Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session A5: Magnetic Resonance and Spin-Dependent Optical Phenomena in SemiconductorsFocus Industry
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Sponsoring Units: GMAG DCMP FIAP Chair: Masashi Shiraishi, U. Kyoto Room: 301 |
Monday, March 14, 2016 8:00AM - 8:12AM |
A5.00001: Optimizing Frequency-Modulated CW EDMR in silicon Lihuang Zhu, Kipp van Schooten, Chandrasekhar Ramanathan Electrically detected magnetic resonance (EDMR) is a powerful method of probing dopant and defect spin states in semiconductor devices. Moreover, at the single dopant level, these spin states are heavily investigated as potential qubit systems, though facile electronic access to single dopants is exceedingly difficult. We therefore characterize detection sensitivities of frequency-modulated CW-EDMR of phosphorus donors in silicon Si:P using a home-built 2.5 GHz system (\textasciitilde 80 mT) at 5 K. An arbitrary waveform generator controls the frequency modulation, allowing us to optimize the signal to noise ratio (SNR) of both the dangling bond and phosphorus donor signals against multiple experimental parameters, such as modulation amplitude and modulation frequency. The optimal range of frequency modulation parameters is constrained by the relaxation time of the phosphorous electron at 5 K, resulting in the same sensitivity limit as field modulated CW-EDMR, but offers some technical advantages; e.g. reducing the relative contribution of magnetic field induced currents and eliminating the need for field modulation coils. We further characterize the EDMR SNR in Si:P as a function of optical excitation energy by using a narrow line laser, tunable across donor exciton and band gap states. [Preview Abstract] |
Monday, March 14, 2016 8:12AM - 8:24AM |
A5.00002: Electron Spin Resonance in a 2D Fermi Liquid with Spin-Orbit Coupling Saurabh Maiti, Muhammad Imran, Dmitrii Maslov Electron spin resonance (ESR) is usually interpreted as a single-particle phenomenon protected from the effect of many-body correlations. We show that this is not the case in a two-dimensional Fermi liquid (FL) with spin-orbit coupling (SOC). Depending on whether the magnetic field is below or above some critical value, ESR in such a system probes –up to three–collective chiral-spin modes, augmented by the presence of the field, or the Larmor mode, augmented both by SOC and FL renormalizations. We argue that ESR can be used as a probe not only for SOC but also for many-body physics. [Preview Abstract] |
Monday, March 14, 2016 8:24AM - 8:36AM |
A5.00003: Electric dipolar spin resonance in systems with a valley dependent \emph{g}-factor Marko Rancic, Guido Burkard We theoretically investigate the electric dipole spin resonance (EDSR) in a single Si/SiGe quantum dot in the presence of a magnetic field gradient, e.g., produced by a micromagnet. The control of electron spin states can be achieved by applying an oscillatory electric field, which induces periodic oscillations in real space of the electron spin inside the quantum dot. This motion inside a magnetic field gradient produces an effective periodic in-plane magnetic field, and allows for driven spin rotations near resonance. The magnetic field gradient induces a valley dependent $g$-factor and a valley dependent Rabi frequency. Our first goal is to quantitatively and qualitatively describe valley dependent $g$-factors and a valley dependent Rabi frequencies using a microscopic model. A valley dependent $g$-factor combined with inter-valley scattering gives rise to a novel electron spin decoherence mechanism. The second goal of our study is to describe the drop of coherence in the presence of inver-valley scattering, and furthermore, to discuss the interplay between valley and spin relaxation. All relevant decoherence mechanisms are quantitatively evaluated by solving a Lindblad master equation. [Preview Abstract] |
Monday, March 14, 2016 8:36AM - 8:48AM |
A5.00004: Electrically and optically detected spin echo of hopping carriers in organic semiconductors Vagharsh Mkhitaryan, Viatcheslav Dobrovitski We develop a theory for electrically and optically detected primary (2-pulse) and stimulated (3-pulse) spin echo produced by the polaron pairs coupled to the nuclear spins in organic semiconductors. The theory employs fully quantum description of the nuclear and polaron spins, and explains how the structure of the echo signal (electron spin echo envelope modulation, ESEEM) depends on the statistics and rate of the polaron hopping. For the primary spin echo the envelope modulation is strong for slow hopping; both modulation amplitude and dephasing time $T_2$ decrease with increasing hopping rate. As the hopping rate increases further, $T_2$ starts to increase again due to motional narrowing, while the primary echo signal becomes exponential without modulation. The stimulated spin echo signal also shows strong envelope modulation for slow polaron hopping. For faster hopping the stimulated echo (unlike the primary echo) shows a modulation which does not disappear for fast hopping, and has the frequency of the nuclear Larmor precession. Besides describing the recent spin echo measurements in $\pi$-conjugated polymers [1], our work provides a way to directly determine the polaron hopping dynamics from the spin echo experiments. [1] H. Malissa et al, Science 345, 1487 (2014). [Preview Abstract] |
Monday, March 14, 2016 8:48AM - 9:00AM |
A5.00005: Tuning magnetic exchange interactions in crystalline thin films of substituted Cobalt Phthalocyanine Naveen Rawat, Lane Manning, Kim-Ngan Hua, Randall Headrick, Michael Bishop, Stephen McGill, Rory Waterman, Madalina Furis Magnetic exchange interactions in diluted organometallic crystalline thin film alloys of Phthalocyanines (Pcs) made of a organo-soluble derivatives of Cobalt Pc and metal-free (H$_2$Pc) molecule and is investigated. To this end, we synthesized a organosoluble CoPc and successfully employed a novel solution-based pen-writing deposition technique to fabricate long range ordered thin films of mixtures of different ratios ranging from 1:1 to 10:1 H$ _{2} $Pc:CoPc. Our previous magnetic circular dichroism (MCD) results on the parent CoPc crystalline thin films identified different electronic states mediating exchange interactions and indirect exchange interaction competing with superexchange interaction. This understanding of spin-dependent exchange interaction between delocalized $ \pi $-electrons with unpaired d spins along with the excitonic delocalization character enabled the further tuning of these interactions by essentially varying the spatial distance between the spins. Furthermore, high magnetic field (B $ < $ 25 T) MCD and magneto-photoluminescence show evidence of spin-polarized band-edge excitons in the same materials. [Preview Abstract] |
Monday, March 14, 2016 9:00AM - 9:12AM |
A5.00006: Measurement of hyperfine fields and the $\Delta $g-effect in $\pi $-conjugated polymer-based OLEDs using multi-frequency electrically detected magnetic resonance Gajadhar Joshi, Hans Malissa, Richard Miller, Lillie Ogden, Douglas Baird, Shirin Jamali, Marzieh Kavand, Kapil Ambal, Johan van Tol, John Lupton, Christoph Boehme Magneto-opto-electronic properties of organic semiconductors, such as organic magnetoresistance or magneto-electroluminescence, are strongly influenced by the interplay of proton induced hyperfine fields to which charge carrier spins are coupled [Nguyen et al., Nat. Mater. 9, 345-352 (2010), McCamey et al. Phys. Rev. Lett. 104, 017601 (2010)]. In addition, the weak but non-negligible and highly inhomogeneously distributed spin-orbit effects caused by the material's structural disorder can affect spin-dependent processes. In order to quantitatively access and discriminate between these mechanisms, we investigate the inhomogeneous broadening of polaron spin-resonances using electrically detected magnetic resonance (EDMR) spectroscopy at various magnetic fields between 3mT and 12T. While random local hyperfine fields cause an external magnetic field-independent line broadening, spin-orbit contributions give rise to a distribution of the charge carrier g-factors. This $\Delta $g effect leads to a resonance line-width contribution that is proportional to the external magnetic field. We observe an EDMR line that is largely field-independent in the low-magnetic field, but shows substantial broadening of line shape at higher fields. [Preview Abstract] |
Monday, March 14, 2016 9:12AM - 9:24AM |
A5.00007: Magnetoresistance detected spin collectivity in organic light emitting diodes Hans Malissa, David P Waters, Gajadhar Joshi, Marzieh Kavand, Mark E Limes, Paul L Burn, John M Lupton, Christoph Boehme Organic magnetoresistance (OMAR) typically refers to the significant change in the conductivity of thin layers of organic semiconductors at low static magnetic fields ($<10$ mT). When radio frequency (rf) radiation is applied to an organic semiconductor under bipolar injection, and in the presence of small magnetic fields $B$, magnetic resonance can occur, which is observed as a change of the OMAR effect [Baker et al., Nat. Commun. 3, 898 (2012)]. When $B$ and the resonant driving field are stronger than local hyperfine fields, an ultrastrong coupling regime emerges, which is marked by collective spin effects analogous to the optical Dicke effect [Roundy and Raikh, Phys. Rev. B 88, 125206 (2013)]. Experimentally, this collective behavior of spins can be probed in the steady state OMAR of organic light-emitting diodes (OLEDs) at room temperature by observation of a sign reversal of the OMAR change under rf irradiation. Furthermore, in the presence of strong driving fields, an ac Zeeman effect can be observed through OMAR [Waters et al., Nat. Phys. 11, 910 (2015)], a unique window to observe room temperature macroscopic spin quantum coherence. [Preview Abstract] |
Monday, March 14, 2016 9:24AM - 9:36AM |
A5.00008: Microwave frequency modulation for improving polarization transfer in DNP experiments Mallory Guy, Chandrasekhar Ramanathan Dynamic nuclear polarization (DNP) is a driven process that transfers the inherently high electron polarization to surrounding nuclear spins via microwave irradiation at or near the electron Larmor frequency. In a typical DNP experiment, the amplitude and frequency of the applied microwaves are constant. However, by adding time dependence in the form of frequency modulation, the electron excitation bandwidth is increased, thereby increasing the number of electron spins active in the polarization transfer process and improving overall efficiency. Both triangular and sinusoidal modulation show a 3 fold improvement over monochromatic irradiation. In the present study, we compare the nuclear spin polarization after DNP experiments with no modulation of the applied microwaves, triangular and sinusoidal modulation, and modulation schemes derived from the sample's ESR spectrum. We characterize the polarization as a function of the modulation amplitude and frequency and compare the optimal results from each modulation scheme. Working at a field of 3.34 T and at a temperature of 4 K, we show that by using a modulation scheme tailored to the electronic environment of the sample, polarization transfer is improved over other modulation schemes. Small-scale simulations of the spin system are developed to gain further insight into the dynamics of this~driven open system. This understanding could enable the design of modulation schemes to~achieve even higher polarization transfer efficiencies. [Preview Abstract] |
Monday, March 14, 2016 9:36AM - 9:48AM |
A5.00009: Resonant and Time-Resolved Spin Noise Spectroscopy Xinlin Song, Brennan Pursley, Vanessa Sih \noindent Spin noise spectroscopy is a technique which can probe the system while it remains in equilibrium. It was first demonstrated in atomic gases and then in solid state systems. Most existing spin noise measurement setups digitize the spin fluctuation signal and then analyze the power spectrum. Recently, pulsed lasers have been used to expand the bandwidth of accessible dynamics and allow direct time-domain correlation measurements. Here we develop and test a model for ultrafast pulsed laser spin noise measurements as well as a scheme to measure spin lifetimes longer than the laser repetition period. For the resonant spin noise technique, analog electronics are used to capture correlations from the extended pulse train, and the signal at a fixed time delay is measured as a function of applied magnetic field. [1]\\ \\ \\ \noindent [1] B. C. Pursley, X. Song, and V. Sih, arXiv: 1508.07383 [Preview Abstract] |
Monday, March 14, 2016 9:48AM - 10:00AM |
A5.00010: Modification of electron spin properties in a GaAs epilayer by an in-plane electric field Michael Macmahon, Vanessa Sih The interaction of electron spins with accelerating electric fields in bulk gallium arsenide results in many effects that are relevant to proposed spin-based devices. For example, in-plane electric fields have been shown to change the g-factor\footnote{M. Luengo-Kovac et al., {\it Phys. Rev. B} {\bf 91}, 201110 (2015)}, generate spin polarization\footnote{B. M. Norman et al., {\it Phys. Rev. Lett.} {\bf 112}, 056601 (2014)}, and decrease the spin lifetime\footnote{M. Furis et al., {\it Appl. Phys. Lett.} {\bf 89}, 102102 (2006)}. Most such studies have used only very low electric fields, typically less than 100 V/cm. We investigate the dependence of spin lifetime on electric field at high electric fields and separate the contribution due to heating. [Preview Abstract] |
Monday, March 14, 2016 10:00AM - 10:12AM |
A5.00011: 3T and nonlocal 4T Hanle measurements of spin accumulation in the persistent photoconductor Al$_{\mathrm{0.3}}$Ga$_{\mathrm{0.7}}$As:Si Joon-Il Kim, K. Kountouriotis, T. Liu, S. von Molnar, P. Xiong, J. Lu, X.Z. Yu, J.H. Zhao 3-terminal (3T) and nonlocal 4-terminal (4T) Hanle measurements have been performed on a spin injection/detection device with patterned Fe electrodes and Al$_{\mathrm{0.3}}$Ga$_{\mathrm{0.7}}$As:Si, a persistent photoconductor, as the channel. The persistent photoconductivity facilitates \textit{in situ} incremental photo-doping of the AlGaAs channel, which enables direct comparisons of the 3T and 4T Hanle results on the same device over a broad range of carrier densities across the insulator-metal transition. Although their magnitudes differ by about an order of magnitude, the 3T and 4T Hanle signals exhibit broad similarities in their dependencies on the injection current and carrier density, as well as the resulting spin lifetimes. Specifically, at each bias current, the magnitudes of both the 3T and 4T Hanle signals are observed to decrease exponentially with increasing carrier density of the AlGaAs deep into the metallic state. The spin lifetimes extracted from the 3T and 4T Hanle curves, both via the FWHM of the Lorentzian fit and the 1D spin drift-diffusion model analysis, show similar values and evolution with the carrier density. [Preview Abstract] |
Monday, March 14, 2016 10:12AM - 10:24AM |
A5.00012: Wurtzite Spin-Lasers GAOFENG XU, Paulo E. Faria Junior, Guilherme M. Sipahi, Igor Zutic Lasers in which spin-polarized carriers are injected provide paths to different practical room temperature spintronic devices, not limited to magnetoresistive effects [1]. While theoretical studies of such spin-lasers have focused on zinc-blende semiconductors as their active regions, the first electrically injected carriers at room temperature were recently demonstrated in GaN-based wurtzite semiconductors [2], recognized also for the key role as highly-efficient light emitting diodes [3]. By focusing on a wurtzite quantum well-based spin-laser, we use accurate electronic structure calculations to develop a microscopic description for its lasing properties. We discuss important differences between wurtzite and zinc-blende spin-lasers. [1] P. E. Faria Junior, G. Xu, J. Lee, N. C. Gerhardt, G. M. Sipahi, and I. Zutic, Phys. Rev. B 92, 075311 (2015). [2] J.-Y. Cheng, T.-M.Wond, C.-W. Chang, C.-Y. Dong, and Y.-F. Chen, Nat. Nanotech. 9, 845 (2014); I. Zutic and P. E. Faria Junior, Nat. Nanotech. 9, 750 (2014). [3] S. Nakamura, Rev. Mod. Phys. 87, 1139 (2015). [Preview Abstract] |
Monday, March 14, 2016 10:24AM - 10:36AM |
A5.00013: Manipulation of spin transfer torque using light Massimo Rontani, Karsten Vendelbjerg, Lu Sham We show that the spin transfer torque induced by a spin-polarized current on a nanomagnet as the current flows through a semiconductor-nanomagnet-semiconductor junction is externally controlled by shining the junction off-resonantly with a strong laser beam. The excitonic coherence driven by the laser dresses the virtual electron-hole pairs coupling conduction and valence bands and inducing an evanescent state in the proximity of the nanomagnet. The Fano-like quantum interference between this localized state and the continuum spectrum is different in the two spin channels and hence it dramatically alters the spin transport, leading to the coherent control of the spin transfer torque. [Preview Abstract] |
Monday, March 14, 2016 10:36AM - 10:48AM |
A5.00014: Microscopic description of a spin laser Paulo E. Faria Junior, Gaofeng Xu, Jeongsu Lee, Nils C. Gerhardt, Guilherme M. Sipahi, Igor Zutic Spin lasers provide interesting possibilities for spintronic applications at room temperature[1]. They have the same elements of a conventional laser, but the injected carriers are spin polarized which allows the output light polarization to have either positive or negative helicity. These devices are commonly implemented as VCSELs, which have the advantage of tuning the photon energy by the cavity design. We investigate a spin VCSEL with a AlGaAs/GaAs quantum well active region using band structure calculations and spin-dependent optical gain. In addition to the desirable properties for steady-state operation and cavity designs, we also show that by applying a uniaxial strain, large values of birefringence $>$ 200 GHz can be achieved[2]. Combined with spin injected carriers, the birefringence in the device allows polarization dynamics much faster than photon intensity dynamics[3]. Our theoretical prediction for high-frequency birefringence was experimentally demonstrated in similar spin VCSELs[4]. [1] I. Zutic and P. E. Faria Junior, Nat. Nanotech. 9, 750 (2014). [2] P. E. Faria Junior et al., PRB 92, 075311 (2015). [3] N. C. Gerhardt et al., APL 99, 151107 (2011). [4] T. Pusch et al., Electron. Lett. 51, 1600 (2015). [Preview Abstract] |
Monday, March 14, 2016 10:48AM - 11:00AM |
A5.00015: Modulating Spin Relaxation with Light and a Novel Spintronic Room Temperature Infrared Photodetector Md Iftekhar Hossain, Saumil Bandyopadhyay, Jayasimha Atulasimha, Supriyo Bandyopadhyay We report modulating the spin relaxation rate in an InSb nanowire with infrared (IR) light. The nanowire is fashioned into a spin valve with cobalt and nickel contacts using electrochemical self-assembly. The spin relaxation length is long in the dark since \textasciitilde 96{\%} of the electrons occupy the lowest conduction subband at room temperature, which results in near elimination of the D'yakonov-Perel' (DP) spin relaxation. Under IR illumination, electrons are excited to higher subbands by IR photons, resulting in the revival of the DP relaxation and a threefold shortening of the spin relaxation length [1]. This changes the resistance of the spin valve and therefore has applications in a novel spintronic IR photodetector that can ideally work at room temperature with infinite light-to-dark contrast ratio, infinite detectivity and zero dark current if all other spin relaxation mechanisms are eliminated and spins can be injected into the nanowire and detected with 100{\%} efficiency. This work is supported by the NSF under grant CMMI-1301013. [1] M. I. Hossain, et al., Nanotechnology, \underline {26}, 281001(2015) [Preview Abstract] |
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