Category Archives: Publications

Time-reversal symmetry (TRS) is pivotal for materials’ optical, magnetic, topological, and transport properties. Chiral phonons, characterized by atoms rotating unidirectionally around their equilibrium positions, generate dynamic lattice structures that break TRS. Here, in our recent work published on Science, we report that coherent chiral phonons, driven by circularly polarized terahertz light pulses, will polarize the paramagnetic spins in cerium fluoride in a manner similar to that of a quasi-static magnetic field on the order of 1 tesla. Through time-resolved Faraday rotation and Kerr ellipticity, we found that the transient magnetization is only excited by pulses resonant with phonons, proportional to the angular momentum of the phonons, and growing with magnetic susceptibility at cryogenic temperatures. The observation quantitatively agrees with our spin-phonon coupling model and may enable new routes to investigating ultrafast magnetism, energy-efficient spintronics, and nonequilibrium phases of matter with broken TRS.

Check out more fun and viewpoints from this perspective, Rice News, futurity.org, phys.org, EurekAlert!, NSF highlights, DeepTech深科技, physics world.

Mature optical technologies from the microwave frequencies to the X-ray frequencies have dramatically changed our life in the modern society, but there is a notable gap between the mid- and far-infrared light, roughly the frequencies of 5 – 15 terahertz, where there are no good commercial solutions to control light. In our new work published on Advanced Materials, we use quantum paraelectric perovskite SrTiO3, whose atoms couple with terahertz light so strongly that form a new particle called “phonon-polaritons” to realize sub-wavelength THz field focus. Unlike other materials that also support phonon-polaritons in other frequencies and usually in a very narrow range, SrTiO3 works for the entire “new terahertz gap” due to its quantum paraelectricity property. We prove the concept of SrTiO3 phonon-polariton devices in the frequency range of 7 – 13 terahertz by designing and fabricating ultrafast THz field concentrators to achieve a strong transient electric field around gigavolts per meter. Such a strong field may be used to change the materials’ structure to create new electronic properties, or create new nonlinear optical response from trace amounts of specific molecules to be detected by a common optical microscope. Our methodology framework is also applicable for a broad range of phonon-polaritonic materials to work for photonic devices in 3 – 19 terahertz, opening a new route to control light in one of the last under-explored territories. More details in Rice news.

Check out our new results on Nano Letters! We have observed coherent hybridization of magnons and phonons first ever in monolayer antiferromagnet FePSe₃ by cavity-enhanced magneto-Raman spectroscopy. The coupling remains robust even in zero magnetic field down to 2D limit in a single atomic layer. We also discovered theoretically that such magnon-phonon hybridization enables nontrivial band inversion between longitudinal and transverse optical phonons and guarantee magnetic-field-controlled topological phase transition. The 2D topological magnon–phonon hybridization would potentially offer a new route toward quantum phononics and magnonics with an ultrasmall footprint.

Phase boundaries can exhibit new properties that neither of the two sides possess. In ultrathin 2D crystals of MoTe₂, both the 2H and 1T’ phases have very weak, if any, inversion symmetry breaking, and thus do not have strong piezoelectricity. However, the metal-semiconductor junction between these two phases has a build-in electric field, so the wavefunctions are asymmetric at the boundary. In this work, we extracted the tiny displacement of these boundary atoms as a result of interfacial piezoelectricity in the CVD-grown 2H-1T’ MoTe2 homojunctions by vector piezoresponse force microscopy. The additional torque from the atoms was distinguished from electrostatic background, and qualitatively agreed with the first-principle calculations. Our findings demonstrate the versatility of boundary engineering in nanoscale electromechanical devices. See more news and details of this work in collaboration with Ajayan group at Rice MSNE.

Elemental tellurium is a fascinating chiral semiconductor with high refractive index, excellent infrared transmission and strong spin orbit coupling, and has great potential for photonic and spintronic devices. The as-synthesized tellurium crystals, however, are typically racemic mixtures of chiral domains. In this work, we designed a simple and scalable optical imaging method based on polarization resolved nonlinear harmonic generation to identify the chirality in tellurium nanowires, which correlates with the structural helicity of the wires observed in transmission electron microscope. The ability to quickly identify chiral structures and domain boundaries may enable more studies on their novel transport properties. This work was performed in collaboration with Pettes group in Los Alamos National Laboratory and published in Nanoscale.

Valleytronics, a new and potentially energy-efficient form of computing hardware, was previously thought to be incompatible with perovskite, a promising energy harvesting semiconductors. Having both functionalities in one single-crystal material is beneficial for monolithically integrated information technology in industrial fabrication. In this work, we identified the necessary atomic lattice symmetry for valleytronics in newly synthesized ultrathin perovskite with nonlinear optical spectroscopy. The finding of entirely different symmetry properties from bulk materials led to the discovery of robust optical valley memory effect, which is surprising at the first glance but understandable from first principle calculation. This collaborative work with Lou group at Rice MSNE and Qian group at TAMU MSE is now published in Advanced Materials. Read more at Rice News.

Light-controlled polymerization has wide applications in additive manufacturing and medicine, but the common catalysts work under UV-blue light, which does not penetrate beyond the surface of the materials or tissues, and may have negative physiological effects. In this work, we demonstrated that perovskite nanocrystals are excellent nonlinear photocatalyst that can be activated with near-infrared light. The giant two-photon absorption coefficient of the nanocrystal, compared with dye molecule catalysts, allows efficient volumetric reaction. Applying a special type of polymerization, PET-RAFT, we showed that the entire solution volume of 1 mL contains polymer chains with highly uniform molecular length (dispersity of 1.07 is very close to unity), which is challenging for conventional laser direct writing. This collaborative work with Egap group at Rice MSNE is published in ACS Macro Letters.

Copyright 2020 American Chemical Society

For the first time, we have experimentally demonstrated that the vibration of atomic lattice may possess topological attributes. These chiral phonons are promising for quantum memories and transducers (Zhu et al, Science 359, 579).

In a 2D asymmetric hexagonal lattice, such as monolayer WSe2, the atoms rotate, instead of shaking back and forth, for phonons propagating with a momentum of K and -K.

The valley optical phonon mode LO(K) contains unidirectional atomic rotation.

The direction of the rotation is determined by the direction of the momentum, as seen from the circular polarization of the light absorption that excites the phonons.

Phonons created by a pair of light pulses show circular dichroism.

For more introduction and stories about this research, please visit LBL News Center.

Piezoelectric materials, which convert energy between electricity and mechanical force, are the core of clocks, ultrasonics and microbalances. We have experimentally demonstrated that a single-layer crystal of less than 1-nm thick can be piezoelectric, and are tough enough for constructing Nano-ElectroMechanical Systems (Zhu et al, Nature Nanotechnology 10, 151).

The freestanding single-layer MoS2 NEMS device. A force is generated by electric field and picked up by Atomic Force Microscope.

The ratio between the in-plane stress and the electric field, i.e. piezoelectric coefficient, for this 2D crystal is comparable with conventional 3D crystals like quartz and AlN. The crystal can be chemically engineered to show out-of-plane piezoelectricity as well (Lu et al, Nature Nanotechnology 12, 744).