Session Z04: Flat Band in Topological Semimetals

Electronic structure in the Dirac nodal-line semimetals TaNiTe5 and TaPtTe5

Among the nodal-line semimetals (NLSMs), Dirac nodal lines (DNLs) that are robust against spin-orbit coupling (SOC) have been increasingly shown to exist in three- and two-dimensional systems, but rarely occur in (quasi) one-dimensional materials. The latter can be found in two-dimensional systems with in-plane anisotropy [1], since additional crystalline symmetries such as mirror or nonsymmorphic symmetries are required to protect a nodal line band crossings [2]. A family of exfoliatable, strong in-plane anisotropic, nonmagnetic, ternary transition semimetal tellurides, Ta-based TaMTe₅ (M=Ni, Pd, Pt) [1,3,4], has recently been shown to host nodal lines with fourfold degeneracy.

Here [5] we investigated the Fermi surface and carrier mass in TaNiTe₅ and TaPtTe₅ using magnetization and magnetic torque measurements in high-quality single crystals at fields of up to 15T. Strongly pronounced de Haas-van Alphen (dHvA) oscillations can clearly be resolved, reaching to temperatures of up to 22 K for some orbits. Quantum oscillations have been tracked for fields along the three crystallographic axes, supplemented by rotation studies in the b-a and b-c planes, and were interpreted with reference to band structure calculations within density functional theory. The extracted values for the Berry phase of the obtained Fermi pockets approach Pi, demonstrating nontrivial topological electronic properties and ultra-high mobility of Dirac carriers in the conduction band.   

Second Harmonic Generation in Correlated Topological Semimetals

Nonlinear responses have proved a powerful way to reveal, understand, and tune the wavefunction geometry and/or topology, driven by the recent discovery and engineering of quantum materials. On the other hand, the interplay between topology and electronic correlations may offer a rich avenue for discovering emergent quantum phenomena in condensed matter systems. In this work, we explore the role of electronic correlation on the nonlinear optical response, specifically, the second harmonic generation (SHG). We identify six distinct contributions (e.g., shift, injection, Berry curvature dipole) to the SHG within the quantum kinetic theory framework. Furthermore, we apply our analysis to the Weyl-Hubbard system to explore the effect of electron-electron correlation on the various components of the SHG signal. Our study reveals the possible mechanisms for correlation-driven SHG enhancement, which have direct experimental consequences.   

Flat Bands and Correlated States in Frustrated Lattice Metals

The notion of an electronic flat band refers to a collectively degenerate set of quantum mechanical eigenstates in periodic solids. The vanishing kinetic energy of flat bands relative to the electron-electron interaction is expected to result in a variety of many-body quantum phases of matter. Here we present recent developments in realizing flat bands in transition element-based frustrated lattice metals. We will present recent experiments in which a partial filling of a flat band is associated with correlated transport and thermodynamic responses. We will also comment on the potential role of band topology and prospects for using similar lattice and orbital engineering to realize new electronic phenomena.   

Tunable topological effects in Ga doped Mn3Sn thin films

Mn₃X(X=Sn, Ga, Ge) antiferromagnetic (AFM) Weyl semimetals have attracted significant research due to their large Berry phase effects and potential for applications[1,2]. The temperature range for the anti-chiral magnetic phase with large topological effects varies for each of these materials, the highest temperature being the Neel temperature (T_N). Thin film forms of Mn₃Sn are routinely studied [3], however, in Mn₃Sn the anti-chiral magnetic phase has a limited temperature range (≈260-420K). To fully exploit the device possibilities, ranging from AFM spintronics [1-3] to unconventional superconducting spintronics based on Mn₃X/superconductor structures [4], one needs to engineer these Mn₃X thin film systems maximizing the Berry phase effects over a wide temperature range. In this work we present sputtered Mn₃Sn_1-xGa_x thin films in which the anti-chiral magnetic phase of Mn₃Sn is systematically modified with Ga doping. Our detailed structural characterization of Mn₃Sn_1-xGa_x reveals a systematic linear variation of the lattice parameters between Mn₃Sn (for x=0) and Mn₃Ga (for x=1) satisfying Vegard’s law of a substitutional solid solution. Temperature dependent Hall effect and magnetization measurements confirm a giant enhancement of anomalous Hall conductivity over a wide temperature range (extending from »500K down to at least 10K), and a systematic enhancement of T_N (from ≈420K to ≈500K) with Ga addition. In conclusion, our work presents a route for engineering topological effects in Mn₃X systems.    

Revealing the anomalous Hall like behavior of Kagome ScV6Sn6

Compounds with Kagome lattice structure are known to exhibit nontrivial topology in electronic band structures, leading to several versatile quantum phenomena. One of them is the anomalous Hall like behavior in ScV₆Sn₆, a non-magnetic charge density wave (CDW) compound. The measured value Hall conductivity is of the order of 10⁴ Ω^-1cm^-1, which is fully entangled with the CDW phase. With the help of external and internal pressures, we disentangled the anomalous Hall behavior from the CDW phase, where the CDW phase exhibited significant changes while the anomalous Hall behavior remained unchanged.   

Quantum phase transitions in 2D moiré Dirac materials

Moire materials provide a new opportunity for the investigation of quantum phase transitions in two-dimensional Dirac systems due to unprecedented experimental control. In particular, the relative interaction can be tuned via a twist angle so that it is possible to overcome the critical interaction strength needed to induce a quantum phase transition for Dirac fermions. We study possible patterns for spontaneous symmetry breaking in a Dirac fermion model motivated by twisted bilayer graphene at charge neutrality. These include the spontaneous generation of a gap, but also of a splitting in energy or wave vector of the Dirac points. We employ a renormalisation group treatment and discuss quantum critical behavior for different numbers of fermion flavours such as spin or valley.   

Unraveling the mesoscale surface and electronic structure in Co3Sn2S2

Co3Sn2S2 is a Magnetic Weyl Semimetal with multiple mesoscale cleave terminations for its crystal. These terminations have been observed to alter the topological surface states that connect the Weyl points. As a surface senstitive probe, photoemission-based methods are uniquely suited to studying electronic, chemical and atomic structure on the same mesoscales. Using micro-focused synchrotron photoemission, we identify signatures of different surface terminations in angle-resolved photemission, x-ray photoemission and photoelectron diffraction spectroscopies. This multi-technique approach for connecting surfacve atomic and electronicstructure is broadly applicable to any material with characteristic surface electronic structure that depends on variable surface chemistry.   

Terahertz spectroscopy of the Dirac semimetal SrIrO3

We use terahertz time domain spectroscopy on molecular beam epitaxially grown SrIrO₃ on a GdScO₃ substrate to study the terahertz conductivity from 6.5 K to 290 K. SrIrO₃ is of interest because the band structure has a lifted Dirac-like cone at the U point in DFT calculations and ARPES data, with an estimated gap calculated to be 5 meV (1.2 THz) when grown on a lattice matched substrate. Lattice matching of SrIrO₃ can be achieved by growth on a GdScO₃ substrate, where the strain due to lattice mismatch is negligible. Using terahertz time domain spectroscopy allows us to directly measure the band gap at the U point.. In addition, we use near infrared pump-THz probe was used to study relaxation times of the lattice and electrons from 6.5 K to 50 K and use the two-temperature to model our results.   

Terahertz sensing based on layered topological semimetal

The emergent atomically thin layered materials enable the unique control of new phases of matter for high-performance electronics and optoelectronics.  One remarkable example is the recent discovered nonlinear Hall effect (NLHE) in topological semimetals, which is mediated by their diverging quantum geometrical properties [1-3]. It dictates a nontrivial second-order transverse current can be induced by an oscillating electric field, greatly enhanced by large Berry curvature dipoles at the band edges. This effect serves as a measure of the topological attributes inherent in quantum materials.

In this talk, I will present our exploration of such effect to enable a new efficient detection mechanism for electromagnetic waves with elevated frequency. In particular, we investigate the NLHE response in THz regime on a layered type-II Weyl semimetal. Leveraging a custom-designed and in-house fabricated device, we attained high and tunable photoresponsivity in few-layer semimetals. These findings illuminate a novel avenue for low-energy photon harvesting and transduction via quantum properties. Our findings also pave the way towards a wide range of THz applications in communication and imaging based on layered topological semimetals.

 

[1] Q. Ma et al., Observation of the nonlinear Hall effect under time-reversal-symmetric conditions, Nature 565, 337 (2019).

[2] K. Kang et al., Nonlinear anomalous Hall effect in few-layer WTe₂, Nature Materials 18, 324 (2019).

[3] J. Xiao et al., Berry curvature memory through electrically driven stacking transitions, Nature Physics 16, 1028 (2020).   

Highly mobile electron-driven transport in Dirac semimetallic oxide BiRe2O6

5d transition metal oxides provide a fertile playground to explore new phenomena resulting from the interplay between topology, spin-orbit, and electron-electron correlations. To date, however, limited studies of topological properties exist due to the difficulty in synthesizing single crystals of these oxides. Here, I will present magnetotransport and quantum oscillations study on high-quality single crystals of Dirac semimetallic oxide BiRe₂O₆. It crystallizes in a monoclinic structure with space group C2/c (SG#15) and is a Pauli paramagnetic. Intriguingly, it exhibits large positive magnetoresistance of 110³ % at 2 K. Unlike other topological semimetals that show large MR, it evidences high electron carrier density and mobility of 0.510²² cm⁻³ and 110³ cm² V^-1 s^-1, respectively. Moreover, analysis of de Haas–van Alphen oscillation measurements and band structure calculations reveals the Fermi surface comprising of multiple pockets with small effective masses. Our findings may trigger to study of novel phenomena of correlated Dirac electrons in topological materials based on oxides.   

Sb1−xAsx and Bi1−xAsx alloys as Topological Weyl semimetals

Weyl semimetal (WSM) phase in antimony arsenic Sb_1-xAs_x alloys and bismuth arsenic Bi_1-xAs_x  alloys are investigated using density functional theory, varying atomic composition, and structure arrangements. We find WSM states in all As concentrations in SbAs alloys, and for three As concentrations (x=0.5, 0.67, and x=0.83) in BiAs alloys with specific inversion symmetry broken. The Weyl points are located very close to the Fermi level in all compositions, facilitating their observation in experiments and suggesting that they could be characterized by analyzing surface and bulk transport properties. The presence of Fermi arcs also verifies the Weyl semimetal phase in SbAs and BiAs alloys. There is a total of 12 Weyl points located in the Brillouin zone for each composition. The chirality of these points shows that 6 Weyl points are the source and 6 Weyl points are the sink of Berry curvature. Our calculations also show that the surface bands reside within the bulk band gap. Our results illustrate the importance of composition and structure arrangement for determining the Weyl semi-metal phase in the Sb_1-xAs_x and Bi_1-xAs_x alloys.