LOCATION: Seminarroom Sensengasse, Ground floor, Sensengasse 8, 1090 Vienna

Strongly-correlated rare-earth (RE) and actinide compounds are studied by an ab initio framework that combines the density-functional theory (DFT) with a quasi-atomic (Hubbard-I) dynamical treatment for the on-site Coulomb repulsion between localized f states. We first discuss applications of this approach to the crystal-field (CF) splitting on RE ions in hard magnetic transition-metal-RE intermetallics. The CF splitting on the 4f shells in these compounds is known to determine the RE single-ion (SI) magnetic anisotropy, which is crucial for the hard-magnetic behavior of these systems. Within the present approach the 3d magnetism of the TM sublattice is treated by the standard local spin-density approximation (LSDA), while strong correlations on the RE 4f shell are included through Hubbard-I. We carefully remove an unphysical self-interaction contribution to CF due to the LSDA [1]; an optimal choice for the localized basis to represent 4f orbitals in solids is also discussed. The method is applied to RCo5 and RFe11MX families of ferromagnetic intermetallics (R=Nd,Sm, M=Fe,Ti, X=empty, N, Li), for which we analyze the impact of 4f-conduction states hybridization on the CF splitting and magnetic anisotropy.

In the second part of the talk we present a linear-response approach for extracting intersite exchange interactions between localized shells.  Starting from the high-temperature local-moment paramagnetic state described within the DFT+Hubbard-I framework, we derive these exchange interactions by evaluating the response of DFT+ Hubbard-I functional to small fluctuations in atomic configurations on two neighboring sites [2]. Using this approach we evaluate the superexchange coupling between the dipole and quadrupole U moments in UO2 [3]. The calculated superexchange Hamiltonian has a non-collinear 3k antiferromagnetic (AFM) ground state, in agreement with the experimental magnetic structure of UO2. We find that the stabilization of 3k AFM is due to a subtle anisotropy of the quadrupolar superexchange lifting the degeneracy between various AFM structures on the frustrate fcc U sublattice.  Applying the same approach to NpO2 we predict a purely multipolar low-temperature ordered state with no dipole magnetic moments and a primary triakontadipole rank-5 order parameter.

[1] P. Delange et al. PRB 96, 155132 (2017) [2] L. V. Pourovskii PRB 94 115117 (2016) [3] L. V. Pourovskii and S. Khmelevskyi, PRB 99, 094439 (2019).

LOCATION: University of Vienna, Faculty of Physics, Seminar Room A, Währinger Straße 17, 2nd Floor, 1090 Vienna

Recent experiments have clearly demonstrated that the non-equilibrium dynamics can induce many intriguing physics in strongly correlated materials. Among them, the most striking is the discovery of photo-induced transient superconducting behaviors in some of high-T_c cuprates and alkali-doped fullerenes. It has also been shown theoretically that superconductivity can be enhanced or induced by pulse irradiation in models for these materials.

However, the main focus so far, both experimentally and theoretically, is a photo-induced state that may already exist in the equilibrium ground state phase diagram.

Here, employing unbiased numerical methods, we show that pulse irradiation can induce superconductivity as photoinduced excited states in the Mott insulator of the Hubbard model. The superconductivity is due to the η-pairing mechanism and exhibits the pair-density-wave like staggered off-diagonal long-range correlation.

Since the superconductivity is absent in the ground state phase diagram, i.e., not induced by photo-doping of carriers or due to a dynamical phase transition by effectively changing the physical parameters, our finding provides a conceptually different pathway to a non-equilibrium control of unraveling hidden excited states and may also give an alternative interpretation for the enhancement of superconductivity observed in the recent experiments.

This work is in collaboration with T.Kaneko, T. Shirakawa, and S. Sorella.

[1] T. Kaneko, T. Shirakawa, S. Sorella, and S. Yunoki, arXiv: 1809.01865.

LOCATION: TU Vienna, Seminar Room 138C (Freihaus, tower B/yellow, 9. OG)

Complex fluids exhibit both elastic and viscous like properties at different time scales in response to an externally applied shear. This response depends on the interplay of the shear-induced and structural relaxation timescales of the material. In this talk, I will present our results on the rheology of glasses and a mixture of liquid crystals and magnetic nanoparticles.

Glasses, which are characterized as simple yield stress fluids, show transient shear banding, the nature of which is not very well understood. Using extensive molecular dynamics (MD) simulations, we first demonstrate that a directed percolation transition occurs near the yielding of the glass. We, then, quantify the effect of temperature, age and shear rate on transient dynamical heterogeneities in thermal glasses.

The composites of liquid crystals (LC) and magnetic nanoparticles (MNP) are interesting hybrid systems in terms of advancing new functionalities. The equilibrium dynamics and rheology of these mixtures are not very well studied as compared to their equilibrium self-assembly. We investigate, using equilibrium and non-equilibrium MD simulations, the equilibrium dynamics and the effect of shear on a LC-MNP mixture where the sizes of both the species are comparable.

We demonstrate that the anisotropic environment provided by the LC matrix strongly affects the equilibrium translational dynamics of the MNPs. This effect is reflected in the form of a subdiffusive regime at intermediate times in the mean square displacement of the MNPs which gets extended as the strength of the dipolar coupling is increased. Also, when the external shear is applied, the mixture shows a transition from Newtonian to non-Newtonian behavior. The extent of the non-Newtonian regime is increased as the strength of the dipolar coupling among the magnetic particles is increased.

References:

1. "Yielding of glass under shear: a directed percolation transition precedes shear-band formation", G. P. Shrivastav, P. Chaudhuri, J. Horbach, Phys. Rev. E 94, 042605 (2016).

2. "Heterogeneous dynamics during yielding of glasses: Effect of aging", G. P. Shrivastav, P. Chaudhuri, J. Horbach, J. Rheol. 60, 835 (2016).

3. "Anomalous transport of magnetic colloids in a mixture of liquid crystals and magnetic colloids", G. P. Shrivastav, Sabine H. L. Klapp, under review in Soft Matter (2018).

LOCATION: TU Vienna, seminar room 138C (Freihaus, tower B/yellow, 9. OG)

Efficient spin injection into semiconductors remains a formidable and elusive challenge even after almost three decades of major scientific effort, studded by obstacles and only partial workarounds. Few years ago I predicted the possibility of injecting massive ultrashort spin current pulses across a ferromagnetic metal/semiconductor interface [M. Battiato, K. Held, Phys. Rev. Lett. 116, 196601 (2016)]. We have now proved experimentally the prediction.  By injecting strongly out-of-equilibrium sub-picosecond spin current pulses across a bare ferromagnet/semi-conductor interface, we have overcome the crippling problem of impedance mismatch and obtained a massive spin transfer. We demonstrated this by producing ultrashort spin current pulses into cobalt and injecting them into monolayer MoS2. The semiconducting MoS2 layer also acts as a selective converter of the spin current into a charge current, whose THz emission is then measured. As predicted, we measured a giant spin current, orders of magnitude larger than typical injected spin current densities in modern devices.

LOCATION: TU Vienna, seminar room 138C (Freihaus, tower B/yellow, 9. OG)

Many of the most relevant chemical properties of matter depend explicitly on atomistic and electronic details, rendering a first principles approach to chemistry mandatory. Alas, even when using high-performance computers, brute force high-throughput screening of compounds is beyond any capacity for all but the simplest systems and properties due to the combinatorial nature of chemical space, i.e. all compositional, constitutional, and conformational isomers. Consequently, efficient exploration algorithms need to exploit all implicit redundancies present in chemical space. I will discuss recently developed statistical learning approaches for interpolating quantum mechanical observables in compositional and constitutional space. Results for our models indicate remarkable performance in terms of accuracy, speed, universality, and size scalability.        

LOCATION: TU Vienna, Seminar Room 138C (Freihaus, Tower B/yellow, 9. OG)

The family of atomically thin two-dimensional (2D) materials, which started with graphene, has expanded rapidly over the past few years and now includes insulators, semiconductors, metals, ferromagnets, and superconductors. This development has prompted an explosion in envisioned applications ranging from batteries and catalysis to photovoltaics, electronics, and photonics. In parallel with this development, the possibility of stacking different 2D materials into van der Waals heterostructures has opened new routes for designing atomically flat heterostructures with tailored properties.

I will show how the electronic and optical properties of 2D materials and their heterostructures can be accurately predicted by combining classical electrostatic models with many-body quantum mechanics, and high-performance computing. I will give examples from our recent research focusing on 2D structures with tunable band structures, excitons, and plasmons[1]. Finally, I will present our recent efforts to establish a comprehensive database of 2D materials using an automatic high-throughput framework (http://c2db.fysik.dtu.dk) and show how it can be used to identify 2D materials with interesting physical properties such as ferromagnetism and non-trivial topology[2].   

LOCATION: TU Vienna, Seminar Room 138C (Freihaus, Tower B/yellow, 9. OG)

Recently, M. Engel et al [1] demonstrated self-assembly of icosahedral quasicrystals in a model one-component system, governed by parametrized oscillating pair potential (EOPP) [2].

I will show how the same type of interaction -- EOPP -- fitted to ab-initio forces in ternary Al--Cu--Fe alloy, spontaneously forms icosahedral quasicrystals in fully realistic replica-exchange atomistic simulation.

The quasicrystal phase is at high temperatures thermodynamically stable in a unique solid state, in which two kinds of small fundamental clusters permanently reshuffle, while maintaining icosahedral order in a flexible icosahedral network. The figure shows two snapshots of the high-temperature (1200K) structure separated by 2 ns time; the colors scale with occupancy probability by atoms (average over 0.5ns time). Red: Fe, yellow/gray: Al, green/blue : Cu.

[1] M. Engel, P. F. Damasceno, C. L. Phillips and C. Glotzer: {it Computational self-assembly of a one-component icosahedral quasicrystal}; Nat. Mat. 2014, DOI10.1038/NMAT4152\\

[2] M. Mihalkovi\v c, C. L. Henley: {\it Empirical oscillating potentials from ab--inito fits...}; Phys. Rev. B{\bf 85}, 092102 (2014)

LOCATION: Faculty of Physics, Erwin-Schrödinger-Lecture Hall, 5th floor, Boltzmanngasse 5, 1090 Vienna

Neutrons and charged particles produced by nuclear reactions in the fuel assembly of a fission power plant or in the deuterium-tritium plasma of a fusion tokamak produce significant changes in the physical and mechanical properties of materials. These changes result from processes occurring at atomic scale. Fast neutrons initiate collision cascades, in which radiation defects are formed. These collision cascade events do not change the chemical composition of reactor materials and result only in the formation of fairly stable and relatively well localized distortions of atomic structure; these are the radiation defects. The defects migrate, react, coalesce, and grow, resulting in the formation of a particular type of microstructure, occurring only in materials exposed to irradiation. If the kinetic energy of neutrons exceeds a certain threshold, nuclear transmutations may also occur. In contrast to collision cascades, transmutation reactions modify the chemical composition of irradiated materials. For example, initially chemically pure tungsten bombarded by neutrons transmutes into an alloy containing significant amounts of rhenium, osmium, and tantalum. Transmutations also result in the accumulation of helium and other noble gases in the lattice, stimulating swelling and giving rise to embrittlement.

Quantifying the effects of irradiation on materials requires developing multiscale models for microstructural evolution, which describe how the defects evolve and interact, and how the resulting microstructure responds to external mechanical stress, magnetic fields or temperature gradients.  The key parameters defining the response of irradiated materials to macroscopic engineering variables are the operating temperature, irradiation dose and irradiation dose rate. I shall review information about the various properties of candidate fusion materials in the context of fusion reactor design effort, and in relation to the on-going development of models for defect and dislocation microstructure derived from ab initio based atomistic and mesoscopic simulations.

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053 and from the RCUK Energy Programme [grant number EP/P012450/1]. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

Turbulence is the major cause of friction losses in transport processes and it is responsible for a drastic drag increase in flows over bounding surfaces. While much effort is invested into developing ways to control and reduce turbulence intensities, so far no methods exist to altogether eliminate turbulence if velocities are sufficiently large.
We demonstrate for pipe flow that appropriate distortions to the velocity profile lead to a complete collapse of turbulence and subsequently friction losses are reduced by as much as 95%. Counterintuitively, the return to laminar motion is accomplished by initially increasing turbulence intensities or by transiently amplifying wall shear. The usual measures of turbulence levels, such as the Reynolds number (Re) or shear stresses, do not account for the subsequent relaminarization. Instead an amplification mechanism measuring the interaction between eddies and the mean shear is found to set a threshold below which turbulence is suppressed beyond recovery.