Decay Dynamics Research Articles

Learning the Synaptic and Intrinsic Membrane Dynamics Underlying Working Memory in Spiking Neural Network Models.

Recurrent neural network (RNN) models trained to perform cognitive tasks are a useful computational tool for understanding how cortical circuits execute complex computations. However, these models are often composed of units that interact with one another using continuous signals and overlook parameters intrinsic to spiking neurons. Here, we developed a method to directly train not only synaptic-related variables but also membrane-related parameters of a spiking RNN model. Training our model on a wide range of cognitive tasks resulted in diverse yet task-specific synaptic and membrane parameters. We also show that fast membrane time constants and slow synaptic decay dynamics naturally emerge from our model when it is trained on tasks associated with working memory (WM). Further dissecting the optimized parameters revealed that fast membrane properties are important for encoding stimuli, and slow synaptic dynamics are needed for WM maintenance. This approach offers a unique window into how connectivity patterns and intrinsic neuronal properties contribute to complex dynamics in neural populations.

An Expanded SET Model Associated with the Functional Hindrance Dominates the Amide-Directed Distal sp3 C-H Functionalization.

The mechanistic understanding of catalytic radical reactions currently lags behind the flourishing development of new types of catalytic activation. Herein, an innovative single electron transfer (SET) model has been expanded by using the nonadiabatic crossing integrated with the rate-determining step of 1,5-hydrogen atom transfer (HAT) reaction to provide the control mechanism of radical decay dynamics through calculating excited-state relaxation paths of a paradigm example of the amide-directed distal sp3 C-H bond alkylation mediated by Ir-complex-based photocatalysts. The stability of carbon radical intermediates, the functional hindrance associated with the back SET, and the energy inversion between the reactive triplet and closed-shell ground states were verified to be key factors in improving catalytic efficiency via blocking radical inhibition. The expanded SET model associated with the dynamic behaviors and kinetic data could guide the design and manipulation of visible-light-driven inert bond activation by the utilization of photocatalysts bearing more or less electron-withdrawing groups and the comprehensive considerations of kinetic solvent effects and electron-withdrawing effects of substrates.

High-resolution laser induced excitation spectroscopy and decay dynamics of YNbO4:Er3+

The rare earth materials have attracted intensive attention due to their strong luminescent characteristic. However, the split fine Stark levels are difficult to be determined. Here we report a room-temperature detection for Stark levels of YNbO4: Er3+ using established laser-induced spectroscopy system with dye laser of superhigh resolution of wavelength at 0.005 nm. From excitation spectra, six split Stark levels of 4G11/2 (Er3+) were directly detected. Moreover, nonradiative relaxations of 4G9/2→4G11/2 and 4G11/2→2H11/2/ 4S3/2 have been observed with weighed lifetimes of 0.70 μs and 6.15 μs, and characteristic green emission of Er3+ (@555 nm) yields lifetime of 31.78 μs.

Making or un-making states: when does war have formative effects?

“War made the state, and the state made war” is Charles Tilly’s famous dictum that has become highly influential both in comparative macrosociology and in International Relations. An extensive literature suggests that this mechanism has played a pivotal role in European processes of state formation. However, its applicability to warfare in the Global South is controversial. While some argue that the relationship remains the same, others are skeptical of the effects of the bellicist mechanism. Against the background of the debate as to whether war makes or un-makes states in the Global South, this paper examines the conditions under which wars have formative effects and result in state-making. Revisiting the war-making/state-making paradigm, I argue that the mode of economic reproduction of “wielders of coercion” determines whether war has formative effects. Wielders of coercion, or more specifically, non-state armed movements may draw on (1) rents, (2) indirect extraction, or (3) direct extraction to sustain their economic base and organize coercion. However, they institutionalize and develop into a state-like organization only when they rely on direct extraction. To support these claims, I conduct a plausibility probe drawing on evidence from highly successful armed insurgencies: Eritrea’s EPLF, Somaliland’s SNM, and Namibia’s SWAPO. In sum, the bellicist relationship remains valid for the Global South, even if the conditions under which war makes states are rare. A more nuanced reading of the bellicist theory improves our understanding of the dynamics of state formation and decay in the post-colonial world.

Understanding and Tailoring Excited State Properties in Solution-Processable Oligo(p-phenyleneethynylene)s: Highly Fluorescent Hybridized Local and Charge Transfer Character via Experiment and Theory.

Rod-shaped oligo(p-phenyleneethynylene) (OPE) offers an attractive π-framework for the development of solution-processable highly fluorescent molecules having tunable hybridized local and charge transfer (HLCT) excited states and (reverse) intersystem crossing ((R)ISC) channels. Herein, an HLCT oligo(p-phenyleneethynylene) library was studied for the first time in the literature in detail systematically via experiment and theory. The design, synthesis, and full characterization of a new highly fluorescent (ΦPL-solution ∼ 1) sky blue emissive 4',4‴-((2,5-bis((2-ethylhexyl)oxy)-1,4-phenylene)bis(ethyne-2,1-diyl))bis(N,N-diphenyl-[1,1'-biphenyl]-4-amine) (2EHO-TPA-PE) was also reported. The new molecule consists of a D'-Ar-π-D-π-Ar-D' molecular architecture with an extended π-spacer and no acceptor unit, and detailed structural, physicochemical, single-crystal, and optoelectronic characterizations were performed. A high solid-state quantum efficiency (ΦPL-solidstate ∼ 0.8) was achieved as a result of suppressed exciton-phonon/vibronic couplings (no π-π interactions and multiple (14 per dimeric form) strong C-H···π interactions). Strong solution-phase/solid-state dipole-dependent tunable excited state behavior (local excited (LE) → HLCT → charge transfer (CT)) and decay dynamics covering a wide spectral region were demonstrated, and the CT state was observed to be highly fluorescent despite extremely large Stokes shift (∼130 nm)/fwhm (∼125 nm) and significant charge separation (0.75 charge·nm). Employing the Lippert-Mataga model, along with detailed photophysical studies and TDDFT calculations, key relationships between molecular design-electronic structure-exciton characteristics were elucidated with regards to HLCT and hot exciton channel formations. The interstate coupling between CT and LE states and the interplay of this coupling with respect to medium polarity were explored. A key relationship between excited-state symmetry breaking process and the formation of HLCT state was discussed for TPA-ended rod-shaped OPE π-systems. (R)ISC-related delayed fluorescence (τ ∼ 2-6 ns) processes were evident following the prompt decays (∼0.4-0.9 ns) both in the solution and in the solid-state. As a unique observation, the delayed fluorescence could be tuned and facilitated via small dielectric changes in the medium. Our results and the molecular engineering perspectives presented in this study may provide unique insights into the structural and electronic factors governing tunable excited state and hot-exciton channel formations in OPEs for (un)conventional solution-processed luminescence applications.

Effect of Diffusion on Photo-Induced Excited-State Energy Transfer between Fluorescent Semiconducting Molecules: Tris-(8-hydroxyquinoline) Aluminum and 6,13-Bis (Triisopropylsilylethynyl) Pentacene

In the present work, the effect of diffusion on photo-induced excitation energy transfer between fluorescent organic semiconducting molecules tris-(8-hydroxyquinoline) aluminum (AlQ3, n-type donor) and 6,13-bis (triisopropylsilylethynyl) pentacene (TIPS-P, p-type acceptor) at a concentration range of 10–4 to 10–6 M in chloroform solution was studied by steady-state and time-domain fluorescence measurements. The donor–donor interaction strength is significantly weaker than the donor–acceptor strength (the ratio of donor–donor to donor–acceptor interaction strengths is 1.55 × 103) in chloroform solution. Considerable overlap between donor emission and acceptor absorption and high interaction parameters favors direct Förster energy transfer and exciplex (D*A) formation. Excitation energy migration does not take place between donor molecules. However, material diffusion appears to influence the donor decay dynamics by forming charge-transfer exciplex complexes and modulating the excitation energy transfer rate from the metal-to-ligand charge-transfer (MLCT) state of the donor to vibronic states of the acceptor at a low acceptor concentration. At high acceptor concentrations, energy transfer from the excited donor to acceptor occurs through the exchange mechanism along with Förster long-range dipole–dipole interactions, and the corresponding critical transfer distance is found to be ∼42 Å. A blue-shift in AlQ3 emission, a red-shift in the 0–0 vibronic peak of TIPS-P, and quenching of exciplex decay following Stern–Volmer quenching are observed, with the increase in acceptor concentration. No evidence of excimer formation is observed in acceptor molecules.

Study of the standard model with weak decays of charmed hadrons at BESIII.

A comprehensive review of weak decays of charmed hadrons (D0/ +, n}{}D^+_s and n}{}Lambda ^+_c) based on analyses of the threshold data from e+e− annihilation in the BESIII experiment is presented. Current experimental challenges and successes in understanding decays of the charmed hadrons are discussed. Precise calibrations of quantum chromodynamics and tests of the standard model are provided by measurements of purely leptonic and semi-leptonic decays of charmed hadrons, and lepton universality is probed in purely leptonic decays of charmed mesons to three generations of leptons. Quantum correlations in threshold data samples provide access to strong phases in the neutral D meson decays and probe the decay dynamics of the charmed Λc baryon. Charm physics studies with near-threshold production of charmed particle pairs are unique to BESIII, and provide many important opportunities and challenges.

Ultrafast photo-induced carrier dynamics of FAPbI3-MAPbBr3 perovskite films fabricated with additives and a hole transport material

In this study, mixed perovskite films of formamidinium lead triiodide (HC(NH2)2PbI3, FAPbI3) and methylammonium lead tribromide (CH3NH3PbBr3, MAPbBr3) were fabricated differently with solution additive methods and a presence of a hole transport material (HTM). We utilized ultrafast time-resolved transient absorption spectroscopy (TAS) in order to investigate the carrier generation and decay dynamics. The photo-induced carrier dynamics changes according to additive methods and applying the HTM. The HTM affects dominantly a TA decay constant of long-time scale. On the other hand, additive methods reduce a time constant of short-time scale and also increase the density of the photo-induced carrier.

The structural and nutrient chemistry during early-stage decomposition and desiccation of cacti in the Sonoran Desert

Despite the abundance of cacti in the Sonoran Desert, most research focuses on population dynamics and physiology. Relatively little is known about their decomposition dynamics. A better understanding of cactus decomposition is important, considering the number of threatened and endangered cactus species. We measured mass, water, nutrient, and structural chemistry over one year of decomposition of two common cactus species, Opuntia chlorotica and Cylindropuntia acanthocarpa, in the Sonoran Desert of Arizona, U.S.A. Our results demonstrate the contribution of cactus decomposition to carbon and nutrient recycling, with comparable dynamics to woody and herbaceous leaf litter for most elements. We enumerate, however, the particularly important role in calcium dynamics. Despite initial differences, both cacti released nutrients at a statistically equivalent rate, though with altered timing due to temporary mass gain in cholla. The resources released from decaying cacti have a modest influence on underlying soil CO2 flux, secondary to a dominant influence of soil microclimate. Our data provide a baseline for understanding the decay dynamics of two common cactus species and suggest that, while there is still a lack of information pertaining to cactus decomposition, the similarities with leaf litter will aid our predictions of the consequences of future cactus population changes.

High quality quarternary-alloyed ZnCdSSe/ZnS quantum dots with single photoluminescence decay channel and high devise stability

Quantum dots with single PL decay dynamics were promising for their applications in QD-based light-emitting devices. We here report an approach to synthesize high quality green emitting quaternary alloyed CdSeZnS quantum dots (QDs) with a hot-injection method. The quantum yield (QY) of as-synthesized ZnCdSSe QD cores is up to 84% with single-exponential PL decay dynamics. After overcoating 4 mono-layers (MLs) ZnS on the ZnCdSSe cores, the resultant ZnCdSSe/ZnS QDs reaches a PL QY as high as 96% at 520 nm and narrow emission spectra (full width at half maximum, FWHM <20 nm). The ZnCdSSe/ZnS QDs are used to fabricate green light emitting diodes (GLEDs) with a 390 nm chip as the backlight. The best GLEDs show bright green light, excellent color purity, and high luminous stability. Such excellent performance is attributed to a high nucleation temperature at 330 °C and the precisely controlled ZnS shells of CdSeZnS/ZnS QDs, which can effectively suppress Auger recombination (AR). This study suggests that the precise nucleation and shell engineering of QDs may provide an effective means to meet requirements for practical display and lighting applications.

Ultrafast Charge-Transfer Dynamics in Twisted MoS2/WSe2 Heterostructures.

Two-dimensional transition metal dichalcogenides offer a fascinating platform for creating van der Waals heterojunctions with exciting physical properties. Because of their typical type-II band alignment, photoexcited electrons and holes can separate via interfacial charge transfer. Furthermore, the relative crystallographic alignment of the individual layers in these heterostructures represents an important degree of freedom. Based on both effects, various fascinating ideas for applications in optoelectronics and valleytronics have been suggested. Despite its utmost importance for the design and efficiency of potential devices, the nature and the dynamics of ultrafast charge transfer are not yet well understood. This is mainly because the charge transfer can be surprisingly fast, usually faster than the temporal resolution of previous experimental approaches. Here, we apply time- and polarization-resolved second-harmonic imaging microscopy to investigate the charge-transfer dynamics for three MoS2/WSe2 heterostructures with different stacking angles at a previously unattainable time resolution of ≈10 fs. For 1.70 eV excitation energy, electron transfer from WSe2 to MoS2 is found to depend considerably on the stacking angle with the fastest transfer time observed to be as short as 12 fs. At 1.85 eV excitation energy, ultrafast hole transfer from MoS2 to hybridized states at the Γ-point and to the K-points of WSe2 has to be considered. Surprisingly, the corresponding decay dynamics show only a minor stacking-angle dependence indicating that radiative recombination of momentum-space indirect Γ-K excitons becomes the dominant decay route for all samples.

Tuning the interfacial stoichiometry of InP core and InP/ZnSe core/shell quantum dots

We demonstrate fine-tuning of the atomic composition of InP/ZnSe quantum dots (QDs) at the core/shell interface. Specifically, we control the stoichiometry of both anions (P, As, S, and Se) and cations (In and Zn) at the InP/ZnSe core/shell interface and correlate these changes with the resultant steady-state and time-resolved optical properties of the nanocrystals. The use of reactive trimethylsilyl reagents results in surface-limited reactions that shift the nanocrystal stoichiometry to anion-rich and improve epitaxial growth of the shell layer. In general, anion deposition on the InP QD surface results in a redshift in the absorption, quenching of the excitonic photoluminescence, and a relative increase in the intensity of broad trap-based photoluminescence, consistent with delocalization of the exciton wavefunction and relaxation of exciton confinement. Time-resolved photoluminescence data for the resulting InP/ZnSe QDs show an overall small change in the decay dynamics on the ns timescale, suggesting that the relatively low photoluminescence quantum yields may be attributed to the creation of new thermally activated charge trap states and likely a dark population that is inseparable from the emissive QDs. Cluster-model density functional theory calculations show that the presence of core/shell interface anions gives rise to electronic defects contributing to the redshift in the absorption. These results highlight a general strategy to atomistically tune the interfacial stoichiometry of InP QDs using surface-limited reaction chemistry allowing for precise correlations with the electronic structure and photophysical properties.

Photoluminescence imaging of defects in TiO2: The influence of grain boundaries and doping on charge carrier dynamics

Understanding the mechanisms of charge generation and their recombination in rutile TiO2 is of key importance in the design of optoelectronic and photocatalytic devices. In this study, we investigate the impact of both extrinsic and intrinsic defects on photoluminescence (PL) decay dynamics. The exploitation of two-photon fluorescence lifetime imaging microscopy (FLIM) enabled differentiation to be made between photoluminescence originating from dislocations and bulk in Nb-doped rutile TiO2. It was found that dislocations pinned at grain boundaries feature lower photoluminescence intensity and faster decay times (by 100 ps) than those in the bulk. This can be reversed upon reduction, whereby trap states are preferentially formed near to dislocation sites. We also evaluated the dependence of the extrinsic doping level on the charge carrier dynamics of rutile, and show that PL lifetimes are governed by predominant Auger processes that are insensitive to reduction, unlike dislocations. We believe that the oxygen deficiency suppression of charge carrier recombination at grain boundaries is the key factor in improving the photocatalytic activity of real TiO2-based materials.

Subradiance in dilute atomic ensembles: Role of pairs and multiple scattering

We study numerically the slow (subradiant) decay of the fluorescence of motionless atoms after a weak pulsed excitation. We show that, in the linear-optics regime and for an excitation detuned by several natural linewidths, the slow decay rate can be dominated by close pairs of atoms (dimers) forming superradiant and subradiant states. However, for a large-enough resonant optical depth and at later time, the dynamics is dominated by collective many-body effects. In this regime, we study the polarization and the spectrum of the emitted light, as well as the spatial distribution of excitation inside the sample, as a function of time during the decay dynamics. The behavior of these observables is consistent with what would be expected for radiation trapping of nearly resonant light. This finding sheds light on subradiance in dilute samples by providing an interpretation based on the light behavior of the system (multiple scattering) which is complementary to the more commonly used picture of the collective atomic Dicke state.

Temporal shaping of single photons by engineering exciton dynamics in a single quantum dot

The majority of photonic quantum information technologies rely on single photons that have high purity and indistinguishability. Although solid-state quantum emitters can serve such single photons on demand, their asymmetric temporal and spatial mode profiles limit the optimal efficiency and fidelity of quantum interaction. Here, we demonstrate single-photon pulses at a telecom wavelength with a Gaussian-like temporal mode profile from a cavity-coupled single quantum dot. Engineering the exciton dynamics via multi-exciton cascade recombination and cavity detuning enables us to modify the rise and decay dynamics of single excitons. Furthermore, the cascade recombination process temporally retards the single-exciton emission from the background emission, leading to possible purification of single photons at high excitation power. In addition, coupling quantum dots into a low Q cavity mode leads to a Gaussian-like spatial mode profile, which brings a high collection efficiency. This approach paves the way for producing single photons with an optimized temporal and spatial waveform.

Optical Transitions and Excited State Absorption Cross Sections of SrLaGaO4 Doped with Ho3+ Ions.

The spectroscopic properties of SrLaGaO4 (SLO) crystal doped with Ho3+ ions were studied in this work. Absorption, emission spectra and decay dynamics of excited states have been measured and discussed using the Judd–Ofelt model. Photoluminescence emissions were attributed to transitions from the excited 3D3, 5S2, 5F5, 5I6 and 5I7 multiplet manifolds. The experimental lifetimes for five excited states have been compared to the theoretical values, calculated using Judd–Ofelt theory, allowing for the determination of the multiphonon relaxation rates (WnR) of the respective states. The experimental data were approximately on a line expressed by WnR = W0 exp(−αΔE) with W0 = 0.5 × 107 s−1 and α = 2.6 × 10−3 cm. To discuss the excited state absorption (ESA) pathways, that originated from several excited levels, we used the Judd–Ofelt formalism allowing determination of the integrated cross section for ESA transitions.

Self-Constructed Multiple Plasmonic Hotspots on an Individual Fractal to Amplify Broadband Hot Electron Generation.

Plasmonic nanoparticles are ideal candidates for hot-electron-assisted applications, but their narrow resonance region and limited hotspot number hindered the energy utilization of broadband solar energy. Inspired by tree branches, we designed and chemically synthesized silver fractals, which enable self-constructed hotspots and multiple plasmonic resonances, extending the broadband generation of hot electrons for better matching with the solar radiation spectrum. We directly revealed the plasmonic origin, the spatial distribution, and the decay dynamics of hot electrons on the single-particle level by using ab initio simulation, dark-field spectroscopy, pump-probe measurements, and electron energy loss spectroscopy. Our results show that fractals with acute tips and narrow gaps can support broadband resonances (400-1100 nm) and a large number of randomly distributed hotspots, which can provide unpolarized enhanced near field and promote hot electron generation. As a proof-of-concept, hot-electron-triggered dimerization of p-nitropthiophenol and hydrogen production are investigated under various irradiations, and the promoted hot electron generation on fractals was confirmed with significantly improved efficiency.

Bright Near-Infrared to Visible Upconversion Double Quantum Dots Based on a Type-II/Type-I Heterostructure

Upconverting semiconductor quantum dots (QDs) combine the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. Here, we present upconverting type-II/type-I colloidal double QDs that enable enhancement of the performance of near-infrared to visible photon upconversion in QDs and broadening the range of relevant materials used. The resulting ZnTe/CdSe@CdS@CdSe/ZnSe type-II/type-I double QDs possess a very high photoluminescence quantum yield, monoexponential decay dynamics, and a narrow line width, approaching those of state-of-the-art upconverting QDs. We quantitatively characterize the upconversion cross section by direct comparison with two-photon absorption when varying the pump frequency across the absorption edge. Finally, we show that these upconversion QDs maintain their optical performance in a much more demanding geometry of a dense solid film and can thus be incorporated in devices as upconversion films. Our design provides guidance for fabricating highly efficient upconverting QDs with potential applications such as security coding and bioimaging.

Arginine methylation and cytoplasmic mRNA fate: An exciting new partnership.

Posttranslational modifications play a crucial role in regulating gene expression. Among these modifications, arginine methylation has recently attracted tremendous attention due to its role in multiple cellular functions. This review discusses the recent advances that have established arginine methylation as a major player in determining cytoplasmic messenger RNA (mRNA) fate. We specifically focus on research that implicates arginine methylation in regulating mRNA translation, decay, and RNA granule dynamics. Based on this research, we highlight a few emerging future avenues that will lead to exciting discoveries in this field.

Dynamics of HIV reservoir decay and naïve CD4 T-cell recovery between immune non-responders and complete responders on long-term antiretroviral treatment

BackgroundThe dynamics of viral reservoir decay and naïve CD4 T-cell recovery between immunological non-responders (INR) and complete responders (CR) during long-term antiretroviral treatment (ART) are not fully known. MethodsTwenty-eight chronic HIV-infected individuals on 5-year ART were divided into two groups: INR (CD4 counts ≤350 cells/μL, n = 13) and CR (CD4 counts ≥500 cells/μL, n = 15). The levels of HIV DNA and cell-associated HIV RNA (CA-RNA), CD4 counts, naïve CD4 counts and their correlations were analyzed at baseline, years 1, 3 and 5 of ART between the two groups. Expression of PD-1 on CD4 T-cells was quantified by flow cytometry. Linear mixed effect models were used to estimate the change procession in repeated measurements over 5 years. Slopes of the above-mentioned indicators were estimated using participant-specific linear regressions, respectively. ResultsINR maintained higher levels of HIV DNA and CA-RNA with higher percentages of PD-1+CD4 T-cells compared with CR during 5-year ART, concurrent with lower naïve CD4 T-cells. However, the rates of HIV DNA and CA-RNA decay in INR were not different from that in CR over time, and INR had higher rates of naïve CD4 T-cell percentage recovery. The baseline levels of HIV DNA were positively associated with the 5-year levels of HIV DNA, but negatively associated with the 5-year naïve CD4 counts. ConclusionsINR maintained significantly higher viral reservoir and lower naïve CD4 T-cells compared with CR during 5-year ART, however, the rates of reservoir decay and naïve CD4 T-cell percentage growth within INR were not lower than that in CR over time.