Search Results (14 total)

The Colorado Learning Attitudes about Science Survey (CLASS) is a new instrument designed to measure student beliefs about physics and about learning physics. This instrument extends previous work by probing additional aspects of student beliefs and by using wording suitable for students in a wide variety of physics courses. The CLASS has been validated using interviews, reliability studies, and extensive statistical analyses of responses from over 5000 students. In addition, a new methodology for determining useful and statistically robust categories of student beliefs has been developed. This paper serves as the foundation for an extensive study of how student beliefs impact and are impacted by their educational experiences. For example, this survey measures the following: that most teaching practices cause substantial drops in student scores; that a student’s likelihood of becoming a physics major correlates with their “Personal Interest” score; and that, for a majority of student populations, women’s scores in some categories, including “Personal Interest” and “Real World Connections,” are significantly different from men’s scores.

We describe an efficient Monte Carlo simulation of profile decay via surface diffusion on a (1+1)D square-lattice system with suppressed roughening. Our algorithm takes advantage of analytical results for a one-dimensional random walker and is valid in the low-temperature limit of noninteracting activated walkers. We find that an initially sinusoidal profile decays nonexponentially with a characteristic decay time which increases with the initial amplitude. At a fixed ratio of initial amplitude to wavelength, the decay time τ increases with wavelength λ as τ∼λ^3.5±0.1. In this model, profile decay arises from irreversible decay of the peak and valley terraces rather than from step-step interactions.

We describe a two-dimensional correlated-percolation model, which we call annealed percolation. In annealed percolation, attractive interactions between near-neighbor occupied sites produce a structure reminiscent of discontinuous metal films. We have carefully determined the percolation threshold p_c and four exponents for this model: the correlation-length exponent ν, the cluster-size-distribution exponent τ, the fractal dimension of the infinite cluster d_f, and the electrical-conductivity exponent t. We find that all these exponents have the same values as in pure (uncorrelated) percolation. We also compute the electrical conductance G versus concentration p for this model and compare with the G vs p curves for pure-site and pure-bond percolation models.

Scanning-tunneling-microscope studies of mass flow on (111) Au have shown that the rate of decay of the area of monolayer islands is linear in time. We demonstrate by Monte Carlo simulation that a proper accounting of the detachment and reattachment rates of kink site atoms produces a linear decay. In our simulation, adatoms emitted from kink sites on mesa edges execute an unbiased random walk and may reattach to the home mesa or be absorbed by an outer ledge, possibly after several reflections from the step down at the outer ledge. The linear decay of mesa areas is found to be rather insensitive to variations in the probability of reflection from the outer ledge.

Metals thermally evaporated onto warm insulating substrates evolve to the thin-film state via the morphological sequence: compact islands, elongated islands, percolation, hole filling, and finally the thin-film state. The coverage at which the metal percolates (p_c) is often considerably higher than that predicted by percolation models, such as inverse swiss cheese or lattice percolation. Using a simple continuum model, we show that high-p_c’s arise naturally in thin films that exhibit a crossover from full coalescence of islands at early stages of growth to partial coalescence at later stages. In this interrupted-coalescence model, full coalescence of islands occurs up to a critical island radius R_c, after which islands overlap, but do not fully coalesce. We present the morphology of films and the critical area coverages generated by this model.

Giant discrete resistance fluctuations are reported in large (50×50 μm²) tunnel junctions of Au-Al₂O₃-Au over the temperature range 4–140 K, with applied dc bias voltages of 0–2 V. The fluctuations are believed to result from single electrons trapped on defect sites within the insulating barrier of the tunnel junction. The temperature and bias-voltage dependence of the characteristic switching lifetimes of these fluctuations have been measured, and departures from previously reported behavior are observed.

In this paper we make the first careful comparison between a computer simulation and an experimental measurement of the conductance of a two-dimensional continuum random conducting medium. In the experiment horizontal and vertical slits are cut in a conducting sheet. The centers of the slits are randomly positioned, and the conductance is measured all the way to percolation. The measurements are consistent with the expected critical exponent for the conductance of t=1.3. The experimental results are compared with computer simulations of ants that are parachuted to random starting points and then diffuse with a Brownian motion. From the behavior at large times, the diffusion constant can be found and hence the conductance, using the Einstein relation. The agreement with experiment is good except near the critical point. We conclude that the analog experiment is superior to the digital computations in this continuum system. This is the reverse of the situation in discrete lattice systems.

Using a new experimental technique, we have measured the fourth moment of the current distribution in two-dimensional random resistor networks. The fourth moment is proportional to relative resistance fluctuations in a network of noisy resistors, and, until now, noise measurements provided the only experimental probe of this higher moment. We show that the fourth moment is simply related to the resistance change due to joule heating in a network of temperature-dependent resistors. We report measurements on both square-lattice and random-void continuum networks, fabricated by scribing computer-generated percolation patterns on sheets of aluminized Mylar.

We have measured the low-field magnetoresistance and current-voltage characteristics of polycrystalline Y₁Ba₂Cu₃O_7-δ near the superconducting-normal transition. The development of a fully superconducting state occurs via an intermediate phase about 2 K in width. Within this intermediate phase, sample dissipation is highly non-Ohmic and exhibits extreme sensitivity to small magnetic fields. The nonlinear I-V characteristics describe a new phase with zero resistance and zero critical current which seems to arise from the presence of strong junction disorder.

The NMR properties of the ¹⁸³W spin system have been measured in the nonstoichiometric compound Na_xTa_yW_1-yO₃, 0<x,y<1. This material, called a bronze, undergoes a metal-insulator (M-I) transition at x-y=0.18 while maintaining a cubic-crystal phase. The ¹⁸³W NMR properties measured include the Knight shift, the line shape, the spin-lattice relaxation time T₁, and the spin-spin phase memory time T₂. The measurements were made in a field of 6.0 T at temperatures of 4.2 and 77 K. As the sample composition is varied from metallic to insulating, the recovery curves of the W nuclear magnetization at 4.2 K evolve smoothly from an exponential curve with a T₁ of a few seconds to a dramatically nonexponential curve with a distribution of relaxation times ranging from seconds to days. The combination of the small moment of ¹⁸³W and wide, inhomogeneously broadened NMR lines inhibits spin diffusion so that spins relax at a rate determined by their immediate electronic environments. The distribution of T₁’s thus probes the distribution of local electronic environments in this disordered material.

Using an analog simulation technique, we have studied the conductivity transition in two-dimensional percolating networks. A computer-controlled x-y plotter scribes a percolating pattern on a sheet of aluminized plastic while the resistance of the sheet is continuously monitored. With this technique, we have measured the conductivity exponent t for two systems: site percolation on a square lattice and random-void continuum percolation. We find t=1.29±0.03 for the lattice and t=1.34±0.07 for the continuum, in agreement with a recent theoretical prediction that the conductivity exponents for these two systems are the same. We have also verified a theoretical estimate of the magnitude of conductance fluctuations due to finite-size effects.

The strength of the Ruderman-Kittel exchange interaction which couples nearest-neighbor ¹⁸³W nuclei in tungsten metal is measured by using a pulsed NMR technique. By measuring the frequency of the ‘‘slow beats’’ in the spin-echo amplitude, the strength of the coupling is found to be J/(2π)=81±1 Hz. The field gradients necessary for the appearance of the slow beats are produced by trace amounts of paramagnetic impurities in the tungsten samples. This sensitivity to trace impurities is a consequence of the smallness of the magnetic moment of ¹⁸³W.

We have measured the dc electrical conductivity σ of cubic Na_xTa_yW_1-yO₃ from 1.6 to 295 K for various values of x and y. A sample with x-y=0.18 appears to fall directly at the metal-insulator transition and shows an unusual temperature dependence, σ(T)∝T, from 1.6 K to room temperature. A model for σ(T) based on the scaling theory of localization, when used to interpret the data on this and other samples, gives a conductivity exponent ν of 1.0. This model assumes that σ(T) is controlled by a thermal smearing of the occupancy of one-electron energy states near the Fermi level rather than by inelastic scattering. Comparison of the conductivity transition in Na_xTa_yW_1-yO₃ with earlier data for the uncompensated material Na_xWO₃ indicates that the additional disorder introduced by Ta doping does not shift the critical value of electron concentration.