2,115 research outputs found
The pace of evolution across fitness valleys
How fast does a population evolve from one fitness peak to another? We study
the dynamics of evolving, asexually reproducing populations in which a certain
number of mutations jointly confer a fitness advantage. We consider the time
until a population has evolved from one fitness peak to another one with a
higher fitness. The order of mutations can either be fixed or random. If the
order of mutations is fixed, then the population follows a metaphorical ridge,
a single path. If the order of mutations is arbitrary, then there are many ways
to evolve to the higher fitness state. We address the time required for
fixation in such scenarios and study how it is affected by the order of
mutations, the population size, the fitness values and the mutation rate
Formation of an Ordered Array of nc-Si Dots by Using a Solution Droplet Evaporation Method
Electronic Orders Induced by Kondo Effect in Non-Kramers f-Electron Systems
This paper clarifies the microscopic nature of the staggered scalar order,
which is specific to even number of f electrons per site. In such systems,
crystalline electric field (CEF) can make a singlet ground state. As exchange
interaction with conduction electrons increases, the CEF singlet at each site
gives way to Kondo singlets. The collective Kondo singlets are identified with
itinerant states that form energy bands. Near the boundary of itinerant and
localized states, a new type of electronic order appears with staggered Kondo
and CEF singlets. We present a phenomenological three-state model that
qualitatively reproduces the characteristic phase diagram, which have been
obtained numerically with use of the continuous-time quantum Monte Carlo
combined with the dynamical mean-field theory. The scalar order observed in
PrFe_4P_{12} is ascribed to this staggered order accompanying charge density
wave (CDW) of conduction electrons. Accurate photoemission and tunneling
spectroscopy should be able to probe sharp peaks below and above the Fermi
level in the ordered phase.Comment: 7 pages, 8 figure
Role of p-f Hybridization in the Metal-Non-Metal Transition of PrRu4P12
Electronic state evolution in the metal-non-metal transition of PrRu4P12 has
been studied by X-ray and polarized neutron diffraction experiments. It has
been revealed that, in the low-temperature non-metallic phase, two inequivalent
crystal-field (CF) schemes of Pr3+ 4f^2 electrons with Gamma_1 and Gamma_4^(2)
ground states are located at Pr1 and Pr2 sites forming the bcc unit cell
surrounded by the smaller and larger cubic Ru-ion sublattices, respectively.
This modulated electronic state can be explained by the p-f hybridization
mechanism taking two intermediate states of 4f^1 and 4f^3. The p-f
hybridization effect plays an important role for the electronic energy gain in
the metal-non-metal transition originated from the Fermi surface nesting.Comment: 5 pages, 5 figures. Accepted by J. Phys. Soc. Jp
Charge transfer excitons in optical absorption spectra of C60-dimers and polymers
Charge-transfer (CT) exciton effects are investigated for the optical
absorption spectra of crosslinked C60 systems by using the intermediate exciton
theory. We consider the C60-dimers, and the two (and three) molecule systems of
the C60-polymers. We use a tight-binding model with long-range Coulomb
interactions among electrons, and the model is treated by the Hartree-Fock
approximation followed by the single-excitation configuration interaction
method. We discuss the variations in the optical spectra by changing the
conjugation parameter between molecules. We find that the total CT-component
increases in smaller conjugations, and saturates at the intermediate
conjugations. It decreases in the large conjugations. We also find that the
CT-components of the doped systems are smaller than those of the neutral
systems, indicating that the electron-hole distance becomes shorter in the
doped C60-polymers.Comment: Figures should be requested to the autho
Quantitative analysis of electronic transport through weakly-coupled metal/organic interfaces
Using single-crystal transistors, we have performed a systematic experimental
study of electronic transport through oxidized copper/rubrene interfaces as a
function of temperature and bias. We find that the measurements can be
reproduced quantitatively in terms of the thermionic emission theory for
Schottky diodes, if the effect of the bias-induced barrier lowering is
included. Our analysis emphasizes the role of the coupling between metal and
molecules, which in our devices is weak due to the presence of an oxide layer
at the surface of the copper electrodes.Comment: 4 pages, 3 figure
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