Research Highlights 2018

Magnetic skyrmions and anti-skyrmions are topologically protected nanoscopic vortices of magnetization that can be stabilized in magnets without inversion symmetry. They have the potential to be used as magnetic bits in high density storage devices such as racetrack memories.  Depending on the helicity of spin rotation, we observed different types of skyrmion textures in Heusler compounds.

Discovery of magnetic anti-skyrmions in tetragonal inverse Heusler compounds

Magnetic skyrmions and anti-skyrmions are topologically protected nanoscopic vortices of magnetization that can be stabilized in magnets without inversion symmetry. They have the potential to be used as magnetic bits in high density storage devices such as racetrack memories.  Depending on the helicity of spin rotation, we observed different types of skyrmion textures in Heusler compounds.

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Single crystals are the pillars for many technological advancements, which begin with acquiring the material. Since different compounds have different physical and chemical properties, different techniques are needed to obtain their single crystals. New classes of quantum materials, from insulators to semimetals, that exhibit non-trivial topologies, have been found. They display a plethora of novel phenomena, including topological surface states, new fermions such as Weyl, Dirac, or Majorana, and non-collinear spin textures such as antiskyrmions. To obtain the crystals and explore the properties of these families of compounds, it is necessary to employ different crystal growth techniques such as the chemical vapour transport method, Bridgman technique, flux growth method, and floating-zone method.

Single crystal growth for topology and beyond

Single crystals are the pillars for many technological advancements, which begin with acquiring the material. Since different compounds have different physical and chemical properties, different techniques are needed to obtain their single crystals. New classes of quantum materials, from insulators to semimetals, that exhibit non-trivial topologies, have been found. They display a plethora of novel phenomena, including topological surface states, new fermions such as Weyl, Dirac, or Majorana, and non-collinear spin textures such as antiskyrmions. To obtain the crystals and explore the properties of these families of compounds, it is necessary to employ different crystal growth techniques such as the chemical vapour transport method, Bridgman technique, flux growth method, and floating-zone method.

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The electronic-structure group investigates magnetic, correlated, and topological materials by angular resolved photoelectron spectroscopy.
The high energy and momentum (angular) resolutions establish it as a leading technique for extensive studies of various topological and correlated materials.

Investigation of topological and correlated materials by angular-resolved photoelectron spectroscopy

The electronic-structure group investigates magnetic, correlated, and topological materials by angular resolved photoelectron spectroscopy.

The high energy and momentum (angular) resolutions establish it as a leading technique for extensive studies of various topological and correlated materials.

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Half-Heusler compounds have a wealth of distinctive characteristics that are promising for use as thermoelectric materials. Here, we developed n-type (Zr,Hf)CoSb-based thermoelectric compounds with a high zT of ≈ 1.0 by considering lanthanide contraction as a design factor to select reasonable alloying atoms. Combined with our previous work, we established (Zr,Hf)CoSb as the first half-Heusler system with matching p-type and n-type thermoelectric performance.
 

Half-Heusler compounds for thermoelectric energy conversion

Half-Heusler compounds have a wealth of distinctive characteristics that are promising for use as thermoelectric materials. Here, we developed n-type (Zr,Hf)CoSb-based thermoelectric compounds with a high zT of ≈ 1.0 by considering lanthanide contraction as a design factor to select reasonable alloying atoms. Combined with our previous work, we established (Zr,Hf)CoSb as the first half-Heusler system with matching p-type and n-type thermoelectric performance.

 

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Topological materials have been essential to discover analogs of fermionic elementary particles and to test fundamental laws predicted in high-energy physics.  Considering the richness of physics that electrical transport in topological bulk materials have recently been provided, our group seeks to unravel thermal transport signatures of new emergent quantum phenomena in topological semimetals. Within the last three years, we have observed signatures of the mixed axial-gravitational anomaly in the Weyl semimetal NbP and of hydrodynamic electron flow at the Planckian bound of dissipation WP2.

Thermal transport in topological semimetals

Topological materials have been essential to discover analogs of fermionic elementary particles and to test fundamental laws predicted in high-energy physics.  Considering the richness of physics that electrical transport in topological bulk materials have recently been provided, our group seeks to unravel thermal transport signatures of new emergent quantum phenomena in topological semimetals. Within the last three years, we have observed signatures of the mixed axial-gravitational anomaly in the Weyl semimetal NbP and of hydrodynamic electron flow at the Planckian bound of dissipation WP2.
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The detailed balance of interactions between spin, charge, orbital and lattice degrees of freedom leads to a large variety of electronic and magnetic ground states in strongly correlated materials. Our studies on the structural and electronic properties of alkalisesqioxides A4O6 with A = Cs, Rb revealed a Verwey-type charge ordering and electron localization transition within the negatively charged oxygen molecule building units. New silver oxoruthenates synthesized by hydrothermal methods were found to feature low-dimensional structural and magnetic properties. 

Oxides with peculiar electronic and magnetic properties

The detailed balance of interactions between spin, charge, orbital and lattice degrees of freedom leads to a large variety of electronic and magnetic ground states in strongly correlated materials. Our studies on the structural and electronic properties of alkalisesqioxides A4O6 with A = Cs, Rb revealed a Verwey-type charge ordering and electron localization transition within the negatively charged oxygen molecule building units. New silver oxoruthenates synthesized by hydrothermal methods were found to feature low-dimensional structural and magnetic properties. 

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Topological materials such as Weyl semimetal are characterized by three-dimensional linear band touching points. This results an ultra-high electrical conductvity, high carrier mobility, as well as a robust surface states. The chiral electrons could couple with the electrons of adsorbates selectively and influence the surface chemical reactions. Our experiments suggest that these topological crystals could act as high efficient catalysts for hydrogen and oxygen evolution reactions.

Topological materials as efficient catalysts for hydrogen evolution reaction

Topological materials such as Weyl semimetal are characterized by three-dimensional linear band touching points. This results an ultra-high electrical conductvity, high carrier mobility, as well as a robust surface states. The chiral electrons could couple with the electrons of adsorbates selectively and influence the surface chemical reactions. Our experiments suggest that these topological crystals could act as high efficient catalysts for hydrogen and oxygen evolution reactions.

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As an edge-cutting topological state, magnetic Weyl semimetals (WSMs) with time-reversal-symmetry breaking are expected to exhibit the fascinating physical behavior of the intrinsic and large anomalous Hall effect (AHE) owing to the nonzero Berry curvature from Weyl nodes, except for the chiral anomaly effect in the bulk and Fermi arc on the surface.Our study establishes magnetic Kagomé-lattice WSM as a key material for fundamental research and device applications connecting topological physics and spintronics.

Magnetic Weyl Semimetals

As an edge-cutting topological state, magnetic Weyl semimetals (WSMs) with time-reversal-symmetry breaking are expected to exhibit the fascinating physical behavior of the intrinsic and large anomalous Hall effect (AHE) owing to the nonzero Berry curvature from Weyl nodes, except for the chiral anomaly effect in the bulk and Fermi arc on the surface.
Our study establishes magnetic Kagomé-lattice WSM as a key material for fundamental research and device applications connecting topological physics and spintronics. [more]
Symmetry engineering plays an important role, enabling us to tune the anomalous Hall conductivity (AHC) in various compounds. Depending upon suitable atomic arrangements or doping, one can alter the band structure in such a way that AHC attains a colossal value or, obtain a band structure that is topologically equivalent to that of a trivial semiconductor with zero AHC. We investigate the magnetic, electrical, and thermal transport properties of various topological magnetic Heusler compounds. We discover the first three-dimensional topological magnet Co2MnGa, showing one of the highest AHC ~ (1600 Ω-1cm-1) at 2 K and highest known room temperature anomalous Hall angle (up to 12%) and anomalous Nernst thermopower  (~6.0 µV K−1 at 1 T magnetic field). Besides, we discover the first metallic intermetallic alloy compound Mn2CoGa, with finite magnetization (2 mB/f.u.), but zero AHC.

Topological Magnetic Heuslers: Role of Symmetry and the Berry Phase

Symmetry engineering plays an important role, enabling us to tune the anomalous Hall conductivity (AHC) in various compounds. Depending upon suitable atomic arrangements or doping, one can alter the band structure in such a way that AHC attains a colossal value or, obtain a band structure that is topologically equivalent to that of a trivial semiconductor with zero AHC. We investigate the magnetic, electrical, and thermal transport properties of various topological magnetic Heusler compounds. We discover the first three-dimensional topological magnet Co2MnGa, showing one of the highest AHC ~ (1600 Ω-1cm-1) at 2 K and highest known room temperature anomalous Hall angle (up to 12%) and anomalous Nernst thermopower  (~6.0 µV K−1 at 1 T magnetic field). Besides, we discover the first metallic intermetallic alloy compound Mn2CoGa, with finite magnetization (2 mB/f.u.), but zero AHC.

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The thin film laboratory focuses on the growth and characterization of thin films and heterostructures of Heusler and other intermetallic compounds, which cover the areas of spintronics, skyrmions, new permanent magnets and antiferromagnets. The physical properties of thin films can be controlled via epitaxial growth and strain engineering, providing a platform to study novel phenomena of interest in both pure and applied sciences. Measurements of thin films allow continued exploration of topologically driven physical effects, as well as the application of these materials in functional devices.

Tunable multifunctional properties in thin films of Heusler and other intermetallic compounds

The thin film laboratory focuses on the growth and characterization of thin films and heterostructures of Heusler and other intermetallic compounds, which cover the areas of spintronics, skyrmions, new permanent magnets and antiferromagnets. The physical properties of thin films can be controlled via epitaxial growth and strain engineering, providing a platform to study novel phenomena of interest in both pure and applied sciences. Measurements of thin films allow continued exploration of topologically driven physical effects, as well as the application of these materials in functional devices.

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With MoTe2 and PdSe2 we identified two compounds which combine topology with superconductivity. MoTe2 with both superconductivity and a topologically non-trivial band structure is probably the first example of a time-reversal invariant topological (Weyl) superconductor. The unusual pressure dependence of superconductivity in pyrite-phase of PdSe2 and topologically nontrivial bulk and surface states in its electronic structure offers a possibility to study the interplay between superconductivity and topological matter.

High-pressure studies of topological materials

With MoTe2 and PdSe2 we identified two compounds which combine topology with superconductivity. MoTe2 with both superconductivity and a topologically non-trivial band structure is probably the first example of a time-reversal invariant topological (Weyl) superconductor. The unusual pressure dependence of superconductivity in pyrite-phase of PdSe2 and topologically nontrivial bulk and surface states in its electronic structure offers a possibility to study the interplay between superconductivity and topological matter.

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A major consequence of topology in materials is the existence of massless fermions in the form of quasiparticles which are responsible for many exotic phenomena and properties for example; chiral anomaly, gravitational anomaly, anomalous Hall effect, planar Hall effect, extremely high mobility, extremely large unsaturated magnetoresistance etc. Existence of fermions in topological materials goes even beyond the Weyl and Dirac, showing nodal line fermion, double Weyl fermion, triple point fermion, sextuple fermion etc. which also underline the physics of unusual properties.

Exceptional transport properties of topological semimetals and metals

A major consequence of topology in materials is the existence of massless fermions in the form of quasiparticles which are responsible for many exotic phenomena and properties for example; chiral anomaly, gravitational anomaly, anomalous Hall effect, planar Hall effect, extremely high mobility, extremely large unsaturated magnetoresistance etc. Existence of fermions in topological materials goes even beyond the Weyl and Dirac, showing nodal line fermion, double Weyl fermion, triple point fermion, sextuple fermion etc. which also underline the physics of unusual properties.

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According to the topological band theory, both insulators and semimetals can be classified into topological trivial and non-trivial states. Protected by the topological invariants from bulk, all the topological matter host interesting surface state, such as two typical examples of Dirac cones in topological insulators and non-closed Fermi arcs in Weyl semimetals. In addition, topological semimetals also exhibit exotic transport properties due to the enhanced Berry curvatures. The research interest in our group is to predict new topological materials and understanding their physical properties in combination topological band theory, model analysis, density functional theory, and linear response theory.

Theoretical study of topological materials

According to the topological band theory, both insulators and semimetals can be classified into topological trivial and non-trivial states. Protected by the topological invariants from bulk, all the topological matter host interesting surface state, such as two typical examples of Dirac cones in topological insulators and non-closed Fermi arcs in Weyl semimetals. In addition, topological semimetals also exhibit exotic transport properties due to the enhanced Berry curvatures. The research interest in our group is to predict new topological materials and understanding their physical properties in combination topological band theory, model analysis, density functional theory, and linear response theory.

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