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.

“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)

 

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.