Limits...
Probing long-range carrier-pair spin-spin interactions in a conjugated polymer by detuning of electrically detected spin beating.

van Schooten KJ, Baird DL, Limes ME, Lupton JM, Boehme C - Nat Commun (2015)

Bottom Line: Spin pairs exhibit persistent spin coherence, allowing minute magnetic fields to perturb spin precession and thus recombination rates and photoreaction yields, giving rise to a range of magneto-optoelectronic effects in devices.Little is known, however, about interparticle magnetic interactions within such pairs.The deviation from uncoupled precession frequencies quantifies both the exchange (<30 neV) and dipolar (23.5±1.5 neV) interaction energies responsible for the pair's zero-field splitting, implying quantum mechanical entanglement of charge-carrier spins over distances of 2.1±0.1 nm.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Astronomy, University of Utah, 115 South 1400 East, Salt Lake City, Utah 84112-0830, USA.

ABSTRACT

Unlabelled: Weakly coupled electron spin pairs that experience weak spin-orbit interaction can control electronic transitions in molecular and solid-state systems. Known to determine radical pair reactions, they have been invoked to explain phenomena ranging from avian magnetoreception to spin-dependent charge-carrier recombination and transport. Spin pairs exhibit persistent spin coherence, allowing minute magnetic fields to perturb spin precession and thus recombination rates and photoreaction yields, giving rise to a range of magneto-optoelectronic effects in devices. Little is known, however, about interparticle magnetic interactions within such pairs. Here we present pulsed electrically detected electron spin resonance experiments on poly(styrene-sulfonate)-doped poly(3,4-ethylenedioxythiophene) (

Pedot: PSS) devices, which show how interparticle spin-spin interactions (magnetic-dipolar and spin-exchange) between charge-carrier spin pairs can be probed through the detuning of spin-Rabi oscillations. The deviation from uncoupled precession frequencies quantifies both the exchange (<30 neV) and dipolar (23.5±1.5 neV) interaction energies responsible for the pair's zero-field splitting, implying quantum mechanical entanglement of charge-carrier spins over distances of 2.1±0.1 nm.

No MeSH data available.


Related in: MedlinePlus

Intrinsic pEDMR spectral linewidth of two conducting polymers with different hyperfine nuclear–electronic coupling strengths.(a) pEDMR lineshapes of MEH-PPV (orange) and PEDOT:PSS (black), at 5 K for an applied microwave field with frequency fMW=9.72691 GHz. PEDOT:PSS has a lower hydrogen content, reducing the effective hyperfine field and narrowing the resonance. (b) pEDMR linewidths as a function of microwave driving field, B1, demonstrating the absence of microwave power broadening for the spectra in a. The data points corresponding to the spectra in a are indicated by red circles. At larger B1 fields, power broadening sets in. (c) Transient dynamics following resonant excitation. The change in current is proportional to the time evolution of eigenstate density operators during the microwave pulse, with the number of resonant charges monitored by integration of these time dynamics, as discussed in the Methods. This integrated current, Q, is recorded as a function of τ, B1 and B0.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4403378&req=5

f2: Intrinsic pEDMR spectral linewidth of two conducting polymers with different hyperfine nuclear–electronic coupling strengths.(a) pEDMR lineshapes of MEH-PPV (orange) and PEDOT:PSS (black), at 5 K for an applied microwave field with frequency fMW=9.72691 GHz. PEDOT:PSS has a lower hydrogen content, reducing the effective hyperfine field and narrowing the resonance. (b) pEDMR linewidths as a function of microwave driving field, B1, demonstrating the absence of microwave power broadening for the spectra in a. The data points corresponding to the spectra in a are indicated by red circles. At larger B1 fields, power broadening sets in. (c) Transient dynamics following resonant excitation. The change in current is proportional to the time evolution of eigenstate density operators during the microwave pulse, with the number of resonant charges monitored by integration of these time dynamics, as discussed in the Methods. This integrated current, Q, is recorded as a function of τ, B1 and B0.

Mentions: To reveal the detuning of Rabi oscillations, minimal inhomogeneous broadening of the resonance line is crucial. The interaction of charge-carrier spins with local hyperfine fields, Bhyp, primarily generated by hydrogen nuclei, is well documented for organic semiconductors1021 and is presumed to be the origin of some of the intriguing magnetic field effects unique to these material systems343536. Typically of order 1 mT, these local Bhyp fields serve to screen the effect of B0, causing a Gaussian distribution of fields, G(B0, Bhyp), to be experienced by the ensemble of paramagnetic moments being probed by spin resonance. This field distribution directly translates to Gaussian disorder in the Larmor frequencies of the ensemble, (ref. 10) which, in turn, broadens the range of Rabi frequencies observed under detuning off-resonance37 (see further discussion in Supplementary Note 2). This disorder has masked the effect of detuning in previous pEDMR studies such as of poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV)27, which is illustrated in Supplementary Fig. 1. In contrast, the π-conjugated thiophene chains in PEDOT are heavily p-doped to support hole transport and are stabilized by ions in the PSS, which itself does not contribute to charge transport33. The ratio of the hydrogen fraction between monomers of PEDOT and MEH-PPV is 1:12, with the actual hyperfine field strength highly dependent on the molecular geometry and the electron wavefunction38. The conductivity of PEDOT:PSS thin films drops strongly from room temperature to 5 K (refs 39, 40), where it exhibits semiconductor characteristics41. Holes are localized to PEDOT domains within the PSS matrix due to limitations in thermally activated hopping transport334042. Figure 2a compares the pEDMR resonance linewidths of MEH-PPV and PEDOT:PSS at 5 K. The wider distribution present in MEH-PPV is due to significant hyperfine broadening of order 1 mT (refs 8, 10, 21, 43). The PEDOT:PSS spectrum, on the other hand, is nearly three times narrower, indicating a reduction in Bhyp by at least the same factor. Note that without measuring the resonance linewidth as a function of both B0 field and microwave frequency, we cannot differentiate hyperfine broadening from broadening due to a distribution in g-factors of the charge-carrier spins, which arises due to spin–orbit coupling8. Thus, the hyperfine fields in PEDOT:PSS may be even smaller than what the linewidth in Fig. 2a indicates. Figure 2b establishes that we measure these spectra in a regime not influenced by power broadening from the microwave B1 field since the linewidth saturates at low B1. We note that identifying polymer-based semiconductors that exhibit such narrow resonances is critical for magnetometry applications utilizing organic semiconductors, for which magnetic field resolution is proportional to linewidth8.


Probing long-range carrier-pair spin-spin interactions in a conjugated polymer by detuning of electrically detected spin beating.

van Schooten KJ, Baird DL, Limes ME, Lupton JM, Boehme C - Nat Commun (2015)

Intrinsic pEDMR spectral linewidth of two conducting polymers with different hyperfine nuclear–electronic coupling strengths.(a) pEDMR lineshapes of MEH-PPV (orange) and PEDOT:PSS (black), at 5 K for an applied microwave field with frequency fMW=9.72691 GHz. PEDOT:PSS has a lower hydrogen content, reducing the effective hyperfine field and narrowing the resonance. (b) pEDMR linewidths as a function of microwave driving field, B1, demonstrating the absence of microwave power broadening for the spectra in a. The data points corresponding to the spectra in a are indicated by red circles. At larger B1 fields, power broadening sets in. (c) Transient dynamics following resonant excitation. The change in current is proportional to the time evolution of eigenstate density operators during the microwave pulse, with the number of resonant charges monitored by integration of these time dynamics, as discussed in the Methods. This integrated current, Q, is recorded as a function of τ, B1 and B0.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4403378&req=5

f2: Intrinsic pEDMR spectral linewidth of two conducting polymers with different hyperfine nuclear–electronic coupling strengths.(a) pEDMR lineshapes of MEH-PPV (orange) and PEDOT:PSS (black), at 5 K for an applied microwave field with frequency fMW=9.72691 GHz. PEDOT:PSS has a lower hydrogen content, reducing the effective hyperfine field and narrowing the resonance. (b) pEDMR linewidths as a function of microwave driving field, B1, demonstrating the absence of microwave power broadening for the spectra in a. The data points corresponding to the spectra in a are indicated by red circles. At larger B1 fields, power broadening sets in. (c) Transient dynamics following resonant excitation. The change in current is proportional to the time evolution of eigenstate density operators during the microwave pulse, with the number of resonant charges monitored by integration of these time dynamics, as discussed in the Methods. This integrated current, Q, is recorded as a function of τ, B1 and B0.
Mentions: To reveal the detuning of Rabi oscillations, minimal inhomogeneous broadening of the resonance line is crucial. The interaction of charge-carrier spins with local hyperfine fields, Bhyp, primarily generated by hydrogen nuclei, is well documented for organic semiconductors1021 and is presumed to be the origin of some of the intriguing magnetic field effects unique to these material systems343536. Typically of order 1 mT, these local Bhyp fields serve to screen the effect of B0, causing a Gaussian distribution of fields, G(B0, Bhyp), to be experienced by the ensemble of paramagnetic moments being probed by spin resonance. This field distribution directly translates to Gaussian disorder in the Larmor frequencies of the ensemble, (ref. 10) which, in turn, broadens the range of Rabi frequencies observed under detuning off-resonance37 (see further discussion in Supplementary Note 2). This disorder has masked the effect of detuning in previous pEDMR studies such as of poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV)27, which is illustrated in Supplementary Fig. 1. In contrast, the π-conjugated thiophene chains in PEDOT are heavily p-doped to support hole transport and are stabilized by ions in the PSS, which itself does not contribute to charge transport33. The ratio of the hydrogen fraction between monomers of PEDOT and MEH-PPV is 1:12, with the actual hyperfine field strength highly dependent on the molecular geometry and the electron wavefunction38. The conductivity of PEDOT:PSS thin films drops strongly from room temperature to 5 K (refs 39, 40), where it exhibits semiconductor characteristics41. Holes are localized to PEDOT domains within the PSS matrix due to limitations in thermally activated hopping transport334042. Figure 2a compares the pEDMR resonance linewidths of MEH-PPV and PEDOT:PSS at 5 K. The wider distribution present in MEH-PPV is due to significant hyperfine broadening of order 1 mT (refs 8, 10, 21, 43). The PEDOT:PSS spectrum, on the other hand, is nearly three times narrower, indicating a reduction in Bhyp by at least the same factor. Note that without measuring the resonance linewidth as a function of both B0 field and microwave frequency, we cannot differentiate hyperfine broadening from broadening due to a distribution in g-factors of the charge-carrier spins, which arises due to spin–orbit coupling8. Thus, the hyperfine fields in PEDOT:PSS may be even smaller than what the linewidth in Fig. 2a indicates. Figure 2b establishes that we measure these spectra in a regime not influenced by power broadening from the microwave B1 field since the linewidth saturates at low B1. We note that identifying polymer-based semiconductors that exhibit such narrow resonances is critical for magnetometry applications utilizing organic semiconductors, for which magnetic field resolution is proportional to linewidth8.

Bottom Line: Spin pairs exhibit persistent spin coherence, allowing minute magnetic fields to perturb spin precession and thus recombination rates and photoreaction yields, giving rise to a range of magneto-optoelectronic effects in devices.Little is known, however, about interparticle magnetic interactions within such pairs.The deviation from uncoupled precession frequencies quantifies both the exchange (<30 neV) and dipolar (23.5±1.5 neV) interaction energies responsible for the pair's zero-field splitting, implying quantum mechanical entanglement of charge-carrier spins over distances of 2.1±0.1 nm.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Astronomy, University of Utah, 115 South 1400 East, Salt Lake City, Utah 84112-0830, USA.

ABSTRACT

Unlabelled: Weakly coupled electron spin pairs that experience weak spin-orbit interaction can control electronic transitions in molecular and solid-state systems. Known to determine radical pair reactions, they have been invoked to explain phenomena ranging from avian magnetoreception to spin-dependent charge-carrier recombination and transport. Spin pairs exhibit persistent spin coherence, allowing minute magnetic fields to perturb spin precession and thus recombination rates and photoreaction yields, giving rise to a range of magneto-optoelectronic effects in devices. Little is known, however, about interparticle magnetic interactions within such pairs. Here we present pulsed electrically detected electron spin resonance experiments on poly(styrene-sulfonate)-doped poly(3,4-ethylenedioxythiophene) (

Pedot: PSS) devices, which show how interparticle spin-spin interactions (magnetic-dipolar and spin-exchange) between charge-carrier spin pairs can be probed through the detuning of spin-Rabi oscillations. The deviation from uncoupled precession frequencies quantifies both the exchange (<30 neV) and dipolar (23.5±1.5 neV) interaction energies responsible for the pair's zero-field splitting, implying quantum mechanical entanglement of charge-carrier spins over distances of 2.1±0.1 nm.

No MeSH data available.


Related in: MedlinePlus