680 research outputs found
Tunnel spectroscopy in ac-driven quantum dot nanoresonators
Electronic transport in a triple quantum dot shuttle device in the presence
of an ac field is analyzed within a fully quantum mechanical framework. A
generalized density matrix formalism is used to describe the time evolution for
electronic state occupations in a dissipative phonon bath. In the presence of
an ac gate voltage, the electronic states are dressed by photons and the
interplay between photon and vibrational sidebands produces current
characteristics that obey selection rules. Varying the ac parameters allows to
tune the tunneling current features. In particular, we show that coherent
destruction of tunneling can be achieved in our device
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The View of Russian Students on Whether Psychology is a Science
The Psychology as Science Scale (Friedrich, 1996) was administered to 525 psychology students from nine Russian universities to assess their beliefs about the nature of the discipline. About half of students (49.6%) generally agreed that psychology may be called a scientific discipline. Specifically, 71. 5% of the students agreed that psychology is a natural science, similar to biology, chemistry, and physics, 39. 9% of students agreed that psychological research is important and training in psychological methodology is necessary, and 43.1% of students agreed that human behavior is highly predictable. Students who took three methodology courses shared significantly stronger beliefs in the need for psychological research and the importance of training in methodology compared to students who did not take any methodology courses. Furthermore, students with a specialist degree had significantly stronger beliefs that psychology is a science compared to students who have just finished school. In terms of the effect of students’ career aspirations, students who wanted to be academic psychologists and clinicians had significantly stronger beliefs that psychology is a science compared to students who did not have clarity about their future careers. Regardless of the study limitations, these findings have potential implications for Russian psychology instructors
Quasienergy spectrum and tunneling current in ac-driven triple quantum dot shuttles
The dynamics of electrons in ac driven double quantum dots have been
extensively analyzed by means of Floquet theory. In these systems, coherent
destruction of tunneling has been shown to occur for certain ac field
parameters. In the present work we analyze, by means of Floquet theory, the
electron dynamics of a triple quantum dot in series attached to electric
contacts, where the central dot position oscillates. In particular, we analyze
the quasienergy spectrum of this ac driven nanoelectromechanical system, as a
function of the intensity and frequency of the ac field and of external dc
voltages. For strong driving fields, we derive, by means of perturbation
theory, analytical expressions for the quasienergies of the driven oscillator
system. From this analysis we discuss the conditions for coherent destruction
of tunneling (CDT) to occur as a function of detuning and field parameters. For
zero detuning, and from the invariance of the Floquet Hamiltonian under a
generalized parity transformation, we find analytical expressions describing
the symmetry properties of the Fourier components of the Floquet states under
such transformation. By using these expressions, we show that in the vicinity
of the CDT condition, the quasienergy spectrum exhibits exact crossings which
can be characterized by the parity properties of the corresponding
eigenvectors
Coping with strong translational noncrystallographic symmetry and extreme anisotropy in molecular replacement with Phaser: human Rab27a
Data pathologies caused by effects such as diffraction anisotropy and translational noncrystallographic symmetry (tNCS) can dramatically complicate the solution of the crystal structures of macromolecules. Such problems were encountered in determining the structure of a mutant form of Rab27a, a member of the Rab GTPases. Mutant Rab27a constructs that crystallize in the free form were designed for use in the discovery of drugs to reduce primary tumour invasiveness and metastasis. One construct, hRab27aMut, crystallized within 24 h and diffracted to 2.82 Å resolution, with a unit cell possessing room for a large number of protein copies. Initial efforts to solve the structure using molecular replacement by Phaser were not successful. Analysis of the data set revealed that the crystals suffered from both extreme anisotropy and strong tNCS. As a result, large numbers of reflections had estimated standard deviations that were much larger than their measured intensities and their expected intensities, revealing problems with the use of such data at the time in Phaser. By eliminating extremely weak reflections with the largest combined effects of anisotropy and tNCS, these problems could be avoided, allowing a molecular-replacement solution to be found. The lessons that were learned in solving this structure have guided improvements in the numerical analysis used in Phaser, particularly in identifying diffraction measurements that convey very little information content. The calculation of information content could also be applied as an alternative to ellipsoidal truncation. The post-mortem analysis also revealed an oversight in accounting for measurement errors in the fast rotation function. While the crystal of mutant Rab27a is not amenable to drug screening, the structure can guide new modifications to obtain more suitable crystal forms
Quantum dynamics, dissipation, and asymmetry effects in quantum dot arrays
We study the role of dissipation and structural defects on the time evolution
of quantum dot arrays with mobile charges under external driving fields. These
structures, proposed as quantum dot cellular automata, exhibit interesting
quantum dynamics which we describe in terms of equations of motion for the
density matrix. Using an open system approach, we study the role of asymmetries
and the microscopic electron-phonon interaction on the general dynamical
behavior of the charge distribution (polarization) of such systems. We find
that the system response to the driving field is improved at low temperatures
(and/or weak phonon coupling), before deteriorating as temperature and
asymmetry increase. In addition to the study of the time evolution of
polarization, we explore the linear entropy of the system in order to gain
further insights into the competition between coherent evolution and
dissipative processes.Comment: 11pages,9 figures(eps), submitted to PR
Conformal Couplings in Induced Gravity
It is found that the induced gravity with conformal couplings requires the
conformal invariance in both classical and quantum levels for consistency. This
is also true for the induced gravity with an extended conformal coupling
interacting with torsion.Comment: 10 pages, Revtex3.0, to appear in General Relativity and Gravitatio
Nutrient and biogenic particulate distributions, primary productivity and nitrogen uptake in the Weddell-Scotia Sea marginal ice zone during winter
During austral winter of 1988, we determined the distributions of inorganic nutrients (nitrate, silicic acid, phosphate, nitrite and ammonium) and particulate materials (chlorophyll, biogenic silica, particulate organic carbon and particulate organic nitrogen), as well as primary productivity and rates of nutrient (NO3− and NH4+) uptake in the upper 150 m of the marginal ice zone of the Weddell-Scotia Sea. Nutrient concentrations were high and particulate matter levels were low throughout the study area, but occasionally nutrient minima and particulate maxima occurred near the ice edge associated with warm-core eddies. Chlorophyll concentrations and primary productivity averaged 0.12 μg l−1 and 32 mg C m−2 d−1, respectively. Surface growth rates calculated from carbon uptake and total particulate organic carbon were very low (ca. 0.03 doublings d−1), but living phytoplankton only comprised about 10% of the POC in the surface layer. Thus, mean phytoplankton growth rates appear to have been between 0.1 and 0.2 doublings d−1. Although nitrate was about 40 times as abundant as ammonium, ammonium was consistently the preferred substrate of the plankton assemblages, accounting for over half of the nitrogen taken up. Paired samples from the same depth and vertically integrated f-ratios averaged 0.43. Both ammonium and nitrate were removed at rates that individually exceeded the apparent nitrogen demand of the phytoplankton, implying significant heterotrophic uptake of inorganic nitrogen
Avaliação de fungicidas, doses e número de aplicações para o manejo da antracnose do sorgo.
Trabalho apresentado no 42º Congresso Brasileiro de Fitopatologia, 2009. Rio de Janeiro, RJ
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