Topological Flat Bands Research Articles

Topological flat bands in functionalized arsenene monolayers

Topological flat bands in functionalized arsenene monolayers

Magic Angles and Fractional Chern Insulators in Twisted Homobilayer Transition Metal Dichalcogenides.

We explain the appearance of magic angles and fractional Chern insulators in twisted K-valley homobilayer transition metal dichalcogenides by mapping their continuum model to a Landau level problem. Our approach relies on an adiabatic approximation for the quantum mechanics of valence band holes in a layer-pseudospin field that is valid for sufficiently small twist angles and on a lowest Landau level approximation that is valid for sufficiently large twist angles. It provides a simple qualitative explanation for the nearly ideal quantum geometry of the lowest moiré miniband at particular twist angles, predicts that topological flat bands occur only when the valley-dependent moiré potential is sufficiently strong compared to the interlayer tunneling amplitude, and provides a convenient starting point for the study of interactions.

Fractional Quantum Anomalous Hall Phase for Raman Superarray of Rydberg Atoms

AbstractRydberg atom arrays offer promising platforms for quantum simulation of correlated quantum matter and raise great interests. This work proposes a novel stripe‐lattice model with “Raman superarray of Rydberg atoms” to realize bosonic fractional quantum anomalous Hall (FQAH) phase. Two types of Rydberg states, arranged in a supperarray configuration and with Raman‐assisted dipole‐exchange couplings, are implemented to realize a minimal QAH model for hard‐core bosons populated into a topological flat band with large bulk gap under proper tunable experimental condition. With this the bosonic FQAH phase can be further achieved and probed feasibly. In particular, a novel quench protocol is proposed to probe the fractionalized excitations by measuring the correlated quench dynamics featured by fractional charge tunneling between bulk and chiral edge modes in the open boundary.

Highly optical anisotropy, electronic and thermodynamic properties of the topological flat bands Kagome Nb3Cl8

This study showcases the intriguing electronic, optical, and thermodynamic properties of Nb3Cl8, an exceptional class of two-dimensional (2D) crystalline materials renowned for their Kagome structure and an exceptional band arrangement featuring remarkably flat energy bands. Nb3Cl8 was found to be a semiconductor with a narrow bandgap energy of 1.23 eV. Our study reveals pronounced anisotropic behavior in various optical aspects, extending within and perpendicular to the ab-plane. We show a distinctive anisotropy in parameters like the refractive index (4.5 in-plane compared to 2.75 out-of-plane), the optical conductivity (8000 (Ω cm)−1 in-plane compared to 100 (Ω cm)−1 in out-of-plane), and the absorption coefficient (105 cm−1 within the plane as opposed to 104 cm−1 in the out-of-plane direction). In addition, the thermodynamic calculations unveil a transition, distinguished by a Schottky anomaly occurring in the specific heat capacity at approximately 100K which is closely associated with the Néel transition of the Nb3Cl8 compound. The presence of robust interactions among quantum spins within Nb3Cl8 was supported by its low thermal conductivity, ranging from 6 to 0.7 W m−1. K−1. This study not only deepens our comprehension of the optical, electronic, and thermodynamic properties of Nb3Cl8 but also establishes a basis for future innovative technological applications.

Fractional quantum anomalous Hall effect in multilayer graphene.

The fractional quantum anomalous Hall effect (FQAHE), the analogue of the fractional quantum Hall effect1 at zero magnetic field, is predicted to exist in topological flat bands under spontaneous time-reversal-symmetry breaking2-6. The demonstration of FQAHE could lead to non-Abelian anyons that form the basis of topological quantum computation7-9. So far, FQAHE has been observed only in twisted MoTe2 at a moiré filling factor v > 1/2 (refs. 10-13). Graphene-based moiré superlattices are believed to host FQAHE with the potential advantage of superior material quality and higher electron mobility. Here we report the observation of integer and fractional QAH effects in a rhombohedral pentalayer graphene-hBN moiré superlattice. At zero magnetic field, we observed plateaus of quantized Hall resistance [Formula: see text] at v = 1, 2/3, 3/5, 4/7, 4/9, 3/7 and 2/5 of the moiré superlattice, respectively, accompanied by clear dips in the longitudinal resistance Rxx. Rxy equals [Formula: see text] at v = 1/2 and varies linearly with v, similar to the composite Fermi liquid in the half-filled lowest Landau level at high magnetic fields14-16. By tuning the gate-displacement field D and v, we observed phase transitions from composite Fermi liquid and FQAH states to other correlated electron states. Our system provides an ideal platform for exploring charge fractionalization and (non-Abelian) anyonic braiding at zero magnetic field7-9,17-19, especially considering a lateral junction between FQAHE and superconducting regions in the same device20-22.

Topological states and flat bands on the maple leaf lattice

Topological states and flat bands on the maple leaf lattice

Negative interatomic spring constant manifested by topological phonon flat band

Negative interatomic spring constant manifested by topological phonon flat band

Strain-Modulated Phase Transition in 2D V3F8 Kagome Lattice with Topological Flat Band

Strain-Modulated Phase Transition in 2D V₃F₈ Kagome Lattice with Topological Flat Band

Hyperbolic topological flat bands

Hyperbolic topological flat bands

Softening of a flat phonon mode in the kagome ScV6Sn6

Geometrically frustrated kagome lattices are raising as novel platforms to engineer correlated topological electron flat bands that are prominent to electronic instabilities. Here, we demonstrate a phonon softening at the kz = π plane in ScV6Sn6. The low energy longitudinal phonon collapses at ~98 K and q = frac{1}{3}frac{1}{3}frac{1}{2} due to the electron-phonon interaction, without the emergence of long-range charge order which sets in at a different propagation vector qCDW = frac{1}{3}frac{1}{3}frac{1}{3}. Theoretical calculations corroborate the experimental finding to indicate that the leading instability is located at frac{1}{3}frac{1}{3}frac{1}{2} of a rather flat mode. We relate the phonon renormalization to the orbital-resolved susceptibility of the trigonal Sn atoms and explain the approximately flat phonon dispersion. Our data report the first example of the collapse of a kagome bosonic mode and promote the 166 compounds of kagomes as primary candidates to explore correlated flat phonon-topological flat electron physics.

Topological Floquet flat bands in irradiated alternating twist multilayer graphene

Topological Floquet flat bands in irradiated alternating twist multilayer graphene

Topological phase transitions and flat bands on an islamic lattice

We construct an islamic lattice by considering the nearest-neighbor (NN) hoppings with staggered magnetic fluxes and the next-NN hoppings. This model supports abundant quantum phases for various values of filling fractions. At filling, Chern insulator (CI) phases with Chern numbers and a zero-Chern-number topological insulator (ZCNTI) phase exist. At filling, several CI phases with Chern numbers and the ZCNTI phase are obtained. For the filling fraction 3/4, CI phases with Chern numbers and two ZCNTI phase areas appear. Interestingly, these ZCNTI phases host both robust corner states and gapless edge states which can be characterized by the quantized polarization and quadrupole moment. We further find that staggered magnetic fluxes can give rise to the ZCNTI state at and fillings. Phase diagrams for filling fractions , , and are presented as well. In addition, flat bands are obtained for various filling fractions by tuning the hopping parameters. At 1/8 filling, a best topological flat band (TFB) with flatness ratio about 12 appears. Several trivial flat bands but with total Chern number emerge in this model and exactly flat band is found at 3/8 filling. We further investigate fractional Chern insulate state when hard-core bosons fill into this TFB model.

Topological flat bands in rhombohedral tetralayer and multilayer graphene on hexagonal boron nitride moiré superlattices

Topological flat bands in rhombohedral tetralayer and multilayer graphene on hexagonal boron nitride moiré superlattices

Raman study of layered breathing kagome lattice semiconductor Nb3Cl8

Niobium chloride (Nb3Cl8) is a layered two-dimensional semiconducting material with many exotic properties including a breathing kagome lattice, a topological flat band in its band structure, and a crystal structure that undergoes a structural and magnetic phase transition at temperatures below 90 K. Despite being a remarkable material with fascinating new physics, the understanding of its phonon properties is at its infancy. In this study, we investigate the phonon dynamics of Nb3Cl8 in bulk and few layer flakes using polarized Raman spectroscopy and density-functional theory (DFT) analysis to determine the material’s vibrational modes, as well as their symmetrical representations and atomic displacements. We experimentally resolved 12 phonon modes, five of which are A 1g modes while the remaining seven are Eg modes, which is in strong agreement with our DFT calculation. Layer-dependent results suggest that the Raman peak positions are mostly insensitive to changes in layer thickness, while peak intensity and full width at half maximum are affected. Raman measurements as a function of excitation wavelength (473–785 nm) show a significant increase of the peak intensities when using a 473 nm excitation source, suggesting a near resonant condition. Temperature-dependent Raman experiments carried out above and below the transition temperature did not show any change in the symmetries of the phonon modes, suggesting that the structural phase transition is likely from the high temperature P 1 phase to the low-temperature R phase. Magneto-Raman measurements carried out at 140 and 2 K between −2 and 2 T show that the Raman modes are not magnetically coupled. Overall, our study presented here significantly advances the fundamental understanding of layered Nb3Cl8 material which can be further exploited for future applications.

Superconductivity in twisted double bilayer graphene stabilized by WSe2.

Identifying the essential components of superconductivity in graphene-based systems remains a critical problem in two-dimensional materials research. This field is connected to the mysteries that underpin investigations of unconventional superconductivity in condensed-matter physics. Superconductivity has been observed in magic-angle twisted stacks of monolayer graphene but conspicuously not in twisted stacks of bilayer graphene, although both systems host topological flat bands and symmetry-broken states. Here we report the discovery of superconductivity in twisted double bilayer graphene (TDBG) in proximity to WSe2. Samples with twist angles 1.24° and 1.37° superconduct in small pockets of the gate-tuned phase diagram within the valence and conduction band, respectively. Superconductivity emerges from unpolarized phases near van Hove singularities and next to regions with broken isospin symmetry. Our results show the correlation between a high density of states and the emergence of superconductivity in TDBG while revealing a possible role for isospin fluctuations in the pairing.

Staggered magnetic flux induced higher-order topological insulators and topological flat bands on the ruby lattice

Staggered magnetic flux induced higher-order topological insulators and topological flat bands on the ruby lattice

Topological Flat Bands in Strained Graphene: Substrate Engineering and Optical Control.

The discovery of correlated phases in twisted moiré superlattices accelerated the search for low-dimensional materials with exotic properties. A promising approach uses engineered substrates to strain the material. However, designing substrates for tailored properties is hindered by the incomplete understanding of the relationship between the substrate's shapes and the electronic properties of the deposited materials. By analyzing effective models of graphene under periodic deformations with generic crystalline profiles, we identify strong C2z symmetry breaking as the critical substrate geometric feature for emerging energy gaps and quasi-flat bands. We find continuous strain profiles producing connected pseudomagnetic field landscapes are important for band topology. We show that the resultant electronic and topological properties from a substrate can be controlled with circularly polarized light, which also offers unique signatures for identifying the band topology imprinted by strain. Our results can guide experiments on strain engineering for exploring interesting transport and topological phenomena.

Hexagonal boron-carbon fullerene heterostructures; Stable two-dimensional semiconductors with remarkable stiffness, low thermal conductivity and flat bands

Among exciting recent advances in the field of two-dimensional (2D) materials, the successful fabrications of the C60 fullerene networks has been a particularly inspiring accomplishment. Motivated by the recent achievements, herein we explore the stability and physical properties of novel hexagonal boron-carbon fullerene 2D heterostructures, on the basis of already synthesized B40 and C36 fullerenes. By performing extensive structural minimizations of diverse atomic configurations using the density functional theory method, for the first time, we could successfully detect thermally and dynamically stable boron-carbon fullerene 2D heterostructures. Density functional theory results confirm that the herein predicted 2D networks exhibit very identical semiconducting electronic natures with topological flat bands. Using the machine learning interatomic potentials, we also investigated the mechanical and thermal transport properties. Despite of different bonding architectures, the room temperature lattice thermal conductivity of the predicted nanoporous fullerene heterostructures was found to range between 4 and 10 W/mK. Boron-carbon fullerene heterostructures are predicted to show anisotropic but also remarkable mechanical properties, with tensile strengths and elastic modulus over 8 and 70 GPa, respectively. This study introduces the possibility of developing a novel class of 2D heterostructures based on the fullerene cages, with attractive electronic, thermal and mechanical features.

Epitaxial Kagome Thin Films as a Platform for Topological Flat Bands.

Systems with flat bands are ideal for studying strongly correlated electronic states and related phenomena. Among them, kagome-structured metals such as CoSn have been recognized as promising candidates due to the proximity between the flat bands and the Fermi level. A key next step will be to realize epitaxial kagome thin films with flat bands to enable tuning of the flat bands across the Fermi level via electrostatic gating or strain. Here, we report the band structures of epitaxial CoSn thin films grown directly on the insulating substrates. Flat bands are observed by using synchrotron-based angle-resolved photoemission spectroscopy (ARPES). The band structure is consistent with density functional theory (DFT) calculations, and the transport properties are quantitatively explained by the band structure and semiclassical transport theory. Our work paves the way to realize flat band-induced phenomena through fine-tuning of flat bands in kagome materials.

Topological phase transitions and topological flat bands on the ruby lattice

We investigate the topological properties of a tight-binding model on the two-dimensional ruby lattice in the presence of staggered fluxes. The variation of the nearest- and next-nearest-neighbor hopping parameters yields tunable Chern-number bands, which may host quantum anomalous Hall insulators at different filling fractions. Interestingly, we obtain topological nontrivial bands with high Chern number C = −4. We show that topological phase transitions among different gapped phases are accompanied with the gap closing and reopening processes. Furthermore, we find topological flat bands with Chern number C=+1, which could be a platform for realizing a fractional quantum Hall effect.