SCIENTIFIC HIGHLIGHTS

Beating the thermodynamic limit with photo-activation of n-doping in organic semiconductors using “hyper-reductants”

Image by Jing Wang and Xin Lin
Doping of semiconductors is a key process for controlling the materials’ charge carrier density, which directly impacts the electrical conductivity. Electronic and optoelectronic devices used in information, communication, energy conversion, and energy storage technologies rely on precise and efficient doping, i.e., the admixture of a small amount of a doping agent into the semiconductor. However, n-type doping of organic semiconductors – electron transfer from the dopant to the semiconductor – is notoriously difficult as the molecular dopants employed presently are highly sensitive to ambient exposure, upon which they react with water and oxygen and are rendered inactive.
In an article that just appeared in Nature Materials, a team of researchers from the Georgia Institute of Technology, the Helmholtz-Zentrum Berlin, Humboldt-Universität zu Berlin, and Princeton University demonstrates a new approach towards n-doping of organic semiconductors, which allows bypassing the dopant sensitivity to the ambient and simultaneously enables doping organic electron transport materials that have been out of reach for n-doping so far. The first step of innovation lies in chemically connecting two organometallic molecular dopants in a dimer that is stable even in air, with reduced ability to dope organic electron transport semiconductors. Consequently, when mixing these into the organic semiconductor, nothing happens at first. The revolutionary step now involves illuminating the mixture with light. A dimer and a semiconductor molecule in immediate proximity absorb a photon, the dimer can dissociate and unfold the full doping power of each dopant in a multi-step process. “By this optical activation of dopants, we could enhance the conductivity of organic electron transport materials by five orders of magnitude. This boosts the efficiency of organic light emitting diodes and solar cells, using rather simple and technologically relevant processing.” says Prof. Antoine Kahn from Princeton University, who coordinated the project. The choice of the article’s title is explained by Prof. Seth Marder from Georgia Tech: “This doping is actually beyond the thermodynamic limit of what the dopant should be able to do, thus once the light is turned off one might naively expect the reverse reaction to occur (rapidly, within seconds perhaps) and the conductivity increase to disappear. However, this is not the case. The reason for this is that the doping process involves multiple steps, and the back-reaction to the starting system involves many uphill intermediate steps creating a kinetic barrier, thus the reverse reaction is extremely slow.” Indeed, no indications of a loss in conductivity upon light-activation after hundreds of hours were found. For these reasons, the compounds are referred to as “hyper-reductants”. The fact that the team demonstrated the beneficial effect of their doped electron transport semiconductors in highly efficient light emitting diodes underlines the huge potential of this approach in device applications. “We believe that our work enables simple processing of n-doped organic semiconductors in numerous device architectures, where the critical step - doping activation - can take place after standard device encapsulation. This will contribute substantially to improved device lifetime and in some case simplify device fabrication.” notes Prof. Norbert Koch from Humboldt-Universität, member of IRIS Adlershof. The work was part of a project within the strategic partnership program of Princeton University and Humboldt-Universität.
 
Beating the thermodynamic limit with photo-activation of n-doping in organic semiconductors using “hyper-reductants”
X. Lin,  B. Wegner,  K.M. Lee,  M.A. Fusella,  F. Zhang, K. Moudgil , B.P. Rand, S. Barlow, S.R. Marder, N. Koch, A. Kahn
Nature Materials, published online


 
Water makes the proton shake -
Ultrafast motions and fleeting geometries in proton hydration

Basic processes in chemistry and biology involve protons in a water environment. Water structures accommodating protons and their motions have so far remained elusive. Applying ultrafast vibrational spectroscopy, Dahms et al. map fluctuating proton transfer motions and provide direct evidence that protons in liquid water are predominantly shared by two water molecules. Femtosecond proton elongations within a hydration site are 10 to 50 times faster than proton hopping to a new site, the elementary proton transfer step in chemistry.

The proton, the positively charged nucleus H+ of a hydrogen atom and smallest chemical species, is a key player in chemistry and biology. Acids release protons into a liquid water environment where they are highly mobile and dominate the transport of electric charge. In biology, the gradient of proton concentration across cell membranes is the mechanism driving the respiration and energy storage of cells. Even after decades of research, however, the molecular geometries in which protons are accommodated in water, and the elementary steps of proton dynamics have remained highly controversial.

Protons in water are commonly described with the help of two limiting structures (Fig. 1A). In the Eigen complex (H9O4+) (left), the proton is part of the central H3O++ ion surrounded by three water molecules. In the Zundel cation (H5O2+) (right), the proton forms strong hydrogen bonds with two flanking water molecules. A description at the molecular level employs the potential energy surface of the proton (Fig. 1B) which is markedly different for the two limiting geometries. As shown in Fig. 1B, one expects an anharmonic single-minimum potential for the Eigen species and a double minimum potential for the Zundel species. In liquid water, such potentials are highly dynamic in nature and undergo very fast fluctuations due to thermal motions of surrounding water molecules and the proton.

Led by Thomas Elsässer, member of  IRIS Adlershof, researchers from the Max Born Institute in Berlin, Germany, and the Ben Gurion University of the Negev in Beer-Sheva, Israel, have now elucidated the ultrafast motions and structural characteristics of protons in water under ambient conditions. They report experimental and theoretical results in Science which identify the Zundel cation as a predominant species in liquid water. The femtosecond (1 fs = 10-15 s) dynamics of proton motions were mapped via vibrational transitions between proton quantum states (red and blue arrows in Fig. 1B). The sophisticated method of two-dimensional vibrational spectroscopy provides the yellow-red and blue contours in Fig. 2A which mark the energy range covered by the two transitions. The blue contour occurs at higher detection frequencies than the red, giving the first direct evidence for the double-minimum character of the proton potential in the native aqueous environment. In contrast, the blue contour is expected to appear at smaller detection frequencies than the red one.

The orientation of the two contours parallel to the vertical frequency axis demonstrates that the two vibrational transitions explore a huge frequency range within less than 100 fs, a hallmark of ultrafast modulations of the shape of proton potential. In other words, the proton explores all locations between the two water molecules within less than 100 fs and very quickly loses the memory of where it has been before. The modulation of the proton potential is caused by the strong electric field imposed by the water molecules in the environment. Their fast thermal motion results in strong field fluctuations and, thus, potential energy modulations on a sub-100 fs time scale. This picture is supported by benchmark experiments with Zundel cations selectively prepared in another solvent and by detailed theoretical simulations of proton dynamics (Fig. 2B).

A specific Zundel cation in water transforms into new proton accommodating geometries by the breaking and reformation of hydrogen bonds. Such processes are much slower than the dithering proton motion and occur on a time scale of a few picoseconds. This new picture of proton dynamics is highly relevant for proton transport by the infamous von Grotthuss mechanism, and for proton translocation mechanisms in biological systems.
      

Figure 1: Chemical structure of hydrated protons in liquid water.


A Schematic of the Eigen cation H9O4+ (left) and the Zundel cation H5O2++ (right). The arrows indicate the O-H bond coordinate r and the (O...H+...O) proton transfer coordinate z. In the Eigen cation a covalent O-H bond localizes the proton whereas in the Zundel cation the proton is delocalized between two water molecules.
B Anharmonic vibrational potential (left) and double minimum potential of the Zundel cation along z (right, red. Distortions by the solvent surrounding impose a modulation of the double minimum potential (right, dotted line). Red and blue arrows indicate transitions between particular quantum states of the proton motion , i.e., the ground-state-to-first-excited-state transition (red) and the first-excited-state-to-second-excited-state transition (blue). The modulation of the potentials leads to spectral shifts of the vibrational transitions which are mapped by two-dimensional infrared spectroscopy.





Fig 2: Femtosecond dynamics of proton motions (1 fs = 10-15 s).



A Two-dimensional vibrational spectra with the ground-state-to-first-excited-state transition (red) at lower detection frequency than the first-excited-state-to-second-excited-state transition (blue). The orientation of both contours parallel to the excitation frequency axis is due to ultrafast frequency fluctuations and the loss of memory in the proton position.
B Simulated real-time dynamics of the proton motions in the Zundel cation. Within less than 100 fs, the proton displays large amplitude excursions along z, the coordinate linking the two water molecules in the Zundel cation. Due to the ultrafast modulation of the shape of proton potential by surrounding solvent molecules, the proton explores all locations between the two water molecules.





Fig 3: Cartoon picture of proton hydration dynamics, visualized with the help of classical physics.


The proton Smiley is sitting in the middle of a sofa with two seats. When shaking the sofa with a mechanical force, the shape of the seating changes and the proton moves forth and back on the sofa. Such motions occur on a time scale shorter than 100 fs (10-13 s). After an average time of 1 ps = 1000 fs = 10-12 s, the sofa breaks and the proton moves to a new site/sofa, including the red halve on the right.

Large-amplitude transfer motion of hydrated excess protons mapped by ultrafast 2D IR spectroscopy
F. Dahms, B.P. Fingerhut, E.T.J. Nibbering, E., and T. Elsaesser
Science, published online


 
   
X-ray "movie" provides insights into the formation of molecular layers

Thin-film technologies that promise control on the atomic and molecular scale have attracted increasing interest in recent years as traditional manufacturing processes reach their fundamental limits. A team from the Department of Physics at the Humboldt-Universität zu Berlin, led by Anton Zykov, Stefan Kowarik and Jürgen P. Rabe (member of  IRIS Adlershof) in collaboration with colleagues from the PETRA III Synchrotron at DESY Hamburg has now studied the non-equilibrium growth of molecular layers using innovative, time-resolved X-ray scattering. The movie sequence of the X-ray scattering during the molecular beam deposition was chosen as the cover image of a special topic issue of the Journal of Chemical Physics on "Atomic and molecular layer processing".

Semiconducting organic molecules have significant potential for future applications such as organic light-emitting diodes (OLED), camera sensors or memory devices. Many of these components are based on ultra-thin layers of functional molecular materials. Their preparation by deposition of molecules from the gas phase is a complex process involving molecular adsorption on a substrate, molecular diffusion and self-assembly. Since many of these processes do not proceed under conditions of local thermodynamic equilibrium, these processes and their velocities are still not well understood.
By means of innovative X-ray measurements of diffuse scattering at the P03 Beamline of the PETRA III synchrotron, the researchers were able to record "movies" of the growth processes on the nanoscale. The measurement makes it possible to follow the nucleation, island growth and the roughness evolution of the layer. The researchers show that the results of the new X-ray technique are consistent with established scanning probe techniques and time-resolved measurements are possible without disturbing the growth. In the study, a significant improvement in the diffusivity of molecules between the first and the subsequent molecular layers was found and the nucleation energy was determined within the framework of recent growth theories. The application of the new X-ray scattering technique will help to take our understanding beyond a recipe-based perspective to that of sound fundamental understanding of molecular growth.

Diffusion and nucleation in multilayer growth of PTCDI-C8 studied with in situ X-ray growth oscillations and real-time small angle X-ray scattering
A. Zykov, S. Bommel, C. Wolf, L. Pithan, C. Weber, P. Beyer, G. Santoro, J.P. Rabe, and S.Kowarik
J. Chem. Phys. 146, 052803 (2017)


 
GLAD makes new organic memory devices possible

Giovanni Ligorio, Marco Vittorio Nardi, and Norbert Koch, member of IRIS Adlershof, have invented a new technique for constructing novel memory devices. The results have now been published in Nano Letters.

Author Dr. Giovanni Ligorio explains: “Novel non-volatile memory devices are currently investigated to overcome the limitation of traditional memory technologies. New materials such as organic semiconductors and new architectures are now considered to address high-density, high-speed, low-fabrication costs and low power-consumption.
Usually nano-devices (traditionally based on inorganic semiconductors) are fabricated via lithography techniques. Here, we show the fabrication of devices with nanometric footprint using a different technique: Glancing Angle Deposition (GLAD).
This technique allows the tailoring of nanostructured morphologies through physical vapor deposition (CVD) via controlling the substrate orientation with respect to the vapor source direction. When thin films are deposited onto stationary substrates under condition of oblique deposition, meaning that the vapor flux is non-perpendicular to the substrate surface, an inclined columnar nanostructured is produced.


(a) Herstellung der Nanosäulen via CVD (b) AFM-Aufnahme der Säulen-columns (c) Skizze der Ansteuerung (d) Skizze eine Säule mit Filament ©G.Ligorio
 
Upon proper bias applied between the two electrodes of the memory device, it is possible to form a conductive path (or filament). The filament shorts the electrodes and drastically changes the resistivity characteristic of the device. Forcing a high current in the device, the filament can be distrust. This programs the device in the original high resistivity state. Since the process can be repeated consecutively we can program the device in a high or low resistive state (i.e. ON or OFF).
We aim for the fabrication of devices in structured arrays (in this publication the nano devices are not ordered in array, but they are randomly distributed.) This allows for connecting via cross bar electrodes, which can be fabricated via printing.
This allows fabricating memory devices with a density of roughly 1 GB/cm² employing novel material for electronics, i.e. organic semiconductors.”
 
Lithography-Free Miniaturization of Resistive Nonvolatile Memory Devices to the 100 nm Scale by Glancing Angle Deposition
G. Ligorio, M. Vittorio Nardi, and N. Koch
Nano Lett. 17 (2017) 1149


 
Color Duality in Photons

Conventionally, quantum frequency conversion is used to as fully as possible transfer photons (coherently) from one frequency to another. This is for different reasons, e.g. to improve detection efficiencies or to convert photons used for quantum communication to the telecommunication band for lowest loss transmission through optical fibers.

But what happens if one stops exactly “half-way” in the conversion process? Quantum mechanically one ends up with a color-super-position state of a single photon being simultaneously at two different color (frequency) states. This is exciting and interesting. But how to proof one really has a proper (coherent) quantum-superposition state? And is this maybe even useful for something?


APS/Alan Stonebraker
The illustration shows the conversion of a photon of one frequency, or color, into a photon that is in a quantum superposition of two colors, and the subsequent verification of this super-positions coherence with Ramsey spectroscopy.

For other quantum systems (like spins of single electrons, single atoms/ions…) such energy superposition states between a ground and excited (electronic) state very are well known. One way to produce them is to apply a so called Pi/2-pulse. By then letting the system freely evolve (and pick up a phase) and subsequently applying another Pi/2-pulse one realizes a so called Ramsey Interferometer. And this type of quantum interferometer is very widely used for a plethora of applications.
So, to demonstrate that one indeed can generate single photons in a genuine quantum super-position of two colors and that this might even be useful for something, the goal was to for the first time realize Ramsey Interference with single photons. The technical challenge of this is, that the quantum frequency conversion has to work so well, that one can cascade two conversion processes and implement a controllable phase in between. And this is exactly what we did.

As a side-remark, in all “traditional” Ramsey Interferometers there is always the possibility that the quantum superposition of the ground and excited state spontaneously decays into the ground state by coupling to the electro-magnetic (vacuum) field. For photons, which are themselves the quanta of the electro-magnetic field, no such decay channel (at least in vacuum) exist, making it fundamentally special.

The paper “Ramsey Interference with Single Photons”[1] and accompanying Viewpoint “Photon Qubit is Made of Two Colors” [2] have been selected as one of the Highlights of the Year 2016 by APS Physics. It was co-authored by Dr. Sven Ramelow, who recently started his Emmy-Noether-Group at the Institute for Physics, Humboldt-University Berlin, and is associated with IRIS Adlershof. While there have numerous highly interesting papers in Physical Review Letters in 2016, APS Physics explains their selection, writing: “It’s no surprise that LIGO’s discovery of gravitational waves tops our list of favorite Physics stories in 2016. The other slots went to research that marked a change in perspective, demonstrated an impressive experimental feat, or simply made us think.”
Incidentally, Dr. Sven Ramelow is working on follow-up ideas of this paper and the corresponding experiments, which he looks forward to soon being implemented at IRIS-Adlershof and the HU Institute for Physics and yielding new intriguing results.


APS/Alan Stonebraker
  [1] Ramsey Interference with Single Photons
S. Clemmen, A. Farsi, S. Ramelow, A.L. Gaeta
Phys. Rev. Lett. 117, 223601 (2016)

[2] Viewpoint: Photon Qubit is Made of Two Colors
P. Treutlein

Physics 9, 135 (2016)


 
Light controls repair of materials

A team of German researchers led by chemists of the Humboldt-Universität zu Berlin has developed a new type of plastic coating, which can heal damages selectively by illumination with light. A heat-induced repair of the material occurs where the damaged area has previously been illuminated with light of a specific color. The promising results of this work have now been published in Nature Communications.



To avoid the environmentally unfriendly as well as expensive replacement of damaged consumer products and constructions, researchers have recently been focusing their efforts on the development of smart materials able to self-repair scratches or cracks. Especially plastic coatings, which are repaired by heat, have yielded promising results in the past. Once subjected to heat, a chemical reaction induces melting and thus enables a homogeneous and complete mending. Upon cooling, the plastic re-establishes its original chemical structure as well as mechanical properties: It hardens and becomes robust again. However, the thermal stress during the healing procedure affects the overall material properties and eventually leads to degradation when applied repeatedly.

To bypass this problem, German researchers from the Humboldt-Universität zu Berlin, the Friedrich-Schiller-University in Jena, the Federal Institute for Materials Research and Testing in Berlin as well as the Helmholtz-Zentrum Geesthacht in Teltow have now developed a smart plastic coating, in which light focusses the thermal healing process to the damaged locations only, without affecting the non-damaged parts.

 

“We aimed to protect intact parts of coatings from degradation.”, says lead researcher Stefan Hecht and adds: “By employing light as stimulus, we now have a true remote control to switch the ability to self-repair ‘on’ or ‘off’ on demand.” Shining light on damaged areas of the coating enables the self-repairing function. This process can be reversed by changing the color of the employed light yielding the original material – but in the healed state.

This seminal development is an important step to future applications in consumer products where light as a remote control facilitates external control over properties of smart materials. This could include the use as latent resists carried through various processing steps in nanofabrication or 3D printing.

Conditional repair by locally switching the thermal healing capability of dynamic covalent polymers with light
A. Fuhrmann, R. Göstl, R. Wendt, J. Kötteritzsch, M.D. Hager, U.S. Schubert, K. Brademann-Jock, A.F. Thünemann, U. Nöchel, M. Behl und S. Hecht
Nature Communications  (2016), published online
DOI: 10.1038/ncomms13623

 
Tapping the sun

A team of researchers from IRIS Adlershof of the Humboldt-Universität zu Berlin and Technische Universiteit Eindhoven in the Netherlands have developed thin plastic films, which continuously move upon exposure to sunlight. These materials are able to convert the sunlight’s energy directly into motion and have great promise for the development of sun-driven active coatings and surfaces, for example self-cleaning windows. These results have been published in Nature Communications.

In order to harvest and utilize the sun’s energy, alternative strategies to circumvent issues with energy storage and directly convert it into mechanical work have been developed over the years. A promising approach has been the design of light-driven molecular systems and machines; however, the collection of the individual molecules’ response and subsequent amplification to macroscopic motion and mechanical work has proven difficult. Furthermore, previous systems required the use of intense high-energy UV light and therefore displayed poor performance in the context of solar energy conversion.

Now a team of German and Dutch chemists combined their expertise and took advantage of specific tetrafluoroazobenzene dyes, which undergo an efficient shape change upon exposure to visible green and blue light, and organized them into ordered liquid crystalline arrays. Fixing the ordered arrangement by polymerization yielded thin plastic sheets, which bend and chaotically oscillate in sunlight.

 


Converting sunlight directly into motion by organizing light-responsive molecules
Figure: Dr. David Bléger



By carefully investigating the individual parameters of the system, the researchers found that the degree of oscillation depends on both the intensity and wavelength of the light and only occurs if both colors, i.e. blue and green triggering the opposite photoreactions, are present. As a result chaotic, macroscopic motion can be realized using “normal” sunlight, without the aid of specific optics or artificial light sources.

The authors foresee immediate practical outdoor applications including self-cleaning coatings and surfaces, for example in windows. In general, these findings should be of great importance for the development of autonomous, sunlight-driven nano- and micromachinery.

A chaotic self-oscillating sunlight-driven polymer
K. Kumar, C. Knie, D. Bléger, M. A. Peletier, H. Friedrich, S. Hecht, D. J. Broer, M. G. Debije and A. P. H. J. Schenning
Nature Communications  (2016), published online
DOI: 10.1038/ncomms11975

 
Enlightening and flexing memories

Researchers from Humboldt-Universität zu Berlin, led by Professor Stefan Hecht, who is a member of IRIS Adlershof, in collaboration with the University of Strasbourg & CNRS (France) and the University of Nova Gorica (Slovenia), have shown that a carefully chosen blend of a small photoswitchable molecule and a semiconducting polymer can be used to fabricate high-performance memory devices that can be written and erased by light. Such multilevel (8-bit) optical memories have also been implemented on flexible substrates, paving the way to applications in wearable electronics, E-papers, and smart devices. These results have been published in Nature Nanotechnology.

In the quest to improve the data storage capability of everyday electronic devices (random-access memories, hard disk drives, USB flash drives, etc.), alternative strategies to conventional silicon-based technologies need to be developed. The continuous miniaturization of electronic circuits, leading to the integration of a larger number of memory cells per unit area, has already shown its limitations due to the increased fabrication complexity. Another appealing approach consists in developing memory elements capable of storing not just one but multiple bits of information per device, commonly referred to as multilevel memories.

Now a European team of researchers from Berlin, Strasbourg, and Nova Gorica developed a light-responsive organic thin-film transistor by blending a custom-designed molecule serving as miniaturized optical switch with a high-performance semiconducting polymer. Upon illumination with ultraviolet and green light to “write” and “erase” information, respectively, the molecular switch undergoes a reversible interconversion between two distinct forms, one enabling and the other one preventing current to flow through the surrounding semiconducting polymer.

 

 



By integrating these components into transistor devices and using short laser pulses the researchers were able to construct multilevel memories with a data storage capacity of 8 bits. Importantly, their prototype devices combine high endurance over 70 write–erase cycles and data retention times exceeding 500 days.

Taking the work yet to another level, the team could transfer the device concept to flexible and light-weight polymer substrates, such as polyethylene terephthalate, to replace the commonly used rigid silicon. The resulting “soft” architecture preserves its electrical characteristics after 1000 bending cycles, thereby demonstrating its robustness and suitability for flexible electronics.
 
These findings are of great importance for the realization of high-performance smart and foldable electronic (nano)devices programmed by light with potential applications in flexible, multilevel high-density optical memories, logic circuits, and more generally in the next generation optoelectronics.

Flexible non-volatile optical memory thin-film transistor device with over 256 distinct levels based on an organic bicomponent blend
T. Leydecker, M. Herder, E. Pavlica, G. Bratina, S. Hecht, E. Orgiu, and P. Samorì
Nature Nanotechnology  (2016), DOI: 10.1038/nnano.2016.87

 
Reproducibility in density functional theory calculations of solids

The success and widespread popularity of density-functional theory (DFT) over the last decades has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. In this article, the results of a community-wide effort is reported, comparing 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the equations of state for 71 elemental crystals. The overall conclusion is that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Results of older methods, however, show stronger discrepancies.

exciting, the program package [1,2] developed in the group of Claudia Draxl at the Humboldt-Universität zu Berlin (Physics Department and IRIS Adlershof) represents one of the all-electron full-potential implementations of DFT. It employs the linearized-augmented planewave basis, which is considered the gold standard within the condensed-matter community. Within this study, exciting has proven to be among the three most precise packages, with nearly negligible differences between them.

 


exciting
has not only evolved into a benckmark code for DFT but has a strong focus on excitations that are treated within time-dependent DFT and many-body perturbation theory. In August 2016, HoW exciting! 2016 [3] will take place at the Campus Adlershof, consisting of an international workshop on excitations in solids and a hands-on course  employing exciting.

[1]   exciting-code.org
[2]  
exciting: a full-potential all-electron package implementing density-functional theory and many-body perturbation theory
A. Gulans, S. Kontur, C. Meisenbichler, D. Nabok, P. Pavone, S. Rigamonti, S. Sagmeister, U. Werner, and C. Draxl
J. Phys: Condes. Matter (Topical Review) 26 (2014) 363202
DOI: 10.1088/0953-8984/26/36/363202
[3]   how-exciting-2016.physik.hu-berlin.de/
     
Reproducibility in density functional theory calculations of solids
K. Lejaeghere, G. Bihlmayer, T. Bjoerkman, P. Blaha, S. Bluegel, V. Blum, D. Caliste, I. E. Castelli, S. J. Clark, A. Dal Corso, S. de Gironcoli, T. Deutsch, J. K. Dewhurst, I. Di Marco, C. Draxl, M. Dulak, O. Eriksson, J. A. Flores-Livas, K. F. Garrity, L. Genovese, P. Giannozzi, M. Giantomassi, S. Goedecker, X. Gonze, O. Granaes, E. K. U. Gross, A. Gulans, F. Gygi, D. R. Hamann, P. J. Hasnip, N. A. W. Holzwarth, D. Iusan, D. B. Jochym, F. Jollet, D. Jones, G. Kresse, K. Koepernik, E. Kuecuekbenli, Y. O. Kvashnin, I. L. M. Locht, S. Lubeck, M. Marsman, N. Marzari, U. Nitzsche, L. Nordstrom, T. Ozaki, L. Paulatto, C. J. Pickard, W. Poelmans, M. I. J. Probert, K. Refson, M. Richter, G.-M. Rignanese, S. Saha, M. Scheffler, M. Schlipf, K. Schwarz, S. Sharma, F. Tavazza, P. Thunstroem, A. Tkatchenko, M. Torrent, D. Vanderbilt, M. J. van Setten, V. Van Speybroeck, J. M. Wills, J. R. Yates, G.-X. Zhang, and S. Cottenier
Science 351 (2016), 1415
DOI: 10.1126/science.aad3000

 


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