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 M60: Experimental Optical Spectroscopic Measurements of 2D Materials IILive
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Sponsoring Units: GIMS Chair: Elias Garratt, Michigan State University |
Wednesday, March 17, 2021 11:30AM - 11:42AM Live |
M60.00001: Optical properties and surface termination of pristine and intercalated GeI2 Archit Dhingra, Katerina Chagoya, Joseph Dalton, Richard Blair, Peter A Dowben GeI2 is an understudied layered material that can be exfoliated easily down to a monolayer, owing to its low cleavage energy of 0.16 J/m2 [1], which is smaller than that of graphite. Large single crystals of GeI2 (space group 164, P-3m1) have now been grown, including crystals intercalated with pyridine, aniline and triethylamine. GeI2 has a thermal stability even at 600 K and the band gap is tunable through intercalation, which makes GeI2 an interesting optoelectronic material. We have confirmed that GeI2 multilayer has an intrinsic band gap of 2.59 ± 0.01 eV, which is consistent with theoretical expectations [1]. Ergo, electronics based on GeI2 may find applications in high temperature devices. Furthermore, angle resolved x-ray photoemission spectroscopy (ARXPS) measurements demonstrate that GeI2 terminates in iodine, and multilayer GeI2 has a significant surface to bulk core level shift. While we have not yet confirmed ferromagnetism in GeI2 single crystals, it does have a temperature dependent moment, and the impact of phonons is being explored. |
Wednesday, March 17, 2021 11:42AM - 11:54AM Live |
M60.00002: Non-Reciprocal Raman response of a Charge Density Wave in a high mobility, 2D antiferromagnet Md Mofazzel Hosen, Yiping Wang, Shiming Lei, Leslie M Schoop, Kenneth Burch Combining magnetism with charge density waves (CDW) offers a unique regime for strong correlations. Here we study a 2D material with high mobility, despite the presence of antiferromagnetic order and charge density wave. We detail comprehensive Raman measurements that show a strong interaction between the lattice and CDW modes. These experiments also demonstrate a surprising non-reciprocal response, even well above the magnetic ordering temperature. |
Wednesday, March 17, 2021 11:54AM - 12:06PM Live |
M60.00003: Resonant Raman Spectroscopy of the Chiral Antiferromagnet CoNb3S6 Nora Hassan, Thuc Mai, Kamal Choudhary, Nirmal Jeevi Ghimire, Angela Hight Walker We report the first full Raman characterization of the chiral antiferromagnet CoNb3S6; i.e. cobalt-doped NbS2. CoNb3S6 exhibits a large c-axis anomalous Hall effect (AHE) not entirely attributable to the small intrinsic ferromagnetic component (Co) along the c-axis [1]. This interesting behavior suggests that the enhancement in AHE may be because of a combination of magnetic field in the presence of the near-Fermi energy Weyl nodes as predicted [2]. Neutron scattering experiments show incommensurate peaks; however, it was not clear whether the peaks are due to a spin spiral or a spin density wave. Magneto-Raman spectroscopy represents an optimal method to differentiate these structures. Wavelength- and polarized-dependent Raman spectra were collected at room temperature from CoNb3S6 flakes. DFT calculations of modes and band structure were performed and compared to experimental results. Experiments of temperature- and magnetic field-dependent Raman spectroscopy to seek magnon (spin wave) signatures detected no change in the spectral weight below the Neel temperature implying absence of spin density waves, thereby identifying the magnetic structure as helical. |
Wednesday, March 17, 2021 12:06PM - 12:18PM Live |
M60.00004: Electrically tunable photonic dispersion in multilayer black phosphorus. Souvik Biswas, William S Whitney, Meir Yoel Grajower, Kenji Watanabe, Takashi Taniguchi, Hans A Bechtel, George R Rossman, Harry Atwater Tuning the optical dispersion or topology of iso-frequency contours (IFC) in photonic/plasmonic structures poses an important challenge and has been addressed until now with complex assemblies of multilayer metal-dielectric stacks based metamaterials. In this work, we demonstrate electrostatically tunable photonic dispersion for intraband transitions in multilayer black phosphorus (BP) by experimentally measuring the complex dielectric function below the optical gap along the two principle axes (armchair and zigzag) of BP as a function of carrier density. The underlying mechanism of the tunable optical dispersion stems from the Fermi level-dependent susceptibility of the free-carrier Drude response, whereas the anisotropy comes from the difference in the fermionic masses arising due to asymmetry in the band-structure. Our results show that BP is a promising system to actively access different optical topologies (such as elliptical and hyperbolic) in the mid to far-infrared. |
Wednesday, March 17, 2021 12:18PM - 12:30PM Live |
M60.00005: High Performance Avalanche Photodiodes from Bi2O2Se Vinod Sangwan, Joohoon Kang, Jan Luxa, James Male, Jeff Snyder, Zdenek Sofer, Mark C Hersam Layered semiconductors consisting of van der Waals and non-van der Waals stacking promise high-performance optoelectronics due to their high mobility, strong light-matter interaction, gate-tunability, and strong in-plane and valley anisotropy. Here, we present metal-semiconductor-metal avalanche photodiodes (APDs) fabricated from layered Bi2O2Se crystals, which consist of alternating [Bi2O2]2+ and [Se]2- layers that are held together by electrostatic interactions. An efficient inverse Auger process yields an intrinsic carrier multiplication factor up to 400 at 7 K under optical excitation at a wavelength of 515.6 nm, resulting in photodiodes with a responsivity gain in excess of 3,000 A/W at a bandwidth of ~400 kHz. Furthermore, exceptionally low dark currents (200 pA) result in high detectivities as high as 4.6 x 1014 Jones. The physics that underlies this superlative performance is elucidated by the temperature and bias dependence of carrier multiplication in reverse-biased Schottky diodes (barrier ≈ 44 meV) with InSe deformation potential of 27 – 33 eV. In contrast with charge trap-based extrinsic gain in phototransistors, the intrinsic gain in Bi2O2Se APDs paves the way for high-speed and low-noise photodetectors. |
Wednesday, March 17, 2021 12:30PM - 12:42PM Live |
M60.00006: Correlative imaging of transition metal dichalcogenides Ute Schmidt, Jan Englert, Olaf Hollricher, Thomas Dieing Transition metal dichalcogenides (TMDCs), belong to a new class of nanomaterials with great potential for optoelectronic device applications. These single or few layer materials exhibit unique electronic and optical properties, which are significantly different from the bulk precursor. Bandgaps of semiconducting TMDCs can be tuned by changing the number of layers, the chemical compositions, or the strain of the materials Thickness, changes in crystal symmetry, and growth defects are a few parameters that define the electronic, optical, and thermal properties of 2D crystals and for which different characterization techniques are required. In this contribution the combination of various techniques for investigation of the same crystals shows that various processes involved in producing the photoluminescence signal are correlated with localized strain as observable by confocal Raman imaging and topographic homogeneity as determined by atomic force microscopy. The edge of the 2D crystals show especially strong variations, which can be explained by the correlation of the various techniques. Second harmonic generation measurements identify grain boundaries as potential sources of strain relief, which is in agreement with both the confocal Raman as well as the confocal PL results. |
Wednesday, March 17, 2021 12:42PM - 12:54PM Live |
M60.00007: Cavity quantum electrodynamics with cyclotron resonance transitions in graphene Yashika Kapoor, Jordan Russell, Ellie I. Hunt, Takashi Taniguchi, Kenji Watanabe, Erik Henriksen Cyclotron resonance transitions between highly - degenerate Landau levels are good candidates to achieve the ultrastrong coupling regime in cavity QED, as shown recently for GaAs quantum wells [1]. Graphene provides a similarly ideal platform, with additional advantage of a strongly anharmonic Landau level spectrum so that a two-level system can be readily explored. We have recently demonstrated graphene cyclotron transitions with Q ~ 200 at 8T magnetic field. Incorporated into cavities with either thin metal or dielectric mirrors, we estimate that cooperativities of C ~ 100 and reduced coupling strengths g/ω ~ 0.2 can be reached using graphene in an infrared cavity. Coupling strength can be increased by improving the quality factor and reducing the mode volume of the designed cavities which leads to stronger confinement of light. We will present initial results of measurements on such graphene cavity QED devices and outline prospects for reaching higher coupling strengths. |
Wednesday, March 17, 2021 12:54PM - 1:06PM Live |
M60.00008: Cyclotron Resonance in Dual-Gated Bilayer Graphene Jordan Russell, Jesse Balgley, Matheus Schossler, Takashi Taniguchi, Kenji Watanabe, Alexander Seidel, Erik Henriksen We report observations of the far-infrared cyclotron transitions involving the quasi-zero energy Landau level octet in a dual-gated, encapsulated graphene bilayer. Working at a fixed magnetic field of 13 tesla, we systematically study the effects of varying the Landau level filling factor, ν, and the electric displacement field applied perpendicular to the sheet. Varying the displacement field yields a direct measurement of the valley splitting in the lowest Landau level at ν = 4, and signatures of a transition between an interaction-driven and layer polarized ground state at ν = 0. At zero displacement field, weak electron-hole asymmetry and multiple resonances are observed as the filling is tuned between ν = -6 and +6. Numerical calculations of the single particle Landau level energies including all non-leading band parameters fail to account for several of our observations. |
Wednesday, March 17, 2021 1:06PM - 1:18PM Live |
M60.00009: Multiphonon processes and resonant Raman scattering in bilayer MoS2/WS2 Yue Yu, Jun Jiang, Liangbo Liang, Georgios D Barmparis, Sokrates T Pantelides, Xiaoguang Zhang Resonant Raman scattering can be a powerful complement to the commonly used non-resonant, but its utility is limited by the absence of practical calculational schemes of resonant Raman intensities. By combining the Kramers-Heisenberg-Dirac (KHD) formula with the path-integral technique, we express the Resonant Raman intensity as a triple integral in time domain, analytically including all contributions from multiphonon processes. We calculate the resonant Raman intensities of a bilayer MoS2/WS2 vertical heterostructure, which features strong interlayer charge transfer in the excited state. We report multiple resonance peaks in Raman intensity as a function of the laser energy. The results constitute strong evidence of involvement of multiple phonon modes. We also demonstrate that a resonant Raman spectrum can distinguish between different stacking sequences of the bilayer. |
Wednesday, March 17, 2021 1:18PM - 1:30PM Live |
M60.00010: Spectrophotometry detection of Color centers within MoS2 ultrathin film samples Mehdi Pakmehr Molybdenite as a semiconducting material has interesting properties both optically and electronically in 2D morphology, make it a suitable candidate for (opto) electronic applications. Ultrathin films of MoS2 grown on sapphire were investigated for possible detection of color centers. Due to fabrication process of our samples (which based on sulfurization of MoO3 ultrathin films grown on sapphire within tube furnace), many void defects contained within them. Optical micrograph of our sample proves the nonhomogeneous nature of the grown film, while absorption spectra confirms the ultrathin formation of layer on sapphire observing excitonic peaks at 1.85 & 2 eV. Minor absorption peaks observed at 1.12 & 1.62 eV (FHWM=0.2 eV) through spectrophotometry and through FTIR spectroscopy. These absorptions attributed to the void/color centers within the samples. We plan to present our findings through a talk at coming APS March meeting. |
Wednesday, March 17, 2021 1:30PM - 1:42PM Live |
M60.00011: Substrate-Mediated Hyperbolic Phonon Polaritons in MoO3 Jeffrey Schwartz, Son Le, Sergiy Krylyuk, Curt Richter, Albert V. Davydov, Andrea Centrone Hyperbolic phonon polaritons (HPhPs) are hybrid excitations of light and coherent charge oscillations that exist in strongly optically anisotropic 2D materials (e.g., MoO3). These polaritons propagate through the material’s volume with long lifetimes, enabling novel mid-infrared nanophotonic applications by compressing light to sub-diffraction dimensions. Here, the dispersion relations and HPhP lifetimes (several ps) in single-crystal α-MoO3 are determined by Fourier analysis of real-space, nanoscale-resolution polariton images obtained with the photothermal induced resonance (PTIR) technique. Measurements of MoO3 crystals deposited on periodic gratings showed longer HPhPs propagation lengths (≈ 2 ×) and lower optical compressions in suspended regions compared to regions in direct contact with the substrate. Additionally, PTIR data reveal polymeric contaminants, resulting from sample preparation, localized under parts of the MoO3 crystals. This work enhances the ability to engineer nanophotonic devices by leveraging substrate morphology to control polariton propagation. |
Wednesday, March 17, 2021 1:42PM - 1:54PM On Demand |
M60.00012: Nanocavity clock spectroscopy: resolving competing exciton dynamics in WSe2/MoSe2 heterobilayers Molly May, Tao Jiang, Chenfeng Du, Kyoung-Duck Park, Xiaodong Xu, Alexey Belyanin, Markus Raschke Long lived interlayer excitons in transition metal dichalcogenide heterobilayers hold promise for applications from high temperature exciton condensates to nano-lasers with extended spatial coherence and other 2D optoelectronic devices. However, their exciton dynamics are difficult to disentangle due to multiple competing processes with timescales varying over many orders of magnitude. Using a configurable nano-optical cavity based on a plasmonic scanning probe tip, we can control the radiative and nonradiative relaxation of intra- and interlayer excitons. Tuning their relative rates in a WSe2/MoSe2 heterobilayer over six orders of magnitude in tip-enhanced photoluminescence spectroscopy (TEPL) reveals cavity-induced crossover from nonradiative quenching to Purcell-enhanced radiation. Rate equation modeling with the interlayer charge transfer time as a reference clock allows for comprehensive determination from the long interlayer exciton (IX) radiative lifetime tIXrad = (94+/-27) ns to the five orders of magnitude faster competing nonradiative lifetime tIXnrad=(0.6+/-0.2) ps. This approach of nanocavity clock spectroscopy is generally applicable to a wide range of excitonic systems with competing decay pathways. |
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