Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F55: Electronic, Optical and Spin Dependent Properties of Transition Metal DichalcogenidesRecordings Available
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Sponsoring Units: DCMP Chair: I-Hsuan Kao, Carnegie Mellon University Room: Hyatt Regency Hotel -Adler |
Tuesday, March 15, 2022 8:00AM - 8:12AM |
F55.00001: Excitons and magnetoexcitons in transitional metal trichalcogenides and phosphorene Roman Y Kezerashvili, Anastasia Spiridonova We study direct and indirect excitons and magnetoexcitons in Rydberg states in monolayers and van der Waals (vdW) heterostructure composed of transitional metal trichalcogenides (TMTCs) and phosphorene within the framework of the effective mass approximation. Binding energies of excitons and magnetoexcitons are calculated employing the Rytova-Keldysh potential for direct excitons and magnetoexcitons and both the Rytova-Keldysh and Coulomb potentials for indirect one. We report the magnetic field energy contribution to the binding energies and diamagnetic coefficients (DMCs) for magnetoexcitons and show their strong depends on the anisotropy of effective masses of electron and hole. It is shown tunability of the binding energies of magnetoexcitons by external magnetic field and control the binding energies and DMCs by the number of hBN layers separated two TMTC sheets. We demonstrate that phosphorene has quasi-one-dimensional structure while TMTC behaves more like a symmetric system despite having quasi-one-dimensional structure. |
Tuesday, March 15, 2022 8:12AM - 8:24AM |
F55.00002: Quantum Point Contacts in Monolayer WSe2 Yuan Song, Augusto Ghiotto, Song Liu, Abhay N Pasupathy, Cory R Dean, James C Hone, Kenji Watanabe, Takashi Taniguchi Quantum point contacts (QPCs) have been realized in III-IV semiconductor heterostructures, and more recently in bilayer graphene and trilayer WSe2. These QPCs are fabricated in these two-dimensional systems using electrostatic gates, and require ballistic transport from source to drain contact to observe conductance quantization. In this work, we show that it is possible to achieve high-quality QPCs on monolayer WSe2 due to improvements in materials synthesis and contact quality. Measurements down to mK temperatures show clear conductance quantization over micron-sized source-drain length scales. In WSe2, at zero field, a two-fold degeneracy is expected at the top of the valence band due to the presence of strong spin-orbit interactions. Surprisingly, we find that the observed conductance plateaux are quantized in units of e2/h, indicating that the spin-valley degeneracy is lifted even without the application of magnetic fields. Further, the first plateau has a systematic dependence on charge carrier density and on applied magnetic field, in a manner similar to the "0.7-anomaly" reported in previous experiments on III-V semiconductors. We will discuss the nature of the anomaly seen here and the relevance of spin-orbit coupling to transport through this system. |
Tuesday, March 15, 2022 8:24AM - 8:36AM |
F55.00003: Emergence of second electronic subband in ultrathin InSe. Dmitry L Shcherbakov, Greyson Voigt, Shahriar Memaran, Kenji Watanabe, Takashi Taniguchi, Luis Balicas, Chun Ning Lau In 2D materials, the energy separation between electronic or hole subbands depends on thickness. We demonstrate the population of the second electronic subband by utilizing high mobility InSe thin enough to host a single layer of two-dimensional electron gas, but at the same time thick enough to allow for electrostatic doping of the second electronic subband. We explore the evolution of Landau levels from both subbands with magnetic and electric fields and investigate the tunability of onset charge density and effective mass of the second subband by perpendicular electric field. At high magnetic fields, the ring-shaped crossings of Landau levels of the two subbands suggest transitions from paramagnetic to ferromagnetic states. We find the electrons in the second subband to manifest tunability unattainable in conventional charge transport. |
Tuesday, March 15, 2022 8:36AM - 8:48AM Withdrawn |
F55.00004: Ultrahigh thermal anisotropy in 2D van der Waals films with interlayer rotation Shi En Kim, Fauzia Mujid, Fredrik Eriksson, Akash Rai, Joonki Suh, Paul Erhart, David G Cahill, Jiwoong Park The miniaturization and densification of modern-day electronics call for the concomitant advancement of thermal management strategies, including the discovery of novel, extreme materials that can serve as heat spreaders or insulators. Thermally anisotropic materials can perform both functions simultaneously; however, the ratios between heat transport along different axes in existing engineered anisotropic materials are usually around 20 or less. Here, we introduce layer-by-layer stacked transition metal dichalcogenide films with interlayer rotation, which boast a record-breaking thermal conductivity anisotropy ratio of nearly 900. Interlayer rotation maintains the crystallinity within each constituent monolayer and breaks the through-plane crystal symmetry. Notably, we achieve an ultralow through-plane thermal conductivity that is only twice the value of air. We interrogate the effect of interlayer rotation on thermal transport via molecular dynamics simulations, which reveal a 1D glass-like transport. Our films can spread heat from current carrying gold nanoelectrodes while insulating their surfaces. We show that interlayer rotation can be a new degree of freedom in manipulating thermal transport in crystalline, layered solids toward the realization of 2D phonon transport. |
Tuesday, March 15, 2022 8:48AM - 9:00AM |
F55.00005: Metal to correlated insulator crossover in 1T-TaSe2 Siqi Wang, Yi Chen, Peiyao Zhang, Sui Yang, Takashi Taniguchi, Kenji Watanabe, Kai Rossnagel, Michael F Crommie, Xiang Zhang Materials hosting charge density wave (CDW) may exhibit drastically different properties as compared with their normal states. Among them, 1T tantalum dichalcogenides develop a unique star-of-David CDW and are predicted to be Mott insulators and hosts of quantum spin liquids (QSL). 1T-TaSe2 is a promising material platform in this family with a robust CDW order and a much cleaner phase diagram, despite the observed metallic behavior in the bulk. Recent results have showed interlayer interactions and stacking order may obfuscate the physics and mask out the strong correlation in the system. On the other hand, advances in layered van der Waals materials provides direct access to individual layer properties and therefore make the “bottom-up” study of complicated systems possible. With TaSe2 devices down to bilayer thickness, we observed a crossover from metallic properties in the bulk to correlated insulating properties in the atomically thin 1T-TaSe2 due to the suppression of the hybridization between Ta 5dz2 orbitals and Se 4pz orbitals with transport and tunneling measurements. We further investigated the magnetic ground state of monolayer 1T-TaSe2 with thermally induced 1T/1H heterostructures. The existence of localized magnetic moments was confirmed, and we established the system as a promising quantum spin liquid candidate. |
Tuesday, March 15, 2022 9:00AM - 9:12AM |
F55.00006: Disorder driven Charge Density Wave superlattice structure and potential Quantum Spin Liquid behavior in 1T-TaS2 and 1T-TaSe2 Sharon S Philip, John A Schneeloch, Despina A Louca The transition metal dichalcogenide 1T-TaS2-xSex is an interesting system due to the multiple phase transitions both as a function of doping and temperature, that includes charge density wave (CDW) states, Mott insulator, superconductivity and more recently, quantum spin liquid (QSL) state. 1T-TaS2, undergoes a series of phase transitions upon cooling, acquiring a commensurate CDW (CCDW) phase below 183 K. Moreover, the CCDW transition is accompanied by a periodic lattice distortion that forms a √13×√13 supercell creating the so-called Star-of-David motif. The Star-of-David formation alone cannot explain the electronic behavior of 1T-TaS2 such as the QSL behavior, superconductivity, and the metal-insulator transition (MIT). From our recent work on this system, we observed clear evidence that the low-temperature crystal structure deviates from the long-range symmetry (P-3m1). Pair distribution function analysis suggests in-plane displacements of Ta atoms in 1T-TaS2 in the CDW phase which shows significant differences from the model based on average structure. Distortions at the 12 Ta atoms making up the outer rim of the star are present in both 1T-TaS2 and 1T-TaSe2, which may very well lead to distortions in the spin structure. |
Tuesday, March 15, 2022 9:12AM - 9:24AM |
F55.00007: Novel hysteresis effects in the charge density wave compound 1T-TaS2-xSex Aimee Nevill, Benjamin Smith, Liam Farrar, Charles Sayers, Daniel Wolverson, Simon J Bending, Sara Dale, Enrico da Como The metallic transition metal dichalcogenide (TMD) compound 1T-TaS2-xSex (x = 0 to 2) has been the focus of intense research due to its extremely rich phase diagram as a function of chalcogen doping x. At low temperatures, a variety of strongly correlated phases are observed including commensurate and incommensurate charge density waves (CDWs), superconductivity, and a Mott insulating state. This is particularly interesting as both TaS2 and TaSe2 have very similar crystal structure and CDW superstructures, yet exhibit dramatically different electronic properties. |
Tuesday, March 15, 2022 9:24AM - 9:36AM |
F55.00008: Spin-dependent magneto-transport in bilayer WSe2 En-Min Shih, Qianhui Shi, Daniel Rhodes, Bumho Kim, Kenji Watanabe, Takashi Taniguchi, James C Hone, Cory R Dean We report magneto-transport measurements in bilayer WSe2, showing electrons behave distinctly in Landau level (LL) of different spins. The conductivity at half-filled minority-spin Landau levels is much lower than in majority-spin and diminishes at high field with electrons completely localized. Also, the temperature-dependent conductance behaves opposite in different spin LLs, with an unusual trend of conductance decreasing with lowering temperature at half-filled minority-spin Landau levels. We further observe a similar phenomenon happening in the valley pseudo-spin when changing the degree of polarization with displacement field. The higher pseudo-spin polarization of valence charges results in higher magneto-resistance. These phenomena can be understood with a polaron-like picture. Finally, magneto-transport demonstrates a sharp majority-minority spin-state transition close to the LL crossing density at partial filling LL. The spike of two-probe conductance signals first-order transition and domain walls of two different spin states. |
Tuesday, March 15, 2022 9:36AM - 9:48AM |
F55.00009: Proximity Induced Superconductivity in Monolayer MoS2 Maria Iavarone, Daniel J Trainer, Baokai Wang, Fabrizio Bobba, Jouko Nieminen, Noah Samuelson, Xiaoxing Xi, John F Zasadzinski, Arun Bansil Molybdenum disulphide (MoS2) has emerged as a prototypical materials among the 2D transition metal dichalcogenides for its stability, low cost and unique electronic, optical and mechanical properties. Its electronic properties can be tuned using different control parameters. This great sensitivity presents an opportunity to functionalize its properties through defect engineering, strain or by proximity to another material. We use high resolution low temperature STM/STS to study the local electronic properties of monolayer MoS2 and the proximity induced superconductivity in monolayer MoS2 placed on top of a Pb this film. We find a superconducting coherence peak amplitude modulated spatially in a Moire pattern on the surface of MoS2 . Our study indicates that the local modulation of induced superconductivity in MoS2 could be controlled via geometrically tuning. This study suggests that heterostructures based on MoS2 offer a viable possibility to tune its electronic properties and open unprecedented possibilities of combining them for technological use. |
Tuesday, March 15, 2022 9:48AM - 10:00AM |
F55.00010: Charge transfer in MoS2/WSe2 heterostructures probed by first-principles calculations and ultrafast laser spectroscopy Stephanie M Amos, Pavel Valencia-Acuna, Hui Zhao, Hartwin Peelaers 2D layered materials have attracted significant scientific attention, as their 2D nature allows the stacking of different materials leading to a control of the functionality. However, a detailed understanding of the properties of such heterostructures is still under development. Here we compare the photocarrier dynamics of heterostructures of monolayer MoS2 with monolayer or trilayer WSe2 using first-principles modeling based on hybrid density functional theory and ultrafast laser spectroscopy. |
Tuesday, March 15, 2022 10:00AM - 10:12AM |
F55.00011: Property Tuning of Two-Dimensional Materials by Stacking Order and applications of homostructural devices zexiang shen, Juan Xia, Jiaxu Yan The optical and electronic structures of two-dimensional (2D) materials often show very strong layer-dependent properties. Such properties allow us to achieve various functionalities using different thickness (layer number), stacking order and heterostructures. The stacking-dependent properties allow us to build optoelectronic devices using the same material and same thickness, but it is a much less explored topic. Detailed understanding of the inter-layer interaction via stacking order will help greatly in tailoring the properties of 2D materials for applications, e.g. in pn junction, transistors, solar cells and LEDs. |
Tuesday, March 15, 2022 10:12AM - 10:24AM |
F55.00012: Complete band structure of microscopic MoS2 and WSe2 flakes Sergey Babenkov, Ye Peng, Marie Froidevaux, Willem Boutu, Nicholas Barrett, Hamed Merdji Transition-metal dichalcogenides (TMDC) have a MX2 layered structure with 2H symmetry and M = Mo, W; X = S, Se, Te etc. They occupy a unique place in the class of two-dimensional materials thanks to their fascinating electronic properties [1]. Recently, it has been predicted theoretically = that a topological phase transition can be induced entirely with shaped light fields, such as trefoil polarization states, in conventional hexagonal materials [2]. This opens a route towards a new device based on creating a topologically protected electronic state using ultrashort structured light pulses. By orienting the trefoil symmetry it is possible to populate alternatively K and K’ valleys in the conduction band. With this in mind, we experimentally determined the complete band structure (both occupied and unoccupied states) of single crystal microscopicTMDC flakes using full k-space imaging of photoemission electron microscopy and secondary electron emission, following ref [3]. These results allowed us to accurately calculate the valence and conduction band structures in a single photoemission experiment and are compared with first-principles calculations. They will be used in future pump-probe experiments to explore the electron dynamics in the conduction band and photo-induced multi-topological states. |
Tuesday, March 15, 2022 10:24AM - 10:36AM |
F55.00013: Depth-profiling of chemical and electronic properties of large-area exfoliated two-dimensional materials Henrique P Martins, Ryan Muzzio, Stephanie Lough, Darian Smalley, Jesse E Thompson, Masahiro Ishigami, Giuseppina Conti, Slavomir Nemsak, Jyoti Katoch Two-dimensional materials such as transition metal dichalcogenides (TMDC) can be exfoliated to a desired thickness and then stacked into different van der Waals heterostructures, leading to emergent phenomena. Interlayer and material/substrate coupling significantly alters the electrical, optical, and vibrational properties of these materials. In this study we do a non-destructive depth-profiling of large area exfoliated TMDCs with few layer thicknesses using standing-wave x-ray photoemission spectroscopy (SW-XPS). This technique utilizes the standing-wave generated by the reflection of soft x-rays on the multilayer [Si/Mo] mirror substrate at the Bragg condition [1]. The core level and valence band data collected using SW-XPS provides answers about the evolution of the chemical bonding with depth and the impact of different experimental conditions to the TMDC materials, such as annealing or gaseous environment. |
Tuesday, March 15, 2022 10:36AM - 10:48AM |
F55.00014: Hidden Breathing Kagome Lattice of d Orbitals in Hexagonal TMDs Jun Jung, Yong-Hyun Kim A breathing Kagome lattice is formed by corner-sharing triangles of two different directions and bond strengths. With the stronger inter-site hopping, this lattice can become a higher-order topological insulator (HOTI). Experimental realizations of breathing Kagome lattices have been reported, but there have been no reports on simple natural materials with an electronic breathing Kagome lattice. Here we theoretically prove that a monolayer hexagonal transition metal dichalcogenide (h-TMD) is an electronic breathing Kagome lattice material. The trigonal prismatic structure-driven sp2-like hybrid d orbitals create an electronic breathing Kagome lattice. As the inter-site hopping is found to be stronger than the on-site hopping, the h-TMDs are HOTI on hidden breathing Kagome lattices. We also demonstrate that h-TMD nanoflakes host topologically protected corner states. Because h-TMDs are easily synthesizable and stable at ambient conditions, our findings provide an easily accessible platform for quantum physics on condensed matter systems. |
Tuesday, March 15, 2022 10:48AM - 11:00AM |
F55.00015: Why is the size dependence of the band gap similar in MX2 ( M = Mo , W and X=S,Se )? Sumanti Patra, Dipankar Das Sarma, Priya Mahadevan The size dependence of the electronic structure of semiconductor nanocrystals have been investigated for many years, with band gap changes being seen only upto 20 Å in some instances, while it is upto 200 Å in other instances. This variation has been found to be associated with the excitonic Bohr radius of the semiconductor involved. In contrast, when one considers van der Waals materials like the transition metal dichalcogenides, one finds that the number of layers upto which band gap variations are found in Mo and W-based based transition metal dichalcogenides MX2 (where M=Mo and W; X=S and Se) are very similar. We have mapped the electronic structure of these materials onto a tight binding model and derived a low energy effective hamiltonian by integrating out the high energy bands. We find that the model derived for a trilayer is able to capture the electronic structure upto 10-layers and beyond as revealed by a comparison with the ab-initio results. Within this model, the origin of the similar size dependence across materials has been addressed. |
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