Papers by Nicholas Harmon
Bulletin of the American Physical Society, Mar 3, 2014
The American Physical Society Spin Relaxation Theory in Amorphous Silicon and Germanium 1 NICHOLA... more The American Physical Society Spin Relaxation Theory in Amorphous Silicon and Germanium 1 NICHOLAS HARMON, MICHAEL E. FLATT É, University of Iowa -Research into spintronic devices using amorphous inorganic semiconductors has seen little attention despite the surge of interest in amorphous organic spintronics. In many ways the two materials are similar -for instance hopping transport is observed in both for certain regimes. Amorphous semiconductors such as silicon and germanium offer advantages such as the ability to greatly reduce and control hyperfine field effects by the process of hydrogenation, and considerably higher mobilities. We present a theory of spin relaxation in amorphous semiconductors based on the theory of a continuous-time random walk, and obtain analytic results in several regimes. We also calculate the spin relaxation with a Monte Carlo simulation. We find that the spin-orbit coupling is the primary limit to long spin lifetimes in amorphous silicon and germanium. The theory we introduce is very general and can also be applied to amorphous organic semiconductors. We compare our results for amorphous inorganic and amorphous organic materials.
Physical Review Letters, Dec 17, 2020
We predict strong, dynamical effects in the dc magnetoresistance of current flowing from a spinpo... more We predict strong, dynamical effects in the dc magnetoresistance of current flowing from a spinpolarized electrical contact through a magnetic dopant in a nonmagnetic host. Using the stochastic Liouville formalism we calculate clearly defined resonances in the dc magnetoresistance when the applied magnetic field matches the exchange interaction with a nearby spin. At these resonances spin precession in the applied magnetic field is canceled by spin evolution in the exchange field, preserving a dynamic bottleneck for spin transport through the dopant. Similar features emerge when the dopant spin is coupled to nearby nuclei through the hyperfine interaction. These features provide a precise means of measuring exchange or hyperfine couplings between localized spins near a surface using spin-polarized scanning tunneling microscopy, without any ac electric or magnetic fields, even when the exchange or hyperfine energy is orders of magnitude smaller than the thermal energy.
arXiv (Cornell University), Jun 4, 2019
The long spin coherence times in ambient conditions of color centers in solids, such as nitrogen-... more The long spin coherence times in ambient conditions of color centers in solids, such as nitrogen-vacancy (NV -) centers in diamond, make these systems attractive candidates for quantum sensing. Quantum sensing provides remarkable sensitivity at room temperature to very small external perturbations, including magnetic fields, electric fields, and temperature changes. A photoreceptive molecule, such as those involved in vision, changes its charge state or conformation in response to the absorption of a single photon. We show the resulting change in local electric field modifies the properties of a nearby quantum coherent spin center in a detectable fashion. Using the formalism of positive operator values measurements (POVMs), we analyze the photo-excited electric dipole field and, by extension, the arrival of a photon based on a measured readout, using a fluorescence cycle, from the spin center. We determine the jitter time of photon arrival and the probability of measurement errors. We predict that configuring multiple independent spin sensors around the photoreceptive molecule would dramatically suppresses the measurement error.
Bulletin of the American Physical Society, Mar 13, 2017
The American Physical Society Spin-Orbit Induced Spin Relaxation in Organic Semiconductors 1 STEP... more The American Physical Society Spin-Orbit Induced Spin Relaxation in Organic Semiconductors 1 STEPHEN MCMILLAN, NICHOLAS HARMON, MICHAEL FLATT É, Univ of Iowa -Weak spin-orbit coupling suggest long spin relaxation times in organic semiconductors. The correlations between slow carrier transport and these mechanisms of spin relaxation can yield complex behavior, including large magnetic-field effects on spin and charge dynamics. We use a continuous time random walk approach to investigate the effect of spin-orbit coupling on spin relaxation in organic materials with non-interacting carriers that incoherently hop from place to place. The simulation has been adapted from earlier work in this area [1] to emphasize the quantum nature of the relaxation. Transition rates for spin conserving and spin flipping hops are calculated as functions of the randomly assigned spatial orientation of the molecular sites. In a 3D cubic lattice with nearest-neighbor hopping we observe deviations of 10% when compared to published analytic results [2]. The disparity is due to a correlation between spin flipping hops and spin conserving hops. The time that carriers spend at a given site is determined by the sum of the conserving and flipping rates. Correlations between the two types of rates affect the time between two transport events altering the relaxation time.1.
Bulletin of the American Physical Society, Mar 18, 2013
The importance of random hyperfine fields is now widely acknowledged as a vital ingredient for th... more The importance of random hyperfine fields is now widely acknowledged as a vital ingredient for the phenomena of organic magnetoresistance (OMAR). Recent experiments (Phys. Rev. X 2 021013 (2012)) have shown that another type of random field -fringe fields due to a nearby ferromagnet -can also dramatically affect magnetoconductivity. A theoretical analysis of the fringe field OMAR is challenging due to the different properties of the fringe fields when compared to the hyperfine fields. For instance, the range of fringe field strengths is 1-2 orders of magnitude larger than that of the hyperfine couplings. The correlation length between fringe fields is also larger by the same degree. We use a recent theory of OMAR that is well-suited to numerically calculate the magnetoresistance with both hyperfine and fringe fields present. We find agreement with key features of experimental fringe-field magnetoresistance dependences on applied magnetic field, including the field values of extrema of the magnetoresistance, the region of large magnetoresistance effects from the fringe fields, and the sign of the effect.
Bulletin of the American Physical Society, Mar 18, 2016
Detecting interfacial defects at magnetic/nonmagnetic junctions 1 NICHOLAS HARMON, MICHAEL FLATT ... more Detecting interfacial defects at magnetic/nonmagnetic junctions 1 NICHOLAS HARMON, MICHAEL FLATT É, University of Iowa -Recent three terminal (3T) measurements in Co/LaAlO3/SrTiO3 show that spin-dependent transport through an interfacial defect is occurring instead of Hanle dephasing [1]. We propose extending 3T measurements into a coherent regime where single defects are detected by their local fields. The setup involves defects being situated between biased non-magnetic (NM) and ferromagnetic (FM) contacts. Spin torque on the FM drives an AC magnetization. Due to the large exchange interaction, the ability for charge to enter the FM depends on its spin and FM's relative orientation. As the FM precesses, the spin is dynamically filtered and a precessing spin accumulation remains at the defect. Local fields also precess the defect spin and interfere with the dynamic spin filtering. If the AC and local field are resonant, the spin accumulation is locked anti-parallel to the FM and leads to a dip in current. By adjusting the AC frequency, information on the local field is ascertained which, for hyperfine local fields, tells which nuclei are present at the defect and aids in identifying the defect. In the DC limit, defect spin accumulation leads to modifications in Hanle signals.
Bulletin of the American Physical Society, Mar 18, 2016
flat-panel displays and lighting applications, organic light emitting diodes (OLEDs) have been wi... more flat-panel displays and lighting applications, organic light emitting diodes (OLEDs) have been widely used because of their efficient light emission, low-cost manufacturing and flexibility. The electrons and holes injected from the anode and cathode, respectively, form a tightly bound exciton as they meet at a molecule in organic layer. Excitons occur as spin singlets or triplets and the ratio between singlet and triplet excitons formed is 1:3 based on spin degeneracy. The internal quantum efficiency (IQE) of fluorescent-based OLEDs is limited 25% because only singlet excitons contribute the light emission. To overcome this limitation, thermally activated delayed fluorescent (TADF) materials have been introduced in the field of OLEDs. The exchange splitting between the singlet and triplet states of two-component exciplex systems is comparable to the thermal energy in TADF materials, whereas it is usually much larger in excitons. Reverse intersystem crossing occurs from triplet to singlet exciplex state, and this improves the IQE. An applied small magnetic field can change the spin dynamics of recombination in TADF blends. In this study, magnetic field effects on both excitonic and exciplex OLEDs will be presented and comparison similarities and differences will be made.
Physical Review X, Feb 5, 2016
As carriers slowly move through a disordered energy landscape in organic semiconductors, tiny spa... more As carriers slowly move through a disordered energy landscape in organic semiconductors, tiny spatial variations in spin dynamics relieve spin blocking at transport bottlenecks or in the electron-hole recombination process that produces light. Large room-temperature magnetic-field effects (MFEs) ensue in the conductivity and luminescence. Sources of variable spin dynamics generate much larger MFEs if their spatial structure is correlated on the nanoscale with the energetic sites governing conductivity or luminescence such as in coevaporated organic blends within which the electron resides on one molecule and the hole on the other (an exciplex). Here, we show that exciplex recombination in blends exhibiting thermally activated delayed fluorescence produces MFEs in excess of 60% at room temperature. In addition, effects greater than 4000% can be achieved by tuning the device's current-voltage response curve by device conditioning. Both of these immense MFEs are the largest reported values for their device type at room temperature. Our theory traces this MFE and its unusual temperature dependence to changes in spin mixing between triplet exciplexes and light-emitting singlet exciplexes. In contrast, spin mixing of excitons is energetically suppressed, and thus spin mixing produces comparatively weaker MFEs in materials emitting light from excitons by affecting the precursor pairs. Demonstration of immense MFEs in common organic blends provides a flexible and inexpensive pathway towards magnetic functionality and field sensitivity in current organic devices without patterning the constituent materials on the nanoscale. Magnetic fields increase the power efficiency of unconditioned devices by 30% at room temperature, also showing that magnetic fields may increase the efficiency of the thermally activated delayed fluorescence process.
Bulletin of the American Physical Society, Mar 16, 2016
Time-resolved Kerr/Faraday rotation (TRKR/TRFR) is employed to study GaAs spin dynamics in the re... more Time-resolved Kerr/Faraday rotation (TRKR/TRFR) is employed to study GaAs spin dynamics in the regime of strong and dynamic exchange coupling to an adjacent MgO/Fe layer. This study reveals a dramatic, resonant suppression in the inhomogeneous spin lifetime (T2*) in the GaAs layer. Further investigation of the magnetization dynamics of the neighboring Fe layer, also using TRKR/TRFR, reveals not only the expected Kittel-dispersion but also additional lower frequency modes with very short lifetime ( 65 ps) that are not easily observed with conventional ferromagnetic resonance (FMR) techniques. These results suggest the intriguing possibility of resonant dynamic spin transfer between the GaAs and Fe spin systems. We discuss the potential for this work to establish GaAs spin dynamics as an efficient detector of spin dissipation and transport in the regime of dynamically-driven spin injection in ferromagnet/semiconductor heterostructures.
Bulletin of the American Physical Society, Nov 22, 2014
Bulletin of the American Physical Society, Mar 16, 2017
Gradients in dynamic nuclear polarization naturally develop near donor atoms in doped semiconduct... more Gradients in dynamic nuclear polarization naturally develop near donor atoms in doped semiconductors, like n-GaAs, that are pumped with electronic spin polarization. Recent work has demonstrated that the nuclear gradients play a role in spin dynamics and spin relaxation [1,2]. We predict a new type of spin current to occur when an external magnetic field is appropriately aligned with the gradient of a dynamically polarized nuclear field. In such cases, a linear spin-split dispersion appears in the Landau Hamiltonian which gives rise to a spin-dependent velocity that separates opposite spins and produces a pure spin current. Unlike the spin Hall effect with spin Hall conductivities much less than the charge conductivity, our gradient-driven spin current utilizes the charge conductivity. We propose optical orientation experiments to demonstrate this outcome.
Bulletin of the American Physical Society, 2016
Detecting interfacial defects at magnetic/nonmagnetic junctions 1 NICHOLAS HARMON, MICHAEL FLATT ... more Detecting interfacial defects at magnetic/nonmagnetic junctions 1 NICHOLAS HARMON, MICHAEL FLATT É, University of Iowa -Recent three terminal (3T) measurements in Co/LaAlO3/SrTiO3 show that spin-dependent transport through an interfacial defect is occurring instead of Hanle dephasing [1]. We propose extending 3T measurements into a coherent regime where single defects are detected by their local fields. The setup involves defects being situated between biased non-magnetic (NM) and ferromagnetic (FM) contacts. Spin torque on the FM drives an AC magnetization. Due to the large exchange interaction, the ability for charge to enter the FM depends on its spin and FM's relative orientation. As the FM precesses, the spin is dynamically filtered and a precessing spin accumulation remains at the defect. Local fields also precess the defect spin and interfere with the dynamic spin filtering. If the AC and local field are resonant, the spin accumulation is locked anti-parallel to the FM and leads to a dip in current. By adjusting the AC frequency, information on the local field is ascertained which, for hyperfine local fields, tells which nuclei are present at the defect and aids in identifying the defect. In the DC limit, defect spin accumulation leads to modifications in Hanle signals.
Journal of Materials Chemistry C, 2018
Correction for ‘Impact of the synthesis method on the solid-state charge transport of radical pol... more Correction for ‘Impact of the synthesis method on the solid-state charge transport of radical polymers’ by Yiren Zhang et al., J. Mater. Chem. C, 2018, 6, 111–118.
Applied Physics Letters, 2021
We report on a study that offers fundamental physical insight into an important phenomenon in sol... more We report on a study that offers fundamental physical insight into an important phenomenon in solid state device physics, tunneling in Si/SiO2. We observe near-zero field magnetoresistance via spin-dependent trap-assisted-tunneling in both unpassivated and passivated Si/SiO2 and 28Si/28SiO2 metal–insulator–semiconductor (MIS) capacitors. A previous report, which utilized electrically detected magnetic resonance and NZFMR on these devices, indicates a surprising conclusion: the observed trap-assisted tunneling spectra are dominated by silicon dangling bonds back bonded to silicon at the Si/SiO2 interface, Pb0 and Pb1 centers. In this study, the four sets of samples are virtually identical, apart from the presence or absence of either 1H and 29Si. We observed a substantial narrowing of the NZFMR response with the removal of 29Si nuclei and a substantial broadening with the addition of 1H. Since superhyperfine interactions between 29Si nuclei Pb at the Si/SiO2 interface are a full orde...
Physical Review X, 2015
The observed magnetoresistance (MR) in three-terminal (3T) ferromagnet-nonmagnet (FM-NM) tunnel j... more The observed magnetoresistance (MR) in three-terminal (3T) ferromagnet-nonmagnet (FM-NM) tunnel junctions has historically been assigned to ensemble dephasing (Hanle effect) of a spin accumulation, thus offering a powerful approach for characterizing the spin lifetime of candidate materials for spintronics applications. However, due to crucial discrepancies of the extracted spin parameters with known materials properties, this interpretation has come under intense scrutiny. By employing epitaxial artificial dipoles as the tunnel barrier in oxide heterostructures, the band alignments between the FM and NM channels can be controllably engineered, providing an experimental platform for testing the predictions of the various spin accumulation models. Using this approach, we have been able to definitively rule out spin accumulation as the origin of the 3T MR. Instead, we assign the origin of the magnetoresistance to spin-dependent hopping through defect states in the barrier, a fundamental phenomenon seen across diverse systems. A theoretical framework is developed that can account for the signal amplitude, linewidth, and anisotropy.
The importance of random hyperfine fields is now widely acknowledged as a vital ingredient for th... more The importance of random hyperfine fields is now widely acknowledged as a vital ingredient for the phenomena of organic magnetoresistance (OMAR). Recent experiments (Phys. Rev. X 2 021013 (2012)) have shown that another type of random field -fringe fields due to a nearby ferromagnet -can also dramatically affect magnetoconductivity. A theoretical analysis of the fringe field OMAR is challenging due to the different properties of the fringe fields when compared to the hyperfine fields. For instance, the range of fringe field strengths is 1-2 orders of magnitude larger than that of the hyperfine couplings. The correlation length between fringe fields is also larger by the same degree. We use a recent theory of OMAR that is well-suited to numerically calculate the magnetoresistance with both hyperfine and fringe fields present. We find agreement with key features of experimental fringe-field magnetoresistance dependences on applied magnetic field, including the field values of extrema of the magnetoresistance, the region of large magnetoresistance effects from the fringe fields, and the sign of the effect.
arXiv (Cornell University), Aug 18, 2020
Dielectric interfaces critical for metal-oxide-semiconductor (MOS) electronic devices, such as th... more Dielectric interfaces critical for metal-oxide-semiconductor (MOS) electronic devices, such as the Si/SiO 2 MOS field effect transistor (MOSFET), possess trap states that can be visualized with electrically-detected spin resonance techniques, however the interpretation of such measurements has been hampered by the lack of a general theory of the phenomena. This article presents such a theory for two electrical spin-resonance techniques, electrically detected magnetic resonance (EDMR) and the recently observed near-zero field magnetoresistance (NZFMR), by generalizing Shockley Read Hall trap-assisted recombination current calculations via stochastic Liouville equations. Spin mixing at this dielectric interface occurs via the hyperfine interaction, which we show can be treated either quantum mechanically or semiclassically, yielding distinctive differences in the current across the interface. By analyzing the bias dependence of NZFMR and EDMR, we find that the recombination in a Si/SiO 2 MOSFET is well understood within a semiclassical approach.
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
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Papers by Nicholas Harmon