Session Q70: Frontiers of Topological Materials: Synthesis, Prediction, and Characterization

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For me, fields are best to work in when new theory, experimental characterization and materials discovery happen simultaneously. That was the case in topological materials research, and its fair to assume that this coupling will happen in several unimaginable or barely imaginable areas of science during your research lifetime. (So “be true to yourself, and never give up” is still the best advice that survivors can give to young scientists in my opinion.) Current efforts in topological materials largely appear to center on the elucidation of what from my perspective appear to be complex issues in new physics, and thus are more difficult for materials chemists to find things to do that are both chemically and physically interesting.  Although materials chemists can make thin layers of previously known materials specifically for physicists to study, “where is the chemistry in that” can sometimes be heard sotto voce in the chemistry community about that kind of work. Those things being said, the collaboration between theoretical and experimental physicists, crystal growers and materials chemists to invent, grow, and experimentally explore he properties of materials that have the potential to display new physics has proven to be very productive, and should best continue. Aside from my personal views of research at the chemistry/physics interface, my idea for this talk is to show how chemical and materials science views of materials discovery and optimization can be applied to materials hosting topological electronic states. Since some of you have not yet taken your personal shot at some of the materials of interest for the development of topological physics, there may be materials of interest to your work presented here.   

Innovative Electron Probe Methods for van der Waals Heterostructures Topological Materials Discovery

New Topological Insulators (TI) quantum materials with interesting/useful properties has caused great excitement and promises to transform device design and computation.   Remarkably, some of the quantum phenomena displayed by these materials now persists at room temperature, opening the way for usable devices.   

We present results from studies of TI's such as  Ba₆Nb₁₁S₂₈ . This material naturally realizes vdW coupled heterointerfaces between transition metal dichalcogenide (TMD) monolayers (hexagonal NbS₂, H-NbS₂) and insulating spacers Ba₃NbS₅  Low Energy Electron Diffraction (LEED) taken along the c-axis shows that the hexagonal spacer and TMD layers,  are commensurate.  The electronic band structure can be understood as that resulting from superimposing a periodic potential defined by Ba₃NbS₅ onto monolayer H-NbS₂. This is similar to the mechanism which yields flat bands and correlated physics in twisted-bilayer graphene and TMD heterostructures and is one of the key advances in the prediction of band structure and properties.  We examine these materials with direct electron probe /photoemission methods Low/Photoemission Energy Electron Microoscopy (L/PEEM)  and ARPES. LEEM is an extremly low energy electon probe method that allows fine surface(S) probing down to several layers.   We show that new approaches to probing methods can show that behavior is a consequence of the underlying symmetry properties of the multi-layer system, lattice such as in the spin-orbit coupled ferromagnetic state. 

The analysis of TI quantum materials presents new challenges on how to minimize surface and sample damage, new approaches to corelate materials properties with the pseudo 2D structure , we present some of our multi-modal and electron and photoemission probe techniques approach in this presentation.   

Nonlocal Measurement as a Probe of the Spin Hall Effect in Topological Insulators

Topological insulators (TIs) are promising candidates for alternative computing device designs. In particular, they have great potential for spintronic devices, where utilization of electron spin rather than charge would allow for lower-power and higher-performance computing in next-generation architectures. Efficient conversion between spin and charge signals is crucial to spintronic technology. TIs provide highly efficient spin-to-charge conversion, as a result of their unique topological properties. One way to electrically quantify conversion efficiency is with the spin Hall effect (SHE). Here, we present SHE measurements of the topological insulator Bi₂Te_2.5Se_0.5. Because of the topological nature of this material, we can measure the SHE without the use of ferromagnetic injectors or detectors. Using the nonlocal resistance, we measure spin Hall angles up to 2.4 with spin lifetimes up to 9 ps. Furthermore, ferromagnet-free measurement allows for quick diagnostics of the spin properties without the need to fabricate multilevel devices.   

High mobilities and exotic anisotropic magnetoresistivity in the Ir3Ge7 structure family of compounds

We synthesized via an indium flux method a single crystal Pd₃In₇ compound that belongs to the cubic Ir₃Ge₇ structure type and we explore its interesting topological behavior. This family of compounds has shown to be a promising candidate to explore the result of a complex Dirac-like electronic structure close to the Fermi level. In the case of Pd₃In₇, the electronic structure displays multiple band crossings close or on the Fermi energy ε_F, leading to Dirac type-I and type-II nodes. The analysis of the quantum oscillations due to the de Haas van Alphen effect (dHvA effect) indicates light effective masses of the order of 0.15m_e and very high carrier mobilities, 3000 cm²/V*s, consistent with what is expected from a Dirac material. Notably, the magnetoresistance shows a strong anisotropic behavior as a function of position, even though during the whole rotation the magnetic field is always perpendicular to the direction of the current, leading to a maximum Lorentz force.   

'Growth and Characterization of Pure and Doped Topological Material CoSi'

In CoSi, nontrivial topology and chirality give rise to intriguing physics, such as hosting Weyl fermions, and making it a potential candidate for displaying the quantized circular photogalvanic effect (CPGE). The CPGE is predicted to be quantized in terms of fundamental constants in Weyl-semimetals whose topological nodes lie at different energies, but has not been experimentally observed. But to experimentally realize this phenomena, the Fermi energy of CoSi must be tuned precisely. To this end, we report the growth and characterization of pure and electron doped CoSi. Ni and P substituted CoSi single crystals are synthesized via flux method and characterized by x-ray diffraction and energy dispersive x-ray spectroscopy (EDX). The Fermi surface and carrier mobility are assessed through quantum oscillations and Hall effect.   

Exotic weak topological state in RhBi2

In contrast to the ubiquity of strong topological insulators, material realizations of weak topological insulators (TIs) remain largely elusive due to the inherent challenge of detecting their non-trivial surface states. The tailored synthesis of new materials is a promising path to reduce the experimental gap between strong and weak TIs, and can also lead to the discovery of novel quantum phenomena. Combining material synthesis, ARPES measurements, and analytical modeling, we report the discovery of a weak topological insulating state in triclinic RhBi₂. In this talk, we show that the low crystal symmetry -1 of this material allows for additional exotic features in the non-trivial surface state dispersion. In particular, two saddle points develop in the proximity of the surface Dirac cone, leading to a van Hove singularity in the surface density of states that is expected to dramatically enhance the effect of electronic interactions and potentially drive the systems towards many-body instabilities. As an example, we discuss how surface superconductivity is favored in the weak TI surface state of RhBi₂ as the chemical potential is tuned across the van Hove singularity. The proximity of saddle and Dirac points to the Fermi level makes RhBi₂ is a promising platform to investigate the interplay between electronic correlations and topological surface states.   

Fermi surface studies of type-II Dirac semimetal candidate NiTe2 using de Haas-van Alphen oscillations

We have investigated the Fermi surface of the type-II Dirac semimetal candidate NiTe₂ using torque magnetometry. The torque signal measured under applied fields of 35 T and temperatures down to 0.32 K shows clear de Haas-van Alphen (dHvA) oscillations. The oscillations are well-defined and consist of two major frequencies F₁ ~ 100 T and F₂ ~ 400 T. To investigate the properties of the Fermi surface, we have carried out measurements of angular and temperature dependence of the dHvA oscillations. Both frequencies vary while increasing the angle between the sample surface and the magnetic field. Also, the amplitude of the oscillations decreases at higher temperatures. We have analyzed the temperature-dependent dHvA data using the Lifshitz-Kosevich theory and calculated several physical parameters characterizing the Fermi surface.   

Thermoelectric and thermomagnetic transport in triple-point topological metal Molybdenum Phosphide (MoP)

MoP is a topological metal which had been shown to host a triple point fermion in its band structure with a Weyl point in the bulk bands [1]. This complex band structure is a strong motivation for studying electrical and thermal transport. Single-crystalline MoP previously demonstrated an extremely high electrical conductivity and the potential for hydrodynamic charge flow [2]. In this work, both the Seebeck effect and Nernst effect are investigated for the first time in polycrystalline MoP samples. Additionally, thermal conductivity, electrical resistivity, heat capacity, and the Hall effect are determined as functions of grain size, temperature, and magnetic field. Compared to single-crystalline MoP [2], both thermal and electrical conductivity are found to be 1-2 orders of magnitude smaller in polycrystalline samples. From Wiedemann-Franz law analysis, electronic and phonon thermal conductivity are determined, and the former is found to be dominating at higher temperatures. This is contrary to the single-crystalline MoP, due to more scattering of phonons. Other transport properties show typical metallic behavior with relatively small Seebeck and Nernst effects due to a large charge carrier density.

[1] B. Q. Lv et al. Nature 546 (2017).

[2] N. Kumar et al. Nat. Commun 10 (2019).   

Fermi surface studies of type-II Dirac semimetal ZrTe2 using torque magnetometry

The transition metal dichalcogenides (AX₂, A = Mo, W, Ta, Zr, Hf, etc., and X = S, Se, or Te) are interesting as they provide platforms for exploring rich and tunable physical properties such as charge density wave, superconductivity, non-trivial band topology, etc. Here, we have explored the Fermi surface of ZrTe₂ using torque magnetometry with applied fields up to 35 T and temperatures down to 0.32 K. Our torque data show de Haas-van Alphen (dHvA) oscillations with two distinct frequencies at 88 T and 530 T. The angular and temperature dependence of the oscillations was performed to understand the Fermi surface properties. Also, the density functional theory calculations were carried out to explain our experimental results. We found that the angular dependence of the observed frequencies agrees with our first-principles calculations.   

Large gap high-order topological insulators in superatomic graphene

High-order topological insulators (HOTIs) have been investigated in several two-dimensional (2D) lattices. The inter and intra-hexagon hopping in a Kekulé lattice opens a non-trivial gap protected by the lattice symmetry, leading to the zero-dimensional corner states. However, the realization of the 2D HOTI is still challenging because of the limit of material candidates and the weak signal of the corner states. Here, we introduce the on-site Coulomb repulsion (U) in the Kekulé lattice to enlarge the topological gap. We found that the corner states become more localized, and the intensity of the corner mode is significantly enhanced. We further proposed a series of superatomic graphene lattices superimposed on a Kekulé lattice for the realization of large gap HOTIs with different U/t ratios. Our results provide a promising strategy to generate strong topological corner states.   

Metal-insulator transition in Fe-doped type-II Weyl semimetal WTe2

 The metal-insulator transition (MIT) has been a subject of intensive study in condensed matter physics for decades, providing opportunities to study fundamental physics as well as develop novel switching devices based on multiple resistance states. Doping has been known as an effective tunable parameter to manipulate the MITs. Here we report the Fe doping in WTe₂. We characterized the structure, transport, and thermodynamic properties of Fe-doped WTe₂ single crystals, and found signatures of Kondo localization and MIT. Therefore, this material system offers a fascinating platform to explore emergent quantum phenomenon introduced by magnetic doping in a Weyl semimetal.