465 research outputs found
Recombination dynamics in bacterial photosynthetic reaction centers
The time dependence of magnetic field effects on light absorption by triplet-state and radical ions in quinone-depleted reaction centers of Rhodopseudomonas sphaeroides strain R-26 has been investigated. Measurements on the time scale of the hyperfine interaction in the radical pair [(BChl)2+. ...BPh-.)] provided kinetic data characterizing the recombination process. The results have been interpreted in terms of a recently proposed model that assumes an intermediate electron acceptor (close site) between the bacteriochlorophyll "special pair" (BChl)2 and the bacteriopheophytin BPh (distant site). Recombination is assumed to proceed through this intermediate acceptor. The experiments led to effective recombination rates for the singlet and triplet channel: k(Seff) = 3.9 . 107 s-1 and k(Teff) = 7.4 . 10(8) s-1. These correspond to recombination rates ks = 1 . 10(1) s-1 and kT = 7.1 . 10(11) s-1 in the close configuration. The upper bound of the effective spin dephasing rate k2eff approximately equal to 1 . 10(9) s-1 is identical with the rate of the electron hopping between the distant site of zero spin exchange interaction and the close site of large interaction. Interpretation of data for the case of direct recombination yields the recombination rates, spin dephasing rate, and exchange interaction in a straightforward way
Direct measurement of the magnetic penetration depth by magnetic force microscopy
We present an experimental approach using magnetic force microscopy for measurements of the absolute value of the magnetic penetration depth (lambda) in superconductors. Lambda is obtained in a simple and robust way without introducing any tip modeling procedure via direct comparison of the Meissner response curves for a material of interest to those measured on a reference sample. Using a well characterized Nb film as a reference, we determine the absolute value of lambda in a Ba(Fe0.92Co0.08)2As2 single crystal and a MgB2 thin film through a comparative experiment. Our apparatus features simultaneous loading of multiple samples, and allows straightforward measurement of the absolute value of lambda in superconducting thin film or single crystal samples.Fil: Kim, Jeehoon. No especifĂca;Fil: Civale, L.. No especifĂca;Fil: Nazaretski, E.. No especifĂca;Fil: Haberkorn, Nestor Fabian. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Patagonia Norte; ArgentinaFil: Ronning, F.. No especifĂca;Fil: Sefat, A. S.. No especifĂca;Fil: Tajima, T.. No especifĂca;Fil: Moeckly, B. H.. No especifĂca;Fil: Thompson, J. D.. No especifĂca;Fil: Movshovich, R.. No especifĂca
Diffusion controlled initial recombination
This work addresses nucleation rates in systems with strong initial
recombination. Initial (or `geminate') recombination is a process where a
dissociated structure (anion, vortex, kink etc.) recombines with its twin
brother (cation, anti-vortex, anti-kink) generated in the same nucleation
event. Initial recombination is important if there is an asymptotically
vanishing interaction force instead of a generic saddle-type activation
barrier. At low temperatures, initial recombination strongly dominates
homogeneous recombination. In a first part, we discuss the effect in one-,
two-, and three-dimensional diffusion controlled systems with spherical
symmetry. Since there is no well-defined saddle, we introduce a threshold which
is to some extent arbitrary but which is restricted by physically reasonable
conditions. We show that the dependence of the nucleation rate on the specific
choice of this threshold is strongest for one-dimensional systems and decreases
in higher dimensions. We discuss also the influence of a weak driving force and
show that the transport current is directly determined by the imbalance of the
activation rate in the direction of the field and the rate against this
direction. In a second part, we apply the results to the overdamped sine-Gordon
system at equilibrium. It turns out that diffusive initial recombination is the
essential mechanism which governs the equilibrium kink nucleation rate. We
emphasize analogies between the single particle problem with initial
recombination and the multi-dimensional kink-antikink nucleation problem.Comment: LaTeX, 11 pages, 1 ps-figures Extended versio
Measurement of the magnetic penetration depth of a superconducting MgB thin film with a large intraband diffusivity
We report the temperature dependent magnetic penetration depth
and the superconducting critical field in a 500-nm MgB film.
Our analysis of the experimental results takes into account the two gap nature
of the superconducting state and indicates larger intraband diffusivity in the
three-dimensional (3D) band compared to that in the two-dimensional (2D)
band. Direct comparison of our results with those reported previously
for single crystals indicates that larger intraband scattering in the 3D
band leads to an increase of . We calculated and the
thermodynamic critical field 2000 Oe employing the gap equations
for two-band superconductors. Good agreement between the measured and
calculated value indicates the two independent measurements, such as
magnetic force microscopy and transport, provide a venue for investigating
superconducting properties in multi-band superconductors
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