3,566 research outputs found
Emergence of New Industries and Endogenous Growth Cycles.
This paper constructs a growth model in which monopolistically competing firms choose the characteristic of their own product from an unbounded product space. While consumers wish to satisfy various needs by purchasing a diverse range of goods, production costs are lower for those goods that are more similar to existing ones because of spillover effects in the lesrning-by-doing process.GROWTH MODELS ; MONOPOLIES ; COMPETITION ; COSTS
Optical conductivity in the CuO double chains of PrBa_2Cu_4O_8: Consequences of charge fluctuation
We calculate the optical conductivity of the CuO double chains of
PrBaCuO by the mean-field approximation for the coupled two-chain
Hubbard model around quarter filling. We show that the 40 meV peak
structure, spectral shape, and small Drude weight observed in experiment are
reproduced well by the present calculation provided that the stripe-type charge
ordering presents. We argue that the observed anomalous optical response may be
due to the presence of stripe-type fluctuations of charge carriers in the CuO
double chains; the fast time scale of the optical measurement should enable one
to detect slowly fluctuating order parameters as virtually a long-range order.Comment: 7 pages, 5 eps figure
The great ideas of biology: Exploration through experimentation in an undergraduate lab course
We developed an introductory laboratory course to provide a visceral experience that aims at getting students truly excited about scientific study of the living world. Our vehicle to do that was to focus on what Paul Nurse dubbed âthe great ideas of biologyâ rather than an approach to biology that celebrates specific factual knowledge. To that end, we developed eight diverse experimental
modules, each of which highlights a key biological concept and gives an opportunity to use theory to generate testable hypotheses, to perform high quality measurements to test those hypotheses (some of which are clearly wrong), and to perform sophisticated computational data analysis. Some
modules incorporate modern microscopy and computational techniques in classic experiments, such as bacterial growth and the LuriaâDelbrĂŒck experiment, while others address current research questions using methods like optogenetics and single molecule measurements. We have offered the course eight times, and in the most recent edition of the course, we conducted pre/postâcourse interviews and
attitude surveys. The students, both bio and nonâbio majors alike, reported being captivated by seeing life occur across the broad range of experiments and model organisms. We observed demonstrable development of their curiosity and enthusiasm for biology. Additionally, we found that
prior to the course, students had only vague notions about what it means to make quantitative biological measurements and interpret them. They completed the course with a clearer understanding of scientific inquiry in biology and the skills and confidence to actually perform and interpret measurements in living systems
Anomalous behaviors of the charge and spin degrees of freedom in the CuO double chains of PrBaCuO
The density-matrix renormalization-group method is used to study the
electronic states of a two-chain Hubbard model for CuO double chains of
PrBaCuO. We show that the model at quarter filling has the charge
ordered phases with stripe-type and in-line--type patterns in the parameter
space, and in-between, there appears a wide region of vanishing charge gap; the
latter phase is characteristic of either Tomonaga-Luttinger liquid or a
metallic state with a spin gap. We argue that the low-energy electronic state
of the CuO double chains of PrBaCuO should be in the metallic state
with a possibly small spin gap.Comment: REVTEX 4, 10 pages, 9 figures; submitted to PR
Spin dynamics and antiferromagnetic order in PrBa2Cu4O8 studied by Cu nuclear respnance
Results of the nuclear resonance experiments for the planar Cu sites in
PrBa2Cu4O8 are presented. The NMR spectrum at 1.5 K in zero magnetic field
revealed an internal field of 6.1 T, providing evidence for an
antiferromagnetic order of the planar Cu spins. This confirms that the CuO2
planes are insulating, therefore, the metallic conduction in this material is
entirely due to the one-dimensional zigzag Cu2O2 chains. The results of the
spin-lattice relaxation rates measured by zero field NQR above 245 K in the
paramagnetic state are explained by the theory for a Heisenberg model on a
square lattice.Comment: 4 pages, 2 figure
Angle-resolved photoemission study of insulating and metallic Cu-O chains in PrBaCuO and PrBaCuO
We compare the angle-resolved photoemission spectra of the hole-doped Cu-O
chains in PrBaCuO (Pr123) and in PrBaCuO (Pr124).
While, in Pr123, a dispersive feature from the chain takes a band maximum at
(momentum along the chain) and loses its spectral weight
around the Fermi level, it reaches the Fermi level at in
Pr124. Although the chains in Pr123 and Pr124 are approximately 1/4-filled,
they show contrasting behaviors: While the chains in Pr123 have an instability
to charge ordering, those in Pr124 avoid it and show an interesting spectral
feature of a metallic coupled-chain system.Comment: 4 pages, 5 figures, to be published in PR
Influence of pulsed laser deposition growth conditions on the thermoelectric properties of Ca3Co4O9 thin films
Thin films of the misfit cobaltite Ca3Co4O9 were grown on (0001)-oriented
(c-cut) sapphire substrates, using the pulsed-laser deposition techniques. The
dependence of the thermoelectric/transport properties on the film growth
conditions was investigatedComment: To be published in J. Appl. Phy
Asymmetric total synthesis of (+)-virosine a via sequential nucleophilic cyclizations onto an activated formamide
Abstract: The first synthesis of tetracyclic alkaloid virosine A is reported. The natural alkaloid was prepared in only 13 steps, in an enantioenriched form. The azabicyclo[2.2.2]octane core was efficiently assembled using a key VilsmeierâHaack and Mannich cyclizations sequence performed in one pot
Possible high temperature superconductivity in Ti-doped A-Sc-Fe-As-O (A= Ca, Sr) system
We report a systematic study on the effect of partial substitution of
Sc by Ti in SrScFeAsO, CaScFeAsO and
SrScFeAsO on their electrical properties. High
level of doping results in an increased carrier concentration and leads to the
appearance of superconductivity with the onset of T up to 45 K.Comment: 8 pages, 4 figures, 2 new figure
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