E. D. Adams

Electron correlation in the second Landau level: a competition between many nearly degenerate quantum phases.

At a very low-temperature of 9 mK, electrons in the second Landau level of an extremely high-mobility two-dimensional electron system exhibit a very complex electronic behavior. With a varying filling factor, quantum liquids of different origins compete with several insulating phases leading to an irregular pattern in the transport parameters. We observe a fully developed nu=2+2/5 state separated from the even-denominator nu=2+1/2 state by an insulating phase and a nu=2+2/7 and nu=2+1/5 state surrounded by such phases. A developing plateau at nu=2+3/8 points to the existence of other even-denominator states.

The Provisional Low Temperature Scale from 0.9 mK to 1 K, PLTS-2000

An internationally-accepted ultra-low temperature scale is needed to provide the basis for reliable thermometry in the temperature range in which commercial dilution refrigerators operate, and in experiments investigating the properties of 3He and other condensed matter. Several laboratories have developed 3He melting-pressure scales, but there are substantial differences even between the most recent of them. These amount to about 0.3% of T near 500mK, rising to about 6% of T at 0.9mK. In 1996 a collaboration was initiated between low temperature physicists in national laboratories and elsewhere to derive an equation for the melting pressure of 3He which could be accepted for international use from 1K to 0.9mK, the Néel temperature of solid 3He. After an open workshop in Leiden in 1998, discussions took place to see if thermodynamic calculation of 3He melting pressures could resolve the differences. In January 2000 the authors (apart from ALR and GS) met at NIST and were able to reach a compromise on the Provisional Low Temperature Scale, PLTS-2000. Its 1-sigma uncertainty is estimated to be 0.3% of T (up to a maximum of 0.5mK), but this rises to about 2% of T at 0.9mK. The provisional status recognizes that the PLTS-2000 is a compromise, rather than a true consensus, but it is likely to be some years before it can be replaced by a more accurate scale. The scale was announced at the Quantum Fluids and Solids conference in Minnesota, USA, in June 2000, and was formally adopted by the Comité International des Poids et Mesures in October 2000.

Thermal expansion and isothermal compressibility of solid nitrogen

The coefficients of linear thermal expansion and of isothermal compressibility have been measured for solid nitrogen in the temperature intervals from 4.2° to 40° K and 8° to 40° K, respectively. Length changes of the sample were detected with a resolution of 10−9 cm using a dilatometer of the three-terminal capacitor variety contained inside a pressure vessel. Apparatus and experimental technique are described in detail. The thermal expansion and compressibility of α-N2 increase rapidly with increasing temperature above 20° K, and in the former case the length increase is exponential in 1/T above 24° K. A reproducible value for the volume discontinuity at the α-β crystallographic transformation at 35.6° K was obtained only if many hours were taken to effect the transformation. The data are discussed in relation to recent experimental and theoretical information obtained from studies of Raman and infrared spectra, nuclear quadrupole resonance, and specific heat. The anomalous thermal expansion is attributed to excited librational and translational states of large amplitude and appreciable anharmonic content. The adiabatic compressibility calculated from the experimental data agrees well with published values from sound velocity experiments. The Grüneisen parameter of α-N2 is found to be strongly temperature dependent.

Melting curve and related thermodynamic properties of He3 below 1K

Measurements of the melting curve of He3 have been made between 16.6° mK and 0.7°K. The liquid and solid molar volumes at melting have been extended from 0.33°K to below 0.03°K, and the molar volume change on melting ΔVm obtained. An increase in ΔVm from 1.20 cm3/mole at 0.33°K to 1.28 cm3/mole at 0.06°K is found, with an extrapolation to 1.27 cm3/mole atT=0. Examination of the thermodynamic consistency of the data indicates that our melting curve slopes are accurate, within experimental error, down to a temperature of 0.04°K. The demagnetization apparatus is described, with considerable attention devoted to thermometry, thermal equilibrium, and experimental techniques. Construction details of the high-sensitivity capacitive strain gauge are given.

Experimental studies of the fractional quantum Hall effect in the first excited Landau level

We present a spectrum of experimental data on the fractional quantum Hall effect (FQHE) states in the first excited Landau level, obtained in an ultrahigh mobility two-dimensional electron system and at very low temperatures, and report the following results. For the even-denominator FQHE states, the sample dependence of the

Thermodynamics of freezing and melting of4He in Vycor

\ensuremath{\nu}=5∕2

Nuclear spin ordering of solid3He in applied magnetic fields

state clearly shows that disorder plays an important role in determining the energy gap at

3He melting pressure temperature scale below 25 mK

\ensuremath{\nu}=5∕2

Solidification of helium in confined geometries

. For the developing

Thermal expansion of solid methane

\ensuremath{\nu}=19∕8