Temporal Regimes Research Articles

Ultrafast Carrier-Coupled Interlayer Contraction, Coherent Intralayer Motions, and Phonon Thermalization Dynamics of Black Phosphorus.

Black phosphorus (bP) exhibits highly anisotropic properties and dynamical behavior that are unique even among two-dimensional and van der Waals (vdW) layered materials. Here, we show that an interlayer lattice contraction and concerted, symmetric intralayer vibrations occur concurrently within few picoseconds following the photoinjection and relaxation of carriers, using ultrafast electron diffraction in the reflection geometry to probe the out-of-plane motions. A strong coupling between the photocarriers and bP's puckered structure, with the alignment of the electronic band structure, is at work for such directional atomic motions without a photoinduced phase transition. Three temporal regimes can be identified for the phonon thermalization dynamics where a quasi-equilibrium without anisotropy is reached in about 50 ps, followed by propagation of coherent acoustic phonons and heat diffusion into the bulk. The early time out-of-plane dynamics reported here have important implications for single- and few-layer bP and other vdW materials with strong electronic-lattice correlations.

Spatiotemporal dynamics of ultrarelativistic beam-plasma instabilities

An electron or electron-positron beam streaming through a plasma is notoriously prone to micro-instabilities. For a dilute ultrarelativistic infinite beam, the dominant instability is a mixed mode between longitudinal two-stream and transverse filamentation modes, with a phase velocity oblique to the beam velocity. A spatiotemporal theory describing the linear growth of this oblique mixed instability is proposed, which predicts that spatiotemporal effects generally prevail for finite-length beams, leading to a significantly slower instability evolution than in the usually assumed purely temporal regime. These results are accurately supported by particle-in-cell (PIC) simulations. Furthermore, we show that the self-focusing dynamics caused by the plasma wakefields driven by finite-width beams can compete with the oblique instability. Analyzed through PIC simulations, the interplay of these two processes in realistic systems bears important implications for upcoming accelerator experiments on ultrarelativistic beam-plasma interactions.

Imprint of the temporal envelope of ultra-short laser pulses on the longitudinal momentum spectrum of e+e− pairs

The effect of the temporal pulse shape of intense pulses on the momentum distribution of ${e}^{+}{e}^{\ensuremath{-}}$ pairs is studied using the quantum kinetic equation. Two closely resembling temporal envelopes, namely, Gaussian and Sauter, keeping all the other pulse parameters the same, are considered to this end. Contrary to the common perception which can be gauged from the interchangeable use of these temporal profiles, the longitudinal momentum spectrum of the pairs created by the two pulses is found to differ significantly in all the temporal regimes. For the pulses having a few cycles of oscillations, the temporal profile of the pulse is revealed in the oscillatory interference pattern riding over the otherwise smooth longitudinal momentum spectrum at asymptotic times. The onset of the oscillation due to the quantum interference of reflection amplitudes from the scattering potential due to the pulses having a temporal structure of multiple barriers takes place for fewer-cycle oscillations for the Gaussian pulse compared to that for the Sauter pulse. Furthermore, the oscillation amplitude for the same number of oscillations within the pulse duration is larger for the Gaussian pulse. The presence of the carrier-envelope phase and the frequency chirping is found to magnify these differences. In the absence of any appreciable interference effect for the pulses having less than five oscillations, the longitudinal momentum spectrum has a higher peak value for the Sauter pulse at asymptotic times. On the other hand, before the transient stage of evolution, the peak of the spectrum shows the opposite trend.

Civil war and the non‐linearity of time: approaching a Mozambican politics of irreconciliation

AbstractAt least 1 million people died during the Mozambican civil war (1976/7‐92). Unfolding after gaining independence from Portugal (1975) and alongside experiments with Afro‐socialism in the 1980s, the war, despite its brutality, has not been subjected to global templates of reconciliation processes. Thus it comprises a unique case to probe what irreconciliation might mean – both as a political horizon and as an analytical concept. This text juxtaposes ethnographic material from rural, central Mozambique from the late 1990s and early 2000s emphasizing reconciliation with material from the same spaces from the 2010s onwards, where I identify what I term a ‘politics of irreconciliation’. I will make three arguments. First, informed by Hannah Arendt, I approach irreconciliation as fundamentally about the rejection of a world of violence in search of a world shared in common. Second, drawing on recent anthropological theorizing about temporal regimes and chronopolitics, I argue for the salience of a non‐linear understanding of the politics of irreconciliation to grapple with the fact that civil war violence is understood as dangerously uncontained rather than nominally past. Third, within the context of Mozambique, forgiveness and its other, irreconciliation, are not only intimately tied to the temporally past or present; they are also, as I show, produced by a tangible and intense absence of a productive future.

Long-distance movements of Antarctic toothfish (Dissostichus mawsoni) as inferred from tag-recapture data.

Examining the causes and consequences of animal movement is fundamental to understanding the ecology of any species. This analysis focuses on Antarctic toothfish movements in the Southern Ocean as inferred from tagging data collected from 2001 to 2019 with a focus on the characteristics of long-distance movements, defined as an individual recaptured greater than 200 km from their release location. The results of this analysis indicate that while adult Antarctic toothfish are generally quite sedentary a small proportion (~7%) move long distances, consistent with findings from previous studies examining movements of toothfish. There appears to be no relationship between time at liberty and long-distance movements, no strong influence of sex and results indicate a distinct bias in the direction of long-distance travel from release to recapture towards a counter-clockwise direction. Frequency and scale of long-distance movements are likely influenced by localized physical oceanographic processes and life-history traits. Knowledge of these movements patterns remains highly important for stock assessments and the design of spatial and temporal fisheries management regimes.

Inferring work by quantum superposing forward and time-reversal evolutions

The study of thermodynamic fluctuations allows one to relate the free energy difference between two equilibrium states with the work done on a system through processes far from equilibrium. This finding plays a crucial role in the quantum regime, where the definition of work becomes non-trivial. Based on these relations, here we develop a simple interferometric method allowing a direct estimation of the work distribution and the average dissipative work during a driven thermodynamic process by superposing the forward and time-reversal evolutions of the process. We show that our scheme provides useful upper bounds on the average dissipative work even without full control over the thermodynamic process, and we propose methodological variations depending on the possible experimental limitations encountered. Finally, we exemplify its applicability by an experimental proposal for implementing our method on a quantum photonics system, on which the thermodynamic process is performed through polarization rotations induced by liquid crystals acting in a discrete temporal regime.

‘Don’t worry. It’s just the baby blues’: representations of new motherhood in contemporary graphic memoirs

ABSTRACT All bodies are governed by principles of temporality, yet regarding their roles as (potential) mothers, female bodies are kept under particularly strict observation. I argue that comics’ power to offer ‘a more inclusive perspective of medicine’ can be made fruitful to interrogate the impact that implicit temporal regimes that govern pregnancy, birth and the postnatal period have on new mothers. Analysing two autobiographical comics, Teresa Wong’s Dear Scarlet and Lucy Knisley’s Kid Gloves, I examine how Knisley’s struggles with pre-eclampsia and Wong’s postnatal depression turn their time as new mothers into a period of ‘crip time’ in which their chrono-normative time experience is altered. Both artists use different aesthetic means to illustrate experiences that differ from the positive image of an easy birth, swift recovery and blissful postnatal period perpetuated by contemporary discourses. I suggest that the medium of comics is particularly suited to illustrate these shifts in their subjective time experience. My paper explores how Knisley’s and Wong’s graphic narratives create alternative representations of the corporeal experience of the postpartum period, providing a social commentary on the discursive constraints of motherhood and an underrepresented time in women’s lives.

Simulating wet active polymers by multiparticle collision dynamics.

The conformational and dynamical properties of active Brownian polymers embedded in a fluid depend on the nature of the driving mechanism, e.g., self-propulsion or external actuation of the monomers. Implementations of self-propelled and actuated active Brownian polymers in a multiparticle collision (MPC) dynamics fluid are presented, which capture the distinct differences between the two driving mechanisms. The active force-free nature of self-propelled monomers requires adaptations of the MPC simulation scheme, with its streaming and collision steps, where the monomer self-propulsion velocity has to be omitted in the collision step. Comparison of MPC simulation results for active polymers in dilute solution with results of Brownian dynamics simulations accounting for hydrodynamics via the Rotne-Prager-Yamakawa tensor confirm the suitability of the implementation. The polymer conformational and dynamical properties are analyzed by the static and dynamic structure factor, and the scaling behavior of the latter with respect to the wave number and time dependence are discussed. The dynamic structure factor displays various activity-induced temporal regimes, depending on the considered wave number, which reflect the persistent diffusive motion of the whole polymer at small wave numbers, and the activity-enhanced internal dynamics at large wave numbers. The obtained simulation results are compared with theoretical predictions.

Dynamic wavelength control of laser pulse profiles at picosecond to nanosecond timescales.

We report on a novel combined laser pulse shaping and dynamic wavelength encoding capability based on a simple architecture implementing direct space to time mapping. There are several potential applications that can be enabled by the ability to control the instantaneous intensity or wavelength of an optical waveform on a picosecond-to-nanosecond timescale. To our knowledge, no known methods can access this temporal regime with a practical architecture. Here, we demonstrate an extension of the Space-Time Induced Linearly Encoded Transcription for Temporal Optimization (STILETTO) technique that can generate optical waveforms with a programmable instantaneous wavelength vs. time. We experimentally demonstrate the technique by generating self-gated spectrograms and show that it can encode dynamic wavelength vs time profiles at timescales not achievable by any other known method.

Synthesizing the impacts of baseflow contribution on concentration–discharge (<i>C</i>–<i>Q</i>) relationships across Australia using a Bayesian hierarchical model

Abstract. Understanding concentration–discharge (C–Q) relationships can inform catchment solute and particulate export processes. Previous studies have shown that the extent to which baseflow contributes to streamflow can affect C–Q relationships in some catchments. However, the current understanding on the effects of baseflow contribution in shaping the C–Q patterns is largely derived from temperate catchments. As such, we still lack quantitative understanding of these effects across a wide range of climates (e.g. arid, tropical and subtropical). The study aims to assess how baseflow contributions, as defined by the median and the range of daily baseflow indices within individual catchments (BFI_m and BFI_range, respectively), influence C–Q slopes across 157 catchments in Australia spanning five climate zones. This study focuses on six water quality variables: electrical conductivity (EC), total phosphorus (TP), soluble reactive phosphorus (SRP), total suspended solids (TSS), the sum of nitrate and nitrite (NOx) and total nitrogen (TN). The impact of baseflow contributions is explored with a novel Bayesian hierarchical model. For sediments and nutrient species (TSS, NOx, TN and TP), we generally see largely positive C–Q slopes, which suggest a dominance of mobilization export patterns. Further, for TSS, NOx and TP we see stronger mobilization (steeper positive C–Q slopes) in catchments with higher values in both the BFI_m and BFI_range, as these two metrics are positively correlated for most catchments. The enhanced mobilization in catchments with higher BFI_m or BFI_range is likely due to the more variable flow pathways that occur in catchments with higher baseflow contributions. These variable flow pathways can lead to higher concentration gradients between low flows and high flows, where the former is generally dominated by groundwater/slow subsurface flow while the latter by surface water sources, respectively. This result highlights the crucial role of flow pathways in determining catchment exports of solutes and particulates. Our study also demonstrates the need for further studies on how the temporal variations of flow regimes and baseflow contributions influence flow pathways and the potential impacts of these flow pathways on catchment C–Q relationships.

СТРАТЕГІЇ ЗБЕРЕЖЕННЯ МІСЦЬ ПАМ'ЯТІ В КОНТЕКСТІ ДОСЛІДЖЕНЬ КУЛЬТУРНОЇ СПАДЩИНИ

The article is focused on the strategies of keeping the places of memory in the focus of the regime of temporality in the contemporary world. The methodological ground for the research is memory studies based on cultural anthropology, in particular, in collective memory regimes of temporality and "present pasts" cultivating (Assmann), as well as the rethinking of the principle of historicism (Lübbe). That presupposes revealing cultural heritage issues through the theme of places of memory (Nora). The new regime of temporality was provided by tragic experience of humankind during the World War II. Today time comprehension regime trough "present pasts" cultivation provides the new form of cultural memory comprehension. It appeals to space artefacts unlike the Modernity form, which worked with texts. Places of memory are particular, through which universal is refracted in the new regime of temporality. The author argues that intellectual search at the crossroads of history, philosophy and culture studies fixes the new regime of temporality, from the "present futures" to the "present pasts". "Present pasts" actualize genetic relation with the past, in comprehension of which is possible understanding current ourselves. Accents are removed from the history of the great accomplishments to the history of daily routine, and attention is paid on individual traumas and mental wounds in the new regime of temporality. That is why the history of memory with appealing to individual, psychological aspects enter the proscenium of historical studies. The past is set in the form of the cultural heritage in the new regime of temporality. Babyn Yar is the place of memory that represents particular where the universal is refracted in in the new regime of temporality. Strategies of keeping of places of memory represented in the private and the state projects reflect concepts "present futures" and "present pasts", respectively.

СМЕНА ТЕМПОРАЛЬНЫХ РЕЖИМОВ И ПРОТИВОРЕЧИВЫЕ ОБРАЗЫ НАЦИОНАЛЬНОГО

The article describes the phenomenon of interference of different cultural practices broadcast in space and time: temporal regimes, images of time as a sphere of possible stories at a given period, and images associated with national self-consciousness and nationalism. The measure of the connection is analyzed between the progressive change in the perception of time described by R. Koselleck, when expectations are separated from experience, and the formation of national self-consciousness at the end of the 18th–19th centuries, as well as between the post-progressive transition from “solid” to “liquid” time described by Z. Bauman and the transformation of national identity into diverse forms of collective self-identifications, self-understandings and identity politics at the end of the 20th−21st centuries. The role in these processes of the transforming attitude to the time of the embodiment of the meaning of history ( airos) is considered: from the unchanging, general and exclusive, associated with the life path of Christ and the idea of the Second Coming, to the changing, multiple and exclusive (the progress of civilization in a given country as airos) and beyond, in the conditions of relativization of the “civilization” concept — to interacting inclusive senses of time, multi-temporality of connected histories or histoire croisée corresponding to multi-level ways of self-identification of the population (synaxis or co-existence, conciliar time). It is these temporal structures that create the prerequisites, first for the creation and strengthening of national self-consciousness, and then for its erosion.

Acceleration theory, temporal regimes, and politics today. An interview with Hartmut Rosa

Acceleration theory, temporal regimes, and politics today. An interview with Hartmut Rosa

Racism and Confederate Monument Construction: Temporal Regimes Distinguishing the 1900s, 1960s, and Contemporary Decades

The bulk of Confederate monuments were constructed by White southerners in the early 1900s, but some were built much later. Recent research has assessed average relationships across the decades, but comparable evidence for distinct peaks in construction is lacking. My objective is to determine whether the timing of monument construction is connected to unique social contexts, particularly different manifestations of racism. I use multinomial regression analysis and a rich dataset spanning the U.S. South. Results confirm the central role of the United Daughters of the Confederacy (UDC), but also suggest stronger attachments to slavery and greater reliance on lynching increase the risk of erecting a monument in the early 1900s. In contrast, the resurgence of construction in the 1960s is unrelated to the presence of a UDC chapter and positively related to the presence of an National Association for the Advancement of Colored People (NAACP) chapter. Construction in the decades after the 1960s shift again, suggesting the renewed importance of the UDC (in addition to the location of Civil War battles), but no role of racialized dynamics. Results suggest three distinct regimes of Confederate monument construction that broadly reflect the structural racism that dominated the early 1900s; the group threat/countermovement dynamics of the 1960s; and the “colorblind” era of racism associated with contemporary decades. This research contributes to knowledge of the factors associated with Confederate monument construction and provides a foundation for public and academic discussions of how racism is intertwined with these divisive public symbols.

Raman random lasing in Ba(NO3)2 powder

Random lasing in Raman active material – barium nitrate powder – has been registered in two temporal regimes: nanosecond and picosecond. Stimulated Raman scattering (SRS) in Ba(NO3)2 for picosecond excitation has been shown to have much lower threshold and spectrum containing more components than for nanosecond excitation opposite to the case of a bulk medium. In picosecond regime SRS intensity increased with temperature decreasing and redistributed in favor of the higher order components. SRS pulse duration in picosecond regime was estimated to be in the range 16.5-22 ps. High conversion efficiency of SRS in barium nitrate powder in picosecond regime and its larger stability under laser impact than for bulk samples gives possibility to use it as an active material in Raman converters.

Exact solution for the Anisotropic Ornstein–Uhlenbeck process

Active-Matter models commonly consider particles with overdamped dynamics subject to a force (speed) with constant modulus and random direction. Some models also include random noise in particle displacement (a Wiener process), resulting in diffusive motion at short time scales. On the other hand, Ornstein–Uhlenbeck processes apply Langevin dynamics to the particles’ velocity and predict motion that is not diffusive at short time scales. Experiments show that migrating cells have gradually varying speeds at intermediate and long time scales, with short-time diffusive behavior. While Ornstein–Uhlenbeck processes can describe the moderate-and long-time speed variation, Active-Matter models for over-damped particles can explain the short-time diffusive behavior. Isotropic models cannot explain both regimes, because short-time diffusion renders instantaneous velocity ill-defined, and prevents the use of dynamical equations that require velocity time-derivatives. On the other hand, both models correctly describe some of the different temporal regimes seen in migrating biological cells and must, in the appropriate limit, yield the same observable predictions. Here we propose and solve analytically an Anisotropic Ornstein–Uhlenbeck process for polarized particles, with Langevin dynamics governing the particle’s movement in the polarization direction and a Wiener process governing displacement in the orthogonal direction. Our characterization provides a theoretically robust way to compare movement in dimensionless simulations to movement in experiments in which measurements have meaningful space and time units. We also propose an approach to deal with inevitable finite-precision effects in experiments and simulations.

Dynamics of A+B→C reaction fronts under radial advection in a Poiseuille flow.

A+B→C reaction fronts describe a wide variety of natural and engineered dynamics, according to the specific nature of reactants and product. Recent works have shown that the properties of such reaction fronts depend on the system geometry, by focusing on one-dimensional plug flow radial injection. Here, we extend the theoretical formulation to radial deformation in two-dimensional systems. Specifically, we study the effect of a Poiseuille advective velocity profile on A+B→C fronts when A is injected radially into B at a constant flow rate in a confined axisymmetric system consisting of two parallel impermeable plates separated by a thin gap. We analyze the front dynamics by computing the temporal evolution of the average over the gap of the front position, the maximum production rate, and the front width. We further quantify the effects of the nonuniform flow on the total amount of product, as well as on its radial concentration profile. Through analytical and numerical analyses, we identify three distinct temporal regimes, namely (i) the early-time regime where the front dynamics is independent of the reaction, (ii) the transient regime where the front properties result from the interplay of reaction, diffusion that smooths the concentration gradients and advection, which stretches the spatial distribution of the chemicals, and (iii) the long-time regime where Taylor dispersion occurs and the system becomes equivalent to the one-dimensional plug flow case.

Time and temporality in organisations: The case of Eskom

ABSTRACT This paper proposes analysing organisations as temporal phenomena composed of multiple temporalities. We argue that the likelihood that an organisation is well placed to function instrumentally with regard to its formal mandate is when what we will call ‘operational time’ is dominant in the temporal regime. We propose that organisations perform poorly when other temporalities come to dominate the temporal regime and/or when the temporal regime is chronically disrupted. We apply this framework to a study of Eskom, the state power company in South Africa. In Eskom, for example, we show how operational time was displaced by a political temporality that ultimately destabilised the temporal regime of the organisation as a whole.

The connectome spectrum as a canonical basis for a sparse representation of fast brain activity

The functional organization of neural processes is constrained by the brain's intrinsic structural connectivity, i.e., the connectome. Here, we explore how structural connectivity can improve the representation of brain activity signals and their dynamics. Using a multi-modal imaging dataset (electroencephalography, structural MRI, and diffusion MRI), we represent electrical brain activity at the cortical surface as a time-varying composition of harmonic modes of structural connectivity. These harmonic modes are known as connectome harmonics. Here we describe brain activity signal as a time-varying combination of connectome harmonics. We term this description as the connectome spectrum of the signal. We found that: first, the brain activity signal is represented more compactly by the connectome spectrum than by the traditional area-based representation; second, the connectome spectrum characterizes fast brain dynamics in terms of signal broadcasting profile, revealing different temporal regimes of integration and segregation that are consistent across participants. And last, the connectome spectrum characterizes fast brain dynamics with fewer degrees of freedom than area-based signal representations. Specifically, we show that a smaller number of dimensions capture the differences between low-level and high-level visual processing in the connectome spectrum. Also, we demonstrate that connectome harmonics capture more sensitively the topological properties of brain activity. In summary, this work provides statistical, functional, and topological evidence indicating that the description of brain activity in terms of structural connectivity fosters a more comprehensive understanding of large-scale dynamic neural functioning.

Single-file dynamics of colloids in circular channels: Time scales, scaling laws and their universality

In colloidal systems, Brownian motion emerges from the massive separation of time and length scales associated to characteristic dynamics of the solute and solvent constituents. This separation of scales produces several temporal regimes in the colloidal dynamics when combined with the effects of the interaction between the particles, confinement conditions, and state variables, such as density and temperature. Some examples are the short- and long-time regimes in two- and three-dimensional open systems and the diffusive and sub-diffusive regimes observed in the single-file dynamics along a straight line. This work studies the way in which a confining geometry induces new time scales. We report on the dynamics of interacting colloidal particles moving along a circle by combining a heuristic theoretical analysis of the involved scales, Brownian Dynamics computer simulations, and video-microscopy experiments with paramagnetic colloids confined to lithographic circular channels subjected to an external magnetic field. The systems display four temporal regimes in this order: one-dimensional free diffusion, single-file sub-diffusion, free-cluster rotational diffusion, and the expected saturation due to the confinement. We also report analytical expressions for the mean-square angular displacement and crossover times obtained from scaling arguments, which accurately reproduce both experiments and simulations. Our generic approach can be used to predict the long-time dynamics of many other confined physical systems.