Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H39: Dynamic Nuclear Polarization and Magnetism in SemiconductorsFocus
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Sponsoring Units: GMAG DMP Chair: Paul Crowell, University of Minnesota Room: BCEC 207 |
Tuesday, March 5, 2019 2:30PM - 2:42PM |
H39.00001: History-Dependent Dynamic Nuclear Polarization in Gallium Arsenide in the Resonant Spin Amplification Regime Joseph Iafrate, Michael Macmahon, Michael J Dominguez, Hua-Wei Hsu, Vanessa Sih Dynamic nuclear polarization (DNP) arises through the interaction of atomic nuclei in a material with spin-polarized electrons. An optically-driven electron system transfers spin polarization to the nuclei. In turn, nuclear spins produce a magnetic field that affects electron spin precession frequency. We use a pulsed laser to excite and measure electron spin polarization in a gallium arsenide epilayer, in the regime of resonant spin amplification. By measuring Kerr rotation as a function of applied magnetic field, we can detect the influence of the nuclear spins. We report an unexpected dependence of DNP on both magnetic field history and transverse electron spin polarization [1]. Variations of the applied field sequence and duration reveal a minutes-long precise memory of the particular field history of the system. |
Tuesday, March 5, 2019 2:42PM - 2:54PM |
H39.00002: Spin Dynamics in the Thermoelectric Li:MnTe Antiferromagnetic Semiconductor Raphael Hermann, Michael Manley, Vasile O Garlea, Douglas L Abernathy, Huaizhou Zhao, Joseph P C Heremans We report the magnetic structure and excitation spectrum of Li0.03:MnTe, a material related to the MnTe antiferromagnetic semiconductor with a Néel temperature of ~307 K. MnTe was proposed to exhibits a strong contribution of magnon drag to the thermopower[1]. Neutron scattering reveals the magnetic structure and spin-spin correlations in the ordered and disordered phases of Li0.03:MnTe. Above the magnetic ordering temperature, strong quasielastic paramagnon scattering is observed. The characteristic energy width of the scattering is temperature independent and provides evidence for slow, ~30 fs, magnetic fluctuations. These data provide necessary input for modelling of the transport properties. |
Tuesday, March 5, 2019 2:54PM - 3:06PM |
H39.00003: Dependence of dynamic nuclear polarization on resonant spin amplification and external field history under periodic optical electron spin pumping Michael Macmahon, Joseph Iafrate, Michael J Dominguez, Hua-Wei Hsu, Vanessa Sih Greater control of nuclear spin polarization could provide breakthroughs in both classical and quantum information storage and processing. We use optical pump-probe techniques in a gallium arsenide epilayer to manipulate an electron spin polarization that persists over several pulse cycles1. The interference of spins excited from different pulses in a swept external magnetic field results in resonant spin amplification (RSA), and we demonstrate a dynamic nuclear polarization perpendicular to the electron spin polarization that actively responds to the external field sweep direction and magnitude of RSA. We also show that the electron-nuclear spin system retains memory of the external field history, including interruptions and reversals in magnetic field sweeps, and present a model that explains key features of these results. |
Tuesday, March 5, 2019 3:06PM - 3:42PM |
H39.00004: Universal nuclear focusing of confined electron spins Invited Speaker: Gian Salis For spin-based quantum computation in semiconductors, dephasing of electron spins by a fluctuating background of nuclear spins is a main obstacle. It has been shown that in self-assembled InAs quantum dots, this nuclear background can be precisely controlled by periodically exciting single electron spins using optical laser pulses. A feedback mechanism between the electron spin polarization and the nuclear system focuses the electron spin precession frequency into discrete modes that are commensurate with the laser repetition rate. In such a spin-mode-locked system, the electron spin lifetime within individual dots can surpass the limit given by nuclear background fluctuations. Here we show that spin mode-locking is a universal phenomenon that also occurs in ensembles of lithographically-defined many-electron GaAs/AlGaAs dots [1]. This opens the door to achieve long electron spin coherence times also in systems that can be controlled in shape, size and position. We discuss possible mechanisms of spin mode-locking and show experiments that suggest that the optical Stark effect plays an important role in the nuclear focusing of our many-electron dots. |
Tuesday, March 5, 2019 3:42PM - 3:54PM |
H39.00005: Study of the dynamics of spin-dependent charge carrier recombination in Tris(8-hydroxyquinolinato) aluminium (Alq3) with pulsed electrically detected magnetic resonance Henna Popli, Xiaojie Liu, Taniya Hansika Tennahewa, Mandefro Teferi, Shirin Jamali, Hans Malissa, Zeev Vardeny, Christoph Boehme We have studied the dynamics of previously reported spin-dependent charge carrier recombination transitions in Tris(8-hydroxyquinolinato) aluminium (Alq3) [1] thin films with both continuous wave [2] as well as pulsed [3] electrically detected magnetic resonance (EDMR) spectroscopy under bipolar injection conditions. The measurements revealed transverse charge carrier spin relaxation times, T2 shorter, but of the same order of magnitude to those previously observed in other organic semiconductors thin films such as various π-conjugated polymers, spin-orbit induced g-strain of the charge carrier magnetic resonances, as well as the local hyperfine field distribution magnitudes of both the electrons and the holes. Our measurements show that, qualitatively, the nature of spin-dependent recombination transitions in Alq3 film is similar to that of the polymer materials and metal-organic complex Zinc phthalocyanine(ZnPc) [4]. [1] C.F.O. Graeff et al., Eur. Phys. J. E, 18, 21 (2005); [2] G Joshi et al., Appl. Phys. Lett., 109, 103303 (2016); [3] D. R. McCamey et al., Nature Mat., 7, 723 (2008); [4] S. Schaefer et al., phys. stat. sol. (b) 245 (10) 2120 (2008). |
Tuesday, March 5, 2019 3:54PM - 4:06PM |
H39.00006: Effect of Charge Localization on the Effective Hyperfine Interaction in Organic Semiconducting Polymers Tho Nguyen, Rugang Geng, Ram Subedi, Hoang Luong, Lawrence Hornak Organic semiconductors (OSECs) possess weak spin-orbit coupling and hyperfine interaction (HFI); therefore, their spin lifetimes are incredibly long for spin based devices including qubits. In this talk, we will show that the spin lifetime in OSECs can be further prolonged by improving the charge delocalization. HFI, originating from the coupling between electron spins and nuclei, has been demonstrated to strongly influence the spin dynamics in OSECs. Nevertheless, the role of charge localization on the HFI strength in organic thin films has not yet been experimentally investigated. In our PRL (120, 086602, 2018), the statistical relation hypothesis that the effective HFI of holes in regioregular poly(3-hexylthiophene) (P3HT) is proportional to 1/N0.5 has been examined, where N is the number of the random nuclear spins within the envelope of the hole wave function. First, we verify that HFI is the dominant spin interaction in P3HT. Second, assuming that holes delocalize fully over the P3HT polycrystalline domain, the strength of HFI is experimentally demonstrated to be proportional to 1/N0.52 in excellent agreement with the statistical relation. Finally, the HFI of electrons in P3HT is about 3 times stronger than that of holes due to the stronger localization of the electrons. |
Tuesday, March 5, 2019 4:06PM - 4:18PM |
H39.00007: Two-electron-spin ratchets as a platform for microwave-free dynamic nuclear polarization of arbitrary material targets Pablo Zangara, Jacob Henshaw, Daniela Pagliero, Ashok Ajoy, Jeffrey A Reimer, Alexander Pines, Carlos Meriles Recently, several schemes centered around the negatively charged Nitrogen Vacancy(NV-) color center in diamond as means to hyperpolarize nuclei have been realized1–2. Here we theoretically consider the case of the NV and the substitutional nitrogen (P1) centers in diamond to show that outside protons spin-polarize efficiently upon a magnetic field sweep across the NV–P1 level anti-crossing. In particular, the nuclear polarization buildup during the low-to-high and high-to-low stages of the sweep cycle adds constructively regardless the relative sweep velocities. The system dynamics can be interpreted in terms of an NV–P1 spin ratchet whose sign depends on the relative timing of an accompanying optical excitation pulse. Our calculations indicate that the polarization transfer process is insensitive to the NV axis orientation, and efficient over a broad range of electron-electron and electron-nuclear spin couplings, even if the proxy spins suffers from short relaxation times. |
Tuesday, March 5, 2019 4:18PM - 4:30PM |
H39.00008: Spin-orbit coupling in antiferromagentic MnTe Gen Yin, Jie-Xiang Yu, Yizhou Liu, Roger Lake, Jiadong Zang, Kang Wang We show that the spin-orbit coupling (SOC) in α-MnTe impacts the transport behavior by generating an anisotropic valence-band splitting, resulting in four spin-polarized pockets near Gamma. A minimal k-dot-p model is constructed to capture this splitting by group theory analysis, a tight-binding model and ab initio calculations. The model is shown to describe the rotation symmetry of the zero-field planer Hall e ect (PHE). The PHE percentage is determined by the SOC induced band shape, and is quantitatively estimated to be 25% ∼ 31% for an ideal thin film with a single antiferromagnetic domain. The k-dot-p Hamiltonian given by this research is not only effective, but also minimal. The quartic spin-orbit coupling terms in the model is necessary, in the absence of which, the extra C4T symmetry rules out any Hall effect. The predicted value of PHE percentage is an order of magnitude greater than previous experimental observations, suggesting a vast space to optimize the material for device applications. |
Tuesday, March 5, 2019 4:30PM - 4:42PM |
H39.00009: Paramagnon drag yields a high thermoelectric figure of merit in Li-doped MnTe Yuanhua Zheng, Tianqi Lu, Md Mobarak Hossain Polash, Morteza Rasoulianboroujeni, Ning Liu, Michael Manley, Yuan Deng, Peijie Sun, Xiaolong Chen, Raphael Hermann, Daryoosh Vashaee, Joseph P C Heremans, Huaizhou Zhao MnTe is an antiferromagnetic semiconductor with a Neel temperature of 307 K. In the antiferromagnet phase, the magnon-electron interaction gives rise to a magnon-drag contribution to thermopower. Surprisingly this magnon-drag thermopower extends to the paramagnetic phase. Experimental results provide evidence for the effect of locally ordered thermal fluctuations of magnetization with finite lifetime (paramagnons). When the lifetime of the paramagnons is longer than the electron-paramagnon scattering time, the paramagnons can effectively push the electrons and contributes to thermoelectric power. By analyzing the measured thermopower, resistivity, Hall coefficient, specific heat and magnetization, a quantitative explanation is given for thermopower of both the AFM and PM regime based upon the hydrodynamic theory [1]. As a result, the ZT of the best sample exceeds 1 at 950 K. |
Tuesday, March 5, 2019 4:42PM - 4:54PM |
H39.00010: Modification of the Heavy Hole Wave-function in Multiply Occupied Magnetic Quantum Dots James Pientka, Peiyao Zhang, Tenzin Norden, Arman Najafi, Biplob Barman, Yutsung Tsai, Bruce McCombe, Jong E Han, Igor Zutic, Athos Petrou, Rafal M Oszwaldowski, Wen-Chung Fan, Wu-Ching Chou We compare the magnetic field PL peak energy red shift of ZnTe Quantum Dots (QDs) embedded in a ZnMnSe matrix using two types of laser excitation: (a) Excitation at 405 nm (3.06 eV) with photon energy above the ZnMnSe matrix gap. This leads to electron-hole pair generation mainly in the matrix. (b) Excitation at 488 nm (2.54 eV) with photon energy below the ZnMnSe matrix gap but above the bandgap of the ZnTe QDs. In the latter case, carriers are excited directly into the ZnTe QDs. |
Tuesday, March 5, 2019 4:54PM - 5:06PM |
H39.00011: Ferromagnetic Contacts to InSb nanowires Yifan Jiang, Zedong Yang, Diana Car, Sasa Gazibegovic, Badawy Ghada, Roy L. M. Op het Veld, Sébastien Plissard, Erik P. A. M. Bakkers, Vlad S Pribiag, Sergey M Frolov InSb nanowires are versatile platforms for a variety of quantum transport experiments. Here we develop ferromagnetic contacts onto InSb nanowires. We fabricate four terminal devices (spin-valve geometry) using both Ti/Au and CoFe contacts. At low temperature, both local and non-local experiments demonstrate hysteretic spin-valve like signals. Besides spin injection, we are considering other explanations, for example, the magneto-Coulomb effect. In addition, magnetized ferromagnetic contacts themselves create local magnetic fields, which may affect conductance in high g-factor InSb nanowires. |
Tuesday, March 5, 2019 5:06PM - 5:18PM |
H39.00012: Electrical Spin Injection into Silicon Nanowires with Axial Doping Gradient Konstantinos Kountouriotis, Jorge L Barreda, Timothy Keiper, Mei Zhang, Peng Xiong For nanoscopic semiconductor spintronic devices, the all-important issue of the ferromagnet/semiconductor (FM/SC) interface becomes even more critical. Here we elucidate the effects of the FM/SC nano interface on electrical spin injection and detection, utilizing a unique type of Si nanowires (NWs) with an inherent axial doping gradient. Two-terminal and nonlocal four-terminal lateral spin-valve measurements were performed using different combinations from a series of FM contacts positioned along the same NW. The data are analyzed with a general model of spin accumulation in a normal channel under electrical spin injection from a FM, which reveals a distinct correlation of decreasing spin-valve signal with increasing injector junction resistance.1 The observation is attributed to the diminishing contribution of the d-electrons in the FM to the injected current spin polarization with increasing Schottky barrier width. The results demonstrate that there is a window of interface parameters for optimal spin injection efficiency and current spin polarization, which provides important design guidelines for nano-spintronic devices with quasi-1D semiconductor channels. |
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