Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: Dissipation in mechanics, optics, acoustics, and electronic circuits is nowadays recognized to be not always detrimental but can be exploited to achieve non-Hermitian topological phases with functionalities for potential device applications, ranging from sensors with unprecedented sensitivity, light funneling, wave isolators, non-reciprocal amplification, to dissipation induced phase transition. As elementary excitations of ordered magnetic moments that exist in various magnetic materials, magnons are the information carrier in magnonic devices with low-energy consumption for reprogrammable logic, non-reciprocal communication, and non-volatile memory functionalities. Non-Hermitian topological magnonics deals with the engineering of dissipation for non-Hermitian topological phases in magnets that are not achievable in the conventional Hermitian scenario, with associated functionalities cross-fertilized with their electronic, acoustic, optic, and mechanic counterparts, such as giant enhancement of magnonic frequency combs, magnon amplification, (quantum) sensing of the magnetic field with unprecedented sensitivity, magnon accumulation, and perfect absorption of microwaves. In this review article, we introduce the unified basic physics and provide a comprehensive overview of the recent theoretical and experimental progress towards achieving distinct non-Hermitian topological phases in magnonic devices, including exceptional points, exceptional nodal phases, non-Hermitian magnonic SSH model, and non-Hermitian skin effect. We emphasize the non-Hermitian Hamiltonian approach based on the Lindbladian or self-energy of the magnonic subsystem but address the physics beyond it as well, such as the crucial quantum jump effect in the quantum regime and non-Markovian dynamics. We provide a perspective for future opportunities and challenges before concluding this article.

Abstract: Charge density wave (CDW) ordering has been an important topic of study for a long time owing to its connection with other exotic phases such as superconductivity and magnetism. The RTe3 (R = rare-earth elements) family of materials provides a fertile ground to study the dynamics of CDW in van der Waals layered materials, and the presence of magnetism in these materials allows to explore the interplay among CDW and long range magnetic ordering. Here, we have carried out a high-resolution angle-resolved photoemission spectroscopy (ARPES) study of a CDW material GdTe3, which is antiferromagnetic below 12 K, along with thermodynamic, electrical transport, magnetic, and Raman measurements. Our Raman spectroscopy measurements show the presence of CDW amplitude mode at room temperature, which remains prominent when the sample is thinned down to 4-layers by exfoliation. Our ARPES data show a two-fold symmetric Fermi surface with both gapped and ungapped regions indicative of the partial nesting. The gap is momentum dependent, maximum along G-Z and gradually decreases going towards G - M. Our study provides a platform to study the dynamics of CDW and its interaction with other physical orders in two- and three-dimensions.

Abstract: Quantum anomalous Hall effect (QAHE) was discovered a decade ago, but is still not utilized beyond a handful of research groups, due to numerous limitations such as extremely low temperature, electric field-effect gating requirement, small sample sizes and environmental aging effect. Here, we present a robust platform that provides effective solutions to these problems. Specifically, on this platform, we observe QAH signatures at record high temperatures, with the Hall conductance of 1.00 e2/h at 2.0 K, 0.98 e2/h at 4.2 K, and 0.92 e2/h at 10 K, on centimeter-scale substrates, without electric-field-effect gating. The key ingredient is an active CrOx capping layer, which substantially boosts the ferromagnetism while suppressing environmental degradation. With this development, QAHE will now be accessible to much broader applications than before.

Abstract: Twisted bilayer graphene (TBLG) subject to a sequence of commensurate external periodic potentials reveals the formation of moir\'e fractals that share striking similarities with the central place theory (CPT) of economic geography, thus uncovering a remarkable connection between twistronics and the geometry of economic zones. The moir\'e fractals arise from the self-similarity of the hierarchy of Brillouin zones (BZ) so formed, forming a nested subband structure within the bandwidth of the original moir\'e bands. The fractal generators for TBLG under these external potentials are derived and we explore their impact on the hierarchy of the BZ edges. Furthermore, we uncover parallels between the modification of the BZ hierarchy and magnetic BZ formation in the Hofstadter butterfly, allowing us to construct an incommensurability measure for moir\'e fractals as a function of the twist angle. The resulting band structure hierarchy bolsters correlation effects, pushing more bands within the same energy window for both commensurate and incommensurate structures.