Quantum Materials Workshop 2024
양자물성 워크숍 2024

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Looking at the current global energy crisis, the development of technology for capturing and converting otherwise wasted heat into useful electrical power will be of utmost importance. The class of materials capable of converting thermal gradients into electrical energy or vice versa by virtue of a unique combination of electrical and thermal properties are called thermoelectric (TE) materials. Innovative design of solid state structures and compositions with low thermal conductivity while maintaining the high electrical transport is the way forward to high performance thermoelectric (TE) materials, which offer an environment friendly solution for recovery of waste heat in the form of electricity. With the coming years TE materials are stated to play a significant role in the energy management. The crux of improving a material’s thermo- electric performance involves essentially the optimization of three interdependent material properties: electrical conductivity (σ), Seebeck coefficient (S) and thermal conductivity (κ_total= electronic (κ_el) + lattice (κ_lat) thermal conductivity) which govern the dimensionless thermoelectric figure of merit, zT=σS^2 T/(κ_el+κ_lat). Bismuth-antimony (BiSb) alloys are promising thermoelectric materials for cooling applications. However, their performance in polycrystalline form and the impact of doping remain underexplored. In this work, we report a schematic study of the thermoelectric properties of BiSb utilizing the substitution of Ti. Our experimental collaborator, Prof. Kim performed experiments.

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Our study investigates hexagonally-shaped flakes of hexagonal boron nitride and graphene across a range of sizes (25 to 250 nm) and twist angles to elucidate the interplay between geometric configuration and system stability. Our findings indicate that the armchair edge configuration is more stable than zigzag and shows a distinctive beating pattern and significant energy barriers persisting at large twist angles. In contrast, the zigzag configuration shows minimal energy differences and lacks a discernible beating pattern. These insights highlight the potential of energy barriers as effective control mechanisms for manipulating flakes. Our study provides valuable guidance to experimentalists, offering deeper insights into flake properties and associated barrier heights crucial for precise control in applications involving 2D materials.

We study the plasmon modes of alternating-twist multilayer graphene within the random phase approximation. Using both numerical and analytical methods, we demonstrate the existence of a gapped out-of-phase plasmon mode, distinct from the usual center-of-mass mode, that remains undamped even in the low-density regime. Additionally, we investigate the effect of a perpendicular electric field on this mode and show how its dispersion can be tuned by applying a gate voltage.

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The relation between Landau levels ($E$) and the magnitude of the magnetic field ($B$) is closely tied to the electronic structure, and magneto-optical conductivity reveals these Landau level structures. In nodal line semimetals, many experiments have reported graphene-like Landau levels ($E \sim \sqrt{B}$). In this study, we investigate the magneto-optical conductivity of the nodal ring semimetal SrAs$_3$. We measure the magneto-optical conductivity in two directions: orthogonal to the ring axis and parallel to the ring axis. The results reveal the various structures, including electron (hole) gas ($E \sim B$), graphene ($E \sim \sqrt{B}$), and semi-Dirac material ($E \sim B^{2/3}$). Notably, the semi-Dirac contribution is newly observed in conventional nodal line semimetals.

Recent years have seen active discussion on various effects of the electrically induced Dzyaloshinskii-Moriya (DM) interaction on the Kitaev spin liquid. It has been pointed out that one possible effect of such interaction is the induced the long-range Majorana fermion hoppings. This talk will discuss how the higher-Chern number chiral spin liquid phase can arise from such long-range hoppings.

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Rhombohedral multilayer graphene (RnG) featuring partially flat bands has emerged as an important platform to probe strong Coulomb correlation effects. Theoretical consideration of local electron-electron interactions are of particular importance for electronic eigenstates with a tendency to spatially localize. We present a method to incorporate mean-field electron-electron interaction corrections in the tight-binding hopping parameters of the band Hamiltonian within the extended Hubbard model that incorporates ab initio estimates of on-site (U) and inter-site (V) Hubbard interactions for the π bands of RnG. Our Coulomb-interaction renormalized band structures feature electron-hole asymmetry, band flatness, band gap, and anti-ferromagnetic ground states in excellent agreement with available experiments for n ≥ 4. We reinterpret the putative gaps proposed in n = 3 systems in terms of shifting electron and hole density of states peaks depending on the range of the Coulomb interaction models.

*Ref: ^[1] Lee, D., Yang, W., Son, Y.-W. & Jung, J. Extended Hubbard corrected tight-binding model for rhombohedral few-layer graphene. Preprint at https://doi.org/10.48550/arXiv.2403.00530 (2024).

These electron density profiles have been quantified through parametrization employing third-order harmonic expansions in twiste bilayer hBN(t2BN) of parallel configurarion(BN/BN), enhancing their adaptability for future research endeavors. Based on these electron density profiles, we explore the moiré potential induced in adjacent materials contacted by t2BN in the BN/BN configurations. The induced moiré potential from BN/BN has been simulated through a real-space tight-binding model for both rigid and relaxed moiré patterns. The electronic potential for small twist angles has been approximated using a harmonic approximation. Specifically, the relaxed model considered first and third-order harmonic approximations, which effectively provide the interface electron density profiles of the moiré patterns. This indicates potential avenues for manipulating the properties of neighboring functional layers by leveraging the surface potential of a t2BN substrate.

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We show that rhombohedral four-layer graphene (4LG) nearly aligned with a hexagonal boron nitride (hBN) substrate often develops nearly flat isolated low-energy bands with nonzero valley Chern numbers. The bandwidths of the isolated flat bands are controllable through an electric field and twist angle, becoming as narrow as ∼10 meV for interlayer potential differences between top and bottom layers of |Δ|≈10–15 meV and 𝜃∼0.5∘ at the graphene and boron nitride interface. The local density of states analysis shows that the nearly flat band states are associated to the nondimer low-energy sublattice sites at the top or bottom graphene layers and their degree of localization in the moiré superlattice is strongly gate tunable, exhibiting at times large delocalization despite the narrow bandwidth. We verified that the first valence band’s valley Chern numbers are 𝐶𝜈=±1 𝑉⁢1=±𝑛, proportional to layer number for 𝑛⁢LG/BN systems up to 𝑛=8 rhombohedral multilayers.

We calculate the electronic band structure of rhombohedral graphene stacks ranging from N = 1 to N = 5 layers. We engineer the low energy states to form flat bands and band gaps by encapsulating the system with chirally rotated hBN layers to maximize the coupling between hBN and the outer graphene layers. By highlighting the energetic stability of the stable configurations of the system, we calculate their electronic spectrum. hBN-superlattice allows to isolate the flattened rhombohedral graphene from the rest of the spectrum where band isolation becomes increasingly hard for an increasing number of graphene layers due to lattice reconstruction effects. We observe that comparatively,theta_1 = – theta_2 = 0.61 degree has a better chance at isolating the flat bands from the rest of the spectrum than the perfectly aligned case with theta_1 = theta_2 = 0 degree. Additionally, we observe that an external electric field for theta_1 = theta_2 = 0 degree case the two peaks corresponding to the low energy flat bands for N = [2,4] evolve into four peaks for N=5 due to additional flatting of higher energy minibands, while for theta_1 = -theta_2 = 0.61 degree very flat bands with a conduction bandwidth of 5 meV and a valence bandwidth of 11 meV can be observed for N=5. For the larger commensurate twist angle combination of theta_1 =-theta_2 = 1.81 degree, high-energy superlattice effects are negligible due to weak coupling between the layers and only small variations are observed in terms of the primary gap amplitude.

Recently, the alternating-twist multilayer graphene (ATMG) has attracted considerable attention due to its observed strongly correlated electron phenomena, similar to those in twisted bilayer graphene (TBG). Among these, the tetralayer (AT4G) case exhibits some peculiarities such as the single-particle band gap upon application of a vertical electric field, while its structure remains only slightly more complex than that of the trilayer.

*Ref: Sliding-dependent electronic structures of alternating-twist tetralayer graphene Kyungjin Shin*, Jiseon Shin*, Yoonsung Lee, Hongki Min†, and Jeil Jung† arXiv:2406.11527 (2024)

Including the effect of the trivial band near Weyl nodes, we evaluate the longitudinal magnetoconductivity (LMC) of Weyl semimetals along the magnetic field direction using the Boltzmann magnetotransport theory and study its dependence on the magnetic field, Fermi energy, and temperature. We find that for weak internode and node-trivial band scatterings, the LMC is quadratic in the magnetic field and is inversely proportional to the fourth power of the Fermi energy at high densities due to internode scatterings and to the square of the Fermi energy at low densities due to scatterings between a Weyl node and the trivial band. In the case of strong internode and node-trivial band scatterings, the magnetic field-driven anisotropy induced by the phase-space volume element and the orbital magnetic moment cannot be neglected. As a result, the LMC exhibits a significantly different trend compared to that in the weak internode and node-trivial band scattering limit. Finally, we calculate the temperature dependence of the LMC in the strong inelastic scattering limit and obtain its asymptotic behaviors at low and high temperatures, respectively, demonstrating that the temperature dependence is strongly affected by the existence of the trivial band.

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