science identify ‘laws’, which lead to new insight and new technologies. However, several recent studies have pointed out that such reproducibility does not always come spontaneously. In scientific areas as diverse as psychology research and genetic research, cases were identified where repeating previous experiments led to very different results. Even predictions by computer codes require caution, since the way in which theoretical models are implemented may affect simulation results. This is a reason for concern in any field of research that critically depends on computer simulations. For the study and design of materials, for instance, there are several independent software packages available based on quantum physics. They are moreover being used increasingly often in automated procedures with limited human supervision. It is therefore essential to know to what extent predicted materials properties depend on the code that has been used.

Online collaboration brings experts together

Despite the need for reliable property predictions of materials, the reproducibility of quantum simulations had not been investigated systematically before. This is mainly because there is no single person sufficiently skilled in all existing codes. Scientists from the University of Basel and Max-Planck Institute for Micstructructure Physics therefore joined forces with more than 60 colleagues, bringing together the know-how of over 30 prominent institutions. The researchers investigated 40 different methods to describe the influence of pressure in 71 different crystals. Among these methods was the ELK, Abnit and BigDFT codes. The team can now demonstrate that, although a few of the older methods clearly yield deviating results, predictions by recent codes are entirely equivalent. They moreover define a quality criterion that allows the verification of future software developments against their extensive database. New test data are continuously added to a publicly available website (http://molmod.ugent.be/DeltaCodesDFT).

The researchers involved hope that their work will  contribute to higher standards for materials property simulations, and that it will facilitate the development of improved simulation codes and methods: essential for the intelligent design of novel materials to address society’s challenges in the 21st Century. 

I have had the great opportunity to be part of this work “Reproducibility in density functional theory calculations of solids” which is now published in Science. If you need access to the paper do not hesitate.

Google’s neural network beats humans at the ancient game of Go

A neural net system built by Google has beaten the European champion in the Chinese game of Go, winning five out of five games and crossing a new threshold for machine intelligence.
In a recent paper published in Nature,  DeepMind researchers revealed how the system was constructed and how it was able to succeed where decades of previous Go systems have failed. Go has long been considered one of the hardest games to automate, making the new DeepMind system particularly interesting for artificial intelligence researchers. The game of Go has long been viewed as the most challenging of classic games for artificial intelligence owing to its enormous search space and the difficulty of evaluating board positions and moves. Here we introduce a new approach to computer Go that uses ‘value networks’ to evaluate board positions and ‘policy networks’ to select moves.

They used a neural networks trained by combinations of supervised learning from human expert games, and reinforcement learning from games of self-play. “Without any lookahead search, the neural networks play Go at the level of state-of-the-art Monte Carlo tree search programs that simulate thousands of random games of self-play”, they said.

New hints on the metallization of Hydrogen

Recently scientist from the Edinburgh’s Centre for Science at Extreme Conditions  have come close to creating a long-sought new state for hydrogen. The group used a set-up called a diamond anvil cell to compress its sample of molecular hydrogen. This apparatus is essentially two gems that have been placed in opposition to each other. Their polished tips, comparable in size to the width of a human hair, are made to press into a cavity containing the sample.

In their experiments, the scientists are able to achieve in excess of 350 gigapascals (3.5 million atmospheres) at room temperature. These pressures are not dissimilar to what would be experienced at the centre of the Earth. The big squeeze on the molecules of hydrogen gas turns them first into a liquid and then into a solid.

blue_liquid_metal

As the pressure gets ever more intense, the atoms in the hydrogen molecules pack closer and closer together,
and the electrical conductivity in the crystalline material increases.
Ultimately, the hydrogen atoms should stack so efficiently that their electrons become shared -just as in a metal. However, the team does not quite see this phase, but rather something that is probably just short of it. The work puts new constraints on where the full metallic hydrogen phase might exist:  possibly below 450 gigapascals at room temperature.

The ambient temperature is very significant, because if metallic hydrogen can ultimately be produced this way it opens the door potentially to a new type of perfect (zero resistance) conductor – a material to boost the performance of next-generation computers. “It’s been predicted that metallic hydrogen could be a room-temperature superconductor, which is still yet to be achieved with any material…
Scientists are also fascinated by metallic hydrogen because they think it may account for a large fraction of the internal composition of planets such as Jupiter.
Link to the original work published in Nature (letter) on January 7th, 2016.
BBC reportage.

 

Yet another superconductor above 100 K ?

Following the recent discovery of very high temperature conventional superconductivity in sulfur hydride (critical temperature Tc of 203 K). Experimentalist have shown results on the covalent hydride phosphine (PH3) which also exhibits a high Tc > 100 K at pressure P > 200 GPa. However, neither the exact composition nor the crystal structure of the superconducting phase have been conclusively determined. In our latest work (December 7, 2015) the phase diagram of P-H was extensively explored by means of ab initio crystal structure predictions. The results did not supported the existence of thermodynamically stable PHx compounds, which exhibit a tendency for elemental decomposition at high pressure even when vibrational contributions to the free energies are taken into account. Although the lowest energy phases of PH1,2,3 display Tc’s comparable to experiments, it remains uncertain if the measured values of Tc can be fully attributed to a phase-pure compound of PH. Link to our work here. This work is featured as en Editor’s Suggestion published in  Rapid. Comm. Phys Rev. B (2016).

Structures_ELF

Superconductivity under high pressure at minus 70 degrees Celsius

Researchers at the Max Planck Institute for Chemistry in Mainz and the Johannes Gutenberg University Mainz observed that hydrogen sulfide becomes superconductive at minus 70 degree Celsius —when the substance is placed under a pressure of 1.5 million bar. This corresponds to half of the pressure of the earth’s core. With their high pressure experiments the researchers in Mainz have thus not only set a new record for superconductivity— their findings have also highlighted a potential new way to transport current at room temperature with no loss.

The work was published in Nature Materials Journal (link) on August 17, 2015. The first original results were released in Cond. Mat. (arxiv) back on December 2014.

fig2

Soon after the discovery in the H-S system, we predicted another system with fairly similar characteristics: transition temperatures of the order of 100 K could be reached in principle in H-Se system at lower pressures. This research is publicly available in arxiv and it was first show on 26 January 2014.

Superconductivity in group-IV honeycomb materials

Our work has just been published in Physical Review B  !

disilicidesIn this theoretical work, we studied in detail the electron-phonon superconductivity of honeycomb MX_2 layered structures where X is one element of group IV (C, Si, or Ge) and M is an alkali or an alkaline-earth metal.  Among the studied compositions we predict a novel compounds energetically stable and showing relatively high transition temperatures of superconductivity:  7 K in RbGe_2 , 9 K in RbSi_2, and 11 K in SrC_2. All these compounds feature a strongly anisotropic superconducting gap. Our results show that despite the different doping levels and structural properties, the three families of materials fall into a similar description of their superconducting behavior. This allows us to estimate an upper critical temperature of about 20 K for the class of intercalated group-IV structures, including intercalated graphite and doped graphene.

 

Thesis defense

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Bonjour a tous! Je suis très content de pouvoir inviter toutes les personnes intéressantes que j’ai rencontré ici, à Lyon et de manière générale en France à deux événement très importants dans ma vie. D’abord, la soutenance publique de ma thèse, et deuxièmement le fameux pot de thèse. La soutenance de thèse se dérouleront dans la salle de conférence de la bibliothèque universitaire de Lyon 1 sur le campus de la DOUA le mardi 18 septembre 2012 à 14:00 heures. Et après, le pot de thèse (si tout se passe comme il faut) sera dans un autre bâtiment vers, 16:30. Merci a la Pequetshina!

Kick-off ! Fysik-Aztek

website-edit-2

Kick-off ! I am delighted to welcome you to my webpage.

Officially opened on Monday 27 august of 2012 at 09:00 local time (Paris).

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