The Use of a Laser Doppler Velocimeter in a Standard Flammability Tube

The use of the Laser Doppler Velocimeter, (LDV), to measure the flow associated with the passage of a flame through a standard flammability limit tube (SFLT) was studied. Four major results are presented: (1) it is shown that by using standard ray tracing calculations, the displacement of the LDV volume and the fringe rotation within the experimental error of measurement can be predicted; (2) the flow velocity vector field associated with passage of an upward propagating flame in an SFLT is determined; (3) it is determined that the use of a light interruption technique to track particles is not feasible; and (4) it is shown that a 25 mW laser is adequate for LDV measurements in the Shuttle or Spacelab

Magnetic Field Dependent Tunneling in Glasses

We report on experiments giving evidence for quantum effects of electromagnetic flux in barium alumosilicate glass. In contrast to expectation, below 100 mK the dielectric response becomes sensitive to magnetic fields. The experimental findings include both, the complete lifting of the dielectric saturation by weak magnetic fields and oscillations of the dielectric response in the low temperature resonant regime. As origin of these effects we suggest that the magnetic induction field violates the time reversal invariance leading to a flux periodicity in the energy levels of tunneling systems. At low temperatures, this effect is strongly enhanced by the interaction between tunneling systems and thus becomes measurable.Comment: 4 pages, 4 figure

Parity Effects in Stacked Nanoscopic Quantum Rings

The ground state and the dielectric response of stacked quantum rings are investigated in the presence of an applied magnetic field along the ring axis. For odd number $N$ of rings and an electric field perpendicular to the axis, a linear Stark effect occurs at distinct values of the magnetic field. At those fields energy levels cross in the absence of electric field. For even values of $N$ a quadratic Stark effect is expected in all cases, but the induced electric polarization is discontinuous at those special magnetic fields. Experimental consequences for related nanostructures are discussed.Comment: typos corrected, to appear Phys. Rev. B (Rapid Communication) 15 Au

A Classroom Demonstration of Levitation and Suspension of a Superconductor over a Magnetic Track

The suspension and levitation of superconductors by permanent magnets is one of the most fascinating consequences of superconductivity, and a wonderful instrument for generating interest in low temperature physics and electrodynamics. We present a novel classroom demonstration of the levitation/suspension of a superconductor over a magnetic track that maximizes levitation/suspension time, separation distance between the magnetic track and superconductor and also insulator aesthetics. The demonstration as described is both inexpensive and easy to construct.Comment: 5 page

Comparative study of the effects of electron irradiation and natural disorder in single crystals of SrFe2_{2}(As1−x_{1-x}Px_x)2_2 (x=x=0.35) superconductor

London penetration depth, $\lambda(T)$, was measured in single crystals of SrFe$_2$(As$_{1-x}$P$_x$)$_2$ ($x=$0.35) iron - based superconductor. The influence of disorder on the transition temperature, $T_c$, and on $\lambda(T)$ was investigated. The effects of scattering controlled by the annealing of as-grown crystals was compared with the effects of artificial disorder introduced by 2.5~MeV electron irradiation. The low temperature behavior of $\lambda(T)$ can be described by a power-law function, $\Delta \lambda (T)=AT^n$, with the exponent $n$ close to one in pristine annealed samples, as expected for superconducting gap with line nodes. Upon $1.2 \times 10^{19}$ \ecm irradiation, the exponent $n$ increases rapidly exceeding a dirty limit value of $n=$ 2 implying that the nodes in the superconducting gap are accidental and can be lifted by the disorder. The variation of the exponent $n$ with $T_c$ is much stronger in the irradiated crystals compared to the crystals in which disorder was controlled by the annealing of the growth defects. We discuss the results in terms of different influence of different types of disorder on intra- and inter- band scattering

Correlated Persistent Tunneling Currents in Glasses

Low temperature properties of glasses are derived within a generalized tunneling model, considering the motion of charged particles on a closed path in a double-well potential. The presence of a magnetic induction field B violates the time reversal invariance due to the Aharonov-Bohm phase, and leads to flux periodic energy levels. At low temperature, this effect is shown to be strongly enhanced by dipole-dipole and elastic interactions between tunneling systems and becomes measurable. Thus, the recently observed strong sensitivity of the electric permittivity to weak magnetic fields can be explained. In addition, superimposed oscillations as a function of the magnetic field are predicted.Comment: 4 page

Magnetic field effect on the dielectric constant of glasses: Evidence of disorder within tunneling barriers

The magnetic field dependence of the low frequency dielectric constant $e_r$(H) of a structural glass a - SiO2 + xCyHz was studied from 400 mK to 50 mK and for H up to 3T. Measurement of both the real and the imaginary parts of $e_r$ is used to eliminate the difficult question of keeping constant the temperature of the sample while increasing H: a non-zero $e_r$(H) dependence is reported in the same range as that one very recently reported on multicomponent glasses. In addition to the recently proposed explanation based on interactions, the reported $e_r$(H) is interpreted quantitatively as a consequence of the disorder lying within the nanometric barriers of the elementary tunneling systems of the glass.Comment: latex Bcorrige1.tex, 5 files, 4 figures, 7 pages [SPEC-S02/009

Superconducting Nb-film LC resonator

Sputtered Nb thin-film LC resonators for low frequencies at 0.5 MHz have been fabricated and tested in the temperature range 0.05--1 K in magnetic fields up to 30 mT. Their Q value increases towards decreasing temperature as sqrt(T) and reaches 10^3 at 0.05 K. As a function of magnetic field Q is unstable and displays variations up to 50%, which are reproducible from one field sweep to the next. These instabilities are attributed to dielectric losses in the plasma deposited SiO_2 insulation layer, since the thin-film coil alone reaches a Q > 10^5 at 0.05 K.Comment: 6 pages, 7 figures, submitted to Review of Scientific Instrument