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The body–boundary correspondence refers to the relationship between the body and boundary states of topological insulators (TIs). In TIs, the presence of boundary states is connected to the bulk topological properties of the material. The topology can be identified by studying the energy of the topological modes within the bulk bandgap. However, not all topological materials exhibit boundary states within the insulating energy gap. In many cases, the presence of boundary states can be hidden or masked by the bulk energy bands, making it difficult to measure TIs. Recent experiments have shown that defects, which are commonly found in crystalline materials, can be used as probes to explore higher-order topologies that have been recently realized on various platforms. These defects can generate fractional charges and stable bound states in the dispersion region, allowing us to observe the clear body–disclination correspondence. We have performed simulations using a coupled acoustic cavity system with C3 and C5 symmetries to investigate the body–disclination correspondence in topological crystal insulators (TCIs) in the field of acoustics. Simulation and theoretical results have demonstrated that defects such as disclinations can be used to probe higher-order topologies that were previously unobservable in three-dimensional structures. This approach allows us to detect fractional mode charges and stable bound states, which are crucial for understanding the topological nature of TCIs. Our work demonstrates the potential of using disclination defects to study the intricate relationship between the body and boundary states in topological materials, particularly in the context of acoustics. [ABSTRACT FROM AUTHOR]

The measurement of topological insulators is usually achieved by measuring spectra, which is inadequate in higher-order topological insulators because the separation of bandgaps may be blurred, such as body boundaries and corner state hybridization, and the above measurement requires that the energy of the topological mode must be located in the bulk bandgap to be measured. Recent experiments have shown a strong relationship between the fractional mode charge and the location of the Wannier center in the crystal body. The fractional quantization of the corner charge originates from the filling anomaly, and the fractional mode charge can be calculated from experimental measurements of the local density of states to describe the topological state of the system, which has been demonstrated in optical systems. We have achieved a similar effect in acoustics by utilizing a coupled acoustic-cavity system featuring C3, C4, and C6 symmetries for simulation. Furthermore, we extended our study to 3D third-order topological crystalline insulators to validate the fractional-corner charge approach. The simulation and theoretical results demonstrate that the fractional mode charge can clearly identify the high-order topology, even without gap states. [ABSTRACT FROM AUTHOR]

Non-Hermitian factors play a key role in introducing changes to the topology of acoustic systems, by significantly expanding the classification of topological phases beyond traditional Hermitian theory and presenting challenges to the conventional bulk–boundary correspondence. Traditional Hermitian theory cannot fully describe the complicated behavior of systems interacting with their environment, thus non-Hermitian theory emerged. Currently, most non-Hermitian topological systems derive their phases from Hermitian components, which entails concurrently considering the gain and loss of the system. In the study of higher-order topological insulators, higher-order topological states are usually achieved by modulating coupling strengths. While traditionally it is viewed that dissipation in topological systems is detrimental to the topological states, recent research studies have challenged this perspective. The nontrivial topology can arise by introducing loss of a specific configuration in a trivial phase. In this study, through finite-element simulations of coupled acoustic cavity systems from one-dimensional to three-dimensional structure, it is illustrated how intentionally introducing non-Hermitian loss can induce the higher-order topology. More crucially, we have simulated the acoustic local density of states (LDOS) and calculated fractional charge modes and can directly observe and prove through LDOS that introducing loss can convert a trivial system into a topological one. Both theoretical and simulated results show that the LDOS can be used to calculate fractional charge modes, effectively characterizing higher-order topological states in non-Hermitian acoustic systems. This study carves out a research pathway for the future of non-Hermitian acoustic topology. [ABSTRACT FROM AUTHOR]

The use of femtosecond laser pulses for ultrafast triggering of magnetization switching is of great interest in multilayer systems with great tunability. At present, the impact of interfacial mixing on magnetization switching has not been thoroughly investigated. In this paper, the impact of interfacial mixing on magnetization dynamics in multilayer systems is investigated by a combination of atomic spin dynamics and two-temperature models. Our results show that interfacial mixing in multilayer systems not only reduces the energy density required for magnetization switching but also expands the range of pulse durations that can trigger magnetization switching. In addition, we have investigated the dependence of the switching time on the interfacial mixing in multilayer systems. The results show that interfacial mixing can accelerate the process of magnetization dynamics, thus providing a theoretical basis for the design of faster speed optically controlled magnetic tunnel junctions. [ABSTRACT FROM AUTHOR]

An attractive avenue for a memory device is offered by thermally induced magnetization switching (TIMS), which is expected to increase the storage density. As an essential step in TIMS, the transient ferromagnetic-like state (TFLS) has attracted much attention. Here, our research analyzes the TFLS from the perspective of magnetization rate based on extensive atomic spin simulations in a parameter space necessary for the formation of TIMS. The results demonstrate that the evolution of the magnetization rate for both Fe and Gd sublattices has two peaks at different timescales, and the two peaks are driven by the Langevin field and the exchange field, respectively. By modulating the pulse fluence, the initial temperature, the pulse duration, the electron–phonon coupling factor, and the damping constant, we find that the first peak of the magnetization rate depends not only on the electron thermal bath and temperature of the phonon but also on the energy transfer rate between the electronic system and the spin system. Moreover, we elucidate the role played by the Langevin field and the exchange field in TFLS formation when modulating the pulse fluence, initial temperature, and pulse duration. The simulation results show that the enhancement of the Langevin field prolongs the duration of TFLS. The enhancement of the exchange field at a long pulse duration before the electron system cools down leads to the shortening of the TFLS duration. However, the time required for Gd demagnetization after the electron system cools down to the Curie temperature changes only slightly when modulating these simulated parameters. [ABSTRACT FROM AUTHOR]

Femtosecond laser ultrafast thermally induced magnetization switching (TIMS) has also attracted much attention due to its ability to trigger a single switching at the picosecond timescale. Current studies have shown that after a TIMS excited by a laser pulse, excitation of the switch again via TIMS does not require equilibrium between the subsystems. In this work, the main investigation is on the various possible cases of magnetization dynamics in GdFeCo under two short-delayed pulse excitations, as well as the factors limiting the minimum separation for double TIMS. These conditions are relevant for the potential application of TIMS to memory devices as it affects both the speed limit at which rewritten data is available and demonstrates the importance of spatial confinement of a laser pulse to bit size. The results show that low energy and short pulse duration lasers are prerequisites for double TIMS in GdFeCo based on simulations of atomic spin dynamics. By changing the damping constants of the alloy, we can shorten the minimum pulse separation between two pump pulses for double TIMS to 2 ps to approach terahertz frequency of write/erase cycles. [ABSTRACT FROM AUTHOR]

Ultrafast magnetization switching induced by a single femtosecond laser pulse in the absence of an applied magnetic field has attracted extensive attention due to its great potential for low energy and ultrafast storage applications. However, pulse helicity-independent switching is mainly used for Gd-based materials, and it is now necessary to explore the possibility of further optimization depending on the composition and damping coefficients. For the GdFe alloy with low Gd concentration, we modulate the damping constant for a comparative analysis based on atomic spin simulations. The simulation results show that an appropriate increase in the high damping ratio αFe/αGd not only helps to extend the pulse fluence range for switching but also reduces the minimum Gd concentration requirement for transient ferromagnetic-like state. The results show that the pulse fluence threshold of switching at low Gd concentration is more sensitive to the damping ratio αFe/αGd. On the contrary, modulating the damping ratio αFe/αGd at high Gd concentrations almost does not cause the shift of pulse fluence threshold. The lower Gd concentration for thermally induced magnetization switching is expected to be unlocked by the engineering of element-specific damping. [ABSTRACT FROM AUTHOR]

The local density of states (LDOS), which describes the space-resolved density of states, has been studied in the context of exploring topological materials. Here, we report the observation of the fractional mode charge of topological states in a four-dimensional synthetic acoustic system, which is based on coupled acoustic cavities. We experimentally measured the LDOS of topological boundary states and corner states of this system and observed one-half and one-quarter fractional mode charges. In addition, we also obtained the LDOS in a three-dimensional acoustic system by simulation and observed the one-eighth fractional mode charge of the system. The results show that the appearance of topological states can correspond to the fractional charges, which is useful for the understanding of topology in acoustic systems and provides a new method for the characterization of topological states in topological acoustic systems. [ABSTRACT FROM AUTHOR]