Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session U41: Quantum Spins in SemiconductorsFocus
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Sponsoring Units: GMAG DMP FIAP DCOMP Chair: Denis Candido, Univ of Iowa Room: 707 |
Thursday, March 5, 2020 2:30PM - 3:06PM |
U41.00001: Spatiotemporal mapping of photocurrent in 2D materials using diamond quantum sensors Invited Speaker: Brian Zhou Photocurrents are conventionally detected by counting the charge that flows between two contacts, but electrical detection cannot resolve the path that photocurrents travel within a material. Here, we leverage nitrogen-vacancy (NV) center magnetometers to resolve the spatial distribution of photocurrent flow in a 2D material by measuring the magnetic field profile produced by the photocurrents [1]. We reveal that photocurrent in monolayer MoS2 circulates as a micron-scale vortex under an external magnetic field due to a strong photo-Nernst effect. By synchronizing dynamical decoupling of the sensor spin with pulsed photoexcitation, we significantly enhance sensitivity and resolve current densities as small as 20 nA/μm. Importantly, our pulsed approach allows probing of the temporal dynamics of photocurrent generation with sub-microsecond resolution. This combined spatiotemporal resolution is invaluable for understanding how novel photocurrent generation mechanisms and local variations control the flow of photocurrent in next-generation optoelectronic devices. |
Thursday, March 5, 2020 3:06PM - 3:18PM |
U41.00002: Optically pumped spin polarization as a probe of many-body thermalization Daniela Pagliero, Pablo Zangara, Jacob Henshaw, Ashok Ajoy, Rodolfo Acosta, Jeffrey A Reimer, Alexander Pines, Carlos Meriles The interplay between disorder and transport is a problem central to the understanding of thermalization. Disorder and many body interactions are known to compete, with the dominance of one or the other giving rise to fundamentally different dynamical phases. Here we investigate the spin diffusion dynamics of 13C in diamond, which we dynamically polarize at room temperature via optical spin pumping of engineered color centers. We focus on low-abundance, strongly hyperfine-coupled nuclei, whose role in the polarization transport we expose through the integrated impact of variable radio-frequency excitation on the observable bulk 13C magnetic resonance signal. Unexpectedly, we find excellent thermal contact throughout the nuclear spin bath, regardless the hyperfine coupling strength, which we attribute to effective carbon-carbon interactions mediated by the electronic spin ensemble. In particular, observations across the full range of hyperfine couplings suggest the nuclear spin diffusion constant is approximately uniform, taking values up to two orders of magnitude greater than that expected from homo-nuclear spin couplings. Our results open intriguing opportunities to study the onset of thermalization in a system by controlling the internal interactions within the bath. |
Thursday, March 5, 2020 3:18PM - 3:30PM |
U41.00003: Nanoscale structure of the orbital magnetic moment of a single dopant spin in a semiconductor Adonai Rodrigues da Cruz, Michael Flatté The localized electron spin of a single impurity in a semiconductor is a promising system to realize quantum information schemes. In this work we investigate the orbital contribution to the magnetic moment originated from the spin-orbit induced circulating current [1] associated with the ground state of a single magnetic impurity in III-V semiconductors. In this project we developed a formalism employing Green's functions obtained by the Koster-Slater technique [2] with a sp3d5s* empirical tight-binding Hamiltonian [3] to describe the host material. We calculated the circulating current and orbital moments of a single Mn dopant in GaAs. The spin-correlated orbital moments originates from the hybridization between the Mn(d5) spin-polarized electrons and the As dangling bonds leading to t2-symmetric triplet acceptor states in the band-gap above the valence band edge. |
Thursday, March 5, 2020 3:30PM - 3:42PM |
U41.00004: Pulsed Electrically Detected Magnetic Resonance (pEDMR) at Low Magnetic Fields using an Arbitrary Wave Form Generator (AWG) Taniya Tennahewa, Hans Malissa, Henna Popli, Sabastian Atwood, Sanaz Hosseinzadeh, John M Lupton, Christoph M Boehme We use pulsed electrically detected magnetic resonance (pEDMR) in order to observe the coherent propagation of electron spins under strong drive magnetic resonance conditions in organic materials, where excitation frequencies (~100 MHz) and Zeeman fields (~3 mT) are low. In these materials, transverse relaxation times of electron spins are known to be in the on the order to 1µs [1], requiring the bandwidths of the pulse experiments to be in the 10-100 MHz range in order to accommodate effective pulse sequences. Thus, at low excitation frequencies [2,3], the duration of spin manipulation pulsed becomes comparable to the carrier-wave frequency, and conventional methods for generating RF pulses become impossible. We therefore utilize an arbitrary waveform generator (AWG) to directly synthesize RF pulses with a shape derived directly from the desired excitation spectrum. We evaluate the fidelity of such pulsed low-frequency excitation and conduct experiments that demonstrate electrically detected coherent spin motion under the given low-field conditions.[1] R. Miller, et al.,Phys. Rev. B, 94, 214202 (2016); [2] S. Jamali et al., Nano Lett. 4648 (2017); [3] D. P. Waters et al., Nature Physics 11, 910 (2015). |
Thursday, March 5, 2020 3:42PM - 3:54PM |
U41.00005: Detection of electron spin resonance in the strong, non-linear drive regime using spin-dependent charge carrier recombination currents and an amplitude-modulated continuous wave electrically detected magnetic resonance scheme Sabastian Atwood, Adnan Nahlawi, Sanaz Hosseinzadeh, Taniya Tennahewa, Henna Popli, Hans Malissa, John M Lupton, Christoph M Boehme Electrically detected magnetic resonance spectroscopy of organic light-emitting diodes with conductive polymers as active layers allows for the study of high-magnetic resonance drive regimes of electron spins in which Zeeman fields are on the same magnitude as drive field amplitudes [1, 2]. Drive amplitude limits of such non-linear magnetic resonance experiments are posed by the superposition of the studied spin-dependent electric currents with other, randomly occurring, radiation-induced artifact signals, possibly due to electric dipole transitions in lowest unoccupied molecular orbitals. Here, we demonstrate the use of amplitude-modulated lock-in detection for the isolation of these two electric current signatures, taking advantage of their different dynamic natures. We validate this approach by analyzing the dependence of the lock-in detected signals on modulation phase and frequency and demonstrate a significant signal improvement allowing for the detection of multi-photon magnetic dipole transitions and the Bloch-Siegert shift. [1] S. Jamali et. al., Nano Lett., 17, 4648 (2017), [2] S. Jamali et. al. (unpublished). |
Thursday, March 5, 2020 3:54PM - 4:06PM |
U41.00006: Probing spin-dependent electronic transitions in a conjugated polymer using electrically detected magnetic resonance (EDMR) under hole injection with MoO3 electrodes Sanaz Hosseinzadeh, Hans Malissa, Adnan Nahlawi, Christoph M Boehme Spin-dependent recombination in organic light emitting diodes (OLEDs) occurs when an active polymer layer is sandwiched between an electron and hole charge carrier injection layers. As spin-dependent electronic transitions rates change under magnetic resonant excitation, spin-dependent recombination can be studied using a method called electrically detected magnetic resonance spectroscopy (EDMR) [1-2]. Most past EDMR studies used the copolymer blend poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) as hole injector layer. Here, we report EDMR spectroscopy of OLEDs based on various π-conjugated poly-phenylenevinylenes as active materials and the Ca and MoO3 layers as electron and hole injectors, respectively, in order to test whether Mo, (atomic number Z=42) is able to affect the charge carrier spin-orbit coupling (SOC) in the active layer, e.g. by suppression of spin-dependent processes. Our results indicate that no SOC effects can be observed and, thus, that SOC in hole injection layers does not affect pEDMR signals in OLEDs. [1] D. McCamey et al., Nature Materials, 7 (9), 723 (2008); [2] H. Malissa et al., Science 345 (6203), 1487 (2014). |
Thursday, March 5, 2020 4:06PM - 4:18PM |
U41.00007: Manipulation of Dynamic Nuclear Polarization in Gallium Arsenide under periodic optical electron spin pumping Michael Dominguez, Joseph Iafrate, Vanessa A Sih Optically oriented electron spins in gallium arsenide polarize the nuclear spin system through a process called dynamic nuclear polarization. The polarized nuclear system will create an effective magnetic field back onto the electron spin system and affect the Larmor precession frequency. The nuclear polarization can be calculated from the change in electron spin polarization using periodic optical pump probe Kerr rotation measurements[1]. We demonstrate that the coupled electron-nuclear spin systems can be manipulated by varying the excitation energy and sweeping an external magnetic field. These dependencies are corroborated by numerical calculations from a model incorporating optical orientation and the optical Stark effect. This model is justified by its correspondence to our experimental observations and can be used to predict the behavior of the nuclear spin polarization. |
Thursday, March 5, 2020 4:18PM - 4:30PM |
U41.00008: The impact of dynamic nuclear polarization on non-local spin valve and inverse spin Hall Hanle measurements in n-GaAs Zhen Jiang, Sahil Patel, Paul Crowell, Chris J Palmstrom Non-local spin valve (NLSV) and inverse spin Hall (ISHE) Hanle measurements are important tools to probe spin-transport parameters such as the spin lifetime and spin Hall angle. We carried out such measurements on Fe/n-GaAs heterostructure samples with varying doping (3x1016 to 7x1016 cm-3) and at temperatures from 2 K to 110 K. The Hanle curves are very sensitive to the hyperfine field generated by dynamic nuclear polarization (DNP). Below a threshold temperature that depends on doping, the hyperfine field leads to distortion of the Hanle line-shapes for both NLSV and ISHE measurements and can enhance the ISHE signal by one order of magnitude. We have developed a pulsed-current technique which utilizes the vast difference between electron and nuclear spin relaxation time scales (about ns and s respectively) to eliminate the impact of the steady-state hyperfine field, and the obtained Hanle curves then match expectations based on simple drift-diffusion models. We will discuss the extracted spin lifetime and spin Hall angle, numerical modeling of the Hanle curves and the role of the hyperfine field in enhancing the apparent spin Hall signal. |
Thursday, March 5, 2020 4:30PM - 4:42PM |
U41.00009: Magnetic properties of a single micrometer-sized YIG/FGT multilayer heterostructure revealed by ferromagnetic resonance Bassim Arkook, Mohammed Alghamdi, Victor Ortiz, Jing Shi, Igor Barsukov Multilayer heterostructures consisting of a magnetic insulator and 2D spin system offer a promising platform for next-generation spintronic applications. Understanding interfacial spin coupling and transport in such heterostructures require a local microwave spectroscopy technique since the flakes of 2D materials obtained by exfoliation are limited to micrometers in size. Here, we fabricate disks of 20um diameter from epitaxial ferrimagnetic insulator, yttrium iron garnet (YIG), thin-film using pulsed laser deposition, e-beam lithography, and lift-off. We place few-nm thick flakes of van-der-Waals material Fe3GeTe2 (FGT) on the pre-patterned YIG disks and transfer a single disk onto a planar microwave resonator. Using angle-dependent ferromagnetic resonance, we find low-energy spin-wave modes in YIG at several microwave frequencies, allowing for the evaluation of magnetic anisotropy and damping. Using cryogenic measurements, we observe an anomalous temperature dependence of effective magnetic anisotropy. The results shed light on spin phenomena at YIG/FGT interfaces and present an experimental approach for studies of micro-scale magnetic insulator/2D heterostructures. |
Thursday, March 5, 2020 4:42PM - 4:54PM |
U41.00010: Dynamics of Spin Waves in 1D and 2D Magnonic Crystals of V[TCNE]x~2 with YIG or Cobalt Kwangyul Hu, Michael Flatté A semiconducting organic ferrimagnet V[TCNE]x~2 is a newly emerging magnonic material. Recent studies have revealed that V[TCNE]x~2 has narrow linewidth, long spin lifetime and high Q factor[1-3]. In addition, deposition of high quality V[TCNE]x~2 does not require specific substrates or high temperature which is a significant advantage compared to YIG[1]. These properties of V[TCNE]x~2 indicate that it is attractive magnonic media. Here, we calculate and present the magnonic dispersions and linewidths of quasi one-dimensional and two-dimensional magnonic crystals consisting of V[TCNE]x~2 with YIG or cobalt. In each case, we consider infinitely periodic lattice structures with a finite thickness. For the calculation, the linearized Landau-Lifshitz-Gilbert equation with the plane-wave method is used[4]. We anticipate that these results will provide fundamental understanding of how to design magnonic devices based on V[TCNE]x~2. |
Thursday, March 5, 2020 4:54PM - 5:06PM |
U41.00011: Impact of hidden structural order on low temperature magnetic resonance in V[TCNE]x Huma Yusuf, Andrew Franson, Seth Kurfman, Ezekiel Johnston-Halperin The coordination compound V[TCNE]x is a promising magnetic material for integration into solid-state quantum information systems due to its low damping (α = (3.98 ± 0.22) × 10-5), high quality factor (Q up to 8,000) microwave resonance, ease of patterning, and compatibility with a wide variety of substrates. Since current quantum information systems require low temperature operation, a comprehensive study of the low temperature magnetic behavior of V[TCNE]x is essential for realizing this promise. Here we report the observation of anomalous temperature dependent anisotropy in thin films of V[TCNE]x at temperatures down to 5 K. The resonance linewidth at 5K is larger by a factor of 3 than at room temperature, competitive with the very best liquid phase epitaxy (LPE) yittrium iron garnet (YIG) films. Further, we observe temperature-dependent changes in the magnetic anisotropy that suggest that this modest increase in linewidth may in fact arise from inhomogeneous thermally-induced strain in the thin film. This result promises further reductions in the low temperature linewidth in structures engineered to minimize differences in thermal contraction between the sample and the substrate. |
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