Despite the simplest possible electronic structure of hydrogen, tremendous complexity can arise when it participates in the formation of solids. We have predicted crystal structures of metal hydrides under pressure and utilized the stochastic quenching method to understand the role that hydrogen can play in affecting electronic properties of certain amorphous materials.

We have predicted crystal structures of metal hydrides under pressure to study structural transformation and phenomena such as metallization which allows us to model metallic hydrogen and associated high-temperature superconductivity. Furthermore, very fundamental aspects of hydrogen in metals have been investigated by us, shedding light on the interplay between interstitial sites occupation and lattice strain. Most recently, we have also utilized the stochastic quenching method to improve our understanding of certain amorphous materials and the role that hydrogen can play in affecting their electronic properties.

References

1. “A new probe of the electronic structure of Amorphous materials”, M. Strømme, R. Ahuja and G. A. Niklasson, Phys. Rev. Lett. 93 206403 (2004).
2. “Cubic metallic phase of aluminum hydride showing improved hydrogen desorption”, R. H. Scheicher, D. Y. Kim, S. Lebègue, B. Arnaud, M. Alouani, and R. Ahuja, Applied Physics Letters 92, 201903 (2008).
3. “Crystal structure of the pressure-induced metallic phase of SiH4 from ab initio theory”, D. Y. Kim, R. H. Scheicher, S. Lebègue, J. Prasongkit, B. Arnaud, M. Alouani, and R. Ahuja, Proceedings of the National Academy of Sciences of the USA 105, 16454 (2008).
4. “Predicted High-Temperature Superconducting State in the Hydrogen-Dense Transition-Metal Hydride YH3 at 40 K and 17.7 GPa”, D. Y. Kim, R. H. Scheicher, and R. Ahuja, Physical Review Letters 103, 077002 (2009).
5. “General trend for pressurized superconducting hydrogen-dense materials”, D. Y. Kim, R. H. Scheicher, H.-K. Mao, T. W. Kang, and R. Ahuja, Proceedings of the National Academy of Sciences of the USA 107, 2793 (2010).
6. “Predicted Formation of Superconducting Platinum-Hydride Crystals under Pressure in the Presence of Molecular Hydrogen”, D. Y. Kim, R. H. Scheicher, C. J. Pickard, R. J. Needs, and R. Ahuja, Physical Review Letters 107, 117002 (2011).
7. “Unveiling the complex electronic structure of amorphous metal oxides”, C. Århammar, A. Pietzsch, N. Bock, E. Holmström, C.M. Araujo, J. Gråsjö, S. Zhao, S. Green, T. Peery, F. Hennies, S. Amerioun, A. Föhlisch, J. Schlappa, T. Schmitt, V. N. Strocov, G. A. Niklasson, D. C. Wallace, J.E. Rubensson, B. Johansson and R. Ahuja, PNAS (the Proceedings of the National Academy of Sciences, USA) 108, 6355 (2011).
8. “Atomic Diffusion in Solid Molecular Hydrogen”, A. B. Belonoshko, M. Ramzan, H. K. Mao and R. Ahuja, Nature-Scientific Reports 3, 2340 (2013).
9. “Effect of uniaxial strain on the site occupancy of hydrogen in vanadium from density-functional calculations”, R. Johansson, R. Ahuja, O. Eriksson, B. Hjörvarsson, and R. H. Scheicher, Scientific Reports 5, 10301 (2015).
10. “Structural characterization of amorphous Fe(1-x)Zrx”, R. Johansson, G. Muscas, S. George, M. Ahlberg, K. Kádas, D. Arvanitis, R. Ahuja, O. Eriksson, R. H. Scheicher and P. Jönsson, in manuscript.

Physical properties of atoms, molecules and solids are determined by the collective behavior of electrons interacting with each other and with the nuclei. Unfortunately, an analytical solution of the electronic problem in a realistic system is often out of reach and one must resort to a numerical approach. In our group we contribute to developing various computational methods to solve the electronic problem in atoms, molecules and solids.

The most substantial part of our activity is focused on methods to solve the fundamental equation of density functional theory, the Kohn-Sham equation. They comprehend full-potential methods like Elk and RSPt, which are based on linear augmented plane waves and linear muffin-tin orbitals, respectively. In RSPt, we also develop computational techniques for treating strongly correlated materials, based on the dynamical mean-field theory (DMFT). A database of electronic structure has also been developed. Other methodological developments are also being pursued, e.g. a real-space electronic structure method that calculates the Green's function of the valence band states using recursion expressions and the continued fraction method, as well as the exact muffin-tin orbitals method (EMTO). The division also hosts developments of tight-binding methods that are used for calculations of e.g. topological states or the damping parameter of the Landau-Lifshitz-Gilbert equation.

Magnetic materials are an essential part of our everyday life. Data storage, communication, and especially green energy production and electrical engines rely on magnetic materials based on high performance magnets which often contain critical components. Huge efforts are made to find suitable non-hazardous replacements. A new emerging field for magnetic materials is magnetic refrigeration. This can be an efficient way to cool environments, both homes as well as fridges and freezers in future.

Today computational simulation plays an inherent role in understanding and optimizing materials properties. We study fundamental properties of magnetic materials for applications in permanent magnets (PM), magnetic cooling (magneto-caloric effect (MCE)), and spintronics based on density functional theory in combination with atomistic spin-dynamics, and Monte Carlo solvers. For PM the key task is to identify new uniaxial phases which have the same characteristics as today's PM but contain much less critical material i.e. rare earth. In case of the MCE fundamental understanding is addressed by investigating materials which show first order (often metamagnetic) magnetic phase transition.

The unique theoretical understanding available for correlated 1D electrons can be leveraged to design superconductors where electron pairing from repulsive interactions is understood, and the critical temperature for the onset of superconductivity could be increased based on accurate theory. This theory can not just be applied to propose novel superconducting devices and bulk materials, but also to understand pairing from repulsive electrons in existing unconventionally superconducting (USC) materials that are built from 1D structures (Bechgaard and Fabre salts, Sr-ladder compounds, K2Cr3As3).

We are developing theory leveraging the unique theoretical understanding available for correlated 1D electrons in order to

• design superconductors where electron pairing from repulsive interactions is understood, and the critical temperature for the onset of superconductivity could be increased based on accurate theory
• design 1D nanowires that are effectively superconducting, and would be so at high temperatures
• understand pairing from repulsive electrons in existing unconventionally superconducting (USC) materials that are built from 1D structures (Bechgaard and Fabre salts, Sr-ladder compounds, K2Cr3As3)
• transfer the mechanism to stabilize superconductivity from 1D to 2D materials, such as doped graphene

Method-wise, these aims will be achieved by combining parallel density matrix renormalization group numerics (pDMRG) with analytical approaches. More information about this work can be found at Theory Group Adrian Kantian.

Background

In condensed matter physics superconducting (SC) materials are of great fundamental and technological interest. In a superconductor, electrons show correlated motion, as opposed to their effectively independent behaviour in a conventional metal – we speak of electron pairing. When the phases of macroscopically many of these pairs become coherent at low enough temperature, they exhibit off-diagonal long range order (ODLRO). This allows for lossless transport of current and energy, which crucially is protected by a SC gap.

The unconventionally superconducting (USC) materials are especially important to condensed matter physics then. As in them pairing results from repulsive electron-electron interactions, they can work at much higher temperatures than conventionally SC materials (where phonon-mediated pairing is just a few Kelvin strong). But no matter which USC material is studied – whether the high-Tc materials, or the organic Bechgaard and Fabre salts (the first materials discovered to be USC) – their fundamental electron pairing mechanism is uniformly very difficult to understand. This prevents theory from guiding any deliberate and systematic increase of critical temperatures for superconductivity in USC materials.

In this context, one-dimensional (1D) electrons have two unique advantages over higher-dimensional ones: in 1D, electron interactions are effectively the strongest, crucial to stability against thermal pair-breaking. And the theory of 1D electrons has a major edge, as it can solve and understand models of USC materials which in higher spatial dimensions remain unsolved, or admit solutions only under strong approximations not widely accepted. The drawback that so far has prevented 1D materials to be used as superconductors are the strong quantum and thermal fluctuations in 1D systems that prevent macroscopic phase coherence of pairs, and thus suppress superconductivity.

This project is based on eliminating this drawback of 1D electron systems via several different mechanisms, each corresponding to concrete physical set-ups. The systems thus enlarged are no longer strictly one-dimensional, but remain in some ways dominated by their original 1D character. The novel theory framework this project is developing will make it possible to gain qualitative and quantitative understanding of both existing and new to-be-proposed USC devices and materials. This understanding will be significantly beyond that currently available for e.g. fully 2D or 3D models and materials, and allow to systematically improve key properties like the critical temperature for superconductivity based on well-controlled and accurate theory.

Modern photon and electron beam sources make it possible to probe excitation spectra in many systems, ranging from atoms and molecules to bulk materials and complex interfaces. The interpretation of the experimental data is, however, problematic without an adequate theoretical support. To address this, we develop theories for predicting and analyzing spectra measured in different experiments, as e.g. (angle-resolved) photoemission spectroscopy, magneto-optical spectroscopy, X-ray emission and X-ray absorption, electron energy loss spectra, as well as resonant inelastic X-ray scattering.

The topics of our interest are fundamental theory of spectroscopy, method development, as well as understanding of functional materials, for energy conversion and molecular electronics, or where complex interactions play a role. Examples are Li based batteries and Ru based catalysts for the productions of solar fuels and correlated oxides such as high-temperature superconductors. The theoretical tools we employ involve density functional theory (DFT), linear-response theory, dynamical mean field theory (DMFT), Mahan-Nozieres-De Dominicis theory, and multi-configurational methods used in quantum chemistry.

Name of Barbara’s project

DMFT and RSPt

Name of Jan’s project: Electron magnetic circular dichroism

Name of Olle’s project

Name of Peter’s project: Theory of ultrafast magneto-X-ray spectroscopy

In order to meet the challenges of modern synchrotron radiation facilities, we develop theories for analyzing the experimental results that are produced in these facilities. This involves calculations of x-ray and photoelectron spectra of molecules, interfaces and bulk materials.

The theory is focused not only on functional materials used for instance in energy conversions and molecular electronics, but also on materials in which the fundamental understanding of complex interactions are in focus. Examples of the former are Li based batteries and Ru based catalysts for the productions of solar fuels, and of the latter are correlated oxides such as high-temperature superconductors. The spectra we focus on involve angular resolved photoemission spectroscopy, angle integrated valence band spectra, optical excitations, x-ray emission and x-ray absorption, as well as Resonant Inelastic X-ray Scattering (RIXS). The theoretical tools we employ involve density functional theory, dynamical mean field theory, Mahan-Nozieres-De Dominicis theory, and multi-configurational methods. Examples of this research can be found in Refs. [1]-[6].

References:

1. A. Grechnev, I. Di Marco, M. I. Katsnelson, A. I. Lichtenstein, J. Wills and O. Eriksson, “Theory of quasiparticle spectra of Fe, Co and Ni; bulk and surface”, Phys. Rev. B 76, 35107 (2007).
2. P. Thunström, I. Di Marco and O. Eriksson, “Electronic Entanglement in Late Transition Metal Oxides”, Phys. Rev Letters 109, 186401 (2012).
3. I. Di Marco, P. Thunström, M. I. Katsnelson, J. Sadowski, K. Karlsson, S. Lebegue, J. Kanski, O. Eriksson, “Electron correlations in MnxGa1-xAs as seen by resonant electron spectroscopy and dynamical mean field theory”, Nat. Commun. 4, 2645 (2013).
4. B. Brena, C. Puglia, M. de Simone, M. Coreno, K. Tarafder, V. Feyer, R. Banerjee, E. Göthelid, B. Sanyal, P. M. Oppeneer, O. Eriksson, “Valence-Band Electronic Structure of Iron Phthalocyanine: an Experimental and Theoretical Photo Electron Spectroscopy Study”, J. of Chemical Physics 134, 074312 (2011).
5. S. Bhowmick, J. Rusz and O. Eriksson, “X-ray absorption spectra: graphene, h-BN and their alloy BC2N”, Phys. Rev. B 87, 155108 (2013).
6. R. Sanchez-de-Armas, B. Brena, I. Rivalta and C. Moyses Araujo, “Soft X-ray spectroscopic properties of ruthenium complex catalyst under CO2 electrochemical reduction conditions: a first-principles study”, J. Phys. Chem. C 119, 22899 (2015).

Spin-bearing metalorganic molecules provide a unique platform for exploiting spin properties, leading to molecular spintronics and on-surface magnetochemistry, an emergent area in which reversible switching of the molecule’s spin is utilized.

To prepare the way for spin-switchable single-molecule electronic devices, we perform fundamental investigations of the complex interplay of chemical and magnetic interactions in metalorganic systems, using density-functional-theory-based methodologies.

Organic molecules are cheap, multifunctional materials that are promising for many technological applications. In the present focus for molecular spintronics applications stand, in particular, magnetic planar organic molecules adsorbed on substrates, such as metal-porphyrins and phthalocyanines and spin-crossover complexes. To investigate how efficient spin manipulation of spin-bearing molecules can be achieved, we use ab initio calculations that can provide valuable, novel insight into the atomic origins of molecular spin polarization and reveal promising routes to achieve active spin control in future molecular devices.

Name of Peter’s project [Link to Peter’s page]

Name of Biplab’s project

Nanobiotechnology is a hot new research field in which nano-sized artificial structures are combined with natural biological systems for a broad range of purposes, among them bio-sensing and drug delivery. We are active in both these research areas.

For sensing applications, our focus is on nanopores as a new approach for sequencing DNA. We use molecular dynamics simulations to explore the movement of DNA in the nanopore channel, density functional theory to determine the electronic structure of the nano-bio device, and quantum transport methods to calculate the electrical conductance which constitutes the output signal to distinguish between the four different bases in DNA.

Our other research focus is the use of 2D nanomaterials as targeted drug delivery agents for the treatment of cancer, antibiotic resistance, neurodegenerative diseases like Alzheimer’s etc. Here, ab initio simulations are used by us to mainly understand the adsorption characteristics of the drug molecules in the context of binding energies, electronic structure and optical properties.

Nanopores for analysis of DNA and proteins [Link to Ralph’s page]

Name of Biplab’s project [Link to Biplab’s page]

Spin-bearing metalorganic molecules provide a unique platform for exploiting spin properties, leading to molecular spintronics and on-surface magnetochemistry, an emergent area in which reversible switching of the molecule’s spin is utilized.

To prepare the way for spin-switchable single-molecule electronic devices, we perform fundamental investigations of the complex interplay of chemical and magnetic interactions in metalorganic systems, using density-functional-theory-based methodologies.

Organic molecules are cheap, multifunctional materials that are promising for many technological applications. In the present focus for molecular spintronics applications stand, in particular, magnetic planar organic molecules adsorbed on substrates, such as metal-porphyrins and phthalocyanines and spin-crossover complexes. To investigate how efficient spin manipulation of spin-bearing molecules can be achieved, we use ab initio calculations that can provide valuable, novel insight into the atomic origins of molecular spin polarization and reveal promising routes to achieve active spin control in future molecular devices.

Name of Peter’s project [Link to Peter’s page]

Name of Biplab’s project