A1 and A2 transitions in superfluid 3He in 98% porosity aerogel

Superfluid 3He in high porosity aerogel is the system in which the effects of static impurities on a p-wave superfluid can be investigated in a systematic manner. We performed shear acoustic impedance measurements on this system (98% porosity aerogel) in the presence of magnetic fields up to 15 T at the sample pressures of 28.4 and 33.5 bars. We observed the splitting of the superfluid transition into two transitions in high fields in both bulk and liquid in aerogel. The field dependence of the splitting in aerogel resembles that of the bulk superfluid 3He caused by the presence and growth of the A1 phase. Our results provide the first evidence of the A1 phase in superfluid (3)He/aerogel.     

Specific heat of disordered superfluid 3He

The specific heat of superfluid 3He, disordered by a silica aerogel, is found to have a sharp discontinuity marking the thermodynamic transition to superfluidity at a temperature reduced from that of bulk 3He. The magnitude of the discontinuity is also suppressed. This disorder effect can be understood from the Ginzburg-Landau theory which takes into account elastic quasiparticle scattering suppressing both the transition temperature and the amplitude of the order parameter. We infer that the limiting temperature dependence of the specific heat is linear at low temperatures in the disordered superfluid state, consistent with predictions of gapless excitations everywhere on the Fermi surface.     

Nonuniversal behavior of scattering between fractional quantum Hall edges

In a fractional quantum Hall system with a narrow constriction, tunneling of quasiparticles between states at different edges can lead to resistance and to shot noise. The ratio of the shot noise to the backscattered current, in the weak scattering regime, measures the fractional charge of the quasiparticle, which has been confirmed in several experiments. However, the predicted nonlinearity of the resistance was apparently not observed in some of these cases. As a possible explanation, we consider a model where coupling between the current carrying edge mode and additional phononlike edge modes can lead to nonuniversal exponents in the current-voltage characteristic.     

The Serre spectral sequence of a multiplicative fibration

In a fibration we show that finiteness conditions on force the homology Serre spectral sequence with -coefficients to collapse at some finite term. This in particular implies that as graded vector spaces, is ``almost' isomorphic to . One consequence is the conclusion that is elliptic if and only if and are.

     

Loop space homology of spaces of LS category one and two

Partially supported by a NATO travel grant     

Inductive shielding of NMR phase noise

We report on a solution to the problem of phase noise in nuclear magnetic resonance (NMR) experiments. Phase noise refers to the variation in the phases of NMR signals from successive acquisitions due to an unstable applied field. Such a situation exists in high-field resistive Bitter magnets and, for sufficiently long timescales, can cause serious signal degradation upon signal averaging. An inductive shield, formed by a highly conducting metal tube placed around the sample and along the applied field, provides screening of the AC components of the applied field and thereby retains phase coherence over long periods. Although simple in principle there are technical difficulties for practical implementation of this method. We present demonstrations of the utility of this approach. In particular, we show a significant extension of the effective transverse coherence time of the ¹³C resonance in doubly ¹³C-labeled glycerol in a resistive Bitter magnet. This was accomplished through the use of a highly conducting aluminum shield, cooled to 4 K with liquid helium.     

The thermoluminescence (TL), phosphorescence and cryoluminescence of n-type hexagonal (6H) SiC crystals

The low temperature (down to liquid helium temperature) TL, phosphorescence and cryoluminescence of n-type 6H SiC crystals is described. The crystals contained nitrogen as the major impurity at concentrations of about 10¹⁶ cm^-3. The glow curves exhibited peaks at about 25, 45, 70 and 90°K (in addition to a peak at 250°K). Thermal activation energies for the above peaks ranged between 0.02 and 0.14 eV (0.30 eV for the 250°K peak). These are much lower than energies reported earlier for nitrogen donor levels in 6H SiC. The values obtained for the 70–90°K peaks (0.08–0.14 eV) fit quite well those obtained by electrical transport measurements and Raman scattering.The crystals exhibited strong phosphorescence even at liquid helium temperature. This was shown to be only partly due to thermal release from traps, the other components being due to pair-recombination and optical release from the shallow traps by the black body radiation (BBR) from the walls of the cryostat. This BBR was found to be responsible also for the observed cryoluminescence.