Manipulating spin in organic spintronics Peter A. Bobbert Science 345, 1450 (2014); DOI: 10.1126/science.1259655
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INSIGHTS | P E R S P E C T I V E S
Manipulating spin in organic spintronics Probing the interplay between the electronic and nuclear spins in organic semiconductors By Peter A. Bobbert
Department of Applied Physics, Technische Universiteit Eindhoven, 5600 MB Eindhoven, Netherlands. E-mail: [email protected]
Hyperfne interaction S spin pairs
T spin pairs
T excitons Balance between singlet and triplet spin pairs. A key ingredient in the work of Malissa et al. is the presence of singlet (S) and triplet (T) spin pairs of an electron and a hole (blue and red arrows) that have almost no mutual spin interaction but can form S and T excitons (electron-hole bound states) with different rates. The balance between the amounts of S and T spin pairs is then a measure of their rate of interconversion, which can occur by rotation of one of the spins driven by a microwave pulse or by hyperfine interaction with a proton spin (green arrow). Precession of the proton spin modulates the interconversion and therefore the balance, which is observed in a modulation of the current.
encounters of electrons and holes injected into the polymer by opposite electrodes. A key aspect of the process is a decoupling of the evolution of the spins in the pair. Small external magnetic couplings of the spins to a static or time-varying magnetic field and to surrounding nuclear magnetic moments via the hyperfine coupling are then able to change the interconversion rate between the S and T spin-pair states. Because of the different reaction rates to form an S or T state with strongly coupled spins (excitons, or strongly bound electron-hole pairs, in the OLEDs of Malissa et al.), the balance between the amounts of S and T spin pairs is a measure for the S↔T interconversion rate in the spin pair and therefore of external magnetic couplings of the spins. In the bird’s compass, the S↔T spin-pair balance is assumed to be “measured” by chemical detection of a signaling reaction product (2). In the OLED of Malissa et al., a change in the S↔T balance changes the current, allowing the convenient electrical detection of
external magnetic couplings. It has become clear that the initially very puzzling effect of organic magnetoresistance in OLEDs and other organic devices (4) is based on a very similar mechanism (5). By a clever combination of microwave pulses in the presence of a static magnetic field B0 (creating an easily detectable feature called a “Hahn echo” in the current, in an electron-spin resonance setup), Malissa et al. deliberately manipulate the electron spin by rotating it over prescribed angles and detect the resulting disturbed S↔T spin-pair balance by monitoring the current as a function of a delay time between two of the pulses. The fascinating aspect of this experiment is that a modulation in the current is observed that corresponds precisely with the nuclear spin precession frequency of a proton in the field B0: It appears that the rotation of electron spins has brought proton spins into precessional motion around B0 via the hyperfine interaction, “kicking back” with their precession sciencemag.org SCIENCE
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he growing interest in spin manipulation in the field of spin electronics, or “spintronics,” is due to the wealth of exciting possibilities that it offers in areas of magnetic sensing, new types of information storage, lowpower electronics, and quantum information processing. Nuclear spin manipulation is especially attractive, as nuclear spin lifetimes can reach minutes or more. Addressing nuclear spins in a direct way is, however, difficult. Fortunately, indirect addressing is possible via their hyperfine interaction with electronic spins, which can be conveniently probed electronically or optically. To date, research efforts have focused mainly on conventional semiconductors such as silicon, germanium, diamond, and III–V compound semiconductors. On page 1487 of this issue, Malissa et al. (1) demonstrate the combined manipulation of the electronic and nuclear spins in an organic semiconductor. The use of organic materials for organic spintronics opens many new roads, mainly because of the sheer number of varieties in which they can be synthesized. It even appears that nature utilizes organic spintronics in a quantum-biological compass that allows organisms to probe Earth’s magnetic field (2). A clear indication of a spinrelated compass is the disruption of the orientation capability of migratory birds by electromagnetic noise (3). The proposed functioning mechanism of this biological compass and the detection mechanism in the spin manipulation of Malissa et al. are closely related. The common key ingredient is pairs of electronic spins undergoing spinselective reactions to a spin-0 (singlet, S) or a spin-1 (triplet, T) state (see the figure). In the compass of the bird, these spin pairs are believed to be created by optical excitation of a donor-acceptor protein complex in the retina, where a negatively charged electron is transferred from the donor to the acceptor, leaving a positively charged “hole” on the donor. Malissa et al. conduct their experiments on organic light-emitting diodes (OLEDs) with a conjugated polymer as the active layer, where spin pairs are created by
ILLUSTRATION: ADAPTED BY P. HUEY/SCIENCE
frequency at the electron spins via the same interaction. Performing the experiment with a deuterated instead of hydrogenated polymer changes the modulation frequency to the deuteron precession frequency (a factor of 6.6 times smaller), which is a nice confirmation of this picture. In a second experiment, Malissa et al. manipulate both the electron and nuclear spins (in an electron-nuclear double resonance setup) by using, in addition to the microwave pulses, a radiofrequency pulse to rotate the proton spins. Because the hyperfine couplings with surrounding proton spins create a small random magnetic field, the field B0 at which resonance with the microwave field occurs is slightly different for each electron spin. A specific field B0 can therefore select a subensemble of electron spins coupling predominantly to proton spins that are directed, say, upward. After a radiofrequency pulse that swaps the proton spins by 180° downward, the subensemble of these electron spins is no longer in resonance with the microwave field, and therefore these spins become insensitive to subsequent microwave pulses. The swapping of the proton spins can then be observed via the current. This provides an intriguing new approach to detect nuclear spin manipulation electrically. The study of Malissa et al. is an important step toward the use of organic materials in quantum-coherent spin manipulation. A great advantage as compared to other spin manipulation approaches is the operation at room temperature. Nature may have paved the way here for us in the development of a quantum-biological compass, obviously operating at ambient temperature. However, the disadvantages at present are the lack of control over the involved electronic processes, which are mostly due to the morphological disorder of the used conjugated polymer. Coherence times are presently limited by the uncontrolled lifetime of the spin pairs. Increasing and controlling morphological order by embedding a semiconducting organic molecule in the pores of a zeolite crystal have led to a spectacular increase in organic magnetoresistance and the option to address only a few molecules (6). This may also be the way to bring spin manipulation in organic materials to a next level of control. ■ REFERENCES
1. 2. 3. 4.
H. Malissa et al., Science 345, 1487 (2014). T. Ritz et al., Biophys. J. 96, 3451 (2009). S. Engels et al., Nature 509, 353 (2014). T. L. Francis, Ö. Mermer, G. Veeraraghavan, M. Wohlgenannt, New J. Phys. 6, 185 (2004). 5. M. Wohlgenannt, P. A. Bobbert, B. Koopmans, MRS Bull. 39, 590 (2014). 6. R. N. Mahato et al., Science 341, 257 (2013). 10.1126/science.1259655
Na Mg 19
Rb Sr 55
Cr Mn Fe
Nb Mo 73
Db Sg 59
Bh Hs Mt
Nd Pm Sm Eu
Np Pu Am Cm Bk
Zn Ga Ge
Cn Uut Fl
Uup Lv Uus Uuo
Dy Ho Cf
Es Fm Md No
The modern Periodic Table. Similar chemical behavior might be expected for the homologs (Mo, W, Sg), (Hg, Cn) and (Pb, Fl), but relativistic effects can cause deviations from the expected behavior.
Superheavy carbonyls The radioactive superheavy element seaborgium can form a carbonyl compound during its short lifetime By Walter Loveland
lements with atomic numbers Z > 94 are radioactive and cannot be found naturally in Earth. Efforts to create these elements, particularly the superheavy elements (Z ≥ 104), in the laboratory typically use “hot fusion” reactions involving extremely high temperatures. On page 1491 of this issue, Even et al. (1) show how these atoms can be cooled down and used to synthesize a new class of chemical compounds, the superheavy metal carbonyls. The results confirm a 15-yearold prediction of relativistic quantum chemistry. The known superheavy elements, referred to as the transactinides, range in atomic number from 104 (Rf ) to the as yet unnamed element 118 (see the first figure). The lifetimes of many of these elements range from milliseconds to minutes. In the case of Z = 112 (Cn) to Z = 118, current methods only allow the production of a few atoms per week. Chemistry must be done on a “one atom at a time” basis, making these studies very difficult. Relativistic effects shape the chemistry of these elements. The speeds of the innermost electrons in these atoms approach the speed of light, requiring the use of relativistic quantum mechanics to describe the atoms and molecules. The relativistic effects consist of a contraction and stabilization of the innermost s and p orbitals, the splitting
of electron energy levels due to spin-orbit coupling, and an expansion (and destabilization) of the outer d orbital and all f orbitals. In the transactinides, stabilization of the 7s orbital of Cn doubles its binding energy. In Sg, the 7s and 6d orbital ordering is inverted, possibly changing its oxidation properties. The spin-orbit splitting, like the other relativistic properties, increases roughly as Z 2, approaching the energies of chemical bonds in magnitude. The periodic table of the elements is not a list of the elements ordered by increasing atomic number, but rather a grouping of the elements by chemical properties. It is thus a living document. The placement of new elements such as the transactinides in the table depends on measurements of their chemical properties. Experimental studies of the chemistry of the transactinides (2) support the placement of the elements Rf (104), Db (105), Sg (106), Bh (107), Hs (108), and Cn (112) into groups 4, 5, 6, 7, 8, and 12 of the periodic table, respectively, forming the fourth row of transition metals; however, the real possibility of deviations exists. For example, element 112 (Cn) is nominally a homolog of Hg; element 114 (Fl) is a homolog of Pb; thermochromatography experiments have shown that Cn behaves like Hg (3); but it has been difficult to determine whether Fl is a noble gas or a noble Oregon State University, Corvallis, OR 97331, USA. E-mail: [email protected]
19 SEP TEMBER 2014 • VOL 345 ISSUE 6203
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