Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session X17: 2D Semiconductor Physics IIIFocus
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Sponsoring Units: DMP Chair: Yu Ye, University of California, Berkeley Room: 316 |
Friday, March 18, 2016 8:00AM - 8:12AM |
X17.00001: Enhanced Valley Zeeman Splitting in MoS2/EuS due to interfacial exchange field Chuan Zhao, Thomas Scrace, Payam Taheri, Peiyao Zhang, Tenzin Norden, Brett Blizzard, Athos Petrou, Hao Zeng, Puqin Zhao, George Kioseoglou A monolayer transition metal dichalcogenides such as MoS$_{\mathrm{2}}$ with broken inversion symmetry possesses two degenerate yet inequivalent valleys that can be selectively excited by circularly polarized light. The ability to manipulate valley degrees of freedom with light or external magnetic field makes them attractive for optoelectronic and spintronic applications. On the other hand, it has been demonstrated recently that a magnetic insulator such as EuS can induce magnetic exchange field (MEF) on graphene through proximity effect. Thus, construction of a magnetic insulator/TMDC heterostructure may induce large MEF on TMDC, which may lead to giant valley Zeeman splitting. In this work, we report the observation of valley Zeeman splitting in monolayer MoS$_{\mathrm{2}}$ and other TMDCs due to the MEF from EuS substrates. Using magneto-reflectivity, we measured a Zeeman splitting of valley exciton of 2 meV at 7 tesla and 4 K, for monolayer MoS$_{\mathrm{2}}$ on a SiO$_{\mathrm{2}}$ substrate. This is consistent with values reported in monolayer WSe$_{\mathrm{2}}$. However, when EuS is used as the substrate, we observed an increase of valley splitting from 2 to 10 meV. We attribute this enhanced valley splitting to the MEF from the EuS substrate. Utilizing MEF of a magnetic insulator can induce magnetic ordering and giant Zeeman splitting in 2D TMDCs, which might enable novel spintronics applications. [Preview Abstract] |
Friday, March 18, 2016 8:12AM - 8:24AM |
X17.00002: Exciton diamagnetic shifts and valley Zeeman effect in monolayer WS$_{2}$ and MoS$_{2}$ to 65Tesla A. V. Stier, K. A. McCreary, B. T. Jonker, J. Kono, S. A. Crooker We report circularly-polarized optical reflection spectroscopy of monolayer WS$_{2}$ and MoS$_{2}$ at low temperatures (4 K) and in high magnetic fields to 65 T [1]. Both the A and the B exciton transitions exhibit a clear and very similar Zeeman splitting of approximately $-$230 $\mu $eV/T (g $\simeq \quad -$4), providing the first measurements of the valley Zeeman effect and associated g-factors in monolayer transition-metal disulphides. These results complement and are compared with recent low-field photoluminescence measurements of valley degeneracy breaking in the monolayer diselenides MoSe$_{2}$ and WSe$_{2}$. Further, the very large magnetic fields used in our studies allows us to observe the small quadratic diamagnetic shifts of the A and B excitons in monolayer WS2 (0.32 and 0.11 $\mu $eV/T$^{2}$, respectively), from which we calculate exciton radii of 1.53 nm and 1.16 nm. When analyzed within a model of non-local dielectric screening in monolayer semiconductors, these diamagnetic shifts also constrain and provide estimates of the exciton binding energies (410 meV and 470 meV for the A and B excitons, respectively), further highlighting the utility of high magnetic fields for understanding new 2D materials. [1] A. V. Stier et al., submitted, arxiv:1510.07022 (2015) [Preview Abstract] |
Friday, March 18, 2016 8:24AM - 8:36AM |
X17.00003: Valley selective high field magneto-spectroscopy of monolayer MoSe$_2$ Jonathan Ludwig, Y. Li, Z. Lu, X.X. Zhang, X. Cui, J. Hone, T.F. Heinz, D. Smirnov Monolayer transition metal dichalcogenides (TMDs) have recently emerged as a new class of direct bandgap 2D semiconductors with valleys at the $\pm$K points in the Brillouin zone. Due to the broken inversion symmetry in monolayer TMDs, this valley degree of freedom can be selectively addressed by optical helicity. We report on circularly polarization resolved photoluminescence on gated monolayer MoSe$_2$ in perpendicular and parallel magnetic fields up to 30T. In a perpendicular field at low carrier density, the PL energies of both the trion and exciton experience a linear shift with a slope of $\approx \pm 2 \mu_B/T$ for the $\pm$K valleys, demonstrating valley degeneracy lifting. This is in contrast to the measurements in parallel field, where no such linear splitting occurs. In addition, we report quadratic corrections to the linear magnetic field dependence of the trion and excition energy in the perpendicular configuration. [Preview Abstract] |
Friday, March 18, 2016 8:36AM - 9:12AM |
X17.00004: Valley-selective optical Stark effect in monolayer WS$_{\mathrm{2}}$ Invited Speaker: Nuh Gedik Monolayer semiconducting transition-metal dichalcogenides (TMDs) have a pair of valleys that, by time-reversal symmetry, are energetically degenerate. Lifting the valley degeneracy in these materials is of great interest because it would allow for valley specific band engineering and offer additional control in valleytronic applications. In this talk, I will show that circularly polarized light, which breaks time-reversal symmetry, can be used to lift the valley degeneracy by means of the optical Stark effect [1]. We demonstrate that this effect is capable of raising the exciton level in monolayer TMD WS$_{\mathrm{2}}$ by as much as 18 meV in a controllable valley-selective manner. The resulting energy shift is extremely large, comparable to the shift that would be obtained using a very high magnetic field ( approximately 100 Tesla). These results offer a novel way to control valley degree of freedom, and may provide a means to realize new valley-selective Floquet topological state of matter. [1] E J Sie, J W McIver, Y H Lee, L Fu, J Kong and N Gedik, Nature Materials 14, 290 (2015) [Preview Abstract] |
Friday, March 18, 2016 9:12AM - 9:24AM |
X17.00005: Valley-Polarized Interlayer Excitons in 2D Semiconductor Heterostructures Pasqual Rivera, Kyle Seyler, Hongyi Yu, John Schaibley, Jiaqiang Yan, David Mandrus, Xiaodong Xu Vertically stacked monolayers of MoSe$_{\mathrm{2}}$~and WSe$_{\mathrm{2}}$~feature a type-II band alignment causing the formation of interlayer excitons, where the Coulomb bound hole and electron reside in different layers. This species of exciton has lifetime many orders of magnitude longer than intralayer valley excitons, providing a unique and advantageous system for investigating valley exciton physics. Here, we optically pump the MoSe$_{\mathrm{2}}$-WSe$_{\mathrm{2}}$~heterostructure with circularly polarized light, creating interlayer valley excitons with gate-tunable spin-valley polarization lifetime up to 40 ns. This long valley lifetime enables the diffusion of the interlayer valley exciton gas to be visualized. Under increasing excitation power we observe the formation of a ring in the spatial distribution of the valley polarization, a manifestation of significant valley-selective exchange interactions at high exciton densities. The combination of long valley polarization and spatial diffusion makes the interlayer exciton in semiconductor heterostructures an exciting platform for studies of valley exciton physics. [Preview Abstract] |
Friday, March 18, 2016 9:24AM - 9:36AM |
X17.00006: Valley-Polarized Exciton-Polaritons in a Monolayer Semiconductor Embedded in a Microcavity Yen-J. Chen, Teodor K. Stanev, Nathaniel P. Stern, Jeffrey D. Cain, Vinayak P. Dravid Two-dimensional transition metal dichalcogenides (TMDs) are semiconductors that exhibit degenerate, but inequivalent, valleys at their $K$-point band gaps which selectively couple to circularly-polarized light fields. Coherent hybrid states of light and matter, exciton-polaritons, have been observed when monolayer TMDs strongly interact with photon fields in a microcavity~\footnote{X. Liu, T. Galfsky, Z. Sun, F. Xia, E.-C. Lin, Y.-H. Lee, S. K\”{e}na-Cohen, and V. M. Menon. \textit{Nature. Photon.} \textbf{9}, 30 (2015)}. The degree of polarization is determined by the relative rates of exciton and intervalley relaxation, which can be modified for microcavity exciton-polaritons. Preservation of valley-polarization in a microcavity at room temperature is compared to the nearly zero polarization for bare monolayer MoS$_2$ on SiO$_2$, demonstrating cavity-modified relaxation dynamics of the coherent valley-specific exciton-polaritons. These results suggest promising opto-electronic applications for valley-based polaritonic and photonic devices integrating monolayer TMDs. [Preview Abstract] |
Friday, March 18, 2016 9:36AM - 9:48AM |
X17.00007: Valley Polarization in Size-Tunable Monolayer Semiconductor Quantum Dots Guohua Wei, David A. Czaplewski, Il Woong Jung, Erik J. Lenferink, Teodor K. Stanev, Nathaniel P. Stern Controlling the size of semiconductor nanostructures allows manipulation of the optical and electrical properties of band carriers. We show that laterally-confined monolayer MoS$_2$ quantum dots can be created through top-down nanopatterning of an atomically-thin two-dimensional semiconductor. Semiconductor-compatible nanofabrication processing allows for these low-dimensional materials to be integrated into complex systems that harness their controllable optical properties. Size-dependent exciton energy shifts and linewidths are observed, demonstrating the influence of quantum confinement. The patterned dots exhibit the same valley polarization characteristics as in a continuous MoS$_2$ sheet, suggesting that monolayer semiconductor quantum dots could have potential for advancing quantum information applications. [Preview Abstract] |
Friday, March 18, 2016 9:48AM - 10:00AM |
X17.00008: Valley-selective harmonic generations in transition metal dichalcogenide monolayers Jingxin Cheng, Tao Jiang, Yuwei Shan, Yingguo Li, Xianhui Chen, Y.R. Shen, Weitao Liu, Shiwei Wu Transition metal dichalcogenide monolayer has emerged as another star in the family of atomically thin two dimensional materials. Different from graphene, the two sublattices in its honeycomb-like structure are occupied by different atoms, leading to the reduced rotational symmetry from six fold to three fold. The reduced symmetry and dimension not only result in many intriguing physics such as valley and excitons, but also lead to rich nonlinear optical phenomena such as strong second harmonic generation. In this talk, we will present a systematic study on linearly and circularly polarized harmonic generations in this wonder material. We show that both the second and third harmonic generations follow the conservation of angular momentum and are valley-selective. Furthermore, these nonlinear optical processes could be used as a powerful imaging tool for studying transition metal dichalcogenide monolayers and other similar 2D materials. [Preview Abstract] |
Friday, March 18, 2016 10:00AM - 10:12AM |
X17.00009: Efficient evaluation of epitaxial MoS$_{\mathrm{2}}$ on sapphire by direct band structure imaging HoKwon Kim, Dumitru Dumcenco, Mathieu Fregnaux, Anass Benayad, Yen-Cheng Kung, Andras Kis, Olivier Renault The electronic band structure evaluation of two-dimensional metal dichalcogenides is critical as the band structure can be greatly influenced by the film thickness, strain, and substrate. Here, we performed a direct measurement of the band structure of as-grown monolayer MoS$_{\mathrm{2}}$ on single crystalline sapphire by reciprocal-space photoelectron emission microscopy with a conventional laboratory ultra-violet He I light source. Arrays of gold electrodes were deposited onto the sample in order to avoid charging effects due to the insulating substrate. This allowed the high resolution mapping ($\Delta E = $0.2 eV$; \Delta k = $0.05 {\AA}$^{\mathrm{-1}})$ of the valence states in momentum space down to 7 eV below the Fermi level. The high degree of the epitaxial alignment of the single crystalline MoS$_{\mathrm{2}}$ nuclei was verified by the direct momentum space imaging over a large area containing multiple nuclei. The derived values of the hole effective mass were 2.41 \textpm 0.05 m$_{\mathrm{0}}$ and 0.81 \textpm 0.05 m$_{\mathrm{0}}$, respectively at $\Gamma $ and K points, consistent with the theoretical values of the freestanding monolayer MoS$_{\mathrm{2}}$ reported in the literature. [Preview Abstract] |
Friday, March 18, 2016 10:12AM - 10:24AM |
X17.00010: ABSTRACT WITHDRAWN |
Friday, March 18, 2016 10:24AM - 10:36AM |
X17.00011: Nanoscale Photoconductivity Imaging of Thin-film Semiconductors by Laser-assisted Microwave Impedance Microscopy Zhaodong Chu, Di Wu, Yuan Ren, Seungcheol Yang, Liuyang Sun, Xiaoqin Li, Keji Lai The photo-response of semiconductors is usually studied by detecting the photocurrent across source-drain electrodes under light illumination. By integrating the microwave impedance microscopy (MIM) technique with focused-laser stimulation, we are able to perform the real-space photoconductivity mapping of photo-sensitive materials without the need of patterning contact electrodes. Here, we report the MIM results of various thin-film materials, such as In2Se3 nano-sheets and transition metal dichalcogenides (TMD) flakes, illuminated by laser beams of different wavelengths in the ambient condition. With no or below-gap illumination, the samples were highly resistive, as indicated by the low MIM signals. The MIM contrast emerges under above-gap light and increases as increasing laser intensity, which clearly demonstrates the local imaging of photoconductivity rather than the transport photocurrent. Interestingly, clear domain structures with mesoscopic length scales were seen in the data due to the coexistence of multiple phases in In2Se3. The unique combination of MIM and laser stimulation thus provides a new direction to explore the microscopic origin of various light-driven phenomena in complex systems. [Preview Abstract] |
Friday, March 18, 2016 10:36AM - 10:48AM |
X17.00012: High-harmonic generation from an atomically thin semiconductor Hanzhe Liu, Yilei Li, Shambhu Ghimire, Tony Heinz, David Reis The process of high-harmonic generation (HHG) from ultrashort laser pulses has recently been observed in bulk solids, complementing the well-established process in the gas phase. HHG is of interest both as a source of ultrashort pulses in the attosecond regime that has photon energies extending up to the soft x-ray region and as a method of probing material response outside the regime of perturbative nonlinear optics. In this paper, we present the observation of HHG from a single atomic layer of MoS$_2$ driven by a strong infrared pulse of 100 fs duration and 0.3 eV photon energy. We observe distinct harmonics up to the 13$^{\textrm{th}}$ order of the infrared excitation. The non-perturbative nature of the HHG process is demonstrated by the weak power dependence of the harmonic intensities. To gain further insight into the process, we have investigated the variation of the HHG signal with sample orientation and the ellipticity of pump excitation. We compare and contrast the process with that from the bulk MoS$_2$ crystal. We find significant differences in the response for the monolayer and bulk crystal, which can be understood in terms of the distinct crystallographic symmetries in the two cases. [Preview Abstract] |
Friday, March 18, 2016 10:48AM - 11:00AM |
X17.00013: Optical spectroscopy and imaging of the higher energy excitons and bandgap of monolayer MoS$_{\mathrm{2}}$ Nicholas Borys, Wei Bao, Edward Barnard, Changhyun Ko, Sefaatin Tongay, Junqiao Wu, Li Yang, P. James Schuck Monolayer MoS$_{\mathrm{2}}$ (ML-MoS$_{\mathrm{2}})$ exhibits a rich manifold of excitons that dictate optoelectronic performance and functionality. Disentangling these states, which include the quasi-particle bandgap, is critical for developing 2D optoelectronic devices that operate beyond the optical bandgap. Whereas photoluminescence (PL) spectroscopy only probes the lowest-energy radiative state and absorption spectroscopy fails to discriminate energetically degenerate states, photoluminescence excitation (PLE) spectroscopy selectively probes only the excited states that thermalize to the emissive ground state exciton. Using PLE spectroscopy of ML-MoS$_{\mathrm{2,}}$ we identify the Rydberg series of the exciton A and exciton B states as well as signatures of the quasi-particle bandgap and coupling between the indirect C exciton and the lowest-energy A exciton, which have eluded previous PLE studies. The assignment of these states is confirmed with density functional theory. Mapping the PLE spectrum reveals spatial variations of the higher-energy exciton manifold and quasi-particle bandgap which mirror the heterogeneity in the PL but also indicate variations in local exciton thermalization processes and chemical potentials. [Preview Abstract] |
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