Nicholas Harmon | Coastal Carolina University (original) (raw)

Papers by Nicholas Harmon

Bulletin of the American Physical Society, 2016

Submitted for the PSF16 Meeting of The American Physical Society Magnetic field effects in regio-... more Submitted for the PSF16 Meeting of The American Physical Society Magnetic field effects in regio-regular polythiophene based devices and thermally activated delayed fluorescence organic light emitting diodes KEVSER SAHIN TIRAS, YIFEI WANG

Bulletin of the American Physical Society, 2018

Physical Review B, Oct 24, 2018

We present a simultaneous investigation of coherent spin dynamics in both localized and itinerant... more We present a simultaneous investigation of coherent spin dynamics in both localized and itinerant carriers in Fe/GaAs heterostructures using ultrafast and spin-resolved pump-probe spectroscopy. We find that for excitation densities that push the transient Fermi energy of photocarriers above the mobility edge there exist two distinct precession frequencies in the observed spin dynamics, allowing us to simultaneously monitor both localized and itinerant states. For low magnetic fields (below 3 T) the beat frequency between these two excitations evolves linearly, indicating that the nuclear polarization is saturated almost immediately and that the hyperfine coupling to these two states is comparable, despite the 100x enhancement in nuclear polarization provided by the presence of the Fe layer. At higher magnetic fields (above 3 T) the Zeeman energy drives reentrant localization of the photocarriers. Subtracting the constant hyperfine contribution from both sets of data allows us to extract the Lande g-factor for each state and estimate their energy relative to the bottom of the conduction band, yielding-2.16 meV and 17 meV for localized and

SPIE Proceedings, 2016

Large magnetic field effects, either in conduction or luminescence, have been observed in organic... more Large magnetic field effects, either in conduction or luminescence, have been observed in organic light-emitting diodes (OLEDs) for over a decade now. The physical processes are largely understood when exciton formation and recombination lead to the magnetic field effects. Recently, magnetic field effects in some co-evaporated blends have shown that exciplexes deliver even larger responses. In either case, the magnetic field effects arise from some spin-mixing mechanism and spin-selective processes in either the exciton formation or the exciplex recombination. Precise control of light output is not possible when the spin mixing is either due to hyperfine fields or differences in the Lande g-factor. We theoretically examine the optical output when a patterned magnetic film is deposited near the OLED. The fringe fields from the magnetic layers supply an additionally source of spin mixing that can be easily controlled. In the absence of other spin mixing mechanisms, the luminescence from exciplexes can be modified by 300%. When other spin-mixing mechanisms are present, fringe fields from remanent magnetic states act as a means to either boost or reduce light emission from those mechanisms. Lastly, we examine the influence of spin decoherence on the optical output.

Chemistry of Materials, 2018

Synthesizing a stable radical polymer with a conjugated backbone seems like a natural way to intr... more Synthesizing a stable radical polymer with a conjugated backbone seems like a natural way to introduce conductivity to radical polymers, which are traditionally synthesized with insulating, nonconjugated backbones. For charge storage applications that take advantage of the redox-active nature of stable radical polymers, enhanced conductivity would improve performance. To explore the interplay between stable radicals and a conjugated backbone, we prepared and studied soluble polythiophene with high regioregularity and various concentrations of pendent radical groups to systematically examine any change in conductivity with radical incorporation. Using electron paramagnetic resonance and electrical conductivity measurements, we show that there is an exponential decrease in conductivity as we increase the percentage of pendent groups attached to repeating units, which changes the conductivity by 6 orders of magnitude between the nonradical control polythiophene material and the material with the highest radical content (∼80%). These findings serve as an important guide to the future design of radical polymers on conjugated backbones with the goal of tuning conductivity as a function of stable radical content in redox-active energy storage applications.

Physical Review B, 2018

We describe the coherent dynamics of electrical transport through a localized spin-dependent stat... more We describe the coherent dynamics of electrical transport through a localized spin-dependent state, such as is associated with a defect spin, at the interface of a ferromagnet and a non-magnetic material during ferromagnetic resonance. As the ferromagnet's magnetic moment precesses, charge carriers are dynamically spin-filtered by the localized state, leading to a dynamic spin accumulation on the defect. Local effective magnetic fields modify the precession of a spin on the defect, which also modifies the time-integrated total charge current through the defect. We thus identify a new form of current-detected spin resonance that reveals the local magnetic environment of a carrier spin located at a defect, and thus potentially the defect's identity.

Journal of materials chemistry. C, Jan 7, 2018

There are conflicting reports in the literature about the presence of room temperature conductivi... more There are conflicting reports in the literature about the presence of room temperature conductivity in poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), a redox active polymer with radical groups pendent to an insulating backbone. To understand the variability in the findings across the literature and synthetic methods, we prepared PTMA using three living methods - anionic, ATRP and RAFT polymerization. We find that all three synthetic methods produce PTMA with radical yields of 70 - 80%, controlled molecular weight, and low dispersity. Additionally, we used on-chip EPR to probe the robustness of radical content in solid films under ambient air and light, and found negligible change in the radical content over time. Electrically, we found that PTMA is highly insulating - conductivity in the range 10-S/cm - regardless of the synthetic method of preparation. These findings provide greater clarity for potential applications of PTMA in energy storage.

Physical Review B, 2015

The hyperfine field from dynamically polarized nuclei in n-GaAs is very spatially inhomogeneous, ... more The hyperfine field from dynamically polarized nuclei in n-GaAs is very spatially inhomogeneous, as the nuclear polarization process is most efficient near the randomly-distributed donors. Electrons with polarized spins traversing the bulk semiconductor will experience this inhomogeneous hyperfine field as an effective fluctuating spin precession rate, and thus the spin polarization of an electron ensemble normal to the fluctuating hyperfine fields will relax. A theory of spin relaxation based on the theory of random walks is applied to such an ensemble precessing in an oblique magnetic field, and the precise form of the (unequal) longitudinal and transverse spin relaxation is analytically derived. To investigate this mechanism, electrical three-terminal Hanle measurements were performed on epitaxially grown Co 2 MnSi/n-GaAs heterostructures fabricated into electrical spin injection devices. The proposed anisotropic spin relaxation mechanism is required to satisfactorily describe the Hanle lineshapes when the applied field is oriented at large oblique angles.

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2015

Until recently the important role that spin-physics (‘spintronics’) plays in organic light-emitti... more Until recently the important role that spin-physics (‘spintronics’) plays in organic light-emitting devices and photovoltaic cells was not sufficiently recognized. This attitude has begun to change. We review our recent work that shows that spatially rapidly varying local magnetic fields that may be present in the organic layer dramatically affect electronic transport properties and electroluminescence efficiency. Competition between spin-dynamics due to these spatially varying fields and an applied, spatially homogeneous magnetic field leads to large magnetoresistance, even at room temperature where the thermodynamic influences of the resulting nuclear and electronic Zeeman splittings are negligible. Spatially rapidly varying local magnetic fields are naturally present in many organic materials in the form of nuclear hyperfine fields, but we will also review a second method of controlling the electrical conductivity/electroluminescence, using the spatially varying magnetic fringe f...

Applied Physics Letters, 2016

We predict very large changes in the room-temperature electroluminescence of thermally-activated ... more We predict very large changes in the room-temperature electroluminescence of thermally-activated delayed fluorescence organic light emitting diodes near patterned ferromagnetic films. These effects exceed the changes in a uniform magnetic field by as much as a factor of two. We describe optimal ferromagnetic film patterns for enhancing the electroluminescence. A full theory of the spin-mixing processes in exciplex recombination and how they are affected by hyperfine fields, spin-orbit effects, and ferromagnetic fringe field effects is introduced. These spin-mixing processes are used to describe the effect of magnetic domain structures on the luminescence in various regimes. This provides a method of enhancing light emission rates from exciplexes and also a means of efficiently coupling information encoded in the magnetic domains to organic light emitting diode emission.

Physical Review Letters, 2016

rotation (TRKR) is employed to study the exchange coupling between spin ensembles in GaAs and a n... more rotation (TRKR) is employed to study the exchange coupling between spin ensembles in GaAs and a neighboring ferromagnet (FM) in an Fe/MgO/GaAs heterostructure. The time-resolved spin dynamics in GaAs provide local magnetometry, revealing the strength and sign of the exchange field as well as its impact on electron and nuclear spins. Consistent with previous studies, we see a hyperpolarization of the nuclei induced by the dynamic exchange at the Fe/MgO/GaAs interface that results in a large effective nuclear field on the electrons (Bn = 0.2 T). Unexpectedly, we observe that the spin relaxation time in GaAs, T2*, depends on the strength of the exchange-driven nuclear field rather than the applied field. In addition, the temperature dependence of T2* shows a crossover of relaxation mechanism from hyperfine dominated to D'yakonov-Perel' (DP) dominated at temperatures above 40 K. These results not only resolve a long-lasting puzzle of the GaAs spin relaxation mechanism, but further demonstrate the ability to detect exchange-driven dissipation in FM/NM heterostructures. We discuss the potential for this work to define a novel detection scheme for exchange-driven spin injection in FM/semiconductor heterostructures, such as ferromagnetic resonance driven spin pumping.

Journal of Applied Physics, 2014

We predict that singly occupied carrier traps, produced by electrical stress or irradiation withi... more We predict that singly occupied carrier traps, produced by electrical stress or irradiation within organic semiconductors, can cause spin blockades and the large room-temperature magnetoresistance known as organic magnetoresistance. The blockade occurs because many singly occupied traps can only become as doubly occupied in a spin-singlet configuration. Magnetic-field effects on spin mixing during transport dramatically modify the effects of this blockade and produce magnetoresistance. We calculate the quantitative effects of these traps on organic magnetoresistance from percolation theory and find a dramatic nonlinear dependence of the saturated magnetoresistance on trap density, leading to values ∼20%, within the theory's range of validity.

Physical Review B, 2014

As semiconductor spintronics research extends to materials beyond intrinsic or lightly doped semi... more As semiconductor spintronics research extends to materials beyond intrinsic or lightly doped semiconductors (e. g. organic materials, amorphous semiconductors, and impurity bands), the need is readily apparent for new theories of spin relaxation that encompass highly disordered materials, where charge transport is incoherent. We describe a broadly applicable theory of spin relaxation in materials with incoherent charge transport. The theory is based on continuous-time-random-walk theory and can incorporate many different relaxation mechanisms. We focus primarily on spin relaxation caused by spin-orbit and hyperfine effects in conjunction with carrier hopping. Analytic and numerical results from the theory are compared in various regimes with Monte Carlo simulations. Three different systems were examined: a polymer (MEH-PPV) [1], amorphous silicon [2], and heavily doped n-GaAs. In the organic and amorphous systems, we predict spin relaxation and spin diffusion dependences on temperature and disorder for three different mechanisms (hyperfine, hopping-induced spin-orbit, and intra-site spin relaxation). The resulting unique experimental signatures predicted by the theory for each mechanism in these disordered systems provide a prescription for determining the dominant spin relaxation mechanism. We find our theory to be in agreement with available measurements in these materials. We also predict that large disorder modifies certain mechanisms to be algebraic instead of exponential in time. Our results should assist in evaluating the suitability of various disordered materials for spintronic devices. All work done in collaboration with Michael E. Flatté. Timothy Peterson and Paul Crowell collaborated as well on the n-GaAs study.

Physical Review B, 2012

A recent theory of magnetoresistance in positionally disordered organic semiconductors is extende... more A recent theory of magnetoresistance in positionally disordered organic semiconductors is extended to include exchange and dipolar couplings between polarons. Analytic results are discovered when the hyperfine, exchange, and dipolar interactions have little time to operate between hopping events. We find an angle-of-field dependence of the magnetoresistance that agrees with previous experiments and numerical simulations. In addition we report new magnetoresistive behavior that critically depends upon the amount of anisotropy in the dipolar interaction.

Physical Review B, 2012

A recently introduced percolative theory of unipolar organic magnetoresistance is generalized by ... more A recently introduced percolative theory of unipolar organic magnetoresistance is generalized by treating the hyperfine interaction semiclassically for an arbitrary hopping rate. Compact analytic results for the magnetoresistance are achievable when carrier hopping occurs much more frequently than the hyperfine field precession period. In other regimes, the magnetoresistance can be straightforwardly evaluated numerically. Slow and fast hopping magnetoresistance are found to be uniquely characterized by their lineshapes. We find that the threshold hopping distance is analogous a phenomenological two-site model's branching parameter, and that the distinction between slow and fast hopping is contingent on the threshold hopping distance.

SPIE Proceedings, 2012

The understanding of spin transport in organics has been challenged by the discovery of large mag... more The understanding of spin transport in organics has been challenged by the discovery of large magnetic field effects on properties such as conductivity and electroluminescence in a wide array of organic systems. To explain the large organic magnetoresistance (OMAR) phenomenon, we present and solve a model for magnetoresistance in positionally disordered organic materials using percolation theory. The model describes the effects of singlettriplet spin transitions on hopping transport by considering the role of spin dynamics on an effective density of hopping sites. Faster spin transitions open up 'spin-blocked' pathways to become viable conduction channels and hence produce magnetoresistance. We concentrate on spin transitions under the effects of the hyperfine (isotropic and anisotropic), exchange, and dipolar interactions. The magnetoresistance can be found analytically in several regimes and explains several experimental observations.

Physical Review B, 2014

We describe a new regime for low-field magnetoresistance in organic semiconductors, in which the ... more We describe a new regime for low-field magnetoresistance in organic semiconductors, in which the spin-relaxing effects of localized nuclear spins and electronic spins interfere. The regime is studied by the controlled addition of localized electronic spins to a material that exhibits substantial roomtemperature magnetoresistance (∼ 20%). Although initially the magnetoresistance is suppressed by the doping, at intermediate doping there is a regime where the magnetoresistance is insensitive to the doping level. For much greater doping concentrations the magnetoresistance is fully suppressed. The behavior is described within a theoretical model describing the effect of carrier spin dynamics on the current.

Physical Review Letters, 2012

A model for magnetoresistance in positionally disordered organic materials is presented and solve... more A model for magnetoresistance in positionally disordered organic materials is presented and solved using percolation theory. The model describes the effects of spin flips on hopping transport by considering the effect of spin dynamics on an effective density of hopping sites. Faster spin-flip transitions open up 'spin-blocked' pathways to become viable conduction channels and hence produces magnetoresistance. The magnetoresistance can be found analytically in several regimes, including when the spin-flip time is slower than the hopping time. The ratio of hopping time to the hyperfine precession time is a crucial quantity in determining the shape of magnetoresistance curves. Studies of magnetoresistance in known systems with controllable positional disorder would provide a stringent test of this model.

Physical Review Letters, 2013

A theory is introduced for spin relaxation and spin diffusion of hopping carriers in a disordered... more A theory is introduced for spin relaxation and spin diffusion of hopping carriers in a disordered system. For disorder described by a distribution of waiting times between hops (e.g. from multiple traps, site-energy disorder and/or positional disorder) the dominant spin relaxation mechanisms in organic semiconductors (hyperfine, hopping-induced spin-orbit, and intra-site spin relaxation) each produce different characteristic spin relaxation and spin diffusion dependences on temperature. The resulting unique experimental signatures predicted by the theory for each mechanism in organic semiconductors provide a prescription for determining the dominant spin relaxation mechanism.

Bulletin of the American Physical Society, 2016

Submitted for the PSF16 Meeting of The American Physical Society Magnetic field effects in regio-... more Submitted for the PSF16 Meeting of The American Physical Society Magnetic field effects in regio-regular polythiophene based devices and thermally activated delayed fluorescence organic light emitting diodes KEVSER SAHIN TIRAS, YIFEI WANG

Bulletin of the American Physical Society, 2018

Physical Review B, Oct 24, 2018

We present a simultaneous investigation of coherent spin dynamics in both localized and itinerant... more We present a simultaneous investigation of coherent spin dynamics in both localized and itinerant carriers in Fe/GaAs heterostructures using ultrafast and spin-resolved pump-probe spectroscopy. We find that for excitation densities that push the transient Fermi energy of photocarriers above the mobility edge there exist two distinct precession frequencies in the observed spin dynamics, allowing us to simultaneously monitor both localized and itinerant states. For low magnetic fields (below 3 T) the beat frequency between these two excitations evolves linearly, indicating that the nuclear polarization is saturated almost immediately and that the hyperfine coupling to these two states is comparable, despite the 100x enhancement in nuclear polarization provided by the presence of the Fe layer. At higher magnetic fields (above 3 T) the Zeeman energy drives reentrant localization of the photocarriers. Subtracting the constant hyperfine contribution from both sets of data allows us to extract the Lande g-factor for each state and estimate their energy relative to the bottom of the conduction band, yielding-2.16 meV and 17 meV for localized and

SPIE Proceedings, 2016

Large magnetic field effects, either in conduction or luminescence, have been observed in organic... more Large magnetic field effects, either in conduction or luminescence, have been observed in organic light-emitting diodes (OLEDs) for over a decade now. The physical processes are largely understood when exciton formation and recombination lead to the magnetic field effects. Recently, magnetic field effects in some co-evaporated blends have shown that exciplexes deliver even larger responses. In either case, the magnetic field effects arise from some spin-mixing mechanism and spin-selective processes in either the exciton formation or the exciplex recombination. Precise control of light output is not possible when the spin mixing is either due to hyperfine fields or differences in the Lande g-factor. We theoretically examine the optical output when a patterned magnetic film is deposited near the OLED. The fringe fields from the magnetic layers supply an additionally source of spin mixing that can be easily controlled. In the absence of other spin mixing mechanisms, the luminescence from exciplexes can be modified by 300%. When other spin-mixing mechanisms are present, fringe fields from remanent magnetic states act as a means to either boost or reduce light emission from those mechanisms. Lastly, we examine the influence of spin decoherence on the optical output.

Chemistry of Materials, 2018

Synthesizing a stable radical polymer with a conjugated backbone seems like a natural way to intr... more Synthesizing a stable radical polymer with a conjugated backbone seems like a natural way to introduce conductivity to radical polymers, which are traditionally synthesized with insulating, nonconjugated backbones. For charge storage applications that take advantage of the redox-active nature of stable radical polymers, enhanced conductivity would improve performance. To explore the interplay between stable radicals and a conjugated backbone, we prepared and studied soluble polythiophene with high regioregularity and various concentrations of pendent radical groups to systematically examine any change in conductivity with radical incorporation. Using electron paramagnetic resonance and electrical conductivity measurements, we show that there is an exponential decrease in conductivity as we increase the percentage of pendent groups attached to repeating units, which changes the conductivity by 6 orders of magnitude between the nonradical control polythiophene material and the material with the highest radical content (∼80%). These findings serve as an important guide to the future design of radical polymers on conjugated backbones with the goal of tuning conductivity as a function of stable radical content in redox-active energy storage applications.

Physical Review B, 2018

We describe the coherent dynamics of electrical transport through a localized spin-dependent stat... more We describe the coherent dynamics of electrical transport through a localized spin-dependent state, such as is associated with a defect spin, at the interface of a ferromagnet and a non-magnetic material during ferromagnetic resonance. As the ferromagnet's magnetic moment precesses, charge carriers are dynamically spin-filtered by the localized state, leading to a dynamic spin accumulation on the defect. Local effective magnetic fields modify the precession of a spin on the defect, which also modifies the time-integrated total charge current through the defect. We thus identify a new form of current-detected spin resonance that reveals the local magnetic environment of a carrier spin located at a defect, and thus potentially the defect's identity.

Journal of materials chemistry. C, Jan 7, 2018

There are conflicting reports in the literature about the presence of room temperature conductivi... more There are conflicting reports in the literature about the presence of room temperature conductivity in poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), a redox active polymer with radical groups pendent to an insulating backbone. To understand the variability in the findings across the literature and synthetic methods, we prepared PTMA using three living methods - anionic, ATRP and RAFT polymerization. We find that all three synthetic methods produce PTMA with radical yields of 70 - 80%, controlled molecular weight, and low dispersity. Additionally, we used on-chip EPR to probe the robustness of radical content in solid films under ambient air and light, and found negligible change in the radical content over time. Electrically, we found that PTMA is highly insulating - conductivity in the range 10-S/cm - regardless of the synthetic method of preparation. These findings provide greater clarity for potential applications of PTMA in energy storage.

Physical Review B, 2015

The hyperfine field from dynamically polarized nuclei in n-GaAs is very spatially inhomogeneous, ... more The hyperfine field from dynamically polarized nuclei in n-GaAs is very spatially inhomogeneous, as the nuclear polarization process is most efficient near the randomly-distributed donors. Electrons with polarized spins traversing the bulk semiconductor will experience this inhomogeneous hyperfine field as an effective fluctuating spin precession rate, and thus the spin polarization of an electron ensemble normal to the fluctuating hyperfine fields will relax. A theory of spin relaxation based on the theory of random walks is applied to such an ensemble precessing in an oblique magnetic field, and the precise form of the (unequal) longitudinal and transverse spin relaxation is analytically derived. To investigate this mechanism, electrical three-terminal Hanle measurements were performed on epitaxially grown Co 2 MnSi/n-GaAs heterostructures fabricated into electrical spin injection devices. The proposed anisotropic spin relaxation mechanism is required to satisfactorily describe the Hanle lineshapes when the applied field is oriented at large oblique angles.

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2015

Until recently the important role that spin-physics (‘spintronics’) plays in organic light-emitti... more Until recently the important role that spin-physics (‘spintronics’) plays in organic light-emitting devices and photovoltaic cells was not sufficiently recognized. This attitude has begun to change. We review our recent work that shows that spatially rapidly varying local magnetic fields that may be present in the organic layer dramatically affect electronic transport properties and electroluminescence efficiency. Competition between spin-dynamics due to these spatially varying fields and an applied, spatially homogeneous magnetic field leads to large magnetoresistance, even at room temperature where the thermodynamic influences of the resulting nuclear and electronic Zeeman splittings are negligible. Spatially rapidly varying local magnetic fields are naturally present in many organic materials in the form of nuclear hyperfine fields, but we will also review a second method of controlling the electrical conductivity/electroluminescence, using the spatially varying magnetic fringe f...

Applied Physics Letters, 2016

We predict very large changes in the room-temperature electroluminescence of thermally-activated ... more We predict very large changes in the room-temperature electroluminescence of thermally-activated delayed fluorescence organic light emitting diodes near patterned ferromagnetic films. These effects exceed the changes in a uniform magnetic field by as much as a factor of two. We describe optimal ferromagnetic film patterns for enhancing the electroluminescence. A full theory of the spin-mixing processes in exciplex recombination and how they are affected by hyperfine fields, spin-orbit effects, and ferromagnetic fringe field effects is introduced. These spin-mixing processes are used to describe the effect of magnetic domain structures on the luminescence in various regimes. This provides a method of enhancing light emission rates from exciplexes and also a means of efficiently coupling information encoded in the magnetic domains to organic light emitting diode emission.

Physical Review Letters, 2016

rotation (TRKR) is employed to study the exchange coupling between spin ensembles in GaAs and a n... more rotation (TRKR) is employed to study the exchange coupling between spin ensembles in GaAs and a neighboring ferromagnet (FM) in an Fe/MgO/GaAs heterostructure. The time-resolved spin dynamics in GaAs provide local magnetometry, revealing the strength and sign of the exchange field as well as its impact on electron and nuclear spins. Consistent with previous studies, we see a hyperpolarization of the nuclei induced by the dynamic exchange at the Fe/MgO/GaAs interface that results in a large effective nuclear field on the electrons (Bn = 0.2 T). Unexpectedly, we observe that the spin relaxation time in GaAs, T2*, depends on the strength of the exchange-driven nuclear field rather than the applied field. In addition, the temperature dependence of T2* shows a crossover of relaxation mechanism from hyperfine dominated to D'yakonov-Perel' (DP) dominated at temperatures above 40 K. These results not only resolve a long-lasting puzzle of the GaAs spin relaxation mechanism, but further demonstrate the ability to detect exchange-driven dissipation in FM/NM heterostructures. We discuss the potential for this work to define a novel detection scheme for exchange-driven spin injection in FM/semiconductor heterostructures, such as ferromagnetic resonance driven spin pumping.

Journal of Applied Physics, 2014

We predict that singly occupied carrier traps, produced by electrical stress or irradiation withi... more We predict that singly occupied carrier traps, produced by electrical stress or irradiation within organic semiconductors, can cause spin blockades and the large room-temperature magnetoresistance known as organic magnetoresistance. The blockade occurs because many singly occupied traps can only become as doubly occupied in a spin-singlet configuration. Magnetic-field effects on spin mixing during transport dramatically modify the effects of this blockade and produce magnetoresistance. We calculate the quantitative effects of these traps on organic magnetoresistance from percolation theory and find a dramatic nonlinear dependence of the saturated magnetoresistance on trap density, leading to values ∼20%, within the theory's range of validity.

Physical Review B, 2014

As semiconductor spintronics research extends to materials beyond intrinsic or lightly doped semi... more As semiconductor spintronics research extends to materials beyond intrinsic or lightly doped semiconductors (e. g. organic materials, amorphous semiconductors, and impurity bands), the need is readily apparent for new theories of spin relaxation that encompass highly disordered materials, where charge transport is incoherent. We describe a broadly applicable theory of spin relaxation in materials with incoherent charge transport. The theory is based on continuous-time-random-walk theory and can incorporate many different relaxation mechanisms. We focus primarily on spin relaxation caused by spin-orbit and hyperfine effects in conjunction with carrier hopping. Analytic and numerical results from the theory are compared in various regimes with Monte Carlo simulations. Three different systems were examined: a polymer (MEH-PPV) [1], amorphous silicon [2], and heavily doped n-GaAs. In the organic and amorphous systems, we predict spin relaxation and spin diffusion dependences on temperature and disorder for three different mechanisms (hyperfine, hopping-induced spin-orbit, and intra-site spin relaxation). The resulting unique experimental signatures predicted by the theory for each mechanism in these disordered systems provide a prescription for determining the dominant spin relaxation mechanism. We find our theory to be in agreement with available measurements in these materials. We also predict that large disorder modifies certain mechanisms to be algebraic instead of exponential in time. Our results should assist in evaluating the suitability of various disordered materials for spintronic devices. All work done in collaboration with Michael E. Flatté. Timothy Peterson and Paul Crowell collaborated as well on the n-GaAs study.

Physical Review B, 2012

A recent theory of magnetoresistance in positionally disordered organic semiconductors is extende... more A recent theory of magnetoresistance in positionally disordered organic semiconductors is extended to include exchange and dipolar couplings between polarons. Analytic results are discovered when the hyperfine, exchange, and dipolar interactions have little time to operate between hopping events. We find an angle-of-field dependence of the magnetoresistance that agrees with previous experiments and numerical simulations. In addition we report new magnetoresistive behavior that critically depends upon the amount of anisotropy in the dipolar interaction.

Physical Review B, 2012

A recently introduced percolative theory of unipolar organic magnetoresistance is generalized by ... more A recently introduced percolative theory of unipolar organic magnetoresistance is generalized by treating the hyperfine interaction semiclassically for an arbitrary hopping rate. Compact analytic results for the magnetoresistance are achievable when carrier hopping occurs much more frequently than the hyperfine field precession period. In other regimes, the magnetoresistance can be straightforwardly evaluated numerically. Slow and fast hopping magnetoresistance are found to be uniquely characterized by their lineshapes. We find that the threshold hopping distance is analogous a phenomenological two-site model's branching parameter, and that the distinction between slow and fast hopping is contingent on the threshold hopping distance.

SPIE Proceedings, 2012

The understanding of spin transport in organics has been challenged by the discovery of large mag... more The understanding of spin transport in organics has been challenged by the discovery of large magnetic field effects on properties such as conductivity and electroluminescence in a wide array of organic systems. To explain the large organic magnetoresistance (OMAR) phenomenon, we present and solve a model for magnetoresistance in positionally disordered organic materials using percolation theory. The model describes the effects of singlettriplet spin transitions on hopping transport by considering the role of spin dynamics on an effective density of hopping sites. Faster spin transitions open up 'spin-blocked' pathways to become viable conduction channels and hence produce magnetoresistance. We concentrate on spin transitions under the effects of the hyperfine (isotropic and anisotropic), exchange, and dipolar interactions. The magnetoresistance can be found analytically in several regimes and explains several experimental observations.

Physical Review B, 2014

We describe a new regime for low-field magnetoresistance in organic semiconductors, in which the ... more We describe a new regime for low-field magnetoresistance in organic semiconductors, in which the spin-relaxing effects of localized nuclear spins and electronic spins interfere. The regime is studied by the controlled addition of localized electronic spins to a material that exhibits substantial roomtemperature magnetoresistance (∼ 20%). Although initially the magnetoresistance is suppressed by the doping, at intermediate doping there is a regime where the magnetoresistance is insensitive to the doping level. For much greater doping concentrations the magnetoresistance is fully suppressed. The behavior is described within a theoretical model describing the effect of carrier spin dynamics on the current.

Physical Review Letters, 2012

A model for magnetoresistance in positionally disordered organic materials is presented and solve... more A model for magnetoresistance in positionally disordered organic materials is presented and solved using percolation theory. The model describes the effects of spin flips on hopping transport by considering the effect of spin dynamics on an effective density of hopping sites. Faster spin-flip transitions open up 'spin-blocked' pathways to become viable conduction channels and hence produces magnetoresistance. The magnetoresistance can be found analytically in several regimes, including when the spin-flip time is slower than the hopping time. The ratio of hopping time to the hyperfine precession time is a crucial quantity in determining the shape of magnetoresistance curves. Studies of magnetoresistance in known systems with controllable positional disorder would provide a stringent test of this model.

Physical Review Letters, 2013

A theory is introduced for spin relaxation and spin diffusion of hopping carriers in a disordered... more A theory is introduced for spin relaxation and spin diffusion of hopping carriers in a disordered system. For disorder described by a distribution of waiting times between hops (e.g. from multiple traps, site-energy disorder and/or positional disorder) the dominant spin relaxation mechanisms in organic semiconductors (hyperfine, hopping-induced spin-orbit, and intra-site spin relaxation) each produce different characteristic spin relaxation and spin diffusion dependences on temperature. The resulting unique experimental signatures predicted by the theory for each mechanism in organic semiconductors provide a prescription for determining the dominant spin relaxation mechanism.