Highlights 2016

We are dedicated to theoretical prediction of exotic topological quantum materials, such as topological insulators, quantum anomalous Hall insulators, and topological semimetals. Guided by the materials design strategy, we search for topological materials by accurate ab initio calculations and close interactions with solid-state chemists in our institute.

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In the frontier of condensed-matter physics, the Weyl semimetals are the new star materials. The WSM is a semimetal regarded as the 3D analogue of graphene, wherein the electronic bands linearly disperse around pairs of nodes, the Weyl points, of fixed (left or right) chirality. We, an interdisciplinary team of our institute, have been carrying out pioneering experimental research on WSM materials with the guidance of theoretical calculations.

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Manganese-based Heusler compounds are studied with the focus on their magnetocrystalline anisotropy and exchange interaction, in order to obtain a general model for this class of compounds. An in-plane to out-of-plane transition of the magnetization direction has been found to strongly depend on the number of valence electrons. The competing interactions may allow for the appearance of non-collinear order as found in Mn₂RhSn.

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Since 2014 our thin film laboratory has been in function. First thin films are sputtered and characterized. For spintronics and permanent magnets we are searching for hard magnetic materials. Manganese-rich Heusler-compounds with large out-of-plane magnetization and large saturation magnetization have been designed for new magnets whereas a high magneto crystalline anisotropy and low magnetic moment for future magnetic random access technologies (MRAM) and spin torque oscillators for wireless communication.

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Nanodots were fabricated from a thin film of Mn₇₀Ga₃₀ with perpendicular magnetic anisotropy.  The nanolithography was achieved using self-assembled polystyrene beads (500-600 nm in diameter) as a mask.  The resultant nanostructures, after low temperature annealing, exhibit chemical order and perpendicular magnetic anisotropy.  These results suggest this lithography procedure is a promising route to fabricating spin-valve devices.

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Chemical intuition is a powerful tool for predicting novel materials. We have succeeded to derive the governing factors of phase stability for transition metal based Heusler compounds. By applying simple rules combined with first-principles calculations it was possible to predict structure, disorder, magnetic and electrical properties of Mn- and Ni-based Heusler compounds.

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High external pressure is a powerful tool to change electronic properties of matter. Here we have studied the electronic transport properties of β-polymorph of molybdenum ditelluride (MoTe₂). β- MoTe₂ is theoretically predicted to be a Weyl-semimetal with band structure around the Fermi level extremely sensitive to change in the lattice constants. At ambient pressure, β- MoTe₂ is a superconductor with critical temperature of superconductivity (T_c) as low as 0.1 K. We have found, that application of pressure drastically increases the T_c of this Weyl-semimetal up to »8K.

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We have designed ferrimagnetic Heusler materials with compensated magnetic sublattice.  A small amount of ferromagnetic inclusions inside the ferrimagnetic background results in an anomalous zero field cooled exchange bias. Then we combine two oppositely magnetized ferrimagnets to design a fully compensated magnetic state that exhibits an extremely large exchange bias and coercivity.

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We have designed acentric Heusler materials Mn₂RhSn and Mn-Pt-Sn that exhibit noncollinear magnetic structure. With help of Lorentz transmission electron microscopy we have demonstrated the existence of helical and skyrmion phases in these materials. In addition, we also find topological Hall effect related to the noncollinear spin alignment.

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Intermetallic half-Heusler compounds are promising candidates for thermoelectric applications.  Several approaches are used to optimize the thermoelectric figure of merit zT, e.g. such tuning the carrier concentration n. Here we optimized carrier concentration of the p-type half-Heusler alloy Ti_0.3Zr_0.35Hf_0.35CoSb_1−xSn_x by partially substitution of Sb by Sn. A maximal figure of merit of zT = 0.8 was achieved for a substitution level of 15% Sn.

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Solid-state refrigeration based on the magnetocaloric effect (MCE) and barocaloric effect (BCE) represents the promising alternative to gas compression refrigeration, yielding up to 50% improved efficiency. Heusler alloys are extremely interesting materials for applications based on the MCE (both conventional and inverse) and BCE since they can show large entropy and temperature change at their magnetic (second-order transition) and magneto-structural (first-order transition) phase transitions with the application of magnetic field and pressure.

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Ionic liquids as electrolyte gate dielectric is a pathway to reach new metastable phases in oxides. The very intense electric field induced at the liquid-solid interface by the polarized electrolyte drives reversible structural changes and produces distinct electronic properties. In vanadium dioxide, the electrolyte gating induces a metallic state suppressing the metal-insulator transition. The electronic properties and particularities of the transformation are reveled  by hard x-ray photoelectron spectroscopy (HAXPES).

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Competing exchange interactions and magneto-structural coupling lead to different spin structures of comparable energy in 3d-5d double perovskites. Detailed experimental and theoretical investigations provide insights into the nature of interactions which drive the formation of unconventional spin structures in Sr₂Fe³⁺Os⁵⁺O₆ and Sr₂Co²⁺Os⁶⁺O₆.

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Similar as in 3d transition metal compounds, anionic p-electron systems show an intimate interplay between spin, charge, orbital and lattice degrees of freedom. We have isolated a tetragonal modification of the alkalisesquioxide Rb₄O₆ which evidences charge ordering of paramagnetic O₂^- and diamagnetic O₂^2- molecular anions. A structural phase transition between the cubic and tetragonal modification is the origin of anomalies in the magnetic properties of Rb₄O₆ and Cs₄O₆.

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The entanglement between the surface and bulk of topological materials brings about many exotic phenomena, such as never saturated magnetoresistance, novel quantum oscillations, anomalous Hall effect and chiral anomaly. Materials topology creates unique gapless property either on the surface or in the bulk, where electrons travel with relativistic velocity. These materials have low energy excitation and fast moving charge carriers that are suitable for low energy devices.

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Electron localization remains one of the most studied physical phenomena already for more than half of a century, whereas its constructive potential for the Materials Science is just at the opening threshold. Recently, we proposed the new spin-selective version of the localization, which opens new prospectives for spintronics, by allowing to induce a high spin-polarization in tetragonal Mn-based Heusler compounds with a strong magnetocrystalline anisotropy.

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