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H. Hofmann

Publications and source records attributed to H. Hofmann.

17 recordsLinked to original sources
arXiv2009

Polarization dependent spontaneous-emission rate of single quantum dots in photonic crystal membranes

We have measured the variation of the spontaneous emission rate with polarization for self-assembled single quantum dots in two-dimensional photonic crystal membranes. We observe a maximum anisotropy factor of 6 between the decay rates of the two bright exciton states. This large anisotropy is attributed to the substantially different projected local density of optical states for differently oriented dipoles in the photonic crystal.

quant-ph↗
arXiv2004

The importance of few-nucleon physics at low energy

This manuscript originated from the discussion at the workshop on the "Future of Few-body Low Energy Experimental Physics" (FFLEEP), which was held at the University of Trento on December 4-7, 2002 and has been written in its present form on March 19, 2003. It illustrates a selection of theoretical advancements in the nuclear few-body problem, including two- and many-nucleon interactions, the three-nucleon bound and scattering system, the four-body problem, the A-body (A$>$4) problem, and fields of related interest, such as reactions of astrophysical interest and few-neutron systems. Particular attention is called to the contradictory situation one experiences in this field: while theory is currently advancing and has the potential to inspire new experiments, the experimental activity is nevertheless rapidly phasing out. If such a trend will continue, advancements in this area will become critically difficult.

nucl-th↗
arXiv2003

Mean first passage time for nuclear fission and the emission of light particles

The concept of a mean first passage time is used to study the time lapse over which a fissioning system may emit light particles. The influence of the "transient" and "saddle to scission times" on this emission are critically examined. It is argued that within the limits of Kramers' picture of fission no enhancement over that given by his rate formula need to be considered.

nucl-th↗
arXiv2003

Mean first passage time for fission potentials having structure

A schematic model of over-damped motion is presented which permits one to calculate the mean first passage time for nuclear fission. Its asymptotic value may exceed considerably the lifetime suggested by Kramers rate formula, which applies only to very special, favorable potentials and temperatures. The additional time obtained in the more general case is seen to allow for a considerable increment in the emission of light particles.

nucl-th↗
arXiv2002

Time scales for fission at finite temperature

The concept of the "transient effect" is examined in respect of a "mean first passage time". It is demonstrated that the time the fissioning system stays inside the barrier is much larger than suggested by the transient time, and that no enhancement of emission of neutrons over that given by Kramers' rate formula ought to be considered.

nucl-th↗
arXiv2001

Nuclear fission: The "onset of dissipation" from a microscopic point of view

Semi-analytical expressions are suggested for the temperature dependence of those combinations of transport coefficients which govern the fission process. This is based on experience with numerical calculations within the linear response approach and the locally harmonic approximation. A reduced version of the latter is seen to comply with Kramers' simplified picture of fission. It is argued that for variable inertia his formula has to be generalized, as already required by the need that for overdamped motion the inertia must not appear at all. This situation may already occur above T=2 MeV, where the rate is determined by the Smoluchowski equation. Consequently, comparison with experimental results do not give information on the effective damping rate, as often claimed, but on a special combination of local stiffnesses and the friction coefficient calculated at the barrier.

nucl-th↗
arXiv2001

Self-consistent quantal treatment of decay rates within the perturbed static path approximation

The framework of the Perturbed Static Path Approximation (PSPA) is used to calculate the partition function of a finite Fermi system from a Hamiltonian with a separable two body interaction. Therein, the collective degree of freedom is introduced in self-consistent fashion through a Hubbard-Stratonovich transformation. In this way all transport coefficients which dominate the decay of a meta-stable system are defined and calculated microscopically. Otherwise the same formalism is applied as in the Caldeira-Leggett model to deduce the decay rate from the free energy above the so called crossover temperature $T_0$.

cond-mat↗
arXiv1999

On nuclear transport at small excitations

Numerical computations of transport coefficients at low temperatures are presented for shapes typically encountered in nuclear fission. The influence of quantum effects of the nucleonic degrees of freedom is examined, with pair correlations included. Consequences for global collective motion are studied for the case of the decay rate. The range of temperatures is specified above which this motion may be described as a quantal diffusion process.

nucl-th↗
arXiv1999

Pairing and shell effects in the transport coefficients of collective motion

The linear response approach to nuclear transport has been extended to pair correlations. The latter are treated within a mean field approximation to a pairing interaction with constant matrix elements $G$. The constraint of particle number conservation has been accounted for on a time dependent average, which leads to modified response functions, both in the pairing degree of freedom as well as in the shape variable. The former is expressed by the gap parameter $Δ$ and the latter by a $Q$ which specifies the elongation of a fissioning nucleus. The tensors for friction and inertia corresponding to these two collective coordinates are computed along the fission path of $^{224}Th$ for temperatures around $T=1 \mev$ and less. It is seen that dissipation decreases with decreasing temperature and increasing pairing gap and falls well below the values of common "macroscopic models". Both friction and inertia show a sensible dependence on the configurations of the mean field caused both by shell effects as well as by avoided crossings of single-particle levels.

nucl-th↗
arXiv1998

On the macroscopic limit of nuclear collective motion and its relation to chaotic behavior

In this note we concentrate on slow collective motion of isoscalar type at small but finite excitations, as given in nuclear fission, for instance. We report on microscopic results of transport coefficients, review how they depend on temperature and address the interplay of one and two body viscosity. We examine how transport properties might be influenced by chaotic behavior of nucleonic dynamics.

nucl-th↗
arXiv1998

Nuclear Transport at Small Excitations: Thermal and Quantal Aspects

The application of the locally harmonic approximation to large scale collective motion is briefly reviewed. Particular emphasis is paid to issues which might be useful in the more general context, or which are specific to our treatment, like there are: Self-consistency between collective and intrinsic motion, a $T$ dependent coupling constant, the inclusion of quantum effects for the dynamics of fluctuations. Finally, open problems are addressed, like the use of thermodynamic concepts at smaller excitations, the ergodicity question, the variation of transport coefficients with the nuclear shape due to shell structure, the limits of the LHA at smaller $T$.

nucl-th↗
arXiv1997

Warm nuclei: The transition from independent particle motion to collisional dominance

We study large scale collective dynamics of isoscalar type and examine the influence of interactions residual to independent particle motion. It is argued that for excitations which commonly are present in experimental situations such interactions must not be neglected. They even help to justify better the assumption of locality, both in time as well as in phase space, which is necessary not only for such classic approaches to collective motion as the "cranking model" but also for the more general picture of a transport process. With respect to dissipation, our results are contrasted with those of wall friction.

nucl-th↗
arXiv1997

Statistical fluctuations for the fission process on its decent from saddle to scission

We reconsider the importance of statistical fluctuations for fission dynamics beyond the saddle in the light of recent evaluations of transport coefficients for average motion. The size of these fluctuations are estimated by means of the Kramers-Ingold solution for the inverted oscillator, which allows for an inclusion of quantum effects.

nucl-th↗
arXiv1997

A self-consistent treatment of damped motion for stable and unstable collective modes

We address the dynamics of damped collective modes in terms of first and second moments. The modes are introduced in a self-consistent fashion with the help of a suitable application of linear response theory. Quantum effects in the fluctuations are governed by diffusion coefficients D_{μν}. The latter are obtained through a fluctuation dissipation theorem generalized to allow for a treatment of unstable modes. Numerical evaluations of the D_{μν} are presented. We discuss briefly how this picture may be used to describe global motion within a locally harmonic approximation. Relations to other methods are discussed, like "dissipative tunneling", RPA at finite temperature and generalizations of the "Static Path Approximation".

nucl-th↗
arXiv1997

A self-consistent treatment of the dynamics of stable and unstable collective modes

We address the dynamics of damped collective modes in terms of first and second moments. The modes are introduced in a self-consistent fashion with the help of a suitable application of linear response theory. Quantum effects in the fluctuations are governed by diffusion coefficients D_{μν}. The latter are obtained through a fluctuation dissipation theorem generalized to allow for a treatment of unstable modes. Numerical evaluations of the D_{μν} are presented. We discuss briefly how this picture may be used to describe global motion within a locally harmonic approximation.

nucl-th↗
arXiv1997

Transport coefficients for shape degrees in terms of Cassini ovaloids

Previous computations of the potential landscape with the shapes parameterized in terms of Cassini ovaloids are extended to collective dynamics at finite excitations. Taking fission as the most demanding example of large scale collective motion, transport coefficients are evaluated along a fission path. We concentrate on those for average motion, namely stiffness C, friction γand inertia M. Their expressions are formulated within a locally harmonic approximation and the help of linear response theory. Different approximations are examined and comparisons are made both with previous studies, which involved different descriptions of single particle dynamics, as well as with macroscopic models. Special attention is paid to an appropriate definition of the deformation of the nuclear density and its relation to that of the single particle potential. For temperatures above 3 MeV the inertia agrees with that of irrotational flow to less than a factor of two, but shows larger deviations below, in particular in its dependence on the shape. Also friction exhibits large fluctuations along the fission path for small excitations. They get smoothed out above 3 - 4 MeV where γattains values in the range of the wall formula. For T > (or=) 2 MeV the inverse relaxation time β= γ/M turns out to be rather insensitive to the shape and increases with T.

nucl-th↗