Dissipation Strength Research Articles

Surveying post-saddle nuclear dissipation with protons and α particles as probes

Using a Langevin model, we calculate post-saddle proton and α-particle multiplicities as a function of the post-saddle dissipation strength (β) for the heavy systems 234Cf, 240Cf, 246Cf and 240U. We find that, with increasing isospin of the system, the sensitivity of post-saddle light charged-particle multiplicities to β decreases considerably and, moreover, for 240U the charged-particle multiplicities are no longer sensitive to β. These results suggest that in order to determine the post-saddle friction strength more accurately by measuring the multiplicities of pre-scission protons and α particles, it is best to populate those heavy compound systems with low isospin.

Environmentally induced shift of the quantum arrival time

Using a simple model potential, we study the effects of weak Markovian dissipation on the quantum arrival time. The interaction with the environment is incorporated into the dynamics through a Markovian master equation of Lindblad type, which allows us to compare time-of-arrival distributions and approximate crossing probabilities for different dissipation strengths and temperatures. We also establish a connection to an earlier study where quantum tunneling with dissipation was investigated, which leads us to some conclusions concerning the formulation of the continuity equation in the Lindblad theory.

Is Diffusion Anomalous in Two-Dimensional Yukawa Liquids?

There have recently been many predictions of "superdiffusion" in two-dimensional strongly coupled Yukawa systems, both by computer simulations and in dusty plasma experiments, with substantially varying diffusion exponents. Here we show that the results crucially depend on the strength of dissipation and the time instant of the measurement. For sufficiently large friction even subdiffusion is possible. However, there are strong indications that, in the long-time limit, anomalous diffusion vanishes and the system returns to normal diffusion, for dissipative as well as for frictionless systems.

Transport in a Luttinger liquid with dissipation: Two impurities

We consider theoretically the transport in a one-channel spinless Luttinger liquid with two strong impurities in the presence of dissipation. As a difference with respect to the dissipation free case, where the two impurities fully transmit electrons at resonance points, the dissipation prevents complete transmission in the present situation. A rich crossover diagram for the conductance as a function of applied voltage, temperature, dissipation strength, Luttinger liquid parameter K and the deviation from the resonance condition is obtained. For weak dissipation and 1/2<K<1, the conduction shows a non-monotonic increase as a function of temperature or voltage. For strong dissipation the conduction increases monotonically but is exponentially small.

Coherent–incoherent transition in the spin–boson model with a super-ohmic bath

The temperature effect on tunnelling splitting in the spin–boson model with a super-ohmic bath is studied by the small polaron theory. The coherent–incoherent transition temperature is calculated and its dependence on dissipation strength and bare tunnelling splitting is analysed. In additional to the traditional transition point described in textbooks, a new kind of transition is found in the low dissipation region, showing different temperature dependence in the transition. The relation to the corresponding transition in the polaron–phonon system is also discussed.

Role of [formula omitted] in giant dipole resonance γ rays as a probe of post-saddle nuclear dissipation

Using a Langevin model, we calculate the post-saddle giant dipole resonance (GDR) γ-ray multiplicity of the heavy nuclei 240Cf, 246Cf, 252Cf, and 240U at various post-saddle dissipation strengths. We find that with increasing the neutron-to-proton ratio N/Z of the Cf system, the sensitivity of the post-saddle γ emission to the dissipation strength decreases significantly, and it is insensitive to nuclear dissipation for the case of the high N/Z240U. The results suggest that in experiments to accurately obtain information of the post-saddle dissipation strength by measuring the GDR γ-ray multiplicity evaporated during the fission process of heavy nuclei, it is best to populate compound systems with low N/Z.

Isospin effects on light charged particles as probes of nuclear dissipation

The multiplicities of postsaddle protons and {alpha} particles of the heavy systems {sup 234}Cf, {sup 240}Cf, {sup 246}Cf, and {sup 240}U as functions of the postsaddle dissipation strength are calculated in the framework of a dynamical Langevin model coupled with a statistical decay model. It is found that with increasing isospin of the Cf system, the sensitivity of the postsaddle proton and {alpha}-particle multiplicity to the dissipation strength decreases substantially, and it disappears for the {sup 240}U system. We suggest that on the experimental side, to accurately probe the postsaddle dissipation strength by measuring the prescission proton and {alpha}-particle multiplicity, it is best to populate heavy compound systems with low isospin.

Surveying the role of excitation energy in probing nuclear dissipation

A dynamical Langevin model is employed to calculate the excess of the evaporation residue cross sections of the 194Pb nucleus over that predicted by the standard statistical model as a function of nuclear dissipation strength. It is shown that large excitation energy can increase the effects of nuclear dissipation on the excess of the evaporation residues and the sensitivity of this excess to the dissipation strength, and that more higher excitation energies have little contribution to further raising this sensitivity. These results suggest that on the experimental side, producing those compound systems with moderate excitation energy is sufficient for a good determination of the pre-saddle nuclear dissipation strength by measuring the evaporation residue cross section, and that forming an extremely highly excited system does not considerably improve the sensitivity of evaporation residues to the dissipation strength.

Estimation of the spatial decoherence time in circular quantum dots

We propose a simple phenomenological model to estimate the spatial decoherence time in quantum dots. The dissipative phase-space dynamics is described in terms of the density matrix and the corresponding Wigner function, which are derived from a master equation with Lindblad operators linear in the canonical variables. The formalism was initially developed to describe diffusion and dissipation in deep inelastic heavy-ion collisions, but an application to quantum dots is also possible. It allows us to study the dependence of the decoherence rate on the dissipation strength, the temperature, and an external magnetic field, which is demonstrated in illustrative calculations on a circular GaAs one-electron quantum dot.

Skyrmion in spinor condensates and its stability in trap potentials

A necessary condition for the existence of a skyrmion in two-component Bose-Einstein condensates with SU(2) symmetry was recently provided by two of the authors [I. F. Herbut and M. Oshikawa, Phys. Rev. Lett. 97, 080403 (2006)], by mapping the problem to a classical particle in a potential subject to time-dependent dissipation. Here we further elaborate this approach. For two classes of models, we demonstrate the existence of the critical dissipation strength above which the skyrmion solution does not exist. Furthermore, we discuss the local stability of the skyrmion solution by considering the second-order variation. A sufficient condition for the local stability is given in terms of the ground-state energy of a one-dimensional quantum-mechanical Hamiltonian. This condition requires a minimum number of bosons, for a certain class of the trap potential. In the optimal case, the minimum number of bosons can be as small $\ensuremath{\sim}{10}^{4}$.

Isospin effects on neutrons as a probe of nuclear dissipation

Based on a dynamical Langevin equation coupled with a statistical decay model, we calculate the excess of the prescission neutron multiplicity of the heavy nuclei $^{240}\mathrm{Cf}$, $^{246}\mathrm{Cf}$, $^{252}\mathrm{Cf}$, and $^{240}\mathrm{U}$ over that predicted by the standard statistical model as a function of the postsaddle dissipation strength. We find that with increasing isospin of the system, the sensitivity of the excess to the dissipation strength decreases substantially. Moreover, for $^{240}\mathrm{U}$, this excess is no longer sensitive to the nuclear dissipation. These results suggest that, on the experimental side, to accurately obtain information of the postsaddle dissipation strength by measuring the neutron multiplicity evaporated during the fission process of heavy nuclei, it is best to populate those compound systems with low isospin.

Lieb-Liniger model of a dissipation-induced Tonks-Girardeau gas

We show that strong inelastic interactions between bosons in one dimension create a Tonks-Girardeau gas, much as in the case of elastic interactions. We derive a Markovian master equation that describes the loss caused by the inelastic collisions. This yields a loss rate equation and a dissipative Lieb-Liniger model for short times. We obtain an analytic expression for the pair correlation function in the limit of strong dissipation. Numerical calculations show how a diverging dissipation strength leads to a vanishing of the actual loss rate and renders an additional elastic part of the interaction irrelevant.

Finite quantum dissipation: the challenge of obtaining specific heat

We consider a free particle coupled with finite strength to a bath and investigate the evaluation of its specific heat. A harmonic oscillator bath of Drude type with cutoff frequency ωD is employed to model an ohmic friction force with dissipation strength γ. Two scenarios for obtaining specific heat are presented. The first one uses the measurement of the kinetic energy of the free particle, while the second one is based on the reduced partition function. Both descriptions yield results which are consistent with the Third Law of thermodynamics. Nevertheless, the two methods produce different results that disagree even in their leading quantum corrections at high temperatures. We also consider the regime where the cutoff frequency is smaller than the friction strength, i.e. ωD< γ. There, we encounter puzzling results at low temperatures where the specific heat based on the thermodynamic prescription becomes negative. This anomaly is rooted in an ill-defined density of states of the damped free particle, which assumes unphysical negative values when γ/ωD> 1.

Landau–Zener tunnelling in dissipative circuit QED

We investigate the influence of temperature and dissipation on the Landau–Zener transition probability in circuit QED. Dissipation is modelled by coupling the transmission line to a bath of harmonic oscillators, and the reduced density operator is treated within Bloch–Redfield theory. A phase-space representation allows an efficient numerical implementation of the resulting master equation. It provides reliable results which are valid even for rather low temperatures. We find that the spin-flip probability as a function of temperature and dissipation strength exhibits a non-monotonic behaviour. Our numerical results are complemented by analytical solutions for zero temperature and for vanishing dissipation strength.

LANGEVIN DESCRIPTION OF THE EVAPORATION RESIDUE SPIN DISTRIBUTION

The recently measured spin distribution of evaporation residue cross section in the reaction 16 O + 184 W is studied in the framework of Langevin equations. Results show that the Langevin approach can describe this observable very well. Moreover, a pre-saddle nuclear viscosity coefficient of 5 × 1021 s-1 is extracted by comparing theoretical calculations with experimental data. We also explore the effect of the isospin of a system on the spin distribution by evaluating the evaporation residue spin distribution of nuclei 194 Pb , 200 Pb and 206 Pb . It has been found that with increasing the isospin of the system, the sensitivity of the evaporation residue spin distribution to the nuclear viscosity is decreased significantly. This result suggests that choosing a low-isospin compound system favors an accurate determination of pre-saddle dissipation strength.

Role of nuclear dissipation and entrance channel mass asymmetry in pre-scission neutron multiplicity enhancement in fusion-fission reactions

Pre-scission neutron multiplicities are measured for ${}^{12}\mathrm{C} + {}^{204}\mathrm{Pb}$ and ${}^{19}\mathrm{F} + {}^{197}\mathrm{Au}$ reactions at laboratory energies of 75--95 MeV for the $^{12}\mathrm{C}$ beam and 98--118 MeV for the $^{19}\mathrm{F}$ beam. The chosen projectile-target combinations in the present study lie on either side of the Businaro-Gallone mass asymmetry (${\ensuremath{\alpha}}_{\mathrm{BG}}$) and populate the $^{216}\mathrm{Ra}$ compound nucleus. The dissipation strength is deduced after comparing the experimentally measured neutron yield with the statistical model predictions which contains the nuclear viscosity as a free parameter. Present results demonstrate the combined effects of entrance channel mass asymmetry and the dissipative property of nuclear matter on the pre-scission neutron multiplicity in fusion-fission reactions.

DENSITY MATRIX ANALYSIS AND SIMULATION OF ELECTRON DYNAMICS IN MULTIPLE-BRIDGED DONOR/ACCEPTOR MOLECULES UNDER DISSIPATIVE ENVIRONMENTS

Feynman and Vernon path integral approach is adopted to investigate electron transfer dynamics in donor–bridge–acceptor molecules under dissipative environments. Especially, we focus on the solvent effect on the superexchange process of electron transfer. The results reveal that at high enough bridge energies or low enough temperature, electron can transfer with a superexchange mechanism no matter whether solvent is incorporated or not. However, the superexchange changes from coherent to incoherent limits when the dissipative strength increases, and electron transfer rates are much dependent on the dissipative strength.

Spin dependence of the modified Kramers width of nuclear fission

A statistical model calculation for the decay of a compound nucleus is presented where the compound nuclear spin dependence of the Kramers modified fission width is included. Specifically, the spin dependences of the frequencies of the harmonic oscillator potentials osculating the rotating liquid-drop model potential at equilibrium and saddle regions are considered. Results for the {sup 16}O+{sup 208}Pb system show that the energy dependence of the dissipation strength extracted from fitting experimental data is substantially reduced when the spin dependence of the frequencies is properly taken into account.

Quantum-classical transition and decoherence in dissipative double-well potential systems: Monte Carlo algorithm

Localization transition and coherent-incoherent crossover in dissipative double-well potential systems are studied by the path-integral Monte Carlo method. By using an improved Monte Carlo algorithm developed by the authors, we determine a critical dissipation strength of localization transition for Ohmic damping by utilizing finite-size scaling analysis of the Kosterlitz--Thouless transition. We also determine a coherent-incoherent crossover by the disappearance of a peak in dynamical response functions. The obtained phase diagram indicates that both localization and coherent-incoherent crossover occur even for the small-barrier limit, where the system cannot be truncated to a two-state model.

Analytic study of charge tunneling through DNA under dissipation

We investigate charge tunneling through the band gap of DNA molecules of homogeneous sequence. We find that the tunneling probability in the presence of energy dissipation linearly decays with molecule length, while a dissipation-free tunneling exponentially decays. For each, two characteristic decay scales emerge, $8{\ensuremath{\eta}}^{2}/{\ensuremath{\alpha}}^{2}$ and $\text{ln}\text{ }\ensuremath{\alpha}$. With $\ensuremath{\eta}$ taken as an energy damping parameter, we obtain the explicit dependence of $\ensuremath{\alpha}$ on the band structure and charge carrier energy. Those behaviors are also confirmed by numerical computations. Exploiting our results to interpret the recent experiments, we further draw a conjecture on a base (or sequence)-dependent dissipation strength, in relation to preferential occupation of counterions.