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Experimentally measured transition path time distributions are usually analyzed theoretically in terms of a diffusion equation over a free energy barrier. It is though well understood that the free energy profile separating the folded and unfolded states of a protein is characterized as a transition through many stable micro-states which exist between the folded and unfolded states. Why is it then justified to model the transition path dynamics in terms of a diffusion equation, namely the Smoluchowski equation (SE)? In principle, van Kampen has shown that a nearest neighbor Markov chain of thermal jumps between neighboring microstates will lead in a continuum limit to the SE, such that the friction coefficient is proportional to the mean residence time in each micro-state. However, the practical question of how many microstates are needed to justify modeling the transition path dynamics in terms of an SE has not been addressed. This is a central topic of this paper where we compare numerical results for transition paths based on the diffusion equation on the one hand and the nearest neighbor Markov jump model on the other. Comparison of the transition path time distributions shows that one needs at least a few dozen microstates to obtain reasonable agreement between the two approaches. Using the Markov nearest neighbor model one also obtains good agreement with the experimentally measured transition path time distributions for a DNA hairpin and PrP protein. As found previously when using the diffusion equation, the Markov chain model used here also reproduces the experimentally measured long time tail and confirms that the transition path barrier height is ∼3

Several recent studies have interrogated the role of quantum coherence in affecting the transfer efficiency of an optical excitation to the designated "trap" state where the energy can be used for subsequent reactions, as in photosynthesis. However, these studies invoke a Markovian approximation for the time correlation function describing the environment-induced stochastic fluctuations. Here, we employ Kubo's quantum stochastic Liouville equation (QSLE) to include memory effects. We extend the existing QSLE scheme to introduce decay of a newly created excitation due to radiative and nonradiative channels and also by desired trapping toward the targeted chromophore. We show that the theoretical formalism based on the QSLE correctly reproduces the rate equation description in the Markovian limit, with the rate constants determined by an appropriate quantum limiting procedure. We find that under certain conditions, the efficiency of excitation transfer to the trap gains from the combined presence of quantum coherence and temporally correlated stochastic fluctuations. We work out different limiting situations in order to discover and quantify the optimum conditions for the energy transfer to the trapped state. We find that maximum energy transfer efficiency is achieved in the intermediate limit between coherent and incoherent transport.

In the field of magnetoelectric coupling, especially via the spin-dependent metal-ligand d-p hybridization mechanism found in multiferroic Ba2MGe2O7 (M = Mn, Co), detailed knowledge of the microscopic structural parameters is essential, also for the theoretical modeling. In this article, we report a systematic structural study of Ba2MnGe2O7 single crystal under varying temperatures between 110 and 673 K using non-destructive in situ single crystal synchrotron radiation diffraction. The maximum entropy method (MEM) was applied to the X-ray diffraction data for the determination of the deformation in the electron density and the orbital hybridization between the 3d of Mn and 2p of O in the Mn-O bond. Within this entire temperature range, the structure was described in a single crystallographic space group P (4) over bar2(1)m and no structural phase transition has been detected. Interestingly, the forbidden reflections, which arise from multiple diffractions so-called "Renninger effect," were observed at all temperatures without any symmetry lowering. The changes in the structural parameters [bond-lengths, bond-angles, anisotropic displacement parameters, and electron density distributions (EDDs) of the atoms] with temperature are revealed, helping to understand some aspects comprising orbital hybridization in multiferroic Ba2MnGe2O7.

Quantum coherence and entanglement in an extended interacting system where energy levels are nondegenerate and coupled to a dissipative environment is a common occurrence in nature, like in photosynthetic reaction systems and conjugated polymers. The temperature dependence of quantum coherence in a trimer complex (first three subunits of the Fenna-Matthews-Olson complex) is studied using a temperature-dependent quantum stochastic Liouville equation. In the non-Markovian limit, the lowering of temperature induces long-lasting quantum coherence that, in turn, leads to delocalization, whose length grows. The entanglement and coherence length determine the nature of the dynamic localization.

We report a spin-wave study on multiferroic ${\mathrm{Ba}}_{2}{\mathrm{CoGe}}_{2}{\mathrm{O}}_{7}$ under magnetic fields up to 12 T using low-energy inelastic neutron scattering. In-plane transverse $({T}_{1})$ spin-wave modes are highly dispersive along $(h00)$ and rather flat but strong in intensity along $(30l)$. In addition, two dispersive electromagnon modes have been observed around 3.5 meV. Dispersion of the out-of-plane transverse modes $({T}_{2})$ under fields reveals that the single-ion anisotropy constant decreases with increasing magnetic field, which is consistent with the linear spin-wave theory. Our results imply that the field-dependent single-ion anisotropy plays a crucial role in determining the characteristics of ${T}_{2}$ and electromagnon modes in the three-dimensional anisotropic spin-wave spectrum.

We report an inelastic neutron scattering study on the magnetic excitations of ${\mathrm{Pr}}_{2}{\mathrm{NiO}}_{4+\ensuremath{\delta}}$ ($\ensuremath{\delta}\ensuremath{\sim}0.24\ifmmode\pm\else\textpm\fi{}0.01$) at $T=10$ K. Spin stripe ordering becomes pronounced below $\ensuremath{\sim}220$ K with an incommensurability $\ensuremath{\epsilon}\ensuremath{\approx}0.346$, and a strong influence of interstitial oxygen is identified on establishing a long-range spin stripe ordering. Apart from the Goldstone modes emerging from the magnetic satellites (${\mathbf{q}}_{m}$), multiple homologous modes are observed along the spin stripe modulation separated by $\mathrm{\ensuremath{\Delta}}{\mathbf{q}}_{m}\ensuremath{\approx}0.076$ in reciprocal lattice units, which is interpreted by the internal periodicity of the long-range ordered discommensurated spin stripes.

Quantum coherence controls almost every aspect of static and dynamic response of a strongly correlated quantum system, although details of this control have not been elucidated in many cases. We find that, in the presence of a significant static off-diagonal coupling, quantum coherence can survive much longer than the bath correlation time in a noisy environment. In the presence of time correlated noise (non-Markovian limit), coherence could propagate through the excited bath states, and this plays a nontrivial role in giving rise to a noncanonical temperature dependence of population distribution in an extended conjugated polymer-like system. The quantum coherence through the excited bath states vanishes in the high temperature limit, giving rise to the equilibrium Boltzmann distribution. Second, we discuss a role of quantum coherence in exciton localization that bears resemblance to Anderson localization. Calculations have been carried out not only with a chain of conjugated polymer but also with dimer and trimer subunits of the Fenna-Matthews-Olson (FMO) complex. We derive an analytical expression of the relation between steady state coherence and excitionic population distribution. We analytically showed that steady state coherence in equilibrium bath states is governed by interchromophoric coupling (

When density is varied at a constant temperature just above the gas-liquid critical temperature, the system is found to exhibit large scale density fluctuations which are often rationalized in terms of crossing of a Widom line. We use the discrete variable representation (DVR) scheme to construct the spectroscopic maps for transition frequencies and transition dipoles, and obtain the infrared spectrum of the O H stretch in the said temperature-density region of the phase diagram. The infrared lineshape shows a crossover from Lorentzian to Gaussian as we approach the Widom line. The width of the lineshape displays a pronounced maximum.

We report a neutron-scattering study on ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{NiO}}_{4}$ ($x\ensuremath{\approx}0.5$) with magnetic incommensurability $\ensuremath{\epsilon}=0.461$ at 10 K, showing direct evidence for out-of-plane magnetic excitations caused by a non-negligible interlayer spin coupling (${J}_{\ensuremath{\perp}}$). The magnetic spectrum is fully represented by linear spin wave theory considering an unconventional body-centered-like stacking of discommensurated spin stripes. A large easy-plane-type single-ion anisotropy ($\ensuremath{\approx}1.5$ meV) has been found to be responsible for a finite-energy ($\ensuremath{\approx}13$ meV) Goldstone mode at the magnetic zone center. Our results validate the presence of a sizable out-of-plane interaction resulting in an anisotropic three-dimensional nature of spin dynamics in ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{NiO}}_{4}$ near the half-doped region.

Recent experiments [ Engel et al. Nature, 2007, 446, 782-786 ] revealed the existence of surprisingly long-lived quantum coherence in the noisy biological environment of the photosynthetic Fenna-Matthews-Olson (FMO) complex. Such coherence can clearly play an important role in facilitating efficient energy transfer. The occurrence of quantum coherence in quantum transport is also implicated in excitation transport processes in conjugated polymers [ Collini et al. Science, 2009, 323, 369-373 ]. Even though these systems are strongly correlated, most theoretical studies invoke a Markovian approximation where the temporal correlation of bath fluctuations is neglected. We use an elegant nonperturbative method based on Kubo's quantum stochastic Liouville equation (QSLE) to study the effects of correlated non-Markovian bath fluctuations in several different limits and find the interesting result that fluctuations not only destroy coherence but under appropriate conditions can also facilitate it. We show that temperature has the most pronounced effect in the intermediate coupling limit where it can promote transition from coherent to incoherent transfer.

32nd European Crystallography Meeting, ECM 32, Vienna, Austria, 18 Aug 2019 - 23 Aug 2019; Acta crystallographica / A Foundations and advances 75(a2), MS27-04 (2019). doi:10.1107/S2053273319091289 special issue: "August 2019 issue Abstracts of the 32nd European Crystallography Meeting Vienna, Austria, 18-23 August 2019", Published by Blackwell, Oxford [u.a.]

Kubo's fluctuation theory of line shape forms the backbone of our understanding of optical and vibrational line shapes, through such concepts as static heterogeneity and motional narrowing. However, the theory does not properly address the effects of quantum coherences on optical line shape, especially in extended systems where a large number of eigenstates are present. In this work, we study the line shape of an exciton in a one-dimensional lattice consisting of regularly placed and equally separated optical two level systems. We consider both linear array and cyclic ring systems of different sizes. Detailed analytical calculations of line shape have been carried out by using Kubo's stochastic Liouville equation (SLE). We make use of the observation that in the site representation, the Hamiltonian of our system with constant off-diagonal coupling J is a tridiagonal Toeplitz matrix (TDTM) whose eigenvalues and eigenfunctions are known analytically. This identification is particularly useful for long chains where the eigenvalues of TDTM help understanding crossover between static and fast modulation limits. We summarize the new results as follows. (i) In the slow modulation limit when the bath correlation time is large, the effects of spatial correlation are not negligible. Here the line shape is broadened and the number of peaks increases beyond the ones obtained from TDTM (constant off-diagonal coupling element J and no fluctuation). (ii) However, in the fast modulation limit when the bath correlation time is small, the spatial correlation is less important. In this limit, the line shape shows motional narrowing with peaks at the values predicted by TDTM (constant J and no fluctuation). (iii) Importantly, we find that the line shape can capture that quantum coherence affects in the two limits differently. (iv) In addition to linear chains of two level systems, we also consider a cyclic tetramer. The cyclic polymers can be designed for experimental verification. (v) We also build a connection between line shape and population transfer dynamics. In the fast modulation limit, both the line shape and the population relaxation, for both correlated and uncorrelated bath, show similar behavior. However, in slow modulation limit, they show profoundly different behavior. (vi) This study explains the unique role of the rate of fluctuation (inverse of the bath correlation time) in the sustenance and propagation of coherence. We also examine the effects of off-diagonal fluctuation in spectral line shape. Finally, we use Tanimura-Kubo formalism to derive a set of coupled equations to include temperature effects (partly neglected in the SLE employed here) and effects of vibrational mode in energy transfer dynamics. Published by AIP Publishing.

International audience; In this study we demonstrate the feasibility of following up a chemical reaction by single crystal x-ray (synchrotron) diffraction under operando conditions, carried out in a specially designed electrochemical cell mounted on the BM01A at the European Synchrotron Radiation Facility (ESRF). We investigated in detail the electrochemical oxidation of SrFeO 2.5 to SrFeO 3 on a spherical single crystal of 70 µ m diameter by in situ diffraction at an ambient temperature. Complete data sets were obtained by scanning the whole reciprocal space using a 2M Pilatus detector, resulting in 3600 frames with a resolution of 0.1° per data set, each obtained in 18 min. The crystal was mounted in a specially designed electrochemical cell with 1N KOH used as the electrolyte. During the electrochemical oxidation, the reaction proceeds following the phase sequence SrFeO 2.5 /SrFeO 2.75 /SrFeO 2.875 /SrFeO 3 , structurally accompanied by establishing a complex series of long-range oxygen vacancy ordering, which gets instantly organized at ambient temperature. The topotactic reaction pathway is discussed in terms of the evolution of the twin domain structure. The formation of SrFeO 2.875 is accompanied by the formation of diffuse streaks along the [1 0 0]-direction of the perovskite cell, reaching high d -spacings. The diffuse streaks are discussed and are thought to originate from a modified twin structure induced by the SrFeO 2.75 to SrFeO 2.875 transition, and the associated changes in the domain structure, developed during the oxygen intercalation. We equally analysed and discussed in detail the twin structure of all the title compounds. We confirm the ground state of SrFeO 2.5 is able to adopt the Imma space group symmetry, showing stacking faults of the tetrahedral layers along the stacking axis of the brownmillerite unit cell, indicated by the 1D diffuse rods. We showed that in situ single crystal diffraction has huge potential in the study of non-stoichiometric compounds under operando conditions, in order to obtain structural information i.e. about diffuse scattering, and microstructural information related to domain effects such as twinning—information far beyond that which powder diffraction methods allow us to obtain

We study excitation transfer and migration in a one-dimensional lattice characterized by dynamic disorder. The diagonal and off-diagonal energy disorders arise from the coupling of system and bath. We consider both same bath (when baths are spatially correlated) and independent bath (when baths are completely uncorrelated) limits. In the latter case, all diagonal and off-diagonal bath coupling elements fluctuate independently of each other and the dynamics is complicated. We obtain time dependent population distribution by solving Kubo's quantum stochastic Liouville equation. In the Markovian limit, both energy transfer dynamics and mean square displacement of the exciton behave the similar way in same and independent bath cases. However, these two baths can give rise to a markedly different behavior in the non-Markovian limit. We note that previously only the same bath case has been studied in the non-Markovian limit. The other main results of our study include the following. (i) For an average, non-zero off-diagonal coupling value J, exciton migration remains coherent in same bath case even at long times while it becomes incoherent in independent bath case in the Markovian limit. (ii) Coherent transfer is manifested in an oscillatory behavior of the energy transfer dynamics accompanied by faster-than diffusive spread of the exciton from the original position. (iii) Agreement with available analytical expression of mean squared displacement is good in Markovian limit for independent bath (off-diagonal fluctuation) case but only qualitative in non-Markovian limit for which no complete analytical solution is available. (iv) We observe transition from coherent to incoherent transport in independent bath (diagonal fluctuation) case when the bath is made progressively more Markovian. We present an analytical study that shows coherence to propagate through excited bath states. (v) The correlation time of the bath plays a unique role in dictating the diffusive spread that is not anticipated in a Markovian treatment.

Polysilicon gates have replaced the metal gates in CMOS technology. If the doping is not high enough in polysilicon then the flatband voltage should be corrected. Polysilicon gates are also depleted with the application of gate voltage. The dependence of MOSFET current (I ds ) on polysilicon gate concentration is studied using the results simulated by Sentaurus TCAD tool in case of N channel MOSFETs. With the decrease in polysilicon gate doping concentration (N d ), the drain current is more degraded. A theory is developed, in order to explain the simulation results that take into consideration the correction in flatband voltage and the voltage drop due to polysilicon depletion. From the analysis of simulated and theoretical curves, an inversion region is suspected to occur at the polysilicon gate for low doping concentration at high gate voltage.

Magnetic nanoparticles offer some attractive possibilities in biomedicine with local heat generation abilities. Here we report on the realization of highest specific absorption rate (~2 kW g−1) and a stable dynamic heat production (42–46 °C) using oleic acid coated CoFe2O4 based ferrofluids which are very promising for hyperthermia applications. CoFe2O4 nanoparticles with different sizes were prepared via co-precipitation method followed by heat treatment in the temperature range 100–600 °C to vary the particle sizes from 12–24 nm. Structural analysis using high resolution transmission electron microscopy (HRTEM) shows well separated oleic acid coated CoFe2O4 nanoparticles with a tapered spherical nature and narrow size (3 nm) distribution. Thermo-gravimetric analysis reveals the strong bonding of oleic acid to the CoFe2O4 nanoparticles. Magnetization studies show oleic acid coated CoFe2O4 nanoparticles have high saturation magnetization and reduced surface spin randomization compared to bare particles. Heat production efficiency was studied near the superparamagnetic limit of CoFe2O4 as a function of magnetic core size and ferrofluid concentration. For a given particle size and concentration, the maximum specific absorption rate varies as the square of the magnetic field applied. Notable increase in the maximum specific absorption rate was found with decrease in particle size and concentration. MTT assay studies with L-929 cells using oleic acid coated CoFe2O4 nanoparticles reveals that the particles with a size of 12 nm are more biocompatible compared to particles with a size of 24 nm.