579 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
Influence of ion implantation on the magnetic and transport properties of manganite films
We have used oxygen ions irradiation to generate controlled structural
disorder in thin manganite films. Conductive atomic force microscopy CAFM),
transport and magnetic measurements were performed to analyze the influence of
the implantation process in the physical properties of the films. CAFM images
show regions with different conductivity values, probably due to the random
distribution of point defect or inhomogeneous changes of the local Mn3+/4+
ratio to reduce lattice strains of the irradiated areas. The transport and
magnetic properties of these systems are interpreted in this context.
Metal-insulator transition can be described in the frame of a percolative
model. Disorder increases the distance between conducting regions, lowering the
observed TMI. Point defect disorder increases localization of the carriers due
to increased disorder and locally enhanced strain field. Remarkably, even with
the inhomogeneous nature of the samples, no sign of low field magnetoresistance
was found. Point defect disorder decreases the system magnetization but doesn t
seem to change the magnetic transition temperature. As a consequence, an
important decoupling between the magnetic and the metal-insulator transition is
found for ion irradiated films as opposed to the classical double exchange
model scenario.Comment: 27 pages, 11 Figure
Impurity band in clean superconducting weak links
Weak impurity scattering produces a narrow band with a finite density of
states near the phase difference in the mid-gap energy spectrum of
a macroscopic superconducting weak link. The equivalent distribution of
transmission coefficients of various cunducting quantum channels is found.Comment: 4 pages, 4 figures, changed conten
Low-temperature thermal expansion of the topological material candidates -PtBi and -BiPd
We report on the low-temperature ( 120 K) thermal expansion of the
bismuth-based topological semimetal -PtBi and topological
superconductor -BiPd candidates. The linear thermal-expansion
coefficient of tetragonal -BiPd shows a pronounced anisotropy
between the - and -axis while the volume thermal-expansion coefficient
(-BiPd) is considerable larger than
(-PtBi). The coefficient (-PtBi)
nearly matches the experimental specific heat, from which a Debye temperature
199 K is obtained. On the other hand, (-BiPd)
reasonably fits the Debye model with 138 K, extracted from the
low-temperature specific heat. An almost constant Gr\"uneisen parameter 2 is obtained for both compounds. No magnetostriction is observed in
any of both compounds up to 16 T. We compare our results with other
Bi-based topological materials.Comment: 7 pages, 3 figure
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