Finite Energy Transfer Research Articles

Renormalization formalism for superconducting phase transition with inner-Cooper-pair dynamics

As charge carrier of the macroscopic superconductivity, the Cooper pair is a composite particle of two paired electrons, which has both center-of-mass and inner-pair degrees of freedom. In most cases, these two different degrees of freedom can be well described by the macroscopic Ginzburg-Landau theory and the microscopic Bardeen-Cooper-Schrieffer (BCS) theory, respectively. Near the superconducting phase transition where the Cooper pair is fragile and unstable because of the small binding energy, there are non-trivial couplings between these two different degrees of freedom due to such as finite energy and/or momentum transfer. The non-trivial couplings make the original derivation of the Ginzburg-Landau theory from the BCS theory fail in principle as where these two different degrees of freedom should not be decoupled. In this article, we will present a renormalization formalism for an extended Ginzburg-Landau action for the superconducting phase transition where there is finite energy transfer between the center-of-mass and the inner-pair degrees of freedom of Cooper pairs. This renormalization formalism will provide a theoretical tool to study the unusual dynamical effects of the inner-pair time-retarded physics on the superconducting phase transition.

Probing chirality fluctuations in molecules by nonlinear optical spectroscopy.

Symmetry breaking caused by geometric fluctuations can enable processes that are otherwise forbidden. An example is a perylene bisimide dyad whose dipole moments are perpendicular to each other. Förster-type energy transfer is thus forbidden at the equilibrium geometry since the dipolar coupling vanishes. Yet, fluctuations of the geometric arrangement have been shown to induce finite energy transfer that depends on the dipole variance, rather than the mean. We demonstrate an analogous effect associated with chirality symmetry breaking. In its equilibrium geometry, this dimer is non-chiral. The linear chiral response which depends on the average geometry thus vanishes. However, we show that certain 2D chiral optical signals are finite due to geometric fluctuations. Furthermore, the correlation time of these fluctuations can be experimentally revealed by the waiting time dependence of the 2D signal.

Revealing the sequential nature of the Si-nanocluster–Er interaction by variable pulse duration excitation

The sequential nature of the Si-nanocluster--Er interaction is investigated by studying the time evolution of the Er luminescence intensity at $1.54\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$, after exciting the system through high-power optical pulses with different time duration, in the range between 200 ns and $55\phantom{\rule{0.3em}{0ex}}\mathrm{ms}$. It is shown that while Si nanocrystals and Er ions separately embedded in the $\mathrm{Si}{\mathrm{O}}_{2}$ matrix can be approximated by an effective two level model, the Er-doped Si nanocluster system behaves differently under short pulse excitation. Indeed, for time durations shorter than the mean time between two successive excitations of a Si nanocluster, the number of excitable Er ions is shown to be limited by the total number of sensitizing centers and by the finite energy transfer time.

Sensitization of Er luminescence by Si nanoclusters

Sensitization of Er emission by Si nanoclusters (Si-nc) is investigated with pulsed and continuous optical pumping, in and off resonance with excited states of ${\text{Er}}^{3+}$ ion. We show that under high-power pulsed pumping, the excitation process is limited by the finite energy transfer time from Si-nc to ${\text{Er}}^{3+}$ ions. By comparison between pulsed and steady-state excitation, the concentration of sensitizers and average number of ${\text{Er}}^{3+}$ ions coupled to a single nanocluster are independently determined in an experiment. The results clarify conditions needed for efficient sensitization of ${\text{SiO}}_{2}:\text{Er}$ by Si-nc.

Photoisomerization of stilbene - a theoretical study of deuteration shifts and limited internal vibrational redistribution

Very detailed experimental studies of the isomerization of trans-stilbene in the S 1 excited electronic state show features which are not readily explained by current theory, e.g., of RRKM type. In this work the deuteration shifts seen in the series C 14H 12, C 14H 10D 2, C 14D 10H 2, C 14D 12, where in the middle two species the ethylenic bridge positions are listed last, are investi RRKM theory modified so as to allow for a finite energy transfer rate between a critical oscillator and the remaining modes of the molecule. Vibrational frequencies are estimated on the basis of experimental results for the ground state molecule with the help of a product rule for deuteration shifts.

Time-resolved fluorescence spectroscopy of lumazine protein from Photobacterium phosphoreum using synchrotron radiation.

Time-resolved fluorescence on lumazine protein from Photobacterium phosphoreum was performed with synchrotron radiation as a source of continuously tunable excitation. The experiments yielded structural and dynamic details from which two aspects became apparent. From fluorescence anisotropy decay monitoring of lumazine fluorescence with different excitation wavelengths, the average correlation times were shown to change, which must indicate the presence of anistropic motion of the protein. A similar study with 7-oxolumazine as the fluorescent ligand led to comparable results. The other remarkable observation dealt with the buildup of acceptor fluorescence, also observed with 7-oxolumazine although much less pronounced, which is caused by the finite energy transfer process between the single donor tryptophan and the energy accepting lumazine derivatives. Global analytical approaches in data analysis were used to yield realistic correlation times and reciprocal transfer rate constants. It was found that the tryptophan residue has a large motional freedom as also reported previously for this protein and for the related protein from P. leiognathi (Lee et al. 1985; Kulinski et al. 1987). The average distance between the tryptophan residue and the ligand donor-acceptor couple has been determined to be 2.7 nm for the same donor and two different acceptors.

The design and performance of QENS, a medium resolution, inverted geometry, tof quasielastic and inelastic spectrometer at IPNS

A multidetector, medium resolution, quasielastic and inelastic neutron scattering spectrometer has been designed and built at the Intense Pulsed Neutron Source of Argonne National Laboratory. The main objective in the design is to combine a moderate resolution around zero energy transfer with relatively uniform and high resolution at finite energy transfer up to about 1000 wave numbers. By using the principles of time and energy focusing together with pyrolytic graphite monochrometers and small detectors, we achieve a best resolution at the quasielastic peak of 56 μeV at the final energy of E f = 3.63 meV. The relative energy resolution at finite energy transfer, Δ E/ E, is fairly constant varying between 2% and 4%. The spectrometer has three arms which rotate around the sample covering the angular range 20°–155°. Two of the arms have 13 detectors and the third has 9 detectors. The q range at the elastic position varies from 0.4 to 2.6 Å −1. The incident neutron intensity on the sample area of 1 cm×10 cm is about 10 6 neutrons/s. Results of preliminary measurements on n-butane in the plastic phase, on crystallized Kaolinite, and on tetramethylammonium template cations occluded within zeolites are outlined.

Variation of impurity−to−band activation energies with impurity density

A theory of the variation of conduction electron density with the temperature for various impurity concentrations is presented. In addition to previously noted effects of condcution band edge lowering and screening of the impurity potential by the conduction electrons, the influence of a finite energy transfer integral and spatial fluctuation in the potential are included. The results show that for ND≳1017 cm−3 in silicon one must not view the activation as occurring between a single impurity level and a well−defined conduction band edge, but must include the broadening of the impurity level and tailing of the conduction−band density of states. Calculations for the shallow donors P, Sb, and As in Si are found to be in satisfactory agreement with experiment.

Gamma-Ray Correlation Function in the Adiabatic Approximation

The gamma-ray correlation function following inelastic excitation of an even-$A$ nucleus by a spinless projectile has been analyzed employing only the adiabatic approximation and theorems relevant to elastic scattering. The direction making equal angles with the incident and scattered directions in the scattering plane, the adiabatic recoil direction, is a convenient axis for quantization. In particular, the intermediate (excited) nucleus is populated with only even-$M$ states, from which follows that the gamma distribution is unchanged by a rotation of $\ensuremath{\pi}$ about this axis. For a ${0}^{+}$-${2}^{+}$-${0}^{+}$ excitation de-excitation, the gamma distribution in the scattering plane reduces to the form ${sin}^{2}[2({\ensuremath{\theta}}_{\ensuremath{\gamma}}\ensuremath{-}{\ensuremath{\theta}}_{0})]$, where ${\ensuremath{\theta}}_{0}$ is the adiabatic recoil axis. Comparison is made to the similar predictions of plane-wave Born approximation theories (in which the recoil direction for finite energy transfer is the symmetry axis) and to distorted-wave Born approximation calculations (for which, in general, there is no simple expression for the symmetry axis). Analysis of experiments verify the general features of the model, but further data obtained from forward scattering would be desirable to distinguish between the predictions of the adiabatic and Born approximations. Brief comments are made regarding gamma-ray polarization.