Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session S42: Magnetic Order and Flat Bands in Topological SystemsFocus
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Sponsoring Units: DMP Chair: Linda Ye, Stanford University Room: Room 318 |
Thursday, March 9, 2023 8:00AM - 8:36AM |
S42.00001: Interrogating and manipulating exchange pathways in the magnetic topological insulator MnBi2Te4 Invited Speaker: Hari Padmanabhan Magnetic order in topological materials results in phases that exhibit quantized behavior such as the topological magnetoelectric effect, quantum anomalous Hall effect, and more. Central to the phenomenology of these materials are exchange interactions – among localized spins constituting the magnetic order, as well as between localized spins and the topological bands. I will present findings from our recent work1,2 employing optical spectroscopy to interrogate and manipulate these exchange couplings in MnBi2Te4, a quasi-two-dimensional antiferromagnetic topological insulator. In the first part of the talk, we consider the weak exchange coupling between localized Mn spins across the van der Waals gap, an interaction that is key to achieving long-range magnetic order1. Using magneto-Raman spectroscopy, we discover that optical phonons strongly modulate this interlayer coupling in MnBi2Te4. This effect is found to be selective to A1g phonons that simultaneously alter both the interlayer distance and the intralayer superexchange bond angles. In the second part, we quantify the exchange coupling between the Mn localized spins and the Bi-Te topological bands2. This is achieved by employing a pump-probe approach, where we optically excite the topological bands, and study the consequent ultrafast dynamics of the localized spins using a multimodal probe. We find that this exchange coupling is two orders of magnitude larger than the primary in-plane superexchange interaction. I will discuss the implications of our findings for the exchange gap in the surface states, and outline the opportunities they create for ultrafast light-driven coherent control of magnetism and topology in MnBi2Te4. |
Thursday, March 9, 2023 8:36AM - 8:48AM |
S42.00002: A Class of Magnetic Topological Material Candidates with Hypervalent Bi Chains Jason F Khoury, Bingzheng Han, Milena Jovanovic, Ratnadwip Singha, Xiaoyu Song, Raquel Queiroz, N. Phuan Ong, Leslie M Schoop The link between crystal and electronic structure is crucial for understanding structure–property relations in solid-state chemistry. In particular, it has been instrumental in understanding topological materials, where electrons behave differently than they would in conventional solids. Herein, we identify 1D Bi chains as a structural motif of interest for topological materials. We focus on Sm3ZrBi5, a new quasi-one-dimensional (1D) compound in the Ln3MPn5 (Ln = lanthanide; M = metal; Pn = pnictide) family that crystallizes in the P63/mcm space group. Density functional theory calculations indicate a complex, topologically nontrivial electronic structure that changes significantly in the presence of spin–orbit coupling. Magnetic measurements show a quasi-1D antiferromagnetic structure with two magnetic transitions at 11.7 and 10.7 K that are invariant to applied field up to 9 T, indicating magnetically frustrated spins. Heat capacity, electrical, and thermoelectric measurements support this claim and suggest complex scattering behavior in Sm3ZrBi5. This work highlights 1D chains as an unexplored structural motif for identifying topological materials, as well as the potential for rich physical phenomena in the Ln3MPn5 family. |
Thursday, March 9, 2023 8:48AM - 9:00AM |
S42.00003: Charge Density Wave Order and Fluctuations above TCDW and below Superconducting Tc in the Kagome Metal CsV3Sb5-xSnx Qiang Chen, Bruce D Gaulin, Dong Chen, Walter Schnelle, Claudia Felser The newly discovered kagome metal system AV3Sb5 (A = K, Rb, Cs) features both superconducting and charge density wave (CDW) instabilities, and provides a platform for a variety of exotic quantum phenomena that motivate their great current interest. We present a comprehensive set of measurements, including x-ray, bulk magnetization and specific heat measurements, over a broad range of temperatures from 300 K to 2 K on the kagome metal CsV3Sb5 and its hole-doped variation CsV3Sb5-xSnx in single crystal form, allowing us to reveal the temperature evolution of the CDW state and the co-existence of the CDW with superconductivity at low temperatures. |
Thursday, March 9, 2023 9:00AM - 9:12AM |
S42.00004: Study of quantum oscillations in RV6Sn6 Kagome magnets Chetan Dhital, Ganesh Pokharel, Brady Wilson, David E Graf, Stephen D Wilson Kagome metals belonging to chemically diverse AB6X6 (A=alkali/alkali earth/rare earth, B= Transition metal, X=Si, Ge, Sn ) family have attracted tremendous recent interest due to the realization of different topological quantum phenomena such as spin polarized Dirac cones, large anomalous hall effect, Chern topological magnetism, negative magnetoresistance, and non-collinear magnetic structures. One charm of the chemical versatility of the 166 class of compounds is the ability to design materials where magnetic interactions can be tuned independently from the Kagome lattice. Nonmagnetic B-site variants such as V provide this flexibility and allow the interplay between magnetism and the Kagome-derived band structures to be explored. We are investigating the emergent electronic phenomena in RV6Sn6 (R=rare earth, Y) arising from the interactions between triangular planes of R-ions and the Kagome net of non-magnetic V ions. I will present the nature of Fermi surface topology and mass of Fermions based on the analysis of de Haas van Alphen oscillations in RV6Sn6 compounds. |
Thursday, March 9, 2023 9:12AM - 9:24AM |
S42.00005: Coexistence of Multiple Flat Bands in Non-Hermitian Parity-Time Symmetric Lattices Aria Hajikhani, Hamidreza Ramezani Controlling the localization of light has been studied through many approaches, including flat bands in Hermitian and non-Hermitian systems. It has been formerly investigated that in non-Hermitian systems, an entirely flat band can emerge at the exceptional point of a Parity-Time (PT) symmetric lattice. Although these flat bands were known to be bound states in continuum (BIC), we demonstrate that by engineering the lattice couplings, this would not necessarily hold true. We observe a flat band which is a bound state emerging at a critical point before reaching the exceptional point, and the curvature of the dispersive band changes before and beyond this critical point. Also, by implementing similar derivations into an array of waveguides composed of tetramers, we discover the coexistence of two flat bands, one of which is always a BIC. |
Thursday, March 9, 2023 9:24AM - 9:36AM |
S42.00006: Investigating Electronic Structures of AV3Sb5 Kagome Metal family Farnaz Kaboudvand Kagome metals have recently attracted attention due to their fascinating electronic structures. Because of their unique three sublattice geometry and depending on the degree of electron filling, these materials are predicted to host a variety of instabilities like superconductivity (SC), spin liquid states, and charge density waves (CDW). One nonmagnetic kagome family of interest is AV3Sb5 (A = K, Rb, Cs). This family of layered metals exhibits CDW, unconventional superconductivity, and non-trivial band topology due to multiple saddle points near the Fermi level. To aid in understanding this material class, we computationally studied the band structure and Fermi surface of these families and applied the Lindhard susceptibility calculations to study the degree of Fermi surface nesting. Additionally, we experimentally and computationally explore the coupling between CDW and SC states by hole doping the systems. The resulting phase diagrams for AV3Sb5−xSnx reveal that small amounts of carrier doping can have dramatic impacts on SC and CDW order in these systems. |
Thursday, March 9, 2023 9:36AM - 9:48AM |
S42.00007: Realization of ideal flat band by rotated d-orbitals in Kagome metals Dongwook Kim, Feng Liu Recently there has been intense interest in Kagome metals, which are expected to host flat bands (FBs). However, the observed FBs are non-ideal as they are not flat over the whole 2D Brillouin zone and overlap strongly with other bands. Most critically, the theoretical conditions for the existence of ideal FB in Kagome metals, beyond the simplest Kagome lattice model, are unknown. Here, based on tight-binding model analyses of the interplay between d-orbital symmetry and underlying Kagome lattice symmetry, we establish such conditions of minimal FB model. We show that for a pure TM Kagome lattice, only <!--[if gte msEquation 12]> style='font-size:12.0pt;mso-ansi-font-size:12.0pt;mso-bidi-font-size:12.0pt; |
Thursday, March 9, 2023 9:48AM - 10:00AM |
S42.00008: Engineering tilted Dirac cones and topological phase transitions in strained kagome lattices Miguel A Mojarro Ramirez, Sergio E Ulloa We study the effects of a uniform strain on the electronic and topological properties of the 2D kagome lattice using a tight-binding formalism that includes intrinsic and Rashba spin-orbit coupling (SOC). The degeneracy at the Γ point, where a flat-band-parabolic-band touching occurs, evolves into a pair of (tilted) type-I Dirac cones owing to a uniform strain, as shown by effective Hamiltonians, where the anisotropy and tilting of the bands depend in a nontrivial way on the magnitude and direction of the strain field. Interestingly, we find that the Dirac cones become type-III (including flat dispersions) when the strain is applied along the sawtooth direction. As expected, the inclusion of SOC opens a gap at the emergent Dirac points, making the strained flat band to become topological, as characterized by a nontrivial Z2 index. We show that the strain drives the systems into a trivial phase for strains of a few percent (with Grüneisen parameter and Poisson ratio values taken as in graphene), allowing topological transitions via uniform deformations. These findings suggest an alternative way of engineering anisotropic tilted Dirac bands with tunable topological properties in strained kagome lattices. |
Thursday, March 9, 2023 10:00AM - 10:12AM |
S42.00009: Transport properties of kagome metals RV6Sn6 (R=Sc, Lu, Y): What makes Sc special? Shirin Mozaffari, Richa Pokharel Madhogaria, William R Meier, Hasitha Suriya Arachchige, Takahiro Matsuoka, Calen Allen, Jeremy H Driver, John W Villanova, SEOUNGHUN KANG, Mina Yoon, David G Mandrus Kagome lattices are potential hosts to both topologically protected bands as well as non-dispersing or flat bands. This concurrence has led to a variety of magnetic and electronic properties such as anomalous Hall effect, topological Hall effect, charge density waves (CDW) and superconductivity. Some of these effects can coexist as is shown in a series of vanadium antimonide AV3Sb5 (A= K, Rb, Cs). ScV6Sn6 is a new member of kagome metals with a similar CDW below 92 K. Surprisingly, CDW is absent in the sister compounds LuV6Sn6 and YV6Sn6. Our detailed transport measurements reveal strong anisotropy in the ScV6Sn6 compound. We discuss the differences and similarities in the electronic transport properties of these three isoelectronic and isostructural vanadium kagome metals. Our results provide insights on why only ScV6Sn6 hosts the CDW. |
Thursday, March 9, 2023 10:12AM - 10:24AM |
S42.00010: Searching for Electronic Flat Bands Beyond the Kagome Framework Paul M Neves, Shiang Fang, Joshua Wakefield, Ryan McTigue, Joseph G Checkelsky The propensity for flat electronic bands to support topological and correlated behaviors has attracted considerable interest in recent years. It has now been well established that the frustrated hopping present in a kagome lattice manifests two-dimensional topologically non-trivial flat bands. Here, we further consider the role of orbital and lattice motif degrees of freedom to realize different flat band dimensionality in bulk crystalline systems. Building on our previous computational lattice material search results, we discuss recent progress on this development of bulk flat band systems beyond the kagome network, highlighting the role of the orbitally enabled band flattening. |
Thursday, March 9, 2023 10:24AM - 10:36AM |
S42.00011: From Loop-current Chern metal to pair density wave and higher-charge superconductivity in kagome superconductors AV3Sb5 (A=K, Cs, Rb) Ziqiang Wang, Sen Zhou The study of transition-metal kagome lattice materials has leapt forward with the recent discovery of superconductivity in vanadium-based nonmagnetic kagome metals AV3Sb5 (A=K, Cs, Rb). We overview the experimental evidence for possible time-reversal symmetry breaking charge density wave in the normal state, the novel pair density wave (PDW) in the superconducting state, and charge-4e and charge-6e superconductivity. We argue that the essential phenomenology can be captured by a doped orbital Chern insulator with staggered loop currents and Chern Fermi pockets carrying concentrated Berry curvature. Cooper pairing in the Chern metal can generate a roton PDW state characterized by an emergent vortex-antivortex lattice of loop-supercurrents. The possible realizations of higher-charge superconductivity and intrinsic chiral topological PDW superconductors will be discussed. |
Thursday, March 9, 2023 10:36AM - 10:48AM |
S42.00012: Engineering high Chern number insulators Sungjong Woo, Seungbum Woo, Hee Chul Park Realizaition of high Chern number insulator is an important issue for potential applications in technologies. As such example, graphene-like honeycomb lattice with long range hopping was proposed for a high Chern number insulator. Furthermore, multilayer quantum anomalous Hall system was also proposed recently and experimentally realized for high Chern number insulators. However, a systematic method to predict such kind of material is still lacking. Here, we propose a practical method to design the model of the two-dimensional Chern insulators with arbitrary high Chern number using controllable one-dimensional Rice-Mele model. |
Thursday, March 9, 2023 10:48AM - 11:00AM |
S42.00013: Interplay between crystal structure, symmetry and topology in Kagome systems with different stackings Fan Yang, Turan Birol The 2-dimensional Kagome structure, with its pattern of corner-sharing triangles, hosts many interesting features related to frustrated magnetism, topology, and superconductivity. There is a growing attention on the transition-metal-based Kagome systems in the recent years. Bulk of the theoretical work so far focuses on 2D single-layer systems, and the out-of-plane interactions between Kagome layers are under less attention. In this talk, we present our results on the 3D tight-binding models and group theoretical analyses for 3D Kagome systems with different stacking ways that correspond to real materials, such as the the simple stacking (AV3Sb5) and the rhombohedral stacking (Co3S2Sn2). We compare the degeneracies and topological features, predict the effect of uniaxial stress (or strain), and provide materials-specific analyses using first-principles results. |
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