Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session R21: Spin Dynamics in Organic-Inorganic Hybrids and Semiconductor NanostructuresFocus
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Sponsoring Units: GMAG DMP FIAP DCOMP Chair: Ezekiel Johnston-Halperin, Ohio State Univ - Columbus Room: LACC 309 |
Thursday, March 8, 2018 8:00AM - 8:36AM |
R21.00001: Spin-polarized exciton quantum beating in hybrid organic-inorganic perovskites Invited Speaker: Yan Li The hybrid organic-inorganic perovskites are an emerging class of semiconductors that have excellent optoelectronic properties, even being solution processed. These compounds also hold great promise for the field of spintronics due to their large and tunable spin-orbit coupling, spin-dependent optical selection rules, and predicted electrically tunable Rashba spin splitting. Here we demonstrate the optical orientation and quantum beating of spin-polarized excitons in polycrystalline films of MAPbClxI3-x using a time-resolved Faraday rotation measurement [1]. The spin lifetime exceeds 1 ns in zero magnetic field at 4 K, which is quite surprising given that Pb and I exhibit large spin-orbit couplings. Application of a transverse magnetic field causes quantum beating at two distinct frequencies, and the approximate linear relationships give two g-factors, which we assign to electrons and holes as ge = 2.63, and gh = -0.33 according to an exciton model. The energy dependence of the Faraday rotation follows the exciton absorption band at low temperatures, confirming its excitonic origin. Spin relaxation mechanisms and unusual exciton spin physics will also be discussed. |
Thursday, March 8, 2018 8:36AM - 8:48AM |
R21.00002: Spin Lifetime Enhancement by External Magnetic Fields in Organometal Halide Perovskites Nathan Gundlach, Yue Yao, Patrick Odenthal, Chuang Zhang, Zeev Vardeny, Yan Li The organometal halide perovskites represent a new class of semiconductors that are solution processed but have excellent optoelectronic properties. Recent theories and experimental results suggest their exciting potential for spintronic applications as well. Specifically, theories have predicted giant Rashba effects, and our previous research revealed long spin lifetimes (>1 ns) in CH3NH3PbI3, despite large spin-orbit coupling. Therefore, it is critical to understand the spin relaxation mechanisms. Here we study the hyperfine interaction contribution to spin relaxation by measuring time-resolved Faraday rotation in external magnetic fields. In transverse magnetic field, the spin lifetime T2* increases at low fields (<300 G) by up to 100%, and then decreases at higher fields due to spin dephasing. In longitudinal magnetic field, the spin lifetime T1 increases up to a saturation value. The enhancement phenomena gradually disappear at temperatures above 40 K. A plausible picture is that this enhancement is due to the suppression of small random local effective magnetic fields induced by hyperfine interaction. We will present all the evidence supporting this hypothesis. |
Thursday, March 8, 2018 8:48AM - 9:00AM |
R21.00003: Rashba splitting effect in doped 2D organic-inorganic hybrid perovskite multiple quantum wells Eric Amerling, Sangita Baniya, Yaxin Zhai, Evan Lafalce, Luisa Whittaker-Brooks, Zeev Vardeny Two-dimensional (2D) organic–inorganic hybrid perovskite multiple quantum wells which consist of multilayers of alternate organic and inorganic layers exhibit large exciton binding energies due to the dielectric confinement between the inorganic and organic layers. These naturally formed multiple quantum wells have strong spin-orbit coupling (SOC) due to the presence of heavy elements in their crystal structures. The strong SOC may split the spin-degenerate bands in these noncentrosymmetric compounds, thus leading to the Rashba splitting effect. Moreover, the Rashba splitting effect may be enhanced in 2D organic–inorganic hybrid perovskite multiple quantum wells by the introduction of dopants. Here, we have used ultrafast transient, nonlinear optical spectroscopies to study how doping affect exciton and free-carriers in 2D perovskite thin films. Our findings suggest that doped organic-inorganic hybrid perovskites have great potential for sensing and spintronics applications. |
Thursday, March 8, 2018 9:00AM - 9:12AM |
R21.00004: Monolayer Assembly of Chiral Molecules on Semiconductors for Spin Filtering Tianhan Liu, Eric Lochner, Xiaolei Wang, Gang Shi, Fan Gao, Yongqing Li, Jianhua Zhao, Peng Xiong Chirality-induced spin selectivity (CISS) has been observed in chiral molecules including dsDNA and oligopeptide on noble metals.1 The effect may have profound implications in biological processes and as novel means of spin injection and detection without using any magnetic material. Here, we report on a study on the formation of self-assembled monolayer (SAM) of thiolated dsDNA and polyalanine molecules on Au and GaAs by AFM and ellipsometry. On Au, ‘blocking’ by hydrophobic alkanethiol SAM is necessary to achieve assembly of dsDNA oriented away from the substrate. The assembly on GaAs is aided by an ammonium sulfide treatment for oxide removal and surface passivation,2 on which the polyalanines form oriented SAM without any ‘blocking’. The spin filtering of the electrons from a Au electrode by polyalanine SAM is ascertained using perpendicularly magnetized (Ga,Mn)As as a spin analyzer: The magnetoresistance of a (Ga,Mn)As/polyalanine/Au junction is measured in a perpendicular magnetic field. Sharp jumps in the junction resistance are observed at the coercive fields of the (Ga,Mn)As. |
Thursday, March 8, 2018 9:12AM - 9:24AM |
R21.00005: Spin injection into multiwall carbon nanotubes across a molecular barrier Roméo Bonnet, Philippe Lafarge, Maria Luisa Della Rocca, Clement Barraud Carbon-based materials such as graphene and carbon nanotubes are promising candidates for efficiently propagating spin information. It is mainly due to the weak spin-orbit coupling and to the weak hyperfine interactions present in those two nanomaterials. However, it remains challenging to inject spins directly from a ferromagnetic metal due to the so-called "impedance mismatch" problem. In this talk, I will present an experimental study concerning spin transport through large diameter multiwall carbon nanotubes functionalized by diazonium molecules and contacted by Co and Ni electrodes. I will show that the molecules form a conformational film around the nanotubes leading to injection resistance around 10 MΩ. I will then describe the magnetoresistance signal observed at low temperatures and its evolution as a function of bias voltage. |
Thursday, March 8, 2018 9:24AM - 9:36AM |
R21.00006: Spin Valve Behaviors of Cobalt Dioxelene Valence Tautomer Molecular Films William Rice, Frank Tsui, Daniel Stasiw, Robert Schmidt, David Shultz Cobalt dioxolene complexes exhibit molecular magnetic bistability in the form of valence tautomerism (VT), an intramolecular charge transfer correlated with a spin-crossover, which can be triggered by a variety of external stimuli, including temperature and pressure changes, as well as optical excitation. We present a study of spin valve behaviors in molecular films of a photoswitchable VT sandwiched between ferromagnetic electrodes of permalloy (40 nm) and Co (100 nm) in the form of vertical crossbar stacks. The magnetic switching fields of the spin valves exhibit two sets of values versus temperature and light exposure. The observed bistability correlates with the magnetic states of the VT molecule. In addition, the magnetoresistance of the devices also exhibits a distinct asymmetry depending on the current/voltage bias direction. These findings indicate that molecular bistabilities, e.g. the VT transition, can be utilized to produce new switchable modalities in molecular spintronic devices. |
Thursday, March 8, 2018 9:36AM - 10:12AM |
R21.00007: Spin dynamics of carriers interacting with dangling bond spins in colloidal semiconductor nanostructures Invited Speaker: Dmitri Yakovlev Non-magnetic colloidal nanostructures can demonstrate magnetic properties typical for diluted magnetic semiconductors because the spins of dangling bonds at their surface can act as the localized spins of magnetic ions. This idea has been suggested theoretically to explain the recombination mechanism of spin-forbidden dark excitons in colloidal nanocrystals [1]. We aprove it experimentally for CdSe colloidal nanocrystals [2]. Spins of dangling bonds indeed activate emission of the dark exciton. Also their collective interaction with the exciton results in formation of the dangling-bond magnetic polaron, which parameters are examined experimentally and theoretically. In CdSe nanoplatelets presence of the dangling bond spins is visualized via strong polarization of excitons in external magnetic field. [1] A.V. Rodina and Al. L. Efros, Nano Lett. 15, 4214 (2015). [2] L. Biadala, et al., Nature Nanotechnology 12, 569 (2017). |
Thursday, March 8, 2018 10:12AM - 10:24AM |
R21.00008: Optical Hyperpolarization of Phosphorus Donor Nuclei and Bound Exciton Capture Rates in Isotopically Purified Silicon Holger Haas, Rahul Deshpande, Thomas Alexander, David Cory We present optical pumping experiments on phosphorus defects in isotopically purified (28Si) silicon at low temperatures and high magnetic fields. Using a combination of above bandgap laser light along with radio frequency saturation and control fields we study the optical dynamical nuclear polarization (DNP) mechanism and measure the rates of Auger ionization processes associated with bound exciton recombination events. We conclude that the dominant DNP mechanism under our experimental conditions is phononic. This has implications both for optimization of devices utilizing the extremely long coherence times of phosphorus nuclei in isotopically purified silicon, and providing a technique for measuring location dependent, frequency specific information of elastic waves in silicon devices. |
Thursday, March 8, 2018 10:24AM - 10:36AM |
R21.00009: Dynamic Nuclear Polarization Enhanced NMR of Surface Hydrogen on Silicon Microparticles Chandrasekhar Ramanathan, Mallory Guy, Kipp van Schooten, Lihuang Zhu Dynamic nuclear polarization (DNP) is a driven process that transfers higher electron spin polarization to surrounding nuclear spins via microwave irradiation near the electron Larmor frequency. We use frequency-modulated 94 GHz excitation of the endogenous dangling-bond electron spins present at the silicon surface, to acquire DNP-enhanced NMR spectra of the surface hydrogen . The high sensitivity DNP NMR signal detection at 3.34 T and 4 K is enabled by an overall signal enhancement of 4150 over the room temperature NMR signal at the same field. The NMR spectrum from a dry sample of polycrystalline silicon powder (1-5 μm) shows a distinctively narrow Lorentzian-shaped resonance with a width of 6.2 kHz, indicative of a very sparse distribution of protons within a few atomic layers of the silicon surface. The narrow proton spectrum measured from the silicon powder is similar to one of the spectral components observed in previous NMR studies of hydrogenated-amorphous silicon films in device-quality wafers. We also characterize the growth and exchange of these surface protons when the silicon particles were immersed in different solvents. |
Thursday, March 8, 2018 10:36AM - 10:48AM |
R21.00010: Exploring Optical Nuclear Polarization in Ga1-xMnxAs Films John Tokarski, Brenden Magill, Christopher Stanton, Giti Khodaparast, Clifford Bowers Gallium-71 optically-pumped NMR (OPNMR) measurements were performed on thin films (<1 µm thickness) of the dilute magnetic semiconductor Ga1-xMnxAs (where x= 0, 0.01 and 0.05), a candidate material for spintronics applications. The optical irradiation time dependence of the OPNMR signal and dark nuclear spin relaxation were measured to investigate the effects of Mn on the spin polarization dynamics of optical nuclear polarization. The action spectra (OPNMR signal vs. photon energy) acquired in bulk GaAs and a Ga.99Mn.01As film are compared to theoretical simulations based on the electronic band structure calculations. Incorporation of Mn produces strong oscillations in the OPNMR signal amplitude, which are in qualitative agreement with theory. In addition, the Mn sites were characterized by X-band EPR spectroscopy at 5 K. The results are interpreted in terms of a modified optical pumping model and spectral simulations. |
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