We studied the dynamical effects induced in a homeotropic nematic film when a normally incident circularly polarized light beam with an elliptical intensity profile is used. A three-dimensional dynamical model shows that, besides the spin, the orbital angular momentum of photons also plays a role in the reorientation process. Our measurements fairly reproduce the main dynamical features predicted by the model in the near threshold region. The model, however, does not work, as it is, at higher incident laser power where chaotic director rotation was reported [A. Vella, A. Setaro, B. Piccirillo, E. Santamato, Phys. Rev. E 67, 051704 (2003)].

It is well known that overdamped unforced dynamical systems do not oscillate. However, well-designed coupling schemes, together with the appropriate choice of initial conditions, can induce oscillations (corresponding to transitions between the stable steady states of each nonlinear element) when a control parameter exceeds a threshold value. In recent publications [A. Bulsara et al., Phys. Rev. E 70, 036103 (2004); V. In et al., ibid. 72, 045104 (2005)], we demonstrated this behavior in a specific prototype system, a soft-potential mean-field description of the dynamics in a hysteretic “single-domain” ferromagnetic sample. These oscillations are now finding utility in the detection of very weak “target” magnetic signals, via their effect on the oscillation characteristics—e.g., the frequency and asymmetry of the oscillation wave forms. We explore the underlying dynamics of a related system, coupled bistable “standard quartic” dynamic elements; the system shows similarities to, but also significant differences from, our earlier work. dc as well as time-periodic target signals are considered; the latter are shown to induce complex oscillatory behavior in different regimes of the parameter space. In turn, this behavior can be harnessed to quantify the target signal.

We studied complex spectra of a two-level electron system coupled to two phonon (vibron) modes represented by the $\mathrm{E}\otimes \mathrm{e}$ Jahn-Teller model. For particular rotation quantum numbers we found a coexistence of up to three regions of the spectra: (i) the dimerized region of long-range-ordered (extended) pairs of oscillating levels, (ii) the short-range-ordered (localized) “kink lattice” of avoiding levels, and (iii) the intermediate region of kink nucleation with variable range of ordering. This structure appears above a certain critical line as a function of interaction strength. The level clustering and level avoiding generic patterns reflect themselves in several intermittent regions between up to three branches of spectral entropies. Linear scaling behavior of the widths of level curvature probability distributions provides the conventionally adopted indication for the presence of quantum chaos. Level spacing probability distributions show peculiarities of the partial (for fixed quantum angular momentum) as well as of the cumulative (all angular momenta) case. The clustering of levels with two and three dominant spacings at fixed angular momenta causes notable deviations of the cumulative distributions from the Poissonian one.

The analytical theory of explosion triggering by an accelerating flame is developed. The theory describes the structure of a one-dimensional isentropic compression wave pushed by the flame front. The condition of explosion in the gas mixture ahead of the flame front is derived; the instant of the explosion is determined provided that a mechanism of chemical kinetics is known. As an example, it is demonstrated how the problem is solved in the case of a single reaction of Arrhenius type, controlling combustion both inside the flame front and ahead of the flame. The model of an Arrhenius reaction with a cutoff temperature is also considered. The limitations of the theory due to the shock formation in the compression wave are found. Comparison of the theoretical results to the previous numerical simulations shows good agreement.

An electrohydrodynamic phenomenon called the electromeniscus is analyzed. This phenomenon is closely related to the oscillation of a microscale liquid and is analyzed using an interpretation of the Maxwell model coupled with a variational principle. The analysis clearly shows that the electromeniscus phenomenon is generated as a result of mass transformation of the oscillating liquid in order to transform the electric energy at the electrode to kinetic energy of the liquid most efficiently through their resonance. Mass transformation is a characteristic phenomenon of a liquid, and control of this phenomenon demonstrates great potential for self-aligning nanoscale materials and production of functional polymers characterized by specific molecular orientations.

A model that solves simultaneously both the electron and atomic kinetics was used to generate a synthetic ${\mathrm{He}}_{\alpha }$ and satellite x-ray spectra to characterize a high intensity ultrashort laser driven Ar cluster target experiment. In particular, level populations were obtained from a detailed collisional-radiative model where collisional rates were computed from a time varying electron distribution function obtained from the solution of the zero-dimensional Boltzmann equation. In addition, a particle-in-cell simulation was used to model the laser interaction with the cluster target and provided the initial electron energy distribution function (EEDF) for the Boltzmann solver. This study suggests that a high density average, ${⟨N_a⟩}_{\mathit{\text{high}}}$, of $3.2\times {10}^{20}{\mathrm{cm}}^{-3}$ was held by the system for a time, $\delta \tau$, of $5.7\mathrm{ps}$, and during this time the plasma was in a highly nonequilibrium state in both the EEDF and the ion level populations.

The balance between diffraction and index-step guiding in photonic-crystal fibers is controlled by modifying the fiber structure, leading to different wavelength dependences of the effective mode area $S_{\mathrm{eff}}\left(\lambda \right)$ and providing a mechanism to control nonlinear-optical phenomena. In optical fibers with a steep $S_{\mathrm{eff}}\left(\lambda \right)$ profile, the guided mode of the light field tends to become much less compact with an increase in radiation wavelength, slowing down the Raman-induced soliton self-frequency shift of an ultrashort laser pulse. A $100\text{−}\mathrm{nm}$ reduction of the soliton self-frequency shift is demonstrated for $6\text{−}\mathrm{fs}$ input laser pulses.

Solitonic phenomena in hollow photonic-crystal fibers are shown to offer new solutions for the control, transmission, and spectral-temporal transformation of ultrashort high-power laser pulses. With the initial parameters of laser radiation accurately matched with fiber dispersion and nonlinearity, submicrojoule laser pulses are shown to exhibit a self-compression and soliton dynamics in the regime of anomalous dispersion. Regimes of solitonic pulse evolution giving rise to few-cycle field waveforms are demonstrated. Based on simple arguments of soliton theory, we derive semiempirical relations providing interesting insights into the minimum pulse width of the laser field in a hollow photonic-crystal fiber as a function of the input laser energy.

A phase space method is employed for the construction of analytical solitary wave solutions of the nonlinear Kronig-Penney model in a photonic structure. This class of solutions is obtained under quite generic conditions, while the method is applicable to a large variety of systems. The location of the solutions on the spectral band gap structure as well as on the low dimensional space of system’s conserved quantities is studied, and robust solitary wave propagation is shown.

Relativistic plasma with two charge species and radiation at thermodynamic equilibrium is a general system of interest in astrophysics and high-energy physics. We develop a self-consistent quasiparticle model for such a system to take account of the collective behavior of plasma, and thermodynamic properties are derived. It is applied to the ultrarelativistic electron-positron plasma and compared with previous results.

Winfree turbulence is a chaotic wave pattern developing through negative-tension instability of scroll wave filaments in three-dimensional weak excitable media. Here, we investigate the response of Winfree turbulence to a spatiotemporal forcing in the form of a traveling-wave modulation of the medium excitability. It is shown that turbulent waves can be suppressed much more rapidly by this method, in comparison with the space-uniform modulation of the medium excitability. Since the occurrence of Winfree turbulence is currently regarded as one of the possible mechanisms underlying cardiac fibrillation, this method turns out to be suggestive of a possible low-amplitude defibrillation approach.

Pure multiplicative noise-induced stochastic resonance, which appears in an anti-tumor system modulated by a seasonal external field, is studied by using theoretical analyses of the generalized potential and numerical simulations. For optimally selected values of the multiplicative noise intensity stochastic resonance is observed, which is manifested by the quasisymmetry of two potential minima. Theoretical results and numerical simulations are in good quantitative agreement.

We perform extensive molecular dynamics simulations of two-dimensional frictionless granular materials to determine whether these systems can be characterized by a single static yield shear stress. We consider boundary-driven planar shear at constant volume and either constant shear force or constant shear velocity. Under steady flow conditions, these two ensembles give similar results for the average shear stress versus shear velocity. However, near jamming it is possible that the shear stress required to initiate shear flow can differ substantially from the shear stress required to maintain flow. We perform several measurements of the shear stress near the initiation and cessation of flow. At fixed shear velocity, we measure the average shear stress ${\Sigma }_{yv}$ in the limit of zero shear velocity. At fixed shear force, we measure the minimum shear stress ${\Sigma }_{yf}$ required to maintain steady flow at long times. We find that in finite-size systems ${\Sigma }_{yf}>{\Sigma }_{yv}$, which implies that there is a jump discontinuity in the shear velocity from zero to a finite value when these systems begin flowing at constant shear force. However, our simulations suggest that the difference ${\Sigma }_{yf}-{\Sigma }_{yv}$, and thus the discontinuity in the shear velocity, tend to zero in the infinite-system-size limit. Thus, our results imply that in the large-system limit, frictionless granular systems can be characterized by a single static yield shear stress. We also monitor the short-time response of these systems to applied shear and show that the packing fraction of the system and shape of the velocity profile can strongly influence whether or not the shear stress at short times overshoots the long-time average value.

We propose a different, hybrid Monte Carlo algorithm that combines configurational bias particle swaps with parallel tempering. We use this algorithm to simulate a standard model of a glass-forming binary mixture above and below the so-called mode-coupling temperature $T_{MCT}$. We find that an ansatz that was used previously to extrapolate thermodynamic quantities to temperatures below $T_{MCT}$ breaks down in the vicinity of the mode-coupling temperature. We also find that the Adam-Gibbs relations $D\propto \exp (-a∕T{S}_c)$ and $\tau \propto \exp (b∕T{S}_c)$, which connect the diffusion coefficient $D$ and the relaxation time $\tau$ with the configurational entropy $S_c$, are valid for all temperatures for which the configurational and vibrational contributions to the free energy decouple.

We develop a complete set of equations governing the evolution of a sharp interface separating a volatile-solvent/nonvolatile-surfactant solution from a vapor atmosphere. In addition to a sorption isotherm equation and the conventional balances for mass, linear momentum, and energy, these equations include an alternative to the Hertz-Knudsen-Langmuir equation familiar from conventional theories of evaporation and condensation. This additional equation arises from a consideration of configurational forces within a thermodynamical framework. While the notion of configurational forces is well developed and understood for the description of materials that, like crystalline solids, possess natural reference configurations, very little has been done regarding their role in materials, such as viscous fluids, that do not possess preferred reference states. We therefore provide comprehensive developments of configurational forces, the balance of configurational momentum, and configurational thermodynamics. Our treatment does not require a choice of reference configuration. The general evolution equations arising from our theory account for the thermodynamic structure of the solution and the interface and for sources of dissipation related to the transport of surfactant, momentum, and heat in the solution and within the interface along with the transport of solute, momentum, kinetic energy, and heat across the interface. Moreover, the equations account for the Soret and Dufour effects in the solution and on the interface and for observed discontinuities of the temperature and chemical potential across the interface. Due to the complexity of these equations, we provide approximate equations which we compare to equations preexistent in the literature.

We have generalized the McMillan theory of liquid crystalline smectic order in uniaxial particle fluids to biaxial particles. Upon varying the control parameter, a uniaxial nematic phase may: (i) order biaxially first, then smectically; (ii) order smectically first, then biaxially; and (iii) simultaneously order biaxially and smectically. We investigate, in the limit of complete orientational order of the molecular major axes, which of these scenarios are realized for a simple model of particles with the symmetry of rectangular parallelepipeds. We also present a generic variational derivation of the theory based on the identification of the dominant order parameters for the most ordered phase.

Biomimetic membranes that adhere to a solid substrate or another interface via switchable crosslinker molecules are studied theoretically using analytical methods and Monte Carlo simulations. The flexible crosslinkers exhibit two conformations which have a different end-to-end distance and, thus, lead to different local separations of the membrane from the substrate surface. Transitions between the molecular conformations can be induced by light, electric potential, or changes in $p\mathrm{H}$ and lead to active shape fluctuations of the membrane and, thus, to an increased membrane roughness. The forward and backward transitions are characterized by two transition rates, ${\omega }_{+}$ and ${\omega }_{-}$, respectively, which define the average fraction $X={\omega }_{+}∕({\omega }_{+}+{\omega }_{-})$ of $+$ (or on) states and the mean switching rate $\omega =({\omega }_{+}+{\omega }_{-})∕2$. The membrane roughness is explicitly calculated as a function of $X$ and $\omega$. It is shown that the interplay of active and thermal fluctuations is subtle and that it is, in general, not possible to describe the active fluctuations in terms of an effective temperature.

Films of oriented deoxyribonucleic acid (DNA), prepared by the wet spinning method, have been studied using inelastic x-ray scattering. Spectra were recorded within the range of energy transfers $-30<\hslash \omega <30\mathrm{meV}$ at momentum transfers $\hslash \mathbf{Q}$ ranging from 2.5 to $30{\mathrm{nm}}^{-1}$ whereby the direction of $\mathbf{Q}$ essentially coincided with the helical axis. Measurements at ambient temperature cover samples in the $A$, $B$, $C$, and $D$ conformations of DNA. Within the limits of the instrumental resolution, the spectra were analyzed by the response of a damped harmonic oscillator delivering dispersion and damping of modes having displacements with nonzero projections onto $\mathbf{Q}$, i.e., essentially the compression waves traveling along the helical axis. The longitudinal speed of sound resulting from the sinusoidal dispersion varies only weakly with conformation. Our sound speed values are compared to results from Brillouin spectroscopy. The dispersion curves exhibit a minimum at about the inverse rise per residue, which—together with strong elastic scattering—reflect the large degree of disorder. Overdamping of the modes is observed for $Q>5{\mathrm{nm}}^{-1}$. The possibility that the observed large damping parameters are due to several contributing modes is discussed in terms of a simple model calculation for an idealized double helix. Whereas the quasicrystalline approximation for an effective disordered chain could well describe the sinusoidal dispersion, it fails to reproduce the observed damping by one order of magnitude. Our results indicate that the high-frequency dynamics of DNA is liquidlike and is most appropriately described by instantaneous normal modes of short correlation length.

The stochastic Feller neuronal model is studied, and estimators of the model input parameters, depending on the firing regime of the process, are derived. Closed expressions for the first two moments of functionals of the first-passage time (FTP) through a constant boundary in the suprathreshold regime are derived, which are used to calculate moment estimators. In the subthreshold regime, the exponentiality of the FTP is utilized to characterize the input parameters. The methods are illustrated on simulated data. Finally, approximations of the first-passage-time moments are suggested, and biological interpretations and comparisons of the parameters in the Feller and the Ornstein-Uhlenbeck models are discussed.

We diagnose the self-channeled propagation of intense femtosecond pulses over an extended distance in a ${\mathrm{N}}_2\mathrm{O}$ cluster gas target using high resolution kilovolt x-ray pinhole images of the channel and spatially resolved x-ray spectra. The x-ray images are consistent with femtosecond optical scattering, shadowgraphy, and interferometry images. We observe extended plasma channels $(\sim 9\mathrm{mm})$ limited either by the cluster jet length or by absorption, for injected laser intensities in the range of ${10}^{16}–{10}^{17}\mathrm{W}∕{\mathrm{cm}}^2$. Spectral line shapes for the OVII $1s^2\text{−}1s3p$ and OVIII $1s\text{−}2p$ transitions (at 1.8627 and $1.8969\mathrm{nm}$, respectively) show significant broadening to the blue side and with truncated emission on the red side. We attribute this effect to Doppler blueshifted emission from fast ions from exploding clusters moving toward the spectrometer; redshifted emission from the opposite side of the cluster is absorbed.