David G. Haase

Thick film chip resistors for use as low temperature thermometers

Abstract We have measured the resistance and magnetoresistance of commercially produced thick film resistors in the range 80 K-15 mK and in fields up to 20 T. These resistors behave similarly to those produced by another manufacturer and characterized recently by a Japanese group. Between 2.5 and 0.1 K the resistance of a 1 kΩ resistor can be fit to R=A exp (B/T 1 4 ) . A 100 kΩ resistor appears to fit a similar expression between 1.6 and 6 K and fits a T −1 2 law from 8 to 80 K. These resistors are very stable upon thermal cycling, have a small magnetoresistance, and should be useful as thermometers over a wide temperature range.

Effect of cation on the structures, magnetic properties and specific heats of salts of bis(5,6-dihydro-1,4-dithiin-2,3-dithiolato)nickelate(III): the crystal structure of[(CH-3)4N] [Ni(DDDT)2]

Abstract The structure of the tetramethylammonium salt of Ni(DDDT)2- (DDDT2-=5,6-lihydro-l,4-dithiin-2,3-dithiolate) has been determined by X-ray crystallography. The crystal belongs to the orthorhombic space group Pbca with unit cell dimensions a=0.543(2), b=13.937(5), c=27.518(7) A, Z=8. The NiS4 core exhibits square planar coordination with an average NiS bond length 2.146(2) A. The crystal packing can be described as interlocking colums consisting of alternating cations and anions. The columns form a zigzag pattern when viewed down the y axis. The shortest S···S contact is 3.57 A with a total of four contacts less than 3.8 A. The X-ray powder diffraction spectrum of this salt showed the presence of only one crystallographic species, while that of the tetrabutylammonium salt indicated the possibility of a third polymorph. Magnetic susceptibility data for [(CH3)4N] [Ni(DDDT)2] were fit to the Curie-Weiss law by expanding the Weiss constant as an infinite series. The specific heats of the [(CH3)4N]+, [(C2H5)4N]+ and [(C4H9)4N]+ salts of Ni(DDDT)2− were measured between 1 and 25 K. Similar specific heats were observed above 4 K but the values differed significantly below this temperature.

Test of parity-conserving time-reversal invariance using polarized neutrons and nuclear spin aligned holmium.

A test of parity-conserving, time-reversal noninvariance (PC TRNI) has been performed in 5.9 MeV polarized neutron transmission through nuclear spin aligned holmium. The experiment searches for the T-violating fivefold correlation via a double modulation technique{emdash}flipping the neutron spin while rotating the alignment axis of the holmium. Relative cross sections for spin-up and spin-down neutrons are found to be equal to within 1.2{times}10{sup {minus}5} (80{percent} confidence). This is a two orders of magnitude improvement compared to traditional detailed balance studies of time reversal, and represents the most precise test of PC TRNI in a dynamical process, to our knowledge. {copyright} {ital 1997} {ital The American Physical Society}

Direct reaction test of T violation in 2 MeV neutron scattering from aligned 165Ho

Abstract The parity-even time-reversal-odd five-fold correlation has been studied in 2 MeV polarized neutron transmission through a rotating, cryogenically-aligned 165Ho target. The analyzing power was found to be (1 ± 6) × 10−4, consistent with time reversal invariance. We obtain a bound of 5 × 10−3 for αT, the ratio of T-odd to T-even couplings in the effective N−N interaction.

Measurement of current voltage characteristics of single grain boundaries in melt textured bulk YBa2Cu3Ox

We have investigated the electrical properties of small parts of single grain boundaries in melt textured YBa2Cu3Ox. Similar measurements on grain boundary networks were also made to obtain composite properties. The results of Tc measurements and current‐voltage characteristics in zero and small magnetic fields are presented. The R(T) characteristics suggest that there is second phase segregation in some parts of the grain boundary area. The current‐voltage characteristics showed three different types of behavior: Resistive, Josephson‐like, and flux flow. The Ic(B) data of Josephson junctions suggest that nonsuperconducting inclusions divide the grain boundaries into smaller single junctions.

Pressure measurements of the solid hydrogen quadrupolar glass

Abstract We report measurements of the pressure and (∂P/∂T)V in samples of solid hydrogen of orthohydrogen concentrations of 40–55% and 150 mK ⩽ T ⩽ 1 K. No evidence of a first or second order phase transition is seen at temperatures associated with the formation of a quadrupolar glass.

Strong and weak-link behavior of single grain boundaries in melt textured bulk Ag doped YBa2Cu3O7−δ

Abstract Electrical characterizations of single isolated grain boundaries have been carried out in melt textured bulk Ag doped YBa 2 Cu 3 O 7−δ . Various measurements such as I − V characteristics and critical current as a function of applied field are reported for numerous single boundaries as well as ensembles of boundaries. Most of the single boundaries in Ag doped YBCO showed strong-link behavior in contrast to pure YBCO prepared by a similar method, which showed weak-link behavior. One possible explanation for strong-link behavior is the enhanced c -axis texture observed in the Ag doped samples.

An apparatus and techniques of tests for fundamental symmetries in compound-nucleus scattering with epithermal polarized neutron beams

Abstract The epithermal polarized-neutron beam facility used for tests of fundamental symmetries is described. The initial unpolarized beam is obtained from the spallation source at the Los Alamos Neutron Scattering Center. Characteristics of the polarized and unpolarized beams are described, as well as design and performance of a fast-spin reversal system, neutron flux monitor and target cooler. Experimental results for polarized-neutron transmission experiments are given to illustrate the overall quality of the system.

The YouTube Makeup Class

When a college instructor goes out of town and must miss a lecture, the standard options are to cancel the class meeting or to enlist a colleague to fill in. In the former case a teaching opportunity is lost; in the latter the substitute may not lead the class in the same way as the instructor. Some students routinely skip lectures by a guest instructor, in the belief that the material in the substitute lecture will not be covered on the exam. There are other makeup options such as a directed study assignment. For instance, a missed class is sometimes a good opportunity to require students to investigate web-based simulations such as Physlets® that illustrate the class topics. These are most effective if the students are given a clear structure and if there are questions that the students must answer from their investigations with the Physlets. But many students are more comfortable with the audio and visual communication that occurs in the classroom. Web 2.0 technology, e.g., YouTube (http://www.youtube....

PARITY AND TIME REVERSAL VIOLATION IN RESONANCE NEUTRON TOTAL CROSS SECTIONS WITH POLARIZED TARGETS

The formalism for evaluating parity and time reversal violating terms in total cross sections of polarized targets and low energy resonance neutrons is reviewed. A complete set of symmetry violating terms (P-odd, T-even; P-odd, T-odd; and P-even, T-odd) is obtained by analyzing the dependence of the cross section on the statistical tensors describing the beam and target. Results are tabulated in numerical and closed analytic form, using the j-spin representation. P-odd, T-even experiments are classified, with emphasis on experiments with an unpolarized beam and a polarized target where the effect of induced polarization is found to be small. Different combinations of partial neutron widths are shown to enter when P-odd, T-odd effects are compared to P-odd, T-even effects. The results for P-even, T-odd experiments for single level and two level mixing are summarized.