Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session C48: Spin and Valley Dynamics in TMDsFocus
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Sponsoring Units: GMAG DMP DCOMP FIAP Chair: Hanan Dery, University of Rochester Room: 395 |
Monday, March 13, 2017 2:30PM - 2:42PM |
C48.00001: Photoluminescence polarization in monolayer WSe$_{2}$ A.T. Hanbicki, M. Currie, G. Kioseoglou, A.L. Friedman, B.T. Jonker Monolayer materials such as WS$_{2}$ or WSe$_{2}$ are direct gap semiconductors with degenerate, yet inequivalent k-points at K and Kâ. The valence band maxima for K and Kâ have spin states of opposite sense enabling one to selectively populate each valley independently with circularly polarized light. Subsequent valley populations can be determined via the polarization of emitted light. Optical emission is dominated by neutral and charged exciton (trion) features, and changes in emitted polarization provide insight into the fundamental processes of intervalley scattering. We measure the circular polarization of WSe$_{2}$ monolayers as a function of temperature and excitation energy for both continuous wave (cw) and pulsed lasers. We find the temperature dependence of the trion follows the same trend as that of the neutral exciton, however, the initial polarization of the trion is significantly larger. Indeed, when excited with a cw laser, the trion polarization is nearly twice the polarization of the neutral exciton. When a pulsed laser is used as the excitation source however, the initial polarizations become very similar. We discuss the similarities and differences in the spectra for these different excitation sources in terms of carrier densities and screening. [Preview Abstract] |
Monday, March 13, 2017 2:42PM - 2:54PM |
C48.00002: Identifying the Reason for Variations in Circularly Polarized Photoluminescence Values in Monolayer WS$_{\mathbf{2}}$ Kathleen McCreary, Marc Currie, Aubrey Hanbicki, Berend Jonker The unique electronic band structure in single layer WS$_{2}$ provides the ability to selectively populate a desired valley by exciting with circularly polarized light. The valley population is reflected through the circular polarization of photoluminescence (PL) and a high degree of circular polarization has been predicted in WS$_{2}$. Interestingly, experimental work has shown this is not always the case. In particular, recent experimental investigations of monolayer WS$_{2}$ find near zero valley polarized emission from the neutral exciton under near resonant excitation. We investigate the circularly polarized PL in over twenty WS$_{2}$ monolayer samples synthesized using chemical vapor deposition. The room temperature circularly polarized emission (P$_{circ})$ values vary from 0{\%} to 20{\%}. The samples also exhibit considerable variation in exciton lifetime, ranging from 300 ps to \textasciitilde 1.5 ns, as measured by time resolved photoluminescence. Comparing P$_{circ}$ with the exciton lifetimes ($\tau _{r})$ reveals an inverse relation between the $\tau_{r}$ and circular polarization, with samples exhibiting the longest $\tau_{r}$ having the lowest P$_{circ}$ and vice versa. Our findings suggest that 100{\%} circular polarization will be achieved in samples exhibiting short $\tau_{r}$. [Preview Abstract] |
Monday, March 13, 2017 2:54PM - 3:06PM |
C48.00003: Long-Lived Valley Polarization of Intra-Valley Trions in Monolayer WSe2 Kha Tran, Akshay Singh, Mirco Kolarczik, Joe Seifert, Yiping Wang, Kai Hao, Dennis Pleskot, Nathaniel Gabor, Sophia Helmrich, Nina Owschimikow, Ulrike Woggon, Xiaoqin Li We report distinct valley dynamics associated with intervalley and intravalley trions in monolayer WSe2. Using circular two-color pump-probe spectroscopy, we observe no decay of a near-unity valley polarization associated with the intra-valley trions during \textasciitilde 25 ps, while the valley polarization of the inter-valley trions exhibits a fast decay of \textasciitilde 4 ps. Moreover, we show that intrinsic dynamics associated with the two types of trions can only be observed when they are excited resonantly. The exceptionally robust valley polarization associated with resonantly created intravalley trions in monolayer WSe2 may be exploited for applications of valleytronic applications such as the valley Hall effect. [Preview Abstract] |
Monday, March 13, 2017 3:06PM - 3:42PM |
C48.00004: Control of Exciton Valley Coherence in Transition Metal Dichalcogenide Monolayers Invited Speaker: Gang Wang Current research on Transition Metal Dichalcogenide (TMD) Monolayers is stimulated by their strong light-matter interaction and the possibility to use the valley index in addition to spin as an information carrier. The direct gap interband transitions in TMD monolayers are governed by chiral optical selection rules. Determined by laser helicity, optical transitions in either the K+ or K- valley in momentum space are induced. Very recently the optical generation of valley polarization and valley coherence (coherent superposition of valley states) have been reported. In this work we go a step further by discussing the coherent manipulation of valley states. Linearly polarized laser excitation prepares a coherent superposition of valley states. We demonstrate the control of the exciton valley coherence in monolayer WSe2 by tuning the applied magnetic field perpendicular to the monolayer plane [1]. The induced valley Zeeman splitting between K+ and K- results in a change of the oscillation frequency of the superposition of the valley states, which corresponds to a rotation of the exciton valley pseudo-spin. We show rotation of this coherent superposition of valley states by angles as large as 30 degrees in applied fields up to 9T and discuss valley coherence in other TMD monolayer materials. This exciton valley coherence control on ps time scale could be an important step towards complete control of qubits based on the valley degree of freedom. [1] G. Wang et al, Phys. Rev. Lett. 117, 187401 (2016). [Preview Abstract] |
Monday, March 13, 2017 3:42PM - 3:54PM |
C48.00005: Spin/valley dynamics of resident electrons and holes in gated monolayer WSe$_{\mathrm{2}}$ Prasenjit Dey, Luyi Yang, Scott Crooker, Cedric Robert, Gang Wang, Bernhard Urbaszek, Xavier Marie Robust spin-valley coupling is a key prerequisite for next generation spintronic and valleytronic devices. Monolayer transition metal dichalcogenides provide an excellent platform to explore the spin-valley physics of electrons, holes and excitons. In contrast to the short (picosecond) recombination lifetimes of excitons and trions, recent studies of electron-doped MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ monolayers demonstrated surprisingly long (nanosecond) spin lifetimes and spin coherence of resident electrons [1,2]. Here we use continuous wave (CW) and time-resolved Kerr rotation spectroscopy (TRKR) to explore the spin and valley polarization dynamics in electrostatically-gated crystals of exfoliated monolayer WSe$_{\mathrm{2}}$. Long-lived polarization dynamics of both resident electrons and holes are observed. Measurements as a function of applied magnetic field, temperature, and carrier doping density will be presented. [1] Yang \textit{et al.}, \textit{Nature Phys. }\textbf{11}, 830 (2015). [2] Yang \textit{et al.}, \textit{Nano Lett. }\textbf{15}, 8250 (2015). [Preview Abstract] |
Monday, March 13, 2017 3:54PM - 4:06PM |
C48.00006: Valley addressable exciton-polaritons in atomically thin MoSe$_2$ Scott Dufferwiel, T. P. Lyons, D. D. Solynshkov, A. A. P. Trichet, F. Withers, S. Schwarz, G. Malpuech, J. M. Smith, K. S. Novoselov, M. S. Skolnick, D. N. Krizhanovskii, A. I. Tartakovskii While conventional semiconductor technology relies on the manipulation of electrical charge for the implementation of computational logic, additional degrees of freedom such as spin offer alternative avenues for the encoding of information. In TMD monolayers, where spin-valley locking is present, strong retention of valley chirality has been reported for MoS$_2$, WSe$_2$ and WS$_2$ while MoSe$_2$ shows low polarization retention. Here, we show that by placing monolayers of MoSe$_2$ in an optical microcavity in the strong light-matter coupling regime, the valley polarization is regained with an enhancement of up to 7X compared with the bare monolayer. Here, polaritons introduce a fast relaxation mechanism which inhibits full valley spin relaxation of reservoir excitons due to the Maialle-Sham mechanism and allows for increased retention of polarization. A dynamical model reproduces the detuning dependence through the incorporation of cavity-modified exciton relaxation, allowing an estimate of the spin relaxation which is an order of magnitude faster than those reported in other TMDs. The valley addressability demonstrates the prospect of valleytronic devices based upon MoSe$_2$ embedded in photonic structures. [Preview Abstract] |
Monday, March 13, 2017 4:06PM - 4:18PM |
C48.00007: Long-lived hole spin/valley polarization probed by time-resolved Kerr rotation Xinlin Song, Saien Xie, Kibum Kang, Jiwoong Park, Vanessa Sih Time-resolved Kerr rotation and photoluminescence measurements are performed on MOCVD-grown monolayer tungsten diselenide (WSe$_{\mathrm{2}})$. We observe a long-lived Kerr rotation signal (around 80 ns) at 10 K, which is attributed to spin/valley polarization of the resident holes. This polarization is robust to transverse magnetic field (up to 0.3 T) due to spin-orbit spin stabilization. Temperature-dependent photoluminescence measurements show a transition from free exciton emission to localized exciton emission as the temperature decreases. Wavelength-dependent measurements reveal that only excitation near the free exciton energy generates this long-lived spin/valley polarization. The long polarization lifetime supports the promise of transition metal dichalcogenides for proposed spintronic and valleytronic devices and opens new investigations aimed at controlling spin/valley polarization in the valence band. (Nano Lett., 2016, 16(8), pp 5010-5014) [Preview Abstract] |
Monday, March 13, 2017 4:18PM - 4:30PM |
C48.00008: Spin-dependence photocurrent for monolayer MoS2 with ferromagnetic contacts Xiao-Xiao Zhang, Emma Dohner, You Lai, Dmitry Smirnov, Tony Heinz In this paper, we examine charge transport between a monolayer of a semiconducting transition metal dichalcogenide (TMDC) and ferromagnetic contacts. Using excitation of the TMDC (MoS$_2$) monolayer at the optical band gap with circularly polarized light, we preferentially create photocarriers with a given spin state. We then compare the photocurrent collected by the ferromagnetic contacts for different handednesses of the excitation to characterize the spin dependence of the process. Consistent with a previous report in WS$_2$[1], we observe a significant modulation of the photocurrent with carrier spin. This effect is confirmed by switching the alignment of the ferromagnetic domains in the contact by an external magnetic field under the same photoexcitation conditions. We discuss the implications of our results for spin diffusion lengths. In addition, for comparison, samples with non-ferromagnetic contacts were measured as a function of an applied out-of-plane magnetic field. A similar dependence of the photocurrent on the polarization state of the light was observed in this case. This effect is, however, only present at a much higher magnetic field strengths and is of a different physical origin. [1]L. Xie and X. Cui, PNAS. USA 113, 3746â3750 (2014) [Preview Abstract] |
Monday, March 13, 2017 4:30PM - 4:42PM |
C48.00009: Van der Waals Engineering of Ferromagnetic Semiconductor Heterostructures for Spin and Valleytronics Ding Zhong, Kyle Seyler, Xiayu Linpeng, Ran Cheng, Nikhil Sivadas, Bevin Huang, Emma Schmidgall, Takashi Taniguchi, Kenji Watanabe, Michael McGuire, Wang Yao, Di Xiao, Kai-Mei Fu, Xiaodong Xu Monolayer transition metal dichalcogenides host easily accessible spin and valley degrees of freedom that can be used to encode and process information. With the advent of van der Waals heterostructures, there are new opportunities to engineer spin and valleytronic devices with more advanced functionalities. In this talk, we will describe a van der Waals heterostructure composed of a monolayer semiconductor, WSe2, and an ultrathin layered ferromagnetic semiconductor, CrI3. The integration of the two materials enables a strong magnetic proximity effect in WSe2 and spin-selective charge transfer from WSe2 to CrI3. Our photoluminescence measurements reveal large valley splitting at zero applied magnetic field, as well as rapid switching of WSe2 valley splitting and polarization within small changes of the applied magnetic field. Moreover, the photoluminescence detection of WSe2 valley pseudospin provides us with a simple yet powerful tool to probe the magnetization dynamics in the ultrathin CrI3. [Preview Abstract] |
Monday, March 13, 2017 4:42PM - 4:54PM |
C48.00010: Gate-controllable magneto-optic Kerr effect in layered collinear antiferromagnets Nikhil Sivadas, Satoshi Okamoto, Di Xiao Using symmetry arguments and a tight-binding model, we show that for layered collinear antiferromagnets, magneto-optic effects can be generated and manipulated by controlling crystal symmetries through a gate voltage. This provides a promising route for electric field manipulation of the magneto-optic effects without modifying the underlying magnetic structure. We further demonstrate the gate control of magneto-optic Kerr effect (MOKE) in bilayer MnPSe$_3$ using first-principles calculations. The field-induced inversion symmetry breaking effect leads to gate-controllable MOKE whose direction of rotation can be switched by the reversal of the gate voltage. [Preview Abstract] |
Monday, March 13, 2017 4:54PM - 5:06PM |
C48.00011: Manipulating spin-orbit coupled electrons in MoS2 Edward Aris Fajardo, Roland Winkler Using group theory, we derive the invariant expansion for the effective Hamiltonian of the electron states near the $K$ points of monolayer MoS$_2$. Our approach fully includes both spin-orbit coupling and the effect of perturbations such as external (or built-in) electric and magnetic fields and strain. It provides a systematic strategy for manipulating the spin-orbit coupled electron dynamics in MoS$_2$ and other transition metal dichalcogenides. [Preview Abstract] |
Monday, March 13, 2017 5:06PM - 5:18PM |
C48.00012: Exchange field induced valley splitting in monolayer WSe$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ Chuan Zhao, Peiyao Zhang, Tenzin Norden, Renat Sabirianov, George Kioseoglou, Athos Petrou, Hao Zeng Monolayer TMDCs such as WSe$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ are of great interest for valleytronics applications. The broken inversion symmetry leads to two degenerate but inequivalent valleys K and K'. Together with strong spin-orbit coupling which splits the band edge states, K and K' valleys have opposite spin characters, making it possible to selectively excite carriers within a particular valley using circularly polarized light. Lifting the valley degeneracy allows for control of valley polarization by an electric field. We demonstrated recently that the exchange field from a ferromagnetic EuS substrate can induce strongly enhanced valley splitting in monolayer WSe$_{\mathrm{2}}$. In this work we show that exchange field effect is dramatically different from that of an external field. We measured the ``A'' and ``B'' exciton transition energies of both WSe$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ on EuS substrates using magneto-reflectance spectroscopy. We observed enhanced valley splitting for both samples, yet with opposite signs of the splitting. Moreover, the signs of valley splitting are opposite for ``A'' and ``B'' excitons. We attribute these observations to a transition from ferromagnetic to anti-ferromagnetic exchange coupling between EuS and different TMDC materials, since the exchange energy is highly sensitive to interatomic spacing. Using magnetic exchange field therefore provides an attractive avenue for valley control in TMDCs beyond what can be achieved by an external field. [Preview Abstract] |
Monday, March 13, 2017 5:18PM - 5:30PM |
C48.00013: Giant paramagnetism induced valley polarization of electrons in charge tunable monolayer MoSe2 Patrick Back, Meinrad Sidler, Atac Imamoglu Transition metal dichalcogenide monolayers such as MoSe2 are strictly two-dimensional direct band-gap semiconductors with a graphene-like honeycomb lattice structure leading to an emergent valley pseudospin degree of freedom. Even though understanding the limits of controllability of the valley pseudospin degree of freedom is of central interest for applications, progress to date has been hindered by the difficulty in obtaining a high-degree of valley polarization of electrons or holes. In this work, we use optical spectroscopy to demonstrate that application of moderate magnetic fields lead to near-complete valley polarization of electrons with densities as high as 1.6x10$^{\mathrm{12}}$ cm$^{\mathrm{-2}}$. This unexpected behavior is a direct consequence of super paramagnetic (or valleytronic) response of conduction band electrons. Our experiments pave the way for experiments exploiting the valley degree of freedom of charged carriers. [Preview Abstract] |
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