Characteristic Time Constants Research Articles

Quantifying relaxation time constants in MoS2xSe2(1-x) alloys: Impact of Stoichiometry and Si/SiO2 interference

Despite a decade of extensive research on two-dimensional transition metal dichalcogenides, there are still some gaps in our understanding of their remarkable properties, limiting their potential applications. One such gap relates to the properties of these alloys, which offer tunable band gaps and can serve as the basis for various optoelectronic and nonlinear optical devices. A comprehensive understanding of the ultrafast charge carrier excitation and relaxation dynamics as well as their characteristic relaxation time constants, is crucial for the further use of these materials in specific applications. In this work, we experimentally investigate the dynamics of ultrafast relaxation processes in monolayer MoS2xSe2(1-x) alloys on a Si/SiO2 substrate using time-resolved spectroscopy. We determine the characteristic relaxation time constants for structures with different stoichiometric compositions and refine these constants by accounting for interference effects arising from the substrate.

Protocol for certifying entanglement in surface spin systems using a scanning tunneling microscope

Certifying quantum entanglement is a critical step toward realizing quantum-coherent applications. In this work, we show that entanglement of spins can be unambiguously evidenced in a scanning tunneling microscope with electron spin resonance by exploiting the fact that entangled states undergo a free time evolution with a distinct characteristic time constant that clearly distinguishes it from the time evolution of non-entangled states. By implementing a phase control scheme, the phase of this time evolution can be mapped back onto the population of one entangled spin, which can then be read out reliably using a weakly coupled sensor spin in the junction of the scanning tunneling microscope. We demonstrate through open quantum system simulations with currently available spin coherence times of T2 ≈ 300 ns, that a signal directly correlated with the degree of entanglement can be measured at temperatures of 100–400 mK accessible in sub-Kelvin scanning tunneling microscopes.

Distribution of Relaxation Times Analysis of PEMFC through Loewner Framework: A Direct and Regularization Free Approach

The analysis and interpretation of electrochemical impedance spectroscopy (EIS) data are often complex and challenging, despite its popularity within the field of electrochemistry. Hence, recently, we have introduced novel data-driven approach for extracting the distribution of relaxation times (DRT) from impedance data in order to improve further analysis [1, 2]. This approach is based on the Loewner framework, which is a versatile and straightforward data-driven modeling method that computes reduced-order models in a state-space representation directly from frequency response data. Thus, it allows estimation of DRT without need of regularization procedures or iterative optimization algorithms.The effectiveness of this approach is successfully demonstrated by analysing the experimental EIS data of a polymer electrolyte membrane fuel cell (PEMFC). The results show that this method allows clear identification of the individual polarization processes based on their characteristic time constants (see Figure). Furthermore, the peaks observed in the DRT plot are correctly associated with different processes by examining spectra under various operating conditions. This is further validated by employing a physics-based model [3]. In this way, unambiguous fault identification and monitoring of the degradation state of the cell is possible and it can be the foundation for an on-board diagnostic tool.

Emulating Ebbinghaus forgetting behavior in a neuromorphic device based on low dimensional h-BN

Artificial synaptic devices that can mimic the biological synaptic functions of learning and forgetting are essential for the realization of neuromorphic computation, which could replace the von Neumann architecture. In this Letter, we have described a high-performing ultraviolet photodetector (wavelength 375 nm) using thin films of single-layer hexagonal boron nitride (h-BN) for potential use in fabricating a neuromorphic device. Furthermore, the classical Ebbinghaus forgetting curve can be optimized using various parameters such as the optical pulse width, number of pulses, and frequency of pulses. Our results show that the characteristic time constant (τ) has much more variability, indicating better performance control than the Ebbinghaus exponent (β). Furthermore, the performance of the optical synapse is very stable for low energy consumption, as low as 2–3 pJ.

The Evaluation of Interface Quality in HfO2 Films Probed by Time-Dependent Second-Harmonic Generation.

Time-dependent second-harmonic generation (TD-SHG) is an emerging sensitive and fast method to qualitatively evaluate the interface quality of the oxide/Si heterostructures, which is closely related to the interfacial electric field. Here, the TD-SHG is used to explore the interface quality of atomic layer deposited HfO2 films on Si substrates. The critical SHG parameters, such as the initial SHG signal and characteristic time constant, are compared with the fixed charge density (Qox) and the interface state density (Dit) extracted from the conventional electrical characterization method. It reveals that the initial SHG signal linearly decreases with the increase in Qox, while Dit is linearly correlated to the characteristic time constant. It verifies that the TD-SHG is a sensitive and fast method, as well as simple and noncontact, for evaluating the interface quality of oxide/Si heterostructures, which may facilitate the in-line semiconductor test.

Autocombustion Route Derived Zinc Ferrite Nanoparticles as Chemiresistive Sensor for Detection of Alcohol Vapors.

Prolonged exposure to alcohol vapors can have detrimental effects on human health, potentially leading to eye irritation, dizziness, and in some cases, damage to the nervous system. The present article aims to provide a comprehensive understanding on the synthesis and characterization of zinc ferrite (ZnFe2O4) nanoparticles, as well as their interactions with a range of alcohol vapors, including methanol, ethanol, n-propanol, and isopropanol. These alcohols differ in their molecular weight, boiling points, diffusivity, and other properties. The study reveals the semiconducting ZnFe2O4 nanoparticulate sensor's capability for reversible, repeatable, and sensitive detection of alcohol vapors. The sensor exhibits the highest response to ethanol within operating temperature range (225-300 °C). An attempt is made to establish a correlation between the properties of the target analytes and the observed sensing signals. Additionally, the response conductance transients of ZnFe2O4 under the exposure to the studied alcohol vapors are modeled based on the Langmuir-Hinshelwood adsorption mechanism. The characteristic time constants obtained from this modeling are justified with respect to the properties of the analytes.

Carbon dots as a sustainable electrolyte enhancer in aqueous alkaline electrochemical capacitors

In the endeavour to increase the energy density and to widen the potential window of aqueous alkaline electrochemical capacitors (EC), this study explores the role of carbon dots (CDs) as an additive in potassium hydroxide electrolyte. The CDs with an average size of ∼2.2 nm and negative surface potential are synthesized from a dispersion of palm kernel shell powder in water using a low-temperature hydrothermal process. Electrochemical measurements show that the CD–electrolyte ion (K+) interaction has improved counter ion adsorption in porous carbon electrodes via lowering the characteristic resistances and time constants, which significantly improved the fraction of adsorbed charges than diffusively stored. The improved ionic conductivity is attributed to the improved wettability introduced by the hydrophilic functional groups in the CDs. These parametric changes widened the potential window and marked an 80 % increase in the specific energy and over a 10 % increase in specific power in a practical EC with similar mass-loading as commercial devices. The device with CDs demonstrated superior cycling stability and Coulombic efficiency than the control device without them. These findings underscore the potential of CDs as a promising avenue for advancing the performance parameters of aqueous electrolyte EC, aligning with the overarching goal of realising economical, environmentally friendly, and sustainable energy storage solutions.

DTT toroidal field conductor samples test in Sultan: DC and AC characterization

The superconducting magnet system of the Divertor Tokamak Test (DTT) facility, composed of 18 toroidal field (TF) coils, 6 poloidal field coils and a central solenoid, has been designed and many procurements have been launched. Some manufacturing aspects and some conductor features require characterization under relevant close-to-operative conditions. To confirm the design choices in all details, cryogenic tests in qualified facilities have been foreseen. In this work, the results of the TF samples characterization at the SULTAN facility at the Swiss Plasma Centre (SPC, EPFL) are presented. The 3 week test campaign started on July the 8th, 2022. The DTT TF SULTAN sample was made of two Nb3Sn cable-in-conduit conductor ‘legs’, namely ‘TF-A’ and ‘TF-B’, made with wires produced by Kiswire Advanced Technology, differing for the cabling twist pitch sequence only, and designed to work in DTT at 42.5 kA at 11.9 T peak field. The extensive characterization comprised 3000 electro-magnetic (EM) cycles and two warm-up-cool-down (WUCD) steps, and in detail it included: AC measurements on the virgin conductors, on cyclic loaded conductors and after WUCDs; DC tests at 10.85 T/42.5 kA with intermediate EM cycles at 10.85 T/45 kA before and after WUCDs; DC tests using partial Lorentz force loads, and Minimum Quench Energy tests at 9 T/42.5 kA after cycles and WUCDs. The results of the DC measurement analysis verified the design, in terms of current sharing temperature (T cs) and critical current (I c), as both samples are over the minimum acceptance values. In particular, the ‘TF-A’ sample, characterized by a so-called ‘long twist pitch’ cabling sequence, showed higher performance without any degradation with loading and WUCD cycles, whereas sample ‘TF-B’ presented an initial T cs reduction that afterwards substantially remained unchanged. In terms of strain acting at the Nb3Sn filaments level, this result can be described by a lower effective strain in the ‘TF-A’ sample. AC losses were measured with a calorimetric method as a function of frequency for each series of AC sinusoidal pulsing measurements, and the characteristic coupling time constants were determined.

Numerical simulation approach for consideration of ageing effects in PCB substrates by modifying viscoelastic materials properties

During operating time of electronic systems, the used materials in such devices are potentially subjected to ageing effects, which might limit the lifetime. Therefore, knowledge about the used materials and the way the materials are affected by ageing effects is of key importance to develop reliable products.In this study, a simulation approach is discussed that is able to consider ageing effects caused by oxidation at elevated temperature of a printed circuit board material, typically used for high frequency applications. The material was characterized for its thermomechanical properties with state-of-the-art techniques for different ageing durations. Ageing was accelerated by storing the samples in an oven at 175 °C for up to 1000 h.Within the simulation workflow, the thermomechanical properties of the different aged states are defined by modifying the pristine viscoelastic properties. Four exponential functions are derived modifying the initial modulus, the characteristic time constants, the shift function and the coefficient of thermal expansion, all in dependency of ageing time.To demonstrate the approach, the soldered interconnection lifetime of a theoretical chip-size-package on a printed circuit board is studied. State-of-the-art lifetime predictions of such interconnections only include thermomechanical ageing effects, for example by creep effects of the solder. By additionally considering the ageing of the printed circuit board, thermal ageing is combined with thermomechanical ageing.Results in the soldered interconnection are compared between either considering additional ageing effects of the printed circuit board or neglecting this behavior. Thus it is shown that thermal ageing plays a significant role in the development of accumulated creep strain which becomes increasingly important with increasing expected lifetime.

Pandemic Equation and COVID-19 Evolution

The Pandemic Equation describes multiple pandemic waves and has been applied to describe the COVID-19 pandemic. Using the generalized approaches of solid-state physics, we derive the Pandemic Equation, which accounts for the effects of pandemic mitigation measures and multiple pandemic waves. The Pandemic Equation uses slow and fast time scales for “curve flattening” and describing vaccination and mitigation measures and the Scaled Fermi–Dirac distribution functions for describing transitions between pandemic waves. The Pandemic Equation parameters extracted from the pandemic curves can be used for comparing different scenarios of the pandemic evolution and for extrapolating the pandemic evolution curves for the periods of time on the order of the instantaneous Pandemic Equation characteristic time constant. The parameter extraction for multiple locations could also allow for uncertainty quantification for such pandemic evolution predictions.

Novel methodology for characterization of thermoelectric modules and materials

The document presents an innovative methodology that combines forced response and natural response theories in thermoelectric materials and devices. It stands out for expressing the thermoelectric figure of merit in terms of the ratio of two temperatures ZT푇̅=∆T′/∆T, enabling comprehensive testing and precise characterization of thermoelectric modules and materials, including measurements of thermal conductance, electrical resistance, Seebeck coefficient, and figure of merit. Additionally, it addresses the determination of thermal resistances and thermal capacitances related to thermal contacts, as well as the derivation of characteristic time constants and angular frequencies. This approach, applicable to both modular devices and individual samples, allows for the simultaneous measurement of all parameters on a single sample. The experiments considered non-ideal contacts and non-adiabatic conditions at room temperature T= 300K, enhancing the feasibility of in-situ characterization and positioning this methodology as a key tool in thermoelectric research.

Tuning ultrafast demagnetization dynamics in FePdPt ternary alloy films: the role of spin–orbit coupling

The influence of spin–orbit coupling (SOC) on ultrafast demagnetization dynamics is still an obscure issue. Ferromagnetic ternary alloy film containing Platinum element has the advantage that SOC strength can be regulated flexibly by varying the component ratio, thus is an excellent platform for investigating the dynamical effect of SOC. Here the fluence and component dependences of ultrafast demagnetization dynamics in ternary alloy L10-Fe0.5(Pd1-xPtx)0.5 films are studied by using time-resolved magneto-optical Kerr spectroscopy. An excitation-fluence dependent transition of demagnetization dynamics shows the Elliott-Yafet (EY) spin-flip scattering as the dominant mechanism of ultrafast demagnetization. We analyze the dynamics by a compatible fitting model and extract multiple characteristic time constants. It is clearly demonstrated that the gradual enhancement of SOC in fact decreases the spin-flip scattering and slow down the demagnetization process. We further show that the demagnetization time τm scales with the SOC strength ξ approximately under a dependence of τm∝ξ1/3, while a linear dependence exists between the spin–lattice relaxation time τs-l and ξ-6. These findings clarify the role of SOC in EY-dominated demagnetization dynamics and demonstrate an approach for tuning the speeds and duration of ultrafast demagnetization by regulating SOC strength.

Second-Harmonic Nonlinear Electrochemical Impedance Spectroscopy: Part I. Analytical Theory and Equivalent Circuit Representations for Planar and Porous Electrodes

Analytical theory for second harmonic nonlinear electrochemical impedance spectroscopy (2nd-NLEIS) of planar and porous electrodes is developed for interfaces governed by Butler-Volmer kinetics, a Helmholtz (mainly) or Gouy-Chapman (introduced) double layer, and transport by ion migration and diffusion. A continuum of analytical EIS and 2nd-NLEIS models is presented, from nonlinear Randles circuits with or without diffusion impedances to nonlinear macrohomogeneous porous electrode theory that is shown to be analogous to a nonlinear transmission-line model. EIS and 2nd-NLEIS for planar electrodes share classic charge transfer RC and diffusion time-scales, whereas porous electrode EIS and 2nd-NLEIS share three characteristic time constants. In both cases, the magnitude of 2nd-NLEIS is proportional to nonlinear charge transfer asymmetry and thermodynamic curvature parameters. The phase behavior of 2nd-NLEIS is more complex and model-sensitive than in EIS, with half-cell NLEIS spectra potentially traversing all four quadrants of a Nyquist plot. We explore the power of simultaneously analyzing the linear EIS and 2nd-NLEIS spectra for two-electrode configurations, where the full-cell linear EIS signal arises from the sum of the half-cell spectra, while the 2nd-NLEIS signal arises from their difference.

Cell-to-cell variation beyond parameter analysis — Identification and correlation of processes in Lithium-Ion Batteries using a combined distribution of relaxation times analysis

Cell-to-cell variations inside a battery pack can result in inhomogeneities, leading to an accelerated, uneven aging. Until today these variations are solely quantified by parameter-based studies, which only offer a limited interpretability. To tackle this inconclusiveness, cell-to-cell variations and their effects on the electrode processes of lithium-ion batteries are investigated in this work. For this purpose 92 cells are characterized by impedance, pulse, and pseudo open-circuit voltage measurements, and the data obtained is compiled as publicly available data set. The data is examined by a holistic distribution of relaxation times analysis, comprising complementary distribution of relaxation times approaches. This enables a reliable identification of process variations between cells. First, the confidence intervals of both the raw data and the distribution of relaxation times are examined. Subsequently, the individual processes, their characteristic time constants, and their polarization are determined. The analysis reveals that a normal distribution only applies in one of the examined cases and can even be excluded in four cases. Finally, a correlation analysis is conducted, allowing the categorization of the identified processes into cell winding, electrochemical interface processes, and solid-state diffusion. The potential physicochemical reasons behind are discussed and the work thus contributes to a deeper understanding of cell-to-cell variations.

Influence of the Bismuth Content on the Optical Properties and Photoluminescence Decay Time in GaSbBi Films.

We report the optical properties of GaSbBi layers grown on GaSb (100) substrates with different bismuth contents of 5.8 and 8.0% Bi. Fourier-transform photoluminescence spectra were determined to identify the band gaps of the studied materials. Further temperature- and power-dependent photoluminescence measurements indicated the presence of localized states connected to bismuth clustering. Finally, time-resolved photoluminescence measurements based on single-photon counting allowed the determination of characteristic photoluminescence decay time constants. Because of the increasing bismuth content and clustering effects, an increase in the time constant was observed.

Benchmarking metal-organic framework coatings from Large-Temperature-Jump experiments: Learning from trade-offs in mean powers versus characteristic times

Adsorption chillers can substitute the electric energy demand of conventional compression chillers with low-temperature heat. The efficiency and power density of adsorption chillers depend on their adsorbent and refrigerant, which form the working pair. Novel working pairs are therefore actively developed and characterized in dynamic small-scale experiments using a characteristic time constant. However, this evaluation by only few characteristic time constants, does not resolve the trade-off between efficiency and power density of adsorption chillers and novel materials are challenging to compare due to varying size, shape, and density. To fill these gaps, we investigate the trade-off between efficiency and power density by quantifying all characteristic time constants and transferring them to specific mean powers. We perform Large-Temperature-Jump experiments with water as the refrigerant on the two metal–organic frameworks CAU–10–H and aluminum fumarate for a chilling/recooling/regeneration temperature triple of 10/30/80 °C. We compare the metal–organic frameworks’ specific mean powers to the commercially available RD type silica gels Siogel and SG123. Siogel performed best in terms of area-specific mean powers with a maximum value of 3.2 kW/m². For low time constants up to 20% relative loading, corresponding to high power density but lower efficiency, Siogel also provided the highest volume-specific mean power at 8.1 MW/m³. CAU–10–H had the highest mass- and volume-specific mean powers at 19.0–19.9 kW/kg and 7.2–8.6 MW/m3 for characteristic time constants of 35% relative loading and higher. Aluminum fumarate and SG123 showed low specific mean powers for all chosen characteristic time constants. The results show that studying specific mean powers is a useful tool to benchmark adsorption working pairs as they would perform in an adsorption chiller, regardless of mass, shape, or density.

A chemo-viscoelastic model and a numerical scheme to study stress-diffusion interactions in Lithium ion battery electrode particles

A mathematical model and an associated numerical framework is developed for an isotropic viscoelastic electrode particle. The ensuing differential equations are solved using FEM with the help of an open source C++ library (deal.ii). The developed model is then used to understand the effects of time dependent material property (stress relaxation function) on the stress distribution of a viscoelastic electrode particle under potentiostatic boundary conditions. In contrast to a purely elastic particle, it is observed that the radial stresses become compressive and tangential stresses tensile towards the outer end of the particle. On varying the values of the characteristic time constant (τ), it is observed that stress relaxes faster for lower values of τ when compared to larger ones. Furthermore, the interactions between stress evolution, stress relaxation function, and diffusion is studied by varying τ and the electrode particle size. The study reveals that viscoelastic effects are apparent for a range of τ depending on the particle size. Thus, the interactions between the rate of diffusion, which varies with particle sizes, and τ affect the stress relaxation function, resulting in a complex mechanical response.

Describing NMR chemical exchange by effective phase diffusion approach

This paper proposes an effective phase diffusion method to analyze chemical exchange in nuclear magnetic resonance (NMR). The chemical exchange involves spin jumps around different sites where the spin angular frequencies vary, which is a random phase walk viewed from the rotating frame reference. Such a random walk in phase space can be treated by the effective phase diffusion method. Both the coupled and uncoupled normal and fractional phase diffusions are considered. Based on the phase diffusion results, the line shape of NMR exchange spectrum can be obtained. By comparing these theoretical results with the conventional theory, this phase diffusion approach works for fast exchange, ranging from slightly faster than intermediate exchange to very fast exchange. For normal diffusion models, the theoretically predicted curves agree with those predicted from traditional models in the literature, and the characteristic exchange time obtained from phase diffusion with a fixed jump time is the same as that obtained from the conventional model. However, the phase diffusion with a monoexponential time distribution gives a relatively shorter characteristic exchange time constant, which is half of that obtained from the traditional model. Additionally, the fractional diffusion obtains a significantly different line shape than that predicted based on normal diffusion.

Model reduction of fractional impedance spectra for time–frequency analysis of batteries, fuel cells, and supercapacitors

AbstractJoint time–frequency analysis is an emerging method for interpreting the underlying physics in fuel cells, batteries, and supercapacitors. To increase the reliability of time–frequency analysis, a theoretical correlation between frequency‐domain stationary analysis and time‐domain transient analysis is urgently required. The present work formularizes a thorough model reduction of fractional impedance spectra for electrochemical energy devices involving not only the model reduction from fractional‐order models to integer‐order models and from high‐ to low‐order RC circuits but also insight into the evolution of the characteristic time constants during the whole reduction process. The following work has been carried out: (i) the model‐reduction theory is addressed for typical Warburg elements and RC circuits based on the continued fraction expansion theory and the response error minimization technique, respectively; (ii) the order effect on the model reduction of typical Warburg elements is quantitatively evaluated by time–frequency analysis; (iii) the results of time–frequency analysis are confirmed to be useful to determine the reduction order in terms of the kinetic information needed to be captured; and (iv) the results of time–frequency analysis are validated for the model reduction of fractional impedance spectra for lithium‐ion batteries, supercapacitors, and solid oxide fuel cells. In turn, the numerical validation has demonstrated the powerful function of the joint time–frequency analysis. The thorough model reduction of fractional impedance spectra addressed in the present work not only clarifies the relationship between time‐domain transient analysis and frequency‐domain stationary analysis but also enhances the reliability of the joint time–frequency analysis for electrochemical energy devices.

Femtosecond exciton relaxation in chlorosomes of the photosynthetic green bacterium <i>Chloroflexus aurantiacus</i>

In the green bacteria Chloroflexus (Cfx.) aurantiacus, the process of photosynthesis begins with the absorption of light by chlorosomes, peripheral antennas consisting of thousands of bacteriochlorophyll c (BChl c) molecules combined into oligomeric structures. In this case, excited states are formed in BChl c, the energy of which migrates along the chlorosome towards the baseplate and further to the reaction center, where the primary charge separation occurs. Energy migration is accompanied by nonradiative electronic transitions between numerous exciton states, that is exciton relaxation. In this work, we studied the dynamics of exciton relaxation in Cfx. aurantiacus chlorosomes using difference femtosecond spectroscopy at cryogenic temperature (80 K). Chlorosomes were excited by 20 fs light pulses at wavelengths from 660 to 750 nm, and the difference (light-dark) kinetics were measured at a wavelength of 755 nm. Mathematical analysis of the data obtained revealed the kinetic components with characteristic time constants of 140, 220, and 320 fs responsible for the exciton relaxation. As the excitation wavelength decreased, the number of components and their relative contribution increased. Theoretical modeling of the results obtained was carried out on the base of the cylindrical model of BChl c aggregates. Nonradiative transitions between groups of exciton bands were ascribed by the system of kinetic equations. The model in which the energetic and structural disorder was taken into account turned out to be the most adequate.