ENERGY TRANSFER IN TRIMERIC C-PHYCOCYANIN STUDIED BY PICOSECOND FLUORESCENCE KINETICS

The excited state kinetics of trimeric C-phycocyanin from Mastigocladus laminosus has been measured as a function of the emission and excitation wavelength by the single-photon timing technique with picosecond resolution and simultaneous data analysis. A fast decay component of 22 ps (C-phycocyanin with linker peptides) and 36 ps (C-phycocyanin lacking linker peptides) is attributed to efficient energy transfer from sensitizing to fluorescing chromophores. At long detection wavelengths the fast decay components are found to turn into a rise term. This finding further corroborates the concept of intramolecular energy transfer. Previous reports on the conformational heterogeneity of the chromophores and/or proteins in C-phycocyanin are confirmed. Our data also provide indications for the importance of the uncoloured linker peptides for this heterogeneity

Optical Response of Grating-Coupler-Induced Intersubband Resonances: The Role of Wood's Anomalies

Grating-coupler-induced collective intersubband transitions in a quasi-two-dimensional electron system are investigated both experimentally and theoretically. Far-infrared transmission experiments are performed on samples containing a quasi-two-dimensional electron gas quantum-confined in a parabolic quantum well. For rectangular shaped grating couplers of different periods we observe a strong dependence of the transmission line shape and peak height on the period of the grating, i.e. on the wave vector transfer from the diffracted beams to the collective intersubband resonance. It is shown that the line shape transforms with increasing grating period from a Lorentzian into a strongly asymmetric line shape. Theoretically, we treat the problem by using the transfer-matrix method of local optics and apply the modal-expansion method to calculate the influence of the grating. The optically uniaxial quasi-two-dimensional electron gas is described in the long-wavelength limit of the random-phase approximation by a local dielectric tensor, which includes size quantization effects. Our theory reproduces excellently the experimental line shapes. The deformation of the transmission line shapes we explain by the occurrence of both types of Wood's anomalies.Comment: 28 pages, 7 figures. Physical Review B , in pres

Order-disorder transition in nanoscopic semiconductor quantum rings

Using the path integral Monte Carlo technique we show that semiconductor quantum rings with up to six electrons exhibit a temperature, ring diameter, and particle number dependent transition between spin ordered and disordered Wigner crystals. Due to the small number of particles the transition extends over a broad temperature range and is clearly identifiable from the electron pair correlation functions.Comment: 4 pages, 5 figures, For recent information on physics of small systems see http://www.smallsystems.d

Energy levels and far-infrared spectroscopy for two electrons in a semiconductor nanoring

The effects of electron-electron interaction of a two-electron nanoring on the energy levels and far-infrared (FIR) spectroscopy have been investigated based on a model calculation which is performed within the exactly numerical diagonalization. It is found that the interaction changes the energy spectra dramatically, and also shows significant influence on the FIR spectroscopy. The crossings between the lowest spin-singlet and triplet states induced by the coulomb interaction are clearly revealed. Our results are related to the experiment recently carried out by A. Lorke et al. [Phys. Rev. Lett. 84, 2223 (2000)].Comment: 17 pages, 6 figures, revised and accepted by Phys. Rev. B (Dec. 15

Microscopic view on Landau level broadening mechanisms in graphene

Placing a two-dimensional sheet of graphene in an external magnetic field the continuous electronic band structure is discretized due to Landau quantization. The resulting optical transitions are subject to a broadening, which can lead to a significant overlap of Landau levels. We investigate the possible microscopic processes that could cause a broadening of the corresponding peaks in the absorption spectrum of Landau-quantized graphene: (i) radiative decay, (ii) Coulomb interaction, (iii) optical phonons, (iv) acoustic phonons, and (v) impurities. Since recent experiments have shown that independent of the magnetic field the resolvable number of Landau levels is constant, we put a special focus on the dependence of the broadening on the external magnetic field B and the Landau level index n. Our calculations reveal the impurities to be the crucial broadening mechanism, where different regimes of well separated and densely spaced Landau levels need to be taken into account. Furthermore, carrier-carrier and carrier-phonon scattering give rise to a very specific dependence on the Landau level index n that has not been observed yet

A possible cooling effect in high temperature superconductors

We show that an adiabatic increase of the supercurrent along a superconductor with lines of nodes of the order parameter on the Fermi surface can result in a cooling effect. The maximum cooling occurs if the supercurrent increases up to its critical value. The effect can also be observed in a mixed state of a bulk sample. An estimate of the energy dissipation shows that substantial cooling can be performed during a reasonable time even in the microkelvin regime.Comment: 5 pages, to appear in Phys. Rev.

Mean parameter model for the Pekar-Fr\"{o}hlich polaron in a multilayered heterostructure

The polaron energy and the effective mass are calculated for an electron confined in a finite quantum well constructed of $GaAs/Al_x Ga_{1-x} As$ layers. To simplify the study we suggest a model in which parameters of a medium are averaged over the ground-state wave function. The rectangular and the Rosen-Morse potential are used as examples. To describe the confined electron properties explicitly to the second order of perturbations in powers of the electron-phonon coupling constant we use the exact energy-dependent Green function for the Rosen-Morse confining potential. In the case of the rectangular potential, the sum over all intermediate virtual states is calculated. The comparison is made with the often used leading term approximation when only the ground-state is taken into account as a virtual state. It is shown that the results are quite different, so the incorporation of all virtual states and especially those of the continuous spectrum is essential. Our model reproduces the correct three-dimensional asymptotics at both small and large widths. We obtained a rather monotonous behavior of the polaron energy as a function of the confining potential width and found a peak of the effective mass. The comparison is made with theoretical results by other authors. We found that our model gives practically the same (or very close) results as the explicit calculations for potential widths $L \geq 10 \AA$.Comment: 12 pages, LaTeX, including 5 PS-figures, subm. to Phys. Rev. B, new data are discusse

Localized magnetoplasmon modes arising from broken translational symmetry in semiconductor superlattices

The electromagnetic propagator associated with the localized collective magnetoplasmon excitations in a semiconductor superlattice with broken translational symmetry, is calculated analytically within linear response theory. We discuss the properties of these collective excitations in both radiative and non-radiative regimes of the electromagnetic spectra. We find that low frequency retarded modes arise when the surface density of carriers at the symmetry breaking layer is lower than the density at the remaining layers. Otherwise a doublet of localized, high-frequency magnetoplasmon-like modes occurs.Comment: Revtex file + separate pdf figure