407 research outputs found
Parametric resonance of a two-dimensional electron gas under bichromatic irradiation
In an ultrahigh mobility 2D electron gas, even a weak nonparabolicity of the
electron dispersion, by violating Kohn's theorem, can have a drastic effect on
dc magnetotransport under ac drive. We study theoretically the manifestation of
this effect in the dc response to the combined action of two driving ac-fields
(bichromatic irradiation). Compared to the case of monochromatic irradiation,
which is currently intensively studied both experimentally and theoretically,
the presence of a second microwave source provides additional insight into the
properties of an ac-driven 2D electron gas. In particular, we find that
nonparabolicity, being the simplest cause for a violation of Kohn's theorem,
gives rise to new qualitative effects specific to bichromatic irradiation.
Namely, when the frequencies and are well away from the
cyclotron frequency, , our simple classical considerations
demonstrate that the system becomes parametrically unstable with respect to
fluctuations with frequency . As an additional effect of
nonparabolicity, this parametric instability can manifest itself in the dc
properties of the system. This happens when , and
are related as 3:1:2, respectively. Even for weak detuning between
and , the effect of the bichromatic irradiation on the dc
response in the presence of nonparabolicity can differ dramatically from the
monochromatic case. In particular, the equations of motion can acquire
multistable solutions. As a result, the diagonal dc-conductivity can assume
several stable negative values at the same magnetic field.Comment: 11 pages, 10 figure
Theory for phonon-induced superconductivity in MgB
We analyze superonductivity in MgB observed below K resulting
from electron-phonon coupling involving a mode at meV and
most importantly the in-plane B-B vibration at
meV. The quasiparticles originating from - and -states couple
strongly to the low-frequency mode and the -vibrations respectively.
Using two-band Eliashberg theory, and , we calculate the gap functions (,
).
Our results provide an explanation of recent tunneling experiments.
We get .Comment: revised version, accepted for publication in PR
Geometric characterization of nodal domains: the area-to-perimeter ratio
In an attempt to characterize the distribution of forms and shapes of nodal
domains in wave functions, we define a geometric parameter - the ratio
between the area of a domain and its perimeter, measured in units of the
wavelength . We show that the distribution function can
distinguish between domains in which the classical dynamics is regular or
chaotic. For separable surfaces, we compute the limiting distribution, and show
that it is supported by an interval, which is independent of the properties of
the surface. In systems which are chaotic, or in random-waves, the
area-to-perimeter distribution has substantially different features which we
study numerically. We compare the features of the distribution for chaotic wave
functions with the predictions of the percolation model to find agreement, but
only for nodal domains which are big with respect to the wavelength scale. This
work is also closely related to, and provides a new point of view on
isoperimetric inequalities.Comment: 22 pages, 11 figure
Application of BRED technology to construct recombinant D29 reporter phage expressing EGFP
Bacteriophage Recombineering of Electroporated DNA (BRED) has been described for construction of gene deletion and point mutations in mycobacteriophages. Using BRED, we inserted a Phsp60-egfp cassette (1143 bp) into the mycobacteriophage D29 genome to construct a new reporter phage, which was used for detection of mycobacterial cells. The cassette was successfully inserted and recombinant mycobacteriophage purified. DNA sequencing of the cassette did not show any mutations even after several phage generations. Mycobacterium smegmatis mc2155 cells were infected with D29::Phsp60-egfp (MOI of 10) and evaluated for EGFP expression by microscopy. Fluorescence was observed at around 2 h after infection, but dissipated in later times because of cell lysis. We attempted to construct a lysis-defective mutant by deleting the lysA gene, although we were unable to purify the mutant to homogeneity even with complementation. These observations demonstrate the ability of BRED to insert c. 1 kbp-sized DNA segments into mycobacteriophage genomes as a strategy for constructing new diagnostic reporter phages.Fil: Silva, Joas L. da. Universidade de Sao Paulo; BrasilFil: Piuri, Mariana. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de QuĂmica BiolĂłgica de la Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de QuĂmica BiolĂłgica de la Facultad de Ciencias Exactas y Naturales; ArgentinaFil: Broussard, Gregory. University Of Pittsburgh; Estados UnidosFil: Marinelli, Laura J.. University Of Pittsburgh; Estados UnidosFil: Bastos, Gisele M.. Universidade de Sao Paulo; BrasilFil: Hirata, Rosario D. C.. Universidade de Sao Paulo; BrasilFil: Hatfull, Graham F.. University Of Pittsburgh; Estados UnidosFil: Hirata, Mario H.. Universidade de Sao Paulo; Brasi
Ideologies of time: How elite corporate actors engage the future
Our paper deals with how elite corporate actors in a Western capitalist-democratic society conceive of and prepare for the future. Paying attention to how senior officers of ten important Danish companies make sense of the future will help us to identify how particular temporal narratives are ideologically marked. This ideological dimension offers a common sense frame that is structured around a perceived inevitability of capitalism, a market economy as the basic organizational structure of the social and economic order, and an assumption of confident access to the future. Managers envisage their organization?s future and make plans for organizational action in a space where ?business as usual? reigns, and there is little engagement with the future as fundamentally open; as a time-yet-to-come. In using a conceptual lens inspired by the work of Fredric Jameson, we first explore the details of this presentism and a particular colonization of the future, and then linger over small disruptions in the narratives of our interviewees which point to what escapes or jars their common sense frame, explore the implicit meanings they assign to their agency, and also find clues and traces of temporal actions and strategies in their narratives that point to a subtly different engagement with time
Heidegger on creativity: From boredom to re-engagement with the world
Experimental psychologists have discussed whether boredom can help us become more creative. At ïŹrst blush, this would seem to be rather unlikely. When we are bored, we are disengaged; we cannot be bothered and nothing seems worthwhile; we have no interest in the world around us. Such a condition, surely, is not conducive to creativity (Haager et al. 2018). Yet some psychologists disagree (Gasper and Middlewood 2014). Boredom, they explain, breaks down entrenched routines and thought-patterns and provides us with an opportunity to think again and anew. Respondents in âapproach-oriented statesâ such as boredom engage in more âassociative thoughtâ than those in âavoidance-oriented affective states.â This is how boredom comes to encourage âthe quest for meaning and explorationâ (Gasper and Middlewood 2014, pp. 53â55...2-s2.0-8508440789
Random-phase approximation and its applications in computational chemistry and materials science
The random-phase approximation (RPA) as an approach for computing the
electronic correlation energy is reviewed. After a brief account of its basic
concept and historical development, the paper is devoted to the theoretical
formulations of RPA, and its applications to realistic systems. With several
illustrating applications, we discuss the implications of RPA for computational
chemistry and materials science. The computational cost of RPA is also
addressed which is critical for its widespread use in future applications. In
addition, current correction schemes going beyond RPA and directions of further
development will be discussed.Comment: 25 pages, 11 figures, published online in J. Mater. Sci. (2012
Fish consumption patterns and hair mercury levels in children and their mothers in 17 EU countries
The toxicity of methylmercury (MeHg) in humans is well established and the main source of exposure is via the consumption of large marine fish and mammals. Of particular concern are the potential neurodevelopmental effects of early life exposure to low-levels of MeHg. Therefore, it is important that pregnant women, children and women of childbearing age are, as far as possible, protected from MeHg exposure.Within the European project DEMOCOPHES, we have analyzed mercury (Hg) in hair in 1799 motherâchild pairs from 17 European countries using a strictly harmonized protocol for mercury analysis. Parallel, harmonized questionnaires on dietary habits provided information on consumption patterns of fish and marine products. After hierarchical cluster analysis of consumption habits of the motherâchild pairs, the DEMOCOPHES cohort can be classified into two branches of approximately similar size: one with high fish consumption (H) and another with low consumption (L). All countries have representatives in both branches, but Belgium, Denmark, Spain, Portugal and Sweden have twice as many or more motherâchild pairs in H than in L. For Switzerland, Czech Republic, Hungary, Poland, Romania, Slovenia and Slovakia the situation is the opposite, with more representatives in L than H.There is a strong correlation (r=0.72) in hair mercury concentration between the mother and child in the same family, which indicates that they have a similar exposure situation. The clustering of motherâchild pairs on basis of their fish consumption revealed some interesting patterns. One is that for the same sea fish consumption, other food items of marine origin, like seafood products or shellfish, contribute significantly to the mercury levels in hair. We conclude that additional studies are needed to assess and quantify exposure to mercury from seafood products, in particular. The cluster analysis also showed that 95% of mothers who consume once per week fish only, and no other marine products, have mercury levels 0.55 ”g/g. Thus, the 95th percentile of the distribution in this group is only around half the US-EPA recommended threshold of 1 ”g/g mercury in hair. Consumption of freshwater fish played a minor role in contributing to mercury exposure in the studied cohort.The DEMOCOPHES data shows that there are significant differences in MeHg exposure across the EU and that exposure is highly correlated with consumption of fish and marine products. Fish and marine products are key components of a healthy human diet and are important both traditionally and culturally in many parts of Europe. Therefore, the communication of the potential risks of mercury exposure needs to be carefully balanced to take into account traditional and cultural values as well as the potential health benefits from fish consumption. European harmonized human biomonitoring programs provide an additional dimension to national HMB programs and can assist national authorities to tailor mitigation and adaptation strategies (dietary advice, risk communication, etc.) to their countryâs specific requirements
- âŠ