Session EE05: V: Metamaterials and 2D Materials

Transition waves in Tristable Magnetoelastic Lattice

We study the dynamics of phase transition in multistable magnetoelastic lattice. The unit cell consists of uniformly magnetized spherical magnets, leading to a tristable energy landscape for the onsite potential. The switching between multiple stable equilibria gives rise to a nonlinear wave called the transition wave. These dynamic phase boundaries travel within the structure with some unique characteristics. We report the existence of multiple types of transition waves emerging due to the asymmetric tristable energy landscape. We formulate an explicit energy transport equation that links the wave velocity to the asymmetric energy landscape and damping factors for such systems. We then analyze the collision of two similar transition waves that nucleate a new phase and transform the lattice to a new configuration. When two dissimilar transition waves collide, we observe the creation of a domain wall. We confirm the simulated predictions with an experimental demonstration. These results indicate the richness of dynamical phenomena in multistable lattices and could find applications in soft robotics, remote actuation, and reconfigurable structures.   

Stackings and effective models of bilayer dice lattices

We introduce and classify nonequivalent commensurate stackings for bilayer dice or lattice. For each of the four possible stackings, an effective low-energy model is derived. Although the energy spectrum remains always gapless with three bands interesting at the same point, depending on the stacking, different types of quasiparticle spectra arise. They include those with flat, tilted, anisotropic semi-Dirac, and three-fold-corrugated energy bands. We use the derived models to calculate the density of states and the spectral function. The corresponding results reveal drastic redistribution of the spectral weight due to the inter-layer coupling that is unique for each of the stackings.   

Surface acoustic waves-driven magnon spin Hall effect in atomically thin van der Waals antiferromagnets

Intrinsic 2D magnetism had long been believed to hardly survive due to the enhanced thermal fluctuations. However, the recent discovery of exfoliated van der Waals (vdW) magnets has opened up a new avenue for 2D magnetism. Especially, transition metal phosphorus trichalcogenides are a family of easily exfoliatable vdW antiferromagnets. These materials share the same honeycomb structure, but the bulk antiferromagnetic (AFM) phase varies depending on the magnetic elements. Therefore, the investigation of these materials paves the way toward not only the understanding of 2D magnetism, but also future AFM spintronic devices.

However, standard methods are not suitable for the study of atomically thin magnets. Especially, antiferromagnets do not have net magnetization, magneto-optical Kerr effect is not available either. Although recent studies have focused on Raman spectroscopy and second-harmonic generation to detect crystal symmetry lowering associated with the AFM transition, these signals do not provide clear identification in the monolayer limit. Therefore, an inclusive method which suits for exploring 2D antiferromagnets is highly desired.

By focusing on extremely large flexibility of 2D materials, we propose an intrinsic magnon spin Hall current driven by the surface-acoustic waves as a novel probe for such 2D vdW antiferromagnets. Furthermore, our results will overcome the difficulties inherent in the use of antiferromagnets and hence provide a building block for future AFM spintronics.   

Studying Moire magnetism in 2D materials using diamond scanning probe

Moiré superlattices of twisted nonmagnetic two-dimensional (2D) materials are highly controllable

platforms for the engineering of exotic correlated and topological states. We studied intriguing 

magnetic textures in twisted 2D magnetic materials. Using single-spin

quantum magnetometry, we directly visualized nanoscale magnetic domains, and from it, calculated domain magnetization. We

observed unexpected magnetic domains with twisted angle dependence. We attribute the phenomenon to magnetic frustration which create a competition between different type of magnetic domains.   

First Principles Study of Structures and Electronic Properties of Pb(10-x)Cux(PO4)6O

Recently, the experimental realization of potential room-temperature ambient-pressure superconductivity was reported for Cu-substituted lead apatite, Pb_(10-x)Cu_x(PO₄)₆O (x~0.9-1.0), so-called LK-99 material. However, the superconductivity in this material is in question and some relevant questions remain unresolved such as how the arrangements of substituted Cu on Pb sites in the LK-99 structure would minimize system's energy and might affect its electronic structure. In this work, we address these questions to some extent by enumerating possible configurations of Cu in Pb_(10-x)Cu_x(PO₄)₆O at 10% Cu substitution and performing density functional theory with Hubbard U correction (DFT+U) calculations of structural and electronic properties of selected configurations. We find that for (1x1x2) supercell, the most energetically favorable substitution sites are the nearest Pb(1) and Pb(2). The partially filled electronic state calculated at the Brillouin zone center is spatially localized around the Cu atom. For the low-energy configuration of single Cu substitution, we find that one electronic band is very flat with a narrow bandwidth of ~0.06 eV. The bands degeneracy at Γ and A high-symmetry points that is observed for a higher-energy configuration with one Cu substitution, disappears when two Cu atoms form a local dimer in a distorted LK-99 structure.   

Modulated Spin Transport in Two-Dimensional (2D) Nanomagnet CrOBr under Strain Engineering

Recent findings related to two-dimensional (2D) magnets demonstrate the capacity to manipulate and regulate both electronic and magnetic states at room temperature [1, 2, 3, 4]. In this study, we showcase controlled spin transport across different magnetic phases of the 2D semiconductor CrOBr [5] using first-principles density functional theory combined with the Non-Equilibrium Green’s function (NEGF) technique. Our findings reveal a noteworthy connection between its magnetic order and charge carriers. Firstly, we examine the spin-split electronic band structure of 2D - CrOBr under strain engineering. The outcomes reveal that the band structure of monolayer CrOBr displays distinct spin characteristics across various magneto-electric phases, attainable through the application of both uniaxial and biaxial strains. CrOBr exhibits versatile phase tunability, transitioning from a magnetic semiconductor state to half-metallic behavior and further to a magnetic metal, all achieved through strain variation. The transition from ferromagnetic (FM) to antiferromagnetic (AFM) state was also observed to occur with an increase in compressive strain of over -10% along the zigzag direction. In the biaxial strain scenario, the AFM state was found at strain levels of -14%, -16%, -18%, 8%, 10%, and 12%, respectively. Notably, a remarkable spin-filtering effect occurs, achieving a high spin injection efficiency of approximately 100% for transport along the armchair direction. Our research shows that CrOBr displays highly anisotropic transport behavior with perfect spin filtering, presenting new possibilities for strain-engineered magneto-electronic devices.   

Enhancement of spin to charge conversion efficiency at the topological surface state by inserting normal metal spacer layer in the topological insulator based heterostructure

We report efficient spin to charge conversion (SCC) in the topological insulator (TI) based heterostructure (BiSbT e1.5Se1.5/Cu/N i80F e20) by using spin-pumping technique. The SCC, characterized by inverse Edelstein effect length (λIEE) in the TI material alters with the Copper (Cu) interlayer thickness. It offers a new degree of freedom to manipulate SCC efficiency at the topological surface state. The significant enhancement of the measured spin-pumping voltage and the linewidth of ferromagnetic resonance (FMR) absorption spectra due to the insertion of Cu layer at the interface of TI and ferromagnetic metal (FM) indicates a reduction in spin memory loss at the interface that resulted from the presence of exchange coupling between the surface state of TI and the local moments of ferromagnetic metal. The temperature dependence (from 8K to 300K) of the evaluated λIEE data for all the trilayer systems, TI/Cu/FM with different Cu thickness confirms the effect of exchange coupling between the TI and FM layer on the spin to charge conversion efficiency of the topological surface state.   

Collective Quantum Phase-Locking Picture for an Analogy Between Periodically Driven Josephson Oscillation and Quantum Hall Effects

In this report, a comparison between the phase-slippage picture of superconductive Josephson effect and the Quantum Hall effects is examined. Collective quantum phase-locking mechanism can be used to unify these two effects by considering the relative motion between the flux quanta and the electron as a common mechanism. From the phase-locking dynamics, the phase changes of the electron and the flux during their relative motion can be in integer ratios or fractional ratios. The Integer Quantum Hall Effect and the Fractional Quantum Hall Effect can thus be shown as collective quantum phase-locking dynamics, as for the rf-driven superconductive Josephson oscillation. The collective quantization, especially the fractional one, is discussed with the pure dynamical arguments. The chaotic behavior of the collective phase-locking is compared with the probabilistic behavior of traditional quantum mechanical quantization.