Hao Chen

Janus materials provide an intrinsic platform for inversion-symmetry breaking and chirality-related phonon phenomena. Here, based on first-principles calculations, we investigate the impact of out-of-plane asymmetry on chiral phonons in Janus SbTeI. We show that exchanging the upper and lower atomic layers produces distinct chirality responses in different structural phases. In the 2H-phase, the phonon chirality in the in-plane directions is reversed while the out-of-plane component is preserved, whereas in the 1T-phase, all three chirality components are simultaneously flipped. We point out that the distinct responses are fundamentally determined by the mirror symmetry Mz and the spatial inversion symmetry P in different phases. Furthermore, these chirality reversals lead to corresponding changes in phonon angular momentum, magnetization under a temperature gradient, and the phonon Hall response in the presence of a magnetic field. Our findings highlight the essential role of Janus asymmetry in phonon chirality and related transport phenomena.

Chiral phonons have attracted increasing attention, as they play important roles in many different systems and processes. However, a method to control phonon chirality by external fields is still lacking. Here, we propose that in displacement-type ferroelectric materials, an external electric field can reverse the chirality of chiral phonons via ferroelectric switching. Using first-principles calculations, we demonstrate this point in the well-known two-dimensional ferroelectric In2Se3. This reversal may lead to a number of electrically switchable phenomena, such as chiral phonon induced magnetization, the phonon Hall effect, and possible topological interface chiral phonon modes at ferroelectric domain boundaries. Our work offers a way to control chiral phonons, which could be useful for the design and application of thermal or information devices based on them.

As an ideal platform, both the theoretical prediction and first experimental verification of chiral phonons are based on transition-metal dichalcogenide materials. The manipulation of phonon chirality in these materials will have a profound effect on the study of chiral phonons. In this work, we utilize the sliding ferroelectric effect to realize the phonon chirality manipulation mechanism in transition-metal dichalcogenide materials. Based on first-principles calculations, we find the different manipulation effects of interlayer sliding on the phonon chirality and Berry curvature in bilayer and four-layer MoS₂ sliding ferroelectrics. These further affect the phonon angular momentum and magnetization under a temperature gradient and the phonon Hall effect under a magnetic field. Our work connects two emerging fields and opens up a new route to manipulating phonon chirality in transition-metal dichalcogenide materials through the sliding ferroelectric mechanism.

In a semiclassical model, magnetic fields can originate from the orbital magnetic moments of electrons. Here, using the Biot-Savart law, we theoretically predict the effective magnetic fields induced by chiral phonons in the point-charge model. Taking tellurium as an example, it is found that by applying a temperature gradient, the average effective magnetic field per mode for a specific location within a unit cell can reach the order of 0.01 T at room temperature. We prove that the effective magnetic field induced by chiral phonons has the ability to manipulate the Curie temperature and spontaneous magnetization in a ferromagnetic material or even in a general material.

Owing to charge-free property, magnon is promising to achieve dissipationless transport without Joule heating. Valley degree of freedom has also been fully expressed in electronic structures, enabling information coding and manipulation via valley-based qubits. Considering the conceptual importance and application potential, it is highly desirable to couple valley to magnon excitation and realize their modulations. We demonstrate valley magnons and valley modulations in a kagome ferromagnetic lattice, with Dzyaloshinskii-Moriya and staggered exchange interactions. The staggered exchange interaction breaks spatial inversion symmetry, leading to magnon valley Hall effect. When the Dzyaloshinskii-Moriya interaction is further included, the valley degeneracy is lifted, exhibiting net anomalous Hall effect and a series of topological phase transitions. Single-edge heat transport of magnons can occur, with propagating currents along the edge and local circulating currents. These findings give full play to spin and valley degrees of freedom and add another dimension to magnonic device paradigms.

The diode effect means that carriers can only flow in one direction but not the other. While diode effects for electron charge, spin, or photon have been widely discussed, it remains a question whether a chiral phonon diode can be realized, which utilizes the chiral degree of freedom of lattice vibrations. In this work, we reveal an intrinsic connection between the chiralities of a crystal structure and its phonon excitations, which naturally leads to the chiral phonon diode effect in chiral crystals. At a certain frequency, phonons with a definite chirality can propagate only in one direction but not the opposite. We demonstrate the idea in concrete materials including bulk Te and α-quartz (SiO₂). Our work discovers the fundamental physics of chirality coupling between different levels of a system, and the predicted effect will provide a new route to control phonon transport and design information devices.

As a potential degree of freedom, the chirality of phonons is expected to become a carrier of information. Exploring the phonon mode that can stably maintain chirality along a certain transport path is the basis for its realization. In this work, we find a chiral interface mode that is obviously localized at the line defect in the hexagonal boron nitride intralayer heterojunction, which is topologically protected. The existence of mirror symmetry results in the mode having opposite chiralities on both sides of the interface. For the B-B and N-N interface configurations, the chiral mode at the heavy-atom interface is more localized, which is determined by the local symmetry and atomic vibration intensity. By applying different harmonic forces, the topological chiral phonons at two valleys can be selectively excited and can be directionally transmitted without dissipation. With the decrease of the size of the heterojunction nanoribbon, the chiral interface mode always exists in the structurally stable systems. Our work enriches the research of chiral phonons in quasi-one-dimensional systems and can also provide certain references for the design of chiral phonon devices.