Session F59: Correlated Topology I: Kondo Insulators, Kagome Material, etc.

High-frequency oscillator magnetometry measurement in the pulsed magnetic field

Recently, the clear quantum oscillations have been observed in an unusual high-field metallic state of the Kondo insulator YbB₁₂, probed by the proximity detector oscillator which was used for the contactless shubnikov-de Haas (SdH) effect studies. However, the de Haas-van Alphen (dHvA) effect has been barely studied in the pulsed magnetic field because of the eddy current present in the metal cantilevers. High-frequency quartz oscillators can be a solution to probe the dHvA effect in the pulse field. Therefore, we developed a torque differential magnetometry technique based on high-frequency oscillators. In this presentation we report on our progress on the torque carry on the measurement in YbB₁₂.   

Oxygen-18 Substituted Strontium Titanate Capacitive Thermometry for Thermal Transport Measurements in Kondo Insulator YbB12

Oxygen-18 substituted strontium titanate is developed as a capacitive thermometer at cryogenic temperatures under intensive magnetic fields. Conventional resistive thermometers used for cryogenic thermometry vary by up to a few percent under high magnetic fields, causing them to have to be extensively calibrated for such conditions.[1] On the other hand, strontium titanate (SrTiO₃) has shown to be an effective capacitive thermometer that is fairly stable under similar magnetic fields, varying only by a few hundreths of a percent. This has been utilized to measure the thermal Hall effect in Bismuth down to 40 K.[2] However, the sensitivity of SrTiO₃ thermometers drops to zero under 7 K, rendering them unusable for measurements in the milliKelvin range. The capacitance behaviour of strontium titanate changes when oxygen-16 is substituted for oxygen-18, specifically moving the low sensitivity region to higher temperatures.[3] Thus we explore the possibility of using oxygen-18 substituted SrTiO₃ as a capacitive thermometer in the milliKelvin range at high magnetic fields, with the aim to utilize them for thermal transport measurements in Kondo insulator YbB₁₂.   

Lifshitz transitions in Kondo Insulator YbB12

The Landau Level quantizations were observed in both magnetization and electrical resistivity in Kondo insulator YbB₁₂ [1,2]. At the intense magnetic fields, the quantum oscillation patterns resemble that contributions from the crystal bulk [3]. In contrast, at zero-field, the electrical conductance of the micron-size YbB₁₂ is shown to be independent of the sample thickness and thus indicating the origin of the surfaces [4]. To solve this problem, we carried out the Hall effect and Seebeck effect measurements in YbB₁₂ under intense magnetic fields. Our results suggest the magnetic field drives  Lifshitz transitions, which lead to the changes in electronic structures at the intense magnetic fields.   

Anomalous Hall Effect in Uranium-based Kondo semimetal

The combination of strong correlation and band structure topology is an exciting avenue to search for new quantum phases. Topological states have been extensively studied in Kondo Insulators within the past decade, showing that Kondo systems are highly versatile platforms to explore the effect of topology in strongly correlated systems. Multiple Uranium based compounds have been proposed as candidates, due to 5f-electrons participating towards large spin-orbit coupling. Through electric transport measurements on a Uranium based semimetal, with a non-centrosymmetric lattice, we observe a large spontaneous hall effect rooted in Weyl nodes pinned to the Fermi level due to the Kondo interaction, giving a non-zero berry curvature. This system exhibits large Kondo Coherence temperature, providing vast energy domain for studying the effect of strong correlations in non-trivial topological environment.   

On the extension of Topological Quantum Chemistry to strongly correlated crystalline topological Kondo insulators

The recently introduced topological quantum chemistry (TQC) framework describes the topological properties of all band insulators in all space groups based on crystal symmetries and time reversal. Nevertheless, it remains unclear to which extent this formalism can be applied to the analysis of systems with interacting electrons. In this talk, we present the extension of TQC to strongly correlated Kondo systems, which consists on the development of the formalism to a particular type of interacting systems and sets up a powerful framework for the investigation of crystalline topological Kondo insulators. These materials can be seen as the symmetry enriched evolution of topological heavy fermion systems proposed during the past decade whose topological ground states were protected by time reversal [1]. We will also present the study of some representative material.

[1] Topological Kondo Insulators, M. Dzero, K. Sun, V. Galitski and P. Coleman, Phys. Rev. Lett. 104, 106408 (2010).   

Metallic chemical potentials in an insulating topological state

Controlling the charge density inside an insulator via the chemical potential is a cornerstone of modern electronics, enabling electrical conductivity of semiconductors and the emergence of fascinating new properties linked with electronic correlations. The compound SmB₆ has recently drawn widespread attention as the first insulator to feature both strong electronic correlations and topological quantum order, potentially hosting new phenomenologies as charge density is modified. However, chemical potential has not been experimentally controlled in studies to date of its electronic structure. In this talk I will present an angle-resolved photoemission spectromicroscopy (μ-ARPES) study of SmB₆, using the natural inhomogeneity of sample surfaces to create the analogue of a multi-dimensional doping series. I will discuss how we determine the role of correlations by observing interdependencies between topologically ordered electronic states and the chemical potential, and explain the effect of doping and quantum coherence in the electron system. Overall, this talk will set up the stage for a holistic understanding of the interplay between strong correlations and topological features in SmB₆.   

Topological Kondo insulator nanowire as a probe tip for spin-sensitive imaging

Spin-momentum locking in topological surface states (TSS) gives rise to exotic spin-dependent phenomenon in materials like the quantum spin Hall effect, Edelstein effect, etc. Achieving these in experiments have been challenging with garden-variety topological insulators due to the coexistence of highly conducting bulk states and the TSS being buried deep in the bulk bands. However, topological Kondo insulators (TKI) are pertinent for such studies as the TSS are well isolated from the bulk by the Kondo hybridization gap and are usually pinned at the Fermi energy. Nanomaterials of topological insulators furnish enhanced surface properties and allow versatility in spintronics applications. Here, we report the fabrication and use of nanowires of a TKI as probe tips for scanning tunneling microscopy and spectroscopy (STM/S). We exploit the presence of spin-momentum locked TSS at the Fermi energy in this TKI to achieve spin-resolved tunneling. We demonstrate that the direction of spin-polarization is tied to the tunneling direction, a unique feature of TSS-dominated tunneling where the group velocity is locked to the spin direction. Our findings initiate a novel form of STM-based spectroscopy for probing spin degrees of freedom in quantum materials using topological phenomena.   

Epitaxial thin film synthesis of correlated kagome metal Ni3In

The kagome lattice is a promising platform for investigating the interplay between topology and correlation as it naturally hosts both linear Dirac and dispersionless flat bands, as has recently been reported in the quasi-two dimensional transition metal stannide compounds (Fe, Co)Sn [1]. Going beyond the single layer paradigm, it has recently been demonstrated that the AB-stacked kagome system Ni₃In hosts a correlated metallic state in close proximity to a lattice-borne flat band [2]. Here we report the synthesis of high quality Ni₃In thin films grown by molecular beam epitaxy. We will also discuss electronic transport properties of this system in connection with the flat band compared with other kagome metals.   

First principles theory of charge density wave state and magnetism in kagome metals AV3Sb5

The recently discovered Kagome metals AV₃Sb₅ (A = Cs, Rb or K) have attracted great attention due to the coexistence of nontrivial band topology and strong correlations, which produce versatile physics such as charge-density wave (CDW), magnetism, and superconductivity. However, the formation mechanisms of the various correlated states are still under debate. Here we theoretically study the Fermi-surface instabilities of AV₃Sb₅ based on first principles calculations, both for the normal phase and the 22 CDW phase. In particular, we have calculated generalized susceptibilities defined in the charge-orbital-spin space within random phase approximation based on Wannierized tight-binding models, and find that there is no instability mode at M point driven by electron-electron interactions. Instead, we find magnetic instabilities at Γ point, which are further confirmed by unrestricted self-consistent Hartree-Fock calculations including both on-site and inter-site Coulomb interactions. Our results suggest the CDW phase is more likely to be driven by electron-phonon couplings, while electron-electron Coulomb interactions may give rise to magnetism which may contribute to anomalous Hall effect.   

T-breaking density wave order in topological kagome metals

Kagome lattice metals $AV_ 3 Sb_ 5$ (A = Cs, K,  Rb) host charge order derived from partially filled topologically non-trivial bands.  In addition, recent muon spin resonance (muSR) measurements in $CsV_ 3 Sb_ 5$ detect an unambiguous signature of broken time reversal (T-) symmetry in the charge order phase.  Here, we address the question: what is the character of emergent correlated phases derived from partially filled Chern bands?  We advance a framework which enables a low energy tight-binding description of such a system using exponentially localized Wannier orbitals. The key conclusion is the existence of a density wave ground state, called DWt, which spontaneously breaks T - symmetry [1]. We argue that such a T-breaking property of the density wave is a fingerprint of the underlying band topology and is absent in other topologically trivial systems. We propose the DW$_t$ ordering as the mechanism for the charge density wave in the $AV_3Sb_5$ materials, and discuss the implications of our results for other correlated metallic systems that host topological bands.   

Geometrically Frustrated Phonons in a Topological Kagome Metal

Geometric frustration on the kagome lattice results in localized real space modes with flat bands in reciprocal space. Because the ratio of potential energy to kinetic energy of these modes is so high, they are promising platforms for studying the effects of strong interactions. Here, we investigate the consequences of localized phonon modes in a topological kagome lattice metal. We use inelastic neutron scattering (INS) to find unambiguous evidence of the flat kagome phonon bands in reciprocal space with low bandwidth across the Brillouin zone, and explore the impact these localized phonons have on bulk thermodynamic properties including heat capacity, thermal conductivity and thermoelectricity.  By using an extensive suite of experimental probes along with INS and DFT calculations, we find that the localized kagome phonons interact strongly with the electrons in this system, inducing a novel phase transition. We highlight how engineering flat phonon bands generates phases of fundamental intrest and can be promising for applications in thermal energy management.   

Critical spin fluctuations in the Dirac metal CoTe2

Quantum critical points separating weak ferromagnetic metals and metallic paramagnetic phases are associated with a variety of novel physical phenomena.  These include novel ground states like triplet superconductors, non-Fermi liquid scaling, and unusual magnetotransport behavior originating from the presence of dynamical spin fluctuations responsible for not only suppressing the magnetic ordering, but also resulting in the unusual transport properties. Here, by means of magnetic, thermal, transport measurements, and first principles calculations, we show that the orthorhombic CoTe₂ with the Marcasite structure displays a lack of long-range magnetic ordering due to the strong critical spin fluctuations. Further the electronic transport yields metallic behavior with a weak linear magnetoresistance, a response typically indicative of Dirac physics present in the band structure of this material.   

Dark Matter Detection with Strongly Correlated Topological Matter: Flatband Effect

Dirac materials have been proposed as a new class of electron targets for light dark-matter (DM) scattering or absorption, with their sensitivity superior to superconductors and superfluid helium. This superiority is obtained by the gain from the significantly reduced in-medium effect over the suppression of DM scattering phase space at the point-like Fermi surface in a Dirac material.  Here we propose materials with strong electron correlations for DM detection. By considering a strongly correlated Weyl semimetal model system, we demonstrate that the correlation effect can amplify significantly the coupling or detection sensitivity to light DM particles. This amplification comes from the strong correlation-induced band renormalization, which enhances the scattering phase space on the one hand and retain the reduced in-medium effect on the other hand.   

4f-2p hybridization driven electronic structure of tetravalent praseodymium in ternary chalcogenides

Tetravalent praseodymium (Pr⁴⁺) with 4f¹ electron configuration is isoelectronic to Ce³⁺. However, crystal electric field (CEF) measurements on Pr⁴⁺ in different systems indicate the first CEF level is ~230 meV, the largest that has been observed for a lanthanide ion.¹ This brings about the need for an intermediate coupling scheme owing to the comparable energy scales of spin-orbit coupling (SOC) and CEF. In this talk, I will present out work on a series of Pr⁴⁺ systems showing evidence of increased metal-ligand covalency compared to the traditional trivalent lanthanides. A combination of X-ray (L₃-edge XANES, M_4,5-edge XANES/XMCD, O K-edge XANES/XMCD), optical (FIRMS), and neutron (INS) spectroscopies, will be utilized to understand the single-ion properties, exchange interactions and bulk behavior driven by covalency and increased CEF in tetravalent praseodymium. Where appropriate, these analyses will be supported by comparisons to our studies of redox chemistry and electronic structure in mixed valence Pr^3+/4+ ternary chalcogenide systems.