Conductance oscillations in mesoscopic rings: microscopic versus macroscopic picture

The phase of Aharonov-Bohm oscillations in mesoscopic metal rings in the presence of a magnetic field can be modulated by application of a DC-bias current I_DC. We address the question of how a variation of I_DC and hence of the microscopic phases of the electronic wave functions results in the macroscopic phase of the conductance oscillations. Whereas the first one can be varied continuously the latter has to be quantized for a ring in two-wire configuration by virtue of the Onsager symmetry relations. We observe a correlation between a phase flip by +/- pi and the amplitude of the oscillations.Comment: 4 pages, 4 figure

Picosecond time-resolved energy transfer within C-phycocyanin aggregates of Mastigocladus laminosus

We have investigated by picosecond absorption experiments how the size of C-phycocyanin aggregates from Mastigocladus laminosus influences the excitation energy transfer kinetics. Going from C-phycocyanin monomers to trimers the lifetime of the faster energy transfer component decreased from 57 ± 4 to 27 ± 4 ps over most of the wavelength range (580–645 nm) studied. This change was interpreted as the opening of fast transfer channels (α-84 → β-84 and/or β-84 → β-84) between two adjacent monomers in the trimeric unit. The 57 ps lifetime is probably due mainly to the β-155 → β-84 energy transfer step. The intermediate lifetime decreased from about 300 ps in the monomer to 100–120 ps in the trimer. The former is believed to be dominated by the equilibration process α-84 a3 β-84, while the latter probably represents the time required for the excitation energy to reach thermodynamic equilibrium within the trimer. The lifetime of the longest components was about 1 ns in both systems. This indicates that the chromophores in these C-phycocyanin complexes are more exposed to non-radiative processes (like, for instance, isomerization) compared to the chromophores in intact phycobilisomes, where this lifetime typically is about 1.8 ns. The anisotropy relaxation closely followed the isotropic lifetimes in both systems. The anisotropy after the initial fast relaxation, r(∞), was 0.29 ± 0.04 in monomers and decreased to 0.15 ± 0.03 in trimers. Measurements of the steady-state fluorescence excitation anisotropy gave the same results within the experimental error

Femtosecond energy transfer between chromophores in allophycocyanin trimers

Ultrafast energy-transfer processes in allophycocyanin (APC) trimers from Mastigocladus laminosus have been examined by a femtosecond absorption technique. Isotropic absorption recovery kinetics with τ=440±30 fs were observed in APC trimers at 615 nm. In APC monomers such a fast process was not observed. The anisotropy in both samples was constant and close to 0.4 during the first few picoseconds. The results are consistent with a model of the APC trimer in which the two APC chromophores have different absorption spectra with maxima about 600 and 650 nm. The transfer of energy from the 600 nm chromophore to the 650 nm chromophore occurs in 440 fs and is dominated by the Förster dipole—dipole energy-transfer mechanism

Carotenoid triplet state formation in Rhodobacter sphaeroides R-26 reaction centers exchanged with modified bacteriochlorophyll pigments and reconstituted with spheroidene

Triplet state electron paramagnetic resonance (EPR) experiments have been carried out at X-band on Rb. sphaeroides R-26 reaction centers that have been reconstituted with the carotenoid, spheroidene, and exchanged with 132-OH-Zn-bacteriochlorophyll a and [3-vinyl]-132-OH-bacteriochlorophyll a at the monomeric, lsquoaccessoryrsquo bacteriochlorophyll sites BA,B or with pheophytin a at the bacteriopheophytin sites HA,B. The primary donor and carotenoid triplet state EPR signals in the temperature range 95–150 K are compared and contrasted with those from native Rb. sphaeroides wild type and Rb. sphaeroides R-26 reaction centers reconstituted with spheroidene. The temperature dependencies of the EPR signals are strikingly different for the various samples. The data prove that triplet energy transfer from the primary donor to the carotenoid is mediated by the monomeric, BChlB molecule. Furthermore, the data show that triplet energy transfer from the primary donor to the carotenoid is an activated process, the efficiency of which correlates with the estimated triplet state energies of the modified pigments

Andreev scattering in nanoscopic junctions at high magnetic fields

We report on the measurement of multiple Andreev resonances at atomic size point contacts between two superconducting nanostructures of Pb under magnetic fields higher than the bulk critical field, where superconductivity is restricted to a mesoscopic region near the contact. The small number of conduction channels in this type of contacts permits a quantitative comparison with theory through the whole field range. We discuss in detail the physical properties of our structure, in which the normal bulk electrodes induce a proximity effect into the mesoscopic superconducting part.Comment: 4 page

Non-equilibrium current noise in mesoscopic disordered SNS junctions

Current noise in superconductor-normal metal-superconductor (SNS) junctions is calculated within the scattering theory of multiple Andreev reflections (MAR). It is shown that the noise exhibits subharmonic gap singularities at $eV=2\Delta/n$, $n=1,2,...$ both in single-mode junctions with arbitrary transparency $D$ and in multi-mode disordered junctions. The subharmonic structure is superimposed with monotonic increase of the effective transferred charge $q^*=S_I(0)/2I$ with decreasing bias voltage. Other features of the noise include a step-like increase of $q^*$ in junctions with small $D$, and a divergence $S_I(0) \propto V^{-1/2}$ at small voltages and excess noise $S_{ex} = 2eI_{ex}$, where $I_{ex}$ is the excess current, at large voltages, in junctions with diffusive transport.Comment: 5 page

Funcionalidades ambientais de solos altomontanos na Serra da Igreja, Paraná.

Apesar de ainda existirem ecossistemas altomontanos no Paraná em excelente estado de conservação, iminentes ameaças antrópicas e a fragilidade desses ambientes têm sido motivos de preocupação. Este trabalho teve os seguintes objetivos: caracterizar solos de área representativa dos campos e florestas altomontanas ocorrentes na Serra da Igreja; apontar quais os possíveis fatores pedológicos que resultam nessas diferentes fitotipias; e caracterizar algumas das suas funcionalidades ambientais (estoque de C e de água). Os principais solos encontrados nos campos foram Organossolos Fólicos fíbricos/sápricos (líticos e típicos) e Organossolos Háplicos fíbricos/sápricos (típicos e térricos) e, nas florestas altomontanas, Gleissolos Háplicos alíticos típicos. Ambas as classes são de solos distróficos, extremamente ácidos, com alta saturação por Al trocável e altos teores de C orgânico total. A distribuição das florestas altomontanas está fortemente controlada por vales e colos de cumeeiras, os quais estão sujeitos a processos morfogenéticos que resultam em solos com horizontes minerais. Já os campos estão estabelecidos em topos, onde processos pedogenéticos promoveram espessamento de horizontes hísticos, os quais, em função de suas características intrínsecas, aliadas aos fortes ventos, parecem conter com sucesso o avanço da floresta sobre o campo. Os estoques de C por unidade de área nos solos dos campos são superiores aos dos solos das florestas altomontanas, sendo ambos considerados altos quando comparados aos dados de outros ecossistemas, sendo duas a três vezes maiores do que os encontrados em solos de ecossistemas de altitudes mais baixas na mesma latitude. Também foi constatada alta capacidade de retenção hídrica devido à porosidade total verificada nos horizontes hísticos, os quais têm o potencial de reter em média 12 vezes seu volume em água