Non-collective Modes Research Articles

Electric Switching Behaviors and Dielectric Relaxation Properties in Ferroelectric, Antiferroelectric, and Paraelectric Smectic Phases of Bent-Shaped Dimeric Molecules.

This study reports the electric switching behaviors and dielectric properties of the ferroelectric smectic-A (SmAPF), anti-ferroelectric smectic-A (SmAPA), anti-ferroelectric SmCAPA, and smectic-A (SmA) phases formed by mixing the bent-shaped dimeric molecules, α,ω-bis(4-alkoxyanilinebenzylidene-4'-carbonyloxy)pentanes. These four phases each show characteristic features. The SmAPF shows a low threshold electric field for ferroelectric switching and a large dielectric strength due to the collective fluctuation mode of dipoles at around 500 Hz. Both the threshold electric field and dielectric strength are strongly dependent on the cell thickness. The threshold field decreases to 0.1 V μm-1, and the dielectric strength increases up to a huge value of 10,000 as the cell thickness increases up to 80 μm. The SmAPA also shows a similar collective mode at around 2 kHz with a relatively small dielectric strength (around 200), which may be induced by the anti-phase rotation of dipoles in adjacent layers. In these collective modes, the dielectric strength is found to be inversely proportional to the switching threshold field. On the other hand, another anti-ferroelectric SmCAPA as well as the paraelectric SmA show only the non-collective mode (i.e., rotational relaxation of individual molecules around their short axes) at a high frequency of around 100 kHz.

One-meson-loop NJL model: Effect of collective and noncollective excitations on the quark condensate at finite temperature

We explore the effect of including quantum fluctuations in the two flavor Nambu$-$Jona-Lasinio model at finite temperature. This is accomplished, in a symmetry preserving way, by including collective and noncollective modes in the one-meson-loop gap equation which originate from poles and branch cuts in the complex plane, respectively. The inclusion of a boson cutoff, $\Lambda_b$, is necessary to regularize the meson-loop momenta. This new parameter is used to study the influence of going beyond the usual mean field approximation in the quark condensate. As the temperature increases, chiral symmetry tends to get restored, the collective modes melt and only noncollective modes contribute to the quark condensate. With the inclusion of such modes, the quark condensate at finite temperature has a different behavior than at mean field level that will be explored.

Lifetimes of the 41+ states of Po206 and Po204 : A study of the transition from noncollective seniority-like mode to collectivity

Lifetimes of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mn>4</mml:mn><mml:mn>1</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:math> states of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Po</mml:mi><mml:mprescripts /><mml:none /><mml:mn>206</mml:mn></mml:mmultiscripts></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Po</mml:mi><mml:mprescripts /><mml:none /><mml:mn>204</mml:mn></mml:mmultiscripts></mml:math> : A study of the transition from noncollective seniority-like mode to collectivity

Fast neutrino flavor conversion: collective motion vs. decoherence

In an interacting neutrino gas, flavor coherence becomes dynamical and can propagate as a collective mode. In particular, tachyonic instabilities can appear, leading to “fast flavor conversion” that is independent of neutrino masses and mixing angles. On the other hand, without neutrino-neutrino interaction, a prepared wave packet of flavor coherence simply dissipates by kinematical decoherence of infinitely many non-collective modes. We reexamine the dispersion relation for fast flavor modes and show that for any wavenumber, there exists a continuum of non-collective modes besides a few discrete collective ones. So for any initial wave packet, both decoherence and collective motion occurs, although the latter typically dominates for a sufficiently dense gas. We derive explicit eigenfunctions for both collective and non-collective modes. If the angular mode distribution of electron-lepton number crosses between positive and negative values, two non-collective modes can merge to become a tachyonic collective mode. We explicitly calculate the interaction strength for this critical point. As a corollary we find that a single crossing always leads to a tachyonic instability. For an even number of crossings, no instability needs to occur.

The interaction of intense ultrashort laser pulses with cryogenic He planar jets

We study the interaction of intense ultrashort laser pulses with cryogenic He planar jets, i.e., slabs, using 2D3V relativistic particle-in-cell simulations. Of particular interest are laser intensities W cm−2, pulse lengths ≤100 fs, and the wave length regime ∼800 nm for which the slabs are initially transparent and subsequently inhomogeneously ionized. Pulses W cm−2 are found to drive ionization along the slab and outside the laser spot, the ionization front propagates along the slab at a considerable fraction of the speed of light. Within the ionized region, there is a highly transient field which is a result of the charge-neutralizing disturbance at the slab-vacuum interface and which may be interpreted in terms of a two-surface-wave decay. The ionized region is predicted to reach solid-like densities and temperatures of few to hundreds of eV, i.e., it belongs to warm and hot dense matter regimes. Such extreme conditions are relevant for high-energy densities as found, e.g., in shock-wave experiments and inertial confinement fusion studies. The temporal evolution of the ionization is studied considering theoretically a pump–probe x-ray Thomson scattering scheme. We observe plasmon and non-collective modes that are generated in the slab, and their amplitude is proportional to the ionized volume. Our theoretical findings could be tested at free-electron laser facilities such as FLASH and the European XFEL (Hamburg) and the LCLS (Stanford).

RELATIVISTIC CYCLOTRON INSTABILITY IN ANISOTROPIC PLASMAS

ABSTRACT A sufficiently large temperature anisotropy can sometimes drive various types of electromagnetic plasma micro-instabilities, which can play an important role in the dynamics of relativistic pair plasmas in space, astrophysics, and laboratory environments. Here, we provide a detailed description of the cyclotron instability of parallel propagating electromagnetic waves in relativistic pair plasmas on the basis of a relativistic anisotropic distribution function. Using plasma kinetic theory and particle-in-cell simulations, we study the influence of the relativistic temperature and the temperature anisotropy on the collective and noncollective modes of these plasmas. Growth rates and dispersion curves from the linear theory show a good agreement with simulations results.

Growth of spinodal instabilities in nuclear matter. II. Asymmetric matter

As an extension of our previous work, the growth of density fluctuations in the spinodal region of charge asymmetric nuclear matter is investigated in the basis of the stochastic mean-field approach in the non-relativistic framework. A complete treatment of density correlation functions are presented by including collective modes and non-collective modes as well.

Growth of spinodal instabilities in nuclear matter

Early growth of density fluctuations of nuclear matter in spinodal region is investigated employing the stochastic mean-field approach. In contrast to the earlier treatments in which only collective modes were included in the calculations, in the present work non-collective modes are also included, thus providing a complete treatment of the density correlation functions. Calculations are carried out for symmetric matter in non-relativistic framework using a semi-classical approximation.

Electron beam irradiation induced changes in the dielectric and electro-optical properties of a room temperature nematic display material 4-(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzoate (6CHBT)

Dielectric and electro-optical studies of a pure and electron beam irradiated liquid crystalline compound 4-(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzoate have been carried out. Dielectric anisotropy, relaxation frequency, activation energy and distribution parameter of an observed non-collective mode corresponding to the molecular rotation about the short axes have been determined as a function of temperature and irradiation dose whereas threshold and switching voltages, splay elastic constant are determined as a function of irradiation dose. The relaxation frequencies initially decrease up to an irradiation dose of 40 kGy but thereafter increase due to irradiation. The activation energies are increased up to irradiation dose of 40 kGy but around 60 kGy dose of irradiation, we found decrease in the activation energy. Electro-optical measurements show the lowering of the threshold voltage with sufficient improvement in the steepness of the transmission voltage curves due to irradiation. The observed changes in the dielectric and electro-optical properties are related with the cis–trans isomerization due to electron beam irradiation.

Change in Dielectric and Electro-Optical Properties of a Nematic Material (6CHBT) Due to the Dispersion of BaTiO3 Nanoparticles

Thermodynamical, dielectric and electro-optical studies of a ferroelectric nanopowder (0.5 weight % Barium Titanate, BaTiO3) dispersed in 4-(trans-4-n-hexylcyclohexyl) isothiocyanatobenzoate (6CHBT) have been carried out. Change in the dielectric anisotropy, relaxation frequency and activation energy of an observed non-collective relaxation mode, threshold and switching voltages of electro-optic response (transmission-voltage characteristic) have been determined for the dispersed and pure samples in their nematic mesophases. These studies suggest that isotropic to nematic transition temperature for BaTiO3 dispersed 6CHBT is lowered. Changes in other properties are also discussed.

Electron beam irradiation-induced transformations in the electrical properties of 4’-octyl-4-cyanobiphenyl (8CB)

Changes in the dielectric and thermodynamical properties of electron beam-irradiated 4′-octyl-4-cyanobiphenyl (8CB) were studied. Irradiation-induced changes in the phase transition temperature, dielectric anisotropy, relaxation frequency and activation energy of an observed non-collective relaxation mode corresponding to molecular rotation about the short axis were determined in both nematic and smectic Ad phases. In the nematic phase, dielectric anisotropy increased for a small dose but decreased for a relatively high dose, whereas the relaxation frequency increased due to the irradiation. The pure and irradiated samples were characterised by UV–visible spectroscopy, Fourier transform infrared spectroscopy, gas chromatography, gas chromatography coupled with mass spectroscopy and pulse radiolysis. The observed changes in the dielectric parameters are related to the detachment of the CN group from some of the 8CB molecules due to the electron beam irradiation.

Temperature and bias electric field dependence of dielectric parameters of ( S )-(+)-4-(1-methylheptyloxycarbonyl) phenyl-4’-(3-butanoiloxyprop-1-oxy)biphenyl-4’-carboxylate

Temperature and bias electric field-dependent dielectric behaviour of an antiferroelectric liquid crystal compound was investigated in the frequency range 1 Hz to 10 MHz. The investigated compound possessed paraelectric (SmA*) and antiferroelectric () phases, in addition to a monotropic hexatic antiferroelectric smectic () phase. The dielectric study showed the presence of a soft mode in the SmA* phase. In the phase, two weak relaxation modes were observed for a planar orientation of molecules. These two modes were related to antiferroelectric ordering and the helical structure of the phase. Two modes, related to the hexatic order of the phase, were also observed and characterized in terms of bond-orientational order and coupling to the molecular tilt direction. With respect to non-collective modes, rotation around the short axis of molecules was detected in the SmA* and phases for a homeotropic alignment.

Practice Then and Now

Practice Then and Now "Practice theory" has a long history in philosophy, under various names, but current practice theory is a response to failures of projects of modernity or enlightenment which attempt to reduce science or politics to formulae. Heidegger, Oakeshott, and MacIntyre are each examples of philosophers who turned to practice conceptions. Foucault and Bourdieu made similar turns. Practice accounts come in different forms: some emphasize skill-like individual accomplishments, others emphasize the social character or presupposition-like character of the tacit conditions of activities. The Social Theory of Practices problematized the idea of sameness, the idea that participants in an activity had the same tacit possessions, which undermined the idea that practices were collective objects in which individuals participated. Later critics, such as Schatzki and Rouse, emphasized the normative coherence and character of practice, which has a collective aspect. Pickering and others suggested a notion of practices that was de-mentalized and focused on the objects that were part of the practical activity, which provided for the continuity and sociality of practice without collectivizing its mental content. The discovery of mirror neurons suggested a non-collective mode of transmission of practices. The implications of these developments can be seen in connection with ethics, where the conflict between the ethical and the practical can be understood in terms of the intrinsic conflict between the need to behave successfully and our learned ethical intuitions.

Competition between collective and noncollective excitation modes at high spin inBa124

High-spin states in Ba-124 were investigated in two experiments using the Ni-64(Ni-64, 4n)Ba-124 reaction at three different beam energies. In-beam gamma-ray coincidences were measured with the Eur ...

All-Atom Calculation of the Normal Modes of Bacteriorhodopsin Using a Sliding Block Iterative Diagonalization Method.

Conventional normal-mode analysis of molecular vibrations requires computation and storage of the Hessian matrix. For a typical biological system such storage can reach several gigabytes posing difficulties for straightforward implementation. In this work we discuss an iterative block method to carry out full diagonalization of the Hessian while only storing a few vectors in memory. The iterative approach is based on the conjugate gradient formulation of the Davidson algorithm for simultaneous optimization of L roots, where in our case 10 < L < 300. The procedure is modified further by automatically adding a new vector into the search space for each locked (converged) root and keeping the new vector orthogonal to the eigenvectors previously determined. The higher excited states are then converged with the orthonormality constraint to the locked roots by applying a projector which is carried out using a read-rewind step done once per iteration. This allows for convergence of as many roots as desired without increasing the computer memory. The required Hessian-vector products are calculated on the fly as follows, Kp = dgp/dt, where K is the mass weighted Hessian, and gp is the gradient along p. The method has been implemented into the TINKER suite of molecular design codes. Preliminary results are presented for the normal modes of bacteriorhodopsin (bR) up to 300 cm(-)(1) and for the high frequency range between 2840 and 3680 cm(-)(1). There is evidence of a highly localized, noncollective mode at ∼1.4 cm(-)(1), caused by long-range interactions acting between the cytoplasmic and extracellular domains of bR.

Rotational bands and noncollective structures in85Zr

The structure of the transitional nucleus ${}^{85}\mathrm{Zr}$ has been investigated through spectroscopy and lifetime measurements. States up to spins of $\frac{45}{2}\ensuremath{\Elzxh}$ and $\frac{39}{2}\ensuremath{\Elzxh}$ in the positive and negative parity bands, respectively, have been established through $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence measurements. The results indicate that there is a competition between collective and noncollective modes of excitation. Two pronounced band crossings have been observed in the positive parity sequence, while none is evident in the negative parity states. The pattern of shape evolution in the positive and negative parity bands is markedly different. The results of the experiments qualitatively corroborate the predictions of total Routhian surface calculations.

Dielectric Investigations of a Chiral Monomer and Side Chain Polymer

The dynamics of different collective and non-collective modes in a chiral liquid crystalline monomer and its side-chain polymer have been studied by dielectric spectroscopy. The monomer system enabled us to study the temperature dependence of the dielectric permittivity when crossing the transition from the SmA* phase directly to the crystal phase. Although the polymer system has a SmA* to SmC* transition, we found that the temperature dependence of the dielectric permittivity is radically different from the normally observed behaviour in these phases. The contributions from the soft and Goldstone modes, respectively, are extremely low, a result which is related to the very low value of the spontaneous polarization of the material. At lower temperatures in the SmC* phase, a very weak dielectric absorption located around 50 kHz is observed and discussed in terms of the liberational motion of side chains around the polymer main chain.

Interplay of collective and noncollective modes at low excitation energy in spherical nuclei

Interplay of collective and noncollective modes at low excitation energy in spherical nuclei

Kinetic theory of low-frequency Alfvén modes in tokamaks

The kinetic theory of low-frequency Alfvén modes in tokamaks is presented. The inclusion of both diamagnetic effects and finite core-plasma ion compressibility generalizes previous theoretical analyses (Tsai S T and Chen L 1993 Phys. Fluids B 5 3284) of kinetic ballooning modes and clarifies their strong connection to beta-induced Alfvén eigenmodes. The derivation of an analytic mode dispersion relation allows us to study the linear stability of both types of modes as a function of the parameters characterizing the local plasma equilibrium and to demonstrate that the most unstable regime corresponds to a strong coupling between the two branches due to the finite thermal ion temperature gradient. In addition, we also show that, under certain circumstances, non-collective modes may be present in the plasma, formed as a superposition of local oscillations which are quasi-exponentially growing in time.

Ferroelectric and antiferroelectric modes in a new chiral thiobenzoate liquid crystal

Abstract This work concerns the dielectric relaxation study of a new compound containing a thiobenzoate group with a fluoro substituent, which presents a sequence of antiferro-, ferri-, ferroelectric and SCα phases. The polar character of these phases was confirmed by optical hysteresis loops. Three modes with a nearly monodispersive behaviour were identified: the soft mode, the Goldstone mode and a non-collective azimuthal mode. The soft mode softens above the SA - SCα transition and triggers another mode within the SCα phase. Moreover, a collective anti-phase azimuthal mode appears and may be the origin of the onset of the antiferroelectric SCA phase. A strong influence of surface interactions on the thermal hysteresis and on the dielectric constant was evident in this work.