We study the spin dynamics of an electron-hole polaron pair in a random hyperfine magnetic field and an external field, B0, under a resonant drive with frequency ω0 = γB0. The fact that the pair decays by recombination exclusively from a singlet configuration, S, in which the spins of the pairpartners are entangled, makes this dynamics highly nontrivial. Namely, as the amplitude, B1, of the driving field grows, mixing all of the triplet components, the long-living modes do not disappear, but evolve from T+, T− into 1 2 T+ ± √ 2T0 + T− . Upon further increase of B1, the lifetime of the Smode is cut in half, while the T0-mode transforms into an antisymmetric combination(T+ − T−) and acquires a long lifetime, in full analogy to the superradiant and subradiant modes in the Dicke effect. Peculiar spin dynamics translates into a peculiar dependence of the current through an organic device on B1. In particular, at small B1, the radiation-induced correction to the current is linear in B1.
Relaxation, Sz(t) , of the average spin of a carrier in course of hops over sites hosting random hyperfine fields is studied theoretically. In low dimensions, d = 1, 2, the decay of average spin with time is non-exponential at all times. The origin of the effect is that for d = 1, 2 a typical random-walk trajectory exhibits numerous self-intersections. Multiple visits of the carrier to the same site accelerates the relaxation since the corresponding partial rotations of spin during these visits add up. Another consequence of self-intersections of the random-walk trajectories is that, in all dimensions, the average, Sz(t) , becomes sensitive to a weak magnetic field directed along z. Our analytical predictions are complemented by the numerical simulations of Sz(t) .
In an external magnetic field B, the spins of the electron and hole will precess in effective fields be + B and bh + B, where be and bh are random hyperfine fields acting on the electron and hole, respectively. For sparse "soft" pairs the magnitudes of these effective fields coincide. The dynamics of precession for these pairs acquires a slow component, which leads to a slowing down of recombination. We study the effect of soft pairs on organic magnetoresistance, where slow recombination translates into blocking of the passage of current. It appears that when be and bh have identical gaussian distributions the contribution of soft pairs to the current does not depend on B. Amazingly, small inequivalence in the rms values of be and bh gives rise to a magnetic field response, and it becomes progressively stronger as the inequivalence increases. We find the expression for this response by performing the averaging over be, bh analytically. Another source of magnetic field response in the regime when current is dominated by soft pairs is inequivalence of the g-factors of the pair partners. Our analytical calculation indicates that for this mechanism the response has an opposite sign.
Motivated by recent experiments, where the tunnel magnetoresitance (TMR) of a spin valve was measured locally, we theoretically study the distribution of TMR along the surface of magnetized electrodes. We show that, even in the absence of interfacial effects (like hybridization due to donor and acceptor molecules), this distribution is very broad, and the portion of area with negative TMR is appreciable even if on average the TMR is positive. The origin of the local sign reversal is quantum interference of subsequent spin-rotation amplitudes in course of incoherent transport of carriers between the source and the drain. We find the distribution of local TMR exactly by drawing upon formal similarity between evolution of spinors in time and of reflection coefficient along a 1D chain in the Anderson model. The results obtained are confirmed by the numerical simulations.PACS numbers: 72.15. Rn, 72.25.Dc, 75.40.Gb, Introduction. Organic spin valves (OSVs), being one of the most promising applications of organic spintronics, are actively studied experimentally [1][2][3][4][5][6][7][8][9] . The organic active layer of an OSV is sandwiched between two magnetized electrodes. Due to long spin-relaxation times of carriers in organic materials, the net resistance of OSV is sensitive to the relative magnetizations of the electrodes. Among many advantages that OSVs offer, is wide tunability due to e.g. chemical doping, and enormous flexibility. The processes that limit the performance of OSVs can be conventionally divided into two groups: (i) interfacial, which take place at the interfaces between the electrodes and active layer [11][12][13][14][15][16][17][18] , and (ii) intralayer, which exist even if the interfaces are ideal. 19,20 Due to the latter processes the injected polarized electrons, Fig. 1, lose memory about their initial spin orientation while traveling between the electrodes. One of the most prominent mechanisms of this spin-memory loss is the precession of a carrier spin in random hyperfine fields of hydrogen nuclei 5,19,20 . The effectiveness of the OSV performance is quantified by tunnel magnetoresistance (TMR) given by a so-called modified Julliere's formula 22 , see e.g. the review Ref. 21,where P 1 , P 2 stand for polarizations of the electrodes. The difference from the original Julliere's formula 22 is the exponential factor Q = exp(−d/λ s ) describing the spin-memory loss over the active layer of thickness, d. Processes (i) can be incorporated into Eq. (1) by appropriately modifying P 1 , P 2 . For example, in Ref. 11 replacement of P 1 , P 2 by "effective" spin polarizations reflects the relative position of the Fermi level with respect to interfacial donor (acceptor) level. In this way, the "effective" polarization depends on bias, which might explain the sign reversal of TMR [11][12][13][14][15][16][17][18] . Processes (ii), on the other hand, are reflected in Eq. (1) via the factor Q = exp(−d/λ s ), where λ s is the spin diffusion length. The meaning of Q is the polarization of electrons at that the...
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