Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K40: Strain and Optical Properties of Monolayers |
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Sponsoring Units: DCMP Chair: Claudia Ojeda-Aristizabal, Cal State Univ- Long Beach Room: LACC 501C |
Wednesday, March 7, 2018 8:00AM - 8:12AM |
K40.00001: The Effect of Applied Strain on the Quasiparticle Band-Gap of Monolayer MoS2 Daniel Trainer, Yuan Zhang, Fabrizio Bobba, Xiaoxing Xi, Saw Hla, Maria Iavarone Atomically thin materials such as single layer Molybdenum Disulfide (MoS2) have emerged as promising candidates for next generation flexible 2D electronics. However, few studies to date have investigated the electronic properties of these materials as a function of applied strain. In this work we use low temperature scanning tunneling microscopy and spectroscopy (STM/STS) to ellucidate the effect of strain on the quasiparticle band-gap of monolayer MoS2. Controlled strain was achieved by depositing single layer MoS2 on a flat substrate and then bending the substrate with a varying radius of curvature using a custom built sample holder. |
Wednesday, March 7, 2018 8:12AM - 8:24AM |
K40.00002: A method for controllably inducing ultra-high strain in suspended 2D materials Nikhil Tilak, Guohong Li, Eva Andrei Generating strain can significantly alter the electronic properties of 2D materials. Strain-induced changes in Bandgap, Direct-Indirect gap transitions, Pseudo-magnetic fields etc. have been reported in various 2D materials. Traditional methods of inducing strain such as stretching or bending cannot produce strain values of more than a few percent on average. Here we discuss a technique for generating very high local strain in suspended 2D materials in a controlled and reversible fashion. An ultra-compact and modular device was constructed which allows a sharp STM tip to be aligned perpendicular to a 2D material suspended on micron sized pores in a Silicon Nitride mesh. This STM tip can then be used to poke and deform the suspended 2D material from one side while simultaneously monitoring the effect of the strain using various experimental probes from the other side. This opens the possibility for studying the changes in the electronic properties of Graphene and other 2D materials with local probes such as Scanning Tunneling Spectroscopy and micro Raman Spectroscopy under extreme strain conditions close to the breaking point. |
Wednesday, March 7, 2018 8:24AM - 8:36AM |
K40.00003: Transition Metal Dichalcogenides Layers with the Strain and the Charge Density Wave Order: Wannier Electronic Structure Modeling Shiang Fang, Stephen Carr, Yafang Yang, Valla Fatemi, Jonathan Ruhman, Pablo Jarillo-Herrero, Efthimios Kaxiras With the development of the experimental techniques to synthesize and characterize the two-dimensional van der Waals layered materials, intense theoretical investigations have been focusing on the potential device applications. On the other hand, the layered geometry also provides a platform to study the correlated many-body physics in the reduced dimensionality. Among different layer types, the transition metal dichalcogenides are an interesting class of materials which come in various flavors for the structural and electronic properties. Many of them such as TaS2 crystals exhibit charge density wave order and superconductivity. To utilize the layer platform to study the interaction between different electronic orders and their mechanism, it is crucial to have a solid picture of the underlying electronic structure. In our modeling work, the Wannier function method is applied to derive these electronic models without any ad hoc fitting procedures. These Hamiltonians shed light on the physics of the strain and the charge density wave order in the layers, and serve as the starting point for more elaborated many-body theory and ab initio numerical simulations. |
Wednesday, March 7, 2018 8:36AM - 8:48AM |
K40.00004: Strain mapping via second-harmonic generation in two-dimensional crystals Lukas Mennel, Marco Furchi, Stefan Wachter, Matthias Paur, Dmitry Polyushkin, Thomas Mueller Strain engineering is widely used in material science to tune the (opto-)electronic properties of materials and enhance the performance of devices. Two-dimensional (2D) atomic crystals are a versatile playground to study the influence of strain, as they can sustain very large deformations without breaking. Various optical techniques have been employed to probe strain in 2D materials, including micro-Raman and photoluminescence spectroscopy. Here we demonstrate that optical second harmonic generation constitutes an even more powerful technique, as it allows to extract the full strain tensor with a spatial resolution below the optical diffraction limit. Our method is based on the strain-induced modification of the nonlinear susceptibility tensor due to a photoelastic effect. Using a two-point bending technique, we determine the photoelastic tensor elements of various transition metal dichalcogenides. Once identified, these parameters allow us to spatially image the 2D strain field in inhomogeneously strained samples. |
Wednesday, March 7, 2018 8:48AM - 9:00AM |
K40.00005: Antiferromagnetism and Competing Charge Instabilities of Electrons in Strained Graphene from Coulomb Interactions David Sanchez De La Pena, Julian Lichtenstein, Carsten Honerkamp, Michael Scherer We study the quantum many-body ground states of electrons on the half-filled honeycomb lattice with short- and long-ranged density-density interactions as a model for graphene. To this end, we employ the recently developed truncated-unity functional renormalization group (TUfRG) approach which allows for a high resolution of the interaction vertex’ wavevector dependence. We connect to previous lattice quantum Monte Carlo (QMC) results which predict a stabilization of the semimetallic phase for realistic ab initio interaction parameters and confirm that the application of a finite biaxial strain can induce a quantum phase transition towards an ordered ground state. In contrast to lattice QMC simulations, the TU-fRG is not limited in the choice of tight-binding and interaction parameters to avoid the occurrence of a sign problem. Thus, we investigate a range of parameters relevant to the realistic graphene material which are not accessible by numerically exact methods. Although a plethora of charge density waves arises under medium-range interactions, we find the antiferromagnetic spin-density wave to be the prevailing instability for long-range interactions. We further explore the impact of an extended tight-binding Hamiltonian with second-nearest neighbor hopping. |
Wednesday, March 7, 2018 9:00AM - 9:12AM |
K40.00006: Optical manipulation of transition metal dichalcogenide with sub-nanometer metal-insulator-semiconductor cavity Shengxi Huang, Ouri Karni, Rui Yang, Jonathan Fan, Tony Heinz Transition metal dichalcogenides (TMDs) exhibit intriguing optical properties, and integrating them with plasmonic structures offers an effective way to tune the optical response, making TMDs promising for optoelectronic applications. In this work, we report the manipulation of optical properties in WS2 through the integration into metal-insulator-semiconductor (MIS) structure. In particular, we separated WS2 from single-crystal Au surface with insulator gaps of sub- to few-nanometers thick. We observed the intensity quenching and frequency shift of static photoluminescence (PL) of WS2 as a function of MIS geometry, consistent with theoretical models of dielectric screening. An ultrafast pump-probe measurement shows the tuning of WS2 optical response even with pumping energy lower than the optical gap, suggesting the charge and energy transfers in the MIS structure with the assistance of the excited hot carriers. Our observation is useful for the understanding of PL quenching, charge and energy transfer, and the coupling between TMDs and MIS structure. |
Wednesday, March 7, 2018 9:12AM - 9:24AM |
K40.00007: Optical Absorption by Indirect Excitons in TMDC Double Layers Matthew Brunetti, Oleg Berman, Roman Kezerashvili We calculate the binding energy, transition energies, oscillator strength, and absorption coefficient of indirect excitons in transition metal dichalcogenide (TMDC) double layers with interstitial few-layer hexagonal boron nitride. The absorption factor, a dimensionless quantity which gives the fraction of incident photons absorbed by each TMDC double layer, is presented and evaluated. The aforementioned optical quantities are obtained for transitions from the ground state to the first two excited states. All quantities were obtained using the solution to the two-body Schrodinger equation for an electron and hole, where the electron-hole interaction is described by the Keldysh potential. For each material, we choose a combination of the exciton reduced mass and the dielectric screening length from the existing literature which give the largest and the smallest indirect exciton binding energy, which provides upper and lower bounds on all quantities presented. Our findings may be verified experimentally, using two-photon spectroscopy to first create the indirect excitons and then probe their excited states. |
Wednesday, March 7, 2018 9:24AM - 9:36AM |
K40.00008: Gate Dependent Transition Metal Dichalcogenide Photoluminescence and Absorption Spectroscopy Ryan Gelly, Giovanni Scuri, You Zhou, Kristiaan De Greve, Alexander High, Luis Jauregui, Kateryna Pistunova, Andrew Joe, Dominik Wild, Mikhail Lukin, Philip Kim, Hongkun Park Transition metal dichalcogenides (TMDs) are a class of semiconductors which can be easily exfoliated (i.e. cleaved) into atomically thin layers. At the monolayer limit, TMDs become direct-gap semiconductors and possess exciton resonances with large binding energies, on the order of hundreds of millielectronvolts. We fabricate van der Waals heterostructures in which monolayer TMDs are encapsulated between two hexagonal boron nitride flakes. We gate these passivated monolayers by adding an additional graphene monolayer using a dry transfer method. We study the photoluminescence and absorption of these high-quality samples at cryogenic temperatures (T=4K) as a function of the Fermi level, which we modulate via an applied gate voltage. We measure the linewidth of the neutral exciton to be less than 1nm, and find that our samples are spectrally homogeneous (variations within a linewidth) over several micrometers. |
Wednesday, March 7, 2018 9:36AM - 9:48AM |
K40.00009: Electronic structure of Black Phosphorous upon laser excitation revealed by Time- and Angle-Resolved Photoemission Spectroscopy Changhua Bao, Guoliang Wan, Yang Wu, Shuyun Zhou Black phosphorous is an intriguing two-dimensional material which possesses a variety of interesting physical properties such as high quality 2DEG and tunable band gap. Investigating how Light-matter interaction can modify its electronic structure is an important topic. Here I will present our experimental results on electronic structure of black phosphorous upon laser excitation by using time- and angle-resolved photoemission spectroscopy. |
Wednesday, March 7, 2018 9:48AM - 10:00AM |
K40.00010: Direct Observation of Photoinduced Bandgap and Binding Energy reduction in WS2 Paul Cunningham, Aubrey Hanbicki, Kathleen McCreary, Berend Jonker In 2-D semiconductors, strong Coulombic interactions give rise to tightly bound excitons. Control of surrounding dielectrics can screen this interaction. It has long been suggested that photo injected charge and excitons can dynamically screen the Coulombic interaction giving rise to bandgap renormalization. However, direct experimental evidence has only revealed small shifts in the exciton resonance due to an assumed imbalance in the cancellation of bandgap and binding energy reduction. Here we present a novel means of optically determining the electronic band gap and exciton binding energy in monolayer WS2. We use transient absorption spectroscopy to identify the exciton formation from initially photogenerated hot charges. The photon energy threshold for this feature occurs at the electronic bandgap, allowing for direct determination of the exciton binding energy. Using this method, we show the first direct measurement of bandgap renormalization and binding energy reduction upon high fluence photoexcitation in monolayer transition metal dichalcogenides. By changing the fluence form 3x1011 to >1x1012 cm-2 we find that the exciton binding energy in WS2 is reduced from 300 to 220 meV due to dynamic screening. |
Wednesday, March 7, 2018 10:00AM - 10:12AM |
K40.00011: Spectrally Resolved Exciton Emission in Monolayer WSe2 Encapsulated in h-BN Ziliang Ye, Lutz Waldecker, Daniel Rhodes, Abhinandan Antony, Bumho Kim, Jenny Ardelean, Minda Deng, Xiao-Xiao Zhang, James Hone, Tony Heinz The reduced screening of Coulomb interactions in transition metal dichalcogenide (TMDC) monolayers leads to the presence of a wide range of many-body effects in the materials’ optical properties, including tightly bound exciton (two-body), trion (three-body), and biexciton (four-body) states. Recently, new crystal growth methods and encapsulation in hexagonal boron nitride (h-BN) has greatly improved the quality of TMDC samples and given rise to a series of new exciting observations, such as the exciton linewidths near their intrinsic limit, signatures of interlayer electron-phonon couplings, and exciton emission polarized perpendicular to the surface. Here we report on our effort to spectrally resolve all emission peaks in h-BN encapsulated monolayers of WSe2. The narrowest peak we observe is from dark exciton states and exhibits a linewidth in photoluminescence below one meV at cryogenic temperature. We identify the different physical origins of the peaks by means of their dependence on doping and excitation properties, as well as their emission pattern. |
Wednesday, March 7, 2018 10:12AM - 10:24AM |
K40.00012: Stimulated Raman Scattering in Two-Dimensional Materials Leandro M. Malard, Lucas Lafeta, Alisson Cadore, Tiago Grasiano, Kenji Watanabe, Takashi Taniguchi, Leonardo Campos, Ado Jório In this work we will show some of our progress for imaging two dimensional materials Stimulated Raman Scattering (SRS) and with coherent anti-stokes spectrocopy (CARS). The number of different two dimensional materials increases every day, with different interesting physical properties and possible applications. Therefore a fast and non-destructive imaging and spectroscopies technics to find and characterize these materials is highly desirable. We have studied the Coherent Anti-Stokes Raman Spectrum in graphene and h-BN. For the case of h-BN was observed an increase in the intensity of anti-Stokes signal when the energy difference between the pump laser and Stokes is equal to the phonon energy of the h-BN (1365 cm-1) . However, in samples of graphene the signal was decreased when the energy difference between the lasers is equal with the phonon energy in graphene (1590 cm-1). We will address the causes for the observation of this phenomenon based on different electronic structures of these two materials, leading to a Fano lineshape in the spectrum. Finally, we will also discuss the application of SRS imaging in fewlayers of h-BN in order to identify these layers by a fast imaging technique. |
Wednesday, March 7, 2018 10:24AM - 10:36AM |
K40.00013: Catastrophic Ionization in Monolayer Transition Metal Dichalcogenides Robert Younts, Alexander Bataller, Hossein Ardekani, Kenan Gundogdu
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Wednesday, March 7, 2018 10:36AM - 10:48AM |
K40.00014: Elastic and electronic tuning of magnetoresistance in MoTe2 Despina Louca, Junjie Yang, Jun Liu, Gia-wei Chern Quasi-two dimensional transition metal dichalcogenides (TMD) exhibit dramatic properties that may transform electronic and photonic devices. We report on how the anomalously large magnetoresistance (MR) observed under high magnetic field in MoTe2, a type II Weyl semimetal, can be reversibly controlled under tensile strain. The MR is enhanced by as much as ∼ 30 % at low temperatures and high magnetic fields, when uniaxial strain is applied along the a-crystallographic direction and reduced by about the same amount when strain is applied along the b-direction. We show that the large in-plane electric anisotropy is coupled with the structural transition from the 1T' monoclinic to the Td orthorhombic Weyl phase. Controlled switching across the Td - 1T' phase boundary is achieved by minimal tensile strain. The sensitivity of the MR to tensile strain could have its origin to the nontrivial spin-orbital texture of the electron and hole pockets in the vicinity of Weyl points. Our ab initio calculations indeed show a significant orbital mixing on the Fermi surface, which is modified by the tensile strains. |
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