Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J31: Focus Session: Spin-Dependent Phenomena in Semiconductors: Probing Spins in 2D Materials and Semiconductors |
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Sponsoring Units: GMAG DMP FIAP Chair: Aubrey Hanbicki, Naval Research Laboratory Room: 207A |
Tuesday, March 3, 2015 2:30PM - 3:06PM |
J31.00001: Tightly bound excitons in monolayer transition metal dichalcogenides Invited Speaker: Jie Shan Atomically thin transition metal dichalcogenides (TMDs) MX$_{2}$ (M = Mo, W, X = S, Se, Te) have emerged as a new class of two-dimensional (2D) semiconductors. Monolayer TMDs possess a honeycomb lattice structure with broken inversion symmetry that permits interesting valley dependent optical selection rules and Berry curvature effects. Yet, another very distinctive feature of electrons in 2D semiconductors is the significantly reduced dielectric screening of Coulomb interactions. An important consequence of strong Coulomb interactions is the formation of tightly bound excitons. They significantly modify the optical response of the material and play a defining role in the optoelectronic processes. In this talk, I will discuss our recent experiments on the binding energy of excitons and trions and their spin/valley properties in TMDs. [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:18PM |
J31.00002: Direct observation of spin-to-charge conversion in MoS2 monolayer with spin pumping Cheng Cheng, Viktoria Ivanovskaya, Juan-Carlos Rojas Sanchez, Bruno Dlubak, Pierre Seneor, Young Hee Lee, Ganghee Han, Hyun Kim, Heejun Yang, Abdelmadjid Anane Unlike graphene, layered transition-metal dichalcogenides are 2D wide bandgap semiconductors with large intrinsic spin-orbit coupling (SOC) and valley-spin coupling, which makes them a unique playground for spintronics. We present here the first demonstration of spin injection into monolayer MoS2 with spin pumping from a 3D ferromagnetic (FM) film, circumventing the impedance mismatch at the metal-semiconductor interface. We measured the transverse voltage generated by spin-to-charge current conversion in MoS2 with broadband (3 GHz- 9 GHz) ferromagnetic resonance (FMR) setup. The observed symmetric Lorentzian signals are in 1$\mu$V range under small rf excitations well below 1 Oe. This voltage magnitude is unexpected for inverse spin Hall effect and is interpreted in the frame of inverse Rashba-Edelstein effect (iREE) due to strong SOC in MoS2. By applying a moderate gate voltage (up to 10 V) on the MoS2/FM multilayer, we observe clear modulation (up to 30\%) of the linewidth and amplitude of the iREE signal, indicating electrical tuning of the spin mixing conductance. [Preview Abstract] |
Tuesday, March 3, 2015 3:18PM - 3:30PM |
J31.00003: Proximity Induced Exchange Splitting in Graphene Shanshan Su, Yafis Barlas, Roger Lake We perform an ab-initio study of the proximity effect in a two-dimensional (2D) heterostructure composed of graphene and a thin film ferromagnetic insulator (europium oxide, EuO). Two different structures are considered i) graphene on a EuO layer and ii) graphene sandwiched between two EuO layers. Both structures show two-fold degenerate low-energy bands at the $\Gamma $ point in the Brillouin zone, however, the former heterostructure shows a clear energy gap in the spectrum whereas the latter exhibits degenerate band crossings. The two different spectra result from a competition of proximity induced exchange splitting on the graphene sheet and sub-lattice mass induced due to the crystal field effect. Addition of spin-orbit coupling in the sandwiched structure indicates lifting of this two-fold degeneracy leading to band anti-crossings if the inversion symmetry perpendicular to the graphene plane is broken. [Preview Abstract] |
Tuesday, March 3, 2015 3:30PM - 3:42PM |
J31.00004: Gate-voltage controlled spin pumping effects: spin injection from YIG and Co into metal and graphene based 2 D materials Alan Kalitsov, Mairbek Chshiev, Oleg Mryasov Spin current injection into nonmagnetic metals, semiconductors and oxides is crucial component of spintronics. The spin pumping mechanism free from the impedance mismatch is a promising way to inject spin current into nonmagnetic materials [1]. Here we present theory of spin current injected into non-magnetic films which arises from magnetization precession. We apply this theory to two cases (i) insulating yttrium iron garnet ferromagnet/nonmagnetic metal interfaces and (ii) hcp-Co/single layer graphene interface. The electron transport calculations are based on the non-equilibrium Green Function formalism within the tight binding Hamiltonian model [2]. We show that magnitude of the pumped spin current can be efficiently controlled by the gate voltage. \\[4pt] [1] K. Ando, S. Takahashi, J. Ieda, H. Kurebayashi, T. Trypiniotis, C. H. W. Barnes, S. Maekawa and E. Saitoh, Nature Mater. \textbf{10}, 655 (2011).\\[0pt] [2] S.-H. Chen, C.-R. Chang, J. Q. Xiao and B. K. Nikolic, Phys. Rev. B \textbf{79}, 054424 (2009). [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J31.00005: Cross-correlation spin noise spectroscopy of interacting multi-component spin systems Luyi Yang, Dibyendu Roy, Scott Crooker, Nikolai Sinitsyn Interacting multi-component spin systems are ubiquitous in semiconductor spintronics; $e.g$. carrier-mediated ferromagnetism in magnetic semiconductors, or electronic spin coupling to nuclear spin baths. Traditionally, inter-species spin interactions are studied by experimental methods that are necessarily perturbative: $e.g.$, by intentionally polarizing or depolarizing one spin species and detecting the response of the other(s). Here, we show that multi-probe spin noise spectroscopy can reveal interspecies spin-spin interactions -- under conditions of strict thermal equilibrium -- by cross-correlating the stochastic fluctuation signals exhibited by each of the constituent spin species. We develop a theory for such noise cross-correlations in thermal equilibrium. As a proof of principle, we compare the results with an experimental study of a well-understood interacting spin system -- a mixture of warm Rb and Cs vapors -- by applying a new type of \textit{two-color} spin noise spectroscopy [1,2]. Noise correlations directly reveal the presence of inter-species spin exchange interactions. Such non-invasive and noise-based techniques should be generally applicable to any multi-component spin system in which the fluctuations of the constituent components are detectable.\\[4pt] [1] D. Roy, L. Yang, S.A. Crooker, N.A. Sinitsyn, arXiv:1408.5399 (2014).\\[0pt] [2] L. Yang \textit{et al}., Nat. Comm. \textbf{5}:4949 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 3:54PM - 4:06PM |
J31.00006: A method for wide-bandwidth spin noise measurements without a large background Brennan C. Pursley, Xinlin Song, Vanessa Sih Spin noise measurements have rapidly evolved over the last few decades to become a class of highly sensitive characterization tools [1,2]. Presently used methods, though quite sensitive, suffer from large backgrounds. We report on an experiment that yields signal proportional to the spin noise, without further processing. Our demonstration utilizes ultrafast optical techniques, but the signal processing could also be applied to pulsed electrical measurements. We will discuss the experiment, necessary equipment, and data sets from a GaAs sample. \\[4pt] [1] V. S. Zapasskii, Adv. Opt. Photonics 5, 131 (2013).\\[0pt] [2] J. H\"{u}bner, F. Berski, R. Dahbashi, and M. Oestreich, Phys. Status Solidi 251, 1824 (2014). [Preview Abstract] |
(Author Not Attending)
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J31.00007: Detection of quantum entanglement using cross-correlation spin noise spectroscopy Dibyendu Roy, Nikolai A. Sinitsyn Nature of spin-spin correlations between two interacting spin subsystems in thermal equilibrium can be probed using cross-correlation spin noise spectroscopy [1] with two linearly polarized off-resonant laser beams of different wavelength. Here we propose to use such cross-correlator of spins to detect and quantify quantum correlations such as quantum entanglement between the spin subsystems. We demonstrate our proposal for interacting quantum dots. \\[4pt] [1] D. Roy, L. Yang, S. A. Crooker, N. A. Sinitsyn, arXiv:1408.5399 [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J31.00008: Effects of Strain and Quantum Confinement in Optically Pumped Nuclear Magnetic Resonance in GaAs: Interpretation Guided by Spin-Dependent Band Structure Calculations Clifford Bowers, Ryan Wood, Saha Dipta, John Tokarski, Lauren McCarthy, Gary Sanders, Christopher Stanton, Stephen McGill, Arneil Reyes, Phil Kuhns, John Reno A combined experimental-theoretical study of optically pumped NMR (OPNMR) has been performed in a GaAs/Al$_{\mathrm{0.1}}$Ga$_{\mathrm{0.9}}$As quantum well film epoxy bonded to a Si substrate with thermally induced biaxial strain. The photon energy dependence of the Ga OPNMR signal was recorded at magnetic fields of 4.9 and 9.4 T at a temperature of 4.8-5.4 K. The data were compared to the nuclear spin polarization calculated from the differential absorption to spin-up and spin-down states of the electron conduction band using a modified Pidgeon Brown model. Comparison of theory with experiment facilitated the assignment of features in the OPNMR energy dependence to specific interband Landau level transitions. The results provide insight into how effects of strain and quantum confinement are manifested in optical nuclear polarization in semiconductors. [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J31.00009: Orbital Angular Momentum Imprints Studied Using Optical Vortex Pump-probe Spectroscopy M. A. Noyan, A. L. Exarhos, J. M. Kikkawa We introduce a second generation magneto-optical spectroscopy based on orbital angular momentum of light. Our technique is analogous to methods such as time resolved Faraday/Kerr rotation, but instead of utilizing photon spin, we use holographic gratings to pump and probe materials using photons carrying \emph{orbital} angular momentum (OAM). We will discuss our first time resolved experiments studying pump-induced OAM dichroism in bulk semiconductors. 100 fs pump pulses with alternating orbital angular momentum $\pm\hbar$ create OAM imprints whose momentum distributions resemble right- or left-handed ``whirlpools'' or vortices. The OAM memory of the sample is then measured using a probe beam whose $\pm\hbar$ OAM components are detected in a balanced photodiode bridge. We find that in n-GaAs, the dynamical OAM signal shows a unique timescale when compared to population and spin dynamics and, surprisingly, lasts considerably longer than the momentum scattering time. This method should be of further interest for studying non-equilibrium dynamics in a variety of orbitally coherent systems. [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 4:54PM |
J31.00010: Room-Temperature Magneto-Optical Phenomena in Organo-Metal Halide Perovskites. Ting Wu, Yu-Che Hsiao, Mingxing Li, Nam-Goo Kang, Jimmy Mays, Bin Hu Organo-metal halide perovskites have become extremely interesting light-emitting, photovoltaic and lasing materials. The first magneto-optical phenomena, namely magneto-absorption effects, were reported in 1994 for very low temperature and high field (20 Tesla). Here, we report room-temperature magneto-optical phenomena at low field (\textless 200 mT) from such perovskites. We find that room-temperature magneto-optical effects require high excitation intensity. At high excitation intensities we can observe magnetic field effects on photoluminescence, photocurrent, and electroluminescence. The magneto-optical phenomena indicate that both light-emitting and photovoltaic responses undergo a spin-dependent process. Furthermore, at low excitation intensities organo-metal halide perovskites exhibit negligible magnetic field effects. Therefore, we can conclude that the magneto-optical phenomena are from spin-dependent charge recombination in light-emitting and photovoltaic processes. This presents a new mechanism to control the light-emitting and photovoltaic functions in organo-metal halide perovskites by using spins. This presentation will discuss the key parameters in controlling magneto-optical phenomena in organo-metal halide perovskites. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J31.00011: Laser-induced ultrafast spin dynamics and {ERASE} functionality on quasilinear molecular ions Georgios Lefkidis, Chun Li, Shaobin Zhang, Wolfgang H\"{u}bner We present an \emph{ab initio} investigation of the $\Lambda$-process-based ultrafast spin manipulation on positively charged two-magnetic-center molecular ions bridged by non-magnetic oxygen\footnote{C. Li, S. Zhang, G. Lefkidis, and W. H\"{u}bner, Phys. Rev. B {\bf 89}, 184404 (2014)}. Multiple derived spin-switching and spin-transfer scenarios on the quasilinear structure [Fe-O-Co]$^{+}$ are used to build two closed, irreversible spin-dynamics cycles with respect to the spin localization and orientation. A mechanism addressing the ``ERASE'' functionality is proposed by properly exploiting the irreversibility of some laser-induced spin-manipulation scenarios, and the resulting Shannon entropy change is analyzed. We compare with a presiously suggested mechanism based on chirped laser pulses\footnote{G. P. Zhang, G. Lefkidis, W. H\"{u}bner, and Y. Bai, J. Appl. Phys. {\bf 111}, 07C508 (2012)}. Such controllable spin-dynamics cycles and logic functionality demonstrate promising applications in the design of spintronic devices on isolated magnetic molecules\footnote{ W. Jin, F. Rupp, K. Chevalier, M. M. N. Wolf, M. Colindres Rojas, G. Lefkidis, H.-J. Kr\"{u}ger, R. Diller, and W. H\"{u}bner Phys. Rev Lett. {\bf 109}, 267209 (2012) }. [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J31.00012: Towards high-frequency operation of spin-lasers Paulo E. Faria Junior, Gaofeng Xu, Jeongsu Lee, Nils C. Gerhardt, Guilherme M. Sipahi, Igor Zutic Injecting spin-polarized carriers in lasers enables room-temperature spintronic applications,not limited to magnetoresistance. While steady-state operation of such spin-lasers has already revealed an improved operation as compared to their conventional (spin-unpolarized) counterparts [1-4], the main opportunities lie in their-high frequency operation [5-9]. We systematically show how our accurate electronic structure and microscopic gain calculations could guide the improved dynamical operation of spin-lasers at modulation frequencies beyond what is possible in conventional lasers [6-9]. [1] J. Rudolph et al., Appl. Phys. Lett. 87, 241117 (2005). [2] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007). [3] J. Frougier et al., Appl. Phys. Lett. 103, 252402 (2013). [4] J.-Y. Chen et al., Nature Nanotech. 9, 845 (2014). [5] J. Lee et al., Appl. Phys. Lett. 97, 041116 (2010). [6] N. C. Gerhardt, et al., Appl. Phys. Lett. 99, 151107 (2011). [7] H. Hopfner et al., Appl. Phys. Lett. 104, 022409 (2014). [8] G. Boeris et al., Appl. Phys. Lett. 100, 121111 (2012). [9] J. Lee et al., Appl. Phys. Lett. 105, 042411 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 5:18PM - 5:30PM |
J31.00013: Topological transitions in the geometric phase in spin interferometers Henri Saarikoski, Enrique Vazquez, Jose Pablo Baltanas, Diego Frustaglia, Fumiya Nagasawa, Junsaku Nitta An electronic spin transported around a circuit acquires a phase factor that depends on the geometry of the path in the parameter space. In the adiabatic limit this is the Berry phase and it has been argued that it can undergo an abrupt transition via manipulation of the topology of the path [1]. However, spin transport in mesoscopic structures is usually nonadiabatic, which is associated with the Aharonov-Anandan geometric phase. Here we identify the characteristic signatures of topological transitions in nonadiabatic spin transport by 1D and 2D calculations of mesoscopic loops. We find that the topological transition is characterized by an effective Berry phase due to correlations between dynamic and geometric phases close to the region where the transition occurs. This effective Berry phase is related to the topology of the field texture rather than the spin-state structure. The transition manifests as a distinct dislocation of the interference pattern in the quantum conductance. The phenomenon is robust, and can be observed in mesoscopic arrays of loops where phase coherence is significant. [1] Y. Lyanda-Geller, Phys. Rev. Lett. 71, 657 (1993). [Preview Abstract] |
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