Semiclassical description of electron spin motion in radicals including the effect of electron hopping

TL;DR: In this article, the electron spin correlation tensor is derived from the theory of rotational diffusion, and the effect of electron hopping between molecules on the spin correlations is analyzed in the context of a radical pair.

Abstract: The coherent electron spin motion in radicals induced by the hyperfine coupling to nuclear spins is described semiclassically. The nuclear spins are treated as constant classical vectors around which the electron spin precesses. The ensemble average over all nuclear spin configurations is taken yielding the electron spin correlation tensor

TL;DR: It is demonstrated coherent control of a quantum two-level system based on two-electron spin states in a double quantum dot, allowing state preparation, coherent manipulation, and projective readout based on rapid electrical control of the exchange interaction.

Abstract: We demonstrated coherent control of a quantum two-level system based on two-electron spin states in a double quantum dot, allowing state preparation, coherent manipulation, and projective readout. These techniques are based on rapid electrical control of the exchange interaction. Separating and later recombining a singlet spin state provided a measurement of the spin dephasing time, T2*, of E10 nanoseconds, limited by hyperfine interactions with the gallium arsenide host nuclei. Rabi oscillations of two-electron spin states were demonstrated, and spin-echo pulse sequences were used to suppress hyperfine-induced dephasing. Using these quantum control techniques, a coherence time for two-electron spin states exceeding 1 microsecond was observed.

TL;DR: It is shown that electron spin flips are dominated by nuclear interactions and are slowed by several orders of magnitude when a magnetic field of a few millitesla is applied, having significant implications for spin-based information processing.

Abstract: The GaAs double quantum dot is the classic spin qubit widely studied for its potential as information carrier in quantum computers. The discovery that electron spin flips in this system are governed by nuclear interactions, and slowed dramatically by a weak magnetic field, is promising in terms of the control and manipulation of spin-based memory. The spin of a confined electron, when oriented originally in some direction, will lose memory of that orientation after some time. Physical mechanisms leading to this relaxation of spin memory typically involve either coupling of the electron spin to its orbital motion or to nuclear spins1,2,3,4,5,6,7. Relaxation of confined electron spin has been previously measured only for Zeeman or exchange split spin states, where spin-orbit effects dominate relaxation8,9,10; spin flips due to nuclei have been observed in optical spectroscopy studies11. Using an isolated GaAs double quantum dot defined by electrostatic gates and direct time domain measurements, we investigate in detail spin relaxation for arbitrary splitting of spin states. Here we show that electron spin flips are dominated by nuclear interactions and are slowed by several orders of magnitude when a magnetic field of a few millitesla is applied. These results have significant implications for spin-based information processing12.

TL;DR: In this article, a detailed account of investigations on the functioning of the reaction center protein with a number of optical and magnetic resonance spectroscopic techniques is presented, with emphasis on the relation between structure and function.

TL;DR: In this paper, a progress report discusses magnetically sensitive excited states and charge-transport processes involved in magnetic field effects (MFEs) in non-magnetic organic semiconducting materials.

Abstract: It has been experimentally discovered that a low magnetic field (less than 500 mT) can substantially change the electroluminescence, photoluminescence, photocurrent, and electrical-injection current in nonmagnetic organic semiconducting materials, leading to magnetic-field effects (M FEs). Recently, there has been significant driving force in understanding the fundamental mechanisms of magnetic responses from nonmagnetic organic materials because of two potential impacts. First, MFEs can be powerful experimental tools in revealing and elucidating useful and non-useful excited processes occurring in organic electronic, optical, and optoelectronic devices. Second, MFEs can lead to the development of new multifunctional organic devices with integrated electronic, optical, and magnetic properties for energy conversion, optical communication, and sensing technologies. This progress report discusses magnetically sensitive excited states and charge-transport processes involved in MFEs. The discussions focus on both fundamental theories and tuning mechanisms of MFEs in nonmagnetic organic semiconducting materials.

TL;DR: The radical pair mechanism is discussed as a possible route whereby a magnetic field of environmental strength might affect a biological system, and arguments are provided to suggest that the encounters of freely diffusing pairs (F-pairs) of radicals are unlikely to produce significant effects in biology.

Abstract: The radical pair mechanism is discussed as a possible route whereby a magnetic field of environmental strength might affect a biological system. It is well established as the origin of reproducible field effects in chemistry, and these can be observed even at very low magnetic field strengths, including that of the geomagnetic field. Here it is attempted to give a description which might assist experimentalists working in biological laboratories to devize tests of its relevance to their work. The mechanism is well understood and a specific theoretical approach is taken to explore and emphasize the importance of the lifetime of the radical pair and the precise chemical natures of the radicals which comprise it in affecting the size of the lowfield effect. Further subsequent processes are likely necessary to cause this primary effect to attain biological significance. Arguments are provided to suggest that the encounters of freely diffusing pairs (F-pairs) of radicals are unlikely to produce significant eff...

TL;DR: In this article, the Debye model of rotational diffusion by small angular steps is generalized to allow molecular reorientation through angular steps of arbitrarily large size, and the generalized diffusion models are found to give a rather accurate representation of molecular re-orientation in liquids and gases.

Abstract: The Debye model of rotational diffusion by small angular steps is generalized to allow molecular reorientation through angular steps of arbitrarily large size. The generalized diffusion models are found to give a rather accurate representation of molecular reorientation in liquids and gases, as observed in the infrared and Raman spectra of simple molecules. One interesting feature of both the theoretical and experimental correlation functions is that the approach to rotational equilibrium often takes the form of a damped oscillation, rather than the monotonic decay which is usually assumed.

TL;DR: In this article, an approximate version of the Liouville equation is proposed to predict the geminate recombination process, which is based on the hyperfine-coupling-induced coherent motion of the unpaired electron spins.

Abstract: Pairs of radical ions generated in polar solvents by photoinduced electron transfer either recombine within a few nanoseconds or separate. The (geminate) recombination process is governed by a hyperfine‐coupling‐induced coherent motion of the unpaired electron spins which can be modulated by weak external magnetic fields. The process which also generates the well‐known CIDNP and CIDEP effects is described theoretically by a stochastic Liouville equation comprising for realistic systems a large set of coupled diffusion equations. For the integration of these equations a finite‐difference algorithm with space and time discretization is developed. By comparison with exact solutions of the Liouville equation for model systems, it is demonstrated that an approximate Liouville equation which entails only two coupled diffusion equations for singlet and triplet radical pairs, respectively, suffices to predict the geminate recombination yields accurately. The approximate Liouville equation is employed then to stud...

TL;DR: In this paper, a theoretical description of the spin-selective recombination of radicals is provided based on a coherent spin motion superimposed on the diffusive motion of the radicals, induced by the hyperfine coupling between electron and nuclear spins.

Abstract: Pairs of radical ions generated in polar solvents by photoinduced electron transfer either recombine within a few nanoseconds to singlet and triplet products or separate On the basis of recent time‐resolved observations of a magnetic field dependence of the pair recombination a theoretical description of this process is provided The description, similar to the radical pair theory of CIDNP and CIDEP, is founded on a coherent spin motion superimposed on the diffusive motion of the radicals The spin motion is induced by the hyperfine coupling between electron and nuclear spins and can be modulated by low (0–200 G) magnetic fields The spin‐selective recombination of radicals is accounted for by a Feshbach optical potential The diffusion process described by a Smoluchowski operator depends sensitively on the solvent properties For the case of free Brownian motion, simple analytical expressions for the time‐ and magnetic‐field‐dependent recombination yields are derived For the Brownian motion of oppositely charged radical ions a differential–difference approximation is used to demonstrate the dependence of the recombination yields on the viscosity and polarity of the solvent medium as well as on the strength of the hyperfine coupling and on the rate of the electron back transfer