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JumpStart - Physical Science
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Low-Temperature and Condensed Matter Physics* |
| Condensed matter physics is the study of all physical phenomena related to matter that is in its liquid or solid state. The collections of atoms that make up these condensed states react to variations in pressure, temperature, and other external stimuli. Condensed matter physicists seek to understand the rules that govern these responses so that they can create and improve models that can predict how atoms will react in certain circumstances. Some condensed matter physicists have particular interest in the properties of states of matter when they are near a critical point, which is a condition of temperature and pressure that is associated with a transition between two phases (i.e., from a liquid to a solid). When approaching a critical point, the properties of the two phases become very similar, and small fluctuations in thermal energy in the material can cause small regions in the sample to pass from one phase to the other. The fluctuations between the two phases tend to dominate the thermodynamic behavior of the material. Therefore, materials near critical points often display many unique properties called critical phenomena. These near-critical-point conditions also cause large numbers of atoms within materials to group together in clusters, the behaviors of which are easier to observe because they are much larger and move more slowly than individual atoms.
Special methods have been developed to study critical phenomena in materials that require extremely low temperatures to reach their critical points. A significant proportion of this work has focused on superfluid helium. The notable properties of a superfluid are its ability to conduct heat extremely rapidly without any temperature difference in the superfluid itself and its lack of viscosity (resistance to flow). At atmospheric pressure, helium becomes a liquid at 4.2 Kelvin (-269° Celsius, or 4.2° C above absolute zero) and reaches the superfluid state at 2.17 Kelvin. Liquid and superfluid helium also provide an excellent model system for developing theories about other substances, since interactions between the atoms within a liquid helium sample are limited. By eliminating gravity from the equations describing the motions of the atoms, physicists conducting experiments with helium in microgravity can be much more precise in their calculations of the effects of external stimuli, like heat, on a condensed matter state.
Microgravity provides a quiet environment (free of seismic vibrations and relatively free of other disturbances) for working with very sensitive measuring devices that use superconductors. This quiet environment presents fewer obstacles to accurate data collection and can be maintained for a longer period of experimental observation time in space than it can be on Earth. With more precise measuring instruments, increased observation times, and the significant reduction in the force of gravity upon an experiment sample, the "smearing" of data that makes it difficult to distinguish which external stimuli are causing specific effects on a sample is greatly reduced in microgravity.
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Another unique critical phenomenon occurring at low temperature is superconductivity. When a metal becomes a superconductor, it loses all electrical resistance and has the ability to expel magnetic fields. With superconductivity, electric currents can flow through a metal without any loss of energy, much like superfluid helium can flow without resistance. In a ring of superconducting material, a current (the supercurrent) can thus flow indefinitely, provided the ring is always kept below the superconducting transition temperature. Superconducting technology has been used effectively in experiments for purposes ranging from magnetic shielding to temperature measurement.
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