Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session S60: Ferroelectricity and Low-Symmetry SystemsFocus Recordings Available
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Chair: Adina Lucian-Mayer, University of Ottawa Room: Hyatt Regency Hotel -DuSable C |
Thursday, March 17, 2022 8:00AM - 8:36AM |
S60.00001: Ubiquitous defect-induced density wave instability in monolayer graphene Invited Speaker: Christopher Gutierrez Quantum materials are notoriously sensitive to their environments. Seemingly small perturbations (lattice strain, dielectric environment, atomic disorder) can tip a system in favor of one amongst several competing ground states. For example, monolayer graphene has long been predicted to host a rich assortment of competing phases, including a structural bond density wave instability ("Kekulé distortion") that couples electrons at the K/K' valleys and breaks the translational symmetry of graphene. In this talk, I will describe our observations of a ubiquitous Kekulé density wave instability in multiple millimeter-scale graphene systems that can be controllably triggered by an extremely dilute concentration of surface atoms. Combining complementary momentum-sensitive angle-resolved photoemission spectroscopy (ARPES) and low energy electron diffraction (LEED) measurements, we show that the kinetic ordering of mobile surface adatoms produces global Kekulé density wave order that opens an energy gap in graphene's energy spectrum. We further find that this Kekulé phase occurs independent of Fermi surface size and shape, suggesting that this lattice instability may be driven by strong electron-lattice interactions. Our results suggest an unexpected sensitivity of the graphene lattice to dilute surface disorder and open the door to harnessing adsorbed atoms for tailoring the properties of two-dimensional materials. |
Thursday, March 17, 2022 8:36AM - 8:48AM |
S60.00002: Layer Specific Anomalous Screening in an Unconventional Ferroelectric System Zhiren Zheng, Xueqiao Wang, Ziyan Zhu, Stephen T Carr, Sergio C de la Barrera, Kenji Watanabe, Takashi Taniguchi, Raymond C Ashoori, Suyang Xu, Efthimios Kaxiras, Qiong Ma, Pablo Jarillo-Herrero In our previous work, we reported the observation of emergent ferroelectricity in bilayer graphene/BN moiré vdW heterostructures. The rich and complex ferroelectric response as a function of the displacement field and electron filling is distinct from any previously reported 2D ferroelectrics. However, despite its unique features and potential for next-generation electronic devices, much remains to be explored and understood in this new platform of unconventional ferroelectric systems. In order to unveil the underlying mechanism, we have performed a more systematic study. Through a more precise angle control, we aim to narrow down the key ingredients that will allow us to deliberately control and engineer the ferroelectricity. Assisted by detailed Hall density measurements, we manage to find evidence that points toward a layer specific charge reservoir, which could be responsible for the unconventional and rather striking ferroelectric response. Based on our observations, we further propose an intuitive picture in an attempt to capture the rich physics and explain the intricate pattern of the layer specific anomalous screening behavior. With our work, we hope to provoke further interests towards a wider class of unconventional ferroelectric systems originating from a controlled layer specific charge reservoir and subsequently lay the foundation for new potential electronic applications. |
Thursday, March 17, 2022 8:48AM - 9:00AM |
S60.00003: Ferroelectricity, spin-singlet formation and orbital-selective Peierls phase in the dihalide MOX2 family with d1 and d2 configurations Yang Zhang, Ling-Fang Lin, Adriana Moreo, Gonzalo Alvarez, Elbio R Dagotto The van der Waals (vdW) family of layered oxide dihalide MOX2 (M = V, Ta, Nb, Os; X = halogen element) display an interesting geometric structure. Here, we systematically study VOI2 and MoOCl2 with 3d1 and 4d2 electronic configurations, respectively. In VOI2, we found the “pseudo-Jahn-Teller” effect caused by the coupling between empty V (dxz/yz and d3z2−r2) and O 2p states. We proposed this as the mechanism that stabilizes the ferroelectric distortion from the paraelectric phase. Moreover, the half-filled metallic dxy band displays a Peierls instability along the b axis, inducing a V-V dimerization. We also found very short-range antiferromagnetic coupling along the V-V chain due to the formation of nearly decoupled spin singlets in the ground state [1]. In MoOCl2, our main result is that an orbital-selective Peierls phase develops in MoOCl2, resulting in the dimerization of the Mo chain along the b-axis. Specifically, the Mo-dxy orbitals form robust molecular-orbital states inducing localized dxy singlet dimers, while the Mo-dxz/yz orbitals remain delocalized and itinerant. Our study shows that MoOCl2 is globally metallic, with the Mo-dxy orbital bonding-antibonding splitting, opening a gap and the Mo-dxz/yz orbitals contributing to the metallic conductivity [2]. |
Thursday, March 17, 2022 9:00AM - 9:12AM |
S60.00004: Raman spectroscopy and XRD measurement of the ferrielectric-paraelectric phase transition in layered CuInP2S6 and CuInP2S6- In4/3P2S6 heterostructures Rahul Rao, Ryan Selhorst, Benjamin S Conner, Michael A Susner CuInP2S6 (CIPS) is an emerging layered material that is ferrielectric at room temperature (Curie temperature TC ~ 315 K). When synthesized with Cu deficiencies, CIPS spontaneously segregates into CuInP2S6 and In4/3P2S6 domains (CIPS-IPS), which form a self-assembled heterostructures within the individual lamellae. This re-structuring raises the Curie temperature and, depending on the Cu concentration, can be up to ~340 K for the highest Cu deficiency. In both CIPS and CIPS-IPS, the loss of polarization through the ferrielectric-paraelectric transition is driven by the diffusion of Cu ions within the lattice. Here we uncover the microscopic origins underpinning the phase transitions in CIPS and CIPS-IPS by performing a temperature-dependent Raman and XRD study. We measured the frequencies and linewidths of various cation and anion phonon modes, and compared them to the extracted atomic positions from the refinement of the XRD data. Our analysis shows that in addition to the Cu ion diffusion, the anion octahedral cages experience significant strains as they deform to accommodate the redistribution of Cu ions upon heating. This process begins around 315 K and ends around 330 K in CIPS-IPS. |
Thursday, March 17, 2022 9:12AM - 9:24AM |
S60.00005: Controlling the Quadruple-well State in CuIn2P2S6 via Interfacial PbZr0.2Ti0.8O3 Kun Wang, Jia Wang, Yifei Hao, Xia Hong Van der Waals material CuInP2S6 (CIPS) exhibits room temperature ferroelectricity with unconventional quadruple-well. Here we report the effect of a ferroelectric substrate on the domain formation and piezoelectric response of CIPS. We mechanically exfoliated 8-250 nm CIPS flakes and transferred them on top of Si, Au, and 50 nm epitaxial (001) PbZr0.2Ti0.8O3 (PZT) films. Piezoresponse force microscopy (PFM) studies show spontaneous domain formation in CIPS flakes on Si and Au substrates, while flakes on PZT exhibit uniform polarization that is fully aligned with the pre-patterned domain structures in the underlying PZT. We also performed in-situ PFM studies at elevated temperatures, which reveals an enhanced Curie temperature of ~200 ºC for CIPS on PZT. We further extracted the piezoelectric coefficient d33 for these samples. The d33 for thin CIPS on PZT is significantly enhanced compared with those on Au and Si, and changes from 10 pm/V for the 8 nm sample to -8 pm/V for flakes thicker than 25 nm. We attribute this result to the shift of Cu cation from the metastable position to the ground state position in the quadruple-energy-well, which is driven by the interfacial polar coupling with PZT. Our study points to a new strategy to engineering the piezoelectric response of CIPS. |
Thursday, March 17, 2022 9:24AM - 9:36AM |
S60.00006: Doping dependence of ferroelectricity in rhombohedral-stacked bilayer transition metal dichalcogenides Xirui Wang, Kenji Yasuda, Yang Zhang, Song Liu, Kenji Watanabe, Takashi Taniguchi, James C Hone, Liang Fu, Pablo Jarillo-Herrero The recent discoveries of stacking-engineered 2D ferroelectrics in parallel-stacked BN [1,2,3] and rhombohedral-stacked (R-stacked) transition metal dichalcogenides (TMDs) [4,5] have vastly expanded the 2D ferroelectrics family. Especially, semiconducting nature of R-stacked TMDs allows us to track the evolution of polarization while doping the charge carrier through gating. In this study, we will present our doping dependence study of ferroelectricity in R-stacked bilayer TMDs, using graphene as a sensor layer. We will discuss the relationship between the band structure and the doping dependence. |
Thursday, March 17, 2022 9:36AM - 9:48AM |
S60.00007: First principles electronic properties predictions of incommensurately layered 2D transition metal dichalcogenides Drake Niedzielski, Berit H Goodge, Mekhola Sinha, Tyrel M McQueen, Lena F Kourkoutis, Tomas A Arias We present a first principles approach applicable to incommensurate, 2D layered materials with unique electronic properties. Using an improved “sandwich” variant of our Mismatched INterface Theory (MINT) [1], we present results for charge doping, interlayer coupling, strain, and distortions in the single-particle density of states for multilayered NbSe2 systems, contrasting our results with those for the related material NbS2. Our results indicate, for example, that proximity of these TMD systems to rare-earth chalcogen rocksalt 2D materials can dope the TMD by more than 0.1 electron per Nb and can increase the single-particle density of states near the Fermi level. |
Thursday, March 17, 2022 9:48AM - 10:00AM |
S60.00008: Elasticity of two-dimensional ferroelectrics across their paraelectric phase transition Salvador Barraza-Lopez, Joseph Roll, John Davis, John W Villanova Two-dimensional (2D) ferroelectric SnSe monolayers turn paraelectric at finite temperature due to an effective increase in atomistic coordination at their ferroelectric-to-paraelectric transition, whereby the structure turns from a three-fold coordination onto an average five-fold coordination, and from a rectangular unit cell onto a square one. Current understanding for elasticity only looks at zero-temperature structures, and it tends to neglect the strong non-linear mechanical behavior of this material that even leads to phase transitions. In this presentation, we propose a model to estimate the elastic moduli at finite temperature and across the structural transition. |
Thursday, March 17, 2022 10:00AM - 10:12AM |
S60.00009: Probing the bulk electronic states in monolayer WTe2 using impedance and potential measurements Mina Rashetnia, Mina Rashetnia, Xiong Huang, Paul T Malinowski, Jiun-Haw Chu, Yongtao Cui The layered semimetal WTe2 in the monolayer limit behaves as a two-dimensional topological insulator with a bulk gap and conducting edge states. However, the nature of this bulk gap is unclear. Recent measurements suggest it may be an excitonic insulator. To investigate this question, we combine scanning microwave impedance microscopy (MIM), Kelvin probe force microscopy (KPFM), and capacitance measurements on monolayer WTe2 devices. MIM probes the local conductivity; KPFM measures the local chemical potential; and quantum capacitance is associated with the electronic density of states. We measure these quantities as a function of carrier density, magnetic field, and temperature. By correlating these measurements, we will discuss their implications on the bulk electronic structure in monolayer WTe2. |
Thursday, March 17, 2022 10:12AM - 10:24AM |
S60.00010: Ferroelectric switching of superconductivity in magic-angle twisted bilayer graphene Dahlia R Klein, David MacNeill, Liqiao Xia, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero Electrical switching of two-dimensional (2D) superconductivity is critical for nanoscale superconducting circuits including memory elements, rectifiers, and electromagnetic sensing devices. While most field-effect transistors to control superconductivity rely on continuous tuning of carrier density, to date there has not been a bistable switch to turn superconductivity on and off. Recently, researchers uncovered ferroelectricity in Bernal-stacked bilayer graphene aligned to its insulating hexagonal boron nitride (BN) gate dielectrics. Here, we report the observation of similar ferroelectricity in a magic-angle twisted bilayer graphene (MATBG) device with aligned BN layers. This ferroelectric behavior coexists alongside the strongly correlated electron system of MATBG without disrupting its correlated insulator or superconducting states. This all-van der Waals platform enables configurable switching between different electronic states of this rich system. To illustrate this new approach, we demonstrate reproducible bistable switching between the superconducting, metallic, and insulating states of MATBG using gate voltage or electric displacement field. These experiments unlock the potential to broadly incorporate MATBG into highly tunable superconducting electronics. |
Thursday, March 17, 2022 10:24AM - 10:36AM |
S60.00011: Stacking-dependent electronic properties of few-layer and bulk γ-GeSe from GW approximation Han-gyu Kim, Hyoung Joon Choi The group-IV monochalcogenide has attracted interest due to its potentials of ferroelectric and multiferroic properties. Recently, centrosymmetric γ-phase GeSe in the double-layer honeycomb lattice was theoretically predicted, but synthesized γ-phase GeSe showed noncentrosymmetric atomic structure, opening a chance for ferroelectricity and spin-splitting. In our present work, we study electronic band structures of the centrosymmetric and noncentrosymmetric few-layer and bulk γ-GeSe using density functional theory and GW calculations [1]. We discuss stacking-dependent band gaps, spontaneous polarizations, and spin-splitting in centrosymmetric and noncentrosymmetric γ-GeSe which show different stacking sequences of atomic layers. We also investigate ferroelectric switching pathways, ferroelectric energy barriers, and origin of spontaneous polarizations in γ-GeSe from its unique stacking geometries. |
Thursday, March 17, 2022 10:36AM - 10:48AM |
S60.00012: Ultrafast optical manipulation of the stacking order in Td-WTe2 Oscar Grånäs, Shaozheng Ji, Amit K Prasad, Jonas Weissenrieder Subtle changes in stacking order of layered transition metal dichalcogenides may have profound influence on the electronic and optical properties. The intriguing electronic properties of Td-WTe2 can be traced to the break of inversion symmetry resulting from the ground-state stacking sequence. Strategies for perturbation of the stacking order are actively pursued for intentional tuning of material properties, where optical excitation is of specific interest since it holds the potential for integration of ultrafast switches in future device designs. Here we investigate the structural response in Td-WTe2 following ultrafast photoexcitation by time-resolved electron diffraction. Using a combination of pump-probe ultrafast electron diffraction and first-principles modeling, we show that a 515 nm laser pump activates a 0.23 THz shear phonon, involving layer displacement along the b-axis. Pump fluences in excess of a threshold of ∼1 mJ/cm2 result in formation of a new stacking order by layer displacement along the b axis in the direction toward the centrosymmetric 1T* phase. The shear displacement of the layers increases with pump fluence until saturation at ∼8 pm. |
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