The out-of-equilibrium dynamics of finite ultracold bosonic ensembles in periodically driven one-dimensional optical lattices is investigated. Our study reveals that the driving enforces the bosons in different wells to oscillate in-phase and to exhibit a dipole-like mode. A wide range from weak-to-strong driving frequencies is covered and a resonance-like behaviour of the intra-well dynamics is discussed. In the proximity of the resonance a rich intraband excitation spectrum is observed. The single particle excitation mechanisms are studied in the framework of Floquet theory elucidating the role of the driving frequency. The impact of the interatomic repulsive interactions is examined in detail yielding a strong influence on the tunneling period and the excitation probabilities. Finally, the dependence of the resonance upon a variation of the tunable parameters of the optical lattice is examined. Our analysis is based on the ab-initio Multi-Configuration Time-Dependent Hartree Method for bosons.

Time-averaged two-point currents are derived and shown to be spatially invariant within domains of local translation or inversion symmetry for arbitrary time-periodic quantum systems in one dimension. These currents are shown to provide a valuable tool for detecting deformations of a spatial symmetry in static and driven lattices. In the static case the invariance of the two-point currents is related to the presence of time-reversal invariance and/or probability current conservation. The obtained insights into the wavefunctions are further exploited for a symmetry-based convergence check which is applicable for globally broken but locally retained potential symmetries.

We explore the nonequilibrium dynamics of noninteracting classical particles in a one-dimensional driven superlattice which is composed of domains exposed to different time-dependent forces. It is shown how the combination of directed transport and conversion processes from diffusive to ballistic motion causes strong correlations between velocity and phase for particles passing through a superlattice. A detailed understanding of the underlying mechanism allows us to tune the resulting velocity distributions at distinguished points in the superlattice by means of local variations of the applied driving force. As an intriguing application we present a scheme how initially diffusive particles can be transformed into a monoenergetic pulsed particle beam whose parameters such as its energy can be varied.

We explore the impact of weak disorder on the dynamics of classical particles in a periodically oscillating lattice. It is demonstrated that the disorder induces a hopping process from diffusive to regular motion i.e. we observe the counterintuitive phenomenon that disorder leads to regular behaviour. If the disorder is localized in a finite-sized part of the lattice, the described hopping causes initially diffusive particles to even accumulate in regular structures of the corresponding phase space. A hallmark of this accumulation is the emergence of pronounced peaks in the velocity distribution of particles which should be detectable in state of the art experiments e.g. with cold atoms in optical lattices. Introduction. -Time-driven nonequilibrium dynamics is a subject of major interest [1][2][3][4][5], covering many different physical systems such as colloidal particles exposed to periodically modulated ion chains [6], particles moving along a filament with a hydrodynamic coupling to the surrounding solvent [7] or cold polar atoms loaded into optical lattices that are driven by periodic phase modulations of the applied laser beam [8][9][10]. A prototype example for a nonequilibrium phenomenon in driven lattices is the celebrated 'ratchet effect' which is the appearance of directed particle motion in the absence of biased forces due to a breaking of certain spatiotemporal symmetries [11,12]. While the aforementioned setups focus on globally acting time periodic forces, it has been demonstrated recently how spatially varying ac-forces introduce a plethora of effects [13][14][15][16][17] such as the formation of density waves [16], the patterned deposition of particles [15] or the possibility for conversion processes between diffusive and ballistic motion [16,17]. Moreover, long-range interactions have been shown to lead to dynamical current reversals in the absence of any paramter change [18]. In this work we demonstrate how the combination of disorder and driving in a lattice can lead to the emergence of regular motion from an originally chaotic and diffusive ensemble of particles. Disorder-induced autocorrelations and pronounced changes of the velocity distributions are found with major differences occuring for the cases of global versus local disorder.

BackgroundLordosis is the bending of the lumbar spine that gives the vertebral column of humans its characteristic ventrally convex curvature. Infants develop lordosis around the time when they acquire bipedal locomotion. Even macaques develop a lordosis when they are trained to walk bipedally. The aim of this study was to investigate why humans and some animals develop a lumbar lordosis while learning to walk bipedally.ResultsWe developed a musculoskeletal model of the lumbar spine, that includes an asymmetric, dorsally shifted location of the spinal column in the body, realistic moment arms, and physiological cross-sectional areas (PCSA) of the muscles as well as realistic force-length and force-velocity relationships. The model was used to analyze the stability of an upright body posture. According to our results, lordosis reduces the local joint torques necessary for an equilibrium of the vertebral column during an erect posture. At the same time lordosis increases the demands on the global muscles to provide stability.ConclusionsWe conclude that the development of a spinal lordosis is a compromise between the stability requirements of an erect posture and the necessity of torque equilibria at each spinal segment.