Molecular Spintronics

Design, prepare, and study molecules and materials that display coherent spin transport with possible use in quantum computation.

Modern pulsed magnetic resonance techniques provide an important avenue into developing a quantum computer. Single qubit operations have been demonstrated using nuclear spins while two qubit operations can be embodied in two-spin systems that are coupled via a spin-spin exchange interaction. There are several important advantages of using electron spins rather than nuclear spins for quantum computation. 1) The gyromagnetic ratio of the electron, ?e is about a thousand times larger than that of nuclei, so that the inherent spectral crowding in nuclear magnetic resonance (NMR) is relieved in electron paramagnetic resonance (EPR). Thus pure spin states are more accessible by EPR. 2) Due to the larger value of ?e, the sensitivity of EPR is much higher than that of NMR. 3) The relaxation times of electron spins are generally 103-106 times faster than those of nuclear spins, leading to increased computational speed. Decoherence times of electron spins are generally not a problem and can be addressed through appropriate choices of materials having longer transverse relaxation times, T2, which are on the order of 10 ?s, and by the application of high-field, time-resolved EPR (TREPR) with a time resolution of 10 ns. 4) Coherence is maintained at room temperature permitting operation of devices at ambient conditions.

Photogeneration of pairs of correlated electron spins as a result of single electron transfer from an organic donor to an acceptor on a picosecond time scale provides important advantages for the development of materials for quantum computing: 1) Ultrafast photogeneration of organic radical pairs (spin pairs) results in formation of a highly spin-polarized pair in which the initial spin state is well defined. This addresses the need for the CLEAR operation in a quantum computing algorithm. 2) Organic radical pairs (RPs) are capable of exhibiting coherent spin motion over time scales that are typically microseconds, which is considerably longer than coherent phenomena involving photogenerated excited states. This makes it possible that coherent spin motion can provide the basis for new organic information processing devices, which use the interaction of two spin-spin exchange coupled qubits that are regenerated repetitively in a highly spin-polarized initial state using laser pulses.

Immediately following rapid, nonadiabatic charge separation, the correlated electron spins are in a singlet configuration. After times usually in the range of a few nanoseconds, radical pair intersystem crossing (RP-ISC) results in formation of a triplet spin configuration. When hyperfine and exchange interactions are isotropic and spin-spin coupling is weak, each of the three zero-field triplet states of the RP will be nearly degenerate with the singlet and will be populated with nearly equal probability at room temperature. If the spin-spin exchange interaction within the RP is nonzero, the triplet manifold is not initially degenerate with the singlet, but rather separated from the singlet by the energy 2J, Figure 1.

Application of a magnetic field results in Zeeman splitting of the triplet sublevels, which at high fields can be described by the T0 and T?1 states, Figure 1. In the high field limit, population of the RP triplet state occurs exclusively by S-T0 mixing, while T-1 and T+1 remain unpopulated. Electron paramagnetic resonance (EPR) measurements on compounds related to those proposed for study here have confirmed that the triplet levels of the radical pair are higher in energy than the singlet state as would be expected from net antiferromagnetic exchange. When the Zeeman energy from the applied field equals that of the S-T splitting, the low energy triplet state, T-1, crosses the singlet, and the RP-ISC rate is maximized, which produces a resonance in the triplet yield at B2J. An increase in the rate of triplet formation at resonance implies that the RP decay rate also increases. One can therefore monitor the RP population as a function of applied magnetic field and obtain a plot with a minimum at B2J as well.

Page 1 2

Research Main