Search Results (10 total)

Physics education research increasingly makes use of video data for analysis of student learning and teaching practice. Collection of these data is conceptually simple but execution is often fraught with costly and time-consuming complications. This pragmatic paper discusses the development of systems to record and permanently archive audio and video data in real-time. We focus on a system based upon consumer video DVD recorders, but also give an overview of other technologies and detail issues common to all systems. We detail common yet unexpected complications, particularly with regard to sound quality and compatibility with transcription software. Information specific to fixed and transportable systems, other technology options, and generic and specific equipment recommendations are given in supplemental appendices

We measure the temperature and frequency dependence of the complex Hall angle for normal state YBa₂Cu₃O₇ films from dc to far-infrared frequencies (20–250 cm^-1) using a new modulated polarization technique. We determine that the functional dependence of the Hall angle on scattering does not fit the expected Lorentzian response. We find spectral evidence supporting models of the Hall effect where the scattering Γ_H is linear in T, suggesting that a single relaxation rate, linear in temperature, governs transport in the cuprates.

Infrared ( 20–120 and 900–1100 cm^-1) Faraday rotation and circular dichroism are measured in high- T_c superconductors using sensitive polarization modulation techniques. Optimally doped YBa₂Cu₃O₇ thin films are studied at temperatures in the range ( 15<T<300  K) and magnetic fields up to 8 T. At 1000 cm^-1 the Hall conductivity σ_xy varies strongly with temperature in contrast to the longitudinal conductivity σ_xx which is nearly independent of temperature. The Hall scattering rate γ_H has a T² temperature dependence but, unlike a Fermi liquid, depends only weakly on frequency. The experiment puts severe constraints on theories of transport in the normal state of high- T_c superconductors.

A bunch-length measuring method has been developed to measure the subpicosecond electron pulses generated at the Stanford University Short Intense Electron Source (SUNSHINE) facility. This method utilizes a far-infrared Michelson interferometer to measure coherent transition radiation emitted at wavelengths longer than or equal to the bunch length via optical autocorrelation. To analyze the measurement, a simple and systematic way has also been developed, which considers interference effects on the interferogram caused by the beam splitter; hence the electron bunch length can be easily obtained from the measurement. This simple, low-cost, frequency-resolved autocorrelation method demonstrates subpicosecond resolving power that cannot be achieved by existing time-resolved methods.

Stimulated, coherent transition radiation (STR) has been observed at the Stanford SUNSHINE facility. Far-infrared light pulses of coherent transition radiation emitted from femtosecond electron bunches are recycled in a special cavity to arrive back at the radiator coincident with subsequent incoming electron bunches. This overlap enables the electrons to do work on the electromagnetic field, thus stimulating the emission of more radiated energy than would be possible without this external field. The experimental setup to observe STR via cavity detuning measurements and experimental results is discussed.

An electron source has been developed at the Stanford SUNSHINE facility which can produce electron bunches as short as 50 fs (rms) with (2-4.6)×10⁸e^- per microbunch. This source consists of a 2.6 MeV rf gun with a thermionic cathode and an alpha magnet for bunch compression. Coherent transition radiation emitted at wavelengths equal to the bunch length and longer is used in a Michelson interferometer to determine the bunch length by optical autocorrelation. The experimental setup and results of bunch length measurements are described.

Magnetic hysteresis, flux pinning, and flux creep in melt-powder-melt-growth processed YBa₂Cu₃O₇ (Y 1:2:3) containing nominal 0, 25, and 40 mol % concentration of Y₂BaCuO₅ (Y 2:1:1) inclusions were investigated. The strong pinning due to 2:1:1-phase precipitates in these samples allows for characterization of the hysteretic response as a function of pinning-site concentration over a large portion of magnetic-field–temperature space. We have found the following: (i) The curves of effective pinning energy U_eff versus current density J reveal a diverging behavior of U_eff(J) in the low-J regime. This supports the existence of a vortex-glass state, and is a signature of a vanishing resistance as the current density approaches zero. (ii) Both the U_eff and the J values obtained from magnetic hysteresis loops were observed to increase with Y 2:1:1 concentration. The appearance of the butterfly-shaped (or ‘‘fishtail’’) hysteresis loops indicates a J_c that is an increasing function of H (or B). Moreover, it has been demonstrated that the additional pinning leads to an increase in U_eff in an H-T region in which the butterfly is developed.

The derived effective pinning energy is fit, from the instantaneous experimental relaxation data, to the relation, U_eff(J,T,H)=U_i[G(T)/Hⁿ](J_i/J)^μ, where U_i is the scale of the activation energy, G(T) =[1-(T/T_x)²]^m, and T_x is close in value to T_irr(H) (the irreversibility line of the material). This description breaks down in the vicinity of the ‘‘butterfly’’ peak. We observed two power-law regimes of J dependence of U_eff which have μ values that agree qualitatively with the theoretical predictions (=7/9 and 3/2) for a three-dimensional flux-line lattice.

The critical current density J_c in Y_0.9Gd_0.1Ba₂Cu₃O_7-x was found to increase relative to the unirradiated value following neutron irradiations in a mixed-spectrum reactor (total neutron fluences ranged between 1×10¹⁷ and 2×10¹⁸ n/cm²). Additional neutron irradiations of structurally similar GdBa₂Cu₃O_7-x were carried out in either a highly thermalized or a pure fast-neutron environment (in the same reactor). This was done to determine whether enhancements in J_c are to be attributed to defects arising from interactions with thermal neutrons (E_n∼0.025 eV) or with fast neutrons (E_n>0.1 MeV). Magnetic-hysteresis measurements on these samples indicate that flux pinning (and thereby J_c) is enhanced by fast-neutron irradition, but not by thermal-neutron irradiation. On the other hand, the critical temperature T_c is significantly altered by exposure both to thermal and fast neutrons. It is proposed that thermal neutrons induce the formation of Frenkel pair defects on the rare-earth sublattice, but that these point defects do not serve as effective flux-pinning centers.

Magnetic hysteresis and flux creep characteristics in melt-powder-melt-growth YBa₂Cu₃O₇ with Y₂BaCuO₅ inclusions were measured between 5 and 80 K for magnetic fields up to 5 T. The critical magnetization current densities which were calculated using the Bean model and the sample dimension show a weak dependence on magnetic field. The J_c values are of the order of 10⁴ A/cm² at 60 K over the investigated field range. The magnetic relaxation rate Q=-dM/d lnt first drops monotonically with increasing temperatures and then gradually saturates, which is similar to the behavior of the initial magnetization M₀ versus temperature. The normalized relaxation rate S=d lnM/d lnt, plotted as a function of temperature, shows a maximum at around 30 K, and the corresponding values of S and T at the maximum are field dependent. A scaling relationship U_eff(J,H)=U_iG(T)F(J/J_i)/H^0.55, where G(T)=1-(T/T_x)², F(J/J_i)∼(J/J_i)^-n, and U_i and J_i are scaling constants, is proposed to fit the creep data from which a universal curve is attained. The characteristic temperature T_x for the scaling function G(T) is determined from the irreversibility line. The current-dependent behavior of U_eff obtained from this work agrees qualitatively with the result predicted by the theory of collective flux creep and suggests a vortex-glass state at low current density.

We have measured the dependence of the transport critical current density on magnetic field and temperature of a high-J_c Bi-Sr-Ca-Cu-O/Ag tape sample. At low temperature, J_c>10⁴ A/cm² extending to high fields as previously reported by several groups. For T>20 K and with the applied field parallel to the c axis J_c declines precipitously with increasing field and temperature, reflecting the properties of intragranular pinning. The variation of J_c with the angle between B and the tape normal is consistent with a two-dimensional model of the vortex lattice. Independent determination of J_c by magnetic hysteresis measurement shows rough agreement with transport J_c’s at low fields but falls below by a factor that increases with increasing field and temperature. This can be accounted for by a model for thermally activated flux motion.