arXiv subjects

N. J. Harmon

Publications and source records attributed to N. J. Harmon.

12 recordsLinked to original sources

arXiv2019

Theory of Single Photon Detection by a Photoreceptive Molecule and a Quantum Coherent Spin Center

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.

quant-ph↗

arXiv2018

Tuning Spin Dynamics and Localization Near the Metal-Insulator Transition in Fe/GaAs heterostructures

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 ob-served 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 itinerant states, respectively. This work advances our fundamental understanding of spin-spin interactions between electron and nuclear spin species, as well as between localized and itinerant electronics states, and therefore has implications for future work in both spintronics and quantum information/computation.

cond-mat.mes-hall↗

arXiv2015

Anisotropic spin relaxation in $n$-GaAs from strong inhomogeneous hyperfine fields produced by the dynamical polarization of nuclei

The hyperfine field from dynamically polarized nuclei in n-GaAs is very spatially inhomogeneous, as the nu- clear 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 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 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.

cond-mat.dis-nn↗

arXiv2015

Exchange-driven spin relaxation in ferromagnet/oxide/semiconductor heterostructures

We investigate electron spin relaxation in GaAs in the proximity of a Fe/MgO layer using spin-resolved optical pump-probe spectroscopy, revealing a strong dependence of the spin relaxation time on the strength of an exchange-driven hyperfine field. The temperature dependence of this effect reveals a strong correlation with carrier freeze out, implying that at low temperatures the free carrier spin lifetime is dominated by inhomogeneity in the local hyperfine field due to carrier localization. This result resolves a long-standing and contentious question of the origin of the spin relaxation in GaAs at low temperature when a magnetic field is present. Further, this improved fundamental understanding paves the way for future experiments exploring the time-dependent exchange interaction at the ferromagnet/semiconductor interface and its impact on spin dissipation and transport in the regime of dynamically-driven spin pumping.

cond-mat.mes-hall↗

arXiv2014

Anomalous organic magnetoresistance from competing carrier-spin-dependent interactions with localized electronic and nuclear spins

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 room-temperature magnetoresistance ($\sim 20$\%). 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.

cond-mat.mtrl-sci↗

arXiv2013

Distinguishing Spin Relaxation Mechanisms in Organic Semiconductors

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.

cond-mat.mes-hall↗

arXiv2012

The effects of spin-spin interactions on magnetoresistance in disordered organic semiconductors

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.

cond-mat.dis-nn↗

arXiv2011

Semiclassical theory of magnetoresistance in positionally-disordered organic semiconductors

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.

cond-mat.dis-nn↗

arXiv2011

Spin-flip induced magnetoresistance in positionally disordered organic solids

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.

cond-mat.dis-nn↗

arXiv2011

Prediction of extremely long electron spin lifetimes at room temperature in wurtzite semiconductor quantum wells

Many proposed spintronics devices require mobile electrons at room temperature with long spin lifetimes. One route to achieving this is to use quantum wells with tunable spin-orbit (SO) parameters. Research has focused on zinc-blende materials such as GaAs which do not have long spin lifetimes at room temperature. We show that wurtzite (w) materials, which possess smaller SO coupling due to being low-Z, are better suited for spintronics applications. This leads to predictions of spin lifetimes in w-AlN exceeding 2 ms at helium temperatures and, relevant to spintronic devices, spin lifetimes up to 0.5 $μs$ at room temperature.

cond-mat.mes-hall↗

arXiv2010

Theory of Electron Spin Relaxation in n-Doped Quantum Wells

Recent experiments have demonstrated long spin lifetimes in uniformly n-doped quantum wells. The spin dynamics of exciton, localized, and conduction spins are important for understanding these systems. We explain experimental behavior by invoking spin exchange between all spin species. By doing so we explain quantitatively and qualitatively the striking and unusual temperature dependence in (110)-GaAs quantum wells. We discuss possible future experiments to resolve the pertinent localized spin relaxation mechanisms. In addition, our analysis allows us to propose possible experimental scenarios that will optimize spin relaxation times in GaAs and CdTe quantum wells.

cond-mat.mes-hall↗

arXiv2009

Theory of Electron Spin Relaxation in ZnO

Doped ZnO is a promising material for spintronics applications. For such applications, it is important to understand the spin dynamics and particularly the spin coherence of this II-VI semiconductor. The spin lifetime $τ_{s}$ has been measured by optical orientation experiments, and it shows a surprising non-monotonic behavior with temperature. We explain this behavior by invoking spin exchange between localized and extended states. Interestingly, the effects of spin-orbit coupling are by no means negligible, in spite of the relatively small valence band splitting. This is due to the wurtzite crystal structure of ZnO. Detailed analysis allows us to characterize the impurity binding energies and densities, showing that optical orientation experiments can be used as a characterization tool for semiconductor samples.

cond-mat.mtrl-sci↗