Nonlinear elastic and electronic properties of Mo_6S_3I_6 nanowires

The properties of Mo_6S_3I_6 nanowires were investigated with ab initio calculations based on the density-functional theory. The molecules build weakly coupled one-dimensional chains with three sulfur atoms in the bridging planes between the Mo octahedra, each dressed with six iodines. Upon uniaxial strain along the wires, each bridging plane shows two energy minima, one in the ground state with the calculated Young modulus Y=82 GPa, and one in the stretched state with Y=94 GPa. Both values are at least four times smaller than the experimental values and the origin of the discrepancy remains a puzzle. The ideal tensile strength is about 8.4 GPa, the chains break in the Mo-Mo bonds within the octahedra and not in the S bridges. The charge-carrier conductivity is strongly anisotropic and the Mo_6S_3I_6 nanowires behave as quasi-one-dimensional conductors in the whole range of investigated strains. The conductivity is extremely sensitive to strain, making this material very suitable for stain gauges. Very clean nanowires with good contacts may be expected to behave as ballistic quantum wires over lengths of several $\mu$m. On the other hand, with high-impedance contacts they are good candidates for the observation of Luttinger liquid behaviour. The pronounced 1D nature of the Mo_6S_3I_6 nanowires makes them a uniquely versatile and user-friendly system for the investigation of 1D physics.Comment: 7 pages, 8 figures include

Magnetic versus nonmagnetic doping effects on the magnetic ordering in the Haldane chain compound PbNi2V2O8

A study of an impurity driven phase-transition into a magnetically ordered state in the spin-liquid Haldane chain compound PbNi2V2O8 is presented. Both, macroscopic magnetization as well as 51V nuclear magnetic resonance (NMR) measurements reveal that the spin nature of dopants has a crucial role in determining the stability of the induced long-range magnetic order. In the case of nonmagnetic (Mg2+) doping on Ni2+ spin sites (S=1) a metamagnetic transition is observed in relatively low magnetic fields. On the other hand, the magnetic order in magnetically (Co2+) doped compounds survives at much higher magnetic fields and temperatures, which is attributed to a significant anisotropic impurity-host magnetic interaction. The NMR measurements confirm the predicted staggered nature of impurity-liberated spin degrees of freedom, which are responsible for the magnetic ordering. In addition, differences in the broadening of the NMR spectra and the increase of nuclear spin-lattice relaxation in doped samples, indicate a diverse nature of electron spin correlations in magnetically and nonmagnetically doped samples, which begin developing at rather high temperatures with respect to the antiferromagnetic phase transition.Comment: 10 pages, 7 figure

Reconstructed Rough Growing Interfaces; Ridgeline Trapping of Domain Walls

We investigate whether surface reconstruction order exists in stationary growing states, at all length scales or only below a crossover length, $l_{\rm rec}$. The later would be similar to surface roughness in growing crystal surfaces; below the equilibrium roughening temperature they evolve in a layer-by-layer mode within a crossover length scale $l_{\rm R}$, but are always rough at large length scales. We investigate this issue in the context of KPZ type dynamics and a checker board type reconstruction, using the restricted solid-on-solid model with negative mono-atomic step energies. This is a topology where surface reconstruction order is compatible with surface roughness and where a so-called reconstructed rough phase exists in equilibrium. We find that during growth, reconstruction order is absent in the thermodynamic limit, but exists below a crossover length $l_{\rm rec}>l_{\rm R}$, and that this local order fluctuates critically. Domain walls become trapped at the ridge lines of the rough surface, and thus the reconstruction order fluctuations are slaved to the KPZ dynamics

Anomalous thickness dependence of the Hall effect in ultrathin Pb layers on Si(111)

The magnetoconductive properties of ultrathin Pb films deposited on Si(111) are measured and compared with density-functional electronic band-structure calculations on two-dimensional, free-standing, 1 to 8 monolayers thick Pb(111) slabs. A description with free-standing slabs is possible because it turned out that the Hall coefficient is independent of the substrate and of the crystalline order in the film. We show that the oscillations in sign of the Hall coefficient observed as a function of film thickness can be explained directly from the thickness dependent variations of the electronic bandstructure at the Fermi energy.Comment: 4 pages incl. 3 figures, RevTeX, to appear in Phys. Rev.

Disordered Flat Phase and Phase Diagram for Restricted Solid on Solid Models of Fcc(110) Surfaces

We discuss the results of a study of restricted solid-on-solid models for fcc (110) surfaces. These models are simple modifications of the exactly solvable BCSOS model, and are able to describe a $(2\times 1)$ missing-row reconstructed surface as well as an unreconstructed surface. They are studied in two different ways. The first is by mapping the problem onto a quantum spin-1/2 one-dimensional hamiltonian of the Heisenberg type, with competing $S^z_iS^z_j$ couplings. The second is by standard Monte Carlo simulations. We find phase diagrams with the following features, which we believe to be quite generic: (i) two flat, ordered phases (unreconstructed and missing-row reconstructed); a rough, disordered phase; an intermediate disordered flat (DF) phase, characterized by monoatomic steps, whose physics is shown to be akin to that of a dimer spin state. (ii) a transition line from the $(2\times 1)$ reconstructed phase to the DF phase showing exponents which appear to be close, within our numerical accuracy, to the 2D-Ising universality class. (iii) a critical (preroughening) line with variable exponents, separating the unreconstructed phase from the DF phase. Possible signatures and order parameters of the DF phase are investigated.Comment: Revtex (22 pages) + 15 figures (uuencoded file

Structure Factors and Their Distributions in Driven Two-Species Models

We study spatial correlations and structure factors in a three-state stochastic lattice gas, consisting of holes and two oppositely ``charged'' species of particles, subject to an ``electric'' field at zero total charge. The dynamics consists of two nearest-neighbor exchange processes, occuring on different times scales, namely, particle-hole and particle-particle exchanges. Using both, Langevin equations and Monte Carlo simulations, we study the steady-state structure factors and correlation functions in the disordered phase, where density profiles are homogeneous. In contrast to equilibrium systems, the average structure factors here show a discontinuity singularity at the origin. The associated spatial correlation functions exhibit intricate crossovers between exponential decays and power laws of different kinds. The full probability distributions of the structure factors are universal asymmetric exponential distributions.Comment: RevTex, 18 pages, 4 postscript figures included, mistaken half-empty page correcte

Fluctuation - induced forces in critical fluids

The current knowledge about fluctuation - induced long - ranged forces is summarized. Reference is made in particular to fluids near critical points, for which some new insight has been obtained recently. Where appropiate, results of analytic theory are compared with computer simulations and experiments.Comment: Topical review, 24 pages RevTeX, 6 figure

Self-tuning to the Hopf bifurcation in fluctuating systems

The problem of self-tuning a system to the Hopf bifurcation in the presence of noise and periodic external forcing is discussed. We find that the response of the system has a non-monotonic dependence on the noise-strength, and displays an amplified response which is more pronounced for weaker signals. The observed effect is to be distinguished from stochastic resonance. For the feedback we have studied, the unforced self-tuned Hopf oscillator in the presence of fluctuations exhibits sharp peaks in its spectrum. The implications of our general results are briefly discussed in the context of sound detection by the inner ear.Comment: 37 pages, 7 figures (8 figure files

Possible origins of macroscopic left-right asymmetry in organisms

I consider the microscopic mechanisms by which a particular left-right (L/R) asymmetry is generated at the organism level from the microscopic handedness of cytoskeletal molecules. In light of a fundamental symmetry principle, the typical pattern-formation mechanisms of diffusion plus regulation cannot implement the "right-hand rule"; at the microscopic level, the cell's cytoskeleton of chiral filaments seems always to be involved, usually in collective states driven by polymerization forces or molecular motors. It seems particularly easy for handedness to emerge in a shear or rotation in the background of an effectively two-dimensional system, such as the cell membrane or a layer of cells, as this requires no pre-existing axis apart from the layer normal. I detail a scenario involving actin/myosin layers in snails and in C. elegans, and also one about the microtubule layer in plant cells. I also survey the other examples that I am aware of, such as the emergence of handedness such as the emergence of handedness in neurons, in eukaryote cell motility, and in non-flagellated bacteria.Comment: 42 pages, 6 figures, resubmitted to J. Stat. Phys. special issue. Major rewrite, rearranged sections/subsections, new Fig 3 + 6, new physics in Sec 2.4 and 3.4.1, added Sec 5 and subsections of Sec

Direct Observation of the Myosin Va Recovery Stroke That Contributes to Unidirectional Stepping along Actin

Myosins are ATP-driven linear molecular motors that work as cellular force generators, transporters, and force sensors. These functions are driven by large-scale nucleotide-dependent conformational changes, termed “strokes”; the “power stroke” is the force-generating swinging of the myosin light chain–binding “neck” domain relative to the motor domain “head” while bound to actin; the “recovery stroke” is the necessary initial motion that primes, or “cocks,” myosin while detached from actin. Myosin Va is a processive dimer that steps unidirectionally along actin following a “hand over hand” mechanism in which the trailing head detaches and steps forward ∼72 nm. Despite large rotational Brownian motion of the detached head about a free joint adjoining the two necks, unidirectional stepping is achieved, in part by the power stroke of the attached head that moves the joint forward. However, the power stroke alone cannot fully account for preferential forward site binding since the orientation and angle stability of the detached head, which is determined by the properties of the recovery stroke, dictate actin binding site accessibility. Here, we directly observe the recovery stroke dynamics and fluctuations of myosin Va using a novel, transient caged ATP-controlling system that maintains constant ATP levels through stepwise UV-pulse sequences of varying intensity. We immobilized the neck of monomeric myosin Va on a surface and observed real time motions of bead(s) attached site-specifically to the head. ATP induces a transient swing of the neck to the post-recovery stroke conformation, where it remains for ∼40 s, until ATP hydrolysis products are released. Angle distributions indicate that the post-recovery stroke conformation is stabilized by ≥5 kBT of energy. The high kinetic and energetic stability of the post-recovery stroke conformation favors preferential binding of the detached head to a forward site 72 nm away. Thus, the recovery stroke contributes to unidirectional stepping of myosin Va