Room: Fysikhuset E324 Schrödinger. Each talk is 30 minutes plus 10 minutes questions. In the morning and the afternoon we will have a break after the second talk.

9.00-9.40 Natalia Dubrovinskaia

Material Physics and Technology at Extreme Conditions, Laboratory of Crystallography, University of Bayreuth, Bayreuth, Germany

Title: Materials synthesis and crystallography at extreme pressure-temperature conditions revealing remarkable materials properties

During last decades, the impact of high-pressure studies on fundamental physics, chemistry, and Earth and planetary sciences, has been enormous. Modern science and technology rely on the vital knowledge of matter which is provided by crystallographic investigations. The most reliable information about crystal structures of solids and their response to alterations of pressure and temperature is obtained from single-crystal diffraction experiments. Advances in diamond anvil cell (DAC) techniques, designs of double-stage DACs, and in modern X-ray instrumentation and synchrotron facilities have enabled structural research at multimegabar pressures.

We have developed a methodology for performing single-crystal X-ray diffraction experiments in double-side laser-heated DACs and demonstrated that it allows the crystal structure solution and refinement, as well as accurate determination of thermal equations of state above 200 GPa at temperatures of thousands of degrees. Application of this methodology resulted in discoveries of novel compounds with unusual chemical compositions and crystal structures, uncommon crystal chemistry and physical properties. Perspectives of materials synthesis and crystallography at extreme conditions will be outlined.

9.40-10.20 Mikhail I. Katsnelson
Theory of Condensed Matter, Institute for Molecules and Materials, Radboud University, Nijmegen, Netherlands

Title: Theory of magnetic interactions in real materials

Magnetic ordering and related phenomena are of essentially quantum and essentially many-body origin and require strong enough electron-electron interactions. Also, they are very sensitive to the details of electronic structure of specific materials. This makes a truly microscopic description of exchange interactions a challenging task. Long ago we suggested a general scheme of calculations of exchange interactions responsible for magnetism based on the “magnetic  force theorem. It was formulated originally as a method to map the spin-density functional to effective classical Heisenberg model, the exchange parameters turned out to be, in general, essentially dependent on initial magnetic configuration and not universal. However, they are directly related to the spin-wave spectrum and, thus, can be verified experimentally. This approach also lies in the base of “ab initio spin dynamics” within the density functional approach.

It is well known now that this scheme is, in general, insufficient for strongly correlated systems and should be combined with the mapping to the multiband Hubbard model and use of, say, dynamical mean-field theory (DMFT) to treat the latter. Our original approach can be reformulated within the DMFT.

The method can be also modified to calculate Dzialoshinskii-Moriya interactions which play a crucial role in the phenomenon of weak ferromagnetism, in physics of magnetic skyrmions, and in magnonics/spintronics in general.

I will discuss both general methods and their applications to electronic structure and magnetism of various groups of magnetic materials including elemental transition and rare-earth metals, half-metallic ferromagnets, transition metal oxides, molecular magnets, and sp electron magnets based on adatoms on Si surface.

10.20-10.40 Coffee break

10.40-11.20 Janusz Sadowski
Department of physics and electrical Engineering, Linnaeus University, Kalmar Sweden; Institute of Physics, Polish Academy of Science, Warsaw, Poland and Faculty of Physics, University of Warsaw, Poland.

Title: Transition metal dichalcogenides – old materials, new physics

Over the last decade transition metal dichalcogenides (TMD) gained tremendous attention of the materials science research community, due to their unique optical and electric properties. These materials with chemical formula defined as MX2, where M is a heavy transition metal (such as W, Mo, Ta, Ti, V,…) and X- group VI element (such as S, Se, Te) constitute a rich family of layered crystals with weak bonding between subsequent molecular layers (tri-layers with plane of transition metal sandwiched in-between the planes of chalcogen atoms). Most of TMDs have hexagonal (2H) crystal structure and exhibit semiconducting properties, but in the bulk form they have indirect energy-gap which results in poor optical properties. However in the 2-dimensional form of the single molecular layers which can be easily cleaved from bulk crystals due to the weak boding between adjacent chalcogen atoms planes TMDs exhibit excellent optical properties. In 2D geometry semiconducting TMDs have direct energy gap and are excellent light emitters despite the minute volume of the material involved. Hence semiconducting TMDs are outstanding candidates for flexible optoelectronics applications. Some of materials from this reach family exhibit very peculiar metallic properties, constituting so called topological semimetals, where, charge carriers are highly mobile and are protected against back-scattering, due to the quantum effects associated with distinct crystal symmetry properties and chemical composition (heavy transition metal elements involved).

I will review the most interesting properties of TMDs, using as an example thin films of MoTe2, and MoSe2 grown by molecular beam epitaxy.

11.20-13.00 Lunch (on your own)


Afternoon session (15.15-17.45):

15.15-15.55 I. A. Abrikosov
Department of Physics, Chemistry and Biology, Linköping University, Linköping, Sweden

Title: Wide-bandgap semiconductors for the next generation of quantum devices

Quantum devices of the next generation are expected to actively create, manipulate and read out quantum states of matter, processing of quantum information beyond today’s limits. A basic unit of the quantum information processing – qubit – has to be stored in atomic- and subatomic-scale systems. Spins associated with defects in semiconductors are promising room-temperature qubits. In this talk, we demonstrate that SiC can, in principle, combine its well-developed semiconductor technology and the best of classic solid-state qubits, like NV-centres in diamond, allowing for a single optically addressable spin embedded in a high-performance electronic material. Recently, members of our team have for the first time successfully engineered single divacancies [1] and Si vacancies [2] in SiC. We discuss multiple advantages of the defects in SiC, including long spin coherence time even at room temperature with high-fidelity infrared spin-to-photon interface [3,4], highly spin-dependent photoluminescence with intensity contrasts of 15%–36% [7], high fidelity bi-directional nuclear qubit initialization [8] and optical control of the defect charge state [9]. Finally, we demonstrate our strategy for a systematic exploration of the vast yet unexplored alternative materials for the next generation quantum technology via combined theoretical and experimental characterization of common point defects in wide-band gap semiconductors [10,11].

This work is supported by Knut and Alice Wallenberg Foundation project KAW 2018.0071. Contributions from all the team members, V. Ivády, J. Davidsson, R. Armiento, V. Darakchieva, M. Schubert, M. Bourennane, I. G. Ivanov, J. Ul Hassan, and N. T. Son are gratefully acknowledged.

[1] G. Balasubramanian, et al., Nature Mater. 8, 383 (2009).

[2] A. Morello et al., Nature 467, 687 (2010).

[3] D.J. Christle, et al., Nature Materials 14, 160 (2015).

[4] M. Widmann, et al., Nature Materials 14, 164 (2015).

[5] D. J. Christle et al., Phys. Rev. X 7, 021046 (2017).

[6] R. Nagy et al. Nature Commun. 10, 1954 (2019).

[7] A. L. Falk et al., Phys. Rev. Lett. 112, 187601 (2014).

[8] V. Ivády et al., Phys. Rev. Lett. 117, 220503 (2016).

[9] B. Magnusson et al., Phys. Rev. B 98, 195202 (2018).

[10] V. Ivády et al., Phys. Rev. B 96, 161114(R) (2017).

[11] J. Davidsson, et al., New J. Phys. 20, 023035 (2018); Appl. Phys. Lett. 114, 112107 (2019).

15.55-16.35 Paolo Sessi
Max Planck Institute of Microstructure Physics Halle (Saale), Germany

 Title: Functionalized topological materials: visualizing new trends at the atomic scale

The discovery of topological materials represents a milestone in condensed matter physics. It twisted the way we look at the band structure of solids classifying them in terms of well-defined global invariants of their bulk wave function electronic space. When non-trivial, these invariants are associated to the emergence of boundary modes which, because of their topological origin, are protected against weak disorder. In my talk, I will provide an overview of universal trends which emerge in topological materials once interacting with functional perturbations.

I will start by discussing topological insulators interacting with time-reversal symmetry breaking perturbations. I will demonstrate that, contrary to the general belief, magnetic order and gapless states can coexist. By analyzing the quantum anomalous Hall effect platform, I will show that this apparent paradox is associated to a dual nature of the dopants which gives rise to a two fluid behaviour with opposite and competing trends, i.e. gap opening vs. gap closing.

I will then move on topological crystalline insulators where I will report on the discovery of robust 1D spin-polarized channels naturally emerging at TCI surfaces once translational invariance is broken. I will illustrate how these 1D channels can be easily obtained in the prototypical TCI Pb1−xSnxSe compound without the need of any sophisticated preparation technique and demonstrate how, contrary to 1D topological states known so far, their protection mechanisms result in a striking robustness to defects, strong magnetic fields, and elevated temperature.

Finally, I will discuss more recent results on Weyl semimetals, showing how their interaction with atomic scale perturbations give rise to strong and universal spectroscopic signatures associates to their topological states, i.e. Weyl points and Fermi arcs. These signatures are found to be stoichiometry independent, providing a unifying picture of the Weyl phase diagram.

16.35-16.55 Coffee break

16.55-17.35 Yasmine Sassa
Chalmers University of Technology, Department of Physics, Göteborg, Sweden

Title: Kagome silicene: a novel exotic form of two-dimensional epitaxial silicon

Since the discovery of graphene, intensive efforts have been made in search of novel 2D materials. Decreasing the materials dimensionality to their ultimate thinness is a promising route to unveil new physical phenomena, and potentially improve the performance of devices. Among recent 2D materials, analogs of graphene, the group IV elements have attracted much attention for their unexpected and tunable physical properties. Depending on the growth conditions and substrates, several structures of silicene, germanene, and stanene can be formed. Here, we report the synthesis of a Kagome lattice of silicene on AL (111) substrate. We provide evidence of such 2D Si allotrope through STM observations, high-resolution core-level and ARPES measurements, along with DFT calculations. The formation of a Kagome silicene opens up the possibility of realizing high temperature quantum Hall states, which is of broad relevance for the investigation of modern quantum materials and the fabrication of cheaper devices.

17.35 Styrelsemöte