incorporate into the lattices of these materials junction between an s- and a p -state super- Clearly, much work remains to be done, (nonmagnetic impurities come free). Another conductor. However, a poor junction would but (the data now available at least refute the test is based on the fact that a “supercurrent” also kill a supercurrent, and good junctions conventional wisdom of a dichotomy be- would not flow through a loop containing a are extremely difficult to prepare. tween superconductivity and a tendency Getting Close to Absolute Zero iquid helium-4 and helium-3 rank with ature limit is set not by freezing (as it is for a heated still where helium-3 evaporates vacuum as sine qua nons for many normal liquids) but by a rapid decrease in preferentially. For economy the helium-3 is L scientific experiments. Some phenom- vapor pressure. condensed and the liquid returned to the ena occur only at temperatures achievable Even lower temperatures (down to about system. The photograph shows author Jef- with these unusual liquids, and others be- 0.005 kelvin) can be reached with a “dilu- frey O. Willis examining a UPt3 whisker in come much more tractable to theoretical tion” refrigerator. This device exploits the the cryostat of the Physical Metallurgy interpretation. natural tendency of liquid helium-3 to Group’s dilution refrigerator. A dewar con- Gaseous helium-4 occurs on the earth as a “evaporate” into the “mechanical vacuum” taining liquid helium encloses the cryostat product of alpha decay and is found in of liquid helium-4. (These two liquids, despite when the refrigerator is operating, About reasonable concentrations in some natural both consisting of isotopes of the same ele- twenty-four hours are required to cool a gas fields. It was first liquefied in 1908 by ment, interact very weakly because one sample to the desired temperature. Heike Kamerlingh Onnes (whose discovery (helium-4) follows Bose-Einstein statistics Temperatures in helium-3 and helium-4 of superconductivity soon followed). Tem- and the other follows Fermi-Dirac statistics.) evaporation refrigerators are determined peratures between about 1 kelvin and the The atoms of helium-3 absorb heat (cor- simply by measuring the vapor pressure. boiling point of liquid helium (4.2 kelvins) responding to the heat of evaporation) as Thermometry in a dilution refrigerator in- can be attained simply by pumping on the they cross the phase boundary between these volves use of a material whose magnetic liquid. The atoms crossing the liquid-vapor two dissimilar liquids. The lower temperature susceptibility is known to be quite closely phase boundary absorb heat, and the remain- limit is set not by a decrease in the “vapor inversely proportional to the temperature. ing liquid cools. Somewhat lower tem- pressure” as the temperature falls but by a The susceptibility versus temperature curve peratures (routinely down to between 0.5 and decrease in the heat of “evaporation.” for this material is calibrated against vapor 0.3 kelvin, depending on the system) can be The accompanying diagram illustrates pressure measurements in a helium-3 evapo- reached by pumping on liquid helium-3. (This schematically the continuous operation of a ration refrigerator. and lower temperatures stable but naturally extremely rare isotope is dilution refrigerator. Liquid helium-3 dis- are obtained by extrapolation. ■ a by-product of the manufacture of nuclear solves in liquid helium-4 in the mixing weapons.) For both liquids the lower temper- chamber, and the dilute solution is pumped to 66 Spring 1984 Los ALAMOS SCIENCE.