Distribution of Partial Neutron Widths for Nuclei close to a Maximum of the Neutron Strength Function

For nuclei near a maximum of the neutron strength function, the secular dependence on energy E of s-wave partial neutron widths differs from the canonical form given by the square root of E. We derive the universal form of that dependence and show that it is expected to significantly influence the analysis of neutron resonance data.Comment: 4 page

First-Forbidden Beta Decay

In the last few years study of the laws of β decay has been the object of many experimental and theoretical investigations. As a consequence, the form of the nuclear (β-decay interaction is now well-established. We know that β decay violates parity conservation completely, and can be written V-A for electron (negaton) emission (TEN 58)

Random-Matrix Model for Thermalization

An isolated quantum system is said to thermalize if ${\rm Tr} (A \rho(t)) \to {\rm Tr} (A \rho_{\rm eq})$ for time $t \to \infty$. Here $\rho(t)$ is the time-dependent density matrix of the system, $\rho_{\rm eq}$ is the time-independent density matrix that describes statistical equilibrium, and $A$ is a Hermitean operator standing for an observable. We show that for a system governed by a random-matrix Hamiltonian (a member of the time-reversal invariant Gaussian Orthogonal Ensemble (GOE) of random matrices of dimension $N$), all functions ${\rm Tr} (A \rho(t))$ in the ensemble thermalize: For $N \to \infty$ every such function tends to the value ${\rm Tr} (A \rho_{\rm eq}(\infty)) + {\rm Tr} (A \rho(0)) g^2(t)$. Here $\rho_{\rm eq}(\infty)$ is the equilibrium density matrix at infinite temperature. The oscillatory function $g(t)$ is the Fourier transform of the average GOE level density and falls off as $1 / |t|$ for large $t$. With $g(t) = g(-t)$, thermalization is symmetric in time. Analogous results, including the symmetry in time of thermalization, are derived for the time-reversal non-invariant Gaussian Unitary Ensemble (GUE) of random matrices. Comparison with the ``eigenstate thermalization hypothesis'' of Ref.~\cite{Sre99} shows overall agreement but raises significant questions

Neutron Resonance Widths and the Porter-Thomas Distribution

Experimental evidence has recently put the validity of the Porter-Thomas distribution (PTD) for partial neutron widths into question. We identify two terms in the effective Hamiltonian that violate orthogonal invariance (the basis for the PTD). Both are due to the coupling to the decay channels. We show that realistic estimates for the coupling to the neutron channel and for non-statistical gamma decays yield significant modifications of the PTD.Comment: 5 pages, 2 figure

Transport equations for driven many-body systems

Transport equations for autonomous driven Fermionic quantum systems are derived with the help of statistical assumptions and of the Markov approximation. The statistical assumptions hold if the system consists of subsystems within which equilibration is sufficiently fast. The Markov approximation holds if the level density in each subsystem is sufficiently smooth in energy. The transport equation describes both, relaxation of occupation probability among subsystems at equal energy that leads to thermalization, and the transport of the system to higher energy caused by the driving force. The laser-nucleus interaction serves as an example for the applicability and flexibility of the approach.Comment: 12 pages no figure

Rate for Laser-Induced Nuclear Dipole Absorption

Using the Brink-Axel hypothesis we derive the rate $R$ for nuclear dipole excitation by a laser pulse carrying $N \gg 1$ photons with average energy $\hbar \omega_0 \approx 5$ MeV. As expected $R \propto (\hbar \omega_0)^3$. The rate is also proportional to the aperure $\alpha$ of the laser pulse. Perhaps less expected is the fact that $R \propto N$, irrespective of the degree of coherence of the laser pulse. The expression for $R$, derived for a nearly stationary laser pulse, is valid also for short times and can, thus, be used in simulations via rate equations of multiple nuclear dipole excitations by a single pulse. The explicit dependence of $R$ on the parameters of the laser pulse and on nuclear parameters given in the paper should help to optimize experiments on laser-nucleus reactions.Comment: 12 pages, v2 slightly modified to match the published versio

Transmission phase of a quantum dot and statistical fluctuations of partial-width amplitudes

Experimentally, the phase of the amplitude for electron transmission through a quantum dot (transmission phase) shows the same pattern between consecutive resonances. Such universal behavior, found for long sequences of resonances, is caused by correlations of the signs of the partial-width amplitudes of the resonances. We investigate the stability of these correlations in terms of a statistical model. For a classically chaotic dot, the resonance eigenfunctions are assumed to be Gaussian distributed. Under this hypothesis, statistical fluctuations are found to reduce the tendency towards universal phase evolution. Long sequences of resonances with universal behavior only persist in the semiclassical limit of very large electron numbers in the dot and for specific energy intervals. Numerical calculations qualitatively agree with the statistical model but quantitatively are closer to universality.Comment: 8 pages, 4 figure

Laser-Nucleus Interactions: The Quasiadiabatic Regime

The interaction between nuclei and a strong zeptosecond laser pulse with coherent MeV photons is investigated theoretically. We provide a first semi-quantitative study of the quasiadiabatic regime where the photon absorption rate is comparable to the nuclear equilibration rate. In that regime, multiple photon absorption leads to the formation of a compound nucleus in the so-far unexplored regime of excitation energies several hundred MeV above the yrast line. The temporal dynamics of the process is investigated by means of a set of master equations that account for dipole absorption, stimulated dipole emission, neutron decay and induced fission in a chain of nuclei. That set is solved numerically by means of state-of-the-art matrix exponential methods also used in nuclear fuel burnup and radioactivity transport calculations. Our quantitative estimates predict the excitation path and range of nuclei reached by neutron decay and provide relevant information for the layout of future experiments.Comment: 14 pages, 9 figures; v2 minor modifications in text, results unchange

Transmission phase of a quantum dot: Testing the role of population switching

We propose a controlled experiment to clarify the physical mechanism causing phase lapses of the amplitude for electron transmission through nanoscale devices. Such lapses are generically observed in valleys between adjacent Coulomb--blockade peaks. The experiment involves two quantum dots embedded in the same arm of an Aharonov--Bohm interferometer. It offers a decisive test of "population switching", one of the leading contenders for an explanation of the phenomenon.Comment: 4 pages, 4 figure