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[en] We developed the pulse sequence TOMROP (T One by Multiple Read Out Pulses) for determining precisely the spatial distribution of the longitudinal relaxation time T1 in nuclear magnetic resonance (NMR): a series of small-angle selection pulses is used to read out longitudinal magnetization from its initial state till thermal equilibrium. Hence, one measurement will produce several images with different T1 weightings whose pixel brilliance depends exponentially from read-out time. T1 can be determined from these independent of initial magnetization and selection pulse angle. The measuring time corresponds to the time needed in multi-echo imaging for the determination of the transversal relaxation time T2. We demonstrate this new method using head images of volunteers produced with a 0.23 T test facility. (orig./HP)
[de]Zur genauen Bestimmung der raeumlichen Verteilung der Laengsrelaxation T1 in der Kernspintomographie haben wir die Pulssequenz-TOMROP (T One by Multiple Read Out Pulses) entwickelt: Mit einer Serie von Kleinwinkelauslesepulsen wird die Laengsmagnetisierung vom Anfangszustand bis zum thermischen Gleichgewicht abgefragt. In einer Messung ergeben sich also mehrere unterschiedlich T1-gewichtete Bilder, bei denen die Pixelhelligkeit exponentiell von der Abfragezeit abhaengt. Hieraus kann T1 unabhaengig von der Anfangsmagnetisierung und dem Auslesepulswinkel bestimmt werden. Die Messzeit entspricht der einer Multiechoaufnahme zur Bestimmung der Querrelaxationszeit T2. Mit einer 0,23-T-Versuchsanlage demonstrieren wir das neue Verfahren an Kopfaufnahmen von freiwilligen Versuchspersonen. (orig./HP)
[en] The dynamic critical exponent z is determined from numerical simulations for the three-dimensional (3D) lattice Coulomb gas (LCG) and the 3D XY models with relaxational dynamics. It is suggested that the dynamics is characterized by two distinct dynamic critical indices z0 and z related to the divergence of the relaxation time τ by τ∝ξz0 and τ∝k-z , where ξ is the correlation length and k the wave vector. The values determined are z0∼1.5 and z∼1 for the 3D LCG and z0∼1.5 and z∼2 for the 3D XY model. Comparisons with other results are discussed
[en] In the setting of non-reversible Markov chains on finite or countable state space, exact results on the distribution of the first hitting time to a given set G are obtained. A new notion of “conditional strong quasi stationary time” is introduced to describe the local relaxation time. This time is defined via a generalization of the strong stationary time. Rarity of the target set G is not required and the initial distribution can be completely general. The results clarify the the role played by the initial distribution on the exponential law; they are used to give a general notion of metastability and to discuss the relation between the exponential distribution of the first hitting time and metastability.
[en] We review our recent proposal for a universal description of generic single-component viscoelastic systems with a single relaxation time. Foliation preserving diffeomorphisms are introduced as an underlying symmetry which naturally interpolates between the two extreme characters of elasticity and fluidity. The symmetry is found to be powerful enough to determine the dynamics in the first order of strains
[en] The relaxation time approximation (RTA) is a well known method of describing the time evolution of a statistical ensemble by linking distributions of the variables of interest at different stages of their temporal evolution. We show that if all the distributions occurring in the RTA have the same functional form of a quasi-power Tsallis distribution the time evolution of which depends on the time evolution of its control parameter, nonextensivity q(t), then it is more convenient to consider only the time evolution of this control parameter.
[en] We study the entanglement dynamics and relaxation properties of a system of two interacting qubits in the cases of (I) two independent bosonic baths and (II) one common bath. We find that in the case (II) the existence of a decoherence-free subspace (DFS) makes entanglement dynamics very rich. We show that when the system is initially in a state with a component in the DFS the relaxation time is surprisingly long, showing the existence of semi-decoherence free subspaces.
[en] The magnetic inertial dynamics have previously been investigated for one sublattice ferromagnets. Here, we develop the magnetization dynamics in two-sublattice ferromagnets including the intra- and inter-sublattice inertial dynamics. First, we derive the magnetic susceptibility of such a ferromagnet. Next, by finding the poles of the susceptibility, we calculate the precession and nutation resonance frequencies. Our results suggest that while the resonance frequencies show decreasing behavior with the increasing intra-sublattice relaxation time, the effect of inter-sublattice inertial dynamics has an opposite effect. (paper)
[en] We measure the relaxation time of a square lattice Ising ferromagnet that is quenched to zero-temperature from supercritical initial conditions. We reveal an anomalous and seemingly overlooked timescale associated with the relaxation to ‘frozen’ two-stripe states. While close to a power law of the form ∼L ν, we argue this timescale actually grows as ∼L 2 ln L, with L the linear dimension of the system. We uncover the mechanism behind this scaling form by using a synthetic initial condition that replicates the late time ordering of two-stripe states, and subsequently explain it heuristically. (paper)