Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A35: 2D Materials - Spins and ValleysFocus
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Sponsoring Units: DMP Chair: Simranjeet Singh, Ohio State Univ - Columbus Room: LACC 409B |
Monday, March 5, 2018 8:00AM - 8:36AM |
A35.00001: Proximity physics in graphene: spin-orbit coupling, exchange, field effects, and pseudohelical states Invited Speaker: Jaroslav Fabian Graphene and novel 2d materials offer new perspectives for spintronics [1]. Since graphene itself has no band gap, its spintronic applications will be limited as a highly efficient spin transfer channel. Instead, heterostructures of graphene and two-dimensional transition-metal dichalcogenides (TMDC) are emerging as systems in which both orbital and spin properties can be controlled by gating, thus offering a materials basis for spintronic applications, such as optospintronics [2]. But these van der Waals stacks also yield interesting fundamental physics. For example, graphene on WSe2 exhibits giant spin anisotropy, and is predicted to support protected pseudohelical states in flakes [4], with a bulk spin-orbit gap of about 1 meV. Even more fascinating is bilayer graphene on TMDCs, as the spin properties of this material can be controlled (turned ON and OFF) by gate voltage, creating a platform for spin-orbit valves and spin transistors [5]. Finally, I will also talk about magnetic proximity effects in graphene on ferromagnetic insulators. Here too bilayer graphene is predicted to yield field-effect |
Monday, March 5, 2018 8:36AM - 8:48AM |
A35.00002: Large proximity-induced spin lifetime anisotropy in monolayer transition metal dichalcogenide/graphene heterostructures Alexey Kaverzin, Talieh Ghiasi, Josep Ingla-Aynés, Bart Van Wees Van-der-Waals heterostructures of layered materials become more and more popular as they offer a powerful playground that allows for a simple method for combining the properties of two different materials while searching for a desired functionality. An outstanding example is a double layer hybrid that consists of a transition metal dichalcogenide (TMD) and graphene. In this case the excellent transport properties of graphene are enriched with a large and anisotropic spin-orbit coupling of TMD. |
Monday, March 5, 2018 8:48AM - 9:00AM |
A35.00003: Optimal charge-to-spin conversion in graphene on transition metal dichalcogenide Manuel Offidani, Aires Ferreira, Mirco Milletari, Roberto Raimondi When graphene is paired with a semiconducting transition metal dichalcogenide (TMD) monolayer |
Monday, March 5, 2018 9:00AM - 9:12AM |
A35.00004: Spin accumulation and dynamics in inversion-symmetric transition metal dichalcogenides Marcos Guimaraes, Bert Koopmans Transition metal dichalcogenides (TMDs) are fascinating materials for spin-logic devices due to their large spin-orbit coupling and their spin-valley locking. Recently, it has been proposed that optical generation of spin accumulation in inversion symmetric TMDs is possible due to a layer-dependent hidden spin polarization[1,2]. Time-resolved photoemission studies showed that the optically generated spin accumulation in the conduction band of the surface layer of bulk TMD crystals decays very fast, in the order of 100s of fs, due to high scattering rates where the electron loses its layer polarization[3]. By using ultrafast time-resolved Kerr rotation and transient reflectivity experiments in WSe2 and MoSe2 we are able to probe the spin and carrier dynamics of the whole crystal and unveil a spin-layer relaxation time of tens of ps at low temperatures, which is reduced, but still sizeable (a few ps) at room temperature. By temperature dependence measurements and comparing the two materials with different spin-orbit coupling strengths we are able to unveil the different spin relaxation mechanisms in inversion-symmetric TMDs. |
Monday, March 5, 2018 9:12AM - 9:48AM |
A35.00005: Optospintronics and Magnetism with 2D Materials and Heterostructures Invited Speaker: Roland Kawakami I will review our latest developments in spintronics, optospintronics and magnetism in two-dimensional (2D) materials and heterostructures. Graphene continues to exhibit improved properties for spin transport and demonstrates additional functionality through the use of vertically stacked heterostructures. One of the interesting new directions is optospintronics enabled by heterostructures of graphene and transition metal dichalcogenides (TMD). Due to the valley optical selection rules of TMDs and the large spin-orbit coupling, the helicity of the photon is coupled to the valley spin polarization of electrons. Thus, circularly polarized optical excitation into a TMD/graphene heterostructure generates spin polarization in the TMD that subsequently transfers to the graphene. This optical spin injection into graphene is an example of new functionality for the expanding field of 2D spintronics. Also worthy of note within the field is the development of van der Waals magnets in the 2D limit. I will discuss our work along these lines as well as recent efforts to utilize spin-polarized scanning tunneling microscopy for the atomic scale imaging of magnetic order. |
Monday, March 5, 2018 9:48AM - 10:00AM |
A35.00006: Valley and Spin Resolved Photocurrent in Atomically Thin MoS2 Xiaoxiao Zhang, You Lai, Emma Dohner, Dmitry Smirnov, Tony Heinz The ability to access the coupled valley-spin indexes in transition metal dichalcogenide monolayers is one of the most intriguing features of these 2D semiconductors. Control and manipulation of the valley properties have been recently been achieved, but these approaches rely mostly on optical measurements and sensing. Here we demonstrate a new method for electrical readout of optically generated valley polarization based on breaking the valley degeneracy with an out-of-plane magnetic field Bz. At the same time, we detect the spin alignment of carriers electrically using the established approach of spin-sensitive ferromagnetic contacts. In our experiments, the presence of Bz is found to induce a significant modulation in the photocurrent from monolayer MoS2 as a function of the handedness of the circularly polarized excitation. We attribute this effect to the unbalanced transport of valley polarized trions, which experience opposite Zeeman shifts in the K and K’ valleys. Our interpretation is further supported by a comparison to the behavior in samples of the 2H bilayer MoS2, with its higher symmetry. The efficient tuning and detection of the valley-polarized photocurrent opens up new directions to develop optoelectronic devices making use of the valley degree of freedom. |
Monday, March 5, 2018 10:00AM - 10:12AM |
A35.00007: Renormalization of the spin-orbit splitting in single-layer WS2 on h-BN Jyoti Katoch, Søren Ulstrup, Roland Koch, Simon Moser, Kathleen McCreary, Simranjeet Singh, Jinsong Xu, Berend Jonker, Roland Kawakami, Aaron Bostwick, Eli Rotenberg, Chris Jozwiak Transition metal dichalcogenides (TMDs) are of great interest due to a thickness dependent indirect to direct bandgap transition, large spin-orbit coupling, and spin-valley locking. In monolayer limit the TMDs have strong Coulomb interactions due to poor dielectric screening, resulting in strongly bound excitons. Interestingly, the electronic, optical and spin-valley related properties of TMDs can be tuned by surrounding dielectric environment and adatom doping due to many-body effects. We investigated the electronic band structure of single-layer (SL) WS2 on h-BN by angle-resolved photoemission (ARPES) with spatial resolution of 10μm sized spatial resolution of the μARPES endstation at the MAESTRO facility of the Advanced Light Source. We will discuss the effect of screening and ad-atom spin-orbit coupling on the trionic quasiparticles in the spectral function of SL WS2. Upon electron doping via alkali metal we observed giant spin-orbit splitting and bandgap renormalization in SL WS2. |
Monday, March 5, 2018 10:12AM - 10:24AM |
A35.00008: Tuning Spin-orbit Coupling in Few-layer InSe Kasun Viraj Madusanka Nilwala Gamaralalage Premasiri, Sukrit Sucharitakul, Rajesh Kumar, R. Sankar, Fangchang Chou, Yit-Tsong Chen, Xuan Gao Manipulating the spin of electrons with the aid of Rashba spin-orbit coupling (SOC) is an indispensable element of spintronics. Electrostatically gating a material with strong Rashba SOC results in an effective magnetic field which in turn can be used to govern the spin of electrons. Hereupon we report the existence of the Rashba SOC in few-layer InSe; and its ability to be tuned electrostatically. We observed weak-antilocalization (WAL) effect in magnetotransport studies of InSe, which is indicative of strong SOC. The WAL effect was utilized to extract phase coherence length (42-63 nm) and spin-orbit relaxation length (40-51 nm). We also demonstrate gate-controlled Rashba SOC with Rashba coefficient changing from 1.03 to 1.28 (×10-11 eVm). This SOC manipulation of InSe may serve pivotally in devising InSe-based 2D-spintronic devices in the future. |
Monday, March 5, 2018 10:24AM - 10:36AM |
A35.00009: Comparing the spin/valley dynamics of resident carriers in gated WSe2 and MoSe2 monolayers Prasenjit Dey, Mateusz Goryca, Scott Crooker, Cedric Robert, Bernhard Urbaszek, Xavier Marie The ability to populate and probe specific valleys in monolayer transition-metal dichalcogenides (TMDs) using polarized light has revived long-standing interests in valleytronics. An essential question therefore concerns the spin/valley relaxation timescales of the resident electrons and resident holes (not excitons) that exist in n-type or p-type TMDs. Using time-resolved Kerr rotation, we recently demonstrated extraordinarily long (microsecond) valley polarization lifetimes for resident holes in electrostatically-gated WSe2 monolayers in the p-type regime [1]. Here we compare the spin/valley dynamics of resident electrons and holes in monolayer WSe2 with its molybdenum-based counterpart, MoSe2. Importantly, WSe2 and MoSe2 monolayers exhibit conduction band spin-orbit splittings (Δc) of opposite sign, which can influence the measured dynamics, particularly in the weakly n-type regime. [1] P. Dey et al., PRL 119, 137401 (2017). |
Monday, March 5, 2018 10:36AM - 10:48AM |
A35.00010: Universal spin dynamics in quantum wires Edward Aris Fajardo, Ulrich Zuelicke, Roland Winkler We discuss the universal spin dynamics in quasi one-dimensional systems including the real spin in narrow-gap semiconductors like InAs and InSb, the valley pseudospin in staggered single-layer graphene, and the combination of real spin and valley pseudospin characterizing single-layer transition metal dichalcogenides (TMDCs) such as MoS2, WS2, MoSe2, and WSe2. All these systems can be described by the same Dirac-like Hamiltonian. Spin-dependent observable effects in one of these systems thus have counterparts in each of the other systems. Effects discussed in more detail include equilibrium spin currents, current-induced spin polarization (Edelstein effect), and spin currents generated via adiabatic spin pumping. Our work also suggests that a long-debated spin-dependent correction to the position operator in single-band models should be absent. |
Monday, March 5, 2018 10:48AM - 11:00AM |
A35.00011: Optospintronics in graphene via proximity coupling Ahmet Avsar, Dmitrii Unuchek, Jiawei Liu, Oriol Lopez Sanchez, Kenji Watanabe, Takashi Taniguchi, Barbaros Özyilmaz, Andras Kis Observation of micron size spin relaxation lengths makes graphene an ideal material for spintronics applications. However, spin dependent scattering at the contact/graphene interfaces causes low spin injection efficiencies. An attractive solution would be employing optical spin injection schemes but the tiny spin orbit coupling and low optical absorption of graphene is a hurdle. |
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