arXiv subjects

Ethan Angerhofer

Publications and source records attributed to Ethan Angerhofer.

3 recordsLinked to original sources

arXiv2026

Nematic Wigner crystals in rhombohedral multilayer graphene

Recent experiments have reported evidence for Wigner crystals (WCs) in rhombohedral graphene. Here, we investigate Wigner crystallization in rhombohedral tetralayer graphene using projected Hartree-Fock (HF) calculations and time-dependent Hartree-Fock (TDHF) calculations. We first perform HF calculations with one electron per Wigner unit cell, and find nematic WCs (nWCs) that spontaneously break the threefold rotational symmetry $C_3$ and $C_3$-invariant WCs. In particular, there are two nWC regions in the phase diagram: one larger region at large displacement fields and low electron densities, and another smaller region at intermediate fields and high densities. Both the nWCs and the $C_3$-invariant WCs are valley-polarized states with zero Chern number, and have positive indirect gaps in the HF band structure. We then perform TDHF calculations to further test the local stability of the WC states. We find that all $C_3$-invariant WCs and half of the nWCs are locally stable, while the remaining nWCs are unstable towards WCs with two electrons per unit cell or metallic states. The predicted stable nWC phase can be identified experimentally by scanning tunneling microscopy through its anisotropic charge distribution or by angle-resolved transport measurements via a direction-dependent depinning voltage.

cond-mat.str-el↗

arXiv2025

Symmetry-enforced Moir\'e Topology

Topological flat bands in two-dimensional (2D) moir\'e materials have emerged as promising platforms for exploring the interplay between topology and correlation effects. However, realistic calculations of moir\'e band topology using density functional theory (DFT) are computationally inefficient due to the large number of atoms in a single moir\'e unit cell. In this work, we propose a systematic scheme to predict the topology of moir\'e bands from atomic symmetry data and moir\'e symmetry group, both of which can be efficiently extracted from DFT. Specifically, for $\Gamma$-valley electron gases, we find that certain combinations of atomic symmetry data and moir\'e symmetry groups can enforce nontrivial band topology in the low-energy moir\'e bands, as long as the moir\'e band gap is smaller than the atomic band splitting at the moir\'e Brillouin zone boundary. This symmetry-enforced nontrivial moir\'e topology, including both topological insulators and topological semimetals, is robust against various material-specific details such as the precise form and strength of the moir\'e potential or the exact twist angle. By exhaustively scanning all 2D atomic symmetry data and moir\'e symmetry groups, we identify 197 combinations that can yield symmetry-enforced nontrivial moir\'e topology, and we verify one such combination using a moir\'e model with cubic Rashba spin-orbit coupling. By screening the existing 2D material database, we currently identify 92 monolayer materials with (i) the low-energy bands near $\Gamma$ and (ii) the atomic symmetry data that belong to those combinations. Our approach is generalizable to other valleys and provides a useful guideline for experimental efforts to discover and design new topologically nontrivial moir\'e materials.

cond-mat.mes-hall↗

arXiv2024

Aluminum Scandium Nitride as a Functional Material at 1000{\deg}C

Aluminum scandium nitride (AlScN) has emerged as a highly promising material for high-temperature applications due to its robust piezoelectric, ferroelectric, and dielectric properties. This study investigates the behavior of Al0.7Sc0.3N thin films in extreme thermal environments, demonstrating functional stability up to 1000{\deg}C, making it suitable for use in aerospace, hypersonics, deep-well, and nuclear reactor systems. Tantalum silicide (TaSi2)/Al0.7Sc0.3N/TaSi2 capacitors were fabricated and characterized across a wide temperature range, revealing robust ferroelectric and dielectric properties, along with significant enhancement in piezoelectric performance. At 1000{\deg}C, the ferroelectric hysteresis loops showed a substantial reduction in coercive field from 4.3 MV/cm to 1.2 MV/cm, while the longitudinal piezoelectric coefficient increased nearly tenfold, reaching 75.1 pm/V at 800{\deg}C. Structural analysis via scanning and transmission electron microscopy confirmed the integrity of the TaSi2/Al0.7Sc0.3N interfaces, even after exposure to extreme temperatures. Furthermore, the electromechanical coupling coefficient was calculated to increase by over 500%, from 12.9% at room temperature to 82% at 700{\deg}C. These findings establish AlScN as a versatile material for high-temperature ferroelectric, piezoelectric, and dielectric applications, offering unprecedented thermal stability and functional enhancement.

cond-mat.mtrl-sci↗