Relaxation Dynamics Research Articles

Understanding crystallization and amorphization in liquid Ti cooled at different rates: A molecular dynamics simulation study.

Crystallization and amorphization are important processes and different cooling rates cause these transitions. Obtaining pure metals from their molten state is a challenge unless these two are well understood. Here we study both these transitions in liquid Ti using molecular dynamics simulations wherein Ti is modeled with embedded atom potential. At 1bar, Ti crystal is melted and then cooled from 2200 to 300K with cooling rates of 0.1, 1, and 10 K/ps. When cooled at 0.1 and 1 K/ps, molten Ti crystallizes to bcc phase between 1100 and 1000K, and when cooled at 10 K/ps, it amorphizes between these temperatures. From radial distribution functions and Voronoi tessellation, we observe that liquid to bcc transition takes place through short range distorted hcp/bcc-like structures already present in it. Relaxation dynamics is studied using velocity-autocorrelation functions (VACFs), intermediate scattering function, and dynamic structure factor. For all cooling rates, relaxations in VACFs increase with cooling. However, correlations in them are stronger when the system is cooled at 10 K/ps. Relaxation times decrease and increase between 1100 and 1000K for crystallization and amorphization, respectively, thereafter they increase again with further cooling. The dynamic structure factor shows stronger damping in thermal diffusive motion when systems are cooled at 0.1 and 1 K/ps and vibration peaks shift to higher frequencies when crystallization take place. Our findings support Binder's [K. Binder, Proc. Natl. Acad. Sci. U. S. A. 111, 9374 (2014)] argument that if we cool the system faster than the minimum time needed for the liquid to relax, it will amorphize. This also prevents the growth of pre-ordered domains in it to establish long-range order.

Connecting Dynamics and Thermodynamics in Polymer–Resin Cured Systems

This work connects the calorimetric responses of different rubber–resin blends with varying resin contents with their alpha relaxation dynamics. We used differential scanning calorimetry and broadband dielectric spectroscopy to characterize the calorimetric and dielectric responses of styrene–butadiene, polybutadiene, and polyisoprene with different resin contents. To model the results, we used the Gordon–Taylor equation combined with an extension of the Adam–Gibbs approach. Thus, we propose a simple and effective model that allows us to estimate the blend dynamics from the temperature dependence of the relaxation times of the pure components and the calorimetric measurement of the glass transition temperature of only one blend composition. By estimating an effective interaction parameter from calorimetry, we achieved accurate alpha relaxation dynamics predictions for different resin concentrations. Our highly predictive approach provides a realistic description of the expected dynamics. This study offers valuable insights into the dynamic properties of polymer compounds, paving the way for the fast and effective development of advanced and more sustainable materials.

Defect-Mediated Exciton Localization and Relaxation in Monolayer MoS2.

Defects in chemical vapor deposition (CVD)-grown monolayer MoS2 are unavoidable and provide a powerful approach to creating single-photon emitters and quantum information systems through localizing excitons. However, insight into the A- trion and B/C exciton localization in monolayer MoS2 remains elusive. Here, we investigate defect-mediated A- trion and B/C exciton localization and relaxation in CVD-grown monolayer MoS2 samples via transient absorption spectroscopy. The localization rate of A- trions is five times faster than B excitons, which is attributed to the distinctions in the Bohr radius, diffusion rate, and multiphonon emission. Furthermore, we obtain unambiguous experimental evidence for the direct excitation of localized C excitons. Varying gap energy at the band-nesting region revealed by first-principles calculations explains the anomalous dependence of localized C exciton energy on delay time. We also find that the rapid dissociation of localized C excitons features a short characteristic time of ∼0.14 ps, while the measured relaxation time is much longer. Our results provide a comprehensive picture of the defect-mediated excitonic relaxation and localization dynamics in monolayer MoS2.

Unraveling the relaxation dynamics of donor-π-acceptor porphyrins, achieving a promising 10.51% device efficiency in dye-sensitized solar cells

Unraveling the relaxation dynamics of donor-π-acceptor porphyrins, achieving a promising 10.51% device efficiency in dye-sensitized solar cells

Revealing Ultrafast Optical Nonlinearity of Trapped Exciton Polaritons in Atomically Thin Semiconductors.

Nonlinearities are fundamental to modern optical technologies. Exciton polaritons in semiconductor microcavities provide a promising route to strong nonlinearities. Monolayer TMDs, with tightly bound excitons and strong oscillator strength, enable polaritonic phenomena under ambient conditions but face challenges from weak polariton interactions due to small exciton Bohr radius. Although spatial confinement can boost polariton nonlinearity, the dynamics of trapped polaritons remain underexplored. Here we study the transient nonlinearities of confined polaritons in monolayer WS2 mesa cavities. We observe increasingly pronounced blueshifts within the first few picoseconds as trapping sizes decrease or excitonic fractions increase. Furthermore, our findings reveal that exciton-photon detuning, not trapping size, predominantly influences the time to reach the peak of transient nonlinearity. This insight aligns with the experimentally observed and theoretically simulated relaxation dynamics of trapped polaritons. Our findings pave the way for developing ultrafast all-optical polaritonic devices in TMD systems.

Understanding the superconductivity and charge density wave interaction through quasi-static lattice fluctuations

In unconventional superconductors, coupled charge and lattice degrees of freedom can manifest in ordered phases of matter that are intertwined. In the cuprate family, fluctuating short-range charge correlations can coalesce into a longer-range charge density wave (CDW) order which is thought to intertwine with superconductivity, yet the nature of the interaction is still poorly understood. Here, by measuring subtle lattice fluctuations in underdoped YBa2Cu3O6+y on quasi-static timescales (thousands of seconds) through X-ray photon correlation spectroscopy, we report sensitivity to both superconductivity and CDW. The atomic lattice shows remarkably faster relaxational dynamics upon approaching the superconducting transition at Tc ≈ 65 K. By tracking the momentum dependence, we show that the intermediate scattering function almost monotonically scales with the relaxation distance of atoms away from their average positions above Tc and in the presence of the CDW state, while this peculiar trend is reversed for other temperatures. These observations are consistent with an incipient CDW stabilized by local strain. This work provides insights into the crucial role of relaxational atomic fluctuations for understanding the electronic physics cuprates, which are inherently disordered due to carrier doping.

Chlorine Substitution Effect on the S1 Relaxation Dynamics of Chlorobenzene and Chlorophenols.

The S1 state relaxation dynamics of chlorobenzene (CB), 3-chlorophenol (3-CP), 3-CP·H2O, and 2-chlorophenol·H2O (2-CP·H2O) have been investigated by means of picosecond time-resolved pump-probe spectroscopy in a state-specific manner. For CB, the S1 state relaxes via the S1-S0 internal conversion in the low internal energy region (<2000 cm-1), whereas the direct C-Cl bond dissociation channel mediated by the upper-lying repulsive πσCCl* state is opened to give the rather sharp increase of the S1 relaxation rate in the high internal energy region (>2000 cm-1). A similar dynamic feature has been observed for 3-CP in terms of the lifetime behavior with an increase in the S1 internal energy, suggesting that the H atom tunneling dissociation reaction from OH might contribute less compared to the internal conversion, although it is not clear at the present time whether or not the sharp increase of the S1 relaxation rate in the high internal energy region of 3-CP (>1500 cm-1) is entirely due to that of the internal conversion. The fact that the internal conversion is facilitated by the Cl substitution implies that the energetic location of the S1/S0 conical intersection should have been strongly influenced by chlorine substitution on the aromatic ring. The approximate energetic location of the saddle point of the S1(ππ*)/πσCCl* conical intersection along the seam coordinate for CB or 3-CP could be inferred from the energy-dependent S1 lifetime measurements. It is discussed in comparison with the dynamic role of the S1(ππ*)/πσCCl* conical intersection, which is strongly influenced by the O-H···Cl intramolecular hydrogen bond in the rather complicated yet ultrafast S1 relaxation dynamics of the cis-2-CP. The S1 lifetimes of 3-CP·H2O and 2-CP·H2O reveal the importance of the conformational structures, especially in terms of the intramolecular hydrogen bonding.

A {Cr4Ln2} Complex with Exchange Coupled {Cr2} Units: Structural Description and Magnetic Study.

We have prepared and structurally characterized pivalate based {CrIII4LnIII2} complexes with Ln=Dy and Gd as well as the Y analogue, with the overall formula [CrIII4LnIII2(mdea)2(piv)10(OH)4], Ln= Gd, Dy and Y. We are reporting a detailed experimental magnetic properties study, including magnetization relaxation dynamics and calorimetric data, supported with quantum chemical calculations. The synthesis of the Y derivative, allowed to precisely identify the Cr(III)-Cr(III) exchange interaction magnitude which proved moderately strong and in agreement with known magneto-structural correlations. This result agrees with the observed double bridged {Cr2-mOH-mOR} units within the {Cr4Ln2} complexes.The Gd(III) complex magnetic properties can be properly described with the already established exchange coupled {Cr2} units, and a unique Gd(III)-Cr(III) exchange coupling parameter which proved anti-ferromagnetic in nature.The MCE characterization of this complexbased on magnetization and calorimetric data down to 2 K affords a moderate entropy change value, mainly affected by the strong Cr-Cr exchange interaction.The complex with Ln=Dy, showed SMM behaviour below 10 K under 0 DC applied field with negligible field dependence up to 3000 Oe and 10 kHz of AC field frequency. Two relaxation processes are clearly distinguished, with thermal barriers for an Orbach mechanism of ca. 20 cm-1 and 40 cm-1.

Nemaline Myopathy Type 6 Caused by Variants in the KBTBD13 Gene: A Cross-Sectional Study of 24 Patients.

Nemaline myopathy type 6 (NEM6) is the most prevalent type of nemaline myopathy in the Netherlands. Because a detailed clinical characterization is not available yet, we here provide a detailed assessment of 24 patients. In this cross-sectional study, we performed a full clinical assessment (medical history and neurologic examination) in patients with NEM6. Patient demographics, causative variants in the KBTBD13 gene, creatine kinase levels, and the results of previous muscle biopsies were collected. We evaluated experienced health-related quality of life, fatigue severity, prevalence of falls, balance control (Mini-Balance Evaluation Systems Test [Mini-BESTest]), functional motor score (Motor Function Measure [MFM]), and 6-minute walk distance. We used transcranial magnetic stimulation to assess muscle relaxation kinetics. Twenty-four patients were included (19 women [19-76 years]; 5 men [25-57 years]). Key patient-reported symptoms since childhood were muscle weakness (n = 23; 96%), slowness of movements (n = 23; 96%), and difficulties with running (n = 20; 83%). Axial, proximal, and distal muscles showed mild weakness in most patients. Health-related quality of life was significantly lower, and there was a significantly increased fatigue severity compared with controls. Prospectively, in a period of 100 days, 8 patients (33%) fell at least 1 time, of whom 5 patients (21%) fell 2 times or more. The median total score on the Mini-BESTest was 24 (21.0-26.0 [interquartile range]) of 28 and the median total percentage on the MFM was 91% (83.5-95.3), both considered to be mildly abnormal. The 6-minute walk distance was below the lower limit of normal in 4 patients (17%). All patients with NEM6 showed a markedly reduced muscle relaxation rate with a median of 6.5 [4.9-8.1] s-1 (lower limit of normal is 10.1 s-1). This cross-sectional study in patients with NEM6 shows a relatively mild clinical phenotype and mildly abnormal functional tests. However, patients report an important impact on the daily activities, which is illustrated by functional difficulties, reduced quality of life, increased fatigue severity, and increased prevalence of falls. This might be related to delayed muscle relaxation. This study provides a comprehensive overview of the clinical presentation and functional limitations in patients with NEM6.

Density-wave-like gap evolution in La3Ni2O7 under high pressure revealed by ultrafast optical spectroscopy

Density wave (DW) order is believed to be correlated with superconductivity in the recently discovered high-temperature superconductor La3Ni2O7. However, experimental investigations of its evolution under high pressure are still lacking. Here, we explore the quasiparticle dynamics in bilayer nickelate La3Ni2O7 single crystals using ultrafast optical pump-probe spectroscopy under high pressures up to 34.2 GPa. At ambient pressure, the temperature-dependent relaxation dynamics demonstrate a phonon bottleneck effect due to the opening of an energy gap around 151 K. The energy scale of the DW-like gap is determined to be 66 meV by the Rothwarf-Taylor model. Combined with recent experiential results, we propose that this DW-like transition at ambient pressure and low temperature is spin density wave (SDW). With increasing pressure, this SDW order is significantly suppressed up to 13.3 GPa before it completely disappears around 26 GPa. Remarkably, at pressures above 29.4 GPa, we observe the emergence of another DW-like order with a transition temperature of approximately 135 K, which is probably related to the predicted charge density wave (CDW) order. Our study provides the experimental evidences of the evolution of the DW-like gap under high pressure, offering critical insights into the correlation between DW order and superconductivity in La3Ni2O7.

Influence of disordered and anisotropic interactions on relaxation dynamics and propagation of correlations in tweezer arrays of Rydberg dipoles

Influence of disordered and anisotropic interactions on relaxation dynamics and propagation of correlations in tweezer arrays of Rydberg dipoles

Modulating hot carrier relaxation and trapping dynamics in lead halide perovskite nanoplatelets by surface passivation.

Two-dimensional (2D) lead halide perovskite (LHP) nanoplatelets (NPLs) have recently emerged as promising materials for solar cells and light-emitting devices. The reduction of LHP dimensions introduces an abundance of surface defects, which can strongly influence the photophysical properties of these materials. However, an insightful understanding of the effect of surface defects on hot carrier (HC) relaxation, one of the important properties of LHP NPLs, is still inadequate. Herein, the HC relaxation and trapping dynamics in pristine and surface passivated two-layer (2L) CsPbBr3 NPLs have been investigated by using time-resolved spectroscopy. The results reveal that surface defects can trap HCs directly before they relax to the band edge, which accounts for the absence of the hot-phonon bottleneck (HPB) effect in LHP NPLs. After healing surface defects with a passivation agent, the relaxation time of HCs is extended from ∼73 to ∼130 fs in 2L CsPbBr3 NPLs, indicating that the channel of HCs trapped by the surface defects can be effectively blocked. Accordingly, the HPB effect is activated in surface-passivated CsPbBr3 NPLs. The finding of surface defect-related HC relaxation dynamics is important for guiding the development of high-performance LHP NPL devices related to HCs through surface defect engineering.

Efficiency of the dynamic relaxation method in the stabilisation process of bridge and building frame

More and more complex Civil Engineering problems are being considered in computational mechanics with the invention of high-quality computing techniques. In addition, the computational cost and storage requirement for complex and or large structures have increased dramatically, leading to an increased interest in removing the difficulties using any form of parallel computing. The process of applying the preload for parallel computing to any unstable structures is called a stabilising process, such as the Dynamic Relaxation Method (DRM) is one. This method minimises the energy by a simple vector iteration technique, which ultimately leads the structure to a static equilibrium state. The present study aims to highlight the utility of the DRM in the stabilisation process for small structures like building frames and large and or complicated structures such as bridges before actual transient analysis. Therefore, the present manuscript discusses the computational cost, CPU runtime, multiple increases of mass and rigid body displacement of building frames and bridges. The DRM allows an explicit solver to conduct a dynamic analysis by increasing the damping until the kinetic energy drops to a proposed value. The simulation of the DRM starts to find the equilibrium state with minimal dynamic effect, which is required to apply at the beginning of the solution phase to obtain the initial stress and displacement field before the start of the actual analysis.

Bosonic Mpemba effect with non-classical states of light

The Mpemba effect (ME) refers to the surprising observation where, under certain conditions, a far-from-equilibrium state can relax toward equilibrium faster than a state closer to equilibrium. A paradigmatic example is provided by the curious fact that hot water can sometimes freeze faster than cold water. The ME has intrigued scientists for a long time and has been predicted and observed in a variety of classical and quantum systems. Recently, the search for Mpemba-like effects of purely quantum nature has raised a major interest. Here, we predict the emergence of ME in the quantum optics context exploiting non-classical states of light. By analyzing the decay dynamics of photon fields in a leaky optical resonator or waveguide, it is demonstrated that bosonic ME emerges in the context of the quantum nature of light. In particular, the relaxation dynamics are strongly influenced by the photon statistics of the initially trapped light field. The ME is observed when comparing the decay dynamics of classical light fields (coherent states) with certain non-classical states, such as Fock states, squeezed states, and Schrödinger cat states.

Noise-induced peak intensity fluctuations in class B laser systems.

Random perturbations and noise can excite instabilities in population systems that result in large fluctuations. Important examples involve class B lasers, where the dynamics are determined by the number of carriers and photons in a cavity with noise appearing in the electric-field dynamics. When such lasers are brought above threshold, the field intensity grows away from an unstable equilibrium, exhibiting transient relaxation oscillations with fluctuations due to noise. In this work, we focus on the first peak in the intensity during this transient phase in the presence of noise, and calculate its probability distribution using a Wentzel-Kramers-Brillouin approximation. In particular, we show how each value of the first peak is determined by a unique fluctuational momentum, calculate the peak intensity distribution in the limit where the ratio of photon-to-carrier lifetimes is small, and analyze the behavior of small fluctuations with respect to deterministic theory. Our approach is easily extended to the analysis of transient, noise-induced large fluctuations in general population systems exhibiting relaxation dynamics.

Enlightening the blind spot of the Michaelis–Menten rate law: The role of relaxation dynamics in molecular complex formation

The century-long Michaelis–Menten rate law and its modifications in the modeling of biochemical rate processes stand on the assumption that the concentration of the complex of interacting molecules, at each moment, rapidly approaches an equilibrium (quasi-steady state) compared to the pace of molecular concentration changes. Yet, in the case of actively time-varying molecular concentrations with transient or oscillatory dynamics, the deviation of the complex profile from the quasi-steady state becomes relevant. A recent theoretical approach, known as the effective time-delay scheme (ETS), suggests that the delay from the relaxation time of molecular complex formation contributes to the substantial breakdown of the quasi-steady state assumption. Here, we systematically expand this ETS and inquire into the comprehensive roles of relaxation dynamics in complex formation. Through the modeling of rhythmic protein–protein and protein–DNA interactions and the mammalian circadian clock, our analysis reveals the effect of the relaxation dynamics beyond the time delay, which extends to the dampening of changes in the complex concentration with a reduction in the oscillation amplitude compared to the quasi-steady state. Interestingly, the combined effect of the time delay and amplitude reduction shapes both qualitative and quantitative oscillatory patterns such as the emergence and variability of the mammalian circadian rhythms. These findings highlight the downside of the routine assumption of quasi-steady states and enhance the mechanistic understanding of rich time-varying biomolecular processes.

Dinuclear Dysprosium Compounds: The Importance of Rigid Bridges.

We report the synthesis, structures and magnetic behaviour of two isostructural dinuclear Dy3+ complexes where the metal ions of a previously reported monomeric building block are connected by a peroxide (O2 2-) or a pair of fluoride (2×F-) bridges. The nature of the bridge determines the distance between the metal ion dipoles leading to a dipolar coupling in the peroxido bridged compound of only ca. 70 % of that in the bis-fluorido bridged dimer. The sign of the overall coupling between the metals is antiferromagnetic for the peroxido bridged compound and ferromagnetic for the bis-fluorido bridged complex. This in turn influences the magnetisation dynamics. We compare the relaxation characteristics of the dimers with those of the previously reported monomeric building block. The relaxation dynamics for the bis-fluorido system are very fast. On the other hand, comparing the properties of the monomer, the peroxido bridged sample and the corresponding Y-doped sample show that the relaxation properties via a Raman process have very similar parameters. We show that a second dysprosium is important for either tuning or detuning the Single Molecule Magnet (SMM) properties of a system.

The spectrum of experimentally accessible states for melting and freezing transitions in mesoporous materials.

Structural disorder in mesoporous solids gives rise to complex phase behavior for materials confined within their pore spaces. As a result, a wide spectrum of possible phase configurations associated with spatial distributions of thermodynamic phases throughout the pore networks can be realized in experiments. Despite their importance, quantifying these states remains largely unaddressed. By considering solid-liquid equilibria as a representative example and using a simple random network model, we investigate the spectrum of such states accessible in real experiments and relate this spectrum to the structural characteristics of porous solids. We classify these states by their free energies and demonstrate how network effects break degeneracies for specific phase compositions and temperatures. Furthermore, we identify the experimental conditions that delineate boundary free energy states, differentiating accessible from inaccessible states. The insights from this study on solid-liquid equilibria are also equally applicable to gas-liquid equilibria in confined spaces and contribute to a deeper understanding of relaxation dynamics associated with hysteresis.

Investigating the numerical stability of dynamic relaxation methods with automatic load-increment schemes to improve snap-back prediction

Investigating the numerical stability of dynamic relaxation methods with automatic load-increment schemes to improve snap-back prediction

Observation of Hilbert space fragmentation and fractonic excitations in 2D

The relaxation behaviour of isolated quantum systems taken out of equilibrium is among the most intriguing questions in many-body physics1. Quantum systems out of equilibrium typically relax to thermal equilibrium states by scrambling local information and building up entanglement entropy. However, kinetic constraints in the Hamiltonian can lead to a breakdown of this fundamental paradigm owing to a fragmentation of the underlying Hilbert space into dynamically decoupled subsectors in which thermalization can be strongly suppressed2, 3, 4–5. Here we experimentally observe Hilbert space fragmentation in a two-dimensional tilted Bose–Hubbard model. Using quantum gas microscopy, we engineer a wide variety of initial states and find a rich set of manifestations of Hilbert space fragmentation involving bulk states, interfaces and defects, that is, two-, one- and zero-dimensional objects. Specifically, uniform initial states with equal particle number and energy differ strikingly in their relaxation dynamics. Inserting controlled defects on top of a global, non-thermalizing chequerboard state, we observe highly anisotropic, subdimensional dynamics, an immediate signature of their fractonic nature6, 7, 8–9. An interface between localized and thermalizing states in turn shows dynamics depending on its orientation. Our results mark the observation of Hilbert space fragmentation beyond one dimension, as well as the concomitant direct observation of fractons, and pave the way for in-depth studies of microscopic transport phenomena in constrained systems.