Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session X55: Ferroelectricity and Second Harmonic Generation in 2D systemsFocus Session Live
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Sponsoring Units: DMP DCMP Chair: Liuyan Zhao, University of Michigan |
Friday, March 19, 2021 8:00AM - 8:36AM Live |
X55.00001: Coherent many-body exciton in van der Waals antiferromagnet NiPS3 Invited Speaker: Je-Geun Park Excitons are the composite particles of an electron-hole pair and rarely observed for many-body states with strong correlation. We have found that a rare spin-orbital entangled exciton state appears below TN=150 K in NiPS3, an antiferromagnetic (AFM) van der Waals (vdW) material. It arises intrinsically from the archetypal many-body states of Zhang-Rice singlet (ZRS) and reach a coherent state assisted by the AFM order. By combining three spectroscopic tools: resonant inelastic X-ray scattering, photoluminescence, and optical absorption, we demonstrate that this exciton not only exists in NiPS3 but also becomes extremely narrow below 50 K. Using a configuration-interaction theory, we identified the coherent excitonic excitations as arising from a Zhang-Rice triplet to ZRS transition, i.e., a spin-orbital entangled exciton. This discovery of the spin-orbital entangled exciton in AFM vdW NiPS3 opens up a new window of novel opportunities by allowing the manipulation of the new quantum state with real material. |
Friday, March 19, 2021 8:36AM - 8:48AM Live |
X55.00002: Crystal Structures, Phase Stability, and Electronic Properties of Two Dimensional Ferroelectric MXenes Mo Li, Joshua Young Two-dimensional ferroelectrics, or monolayer thick materials that display a switchable electric polarization, have attracted attention during recent years due to their advanced electronic properties that can be used in various practical applications such as non-volatile random access memory. In addition to the widely studied 2D materials like In2Se3 or MoTe2, the MXene Sc2CO2 was also recently predicted to possess ferroelectric properties in a metastable phase. [1] In this work, Density Functional Theory (DFT) and the Berry Phase approach were used to study both the structures and the ferroelectric properties of additional MXene materials. Eight MXenes with chemical formula M2CT2 were carefully studied, where M represents a transition metal (M = Sc, Y, La), C is carbon, and T represents the surface termination group (T = O, F). The spontaneous polarization and the piezoelectric tensors were calculated, and we found that Y2CO2 has a stable ferroelectric ground state and an out-of-plane polarization larger than other materials. Finally, we computed the bad gap of each material and found Y2CO2 exhibits a bandgap of 2.07 eV, which gives it potential as an excellent photocatalytic material. |
Friday, March 19, 2021 8:48AM - 9:00AM Live |
X55.00003: Optical Second-Harmonic Interference in Two-Dimensional Heterostructures Wontaek Kim, Sunmin Ryu Atom-thick two-dimensional (2D) transition metal dichalcogenides (TMDs) with strong excitonic transitions in the NIR and visible range are excellent media for optical second-harmonic generation (SHG). SHG is not only a powerful structural method owing to its sensitivity to crystallographic symmetry, but also a coherent probe because of its instantaneous but non-dissipative response. In this talk, I will present our recent findings on SHG interference occurring in 2D TMD heterostructures. All 2D crystal samples were prepared by mechanical exfoliation and studied with a polarized SHG micro-spectroscopy setup powered by a tunable femtosecond Ti-sapphire laser. Polarization-resolved SHG polar plots of MoS2 homo-bilayers exhibited a six-petal pattern with six angular nodes like those of MoS2 monolayers. Remarkably, the SHG behavior of MoS2/WS2 was much different from that of MoS2 bilayers in that the former lacked angular nodes. This anomalous angular background became larger for more staggered hetero-bilayers but decreased for increased fundamental wavelength. All the results were nicely explained by an SHG interference model considering material-dependent phase delay in their nonlinear response, which was corroborated with phase-resolved interferometric SHG measurements. |
Friday, March 19, 2021 9:00AM - 9:12AM Live |
X55.00004: Stacking-engineered ferroelectricity in bilayer boron nitride Kenji Yasuda, Xirui Wang, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero 2D ferroelectrics with robust polarization in the ultrathin limit, provide novel building blocks for functional van der Waals heterostructures. Experimental reports, however, are limited due to the strict requirement of a layered polar bulk crystal [1]. Here, we demonstrate a general approach to engineering 2D ferroelectrics from a non-ferroelectric parent compound using van der Waals assembly [2]. Parallel-stacked bilayer boron nitride is shown to possess out-of-plane electric polarization that reverses depending on the stacking order, AB or BA. The polarization switching is probed via the induced carrier to an adjacent graphene sheet. Furthermore, small-angle twisted bilayer boron nitride creates moiré ferroelectricity with staggered polarization, which changes ferroelectric switching dynamics. The ferroelectricity persists to room temperature while keeping the high mobility of graphene, paving the way for potential ultrathin nonvolatile memory applications [3]. |
Friday, March 19, 2021 9:12AM - 9:24AM Live |
X55.00005: Tuning Polarization and Electronic Structure via Chemical Substitution in Two Dimensional Ferroelectrics Joshua Young, Mo Li, Olamide Omisakin Two dimensional materials with switchable spontaneous electric polarizations (“2D ferroelectrics”) have recently been garnering attention as components for ultrathin electronic devices. Recently, it was proposed by Chandrasekaran et al. that the functionalized MXene Sc2CO2 has a metastable state with a comparatively large out-of-plane ferroelectric polarization. [1] Here, we used density functional theory (DFT) calculations to investigate additional M2CX2 materials (M = Sc, Y, La; X = O, F). We found that (1) chemical substitution of Sc with Y and/or O with F can preferentially stabilize the ferroelectric phase as the ground state (with the dynamic stability of these phases verified with phonon calculations); (2) such substitution can be used to systematically tune the polarization and band gap; and (3) these monolayers display large piezoelectric coefficients. Finally, we demonstrate how these properties can be continuously tuned by the application of external strain or alloying to create Sr2xY2-2xCO2 monolayers. These findings demonstrate that chemical substitution and external stimuli are powerful ways to stabilize and control the properties of low dimensional ferroelectrics. |
Friday, March 19, 2021 9:24AM - 9:36AM Live |
X55.00006: Theoretical Insights into C-H Bond Activation of Methane by Transition Metal Clusters: The Role of Anharmonic Effects Preeti Bhumla, Saswata Bhattacharya In heterogeneous catalysis, materials' property changes under operational conditions (i.e. temperature (T) and pressure (p) in an atmosphere of reactive molecules). Here we study T, p dependence of the composition, structure, and stability of metal oxide clusters using a prototypical model catalyst having practical applications: free transition metal (Ni) clusters in a combined oxygen and methane atmosphere. A robust methodological approach is employed, starting from systematic scanning of potential energy surface to obtain the global minimum structures using a massively parallel cascade genetic algorithm (cGA) at the hybrid density functional level. The low energy clusters are further analyzed to estimate their thermodynamic stability at realistic T, pO2 and pCH4 using ab initio atomistic thermodynamics. To incorporate the anharmonicity in the vibrational free energy contribution to the configurational entropy, we evaluate the excess free energy of the clusters numerically by thermodynamic integration method with ab initio molecular dynamics (aiMD) simulation inputs. By analyzing a large dataset, we show that the conventional harmonic approximation miserably fails for this class of materials and capturing anharmonic effects is significant in detecting the activation of C-H bond. |
Friday, March 19, 2021 9:36AM - 9:48AM Live |
X55.00007: The most efficient thickness of Si nano film for high-harmonic generation Shunsuke Yamada, Kazuhiro Yabana We performed first-principles calculations of high-harmonic generation (HHG) in reflected and transmitted waves from a Si nano film exposed to an intense light pulse. We develop and utilize first-principles methods coupling Maxwell and time-dependent Kohn-Sham equations with and without course-graining approximation. For a film of thickness less than or equal to a few nm, we may treat the film as 2D materials and employ a microscopic scheme without course-graining. For thicker films, propagation effects become significant and we employ a macroscopic scheme with course-graining. We studied the thickness dependence of HHG taking into account the effects of light propagation and surface electronic structure. Calculations are performed for Si films of thickness up to 200 nm, and reveal a peak structure in the thickness dependence of HHG. The HHG shows the strongest signal at the thickness around 2 to 15 nm for both of reflection and transmission HHG. We found that the most efficient thickness of the Si film for HHG is roughly determined by the dielectric function of bulk Si and the thickness dependence of HHG is correlated with the transmittance of the film. |
Friday, March 19, 2021 9:48AM - 10:00AM Live |
X55.00008: Selective high harmonics generated from a carbon nanotube Walter Furman, Linda E Reichl Selective high-order harmonic radiation can be generated when circularly polarized driving fields interact with single-walled carbon nanotubes. We obtain a gauge-covariant Hamiltonian for electrons confined to a nanotube surface in the presence of a circularly polarized electromagnetic field. Regularized delta-functions are used to approximate the π-bonds in the nanotube unit cells and the electron dynamics, in the presence of the field, is analyzed. The high harmonic radiation occurs for incident field intensities high enough to induce significant nonlinearities and chaos in the electron dynamics. Both (5,5) and (10,10) armchair nanotubes are considered and the electron quantum dynamics is modeled using Floquet-Bloch theory. The behavior of the quasienergy spectrum, Floquet states, and the electron current density are described for varying intensities of the incident field. We show that, for incident radiation with frequency ω, the emitted radiation for a (p,p) armchair nanotube, will be (2p±1)ω, (4p±1)ω, etc. |
Friday, March 19, 2021 10:00AM - 10:12AM Live |
X55.00009: Second Harmonic Generation from a Single Plasmonic Nanorod Strongly Coupled to a WSe2 Monolayer Chentao Li, Xin Lu, Ajit Srivastava, S. David Storm, Rachel Gelfand, Matthew A Pelton, Maxim Sukharev, Hayk Harutyunyan Monolayer transition metal dichalcogenides, coupled to metal plasmonic nanocavities, have recently emerged as new platforms for strong light-matter interactions. These systems are expected to have nonlinear-optical properties that will enable them to be used in many classical and quantum photonic technologies. Here we report the first experimental investigation of the nonlinear properties of these strongly-coupled systems, by observing second harmonic generation from a WSe2 monolayer strongly coupled to a single gold nanorod. The pump-frequency dependence of the second-harmonic signal displays a pronounced splitting that can be explained by a coupled-oscillator model with second-order nonlinearities. Rigorous numerical simulations utilizing a non-perturbative nonlinear hydrodynamic model of conduction electrons support this interpretation and reproduce experimental results. Our study thus lays the groundwork for understanding the nonlinear properties of strongly-coupled nanoscale systems. |
Friday, March 19, 2021 10:12AM - 10:24AM Live |
X55.00010: Enhanced Tunable Second Harmonic Generation from Twistable Interfaces and Vertical Superlattices in Boron Nitride Homostructures Kaiyuan Yao, Nathan R Finney, jin zhang, Samuel Moore, Lede Xian, Nicolas Tancogne-Dejean, Fang Liu, Jenny Ardelean, Xinyi Xu, Dorri Halbertal, Kenji Watanabe, Takashi Taniguchi, Hector Ochoa, Ana Asenjo-Garcia, Xiaoyang Zhu, Dmitri Basov, Angel Rubio, Cory Dean, James Hone, P. James Schuck Broken symmetries induce strong even-order nonlinear optical responses in materials and at interfaces. Unlike conventional covalently bonded nonlinear crystals, van der Waals (vdW) heterostructures feature layers that can be stacked at arbitrary angles, giving complete control over the presence or lack of inversion symmetry at a crystal interface. Here we report highly tunable second harmonic generation (SHG) from nanomechanically rotatable stacks of bulk hexagonal boron nitride (BN) crystals, and introduce the term twistoptics to describe studies of optical properties in twistable vdW systems. By suppressing residual bulk effects, we observe SHG intensity modulated by a factor of more than 50, and polarization patterns determined by moiré interface symmetry. Finally, we demonstrate greatly enhanced conversion efficiency in vdW vertical superlattice structures where multiple BN crystals are joined by symmetry-broken interfaces. Our study paves the way for compact twistoptics architectures aimed at efficient tunable frequency-conversion, and demonstrates SHG as a robust probe of buried vdW interfaces. |
Friday, March 19, 2021 10:24AM - 10:36AM Live |
X55.00011: Ferroelectricity in parallel-stacked transition metal dichalcogenides Xirui Wang, Kenji Yasuda, Yang Zhang, Sergio C De La Barrera, Daniel A Rhodes, Kenji Watanabe, Takashi Taniguchi, James Hone, Liang Fu, Pablo Jarillo-Herrero 2D ferroelectrics have great potential for dense and low-consumption nonvolatile memory applications. Recent reports have shown robust out-of-plane polarization in parallel-stacked bilayer hBN and small-angle-twisted bilayer hBN, which is switchable through interlayer movement [1,2,3]. This method of building ferroelectrics out of non-ferroelectrics can be generalized to other bipartite materials, such as transition metal dichalcogenides (TMDs) [4]. The natural 2H phase of TMDs is centrosymmetric. Here, we demonstrate that robust ferroelectricity exists in parallel-stacked bilayer TMDs. The polarization of the bilayer is probed via an adjacent graphene layer. Our demonstration of ferroelectricity in stacking-engineered TMD bilayers consolidates the feasibility of engineering 2D ferroelectric semiconductors and opens up a broad way of engineering various functional heterostructures out of non-ferroelectrics. |
Friday, March 19, 2021 10:36AM - 10:48AM Live |
X55.00012: Tunable extra Dirac points in one-dimensional graphene superlattice induced by periodic ferroelectric domains Tianlin Li, Hanying Chen, Kun Wang, Yifei Hao, Le Zhang, Qiuchen Wu, Xia Hong In this work, we investigate the transport signature of one-dimensional (1D) superlattice (SL) in monolayer graphene induced by pre-patterned periodic domains in a ferroelectric bottom gate. We work with 50 nm single-crystalline ferroelectric Pb(Zr,Ti)O3(PZT) films deposited on (La,Sr)MnO3 buffered SrTiO3 substrates, and create periodic polarization up (Pup) and down (Pdown) stripe domains on PZT using conductive atomic force microscopy. The domain periodicity varies from 200 nm to 300 nm, and the number of periods changes from 30 to 50. We then transfer hBN-graphene stacks onto the pre-patterned domains and fabricate them into top-gated field-effect devices. The difference in carrier density between the two polarization regions reaches around 3×1013 cm-2 at 2 K due to the pyroelectric effect. We observe extra Dirac points in R(Vg) by applying a voltage to the hBN top gate, which is attributed to the SL modification of the band structure. We discuss the effects of the SL period and the width ratio between the Pup and Pdown domains on the position of the extra Dirac points, and the magnetotransport properties of these 1D SLs. |
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