107,597 research outputs found
Energetics of active fluctuations in living cells
The nonequilibrium activity taking place in a living cell can be monitored
with a tracer embedded in the medium. While microrheology experiments based on
optical manipulation of such probes have become increasingly standard, we put
forward a number of experiments with alternative protocols that, we claim, will
provide new insight into the energetics of active fluctuations. These are based
on either performing thermodynamic--like cycles in control-parameter space, or
on determining response to external perturbations of the confining trap beyond
simple translation. We illustrate our proposals on an active itinerant Brownian
oscillator modeling the dynamics of a probe embedded in a living medium
Automated Selection of Active Orbital Spaces
One of the key challenges of quantum-chemical multi-configuration methods is
the necessity to manually select orbitals for the active space. This selection
requires both expertise and experience and can therefore impose severe
limitations on the applicability of this most general class of ab initio
methods. A poor choice of the active orbital space may yield even qualitatively
wrong results. This is obviously a severe problem, especially for wave function
methods that are designed to be systematically improvable. Here, we show how
the iterative nature of the density matrix renormalization group combined with
its capability to include up to about one hundred orbitals in the active space
can be exploited for a systematic assessment and selection of active orbitals.
These benefits allow us to implement an automated approach for active orbital
space selection, which can turn multi-configuration models into black box
approaches.Comment: 29 pages, 10 figures, 5 table
Quantifying and Controlling Prethermal Nonergodicity in Interacting Floquet Matter
The use of periodic driving for synthesizing many-body quantum states depends crucially on the existence of a prethermal regime, which exhibits drive-tunable properties while forestalling the effects of heating. This dependence motivates the search for direct experimental probes of the underlying localized nonergodic nature of the wave function in this metastable regime. We report experiments on a many-body Floquet system consisting of atoms in an optical lattice subjected to ultrastrong sign-changing amplitude modulation. Using a double-quench protocol, we measure an inverse participation ratio quantifying the degree of prethermal localization as a function of tunable drive parameters and interactions. We obtain a complete prethermal map of the drive-dependent properties of Floquet matter spanning four square decades of parameter space. Following the full time evolution, we observe sequential formation of two prethermal plateaux, interaction-driven ergodicity, and strongly frequency-dependent dynamics of long-time thermalization. The quantitative characterization of the prethermal Floquet matter realized in these experiments, along with the demonstration of control of its properties by variation of drive parameters and interactions, opens a new frontier for probing far-from-equilibrium quantum statistical mechanics and new possibilities for dynamical quantum engineering
Impact of random obstacles on the dynamics of a dense colloidal fluid
Using molecular dynamics simulations we study the slow dynamics of a
colloidal fluid annealed within a matrix of obstacles quenched from an
equilibrated colloidal fluid. We choose all particles to be of the same size
and to interact as hard spheres, thus retaining all features of the porous
confinement while limiting the control parameters to the packing fraction of
the matrix, {\Phi}m, and that of the fluid, {\Phi}f. We conduct detailed
investigations on several dynamic properties, including the tagged-particle and
collective intermediate scattering functions, the mean-squared displacement,
and the van Hove function. We show the confining obstacles to profoundly impact
the relaxation pattern of various quantifiers pertinent to the fluid. Varying
the type of quantifier (tagged-particle or collective) as well as {\Phi}m and
{\Phi}f, we unveil both discontinuous and continuous arrest scenarios.
Furthermore, we discover subdiffusive behavior and demonstrate its close
connection to the matrix structure. Our findings partly confirm the various
predictions of a recent extension of mode-coupling theory to the
quenched-annealed protocol.Comment: 16 pages, 20 figures, minor revision
Sex‐specific activation of SK current by isoproterenol facilitates action potential triangulation and arrhythmogenesis in rabbit ventricles
Sex has a large influence on cardiac electrophysiological properties. Whether sex differences exist in apamin‐sensitive small conductance Ca2+‐activated K+ (SK) current (IKAS) remains unknown. We performed optical mapping, transmembrane potential, patch clamp, western blot and immunostaining in 62 normal rabbit ventricles, including 32 females and 30 males. IKAS blockade by apamin only minimally prolonged action potential (AP) duration (APD) in the basal condition for both sexes, but significantly prolonged APD in the presence of isoproterenol in females. Apamin prolonged APD at the level of 25% repolarization (APD25) more prominently than APD at the level of 80% repolarization (APD80), consequently reversing isoproterenol‐induced AP triangulation in females. In comparison, apamin prolonged APD to a significantly lesser extent in males and failed to restore the AP plateau during isoproterenol infusion. IKAS in males did not respond to the L‐type calcium current agonist BayK8644, but was amplified by the casein kinase 2 (CK2) inhibitor 4,5,6,7‐tetrabromobenzotriazole. In addition, whole‐cell outward IKAS densities in ventricular cardiomyocytes were significantly larger in females than in males. SK channel subtype 2 (SK2) protein expression was higher and the CK2/SK2 ratio was lower in females than in males. IKAS activation in females induced negative intracellular Ca2+–voltage coupling, promoted electromechanically discordant phase 2 repolarization alternans and facilitated ventricular fibrillation (VF). Apamin eliminated the negative Ca2+–voltage coupling, attenuated alternans and reduced VF inducibility, phase singularities and dominant frequencies in females, but not in males. We conclude that β‐adrenergic stimulation activates ventricular IKAS in females to a much greater extent than in males. IKAS activation plays an important role in ventricular arrhythmogenesis in females during sympathetic stimulation
Investigation on energetic optimization problems of stochastic thermodynamics with iterative dynamic programming
The energetic optimization problem, e.g., searching for the optimal switch-
ing protocol of certain system parameters to minimize the input work, has been
extensively studied by stochastic thermodynamics. In current work, we study
this problem numerically with iterative dynamic programming. The model systems
under investigation are toy actuators consisting of spring-linked beads with
loading force imposed on both ending beads. For the simplest case, i.e., a
one-spring actuator driven by tuning the stiffness of the spring, we compare
the optimal control protocol of the stiffness for both the overdamped and the
underdamped situations, and discuss how inertial effects alter the
irreversibility of the driven process and thus modify the optimal protocol.
Then, we study the systems with multiple degrees of freedom by constructing
oligomer actuators, in which the harmonic interaction between the two ending
beads is tuned externally. With the same rated output work, actuators of
different constructions demand different minimal input work, reflecting the
influence of the internal degrees of freedom on the performance of the
actuators.Comment: 14 pages, 7 figures, communications in computational physics, in
pres
Work probability distribution in systems driven out of equilibrium
We derive the differential equation describing the time evolution of the work
probability distribution function of a stochastic system which is driven out of
equilibrium by the manipulation of a parameter. We consider both systems
described by their microscopic state or by a collective variable which
identifies a quasiequilibrium state. We show that the work probability
distribution can be represented by a path integral, which is dominated by
``classical'' paths in the large system size limit. We compare these results
with simulated manipulation of mean-field systems. We discuss the range of
applicability of the Jarzynski equality for evaluating the system free energy
using these out-of-equilibrium manipulations. Large fluctuations in the work
and the shape of the work distribution tails are also discussed
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