Short-time Decay Research Articles

Aspects of the decoherence in high-spin environments: Breakdown of the mean-field approximation

The study of the decoherence of qubits in spin systems is almost restricted to environments whose constituents are spin-$\frac{1}{2}$ particles. In this paper we consider environments that are composed of particles of higher spin, and we investigate the consequences on the dynamics of a qubit coupled to such baths via Heisenberg $XY$ and Ising interactions. It is shown that while the short time decay in both cases gets faster as the magnitude of the spin increases, the asymptotic behavior exhibits an improvement of the suppression of the decoherence when the coupling is through Heisenberg $XY$ interactions. In the case of a transverse Ising model, we find that the mean field approximation breaks down for high values of the spin.

Activated desorption at heterogeneous interfaces and long-time kinetics of hydrocarbon recovery from nanoporous media.

Hydrocarbon recovery from unconventional reservoirs (shale gas) is debated due to its environmental impact and uncertainties on its predictability. But a lack of scientific knowledge impedes the proposal of reliable alternatives. The requirement of hydrofracking, fast recovery decay and ultra-low permeability—inherent to their nanoporosity—are specificities of these reservoirs, which challenge existing frameworks. Here we use molecular simulation and statistical models to show that recovery is hampered by interfacial effects at the wet kerogen surface. Recovery is shown to be thermally activated with an energy barrier modelled from the interface wetting properties. We build a statistical model of the recovery kinetics with a two-regime decline that is consistent with published data: a short time decay, consistent with Darcy description, followed by a fast algebraic decay resulting from increasingly unreachable energy barriers. Replacing water by CO2 or propane eliminates the barriers, therefore raising hopes for clean/efficient recovery.

Loschmidt echo in many-spin systems: contrasting time scales of local and global measurements.

A local excitation in a quantum many-spin system evolves deterministically. A time-reversal procedure, involving the inversion of the signs of every energy and interaction, should produce the excitation revival. This idea, experimentally coined in nuclear magnetic resonance, embodies the concept of the Loschmidt echo (LE). While such an implementation involves a single spin autocorrelation M(1,1), i.e. a local LE, theoretical efforts have focused on the study of the recovery probability of a complete many-body state, referred to here as global or many-body LE MMB Here, we analyse the relation between these magnitudes, with regard to their characteristic time scales and their dependence on the number of spins N We show that the global LE can be understood, to some extent, as the simultaneous occurrence of N independent local LEs, i.e. MMB∼(M(1,1))(N/4) This extensive hypothesis is exact for very short times and confirmed numerically beyond such a regime. Furthermore, we discuss a general picture of the decay of M1,1 as a consequence of the interplay between the time scale that characterizes the reversible interactions (T(2)) and that of the perturbation (τ(Σ)). Our analysis suggests that the short-time decay, characterized by the time scale τ(Σ), is greatly enhanced by the complex processes that occur beyond T(2) This would ultimately lead to the experimentally observed T(3), which was found to be roughly independent of τ(Σ) but closely tied to T(2).

Pr3+:BaY2F8 Crystal Nanoparticles (24 nm) Produced by High-Energy Ball Milling: Spectroscopic Characterization and Comparison with Bulk Properties

Nanocrystals (NC) of Pr3+:BaY2F8 with average diameter of 24 nm have been successfully prepared by high-energy ball milling. The method is versatile, easily scalable, and does not require the use of surfactants or catalysts. NC were prepared starting from high quality single crystal pieces, and their spectroscopic features are analyzed and compared with those of a single bulk crystal. Under 445 nm excitation, we recorded the 10 K and room temperature emission spectra of the two samples. The spectra show the same peak positions and width, and this means that the milling process does not introduce substantial modifications to the crystal structure. Besides, there are strong differences in the relative intensity of the lines emitted toward different lower lying levels in the two samples. In particular, the high-energy transitions seem to be hyper intense in the NC with respect to the bulk sample. On the contrary, the emission lines that end at excited levels are less intense in the NC. In addition, the time evolution of the 3P0 decay shows striking differences between the nanosized materials and the bulk sample. Despite the exponential decay of the latter luminescence (τ = 43 μs), NC possess a strong nonlinear component with a lifetime much shorter than in the bulk. Calculations show that nearly 89% of the excited ions contribute to the short-time decay, which is attributed to ions residing near the NC surface.

Two Roads Converged in a Yellow Dye: Unusual Spectral Broadening in the Emission of Auramine-O Possibly Caused by Low-Friction Intramolecular Rotation

Steady-state and time-resolved optical techniques were employed to study the rather complex relaxation of excited states of Auramine-O (AuO) in several liquids at room temperature. We found three relaxation times in the decay of the pump–probe signals of the excited states of AuO. We focused our study on the short time decay, with a duration of within about 150–300 fs. We found that the temporal changes of the emission band of AuO could be divided into three behaviors, depending on the solvent characteristics. In dimethyl sulfoxide (DMSO), a hydrogen-bond-accepting solvent, AuO, shows, at short times, a relatively broad emission band with small changes in its peak position and width as a function of time. In acetonitrile and in acetic acid, both hydrogen-bond-donating solvents, we found large changes in the band peak and width as a function of time. Dichloromethane is a solvent lacking strong solvent interactions, it is apolar and is neither a strong hydrogen-bond-donator nor a strong hydrogen-bond-acceptor. For AuO in dichloromethane we found an oscillation with a time constant of 200 fs in the time-resolved emission signal. We attribute the rather large changes of the emission band with time in the short time window to the twist of the dimethylamino groups of the aniline groups of AuO.

Simulating Zeno physics by a quantum quench with superconducting circuits

Studying out-of-equilibrium physics in quantum systems under quantum quench is of vast experimental and theoretical interests. Using periodic quantum quenches, we present an experimentally accessible scheme to simulate the quantum Zeno and anti-Zeno effects in an open quantum system of a single superconducting qubit interacting with an array of transmission line resonators. The scheme is based on the following two observations: Firstly, compared with conventional systems, the short-time non-exponential decay in our superconducting circuit system is readily observed; and secondly, a quench-off process mimics an ideal projective measurement when its time duration is sufficiently long. Our results show the active role of quantum quench in quantum simulation and control.

Time-dependent yield of [rad]OH radicals in the low linear energy transfer radiolysis of water between 25 and 350 °C

Monte Carlo simulations were used to calculate time-dependent yields of OH radicals in the low-LET radiolysis of water from 25 to 350°C. The excellent agreement found, at 25°C, with bothOH yields directly measured in picosecond pulse radiolysis and those inferred from scavenger experiments, resolves a long-standing problem in models of the radiation chemistry of water concerning short-time OH decay kinetics. Above ∼200°C, the OH yields markedly increased at long times due to the reaction H+H2O→H2+OH. Our results suggest a way to assess this reaction’s rate constant, which is still controversial.

Short-time behavior of advecting-diffusing scalar fields in Stokes flows

This article addresses the short-term decay of advecting-diffusing scalar fields in Stokes flows. The analysis is developed in two main subparts. In the first part, we present an analytic approach for a class of simple flow systems expressed mathematically by the one-dimensional advection-diffusion equation w(y)∂(ξ)φ=ε∂(y)(2)φ+iV(y)φ-ε'φ, where ξ is either time or axial coordinate and iV(y) an imaginary potential. This class of systems encompasses both open- and closed-flow models and corresponds to the dynamics of a single Fourier mode in parallel flows. We derive an analytic expression for the short-time (short-length) decay of φ, and show that this decay is characterized by a universal behavior that depends solely on the singularity of the ratio of the transverse-to-axial velocity components V(eff)(y)=V(y)/w(y), corresponding to the effective potential in the imaginary potential formulation. If V(eff)(y) is smooth, then ||φ||(L(2))(ξ)=exp(-ε'ξ-bξ(3)), where b>0 is a constant. Conversely, if the effective potential is singular, then ||φ||(L(2))(ξ)=1-aξ(ν) with a>0. The exponent ν attains the value 5/3 at the very early stages of the process, while for intermediate stages its value is 3/5. By summing over all of the Fourier modes, a stretched exponential decay is obtained in the presence of nonimpulsive initial conditions, while impulsive conditions give rise to an early-stage power-law behavior. In the second part, we consider generic, chaotic, and nonchaotic autonomous Stokes flows, providing a kinematic interpretation of the results found in the first part. The kinematic approach grounded on the warped-time transformation complements the analytical theory developed in the first part.

Investigation of chlorine decay of water resource in khanbebein city, Golestan, Iran

Aims: The object of the study was to identify chlorine demand factorsin water and chlorine in Kanbebein City water and eliminating of them. Materials and Methods: Concentration of free and combined residual chlorine with the methods presented in the book Standard Methods for Water and Wastewater Examination were measured. Other parameters such as: Temperature, TOC, TDS, pH, EC, nitrate, ammonia, total hardness, alkalinity, iron and manganese were also examined before and after of chlorination. Results: The Highest chlorine decay constant in the short time were in water well was 0.895 mgL -1 h -1 and the least amount of it was 0.0097 mgL -1 h -1 and the Highest chlorine decay constant in the long time were in water well was-0.092Lmg -1 h -1 and the least amount of it was-0.0017 Lmg -1 h -1 that correlated with nitrite, ammonia and iron concentration. Conclusion: The factors that affected short-time chlorine decay constant (k 2 ) were nitrite, ammonia and iron and The factors that affected on long-time chlorine decay constant (k 2 ) were Nitrite, ammonia and iron. The material removal techniques for them were the use of natural resins, zeolite, ion exchange, membrane and aeration methods, oxidation and sedimentation, and filtration.

Time- and Polarization-Resolved Photoluminescence Decay from Isolated Polythiophene (P3HT) Nanofibers

Isolated nanofibers present unique opportunities to investigate excitonic processes and dynamics in a confined crystalline geometry, where structural order in the transverse (intrachain) and longitudinal (interchain) directions can be tuned by polymer molecular weight, regioregularity, and solvent processing conditions. We report on time- and polarization-resolved photoluminescence (TRPL) studies from isolated crystalline P3HT nanofibers (also known as nanowires), which reveal a highly reproducible short-time decay behavior, appearing as an amplified spontaneous emission process (biexciton annihilation) as signaled by a quadratic excitation power dependence in amplitude and decay rate. In the long-time (5–100 ns) regime, we observed a power-law decay in the photoluminescence similar to that seen in thin films and nanoparticles; however, for certain nanofiber families (prepared from p-xylene) we observe an extremely long-lived PL component which we postulate arises from deeply trapped carriers in chain packing faults within the NF. Finally, we probe depolarization dynamics in individual nanofibers using polarization-resolved TRPL measurements in which both arrival time and (parallel/perpendicular) polarization state relative to the nanofiber axis are resolved, delineating the different dynamics associated with intra- and interchain excitons.

Short-Time Spin Dynamics in Strongly Correlated Few-Fermion Systems

The nonequilibrium spin dynamics of a one-dimensional system of repulsively interacting fermions is studied by means of density-matrix renormalization group simulations. We focus on the short-time decay of the oscillation amplitudes of the centers of mass of spin-up and spin-down fermions. Because of many body effects, the decay is found to evolve from quadratic to linear in time, and eventually back to quadratic as the strength of the interaction increases. The characteristic rate of the decay increases linearly with the strength of repulsion in the weak-coupling regime, while it is inversely proportional to it in the strong-coupling regime. Our predictions can be tested in experiments on tunable ultracold few-fermion systems in one-dimensional traps.

Effect of anisotropic strain on the charge dynamics of Nd0.5Sr0.5MnO3 thin films

We report the transmittance (T) and the pump–probe transmittance change (ΔT) of Nd0.5Sr0.5MnO3 (NSMO) thin films grown on SrTiO3(STO)(100) and STO(110) substrates in 1.3–1.7eV range at room temperature. The strain of NSMO was isotropic on STO(100) and anisotropic on STO(110). Both T and ΔT showed no polarization dependence in NSMO/STO(100) but the intensity of T and ΔT was anisotropic in NSMO/STO(110). Also ΔT showed sharp exponential decay with increasing time delay between the pump and probe pulse. The short time decay behaviour (≲0.3ps) showed no significant polarization direction and substrate dependence, which indicates that the carrier–carrier scattering rate is not sensitive to the strain state in NSMO.

Nonmonotonic short-time decay of the Loschmidt echo in quasi-one-dimensional systems

We study the short-time stability of quantum dynamics in quasi-one-dimensional systems with respect to small localized perturbations of the potential. To this end, we analytically and numerically address the decay of the Loschmidt echo (LE) during times that are short compared to the Ehrenfest time. We find that the LE is generally a nonmonotonic function of time and exhibits strongly pronounced minima and maxima at the instants when the corresponding classical particle traverses the perturbation region. We also show that, under general conditions, the envelope decay of the LE is well approximated by a Gaussian, and we derive explicit analytical formulas for the corresponding decay time. Finally, we demonstrate that the observed nonmonotonic nature of the LE decay is only pertinent to one-dimensional (and, more generally, quasi-one-dimensional) systems, and that the short-time decay of the LE can be monotonic in a higher number of dimensions.

Boxed Molecular Dynamics: Decorrelation Time Scales and the Kinetic Master Equation.

A number of methods proposed in the past few years have been aimed at accelerating the sampling of rare events in molecular dynamics simulations. We recently introduced a method called Boxed Molecular Dynamics (BXD) for accelerating the calculation of thermodynamics and kinetics ( J. Phys. Chem. B 2009 , 113 , 16603 - 16611 ). BXD relies upon confining the system in a series of adjacent "boxes" by inverting the projection of the system velocities along the reaction coordinate. The potential of mean force along the reaction coordinate is obtained from the mean first passage times (MFPTs) for exchange between neighboring boxes, simultaneously providing both kinetics and thermodynamics. In this paper, we investigate BXD in the context of its natural relation to a kinetic master equation and show that the BXD first passage times (FPTs) include different time scales-a fast short time decay due to correlated dynamical motion and slower long time decay arising from phase space diffusion. Correcting the FPTs to remove the fast correlated motion yields accurate thermodynamics and master equation kinetics. We also discuss interrelations between BXD and a recently described Markovian milestoning technique and use a simple application to show that, despite each method producing distinct nonstatistical effects on time scales on the order of dynamical decorrelation, both yield similar long-time kinetics.

Excited-State Intermolecular Proton Transfer of Firefly Luciferin III. Proton Transfer to a Mild Base

Steady-state and time-resolved techniques were employed to study the excited-state proton transfer (ESPT) from d-luciferin, the natural substrate of the firefly luciferase, to the mild acetate base in aqueous solutions. We found that in 1 M aqueous solutions of acetate or higher, a proton transfer (PT) process to the acetate takes place within 30 ps in both H(2)O and D(2)O solutions. The time-resolved emission signal is composed of three components. We found that the short-time component decay time is 300 and 600 fs in H(2)O and D(2)O, respectively. This component is attributed either to a PT process via the shortest water bridged complex available, ROH··H(2)O··Ac(-), or to PT taking place within a contact ion pair. The second time component of 2000 and 3000 fs for H(2)O and D(2)O, respectively, is attributed to ROH* acetate complex, whose proton wire is longer by one water molecule. The decay rate of the third, long-time component is proportional to the acetate concentration. We attribute it to the diffusion-assisted reaction as well as to PT process to the solvent.

Quantum Drude friction for time-dependent density functional theory

Friction is a desired property in quantum dynamics as it allows for localization, prevents backscattering, and is essential in the description of multistage transfer. Practical approaches for friction generally involve memory functionals or interactions with system baths. Here, we start by requiring that a friction term will always reduce the energy of the system; we show that this is automatically true once the Hamiltonian is augmented by a term of the form integral a(q;n(0)) x [partial differential j(q,t)/partial differential t] x J(q)dq, which includes the current operator times the derivative of its expectation value with respect to time, times a local coefficient; the local coefficient will be fitted to experiment, to more sophisticated theories of electron-electron interaction and interaction with nuclear vibrations and the nuclear background, or alternately, will be artificially constructed to prevent backscattering of energy. We relate this term to previous results and to optimal control studies, and generalize it to further operators, i.e., any operator of the form integral a(q;n(0))[partial differential c(q,t)/partial differential t] x C(q)dq (or a discrete sum) will yield friction. Simulations of a small jellium cluster, both in the linear and highly nonlinear excitation regime, demonstrate that the friction always reduces energy. The energy damping is essentially double exponential; the long-time decay is almost an order of magnitude slower than the rapid short-time decay. The friction term stabilizes the propagation (split-operator propagator here), therefore increasing the time-step needed for convergence, i.e., reducing the overall computational cost. The local friction also allows the simulation of a metal cluster in a uniform jellium as the energy loss in the excitation due to the underlying corrugation is accounted for by the friction. We also relate the friction to models of coupling to damped harmonic oscillators, which can be used for a more sophisticated description of the coupling, and to memory functionals. Our results open the way to very simple finite grid description of scattering and multistage conductance using time-dependent density functional theory away from the linear regime, just as absorbing potentials and self-energies are useful for noninteracting systems and leads.

Radiationless Transitions of G4 Wires and dGMP

Steady-state and time-resolved emission techniques were employed to study the nonradiative process of deoxygunaosine monophosphate (dGMP) and novel uniform continuous G4 wires containing hundreds of stacked tetrads. We found that the time-resolved emissions of both dGMP and G4 wires decay nonexponentialy. At room temperature, the short-time decay of the G4 wires is about 10 ps. At low temperatures in ice, the fluorescence quantum yields of both dGMP and G4 wires increase as the temperature decreases. For the G4 wires, the fluorescence quantum yield increases from about 10−3 at room temperature to about 0.03 at liquid-nitrogen temperatures. The asymptotic long-time decay of the lifetime-corrected emission of the G4 wires obeys a power law. At all temperatures, the average fluorescence decay time of G4 wires is longer than that of dGMP. We successfully used an inhomogeneous nonradiative model to fit the experimental results.

Vibrational Spectral Diffusion and Hydrogen Bond Dynamics in Heavy Water from First Principles

We present a first-principles theoretical study of vibrational spectral diffusion and hydrogen bond dynamics in heavy water without using any empirical model potentials. The calculations are based on ab initio molecular dynamics simulations for trajectory generation and a time series analysis using the wavelet method for frequency calculations. It is found that, in deuterated water, although a one-to-one relation does not exist between the instantaneous frequency of an OD bond and the distance of its associated hydrogen bond, such a relation does hold on average. The dynamics of spectral diffusion is investigated by means of frequency-time correlation and spectral hole dynamics calculations. Both of these functions are found to have a short-time decay with a time scale of approximately 100 fs corresponding to dynamics of intact hydrogen bonds and a slower long-time decay with a time constant of approximately 2 ps corresponding to lifetimes of hydrogen bonds. The connection of the slower time scale to the dynamics of local structural relaxation is also discussed. The dynamics of hydrogen bond making is shown to have a rather fast time scale of approximately 100 fs; hence, it can also contribute to the short-time dynamics of spectral diffusion. A damped oscillation is also found at around 150-200 fs, which is shown to have come from underdamped intermolecular vibrations of a hydrogen-bonded water pair. Such assignments are confirmed by independent calculations of power spectra of intermolecular motion and hydrogen bond kinetics using the population correlation function formalism. The details of the time constants of frequency correlations and spectral shifts are found to depend on the frequencies of chosen OD bonds and are analyzed in terms of the dynamics of hydrogen bonds of varying strengths and also of free non-hydrogen-bonded OD groups.

Orientational relaxation in a dispersive dynamic medium: Generalization of the Kubo-Ivanov-Anderson jump-diffusion model to include fractional environmental dynamics

The Ivanov-Anderson model (and an earlier treatment by Kubo) envisages a decay of the orientational correlation by random but large amplitude molecular jumps, as opposed to infinitesimal small jumps assumed in Brownian diffusion. Recent computer simulation studies on water and viscous liquids have shown that large amplitude motions may indeed be more of a rule than exception. Existing theoretical studies on jump diffusion mostly assume an exponential (Poissonian) waiting time distribution for jumps, thereby again leading to an exponential decay. Here we extend the existing formalism of Ivanov and Anderson to include an algebraic waiting time distribution between two jumps. As a result, the first (l=1) and second (l=2) rank orientational time correlation functions show the same long time power law, but their short time decay behavior is quite different. The predicted Cole-Cole plot of dielectric relaxation reproduces various features of non-Debye behavior observed experimentally. We also developed a theory where both unrestricted small jumps and large angular jumps coexist simultaneously. The small jumps are shown to have a large effect on the long time decay, particularly in mitigating the effects of algebraic waiting time distribution, and in giving rise to an exponential-like decay, with a time constant, surprisingly, less than the time constant that arises from small amplitude decay alone.

Ab initiocalculation of time-dependent control dynamics in polyelectronic systems involving bound and resonance states: Application to a quartet spectrum ofHe−

Using a pump-probe scheme with controllable time-delay in the range of $0--73\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, we have solved from first principles the time-dependent Schr\odinger equation (TDSE) describing a hyper-fast excitation process that involves both the discrete and the continuous spectrum of the three-electron atomic negative ion, ${\mathrm{He}}^{\ensuremath{-}}$. Two approaches were implemented, both using state-specific wave functions for two multiply excited discrete states (the initial state, ${\mathrm{He}}^{\ensuremath{-}}\phantom{\rule{0.2em}{0ex}}1s2s2p\phantom{\rule{0.2em}{0ex}}^{4}P^{o}$, and the final state, ${\mathrm{He}}^{\ensuremath{-}}\phantom{\rule{0.2em}{0ex}}2{p}^{3}\phantom{\rule{0.2em}{0ex}}^{4}S^{o}$), and one intermediate state which is a shape resonance (${\mathrm{He}}^{\ensuremath{-}}\phantom{\rule{0.2em}{0ex}}1s2{p}^{2}\phantom{\rule{0.2em}{0ex}}^{4}P$, including the underlying continuum of scattering states. The wavelengths of the two pulses connecting resonantly the states are in the infrared $(10\phantom{\rule{0.2em}{0ex}}080\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$ and in the soft x-ray region $(323\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$. The first approach is analytic and solves the TDSE to first order in perturbation theory. This is acceptable for weak pulses whose duration is of the order of a few tens of femtoseconds. The second approach solves the TDSE nonperturbatively, via the state-specific expansion approach. A series of computations have produced time-dependent probabilities for preparing the triply excited bound state ${\mathrm{He}}^{\ensuremath{-}}2{p}^{3}\phantom{\rule{0.2em}{0ex}}^{4}S^{o}$ for various combinations of the duration of the two pulses. Apart from providing the first such quantitative data, the results suggest the appearance of effects of short-time nonexponential decay of resonances when it becomes possible to monitor excitation with time-delayed short pulses.