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Metastable States (metastable + states)
Selected AbstractsOptical, Magnetic and Structural Properties of the Spin-Crossover Complex [Fe(btr)2(NCS)2]·H2O in the Light-Induced and Thermally Quenched Metastable StatesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 36 2007Vincent Legrand Abstract [Fe(btr)2(NCS)2]·H2O [btr = 4,4,-bis(1,2,4-triazole)] is thearchetype of highly cooperative and low-dimensional spin-crossover complexes, which exhibit low-spin (LS) to high-spin (HS) light-induced conversion at very low temperature. The structural reorganizations related to the light-induced and thermally induced LS,HS transitions were characterized by single-crystal X-ray diffraction below the relaxation temperature (T = 15 K < TLIESST) and at 130 K within the thermal hysteresis loop. We show that the LIESST and thermal spin transitions lead to the same structural variations, namely an elongation of the Fe,N bonds by 0.18 Å (Fe,NNCS) and 0.20 Å (Fe,Nbtr), on going from LS to HS, together with a reorientation of the NCS group by nearly 13°. The atomic displacement amplitudes, derived from the crystal structures, indicate lattice vibration modes of larger amplitudes and correlatively lower vibration frequencies in the HS state. The deformation of the crystal lattice as a function of temperature and laser excitation was quantitatively analyzed in terms of the HS and LS thermal-expansion (,HS and ,LS) and spin-transition spontaneous-strain (,) tensors. The eigendirections and eigenvalues of the , and , tensors correlate well with the weak and strong interactions in the solid and are responsible for the high cooperativity and low-dimensional behaviour. Magnetic and spectroscopic measurements were performed in all the different spin states and related to the structural findings. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Metastable States of Water and Ice during Pressure-Supported Freezing of Potato TissueBIOTECHNOLOGY PROGRESS, Issue 3 2004O. Schlüter Different ice modifications were obtained during freezing processes at several pressure levels from atmospheric pressure up to 300 MPa. In the pressure range between 210 and 240 MPa, a metastable ice I modification area was observed, as the nucleation of ice I crystals in the thermodynamically stable region of ice III was reached. A significant degree of supercooling was obtained before freezing the tissue water to ice III, which has to be considered when designing pressure-supported freezing processes. The effect of supercooling phenomenon on the phase transition time is discussed using a mathematical model based on the solution of the heat transfer governing differential equations. Phase transition and freezing times for the different freezing paths experimented are compared for the processes: freezing at atmospheric pressure, pressure-assisted freezing, and pressure-shift freezing. Different metastable states of liquid water are defined according to their process-dependent stability. [source] Computational significance of transient dynamics in cortical networksEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 1 2008Daniel Durstewitz Abstract Neural responses are most often characterized in terms of the sets of environmental or internal conditions or stimuli with which their firing rate are correlated increases or decreases. Their transient (nonstationary) temporal profiles of activity have received comparatively less attention. Similarly, the computational framework of attractor neural networks puts most emphasis on the representational or computational properties of the stable states of a neural system. Here we review a couple of neurophysiological observations and computational ideas that shift the focus to the transient dynamics of neural systems. We argue that there are many situations in which the transient neural behaviour, while hopping between different attractor states or moving along ,attractor ruins', carries most of the computational and/or behavioural significance, rather than the attractor states eventually reached. Such transients may be related to the computation of temporally precise predictions or the probabilistic transitions among choice options, accounting for Weber's law in decision-making tasks. Finally, we conclude with a more general perspective on the role of transient dynamics in the brain, promoting the view that brain activity is characterized by a high-dimensional chaotic ground state from which transient spatiotemporal patterns (metastable states) briefly emerge. Neural computation has to exploit the itinerant dynamics between these states. [source] On the precision and accuracy of structural analysis of light-induced metastable statesJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2007Vincent Legrand Bragg diffraction data were collected on single crystals of the spin-crossover complex [Fe(phen)2(NCS)2] in its low-spin and light-induced metastable high-spin states. Experimental variables included the temperature (32 and 15,K), the X-ray source (sealed tube and synchrotron), and the time interval between laser light excitation of the sample (, = 647,nm). From a comparison of the structural parameters refined, it is shown that photo-crystallographic measurements suffer significantly and systematically from bias if the probed sample contains residual ground-state species, resulting from an incomplete photo-conversion or a significant metastable- to ground-state relaxation. It follows that a 4% population of species in a different spin state affects the Fe,N bond lengths by more than three standard deviations, and the FeN6 polyhedron volume by as much as seven standard deviations, while the mean atomic position misfit exceeds 0.005,Å. [source] X-ray structure study of the light-induced metastable states of the spin-crossover compound [Fe(mtz)6](BF4)2JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2001Joachim Kusz Iron(II) complexes exhibiting thermal spin-crossover may be converted from the 1A1 low-spin (LS) state to the 5T2 high-spin (HS) state by irradiation with green light (light-induced excited spin-state trapping, LIESST) and from the LS to the HS state by irradiation with red light (reverse LIESST). The lifetime of the metastable LIESST states may be sufficiently long to enable an X-ray diffraction study. The lattice parameters of a single crystal of [Fe(mtz)6](BF4)2 (mtz = methyltetrazole) (space group P21/n) were measured between 300 and 10,K. While one Fe lattice site (A) of the crystal changes from the HS to the LS state near 78,K, the other site (B) remains in the LS state. Using the green light (514,nm) of an argon ion laser the crystal was quantitatively converted to the HS state at 10,K. Irradiation of the crystal at 10,K by red light of a laser diode (820,nm) with site A in the LS and site B in the HS state converts site B almost completely to the LS state. The lattice parameters of both metastable states were measured up to 50,K, where they start to decay on a minute timescale. At 10,K, a full data set for evaluation of the crystal structure was recorded. The volume change of the crystal per complex molecule accompanying the spin transition is 31.5,Å3 at site A and close to zero [,0.21,(14),Å3] at site B. [source] Quantum phase transition in a spin-½ XX chain with three-spin and uniform long-range interactionsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 15 2005Ping Lou Abstract A spin-½ XX chain with three-spin and uniform long-range interactions among the z components of the spins is constructed. Based upon the model, using the Jordan,Wigner transformation, the thermodynamic quantities and spin transports of the system are evaluated exactly. The phase diagram together with the key parameters characterizing quantum phase transitions is presented. It is noted that, similarly to classical first-order phase transitions, there are also the metastable states "supercooled" and "superheated" in the first-order quantum phase transitions. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Metastable States of Water and Ice during Pressure-Supported Freezing of Potato TissueBIOTECHNOLOGY PROGRESS, Issue 3 2004O. Schlüter Different ice modifications were obtained during freezing processes at several pressure levels from atmospheric pressure up to 300 MPa. In the pressure range between 210 and 240 MPa, a metastable ice I modification area was observed, as the nucleation of ice I crystals in the thermodynamically stable region of ice III was reached. A significant degree of supercooling was obtained before freezing the tissue water to ice III, which has to be considered when designing pressure-supported freezing processes. The effect of supercooling phenomenon on the phase transition time is discussed using a mathematical model based on the solution of the heat transfer governing differential equations. Phase transition and freezing times for the different freezing paths experimented are compared for the processes: freezing at atmospheric pressure, pressure-assisted freezing, and pressure-shift freezing. Different metastable states of liquid water are defined according to their process-dependent stability. [source] The geometric minimum action method: A least action principle on the space of curvesCOMMUNICATIONS ON PURE & APPLIED MATHEMATICS, Issue 8 2008Matthias Heymann Freidlin-Wentzell theory of large deviations for the description of the effect of small random perturbations on dynamical systems is exploited as a numerical tool. Specifically, a numerical algorithm is proposed to compute the quasi-potential in the theory, which is the key object to quantify the dynamics on long time scales when the effect of the noise becomes ubiquitous: the equilibrium distribution of the system, the pathways of transition between metastable states and their rate, etc., can all be expressed in terms of the quasi-potential. We propose an algorithm to compute these quantities called the geometric minimum action method (gMAM), which is a blend of the original minimum action method (MAM) and the string method. It is based on a reformulation of the large deviations action functional on the space of curves that allows one to easily perform the double minimization of the original action required to compute the quasi-potential. The theoretical background of the gMAM in the context of large deviations theory is discussed in detail, as well as the algorithmic aspects of the method. The gMAM is then illustrated on several examples: a finite-dimensional system displaying bistability and modeled by a nongradient stochastic ordinary differential equation, an infinite-dimensional analogue of this system modeled by a stochastic partial differential equation, and an example of a bistable genetic switch modeled by a Markov jump process. © 2007 Wiley Periodicals, Inc. [source] | |||||||||||||