Selected as an Emerging leader by the Journal of Physics: Condensed Matter

I am very honoured to be selected as one of the Emerging leaders by the Journal of Physics: Condensed Matter in its Edition of 2019

Accordingly to the guidance, “Exceptional candidates, top research on different fields in Condensed Matter Physics, with a prospect career are nominated by the editorial board of the Journal”. The following publication belongs to the special edition of 2019, “Crystal structure prediction of magnetic materials”.  

https://iopscience.iop.org/journal/0953-8984/page/EmergingLeaders2019

My research was also featured by the PhysicsWorld and feature an interview, you can access it here

What was the motivation for the research?

The motivation was to develop a computational algorithm based on structure prediction methods for a class of materials that have been poorly studied by the community due to their complexity: magnetic materials.

These are a broad set of materials characterized by the presence of an electron spin-degree of freedom. Within this general classification of magnetic materials, there are two large subdivisions: soft and hard magnetic materials. Soft magnetic materials do not stay magnetized, while hard magnetic materials remain magnetized. Hard magnets (or permanent magnets) are essential components in modern technologies, used in many electrical and electronic devices from computers, appliances to medical equipment. But they are also crucial in emerging applications, for instance; in electric vehicles and wind turbines.

The problem is that we depend primarily on three classes of permanent magnets. One of the aims of the work was to further gain insights into the delicate balance between the crystalline structure of permanent magnets and their magnetic properties.

What did you do in the work?

Our research consisted of combining the minima hopping method (for structure prediction) with state-of-the-art first-principles magnetic calculations to scan for potentially new types of permanent magnets.

We faced many technical constraints. The size of simulation cells, convergence problems, and the complexity of the materials was always a significant restraint. This was a computational/theoretical work in which we had to make several approximations and consider simple, well-known cases, gradually working up to the most challenging magnetic materials. One innovative part of the work is how we combined two-level (spin and spin-unpolarized) calculations to overcome the demanding computational overhead. Another point that makes the work interesting for other researches is how it was automated, which provides reliable results within the same theoretical footing for a large number of magnetic systems. In this research, we investigated binary phases made of 3d-transition metals, such as FeNi, FeCo, FeMn and FeCr.

What was the most interesting and/or important finding?

One of the most striking results, apart from the developed computational machinery, was the fact that we could predict an exciting phase of FeNi. There has been much speculation about the tetrataenite phase of FeNi showing important capacities as a hard magnet. However, this phase is only found in meteorites, and experimentally it has been elusive to synthesize in laboratories.

In this work, we report a crystalline structure that has lower energy that the tetrataenite phase and has a saturation magnetization (Ms) of 1.2 M A/m and a magnetic anisotropy energy (MAE) above 1200 k J/m3. This compares with the state-of-the-art hard magnet Nd2Fe14B (Ms of 1.28 M A/m and MAE of 4900 k J/m3).

Theoretically, this system could be a good candidate for permanent magnet applications, especially considering it is free of rare-earth metals and made of abundant elements. Thus, the outcome of our research is appealing for experimental colleagues to explore further this low energy polymorph of FeNi.

Why is this research significant?

The research is significant because it shows it is possible to “access” magnetic materials from the computational point of view. There is a misconception in the community that first-principles calculations fail entirely to describe these systems. While this is well-founded for a specific type of interaction (strongly correlated systems) there are other types of systems that are magnetic and can be accurately described using Kohn-Sham density functional theory.

We hope that this work will further ignite research in magnetic materials, from theory to computational developments to further experimental investigations. An essential part of this research is the transferability of the computational methodology to other types of applications. In future, we foresee the study of topological materials given rise to magnetic anisotropy energies.

What do you plan to do next?

As mentioned before, the study of topological materials and magnetism promises a vast niche for discovering exciting phenomena. However, in the short term, what we plan next is to extend our study to materials showing anti-ferromagnetism.

However, a series of developments must be conducted before reaching that stage. For instance, we need to find a way to reduce the computational overhead further and find a smart idea for initializing antiferromagnetic solutions without the use of large supercells.

This is the next step of our research, and we hope soon to approach this challenging problem.

Scientific visit to Japan spring-2020!

During February (2020), I visited various top-research institutes in Japan. I gave three seminars and one small crash-course on computational methods for structure prediction.

I visited Prof. Ishiwata’s group in Osaka, Prof. Arita’s at The University of Tokyo, Hongo campus and RIKEN in Saitama and Dr’s Tadano group at NIMS in Tsukuba.

This was my third visit to Japan, the longest of all. I enjoyed so much my time there but also wanted to come back to continue implementing the new ideas. I am very sure in coming months the fruitful collaboration will solidify in amazing results. I compile some pictures of my trip:

Quantum fluctuations stabilize high-temperature superconductors

Supercomputers are indispensable in the research of superconductors, materials in which electricity flows without loss. Using simulations on CSCS supercomputer ‘Piz Daint’, an international team of researchers has now shown that physics has not been sufficiently considered in the study of superconductors made of so-called hydrides.

February 6, 2020 – by Simone Ulmer  (This post appeared originally published in CSCS website: here)

The physicist Heike Kamerlingh Onnes discovered in 1911 that the electrical resistance of mercury disappears when it is cooled to a few kelvins (-268.95°Celsius). This discovery marks the opening of the superconductivity research field. Although the field has since set milestones, for example in explaining the phenomena of superconductivity and finding possible candidates for superconductivity at a higher temperature, the holy grail of superconductivity at room temperature has not yet been found. Even though research has increased the temperature at which superconductivity can be observed by more than 200 kelvins, scientists are still searching intensively for materials that allow current to flow at room temperature without losses.

Promising hydrides

Caption: At the centre image, highly symmetric hydrogen cages enclose the lanthanum ions. The top sketch depicts a complex classical energy landscape that would correspond to distorted hydrogen cages. The bottom illustrates a wholly reshaped, much simpler, quantum energy landscape with a single structure: the Fm-3m phase of LaH10 at 250 K superconductor. (Image: José A. Flores-Livas)

More than 100 years after the ground-breaking discovery by Onnes, computer simulations are essential in the search of high-temperature superconductors, as simulations can provide estimates of promising materials. This saves experimental researchers time and resources. In recent years, the so-called hydrides — a composition of hydrogen with other elements — gave new hope to high-temperature superconductivity. Hundreds of stable hydrogen-rich compounds were predicted using modern ab initio calculations based on density functional theory (DFT).

In 2017, theoretical predictions pointed out that lanthanum and hydrogen (LaH10) could be a promising candidate for superconductors near room temperature. However, under pressures that are 230 million times higher than those of the atmosphere, 230 gigapascals, lanthanum and hydrogen should arrange themselves into a highly symmetrical crystal, which makes superconductivity possible. Experimentally, researchers succeeded in 2019 in producing this superconducting compound of lanthanum at a pressure of “only” 130 gigapascals, and superconductivity surprisingly took place at a temperature of 250 kelvin (- 23°Celsius) — this corresponds to the temperature of a conventional freezer.

An international research group led by José A. Flores-Livas, recently appointed Assistant Professor at the Department of Physics at the Sapienza University of Rome, has now clarified the discrepancy between theory and experiment using the supercomputer ‘Piz Daint’ at CSCS. The results of the study are published today in the scientific journal Nature [1].

For the first time, the scientists considered so-called protonic quantum fluctuations in the optimization of the crystalline structure. Quantum mechanics governs the motion of the electrons and nuclei; usually, for the latter, effects are not considered due to its complexity.  Introducing quantum mechanical fluctuations into the calculations — such as the protonic quantum fluctuation (associated with the energy of the nuclei) and its effect on the zero-point energy, the lowest possible energy that a quantum mechanical system may have — turn out to be essential to describe these types of superconductors accurately.

The simulation showed that protonic quantum fluctuations ‘hold’ the symmetric structure of LaH10 in the observed experimental pressure range. “The calculations show that if the nuclei of atoms are treated as classical particles, that is, as simple points in space, many distortions of the structure tend to lower the energy of the system”, Flores-Livas says. This means that the classical energy landscape of the simulated LaH10-crystal is very complex, with many local energy minima. “It looks like a highly deformed mattress, as though many people are standing on it.”

Superconducting critical temperature of LaH10: calculated versus the experimentally reported as a function of pressure. (Image: José A. Flores-Livas)

Quantum fluctuations stabilize crystal structures

When the researchers treated the atoms (nuclei and electrons) like quantum objects, which are described with a delocalized wave function, the energy landscape completely reshapes and only one minimum is evident. “In an illustrative way, quantum effects get rid of everybody on the mattress, and only one foot remains to deform the mattress”, Flores-Livas explains. And exactly this deformation in the high-dimensional energy landscape corresponds to the highly symmetric structure of LaH10.

Furthermore, the estimations of the critical temperature — where superconductivity took place — using the new quantum energy landscape agree with the experimental evidence. This additionally supports the high-symmetry structure as responsible for the superconducting record. “The results demonstrate how protonic quantum fluctuations can stabilize crystal structures even at more than 100 gigapascals below their classical instability pressure”, Flores-Livas emphasizes. “Quantum effects stabilize crystal structures with substantial superconducting temperatures that would otherwise be unstable.” The scientists are convinced that these results give new hope for the discovery of high-temperature superconducting hydrogen compounds at much lower pressures than expected, maybe even approaching the ambient pressure.

Significant consequences for further research

The results call into question predictions of crystal structures of hydrides that were made without considering quantum fluctuations — this means in figures presumably from more than 100 publications, Flores-Livas says [2]. In addition, it could mean a setback for some experimental research work, which has tried to synthesise high-temperature superconductors based on classical simulation results. However, the improved simulations also present a challenge: “The price to be paid to ‘add’ the quantum mechanical fluctuation into the calculations is still elevated and only affordable for relatively small atomic systems and for a subset of pressures”, Flores-Livas says. “Our study would not be possible without the use of a supercomputer such as ‘Piz Daint’”. He sees a need for action in the improvement of algorithms, as well as further assumptions or different approximations to circumvent the computational overhead. “Protonic quantum fluctuations, anharmonicity and adiabaticity, among other phenomena, are also present at ambient conditions of pressure and temperature, and for many systems.”, Flores-Livas says. Therefore, one can argue that all compounds composed of hydrogen will suffer in a more or less dramatic way the consequences of quantum mechanical fluctuations, even at ambient pressure.

Reference:

[1] Errea I at al.: Quantum crystal structure in the 250-kelvin superconducting lanthanum hydride Nature (2019), 578, 66-69; DOI: 10.1038/s41586-020-1955-z

[2] Flores-Livas, J. A. et al.: A Perspective on Conventional High-Temperature Superconductors at High Pressure: Methods and Materials. Review on Physics Reports 2020 (ArXiv version).

Our most recent article is published in Nature!

Together with collaborators in Spain (Prof. Errea and his team), Italy (Prof. Mauri and his team), France (Prof. Calandra), Germany (Dr Sanna), and Japan (Prof. Arita, Prof. Koretsune and Prof. Tadano) we have shown, in our most recent publication that the crystal structure of the record superconducting LaH10 compound is stabilised by nuclear quantum fluctuations. This result suggests that superconductivity approaching room temperature may be possible in hydrogen-rich compounds at much lower pressures than previously expected with classical calculations!

 

The prestigious journal of Nature publishes the results on Feb. 5, 2020. You can see the article here:  https://rdcu.be/b1fCK. 

The possibility of high-temperature superconductivity in LaH10, a super-hydride formed by lanthanum and hydrogen, was anticipated by crystal structure predictions back in 2017. These calculations suggested that above 230 gigapascals a highly symmetric LaH10 compound (Fm-3m space group), with a hydrogen cage enclosing the lanthanum atoms (see figure), would be formed. It was calculated that this structure would distort at lower pressures, breaking the highly symmetric pattern. However, experiments performed in 2019 were able to synthesise the highly symmetric compound at much lower pressures, from 130 to 220 gigapascals, and measure superconductivity around 250 kelvin. The crystal structure of the record superconductor, and thus its superconductivity, remained not entirely clear. Now, thanks to our results, we know that atomic quantum fluctuations “hold” the symmetric structure of LaH10 in the experimental pressure range (in which superconductivity has been observed).

More in detail, the calculations show that if atoms are treated as classical particles, that is, as simple points in space, many distortions of the structure tend to lower the energy of the system. This means that the classical energy landscape is very complex, with many local minima, like a highly deformed mattress because many people are standing on it. However, when atoms (nuclei and electrons) are treated like quantum objects, which are described with a delocalized wave function, the energy landscape is completely reshaped: only one minimum is evident, which corresponds to the highly symmetric Fm-3m structure. Somehow, quantum effects get rid of everybody in the mattress, but only one-foot person’s remain deforming the mattress. 

Furthermore, the estimations of the critical temperature using the quantum energy landscape agree satisfactorily with the experimental evidence. This additional supported the Fm-3m high-symmetry structure as responsible for the superconducting record. The results are especially relevant because they also demonstrate how nuclear quantum fluctuations can stabilise crystal structures even at more than 100 gigapascals below their classical instability pressure. Our work shows that the “classical” instabilities are due to the enormous electron-crystal lattice interaction that makes this compound a record superconductor. In other words, quantum effects stabilise crystal structures with substantial superconducting temperatures that would otherwise be unstable. Consequently, new hopes are opened to discover high-temperature superconducting hydrogen compounds at much lower pressures than expected classically, maybe even at ambient pressure.

Reference: Ion Errea, Francesco Belli, Lorenzo Monacelli, Antonio Sanna, Takashi Koretsune, Terumasa Tadano, Raffaello Bianco, Matteo Calandra, Ryotaro Arita, Francesco Mauri &  José A. Flores-Livas. “Quantum crystal structure in the 250-kelvin superconducting lanthanum hydrideNature 578pages 66–69 (2020) See the article here: https://rdcu.be/b1fCK.

Granted supercomputer project in Europe’s fastest supercomputer!

Our third computing grant, the second one in CSCS (Swiss National supercomputer), has been accepted!

The project proposes a vital study of unconventional high-performance p-type transparent conductor materials. For the following 2 years, we will have access to 1.4 Million computing node hours in the fastest supercomputer in Europe! The supercomputer is hosted by the Swiss National Supercomputing Centre, Centro Svizzero di Calcolo Scientifico (CSCS). 

High-throughput studies are gaining increasing popularity as a tool for predicting p-type transparent conductors. However, the lack of appropriate descriptors makes the efficient screening of materials for this purpose challenge. Yet, to identify and validate novel descriptors, more robust data on non-oxide p-type transparent conductors is needed. Towards this goal, we suggest an in-depth investigation of the p-type propensity of all recently speculated, but not experimentally verified, p-type TC materials through an exhaustive study of native defects. Subsequent optimization of the most promising materials through external doping will allow us to mobilize the experimental community to follow up on the outputs of our studies.  

Coming months surely will be freighted with exciting news!

 

Enhancing transparent copper iodide, available dopants?

In our most recent work, the zinc blende phase of copper iodide (CuI) was investigated using a computational high-throughput approach. This material notably holds the record hole conductivity for intrinsic transparent p-type semiconductor. In our study, eight impurities suitable for n-type doping were discovered. Unfortunately, the work reveals that donor doping is hindered by compensating native defects, making ambipolar doping unlikely.

Atomic and electronic structure of CuI. Left: Atomic structure of the 64 atom CuI supercell used in calculations. The different defect sites are indicated by bright blue spheres, copper atoms are shown in light green, and iodine atoms in red. Right: Electronic band structure (calculated using PBE0) on selected high-symmetry directions and the corresponding partial density of states. Red points around G indicate the shift in energy due to spin-orbit coupling contribution. The resultant change in the bandgap is also indicated.

This article is part of the themed collection: 2019 PCCP HOT Articles, and is free to download for a couple of months. It was selected for the September 2019 Cover of PCCP!  (link)

Review on high-Tc superconductors: Methods and Materials

I am very excited to finally release my latest work that has been running on parallel for more than a year. Notably, the last five months have been of an intense, insane amount of work.

The number of hours invested in this work has been many more from the initially planned. I wish to thank my collaborators, especially the experimental colleagues (M. Eremets) who have recently measured 250 K superconductivity!

The referees of work and other colleagues have remarked that our work is perhaps the most complete and thoroughly detailed Review in this field. This Review is dedicated to one of the hottest subjects: high pressure, hydrogen-based superconductors.

The approximate number of publications in the field of hydrides per decade. The first hydride rush took place right after Ashcroft’s and Ginzburg’s prediction of high-T c superconductivity in hydrogen at the end of the 60s. The second hydride rush started at the dawn of the 2000s, and the third is starting now (2019) after the discovery of LaH10 and will continue through the following decade. The expected number of publications in the field by the year 2030 is above 10,000.

Abstract

Two hydrogen-rich materials, H3S and LaH10, synthesized at megabar pressures, have revolutionized the field of condensed matter physics providing the first glimpse to the solution of the hundred-year-old problem of room temperature superconductivity. The mechanism underlying superconductivity in these exceptional compounds is the conventional electron-phonon coupling. Here we describe recent advances in experimental techniques, superconductivity theory and first-principles computational methods which made this discovery possible. This work aims to provide an up-to-date compendium of the available results on superconducting hydrides and explain how the synergy of different methodologies led to extraordinary discoveries in the field. Besides, in an attempt to evidence empirical rules governing superconductivity in binary hydrides under pressure, we discuss general trends in the electronic structure and chemical bonding. The last part of the Review introduces possible strategies to optimize pressure and transition temperatures in conventional superconducting materials as well as future directions in theoretical, computational and experimental research. This work would appear as a Review on Physics Reports 2020 (Link to ArXiv). 

New playground materials: Magnetic nitrides Perovskites

We propose a novel class of materials: stable nitrides with a perovskite-type structure and magnetic rare-earth metals. These materials are thermodynamically stable and, despite possessing the different atomic environments, retain the magnetic moment of their guest, the rare-earth metal. 

We find both magnetic metals and insulators, with a variable range of magnetic moments and some systems posses record-high magnetic anisotropy energies. Further tuning of the electronic and magnetic properties can also be expected by doping with other rare-earths or by creating solid solutions. The synthesis of these fascinating materials with novel compositions would extend the accepted stability domain of perovskites. This work is published in the newly open-access Journal of Physics: Materials (JPhys Materials, from IOP Science).

The calculated saturation magnetization per guest rare earth metal in a nitride-Perovskite structure.

Transparente Elektronik gewinnt an Fahrt

This research is featured in CSCS (National supercomputer center of Switzerland) here: link and other sites such as PhysOrg and HPCwire!

German version: 

An transparenter Elektronik wird intensiv geforscht. Sie steht für perfektes Design elektronsicher oder optischer Geräte. Forscher der Universität Basel haben nun mit Hilfe von Computersimulation auf dem Supercomputer «Piz Daint» chemische Elemente identifiziert, die der Forschung in diesem Bereich Schub verleihen könnten.

Transparenter Elektronik gehört die Zukunft. Darin sind sich Forscher einig. So auch José A. Flores-Livas und Miglė Graužinytė von der Forschungsgruppe von Stefan Goedecker, Professor für Computational Physics an der Universität Basel. Doch die entsprechende Technologieentwicklung verläuft schleppend. Grund dafür ist, dass es an bestimmten transparenten Halbleitern mit hoher Leitfähigkeit mangelt.

Fremdatome zur Optimierung

Die elektronischen oder optischen Eigenschaften von Halbleitern lassen sich manipulieren und optimieren, indem geeignete Fremdatome in das Material eingesetzt werden. Diese sogenannte Dotierung durch Fremdatome, beispielsweise in Transistoren, verändert die Ladungsträgerdichte und erhöht somit die Leitfähigkeit.

Geeignete Fremdatome im Periodensystem der Elemente zu identifizieren, bedeutet im Labor jedoch nicht selten jahrelange und kostenintensive Experimentierarbeit. Forscher versuchen diesen Prozess zu beschleunigen, indem sie auf Computersimulationen zurückgreifen. Mit ihnen berechnen sie die vielversprechendsten Kandidaten auf der Basis der physikalischen Gesetze, die die Wechselwirkung zwischen Fremdatom und dem eigentlichen Material des Leiters beschreiben. Potentielle Kandidaten können dann gezielt im Labor getestet werden.

Mangel an speziellen leistungsstarken Leitern

Mit dem Ziel, geeignete Fremdatome zu finden, mit denen die gewünschten transparenten Halbleiter dereinst hergestellt werden können, führten die Wissenschaftler der Universität Basel nun mit Hilfe des Supercomputers “Piz Daint“ solche komplexen Simulationen durch. Bei den transparenten Halbleitern fehlt es vor allem an leistungsstarken Leitern des sogenannten p-Typs (positiv Leiter), bei denen das implantierte Fremdatom ein Elektron zu wenig hat. Sogenannten n-Typ-Leiter (negativ Leiter), sind mit Elementen dotiert, die sozusagen ein Elektron übrig haben.

Laut den Forschern zeigte sich erst kürzlich, dass das umweltfreundliche und häufig vorkommende Zinnmonoxid ein vielversprechendes Material für die Gewinnung transparenter und zugleich leistungsstarker p-Typ Leiter sein könnte. Zudem eigen es sich für das sogenanntes bipolares dotieren. In bipolaren Leitern sind sowohl negative wie positive Ladungsträger vereint. Allerdings habe man bis anhin nur eine Handvoll Elemente untersucht, die sich als Fremdatome eigenen könnten, um den Halbleiter auf der Basis von Zinnmonoxid mit den gewünschten Eigenschaften auszustatten.

Vielversprechende Alkalimetalle

Anhand ihrer Berechnungen wurden die Forscher nun bei den Alkalimetallen fündig. Sie konnten fünf Alkalimetalle (Lithium, Natrium, Kalium, Rubidium und Cäsium) identifizieren, die dotiert in Zinnmonoxid leistungsstarke und zugleich transparente p-Typ Halbleiter ermöglichen könnten. Darüber hinaus ermittelten die Berechnungen laut den Forschern 13 Elemente, die sich zur Dotierung von n-Typ Ladungsträgern in Zinnmonoxid eignen. «Wenn sich diese Elemente erfolgreich in Zinnmonoxid dotieren und den gewünschten Halbleiter herstellen lassen, öffnet uns das den Weg für viele transparente Technologien», ist José Flores-Livas überzeugt.

Literaturhinweis:

Graužinytė M, Goedecker S, and Flores-Livas JA: Towards bipolar tin monoxide: Revealing unexplored dopants, Phys. Rev. Materials 2, 104604 – October 2018. DOI:https://doi.org/10.1103/PhysRevMaterials.2.104604

 

Revealing unexplored dopants in semiconducting materials

The advancement of transparent electronics, one of the most anticipated technological developments for the future, is currently inhibited by a shortage of high-performance p-type conductors.
In materials, doping is the intentional introduction of impurities into an intrinsic semiconductor for the purpose of modulating its electrical and optical and structural properties. Recent demonstration of tin monoxide as a successful transparent p-type thin-film transistor and the discovery of its potential for  ambipolar doping, suggests that tin monoxide—an environmentally-friendly earth-abundant material—could offer a solution to this challenge. However, only a handful of elements have been investigated as candidate dopants for this material.
Identifying suitable elements for doping requires an enormous amount of experimentation, that involves expensive trial-and-error approaches and can take years before a suitable substitutional element is found. With the aim of accelerating this process and finding a path for successfully enhancing the electronic properties of SnO, an extensive computational search for useful dopant elements was performed using “Piz Daint” at the CSCS. Employing computer simulations researchers at the University of Basel have discovered that substitutional doping with the family of alkali metals provides an as off yet unexplored possibility to increase the concentration of acceptors (i.e. increase p-type conductivity) without being detrimental to the transparency of SnO.
Over ten shallow donors, which, to the best of our knowledge, have not been previously contemplated, were identified in this accelerated discovery thanks to Piz Daint.  The work presents a detailed analysis of the most promising n-/p -type dopants—offering new insights into the design of an ambipolar semiconductor. If synthesized successfully, such a doped ambipolar oxide could open new avenues for many transparent technologies.
Reference: Miglė Graužinytė, Stefan Goedecker, and José A. Flores-Livas. “Towards bipolar tin monoxide: Revealing unexplored dopants”,  Phys. Rev. Materials 2, 104604 – October 2018.

Emergence of Perovskite (MAPI) phases upon little compression

Perovskites are among the most promising and versatile class of candidate compounds for new or improved materials in energy applications, including photovoltaics, superconductivity, and lasing. With the general formula ABX3, the perovskite structure consists of corner-sharing BX6 octahedra forming a three-dimensional (3D) framework that provides room for the A units in the resulting cuboctahedral cavities.

In our latest work we use classical interatomic potential and coupled with minima hopping method to screen the potential energy surface using simulation cells containing 2, 4, and 8 units (i.e., 24, 48, and 96 atoms), on were more than 140,000 structures were generated.

We also investigated how pressure affects the enthalpy and volume of the experimental and predicted phases of MAPI with respect to the orthorhombic phase (see figure). Based on our careful analysis of the available literature work, none of the experimental high-pressure experiments used the low-temperature orthorhombic phase as the starting material. Instead, our simulated XRD patterns show that the tetragonal phases is present in the samples of experimental work. Based on our calculations, we therefore suggest that either the delta or double delta phases could be synthesized by compressing precursor samples in the orthorhombic (Pnma) phase in low-temperature compression experiments. Since the orthorhombic phase is strongly destabilized upon compression, a transition towards the double-delta or delta phases will be rapidly favored with increasing pressure. Our work is now published in “Phys. Rev. Materials 2, 085201 2018″.

Not all materials are metallic under pressure: tuning the gap !

Highly stable materials are usually wide-gap insulatorswhere covalency dominates the ionic exchange, such as in carbon (diamond), MgO, and LiH, to name a few.  In our latest work,  in collaboration with experimental teams in USA,  UK, and Japan we studied the enhanced stability of Sn3N4 to applied pressure and temperature. Our predicted phase transitions were confirmed by state-of-the-art Synchrotron X-ray diffraction and EXAFS measurements upon different annealing procedures. 

The unique stability of the spinel (Sn3N4) phase  is attributed to its dominant ionic character. This work is now published in Angewandte Chemie (International Edition).  We also demonstrated that the application of pressure allowed us a systematic tuning of the charge density, cleanly, that is, without changes to the chemical composition via dopants.
More importantly we demonstrated the mechanism responsible for the opening of the electronic band upon compression.  A continuous opening of the optical band gap was observed from 1.3 eV to 3.0 eV over a range of 100 GPa, a 540 nm blue‐shift spanning the entire visible spectrum. The pressure‐mediated band gap opening is general to this material across numerous high‐density polymorphs, implicating the predominant ionic bonding in the material as the cause. The rate of decompression to ambient conditions permits access to recoverable metastable states with varying band gaps energies, opening the possibility of pressure‐tuneable electronic properties for future applications.

Possible superconductivity in hydrogenated carbon nanostructures

In this work we present an application of density-functional theory for superconductors (SCDFT) to superconductivity in hydrogenated carbon nanotubes and fullerane (hydrogenated fullerene). We show that these systems are chemically similar to graphane (hydrogenated graphene) and like graphane, upon hole doping, develop a strong electron phonon coupling. This could lead to superconducting states with critical temperatures approaching 100 K. However this possibility depends crucially on if and how metallization is achieved.

(Left) Real space anomalous potential ∆ (R, s) (left) and order parameter χ (R, s) (center and right) as a function of the Cooper pair center of mass R. On the top (a–c) an xy cut of the tube and on the bottom (d–f) a vertical cut of the tube. The doping level (rigid shift) is indicated on the top. The value of the functions is given according to the colorscale in the center (left scale refers to the left plot and the two right scales to the two right plots. A white dashed line in the a–c plots indicates the cut shown on the (d–f) plots.

This work is dedicated to Hardy Gross who, not only jointly invented SCDFT, but also devoted a large effort to develop the theoretical framework into a fully functioning method, investigating functionals, extensions  and transforming it into a useful and predictive tool in material science.

Doping polyethylene, the next superconductor?

High pressure is an exciting field that has evolved incredibly far since the pioneering work of Cailletet, Amagat and Bridgman. A substantial amount of research in the field of high pressure (post-Bridgman era) was triggered by the tantalizing idea of metalizing hydrogen (Wigner and Huntington transition) which dates back to the mid 30’s. The metalization of hydrogen is seen as the holy-grail of high pressure research, it has been a compelling subject of great interest for many scientists ranging from experimental chemists and physicists to theoreticians,  including Prof. Gross (my former boss in Max-Planck, Halle).

It is well understood, that compression of molecular systems at high pressure increases the electron-orbital overlap between neighboring atoms resulting in an increase of the band dispersions consequently closing the electronic band gap. Chemical pre-compression is certainly one promising route to reduce the metalization pressure on insulating elements, but not the only one! Another method to reach metalization is chemical doping under pressure –a path previously used at ambient pressures to render standard insulators superconducting.–  We demonstrated theoretically this approach for H2O.

In our latest article, we investigated the structural stability of polyethylene (H2C)n under pressure. The questions we want to address in the work were: is there a stable polyethylene phase under pressure that can be doped? if yes, is it superconducting? Finally, we dedicated this article to Prof. Hardy Gross, for his 65th birthday. This research article would appear in the Topical Issue “Special issue in honor of Hardy Gross” edited by C.A. Ullrich, F.M.S. Nogueira, A. Rubio, and M.A.L. Marques.

“Cold-Passivation” of Defects in Tin-Based Oxides

Our latest results arising from a collaborative work with experimental groups in École Polytechnique Fédérale de Lausanne (EPFL), Neuchâtel,  University of California Berkeley, National Institute of Advanced Industrial Science and Technology (AIST Japan) and the Institute for Nanotechnology, University of Twente, is published now in Journal of Physical Chemistry C  (link).

In this work we studied transparent conductive oxides (TCOs), which are essential in technologies coupling light and electricity. For Sn-based TCOs, oxygen deficiencies and under-coordinated Sn atoms result in an extended density of states below the conduction band edge. While shallow states provide free carriers necessary for electrical conductivity, deeper states inside the bandgap are detrimental to transparency. In zinc tin oxide (ZTO), the overall optoelectronic properties can be improved by defect passivation via annealing at high temperatures. Yet, the high thermal budget associated with such treatment is incompatible with many applications. In our most recent work, we demonstrate an alternative, low-temperature passivation method, which relies on co-sputtering Sn-based TCOs with silicon dioxide (SiO2). Using amorphous ZTO and amorphous/polycrystalline tin dioxide (SnO2) as representative cases, we demonstrate through optoelectronic characterization and density functional theory simulations that the SiO2 contribution is two-fold. First, oxygen from SiO2 passivates the oxygen deficiencies that form deep defects in SnO2 and ZTO. Secondly, the ionization energy of the remaining deep defect centers is lowered by the presence of silicon atoms. Remarkably, we find that these ionized states do not contribute to subgap absorptance. This simple passivation scheme significantly improves the optical properties without affecting the electrical conductivity, hence overcoming the known transparency-conductivity trade-off in Sn-based TCOs.

Tittle of the article: “New Route for “Cold-Passivation” of Defects in Tin-Based Oxides

Crystal defects for qubits

We had the opportunity to attend the CECAM-Workshop “Crystal defects for qubits, single photon emitters and nanosensors” in Bremen (Germany), an interesting and forefront workshop in crystal defects for quibits.

Since we all know, the leading contender is the nitrogen-vacancy center in diamond which may be considered as a robust quantum tool. Several quantum algorithms and protocols for sensing have been already demonstrated by this center.  However, researchers face many materials science problems in order to maintain the favorable intrinsic properties of this color center that can be perturbed by other defects either in bulk or at the surface of diamond that is difficult to resolve because of its chemical hardness and the concurrent stability of carbon  allotropes.

Theory-driven search for alternative materials could identify other quantum bit candidates in technologically mature wide band gap semiconductors, particularly silicon carbide, that have been recently demonstrated in experiments. However, the knowledge about these color centers is scarce.

World-leaders, experts in the filed presented the state of the art in this research topic,  invited among others:
David Awschalom (University of Chicago, Illinous) Adám Gali (Hungarian Academy of Science, Wigner Research Centre for Physics, Budapest) Audrius Alkauskas (Center for Physical Sciences and Technology, Vilnius)Sophia Economou (Virginia Polytechnic Institute and State University Blacksburg, Virginia) Ronald Hanson (Delft University of Technology) Marcus Doherty (Australian National University, Canberra) Arne Laucht (University of New South Wales, Sydney) Fedor Jelezko (Ulm University).

During this meeting we could exchange ideas of methods to use, and listen to discussions of new developments between scientists working on different aspects of diamond, silicon carbide and related materials. The interdisciplinary character of this workshop was a unique opportunity for me to learn  and to have a better idea on which problems I should focus.

Stay tuned! this research topic is very promising and will lead to nice discoveries!

 

SuperMUC will build a high pressure materials database.

Our computing project has just been accepted to run in the SuperMUC Petascale System.

With more than 2.3 millions of computing cores at our disposition the grant in its first phase will serve to construct a database for materials under pressure. The project will runs until 2020 including a second phase with possible extension of the requested computing allocation.

The supercomputer is SuperMUC ranked top 4 as the fastest supercomputer in Germany. It is composed of more than 147,000 computing cores.  SuperMUC (the suffix ‘MUC’ alludes to the IATA code of Munich’s airport) is operated by the Leibniz Supercomputing Centre, a European centre for supercomputing.

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

Structures of exohedrally decorated C60-Fullerenes

In our most recent publication we studied the exohedrally metal decorated carbon-fullerenes. These systems are a promising material for its good hydrogen adsorption (high concentrations and with optimal binding energies) properties. Since their geometry and type of coverage play a key role in determining the H2 adsorption mechanism, in this paper just accepted in Carbon Journal, we studied in a fully  ab-initio, unbiased structure fashion the configurational space of decorated C60 fullerenes.

Many of the hitherto postulated ground state structures are not ground states. We could determine the energetically lowest configurations for decorations with a varying number of decorating atoms for alkali metals, alkaline-earth metals as well as some other important elements and find that the dense uniform distribution of the decorating atoms over the surface of the C60, desired for hydrogen storage, can be obtained only for a few elements.  An understanding of the behavior of the decorating atoms can be obtained
by analyzing their bonding characteristics.

Searching hole-electron substitutional dopants for TCO technologies

The combination of optical transparency and high electrical conductivity enables transparent conductive oxide (TCO) materials to be used for a wide range of applications -from simple smart window coatings to OLEDs and futuristic see-through displays.  Doped tin-dioxide (SnO2) is an important semiconductor that is already used for these applications. However, in order to uncover the entire potential of this material in more advanced applications of  optoelectronics further improvements in electrical properties are necessary.

We conducted an extensive search for useful substitutional dopants of SnOfor which a novel and well-converged protocol was used. The entire periodic chart was scanned for stable charges and hole-electron dopants.  Our finding are in excellent agreement with current known dopants, besides we predicted other possible substitutional dopants that have been not experimentally examined to date.

This work has been just accepted (link here) and appeared in Chemistry of Materials.

 

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)

 

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