Search Results (131 total)

Motivated by the recent controversy surrounding the Kerr effect measurements in strontium ruthenate [J. Xia , Phys. Rev. Lett. 97, 167002 (2006)], we examine the electromagnetic response of a clean chiral p-wave superconductor. When the contributions of the collective modes are accounted for, the Hall response in a clean chiral superconductor is smaller by several orders of magnitude than previous theoretical predictions and is too small to explain the experiment. We also uncover some unusual features of the collective modes of a chiral superconductor, namely, that they are not purely longitudinal and couple to external transverse fields.

We investigate the complexity of the dynamics of two mutually coupled systems with internal delays and vary the coupling delay over 4 orders of magnitude. Karhunen-Loève decomposition of spatiotemporal representations of fiber laser intensity data is performed to examine the eigenvalue spectrum and significant orthogonal modes. We compute the Shannon information from the eigenvalue spectra to quantify the dynamical complexity. A reduction in complexity occurs for short coupling delays while a logarithmic growth is observed as the coupling delay is increased.

We investigate the spatiotemporal dynamics of a large array of laser oscillators. The oscillators are locally coupled and their natural frequencies are randomly detuned. We show that synchronization of the array elements results in localized excitations wandering along well-defined trajectories.

We propose a basic mechanism for isochronal synchrony and communication with mutually delay-coupled chaotic systems. We show that two Ikeda ring oscillators, mutually coupled with a propagation delay, synchronize isochronally when both are symmetrically driven by a third Ikeda oscillator. This synchronous operation, unstable in the two delay-coupled oscillators alone, facilitates simultaneous, bidirectional communication of messages with chaotic carrier wave forms. This approach to combine both bidirectional and unidirectional coupling represents an application of generalized synchronization using a mediating drive signal for a spatially distributed and internally synchronized multicomponent system.

We experimentally and numerically demonstrate the dual synchronization of chaos in two pairs of one-way-coupled Mackey-Glass electronic circuits with time-delayed feedback. The outputs of the two drive circuits are mixed and used both for the feedback signal to the two drive circuits and for the transmission signal to the two response circuits. We investigate the regions for achieving dual synchronization of chaos when the delay time is mismatched between the drive and response circuits.

Simultaneous measurement of both neutrons and charged particles emitted in the reaction ⁶⁴Zn + ⁶⁴Zn at 45 MeV/nucleon allows comparison of the neutron to proton ratio at midrapidity with that at projectile rapidity. The evolution of N/Z in both rapidity regimes with increasing centrality is examined. For the completely reconstructed midrapidity material one finds that the neutron to proton ratio is above that of the overall ⁶⁴Zn + ⁶⁴Zn system. In contrast, the reconstructed ratio for the quasiprojectile is below that of the overall system. This difference provides the most complete evidence to date of neutron enrichment of midrapidity nuclear matter at the expense of the quasiprojectile.

We study spin-Hall effects in time-reversal-symmetry- (TRS-) broken systems such as triplet chiral superconductors and TRS-preserved ones such as graphene. For chiral triplet superconductors, we show that the edge states carry a quantized spin-Hall current in response to an applied Zeeman magnetic field B along the d vector [A. J. Leggett, Rev. Mod. Phys. 47, 331 (1975)], whereas the edge spin current for B⊥d is screened by the condensate. We also derive the bulk spin-Hall current for chiral triplet superconductors for arbitrary relative orientation of B and d and discuss its relation with the edge spin current. For TRS-invariant system graphene, we show that the bulk effective action, unlike its TRS-broken counterparts, does not support a SU(2) Hopf term but allows a crossed Hopf term in the presence of an external electromagnetic field, which yields a quantized bulk spin-Hall current in response to an electric field. We also present an analytical solution of the edge problem for armchair edges of graphene and contrast the properties of these edge states with their time-reversal-symmetry-broken counterparts in chiral superconductors. We propose possible experiments to test our results.

We investigate experimentally and theoretically the coalescence dynamics of two spreading droplets on a highly wettable substrate. Upon contact, surface tension drives a rapid motion perpendicular to the line of centers that joins the drops and lowers the total surface area. We find that the width of the growing meniscus bridge between the two droplets exhibits power-law behavior, growing at early times as t^1/2. Moreover, the growth rate is highly sensitive to both the radii and heights of the droplets at contact, scaling as h_o^3/2/R_o. This size dependence differs significantly from the behavior of freely suspended droplets, in which the coalescence growth rate depends only weakly on the droplet size. We demonstrate that the scaling behavior is consistent with a model in which the growth of the meniscus bridge is governed by the viscously hindered flux from the droplets.

Synchronization of chaotic systems has been studied extensively, and especially, the possible applications to the communication systems motivated many research areas. We demonstrate the effect of the frequency bandwidth limitations in the communication channel on the synchronization of two unidirectionally coupled Mackey-Glass (MG) analog circuits, both numerically and experimentally. MG system is known to generate high dimensional chaotic signals. The chaotic signal generated from the drive MG system is modified by a low pass filter and is then transmitted to the response MG system. Our results show that the inclusion of the dominant frequency component of the original drive signals is crucial to achieve synchronization between the drive and response circuits. The maximum cross correlation and the corresponding time shift reveal that the frequency-dependent coupling introduced by the low pass filtering effect in the communication channel change the quality of synchronization.

We experimentally study the synchronization and the emergence of leader-follower dynamics in two time-delayed mutually coupled fiber ring lasers. We utilize spatiotemporal representations of time series to establish the roles of leader and follower in the synchronized dynamics.

We report a direct experimental observation of chaotic itinerancy in simultaneous measurements of the light intensity and voltage fluctuations of a laser diode exhibiting low-frequency fluctuations. The distribution of trajectories leading up to (following) an intensity dropout is computed from the experiment and reveals the presence of itinerant mechanisms before (after) dropout initiation. A phase space reconstruction of the trajectory for the optimal path of motion illustrates sudden shifts between low-dimensional attractor ruins and is shown to correspond to simulations of the laser intensity and carrier number.

We look for signals of criticality in multifragment production in heavy-ion collisions using model-independent universal fluctuations theory. The phenomenon is studied as a function of system size, bombarding energy, and impact parameter over a wide range of INDRA data. For very central collisions (b/b_max<0.1) we find evidence that the largest fragment in each event, Z_max, plays the role of an order parameter, defining two different regimes at low and high incident energy, respectively, according to the scaling properties of its fluctuations. Data for a wide range of system masses and incident energies collapse on to an approximately universal scaling function in each regime for the most central collisions. The forms of the scaling functions for the two regimes are established, and their dependence on the total mass and the bombarding energy is mapped out. Data suggest that these regimes are linked to the disappearance of heavy residues in central collisions.

⁵⁸Ni+⁵⁸Ni collisions at 32 MeV/nucleon have been studied with the 4π multidetector INDRA. The evolution from binary (dissipative) collisions to a fusionlike process is evidenced with decreasing impact parameter throughout a set of experimental observables within a discriminant analysis. Preequilibrium effects and characteristics of a single-source emission are discussed. A coexistence (bimodality) between two decay mechanisms is pointed out and examined in the context of a multiple-fragment (particle) emission.

We study the influence of asymmetric coupling strengths on the onset of light intensity oscillations in an experimental system consisting of two semiconductor lasers cross coupled optoelectronically with a time delay. We discover a scaling law that relates the amplitudes of oscillations and the coupling strengths. These observations are in agreement with a theoretical model. These results could be applicable to the population dynamics of other systems, such as the spread of disease in human populations coupled by migration.

By combining data from a charged particle ⁵⁸Ni+⁵⁸Ni experiment at 52 MeV/nucleon with an ³⁶Ar+⁵⁸Ni experiment at 50 MeV/nucleon for which free neutrons have been detected, an increase in the neutron to proton ratio of the whole nuclear material at midrapidity has been experimentally observed in the reaction ⁵⁸Ni+⁵⁸Ni at 52 MeV/nucleon. The neutron-to-proton ratio of the quasi-projectile emission is analyzed for the same reactions and is seen to decrease below the ratio of the initial system. Those observations suggest that an asymmetric exchange of neutrons and protons between the quasiprojectile and the midrapidity region exists.

The phase dynamics of a semiconductor laser with optical feedback is studied by construction of the Hilbert phase from its experimentally measured intensity time series. The Hurst exponent is evaluated for the phase fluctuations and grows from 0.5 to ∼0.7 (indicating fractional Brownian motion) as the feedback strength is increased. A comparison with numerical computations based on a delay-differential equation model shows excellent agreement and reveals the relative roles of spontaneous emission noise and deterministic dynamics for different feedback strengths.

The consistency of a nonlinear system's response to a repeated complex waveform drive signal is an important consideration in classical and quantum systems as diverse as lasers, neuronal networks, and manufacturing plants. We show from a consideration of different characteristic waveforms that there is typically an optimal drive amplitude for the most consistent response; internal noise sources dominate for small amplitude driving while deterministic system nonlinearity reduces consistency for large amplitudes. We test this general concept and its measurement experimentally and numerically on the specific example of a laser system.

A number of groups have reported measurements of the location in the parameter space of bubble size versus acoustic pressure amplitude of shape- and size-stable bubbles. For air/water systems, a general trend emerges: stable bubbles are found on one of two line paths in this space defined by their range of acoustic pressure. Bubbles on the higher-pressure path emit light. There have been few studies of the transition between these two paths. In this work we describe our observations of this transition regime. In this regime, a slow time scale oscillation (period 2–7 s) in the bubble size, position, and phase of flash timing can be observed. At lower dissolved gas concentrations, a hysteresis in the bubble size as a function of acoustic pressure is observed, complementing previous light intensity measurements reported in the literature.

We demonstrate generalized synchronization in a spatiotemporal chaotic system, a liquid crystal spatial light modulator with optoelectronic feedback.

Effects of in-medium cross sections and of optical potential on preequilibrium emission and on formation of a thermal source are investigated by comparing the results of transport simulations with experimental results from the p+¹⁹⁷Au reaction at 6.2–14.6  GeV∕c. The employed transport model includes light-composite-particle production and allows for inclusion of in-medium particle-particle cross-section reduction and of momentum dependence in the particle optical potentials. Compared to the past, the model incorporates improved parametrizations of elementary high-energy processes. The simulations indicate that the majority of energy deposition occurs during the first 25  fm∕c of a reaction. This is followed by a preequilibrium emission and readjustment of system density and momentum distribution toward an equilibrated system. Within different variants of calculations, the best agreement with data, on the d∕p and t∕p yield ratios and on the residue mass and charge numbers, is obtained at the time of about 65  fm∕c from the start of a reaction, for simulations employing reduced in-medium cross sections and momentum-dependent optical potentials. By that time, the preequilibrium nucleon and cluster emission, as well as mean field readjustments, drive the system to a state of depleted average density, ρ∕ρ₀∼1∕4–1∕3 for central collisions, and low-to-moderate excitation, i.e., the region of nuclear liquid-gas phase transition.

The excitation spectrum of a two-dimensional p_x+ip_y fermionic superfluid, such as a thin film of ³He-A, includes a gapless Majorana-Weyl fermion which is confined to the boundary by Andreev reflection. There is also a persistent ground-state boundary current which provides a droplet containing N particles with angular momentum ħN/2. Both of these boundary effects are associated with bulk Chern-Simons effective actions. We show that the gapless edge mode is required for the gauge invariance of the total effective action, but the same is not true of the boundary current.

We investigate correlations of the intensity fluctuations of two-dimensional arrays of nonidentical, locally coupled lasers, numerically and experimentally. We find evidence of a power-law dependence of spatial correlations as a function of laser pair distance (or coupling strength) near the phase-locking threshold.

We demonstrate generalized synchronization of chaos in a two-mode laser system. The total intensity of the laser output (the sum of the individual mode intensities) is used as the drive signal. This lumped variable transmitted to the identical response system does not generate identical synchronization. Generalized synchronization is observed instead of identical synchronization because of the hidden internal degrees of freedom.

Fusionlike events in ⁵⁸Ni+¹²C at 34.5 MeV/nucleon are isolated by use of the statistical discriminant analysis method applied to a set of 20 global variables in complete events with at least 90% of the total charge of the system. Two-fragment reduced-velocity correlation functions are measured for the emission of intermediate mass fragments (IMF’s) and compared to many-body trajectory calculations. Alpha particle emission sequence has been deduced as a function of excitation energy. At the highest excitations, α particles are emitted simultaneously with IMF’s on a time scale of about 200–300 fm/c. In less excited events, they are emitted on a longer time scale. This hybrid deexcitation mechanism, sequential and prompt, is corroborated by comparing the charge of the heaviest and the second heaviest fragments to predictions of GEMINI and SMM simulations.

Characteristics of the primary fragments produced in central collisions of ¹²⁹Xe+^natSn from 32 to 50 A MeV have been obtained. By using the correlation technique for the relative velocity between light charged particles (LCP) and fragments, we were able to extract the multiplicities and average kinetic energy of secondary evaporated LCP. We then reconstructed the size and excitation energy of the primary fragments. For each bombarding energy a constant value of the excitation energy per nucleon over the whole range of fragment charge has been found. This value saturates at 3A MeV for beam energies 39A MeV and above. The corresponding secondary evaporated LCP represent less than 40% of all produced particles and decreases down to 23% for 50A MeV. The experimental characteristics of the primary fragments are compared to the predictions of statistical multifragmentation model (SMM) calculations. Reasonable agreement between the data and the calculation has been found for any given incident energy. However SMM fails to reproduce the trend of the excitation function of the primary fragment excitation energy and the amount of secondary evaporated LCP’s.