23,532 research outputs found
Macroscopic coherence effects in a mesoscopic system: Weak localization of thin silver films in an undergraduate lab
We present an undergraduate lab that investigates weak localization in thin
silver films. The films prepared in our lab have thickness, , between 60-200
\AA, a mesoscopic length scale. At low temperatures, the inelastic dephasing
length for electrons, , exceeds the thickness of the film (), and the films are then quasi-2D in nature. In this situation, theory
predicts specific corrections to the Drude conductivity due to coherent
interference between conducting electrons' wavefunctions, a macroscopically
observable effect known as weak localization. This correction can be destroyed
with the application of a magnetic field, and the resulting magnetoresistance
curve provides information about electron transport in the film. This lab is
suitable for Junior or Senior level students in an advanced undergraduate lab
course.Comment: 16 pages, 9 figures. Replaces earlier version of paper rejected by
Am. J. Phys. because of too much content on vacuum systems. New version deals
with the undergraduate experiment on weak localization onl
Parity Effect in a Small Superconducting Particle
Matveev and Larkin calculated the parity effect on the ground state energy of
a small superconducting particle in the regimes where the mean level spacing is
either large or small compared to the bulk gap. We perform a numerical
calculation which extends their results into the intermediate regime, where the
level spacing is of the same order as the bulk gap.Comment: 6 LaTeX pages, including 2 EPS figures; corrected reference and
spellin
Quantitative measurements of the thermopower of Andreev interferometers
Using a new second derivative technique and thermometers which enable us to
determine the local electron temperature in a mesoscopic metallic sample, we
have obtained quantitative measurements of the low temperature field and
temperature dependent thermopower of Andreev interferometers. As in previous
experiments, the thermopower is found to oscillate as a function of magnetic
field. The temperature dependence of the thermopower is nonmonotonic, with a
minimum at a temperature of K. These results are discussed from the
perspective of Andreev reflection at the normal-metal/superconductor interface.Comment: 6 pages, 4 figure
Studying changes in the practice of two teachers developing assessment for learning
This paper describes changes in the practice of two teachers, observed over an eighteen month period, who were participating in a study intended to support teachers in developing their use of assessment in support of learning. The design of the intervention allowed each teacher to choose for themselves which aspects of their practice to develop. Analysis of lesson observations, journal entries and interviews indicate that both teachers were keen to change their practice, but were concerned about the disruption to their established routines, and in particular about the potential for loss of control of their classes. Both teachers did effect significant changes in their classrooms, but these tended to be developments of existing preferred ways of working, rather than radical innovations. In conclusion, it is suggested that to be most effective, teacher professional development needs to be structured strongly enough to afford teacher growth, but flexible enough to allow different teachers to take their practice in different ways
Prospects for Nuclear Microreactors: A Review of the Technology, Economics, and Regulatory Considerations
The nuclear energy sector is actively developing a new class of very small advanced reactors, called microreactors. This technology has disruptive potential as an alternative to carbon-intensive energy technologies based on its mobility and transportability, resilience, and independence from the grid, as well as its capacity for long refueling intervals and low-carbon emissions. Microreactors may extend nuclear energy to a new set of international customers, many of which are located where energy is at a price premium and/or limited to fossil sources. Developers are creating designs geared toward factory production where quality and costs may be optimized. This paper reviews the existing literature on the technology, potential markets, economic viability, and regulatory and institutional challenges of nuclear microreactors. The technological characteristics are reviewed to describe the wide range of microreactor designs and to distinguish them from large nuclear power plants and small modular reactor (SMR) designs.
The expanding literature on the cost competitiveness of SMRs relative to other nuclear and nonnuclear technologies is also reviewed, with an emphasis on understanding the challenges of making microreactors economically viable. A major part of this study focuses on the deployment potential of microreactors across global markets. Previous work on SMR market assessment is reviewed, and the adaptation of these studies to the deployment of microreactors is more fully examined. Characteristics that differentiate microreactors from SMRs and other energy technologies may make microreactors suitable for unique and localized applications if they can be economically competitive with other energy technologies, as well as meet regulatory and other societal requirements. Recent research on global markets for microreactors is evaluated and extended in this paper to a previously unevaluated use case in which microreactors can play a role in grid resiliency and integration with renewables. Further challenges associated with the commercialization of microreactors, in addition to cost competitiveness, are explored by examining the regulatory and safety challenges of microreactor deployment
Mechanisms for Non-Trivial Magnetization Plateaux of an S=1 Frustrated Spin Ladder
We investigate the non-trivial magnetization plateau at 1/4 of the saturation
magnetization of S=1 spin ladder, especially with reference to recent
experimental results on a new organic tetraradical
3,3',5,5'-tetrakis(N-tert-butylaminoxyl)biphenyl, abbreviated as BIP-TENO. We
propose three mechanisms for the formation of the plateau; the Neel mechanism,
the dimer mechanism and the spin-Peierls mechanism. We also discuss the effect
of four-spin exchange interactions.Comment: 3 pages, 5 figures, Orbital2001 (International Conference on Strongly
Correlated Electrons with Orbital Degrees of Freedom) (September 11-14, 2001.
Sendai
Ground State Properties of One Dimensional S=1/2 Heisenberg Model with Dimerization and Quadrumerization
The one dimensional S=1/2 Heisenberg model with dimerization and
quadrumerization is studied by means of the numerical exact diagonalization of
finite size systems. Using the phenomenological renormalization group and
finite size scaling law, the ground state phase diagram is obtained in the
isotropic case. It exhibits a variety of the ground states which contains the
S=1 Haldane state, S=1 dimer state and S=1/2 dimer state as limiting cases. The
gap exponent is also calculated which coincides with the value for the
dimerization transition of the isotropic Heisenberg chain. In the XY limit, the
phase diagram is obtained analytically and the comparison is made with the
isotropic case.Comment: 4 pages, 7 figure
Investigating the nature of the Fried Egg nebula: CO mm-line and optical spectroscopy of IRAS 17163-3907
Through CO mm-line and optical spectroscopy, we investigate the properties of
the Fried Egg nebula IRAS 17163-3907, which has recently been proposed to be
one of the rare members of the yellow hypergiant class. The CO J=2-1 and J=3-2
emission arises from a region within 20" of the star and is clearly associated
with the circumstellar material. The CO lines show a multi-component
asymmetrical profile, and an unexpected velocity gradient is resolved in the
east-west direction, suggesting a bipolar outflow. This is in contrast with the
apparent symmetry of the dust envelope as observed in the infrared. The optical
spectrum of IRAS 17163-3907 between 5100 and 9000 {\AA} was compared with that
of the archetypal yellow hypergiant IRC+10420 and was found to be very similar.
These results build on previous evidence that IRAS 17163-3907 is a yellow
hypergiant.Comment: 14 pages including appendix, accepted for publication in A&
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