Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session F19: Non-Carbon 2D materials II |
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Sponsoring Units: DCMP Chair: Paul Nguyen, University of Washington Room: Room 211 |
Tuesday, March 7, 2023 8:00AM - 8:12AM |
F19.00001: Anomalous Hall Effect in Ultrathin Crystalline Strontium Ruthenate Membranes Patrick Blah, Edouard Lesne, Mattias Matthiesen, Thierry van Thiel, Jorrit R Hortensius, Ulderico Filippozzi, Graham Kimbell, Yingkai Huang, Herre van der Zant, Peter G Steeneken, Andrea Caviglia SrRuO3 (SRO) is a complex oxide that hosts a plethora of exotic magneto-transport properties due to its strong spin-orbit coupling and itinerant ferromagnetism. In particular it is an excellent candidate to investigate the intrinsic Berry-phase driven Anomalous Hall Effect. A recent breakthrough1 has allowed complex oxides, epitaxially grown via pulsed laser deposition, to be exfoliated and released via a sacrificial layer. These freestanding complex oxide membranes are an exciting new platform for investigating and tuning the interplay between structural and electronic properties2. |
Tuesday, March 7, 2023 8:12AM - 8:24AM |
F19.00002: Valley spin-acoustic resonance in monolayer MoS2 Kabyashree Sonowal, D.V Boev, A. V. Kalameitsev, Vadim Kovalev, Ivan Savenko Monolayer molybdenum disulphide (MoS_2) is a strong candidate material to study spin-valley coupled physics in 2D materials. Its unique band structure consists of spin-split subbands crossing each other at finite momenta with opposite spin orientation in both the valleys. When exposed to Rayleigh surface acoustic waves, strain-induced pseudomagnetic fields couple with spin resulting in spin-phonon interaction. We theoretically predict the occurrence of spin acoustic resonance accompanied by an acoustoelectric current due to spin-flip transitions between the spin-split subbands. We calculate the transition probabilities, obtain the conditions for observing spin acoustic resonance and calculate and study the behaviour of the acoustoelectric current. On breaking time reversal symmetry, both the spin acoustic resonance and acoustoelectric current become valley sensitive paving the way for acousto-electric spectroscopy of valley selective phenomena. |
Tuesday, March 7, 2023 8:24AM - 8:36AM |
F19.00003: Uncovering unconventional hidden Rashba effect in two-dimensional InTe with inversion Sangmin Lee, Young-Kyun Kwon, Miyoung Kim, Gyu-Chul Yi The hidden Rashba (R-2) effect has been observed in centrosymmetric crystals by re-examining their spin polarization which was previously overlooked due to their inversion symmetry. Especially the R-2 effect exhibits an intriguing spin-layer locking (SLL) phenomenon in two-dimensional materials. In this research, we propose a mechanism to manipulate the spin chirality of the R-2 effect. In general, both the R-1 and R-2 effects have opposite spin chiralities in the inner and outer bands. In our newly proposed R-2 system, however, both the inner and outer bands can possess their spins with the same chirality. To materialize the new type of R-2 effect, we demonstrate a centrosymmetric InTe monolayer as a model system using first-principles density functional theory. We show that InTe possesses two pairs of the spin-degenerate R-2 Rashba bands and hosts unconventional spin chirality, with the inner and outer bands exhibiting the same chirality. We also revealed that strong spin-orbit coupling-induced band inversion between the two copies of the R-2 Rashba bands plays an important role in the formation of the unconventional spin texture. Our results also provide an exciting platform to explore the intriguing spin polarization physics from the R-2 effect. |
Tuesday, March 7, 2023 8:36AM - 8:48AM |
F19.00004: High-temperature superconducting diode effect in artificial Josephson junction of Bi2Sr2CaCu2O8+δ Sanat Ghosh, Vilas Patil, Amit Basu, Kuldeep Kuldeep, Digambar A Jangade, Ruta Kulkarni, A Thamizhavel, Mandar M Deshmukh An exciting new development in the field of superconducting electronics is the unearthing of the superconducting diode effect (SDE) manifested due to various mechanisms. The effect is the superconducting analogue of the semiconducting p-n junction diode where a system behaves superconducting for one direction of current flow and dissipative for the other direction. The effect is tunable with a magnetic field in some specific systems. SDEs demonstrated so far, however, require very low temperatures for operation (~ 4 K or less), which impedes their use in technological applications. We demonstrate SDE in an artificial Josephson junction of twisted Bi2Sr2CaCu2O8+δ (BSCCO), a high-Tc van der Waals material. This system shows a diode effect even above 77 K and is independent of the twist angle. As a figure of merit, we also observe the highest asymmetry (60 %) between switching currents for positive and negative bias at 20 K. A very small magnetic field applied perpendicular to the junction plane tunes the diode effect. Interestingly, we observe the SDE originates from the artificial Josephson junction at the interface, which breaks inversion symmetry, and not from the intrinsic Josephson junctions of the bulk BSCCO crystal. |
Tuesday, March 7, 2023 8:48AM - 9:00AM Not Participating |
F19.00005: Materials where crystal structure and electronic structure have different quasi-dimensionality Sinisa Coh We theoretically study materials, such as LaTe3, in which the effective dimensionality of the crystal structure is quasi-two-dimensional while the electronic structure is quasi-one-dimensional. We show this by using a basis of electronic states that are maximally localized both in space and time. Unlike maximally localized Wannier functions, these orbitals have the property that their spread in space is minimally changing over time. We find that in such basis, relevant Te p-like electron orbitals evolve along one-dimensional chains within the LaTe3 plane (other p-like orbitals disperse in the perpendicular direction). We also find large, but less pronounced, quasi-one-dimensionality of electronic structure in NbS3. Interestingly, in NbS3 p-like orbitals on S tend to evolve along one-dimensional chains perpendicular to the ones formed by d-like Nb orbitals. We related these findings to the charge density wave state in both LaTe3 and NbS3. When we apply our approach to graphene, black phosphorene, and MoS2 we find that all three have very isotropic quasi-two-dimensional electronic structures (unlike LaTe3 and NbS3). |
Tuesday, March 7, 2023 9:00AM - 9:12AM |
F19.00006: Mechanical and acoustic properties of graphene and 2D membranes explained thanks to anharmonic effects Josu Diego, Unai Aseginolaza, Aitor Bergara, Ion Errea The phonon properties of graphene, and in general of any 2D material, are still highly debated. The harmonic approximation predicts diverging atomic fluctuations and a constant linewidth of in-plane acoustic phonon modes at small momentum, which implies that graphene cannot propagate sound waves. The origin of these problems is the quadratic dispersion of the acoustic out-of-plane phonon frequencies obtained in the harmonic approximation. By including anharmonicity in a non-perturbative way within the Stochastic Self-Consistent Harmonic Approximation (SSCHA) we show that the physical dispersion expected experimentally for the acoustic out-of-plane mode should indeed be quadratic but actually compatible with well-defined sound waves. We verify this result using both atomistic simulations and a membrane model for graphene. |
Tuesday, March 7, 2023 9:12AM - 9:24AM |
F19.00007: Tracking TaS2 CDW phases with cryo-STEM and in situ electric biasing James L Hart, Saif Siddique, Noah Schnitzer, Lena F Kourkoutis, Judy J Cha 1T-TaS2 is a 2D transition metal dichalcogenide which hosts several charge density wave (CDW) phases that can be accessed in different temperature ranges. Additionally, application of short electric pulses can trigger a metal-to-insulator transition (MIT) in TaS2, which is promising for 2-terminal electronic devices. However, many questions remain unresolved in this system: what is the mechanism of the pulse-induced MIT (thermal or field-driven); how does the pulse-induced MIT couple to the CDW order parameters; and how do the transitions evolve spatiotemporally? In this talk I will discuss transmission electron microscopy (TEM) measurements of TaS2 with cryogenic cooling and in situ electric biasing. Our TEM measurements include 4D-STEM mapping, atomic-resolution imaging, and time-resolved diffraction. This approach allows direct correlation of the CDW structures of TaS2 with the electronic properties, both as a function of temperature and applied electric field. |
Tuesday, March 7, 2023 9:24AM - 9:36AM |
F19.00008: AFM Characterization of Hexagonal Boron Nitride Dielectrics Jeffrey Kwan, Isabelle Y Phinney, Andrew Zimmerman, Zeyu Hao, James Ehrets, Takashi Taniguchi, Kenji Watanabe, Philip Kim Van der Waals heterostructures often rely on electrostatic gating to tune the carrier density in the two-dimensional material of interest. In many cases, large gate voltages are necessary to significantly shift the Fermi level or to apply large displacement fields, which requires ultra-high quality gate dielectrics. In this presentation, we report on using a conductive Atomic Force Microscope (AFM) tip to characterize defects in hexagonal boron nitride (hBN), the primary gate dielectric in van der Waals heterostructures. We initially characterize the hBN flakes by measuring the voltage that must be applied to the AFM tip to break down the hBN dielectric at the point below the tip. Following this procedure, we test whether the breakdown voltage can be improved by electrostatically moving defects using voltage applied to the AFM tip. We present the results of our characterization and defect manipulation to improve hBN dielectric breakdown. |
Tuesday, March 7, 2023 9:36AM - 9:48AM |
F19.00009: Dirac fermions in germanene : bottom-up versus top-down approaches Guy Le Lay, Marco Minissale, Eric Salomon, Thierry Angot Germanene is an artificial two-dimensional graphene-like germanium allotrope predicted to be a near room temperature topological insulator, belonging to the class of so-called Xenes. It was synthesized in 2014, exactly ten years after the isolation of graphene, and just two years after the archetype growth of silicene, the first Xene ever produced. The canonical germanene paper, described its top-down synthesis by Ge deposition onto a Au(111) crystal, but revealed multi-phases. Instead, we will show that a single germanene phase, as evidenced in Scanning Tunneling Microscopy in situ imaging, is obtained by Ge segregation on top of a thin Au(111) film epitaxially grown under ultra-high vacuum on a Ge(111) template, that is, through a bottom-up approach. This segregation process was directly vizualized ex-situ in high-resolution Scanning Electron Microscopy. The most striking signature of this single germanene phase is a characteristic Low Enery Electron Diffraction pattern, curiously featuring 24 spots in 12 doublets forming a ring. This intriguing LEED pattern was already observed in 1971, but remained undeciphered for more than half a century. This unique germanene phase is characterized by sharp Ge 3d and Au 4f core-levels in synchrotron radiation PhotoElectron Spectroscopy, and the emergence in angle-resolved PES measurements of Dirac fermions around the Brillouin zone center with a Fermi velocity of ~0.8 106 ms -1, quite close to that of graphene. |
Tuesday, March 7, 2023 9:48AM - 10:00AM |
F19.00010: Bandgap Tuning for Monolayer TMD Materials by Using STM Tip Chun-Liang Lin, Guan-Hao Chen, Meng-Kai Lin, Joseph A Hlevyack, Chia-Nung Kuo, Tsu-Yi Fu, Juhn-Jong Lin, Chin Shan Lue, Wen-Hao Chang, Noriaki Takagi, Ryuichi Arafune, Tai-Chang Chiang Monolayer transition metal dichalcogenides (TMDs) offer a promising platform for developing advanced electronics beyond graphene. Similar to two-dimensional molecular frameworks, the electronic properties of such monolayers can be sensitive to perturbations from the surroundings; the implied tunability of electronic structure is of great interest. Using scanning tunneling microscopy/spectroscopy, we demonstrated a bandgap engineering technique in two monolayer materials, MoS2 and PtTe2, with the tunneling current as a control parameter [1]. The bandgap of monolayer MoS2 decreases logarithmically by the increasing tunneling current, indicating an electric-field-induced gap renormalization effect. Monolayer PtTe2, by contrast, exhibits a much stronger gap reduction, and a reversible semiconductor-to-metal transition occurs at a moderate tunneling current. This unusual switching behavior of monolayer PtTe2, not seen in bulk semimetallic PtTe2, can be attributed to its surface electronic structure that can readily couple to the tunneling tip, as demonstrated by theoretical calculations. Our findings reported herein have significant implications regarding methods and the principles for bandgap engineering of TMD monolayers. |
Tuesday, March 7, 2023 10:00AM - 10:12AM Author not Attending |
F19.00011: Effect of Dielectric Environmental on Optoelectronic Properties of Mixed-Dimensional van der Waals Heterostructures Atikur Rahman We have developed a simple method of fabricating large-area MoS2 monolayer devices. We will discuss the fabrication of various heterostructures using monolayer MoS2 as one of the components. Due to the 2D nature of MoS2, the coulomb screening is less effective and the electronic and optoelectronic properties of these heterostructures depend strongly on the local environment. We made mixed-dimensional van der Waals heterostructures by combining n-type MoS2 monolayer with p-type silicon. These 2D-3D systems show interesting photoresponse properties. We observed that the photoresponse strongly depends on the surrounding dielectric environment and by tuning the dielectric contestant, one can enhance the photoresponse by several orders of magnitude. We will discuss how various characterization methods such as low-frequency noise, dielectric and transient photoresponse can be used to understand the charge transport mechanism of such systems. |
Tuesday, March 7, 2023 10:12AM - 10:24AM |
F19.00012: Nonlinear Conductance of Monolayer WTe2 Elliott Runburg, Eric K Lester, Jiaqi Cai, Paul T Malinowski, Xiaodong Xu, Jiun-Haw Chu, David H Cobden, Yuva Belarbia Monolayer WTe2 exhibits edge conduction consistent with most of the expected properties of a 2D topological insulator, including a strong and anisotropic response to a magnetic field explained by the expected spin-momentum locking in the edge states. Although at temperatures above about 10 K the edge conductance is roughly temperature independent, as expected from single-particle theory, at lower temperatures it eventually freezes out in linear response. This behavior presents a challenge to theories of conduction in a helical quantum wire. To provide more input for theory, we have made measurements of the nonlinear conductance of monolayer WTe2 down to 50 mK in a vector magnetic field. We find that the differential conductance rises quickly to close to the higher temperature value above a particular level of voltage bias which varies strongly with gate voltage, and the magnetic field anisotropy of the threshold bias is similar to that of the linear-response conductance, suggesting that some kind of disorder- and spin-related gap controls the conductance at low temperatures. |
Tuesday, March 7, 2023 10:24AM - 10:36AM |
F19.00013: Valley resolved moiré superlattices on twisted WSe2 bilayers Yanxing Li, Fan Zhang, Yu-Chuan Lin, Chengye Dong, Hyunsue Kim, Joshua A Robinson, Chih-Kang Shih Moiré superlattices have emerged as a major frontier in tailoring novel 2D electronic and excitonic structures. One approach is to twist homobilayers (TBL) of transition metal dichalcogenide (TMDs). Two groups have reported the observation of moiré superlattices in TBL WSe2. One fundamental issue in TMD TBL is whether the observed electronic superlattices are originated from the interlayer coupling at Γ or K valleys [1-2]. Many theoretical and experimental findings suggest that the moiré flat bands come from Κ valleys. Recent studies, however, suggest that the flat bands belong to Γ valleys [3-4]. Here, we utilize the scanning tunneling microscopy/spectroscopy (STM/STS) to study moiré superlattices in naturally formed twisted bilayer WSe2 at various angles grown by MOCVD. By conducting spectroscopy measurements at different modes, including standard constant height dI/dV, variable Z dI/dV, and <!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>? κ (decay constant) measurements, we are able to distinguish the moiré states from Γ, Κ valleys. Our experimental results have important implications on moiré design of valleytronics using twisted TMDs. |
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