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@HP the case of polyethylene: superconductivity!

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.