Energetic Instability Research Articles

Adsorption of Antifungal Drugs Inside Pristine and Functionalized Fullerenes and Nanotubes: DFT Investigation.

Econazole, sulconazole and tioconazole usage as antifungal agents is limited due to poor pharmacokinetic properties. Pristine and hydroxylated structures of the C240 fullerene and single walled carbon nanotube (SWCNT) were proposed as transporters of these imidazoles potentially enhancing their pharmacokinetics. To assess possibility of creation of the endohedral complexes of the azoles and carbon nanostructures, their adsorption and interaction energies were calculated with the hybrid exchange-correlation density functional B97-1 and 6-31(d,p) basis set. Interactions within the transporter - drug complexes were investigated with the Atoms in Molecules (AIM) Theory and Reduced Density Gradient (RDG). The adsorption energies of the studied azoles depend on type and surface modification of the transporter. Hydroxylation of the fullerene and nanotube surface makes an opportunity for chemisorption of the investigated antifungal drugs. The pristine and hydroxylated nanotube complexes exhibit thermodynamic stability. The complexes of the fullerenes are thermodynamically unstable but its kinetic stability could be significant thus allowing for the such structures to exist. The energetic instability would enhance liberation of the encapsulated molecule from the complex. It is advantageous in the context of drug release.

Dynamical and energetic instabilities of [formula omitted] spinor Bose–Einstein condensates in an optical lattice

The dynamical and energetic instabilities of the F=2 spinor Bose–Einstein condensates in an optical lattice are investigated theoretically and numerically. By analyzing the dynamical response of different carrier waves to an additional linear perturbation, we obtain the instability criteria for the ferromagnetic, uniaxial nematic, biaxial nematic and cyclic states, respectively. When an external magnetic field is taken into account, we find that the linear or quadratic Zeeman effects obviously affect the dynamical instability properties of uniaxial nematic, biaxial nematic and cyclic states, but not for the ferromagnetic one. In particular, it is found that the faster moving F=2 spinor BEC has a larger energetic instability region than lower one in all the four states. In addition, it is seen that for most states there probably exists a critical value kc>0, for which k>kc causes the energetic instability to arise under appreciative parameters.

Persistent currents in coherently coupled Bose-Einstein condensates in a ring trap

We study the stability of persistent currents in a coherently coupled quasi-2D Bose-Einstein condensate confined in a ring trap at T=0. By numerically solving Gross-Pitaevskii equations and by analyzing the excitation spectrum obtained from diagonalization of the Bogoliubov-de Gennes matrix, we describe the mechanisms responsible for the decay of the persistent currents depending on the values of the interaction coupling constants and the Rabi frequency. When the unpolarized system decays due to an energetic instability in the density channel, the spectrum may develop a roton-like minimum, which gives rise to the finite wavelength excitation necessary for vortex nucleation at the inner surface. When decay in the unpolarized system is driven by spin-density excitations, the finite wavelength naturally arises from the existence of a gap in the excitation spectrum. In the polarized phase of the coherently coupled condensate, there is an hybridization of the excitation modes that leads to complex decay dynamics. In particular, close to the phase transition, a state of broken rotational symmetry is found to be stationary and stable.

APTWG: The 5th Asia-Pacific Transport Working Group Meeting

This conference report gives a summary on the contributed papers and discussions presented at the 5th Asia-Pacific Transport Working Group Meeting held at Dalian, China from 9–12 June 2015. The main goal of the working group is to develop a predictive understanding of the basic mechanisms responsible for particle, momentum and energy transport in magnetically confined plasmas. The topics of the meeting in 2015 were organized under five main headings: (1) turbulence suppression and transport barrier formation, (2) effect of magnetic topology on MHD activity and transport, (3) non-diffusive contribution of momentum and particle transport, (4) non-local transport and turbulence spreading and coupling and (5) energetic particles and instability. The Young Researchers’ Forum which was held at this meeting is also described in this report.

First electron density fluctuation measurement via the HCOOH laser far-forward collective scattering on HL-2A tokamak

Based on the fact that the scattered power is proportional to the square of the electron density fluctuation amplitude, the four-channel formic acid (HCOOH, λ=432.5 um) laser interferometer has been firstly upgraded to measure the far-forward collective scattering (FCS) from the electron density fluctuation (ñe) on HL-2A tokamak in 2014–15 experimental campaign. The HCOOH-FCS system provides the line-integrated measurement of fluctuations covering the wavenumber range: k⊥ <1.6 cm−1 (perpendicular to the magnetic field). A lot of electron density fluctuations have been successfully measured by the HCOOH-FCS. In especial, the high-frequency density fluctuation (up to 500 kHz) induced by the core-localized Alfvénic eigenmode (AE) has been observed by the innermost scattering chord. The HCOOH-FCS system will be extremely helpful for the research of energetic particles instability on HL-2A.

Memory-induced sign reversals of the spatial cross-correlation for particles in viscoelastic shear flows

The behavior of shear-induced cross-correlation functions between particle fluctuations along orthogonal directions in the shear plane for harmonically trapped Brownian particles in a viscoelastic shear flow is studied. A generalized Langevin equation with a power-law-type memory kernel is used to model the complex structure of the viscoelastic media. Interaction with fluctuations of environmental parameters is modeled by a multiplicative white Gaussian noise, by an internal fractional Gaussian noise, and by an additive external white noise. It is shown that the presence of a memory has a profound effect on the behavior of the cross-correlation functions. Particularly, memory-induced reentrant sign reversals of the spatial cross-moment between orthogonal random displacements of a particle are established, i.e., an increase of the memory exponent can cause the sign reversal from positive to negative, but by a further increase of the memory exponent a reentrant transition from negative to positive values appears. Similarities and differences between the behavior of the models with additive internal and external noises are considered. It is shown that additive external and internal noises cause qualitatively different dependencies of the cross-correlation functions on the time lag. The occurrence of energetic instability due to the influence of multiplicative noise is also discussed.

Instability of the Superfluid Flow as Black-Hole Lasing Effect.

We show that the critical velocity of a superfluid flow through a penetrable barrier coincides with the onset of the analog black-hole lasing effect. This dynamical instability is triggered by modes resonating in an effective cavity formed by two horizons enclosing the barrier. The location of the horizons is set by v(x)=c(x), with v(x),c(x) being the local fluid velocity and sound speed, respectively. We compute the critical velocity analytically and show that it is univocally determined by the configuration of the horizons. In the limit of broad barriers, the continuous spectrum at the origin of the Hawking-like radiation and of the Landau energetic instability is recovered.

Coarsening dynamics at unstable crystal surfaces

In this paper we focus on crystal surfaces led out of equilibrium by a growth or erosion process. As a consequence of that the surface may undergo morphological instabilities and develop a distinct structure: ondulations, mounds or pyramids, bunches of steps, ripples. The typical size of the emergent pattern may be fixed or it may increase in time through a coarsening process which in turn may last forever or it may be interrupted at some relevant length scale. We study dynamics in three different cases, stressing the main physical ingredients and the main features of coarsening: a kinetic instability, an energetic instability, and an athermal instability.

APTWG: The 4th Asia-Pacific Transport Working Group Meeting

This conference report summarizes the contributions to, and discussions at, the 4th Asia-Pacific Transport Working Group Meeting held at Kyushu University, Japan, during 10–13 June 2014. The topics of the meeting were organized under five main headings: turbulence suppression and transport barrier formation, effect of magnetic topology on MHD activity and transport, non-diffusive contribution of momentum and particle transport, non-local transport and turbulence spreading and coupling, energetic particles and instability. The Young Researchers' Forum which was held in this meeting is also described in this report.

Trinucleotide duplex formation inside a confined nanospace under supercooled conditions

Self-assembly of nucleotides of fewer than three base pairs is often found in protein-nucleotide conjugations, despite their energetic instability, and is regarded as the potential starting point for the creation of artificial hydrogen-bonded supramolecular complexes. Here we report duplex formation of 3-mer DNA fragments confined within silica mesopores modified with a positively charged trimethyl aminopropyl monolayer, and their further stabilization under supercooled conditions (T<273 K). We load 3-mer DNA fragments with donor- or acceptor-dye into modified silica mesopores and examine their hybridization behaviours using FRET measurements. The FRET results clearly reveal that efficient duplex formation through at least two A-T base pairs can be achieved at 233 K. Enthalpy changes for duplex formation are found to be nearly equal between complementary and single-mismatched 3-mer DNA duplexes. These results confirm confined mesoscale cavities to be a novel low-temperature reaction space for hydrogen-bonded supramolecular complexes.

Effect of cation species on surface-induced phase transition observed for platinum complex anions in platinum electrodeposition using nanoporous silicon.

In an earlier work [K. Fukami et al., J. Chem. Phys. 138, 094702 (2013)], we reported a transition phenomenon observed for platinum complex anions in our platinum electrodeposition experiment using nanoporous silicon. The pore wall surface of the silicon electrode was made hydrophobic by covering it with organic molecules. The anions are only weakly hydrated due to their large size and excluded from the bulk aqueous solution to the hydrophobic surface. When the anion concentration in the bulk was gradually increased, at a threshold the deposition behavior exhibited a sudden change, leading to drastic acceleration of the electrochemical deposition. It was shown that this change originates from a surface-induced phase transition: The space within a nanopore is abruptly filled with the second phase in which the anion concentration is orders of magnitude higher than that in the bulk. Here we examine how the platinum electrodeposition behavior is affected by the cation species coexisting with the anions. We compare the experimental results obtained using three different cation species: K(+), (CH3)4N(+), and (C2H5)4N(+). One of the cation species coexists with platinum complex anions [PtCl4](2-). It is shown that the threshold concentration, beyond which the electrochemical deposition within nanopores is drastically accelerated, is considerably dependent on the cation species. The threshold concentration becomes lower as the cation size increases. Our theoretical analysis suggests that not only the anions but also the cations are remarkably enriched in the second phase. The remarkable enrichment of the anions alone would give rise to the energetic instability due to electrostatic repulsive interactions among the anions. We argue that the result obtained cannot be elucidated by the prevailing view based on classical electrochemistry. It is necessitated to consult a statistical-mechanical theory of confined aqueous solutions using a molecular model for water.

Hybrid Boltzmann–Gross-Pitaevskii theory of Bose-Einstein condensation and superfluidity in open driven-dissipative systems

We derive a theoretical model which describes Bose-Einstein condensation in an open driven-dissipative system. It includes external pumping of a thermal reservoir, finite lifetime of the condensed particles, and energy relaxation. The coupling between the reservoir and the condensate is described with semiclassical Boltzmann rates. This results in a dissipative term in the Gross-Pitaevskii equation for the condensate, which is proportional to the energy of the elementary excitations of the system. We analyze the main properties of a condensate described by this hybrid Boltzmann--Gross-Pitaevskii model, namely, dispersion of the elementary excitations, bogolon distribution function, first-order coherence, dynamic and energetic stability, and drag force created by a disorder potential. We find that the dispersion of the elementary excitations of a condensed state fulfills the Landau criterion of superfluidity. The condensate is dynamically and energetically stable as longs as it moves at a velocity smaller than the speed of excitations. First-order spatial coherence of the condensate is found to decay exponentially in one dimension and with a power law in two dimensions, similarly with the case of conservative systems. The coherence lengths are found to be longer due to the finite lifetime of the condensate excitations. We compare these properties with those of a condensate described by the popular ``diffusive'' models in which the dissipative term is proportional to the local condensate density. In the latter, the dispersion of excitations is diffusive which as soon as the condensate is put into motion implies finite mechanical friction and can lead to an energetic instability.

Memory effects for a trapped Brownian particle in viscoelastic shear flows

The long-time limit behavior of the positional distribution for an underdamped Brownian particle in a fluctuating harmonic potential well, which is simultaneously exposed to an oscillatory viscoelastic shear flow is investigated using the generalized Langevin equation with a power-law-type memory kernel. The influence of a fluctuating environment is modeled by a multiplicative white noise (fluctuations of the stiffness of the trapping potential) and by an additive internal fractional Gaussian noise. The exact expressions of the second-order moments of the fluctuating position for the Brownian particle in the shear plane have been calculated. Also, shear-induced cross correlation between particle fluctuations along orthogonal directions as well as the angular momentum are found. It is shown that interplay of shear flow, memory, and multiplicative noise can generate a variety of cooperation effects, such as energetic instability, multiresonance versus the shear frequency, and memory-induced anomalous diffusion in the direction of the shear flow. Particularly, two different critical memory exponents have been found, which mark dynamical transitions from a stationary regime to a subdiffusive (or superdiffusive) regime of the system. Similarities and differences between the behaviors of the models with oscillatory and nonoscillatory shear flow are also discussed.

Calculating the properties of a coreless vortex in a two-component condensate

In a two-component condensate, a vortex in one of the components will induce a density modulation in the other component. This density modulation is centered at the location of the vortex core and can take the form of a peak or trough in density. As such the total density is nonzero and the system is referred to as a ``coreless vortex.'' In this paper we give an analytical form, based on a variational calculation, for the coreless vortex in a miscible homogeneous condensate. In particular we can calculate the density of the coreless vortex at its core, the associated length scales, and the energetic instability of a multicharged coreless vortex.

Linear stability and nonlinear dynamics of the fishbone mode in spherical tokamaks

Extensive linear and nonlinear simulations have been carried out to investigate the energetic particle-driven fishbone instability in spherical tokamak plasmas with weakly reversed q profile and the qmin slightly above unity. The global kinetic-MHD hybrid code M3D-K is used. Numerical results show that a fishbone instability is excited by energetic beam ions preferentially at higher qmin values, consistent with the observed appearance of the fishbone before the “long-lived mode” in MAST and NSTX experiments. In contrast, at lower qmin values, the fishbone tends to be stable. In this case, the beam ion effects are strongly stabilizing for the non-resonant kink mode. Nonlinear simulations show that the fishbone saturates with strong downward frequency chirping as well as radial flattening of the beam ion distribution. An (m, n) = (2, 1) magnetic island is found to be driven nonlinearly by the fishbone instability, which could provide a trigger for the (2, 1) neoclassical tearing mode sometimes observed after the fishbone instability in NSTX.

Dynamic phase diagram for the quantum phase model

We address the stability of superfluid currents in a system of interacting lattice bosons. We consider various Gutzwiller trial states for the quantum phase model which provides a good approximation for the Bose-Hubbard model in the limit of large interactions and boson populations. We thoroughly analyze the current-carrying stationary states of the dynamics ensuing from a Gaussian ansatz, and derive analytical results for the critical lines signaling their modulational and energetic instability, as well as the maximum of the carried current. We show that these analytical results are in good qualitative agreement with those obtained numerically in previous works on the Bose-Hubbard model, and in the present work for the quantum phase model.

Supercurrent and dynamical instability of spin-orbit-coupled ultracold Bose gases

We investigate the stability of supercurrents in a Bose-Einstein condensate with one-dimensional spin-orbit and Raman couplings. The consequence of the lack of Galilean invariance is explicitly discussed. We show that in the plane-wave phase, characterized by a uniform density, the supercurrent state can become dynamically unstable, the instability being associated with the occurrence of a complex sound velocity, in a region where the effective mass is negative. We also discuss the emergence of energetic instability in these supercurrent states. We argue that both the dynamical and the energetic instabilities in these systems can be generated experimentally through excitation of the collective dipole oscillation.

Energetic instability of polygonal micro- and nanowires

Micro- and nanowires are commonly used in biological sciences, micro- and nanoelectronics, and optoelectronics, and their morphological stability needs to be understood and controlled. Previous linear-stability analyses of infinitely-long circular wires have shown that the critical wavelength for instability approaches infinity if the wire surface is facetted. Thus, Rayleigh’s instability is suppressed in finite facetted circular wires and possibly in finite facetted polygonal wires. This work studies the energetic stability of long facetted regular-polygonal and rectangular wires that are not in thermodynamic equilibrium. The long wire is assumed to break up into N identical equilibrium crystals with the same cross-sectional shape as the wire. The length of the wire is Nλ and that of the crystal is L. The surface energy of the system is found to decrease after breakup if λ/L&amp;gt;9/4-2/3N+… for N ≫ 1. This instability criterion holds for regular-polygonal wires of arbitrary sides and for rectangular wires of arbitrary aspect ratios. The criterion also applies to infinitely long equilibrium wires if N is set to infinity. Previous experimental studies of pore channels in sapphire have revealed that the facetted channels break up not by Rayleigh’s instability, but by tip shedding. The observed “crystal” separation to length ratio agrees with the above instability criterion.

Bose—Einstein Condensates in Optical Lattices with Higher-Order Interactions

The higher-order interactions of Bose-Einstein condensate in multi-dimensional optical lattices are discussed both analytically and numerically. It is demonstrated that the effects of the higher-order atomic interactions on the sound speed and the stabilities of Bloch waves strongly depend on the lattice strength. In the presence of higher-order effects, tighter and high-dimensional lattices are confirmed to be two positive factors for maintaining the system's energetic stability, and the dynamical instability of Bloch waves can take place simultaneously with the energetic instability. In addition, we find that the higher-order interactions exhibit a long-range behavior and the long-lived coherent Bloch oscillations in a tilted optical lattice exist. Our results provide an effective way to probe the higher-order interactions in optical lattices.

Interdependence of structure and chemical order in high symmetry (PdAu)N nanoclusters

In this work we study the stabilities of high-symmetry AuN, PdN and (AuPd)N clusters, for N < 1500, using mathematical constructs, a semi-empirical potential with two different parameter sets, and a quasi-Newtonian minimisation technique. For PdN clusters, both parameter sets tested result in preferences for icosahedral (Ih) structures for N < 1000 over other high-symmetry 12-vertex geometries; for AuN clusters we find a tendency towards face-centred cubic (FCC) structures at values of N lower than seen for PdN: parameter set I of Cleri and Rosato [Cleri and Rosato, Phys. Rev. B, 1993, 48, 22] gave a transition at N ≈ 650 to the I-Dh, whilst for parameter set II of Baletto et al. [Baletto et al., J. Chem. Phys., 2002, 116, 3856] this value was lower still. For (AuPd)N clusters we found that the preferred arrangement is (PdcoreAushell)N, with thin (monolayer) surface coverings of Au being most energetically favourable compared to the homogeneous clusters; however for parameter set II multiple layers of Au lead to energetic instability. (AucorePdshell)N clusters are not energetically favourable with thin coatings of Pd, however as the shell coating thickens so the stability improves. Ih structures are unfavourable compared to the Ino-decahedron and cuboctahedron for (AucorePdshell)N, whereas the FCC-type structures are strongly preferred for (PdcoreAushell)N. Overall, the strong tendency towards core–shell segregation is emphasised for parameter set I more than II, agreeing with previous work on smaller (AuPd)N clusters.