Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session A16: Focus Session: Spin-Dependent Physics in Organic Compounds |
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Sponsoring Units: GMAG DMP Chair: Christoph Boehme, University of Utah Room: 318 |
Monday, March 18, 2013 8:00AM - 8:36AM |
A16.00001: Competing Mechanisms in Organic Magnetoresistance Invited Speaker: Bert Koopmans A surprisingly large ``organic magnetoresistance'' (OMAR) has been found in both polymers and small molecule organic semiconductors at relatively small applied magnetic fields ($\sim$ 5 mT) and at room temperature. Unlike spin-injection devices, where the occurrence of a finite \textit{spin polarization} of the current is essential for measuring a finite magnetoresistance, OMAR is generally considered to be due to \textit{spin correlations} between spin carrying particles in the organic material. Although the microscopic mechanisms of hyperfine field induced spin mixing are relatively well understood, it is still intensively debated which particles are involved and how they can affect the current in such a drastic manner. In this presentation recent developments and new insights as to the underlying physics are discussed. Quantitative models will be introduced, based on different pairs of particles and mechanisms, and giving rise to effects at a variety of field scales. It will be discussed how specific device physics causes a non-trivial relation between microscopic spin-dependent reactions and macroscopic device behaviour. Finally, it will be shown how comprehensive studies on especially engineered organic systems, including polymer-fullerene blends and molecular doping, can be used to pinpoint the relevance of different mechanisms in the complementary regimes. The experimentally observed linewidth, sign and amplitude of both ``high-field'' (\textgreater 100 mT) and ``low-field'' ($\sim$ 5 mT) effects, as well as their bias voltage dependence display very pronounced features as a function of fullerene doping. They provide unique fingerprints for which mechanism is of relevance. After careful analysis, this allows for identification of three earlier proposed mechanisms, involving exciton-charge, electron-hole and bipolaron (polarons of like charge) reactions. Present activities are aiming at using this insight for tailoring OMAR response by design. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A16.00002: Recent Advance in Organic Spintronics and Magnetic Field Effect Invited Speaker: Z. Valy Vardeny In this talk several important advances in the field of Organic Spintronics and magnetic field effect (MFE) of organic films and optoelectronic devices that have occurred during the past two years from the Utah group will be surveyed and discussed. (i) Organic Spintronics: We demonstrated spin organic light emitting diode (spin-OLED) using two FM injecting electrodes, where the electroluminescence depends on the mutual orientation of the electrode magnetization directions [1]. This development has opened up research studies into organic spin-valves (OSV) in the space-charge limited current regime. (ii) Magnetic field effect: We demonstrated that the photoinduced absorption spectrum in organic \textit{films} (where current is not involved) show pronounced MFE [2]. This unravels the underlying mechanism of the MFE in organic devices, to be more in agreement with the field of MFE in Biochemistry. (iii) Spin effects in organic optoelectronic devices: We demonstrated that certain spin 1/2 radical additives to donor-acceptor blends substantially enhance the power conversion efficiency of organic photovoltaic (OPV) solar cells [3]. This effect shows that studies of spin response and MFE in OPV devices are promising.\\[4pt] In collaboration with T. Nguyen, E. Ehrenfreund, B. Gautam, Y. Zhang and T. Basel.\\[4pt] [1] Nguyen \textit{et al}., \textit{Science} 337, 204 (2012);\\[0pt] [2] Gautam \textit{et al}. PRB 85, 205207 (2012);\\[0pt] [3] Zhang \textit{et al}. \textit{Nature Commun}. 3, 1043 (2012). [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A16.00003: Spin-dependent charge carrier recombination in PCBM Hiroki Morishita, William J. Baker, David P. Waters, Rachel Baarda, John M. Lupton, Christoph Boehme We present room temperature pulsed electrically detected magnetic resonance (pEDMR) measurements on [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) (electron acceptor) thin film unipolar and bipolar devices. Our study aimed at identifying the dominating spin-dependent transport and recombination processes therein. Experimentally, the devices were operated under a constant positive bias, and the resultant transient current response was then monitored after the application of a short resonant microwave pulse excitation. The measurements did not reveal any observable signal for unipolar electron devices which suggests that spin-dependent transport mechanisms are not dominant in PCBM. However, under bipolar injection, at least two pronounced spin-dependent signals were detected whose magnitudes increased as the devices degraded upon exposure to air. Electrical detection of spin-Rabi beat oscillation revealed that one of these two signals is due to weakly coupled pairs of spins with s$=$1/2. We therefore attribute this signal to electron-hole recombination. This observation shows that while PCBM is a poor hole conductor, hole injection can be significant. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A16.00004: Robust Absolute Magnetometry with Organic Thin-Film Devices David P. Waters, William J. Baker, Kapil Ambal, Rachel Baarda, Hiroki Morishita, Kipp van Schooten, Dane R. McCamey, John M. Lupton, Christoph Boehme Magnetometers based on organic thin film materials have attracted considerable interest in recent years as they can be manufactured at very low cost and on flexible substrates. In spite of these advantages, the technological relevance of such magnetoresistive sensors is limited due to their narrow magnetic field ranges ($\sim$30mT) and the continuous calibration required to compensate temperature fluctuations and materials degradation. Conversely, magnetic resonance based sensors, which utilize fundamental physical relationships for extremely precise measurements of fields, are usually large and expensive. This presentation will discuss an organic magnetic resonance based magnetometer [1], employing spin-dependent electronic transitions in an organic diode, which combines the low-cost thin-film fabrication and integration properties of organic electronics with the precision of a magnetic resonance based sensor.\\[4pt] [1] Baker et al., Nature Commun. 3, 898 (2012). [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A16.00005: Organic magnetoresistance near saturation: mesoscopic effects in small devices Robert Roundy, Zeev Vardeny, Mikhail Raikh In organic light emitting diodes with small area the current may be dominated by a finite number, $N$ of sites in which the electron-hole recombination occurs. As a result, averaging over the hyperfine magnetic fields, ${\mathbf b}_h$, that are generated in these sites by the environment nuclei is incomplete. This creates a random ({\em mesoscopic}) current component, $\delta I({\mathbf B})$, at field ${\mathbf B}$ having relative magnitude $\sim N^{-1/2}$. We demonstrate that mesoscopic fluctuations develop at fields $|{\mathbf B}| \gg |{\mathbf b}_h|$, where the average magnetoresistance is near saturation. These fluctuations originate from the slow beating between $S$ and $T_{0}$ states of the recombining $e$-$h$ spin pair-partners. We identify the most relevant processes responsible for the current fluctuations as due to anomalously slow beatings that develop in sparse $e$-$h$ polaron pairs at sites for which the ${\mathbf b}_h$ projections on the external field direction almost coincide. To find the characteristic period $\Delta {\mathbf B}$ of the fluctuations, we calculate the correlator $K({\mathbf B}, \Delta {\mathbf B}) = \left<\delta I\left( {\mathbf B} - \Delta {\mathbf B} \right) \delta I\left( {\mathbf B} + \Delta {\mathbf B} \right)\right>$. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A16.00006: Photocontrolled spin polarization at hybrid organic-ferromagnetic interfaces Yan Wang, Hai-Ping Cheng We report a first-principles study of magnetic properties at an organic-ferromagnetic interface by placing light-switchable azobenzene molecules on a Fe/W(110) surface. Our calculations clearly demonstrate that the magnetic properties of the hybrid interface, such as the local magnetic moment and spin polarization, change significantly as the azobenzene molecule switches reversibly from the trans to the cis form. The molecule-surface interaction, which determines the feasibility of photo-switching of the azobenzene on the surface, can be altered by chemical functionalization of the molecule. Specifically, we find that substitution of the H atoms with electronegative F atoms substantially reduces the binding energies of the molecule on the Fe surface. This study suggests a new way to manipulate magnetism by application of light at organic-ferromagnetic hybrid interfaces. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A16.00007: The Effects of Fringe Fields on Organic Magnetoresistance Nicholas Harmon, Ferran Maci\`a, Fujian Wang, Markus Wohlgenannt, Andrew Kent, Michael Flatt\'e The importance of random hyperfine fields is now widely acknowledged as a vital ingredient for the phenomena of organic magnetoresistance (OMAR). Recent experiments (Phys. Rev. X 2 021013 (2012)) have shown that another type of random field - fringe fields due to a nearby ferromagnet - can also dramatically affect magnetoconductivity. A theoretical analysis of the fringe field OMAR is challenging due to the different properties of the fringe fields when compared to the hyperfine fields. For instance, the range of fringe field strengths is 1-2 orders of magnitude larger than that of the hyperfine couplings. The correlation length between fringe fields is also larger by the same degree. We use a recent theory of OMAR that is well-suited to numerically calculate the magnetoresistance with both hyperfine and fringe fields present. We find agreement with key features of experimental fringe-field magnetoresistance dependences on applied magnetic field, including the field values of extrema of the magnetoresistance, the region of large magnetoresistance effects from the fringe fields, and the sign of the effect. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A16.00008: Using photoexcited triplet states to probe small-molecule endohedral fullerenes by ESR Vasileia Filidou, Salvatore Mamone, Alessandro Bagno, Federico Rastrelli, Yasujiro Murata, Koichi Komatsu, Xuegong Lei, Yongjun Li, Nicholas J. Turro, Malcolm H. Levitt, John J.L. Morton Ortho to para conversion of molecular hydrogen $\mbox{H}_{2} $ can be catalyzed by the use of a coupled paramagnet such as a fullerene in its triplet state. The recently synthesized endohedral fullerenes $\mbox{H}_{2} @\mbox{C}_{60} $ and $\mbox{H}_{2} @\mbox{C}_{70} $ were photoexcited to their long lived triplet state (S= 1) and probed by electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) spectroscopic techniques. With these techniques we characterized both spin systems by extracting the hyperfine interaction the kinetic parameters of the triplet state and the spin relaxation times. The observed variations of the linewidths and the lineshape are discussed in the context of a dynamic Jahn-Teller effect. Irradiation of the $\mbox{H}_{2} @\mbox{C}_{70} $ at different temperatures reveals that the fullerene triplet state can serve as a spin catalyst for ortho to para interconversion while for the triplet $\mbox{H}_{2} @\mbox{C}_{60} $ no appreciable interconversion is observed [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A16.00009: Electron Spin Relaxation Dynamics in Single-Walled Carbon Nanotubes William Rice, Ralph Weber, Pavel Nikolaev, Sivaram Arepalli, Vladimir Burka, Ah-Lim Tsai, Junichiro Kono We have measured temperature-dependent electron spin resonance (ESR) in an ensemble of single-walled carbon nanotubes. From the linewidths of these traces, we clearly observe that the spin-spin dephasing time, $T_{2}$, decreases by over a factor of two when temperature, $T$, is lowered from 300 K to 3 K, a phenomenon we attribute to motional narrowing. We fit the temperature dependence of $T_{2}$ with a hopping model and obtain a spin hopping frequency of 285 GHz. At selected temperatures below 100 K, we performed microwave power-dependent scans to investigate the saturation behavior of the ESR signal. A homogenously broadened two-level model fit the saturation data well, which allowed us to extract the spin-lattice relaxation times, $T_{1}$, for the investigated temperature range. We observed that the spin-lattice relaxation rate,1/$T_{1}$, is proportional to $T$ from 100 K to 3 K, suggesting that the relaxation occurs via phonon emission. Last, we show that the Dysonian lineshape asymmetry, which is roughly proportional to the conductivity, follows a three-dimensional variable-range hopping behavior from 3 K to 20 K, from which we estimate a spin hopping localization length of 100 nm. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A16.00010: Mechanical read out of a single electron spin in a carbon nanotube Guido Burkard, Heng Wang, Philipp Struck The spin of a single electron in a suspended carbon nanotube can be read out by using its coupling to the nano-mechanical motion of the nanotube. To show this, we consider a single electron confined within a quantum dot formed by the suspended carbon nanotube. The spin-orbit interaction induces a coupling between the spin and one of the bending modes of the suspended part of the nanotube [1]. We simulate the response of the system to the external driving with a Jaynes-Cummings model by solving the quantum master equation. Using parameters comparable to those used in recent experiments, we show how information of the spin state of the system can be acquired by measuring its mechanical motion [2]. The mechanical motion can be detected by observing the current through a nearby charge detector. \\[4pt] [1] A. Palyi, P.R. Struck, M. Rudner, K. Flensberg, G. Burkard, Phys. Rev. Lett. 108, 206811 (2012).\\[0pt] [2] H. Wang, P. R. Struck, G. Burkard, manuscript in preparation. [Preview Abstract] |
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