# Abstracts Invited Speakers

The preliminary abstracts of the talks given by the invited speakers.

# Commercial vs. academic software: a love-hate story

Anders Blom QuantumWise A/S
For over 13 years, QuantumWise has worked on developing commercial software tools for atomic-scale simulations. During this time we have collaborated with academic groups on several levels, involving both pure research into new algorithms and methods, co-implementation of libraries, and integration of existing code of various types. This has given us a lot of insight into the dynamics between academic and commercial software, how they can co-exist, and what works and what doesn't both from the perspective of the distributor (the commercial partner) and the end-user. In my presentation I will explore this relationship, touching on topics like code quality, licensing models, pricing, open/closed source, and the roles of professional software manufacturers vs. that of scientists. In addition you will learn about how our tools can be used for atomic-scale simulations within materials science and nanoelectronics, and in particular how our platform allows users to set up calculations with almost any software packages (like VASP, Abinit, QuantumEspresso, LAMMPS) and analyzing the output results.

# Electronic structure calculations for correlated materials -- A Dynamical Mean Field Theory perspective

Silke Biermann CPHT -- Ecole Polytechnique
Dynamical mean field theory (DMFT), in conjunction with electronic structure techniques such as density functional theory (DFT), has led to tremendous progress in the description of excited state properties of materials with strong electronic Coulomb correlations. The challenge nowadays consists in refining the interface of electronic structure and many-body theory in order to develop quantitatively accurate predictive schemes. In this talk, we focus on recent efforts of incorporating dynamical screening effects into a DMFT-based description of correlated materials [1]. Such effects can stem either from higher energy degrees of freedom that have been integrated out or from nonlocal processes that are effectively backfolded into a local description. This can be conveniently done by combined many-body perturbation theory and dynamical mean field theory ("GW+DMFT") techniques [2]. An analysis of the effects of the different corrections to standard DFT+DMFT schemes leads to new insights also into DFT itself [3, 4].\newline \newline [1] For a review, see S. Biermann, J. Phys.: Condens. Matter 26 173202 (2014). \newline [2] S. Biermann, F. Aryasetiawan, A. Georges, Phys. Rev. Lett. 90 086402 (2003); T. Ayral et al., Phys. Rev. B 87, 125149 (2013); Phys. Rev. Lett. 109 226401 (2012); J.M. Tomczak et al., Europhys. Lett. 100 67001 (2012); Phys. Rev. B 90 165138 (2014). \newline [3] A. van Roekeghem et al., Phys. Rev. Lett. 113 266403 (2014); Europhysics Letters 108 57003 (2014). \newline [4] M. Hirayama et al., arXiv:1511.03757

# Correlated systems at extreme conditions

Theoretical simulations of transition metals and their alloys at extreme conditions of ultra-high pressure and temperature generally rely on a picture of non-magnetic wide-band systems with insignificant local correlations. Upon compression, the overlap between localized states increases and so does the bandwidth W, while the local Coulomb repulsion U between those states is screened more efficiently. The reduction of the U/W ratio is used to rationalize the absence of electronic correlations beyond the local or semi-local approximations within the Density Functional Theory (DFT) at high-pressure conditions. We show that this generally accepted picture is incorrect. Carrying out theoretical simulations at the level of Dynamical Mean-Field Theory combined with DFT (DFT+DMFT) [1] coupled to advanced experimental studies of materials at extreme conditions, we show that many-electron effects may have strong influence on the electronic structure and properties of transition metals, their alloys and compounds. Indeed, it is well known that correlation effects are essential for a description of the pressure induced insulator-to-metal transitions (IMT). We illustrate this by considering IMTs in transition metal oxides [2,3]. Moreover, considering hcp Fe and Os, we show that including correlation effects is necessary for the description of the topological changes of the Fermi surface for valence electrons at high pressure, the so-called electronic topological transition (ETT) [4,5]. Considering Fe at the conditions of the Earth’s core, we show that DFT+DMFT calculations allow one for better understanding of the Earth’s geodynamo [6,7]. Finally, considering Os compressed to over 770 GPa, we discuss the anomaly observed experimentally in the behavior of the unit cells parameters ratio c/a at about 440 GPa. We argue that the anomaly might be related to a new type of electronic transition, the core level crossing (CLC) transition, associated with interactions between the core electrons induced by pressure [5]. \newline \newline [1] A. Georges, G. Kotliar, W. Krauth, and M. J. Rozenberg, Rev. Mod. Phys. {\bf 68}, 13 (1996); https://triqs.ipht.cnrs.fr/ \newline [2] V. Potapkin, L. Dubrovinsky, I. Sergueev, M. Ekholm, I. Kantor, D. Bessas, E. Bykova, V. Prakapenka, R. Hermann, R. R{\"u}ffer, V. Cerantola, J. Jönsson, W. Olovsson, S. Mankovsky, H. Ebert, and I. A. Abrikosov, Phys. Rev. B {\bf 93}, 201110(R) (2016). \newline [3] I. Leonov, L. Pourovskii, A. Georges, and I. A. Abrikosov, in manuscript. \newline [4] K. Glazyrin, L.V. Pourovskii, L. Dubrovinsky, O. Narygina, C. McCammon, B. Hewener, V. Schünemann, J. Wolny, K. Muffler, A. I. Chumakov, W. Crichton, M. Hanfland, V. Prakapenka, F. Tasn{\`a}di, M. Ekholm, M. Aichhorn, V. Vildosola, A. V. Ruban, M. I. Katsnelson, I. A. Abrikosov, Phys. Rev. Lett. {\bf 110}, 117206 (2013). \newline [5] L. Dubrovinsky, N. Dubrovinskaia, E. Bykova, M. Bykov, V. Prakapenka, C. Prescher, K. Glazyrin, H.-P. Liermann, M. Hanfland, M. Ekholm, Q. Feng, L. V. Pourovskii, M. I. Katsnelson, J. M. Wills, and I. A. Abrikosov, Nature {\bf 525}, 226–229 (2015). \newline [6] L. V. Pourovskii, T. Miyake, S. I. Simak, A. V. Ruban, L. Dubrovinky, and I. A. Abrikosov, Phys. Rev. B {\bf 87}, 115130 (2013). \newline [7] L. V. Pourovskii, J. Mravlje, A. Georges, S. I. Simak, I. A. Abrikosov, arXiv:1603.02287 [cond-mat.str-el].

# Imaging and spectroscopy of Low Dimensional Structures to the Ångström Length and Attosecond Time Scales.

Anders Mikkelsen Lund University
We combine microscopy/spectroscopy to the atomic scale, ultra fast laser technology, synchrotron sources and advanced nanoscale device manufacturing. This allow development and use of a new imaging techniques with potential for orders of magnitude better resolution in both time and space for direct studies of materials and devices confined in one or more dimensions - even during operation. Two main themes will be covered: Firstly, we combine the femtosecond and attosecond time resolution of advanced lasers with the nanoscale spatial resolution of PhotoEmission Electron Microscopy (PEEM). Using 5.5 femtosecond laser pulses with a central energy around 1.6 eV in an interferometric time-resolved PEEM setup, we observe differences in near-field enhancement inside a variety of nanostructures already within the first few optical cycles[1,2]. We demonstrate imaging using <100 attosecond laser pulses with central energies between 30-100eV in combination with PEEM (attoPEEM)[3,4] and outline the pathway for using such pulses for direct investigations of photon excitations with sub-femtosecond precision. The combination of advanced lasers and PEEM allows for very sensitive measurements of plasmonics fields, surface chemistry and pump-probe experiments on ultrafast time scales - all in the same picture. Secondly, we have developed and used scanning probe microscopy and high brightness synchrotron based microscopy to determine structure, chemistry and physical properties of III-V semiconductor nanowires. We have developed novel methods to directly image surfaces both inside, outside and topside of nanowires down to the single atom level[5,6]. We have now extended this too direct imaging of nanowire devices in operation[7,8] and have now found that real atomic resolution can be achieved during operation. Using our rather diverse toolbox we can obtain a real understanding of the connections between structure, growth and function of these nanowires. \newline \newline [1] E. Mårsell et.al Nano Lett. 15 (2015) 6601 \newline [2] E. Lorek et.al, Optics express 23(2015)31460 \newline [3] A. Mikkelsen et.al., Rev. Sci. Instrum. 80(2009)123703 \newline [4] ”Imaging Localized Surface Plasmons by Femtosecond to Attosecond Time‐Resolved Photoelectron Emission Microscopy–“ATTO‐PEEM””, Chapter in “Attosecond Nanophysics: From Basic Science to Applications”, Wiley‐VCH Verlag GmbH \& Co (2015) \newline [5] A. Mikkelsen and E. Lundgren, Invited Prospective, Surf. Sci. 607 (2013) 97 \newline [6]. M. Hjort et al, Nano Lett. 13 (2013) 4492 \newline [7] J. Webb et al, Nano Lett. 15 (2015) 4865 \newline [8] O. Persson et al, Nano Lett. 15 (2015) 3684

# Electronic properties of Mg-Ni-H across the metal-to-insulator transition for industrial hydrogen sensing

Dook van Mechelen ABB Corporate Research
Optical hydrogen sensors have a promising future in a society where hydrogen detection becomes increasingly essential. The traditional use of Pd as sensing material is disadvantaged by its small optical contrast and low sensitivity. A solution is found by transition metal hydrides that cross a metal-to-insulator transition upon hydrogenation. Since a typical hydrogen sensor operates in a continuous hydrogen pressure range, knowledge of the optical properties in the vicinity of the metal-to-insulator transition is crucial. Here we present an optical spectroscopy study on Mg-Ni-H where we investigate the approach towards the insulating state. Upon increasing the hydrogen content, the infrared conductivity demonstrates a strongly decreasing intraband peak and upcoming mid-infrared spectral weight. DFT calculations of hydrogen deficient Mg$_2$NiH$_x$ indicate that for hydrogen vacancy concentrations between 0.25 and 2 per unit cell, states appear in the original gap of Mg$_2$NiH$_4$. Transitions involving these states cause significant mid-infrared spectral weight, in agreement with the experiment. In this talk, we put forward how the observed optical properties in relationship with DFT models and the proposed strongly correlated mechanism of the metal-to-insulator transition are of importance for industrial optical hydrogen sensing.

# Twisted excitations in low-dimensional quantum magnets

Mechthild Enderle Institut Laue-Langevin
Many magnetic materials are well described by classical approximations. When conventional magnets are cooled down, their atomic magnetic moments order into a periodic pattern. In quantum magnets, however, the atomic magnetic moments seem to disappear when cooled down close to absolute zero temperature, in spite of their interaction. Here, classical approximations dealing with local moments break down and the collective behavior of the interacting assembly requires a description in terms of a many-body quantum mechanical wave function. While the ground state wave function is usually inaccessible to experimental investigation, its character is nevertheless evidenced in unusual collective excitations which can be probed by inelastic neutron scattering. We present neutron scattering evidence for collective quantum excitations in materials where the magnetic interaction dominates in one or two spatial dimensions.

# Emergent behaviour and magnetic frustration.

Pascale Deen European Spallation Source
Frustrated magnetic materials are promising candidates for new states because lattice geometry suppresses conventional magnetic dipole order. Frustration thus drives novel emergent states. Classical spins on the 3D triangular hyperkagome lattice have long been considered ideal for novel states yet have provided few examples. Gd$_{3}$Ga$_{5}$O$_{12}$ is the canonical frustrated magnet since the compound does not order via the pervasive “order by disorder” mechanism down to the lowest temperatures probed, 25 mK [1 and ref. therein]. Short-range correlations, determined via neutron diffraction, have long been assumed to originate from near neighbour short-range interactions [1]. However in recent work an emergent spin state of decagon looped structures at the lowest temperatures has been uncovered [2,3]. These emergent decagon spin loops remain dominant when attempting to polarise the spin structure by a large applied magnetic field [4]. Neutron inelastic scattering shows a dominant contribution at large momentum transfers from a band of almost dispersionless excitations that correspond to the spin waves localized on ten site rings, expected on the basis of nearest neighbor exchange interaction [4]. These result illustrate the richness and diversity that arise from frustrated exchange on the three-dimensional hyperkagome lattice \newline \newline [1] O. A. Petrenko, C. Ritter, M. Yethiraj, D. McK Paul, Phys. Rev. Lett. {\bf 80}, 4570 (1998). \newline [2] J. A. M. Paddison, A. L. Goodwin, Phys. Rev. Lett. {\bf 108}, 017204 (2012). \newline [3] P. P. Deen, O. Florea, E. Lhotel, and H. Jacobsen, Phys. Rev. B {\bf 91}, 014419 (2015) \newline [4] N. d’Ambrumenil, O. A. Petrenko, H. Mutka, and P. P. Deen, Phys. Rev. Lett. {\bf 114}, 227203 (2015)

# Relativistic ferroelectrics: a novel class of multifunctional materials

Silvia Picozzi Consiglio Naz. Ricerche CNR-SPIN
The discovery of novel properties, effects or microscopic mechanisms in modern materials science is often driven by the quest for the coexistence and/or coupling of several functional properties into a single compound. Within this framework, by exploiting the interplay between spin and dipolar degrees of freedom via spin-orbit coupling in ferroelectric semiconductors, I will focus on the tight link between k-dependent spin-splitting in the electronic structure, spin-texture and electric polarization. Based on density functional simulations, I will show our theoretical predictions of a giant Rashba spin-splitting in “bulk” GeTe[1], prototype of novel multifunctional materials - labeled as Ferro-Electric Rashba Semi-Conductors (FERSC)[2] - where the chirality of the spin texture is one-to-one linked to polarization. As the latter can be induced/controlled/switched via an electric field in a non-volatile way, the integration of semiconductor spintronics with ferroelectricity is envisaged. In the second part of the talk, the connection between ferroelectricity and spin-degrees of freedom will be discussed by providing examples from different materials classes (oxides heterostructures,[3] halides perovskites,[4] chalcogenides, etc), all of them showing strong relativistic effects. \newline \newline [1] D. Di Sante, P. Barone, R. Bertacco and S. Picozzi, Adv. Mater. 25, 509 (2013); M. Liebmann et al, Adv. Mater. {\bf 28}, 560 (2016). \newline [2] S. Picozzi, Front. Physics {\bf 2}, 10 (2014) \newline [3] K. Yamauchi, P. Barone, T. Shishidou, T. Oguchi and S. Picozzi, Phys. Rev. Lett. {\bf 115}, 037602 (2015) \newline [4] A. Stroppa, D. Di Sante, P. Barone, M. Bokdam, G. Kresse, C. Franchini, M.-H. Whangbo, S. Picozzi, Nature Communications {\bf 5}, 5900 (2014)

# Search for new f-electron topological materials

Liu Hao Tjeng MPI CPfS
The interplay of strong spin-orbit coupling and electron-electron correlations in f-electron containing intermetallic systems have recently been shown to allow for the emergence of topologically nontrivial surface bands, thereby bringing into existence the new field of "strongly correlated topological insulators". Using a minimum model consisting of localized f-electrons and dispersive conduction electrons with opposite parity, it has been inferred that f-electron systems with cubic and tetragonal symmetries will open both hybridization plus insulating gaps and host topologically protected metallic surface states, if the ground state multiplet is $\Gamma$ 8 and $\Gamma$ 6 , respectively $^{[1,2,3,4}$. Motivated by this theory, we will present two candidate materials: CeRu$_4$Sn$_6$ $^5$ and SmO $^6$. We will discuss the electronic structure of these materials in detail using both synchrotron-based spectroscopic data and parametrized band structure calculations. \newline \newline [1] M. Dzero, K. Sun, V. Galitski and P. Coleman, Phys. Rev. Lett. {\bf 104}, 106408 (2010). \newline [2] M. Dzero, K. Sun, P. Coleman, and V. Galitski, Phys. Rev. B {\bf 85}, 045130 (2012). \newline [3] M. Dzero, and V. Galitski, J. of Expt. and Theo. Phys. {\bf 117}, 499, (2013). \newline [4] T. Takimoto, J. Phys. Soc. Jpn. {\bf 80}, 123710 (2011). \newline [5] M. Sundermann, F. Strigari, T. Willers, H. Winkler, A. Prokofiev, J.M. Ablett, J.P. Rueff, D. Schmitz, E. Weschke, M. Moretti Sala, A. Al-Zein, A. Tanaka, M.W. Haverkort, D. Kasinathan, L.H. Tjeng, S. Paschen, and A. Severing, Scientific Reports {\bf 5}, 17937 (2015) \newline [6] D. Kasinathan, K. Koepernik, L.H. Tjeng, and M.W. Haverkort, Phys. Rev. B {\bf 91}, 195127 (2015). \newline \newline Work done in collaboration with A. Severing$^1$ , D. Kasinathan$^2$ , M.W. Haverkort$^2$ \newline $^1$ Institute of Physics II, University of Cologne, Germany. \newline $^2$ Max Planck Institute for Chemical Physics of Solids, Dresden, Germany

# Should we really give up with DFT in strongly correlated quantum transport?

Gianluca Stefanucci University of Rome Tor Vergata
Density functional theory (DFT) is certainly not the first method which comes to mind to tackle quantum transport in the strongly correlated regime. Nevertheless, today we have a rigorous DFT-like approach and an accurate exchange-correlation (xc) potential to reproduce the I-V characteristic of the Anderson model, a hallmark of strong correlation, in a wide range of temperatures, gates and biases. In this talk I retrace the steps [1-4] that led us to the current understanding of the problem and discuss some general features of the xc potential to perform ab-initio calculations on strongly correlated nanoscale junctions.\newline ~\newline [1] G. Stefanucci and S. Kurth, Phys. Rev. Lett. {\bf 107}, 216401 (2011). \newline [2] S. Kurth and G. Stefanucci, Phys. Rev. Lett. {\bf 111}, 030601 (2013). \newline [3] G. Stefanucci and S. Kurth, Nano Lett. {\bf 15}, 8020 (2015). \newline [4] S. Kurth and G. Stefanucci, arXiv:1605.09330 (2016).

# Spin dynamics and spin-heat interaction

Anna Delin KTH Royal Institute of Technology
I will present some recent results involving both atomistic and classical spin-dynamics simulations (based on first-principles calculations). These methods can be used to investigate theoretically a range of spin-based phenomena, such as Gilbert damping and spin stiffness. [1,2] Recently, we have turned some focus toward spincaloritronic phenomena, i.e. the interaction between spin and heat. Specifically, we have studied energy and magnetization transport in oscillator trilayers and networks. In the presence of a thermal gradient, we find a rectification of the flows at certain conditions. Our model also predicts that energy and magnetization in certain situations may flow between two sources with the same temperature and chemical potential. This latter effect can be compared to the well-known dc Josephson effect in superconductors. [3,4] \newline \newline [1] Philipp Dürrenfeld, Felicitas Gerhard, Jonathan Chico, Randy K. Dumas, Mojtaba Ranjbar, Anders Bergman, Lars Bergqvist, Anna Delin, Charles Gould, Laurens W. Molenkamp, Johan Åkerman, Phys. Rev. B {\bf 92}, 214324 (2015). \newline [2] Yuli Yin, Fan Pan, Martina Ahlberg, Mojtaba Ranjbar, Philipp Dürrenfeld, Afshin Houshang, Mohammad Haidar, Lars Bergqvist, Ya Zhai, Randy K. Dumas, Anna Delin, Johan Åkerman, Phys. Rev. B {\bf 92}, 024427 (2015). \newline [3] Simone Borlenghi, Weiwei Wang, Hans Fangohr, Lars Bergqvist, Anna Delin, Phys. Rev. Lett. {\bf 112}, 047203 (2014). \newline [4] Simone Borlenghi, Stefano Iubini, Stefano Lepri, Lars Bergqvist, Anna Delin, Jonas Fransson (2015). Coherent energy transport in classical nonlinear oscillators: An analogy with the Josephson effect, Phys. Rev. E {\bf 91}, 040102(R) (2015).

# Recent extensions to the one-step theory of photoemission

Hubert Ebert University of Munich / Dept. Chemistry
During the last decades the one step model of photo-emission was extended in various ways. An important step was the introduction of the Coherent Potential Approximation (CPA) that led to a very reliable description of angle resolved photo emission spectroscopy (ARPES) of transition metal alloys. Making use of the alloy analogy model the CPA has been exploited as well to deal with thermal lattice vibrations treated in a quasi-static manner. This scheme has recently been complemented by a corresponding description of thermal spin fluctuations that are of crucial importance for the properties of magnetic materials at finite temperatures. The second part of the talk will be devoted to the extension of the one-step model to deal with pump-probe photo emission. The approach is based on a general treatment of the photo emission process using the Keldysh formalism for the lesser Green function to describe the real-time evolution of the electronic degrees of freedom in the initial state after a strong pump pulse that drives the system out of equilibrium. The final state is represented by a time-reversed low energy electron diffraction (LEED) state. The formalism allows for a quantitative calculation of the time-dependent photo current that results from core or valence-band excitations, and is applicable to simple metals or more complex materials like topological insulators. So far the scheme has been implemented to deal with two-photon photo emission (2PPE) where both the pump-pulse and the probe pulse are weak in intensity. As a first application results for 2PPE spectra of Ag(100) are presented, where the first image state serves as an intermediate state to probe the occupied band structure below the Fermi level. Application of the approach to more complex situations as well as its application to other types of spectroscopy will be discussed.