Nonequilibrium Spectrum Research Articles

Quantum Cascade Phenomenon inBi2Sr2CaCu2O8+δSingle Crystals

We study interlayer transport in Bi2Sr2CaCu2O(8+delta) cuprates, which represent stacks of atomic scale intrinsic Josephson junctions. A series of resonant dips in conductance is observed at condition when bremsstrahlung and recombination bands in nonequilibrium spectrum of Josephson junctions overlap. The phenomenon is explained in terms of self-detection of a new type of collective strongly nonequilibrium state in natural atomic superlattices, bearing certain resemblance with operation of a quantum cascade laser. Conclusions are supported by in situ generation-detection experiments and by numerical simulations.

MESOSCOPIC SPECTRA IN TWO-DIMENSIONAL DECAYING TURBULENCE

Two-dimensional decaying turbulence is simulated using a lattice Boltzmann model with the Bhatnagar–Gross–Krook collision operator. Auto-power spectra of the one-velocity particle distribution functions are presented. The relation between the spectrum of the kinetic energy and the spectra of the distribution functions is given. An interpretation of the non-equilibrium spectra as a measure of the dissipation in different scales is given. A peak in the spectrum of the resting particle distribution functions is observed exactly at the ultraviolet cutoff. It is shown that the peak can be associated with enhanced acoustic activity, which might be a numerical artifact or a consequence of the compressibility of the lattice Boltzmann fluid.

Mathematical simulation of the spectrum of a nonequilibrium laser plasma

A method is proposed for calculating the spectrum of a nonequilibrium plasma, which is based on a nonequilibrium collision—radiation model including all common line broadening mechanisms (natural, pressure, Doppler, and quadratic Stark effect broadening) and supplemented with the energy balance equations for electrons and ions. The nonequilibrium populations of the ground and excited states of neutral atoms and ions for an arbitrary instant of time are found by solving kinetic equations. The shape of each spectral line is determined by its central core calculated in the collision approximation up to the frequency boundary of its applicability, where the central core is 'joined' with the line wings calculated in the quasi-static approximation. The validity of this theoretical model is confirmed by simulations of a number of experimental studies of emission spectra under the conditions of a local thermodynamic equilibrium. It is shown that the calculated and experimental data obtained for the ground-state lines of the first carbon ion and neutral helium and argon atoms are in good agreement. The nonequilibrium spectrum of the optical breakdown in argon is calculated. Mathematical simulations showed that the intensities of nonequilibrium line spectra can be noticeably (by several times) lower than those of equilibrium spectra.

Difference between the spectra of acoustic gravity waves in daytime and nighttime hours due to nonequilibrium effects in the atmosphere

The singularities of the wave disturbance spectra of the nonequilibrium atmosphere in the range of acoustic gravity waves (AGWs) have been analyzed. Using the dispersion ratio for AGWs in the nonequilibrium atmosphere, it has been established that the spectra in the daytime and nighttime hours are different and this difference, caused by a nonequilibrium spectrum sensitivity to atmospheric temperature, can reach several percent in certain atmospheric regions. For the spectrum of the equilibrium model of the atmosphere, the difference between the daytime and nighttime spectra makes up several fractions of percent. As a result of the spectral treatment of variations in pressure and intensity of cosmic rays (CRs), it has been found out that the daytime AGW spectrum is higher-frequency than the nighttime spectrum. A comparison of the theoretical calculations of the AGW spectrum with observations has made it possible to distinguish the effect of nonequilibrium in the AGW spectral composition.

A new partially integrated transport model for subgrid-scale stresses and dissipation rate for turbulent developing flows

A new subgrid-scale turbulence model involving all the transport equations of the subgrid-scale stresses and including a dissipation rate equation is proposed for large-eddy simulation (LES) of unsteady flows which present nonequilibrium turbulence spectra. Such a situation in flow physics occurs when unsteadiness is created by forced boundary conditions, but also in more complex situations, when natural unsteadiness develops due to the existence of organized eddies. This latter phenomenon explains the instability found in a porous-walled chamber with mass injection. Due to the high value of Reynolds number, the presence of wall boundaries, and the use of relatively coarse grids, the spectral cutoff may be located before the inertial zone of the energy spectrum. The use of transport equations for all the subgrid-scale stress components allows us to take into account more precisely the turbulent processes of production, transfer, pressure redistribution effects, and dissipation, and the concept of turbulent viscosity is no longer necessary. Moreover, some backscatter effects can possibly arise. As a result of modeling in the spectral space, a formally continuous derivation of the model is obtained when the cutoff location is varied, which guaranties compatibility with the two extreme limits that are the full statistical Reynolds stress transport model of Launder and Shima and direct numerical simulation. In the present approach, due to the presence of the subgrid-scale pressure-strain correlation term in the stress equations, the new subgrid model is able to account for history and nonlocal effects of the turbulence interactions, and also to describe more accurately the anisotropy of the turbulence field. The present model is first calibrated on the well-known fully turbulent channel flow. For this test case, the LES simulation reveals that the computed velocities and Reynolds stresses agree very well with the DNS data. The application to the channel flow with wall mass injection which undergoes a transition process from laminar to turbulent regime and the development of natural unsteadiness is then considered for illustrating the potentials of the method. LES results are compared with experimental data including the velocity components, the turbulent stresses, and the transition location. A satisfactory agreement is obtained for both the mean quantities and the turbulent field. In addition, structural information of the flow is provided.

Diffusion and local deconfinement in relativistic systems

The evolution of net-proton rapidity spectra with s N N is heavy relativistic systems is proposed as an indicator for local equilibration, longitudinal expansion, and deconfinement. Bell-shaped distributions in central collisions at AGS energies and double-humped nonequilibrium spectra at SPS ishow substantial collective expansion. The broad midrapidity valley appearing at RHIC indicates longitudinal collective expansion of a locally equilibrated subsystem. In central Au-Au collisions at s N N = 200 GeV , fast local equilibration which is most likely due to deconfinement is estimated in a Relativistic Diffusion Model to occur for 14 per cent of the incoming baryons.

Theory of weak continuous measurements in a strongly driven quantum bit

Continuous spectroscopic measurements of a strongly driven superconducting qubit by means of a high-quality tank circuit (a linear detector) are under study. Output functions of the detector---namely, a spectrum of voltage fluctuations and an impedance---are expressed in terms of the qubit spectrum and magnetic susceptibility. The nonequilibrium spectrum of the current fluctuations in the qubit loop and the linear response function of the driven qubit coupled to a heat bath are calculated with Bloch-Redfield and rotating-wave approximations. Back-action effects of the qubit on the tank and the tank on the qubit are analyzed quantitatively. We show that the voltage spectrum of the tank provides detailed information about the frequency and decay rate of Rabi oscillations in the qubit. It is found that both an efficiency of spectroscopic measurement and measurement-induced decoherence of the qubit demonstrate a resonant behavior as the Rabi frequency approaches the resonant frequency of the tank. We determine conditions when the spectroscopic observation of the Rabi oscillations in the flux qubit with the tank circuit can be considered as a weak continuous quantum measurement.

De Wet Process for the Beneficiation of Zircon: Optimization of the Alkali Fusion Step

Milled zircon, d50 ≈ 9 μm, was fused with caustic soda pearls in open reaction vessels at temperatures between 650 and 850 °C. Fusion times of 1, 2, 4, 24, and 336 h (with intermittent milling) were investigated. The fusion products were characterized by X-ray diffraction and by wet-chemical analysis. For prolonged fusion times, the fusion products approached equilibrium compositions. However, the phases Na2ZrSiO5 and Na4Zr2Si3O12 observed at 850 °C did not form at 650 °C. Because these compounds do not hydrolyze completely in water, they limit the recovery of alkali in the sodium silicate product stream. With short fusion times (2 h), a nonequilibrium product spectrum is obtained. It is dominated by sodium zirconates and sodium silicates, allowing reasonable zirconia yields and high alkali recovery in a sodium silicate product stream. This allows efficient fusion of two moles of NaOH per mole of zircon at 850 °C with a zirconia yield of ca. 57% and an alkali recovery of ca. 78%.

LES of unsteady turbulence via a one-equation subgrid-scale transport model

The study of unsteady turbulent flows can lead to non-equilibrium turbulence spectra and there is a need to develop new subgrid-scale models that can be efficient in such situations. This is particularly necessary if the grid is coarse and the cutoff lies before the inertial zone of the spectrum. The model should be able to behave like a standard LES when the filter is sharp and like RANS models when the filter is wide, in a progressive way. The present work is a first step in that direction made by using transport equations for the subgrid-scale turbulence. We have used a transposition of the statistical model of Hassid and Poreh. The results of the simulations show that the one-equation model can predict the lag effects in the turbulent field response in pulsed channel flow, while the usual Smagorinsky model fails in several aspects.

Spin-Injection Quasiparticle Nonequilibrium in Cuprate/Manganite Heterostructures

Quasiparticle nonequilibrium due to spin-polarized current injection in perovskite superconductor/ferromagnet (S/F) thin-film heterostructures has been studied with the technique of cryogenic scanning tunneling spectroscopy. The spin-injection heterostructures consisted of epitaxial bilayers of the high-Tc superconductor YBa2Cu3O7 − δ and the half-metallic ferromagnet La0.7Ca0.3MnO3, with a spin-polarized quasiparticle current injected into the cuprate layer from the manganite layer. The tunneling conductance measured at 4.2 K on the cuprate layer showed a distinctly nonequilibrium quasiparticle spectrum as a result of the spin injection. Quantitative analysis of the tunneling spectral evolution versus the injection current yielded an estimate of the quasiparticle spin-diffusion depth and the spin-relaxation time in the superconducting cuprate.

Non-thermal pumping of the H-like ions by Z-pinch discharge

The paper describes experimental studies of x-ray pulses generated by implosions of current-sheaths onto sufficiently thick carbon fibers with the focus on a possibility of pumping the inverse population of hydrogen like ions levels. The stabilizing role of carbon fibers on the pinched-phase plasma is suggested and documented by the pinhole and streak camera pictures, PIN diode signals, and detected non-equilibrium spectrum of the Lyman lines of hydrogen-like carbon ions.

A Useful Approximation to the Cooling Coefficient of Trace Elements

Radiative cooling is an important ingredient in hydrodynamical models involving evolution of high-temperature plasmas. Unfortunately, calculating an accurate cooling coefficient generally requires the solution of over 100 differential equations to follow the ionization. We discuss here a simple two-parameter approximation for the cooling coefficient due to elements heavier than H and He, for the temperature range T = 104-108 K. Tests of the method show that it successfully tracks the ionization level in severe dynamical environments and accurately approximates the nonequilibrium cooling coefficient of the trace elements, usually to within 10% in all cases for which cooling is actually important. The error is large only when the temperature is dropping so rapidly due to expansion that radiative cooling is negligible, but even in this situation, the ionization level is followed sufficiently accurately. The current approximation is fully implemented in publicly available FORTRAN code. A second paper will discuss general approaches to approximation methods of this type, other realizations that could be even more accurate, and the potential for extension to calculations of nonequilibrium spectra.

Nonequilibrium plasmons in optically excited semiconductors

An analysis of the nonequilibrium plasmon spectrum of optically excited semiconductors is presented. It is shown that semiconductors with preexisting carrier populations, due, e.g., to a prepump or doping, may exhibit a rich collective excitation spectrum including additional plasmon modes. If these modes are weakly damped they give rise to an essential acceleration of thermalization processes. It is found that the most favorable conditions for this effect to appear are low temperature and p doping. These theoretical predictions are fully confirmed by results of comprehensive pump-probe experiments on bulk GaAs in the presence of a prepump and in doped samples.

Heat conduction in one-dimensional chains and nonequilibrium Lyapunov spectrum

We define and study the heat conductivity \ensuremath{\kappa} and the Lyapunov spectrum for a modified ``ding-a-ling'' chain undergoing steady heat flow. Free and bound particles alternate along a chain. In the present work, we use a linear gravitational potential to bind all the even-numbered particles to their lattice sites. The chain is bounded by two stochastic heat reservoirs, one hot and one cold. The Fourier conductivity of the chain decreases smoothly to a finite large-system limit. Special treatment of satellite collisions with the stochastic boundaries is required to obtain Lyapunov spectra. The summed spectra are negative, and correspond to a relatively small contraction in phase space, with the formation of a multifractal strange attractor. The largest of the Lyapunov exponents for the ding-a-ling chain appears to converge to a limiting value with increasing chain length, so that the large-system Lyapunov spectrum has a finite limit.

Non-equilibrium energy-loss spectra of fast ions propagating through matter

A method of coupled integral equations for partial (charge selected) energy distributions of fast ions passing through matter under conditions of strong electron exchange with a medium is applied to the theoretical interpretation of recent experiments [H. Ogawa et al., Nucl. Instrum. Methods B 115 (1996) 66] on the evolution of the shape of energy-loss spectra of fast Li 2+ ions passing through thin carbon foils.

On the stationary energy spectra for unidirectionally propagating internal gravity waves

Starting from the Euler equations in the Boussinesq approximation for a stratified fluid in two dimensions we establish a kinetic equation for a statistical ensemble of weakly non-linear internal gravity waves propagating in the same horizontal direction. We find a transformation of this equation to a standard form adopted in the weak turbulence theory and then, limiting ourselves to high vertical-low horizontal wavenumber waves, get equilibrium (Rayleigh-Jeans) and non-equilibrium (Kolmogorov) stationary energy spectra, among them a k −3 vertical wavenumber spectrum.

Energy distributions of 2D excitons in the presence of nonequilibrium phonons

For the first time we present the results of an analysis of the energy distribution function of 2D excitons for fixed nonequilibrium phonon spectra. The calculations are based on the kinetic equation for the interacting 2D exciton and 3D acoustic phonon gases. For the low-density exciton gas in GaAs/AlGaAs quantum wells (QW), we find a strong dependence of the exciton energy distribution on the QW width and nonequilibrium phonon spectrum.

Kinetic equations for electron and heavy fermion excitations

A system of kinetic equations describing the relaxation of electron and heavy quasy-particle excitations in the periodic Anderson Hamiltonian model is obtained. It is shown that a complete system of equations in the second and higher orders of perturbation theory in the spatially homogeneous case includes, in addition to kinetic equations for “light” and “heavy” quasi-particle, also a system of equations for the nonequilibrium spectrum of quasi-particles, the nonequilibrium renormalization parameter and the f-level of local electrons.

Field-induced nonequilibrium carrier distributions in GaAs probed by electronic Raman scattering.

Inelastic light scattering is used to probe nonequilibrium carrier distributions in active GaAs devices. Unlike many photoexcited nonequilibrium studies, this work examines carrier distributions in electrically biased ${\mathit{n}}^{+}$-${\mathit{n}}^{\mathrm{\ensuremath{-}}}$-${\mathit{n}}^{+}$ GaAs devices. Using a laser with photon energy lower than, but within 100 meV of, the direct band gap, single-particle and collective scattering events were observed in an effort to determine the hot-electron distribution function. Results are presented for various bias conditions. Due to the number and complexity of scattering mechanisms that contribute to the observed spectra, the effects of the applied field were more accurately analyzed by examining the differences between the equilibrium and nonequilibrium spectra. The carrier temperature and average drift velocity as a function of field strength were determined by fitting a theoretical model of the scattering cross section to the observed spectra.

Path-integral approach to transient transport of a double-barrier resonant-tunneling system.

Based on a many-body tunneling Hamiltonian for a double-barrier resonant-tunneling system, we demonstrate the evolution of the tunneling current in terms of the nonequilibrium distribution, spectrum, and dissipation of tunneling electrons. We find two time scales relevant to the transient process: the lifetime of resonance level (${\mathrm{\ensuremath{\tau}}}_{\mathit{R}}$) and the dissipation time (1/\ensuremath{\nu}) of electrons in the two leads. ${\mathrm{\ensuremath{\tau}}}_{\mathit{R}}$ is determined as a function of the dissipation of lead electrons as well as the barrier profile, including the bias effect.