Low-Temperature Physics
Low-temperature physics studies the behavior of matter at temperatures approaching absolute zero (-273.15°C) with the use of experimental cryogenic methods. At such extremely cold temperatures, the optical, thermal, electric, and magnetic properties of materials undergo significant changes compared to those properties at warmer temperatures. Two classic examples of these changes are superconductivity and superfluidity. Major advances in low-temperature physics were made during the early twentieth century. Superconductivity was discovered in 1911 by Dutch physicist H. K. Onnes, who spent his scientific career developing cryogenic methods and investigating the properties of very cold materials. In 1908, he succeeded in liquefying helium by cooling it to -269°C (4K), thus providing experimentalists with the possibility to work at previously unreachable temperatures. By immersing samples in liquid helium with the use of cryostats or refrigerating units, physicists were able to cool other materials to temperatures approaching absolute zero (0K), the coldest temperature possible and the temperature at which the energy of material becomes as small as possible. In 1911, Onnes began to study the electrical properties of metals at liquid helium temperatures. It was known that the electrical resistance of metals decreased with temperature, but the limiting value of their resistance was unknown, especially as the temperature approached zero k. William Thomson Kelvin held that the electron flow would completely stop at such temperatures. Onnes believed that the resistance would dissipate, i.e., that there would be an incremental and gradual decrease in resistance and a corresponding increase in the conductivity of the cooled material. He passed a current through a mercury wire of high purity and measured its resistance as he gradually lowered the temperature to 4.2K, at which point the resistance suddenly disappeared. Superconductivity had been discovered and in 1972, the Nobel Prize for physics was conferred to John Bardeen, L. N. Cooper, and J. R.
Schrieffer for formulating a fundamental theory of superconductivity, now called BCS theory in their honor.
The discovery of superfluidity in helium represents the other success story of low-temperature physics. Superfluidity is a consequence of the atomic structure of helium, which naturally occurs as two isotopes, 4-helium and 3-helium. With an even number of four particles in its nucleus, four-helium is a boson and can thus undergo Bose-Einstein condensation, which brings it to a state of lowest possible energy where friction vanishes, allowing the helium to behave as a superfluid with the ability to flow through any opening, no matter how small. 4-helium superfluidity was discovered in 1938 by the Russian physicist Pyotr Kapitsa and 3-helium superfluidity was first observed by D. M. Lee, D. D. Osheroff and R. C. Richardson in the 1970s. Unlike 4-helium, 3-helium is a fermion because its nucleus has an odd number of particles and theoretically it should not be able to undergo Bose-Einstein condensation. However, BCS theory proposed a likely mechanism by which the 3-helium fermions could combine to form pairs and thus behave as bosons that could become superfluid at very low temperatures.
Present-day low-temperature physics focuses its efforts on investigating the properties of insulators, semiconductors, conductors, and superconductors in a dynamic search for improved materials and more fundamental insights into superconducting theory. For example, direct tunneling spectroscopy of the single-particle band gap in bulk Si:B was recently reported and the interesting question of how Coulomb interactions evolve across the metal-insulator transition could be examined. With advances in cryogenic technology, cryostats are now capable of cooling materials to temperatures of a few millikelvin. This is achieved by insulating the actual sample compartment from the surrounding room temperature within a concentric chamber, itself cooled with liquid nitrogen (77K). In other applications, low-temperature scanning tunneling microscopes have been developed to study the topographic and electronic structure of surfaces on the atomic scale. Such studies combine helium temperature measurements with the application of external magnetic fields reaching up to 7 tesla to investigate semiconducting materials, high superconducting crystals, spin glasses, nanotubes, and gold clusters. Techniques combining ultrahigh spatial and temporal resolution, based on near-field optical spectroscopy and femtosecond laser systems are also being used to study phenomena that are at the forefront of quantum physics, such as quantum wells, wires, and dots on the ultrafast time scales required, i.e., ranging from some hundred picoseconds to few femtoseconds. Another incentive for using low-temperature experimental methods in physics is that, whereas not all materials exhibit interesting properties that they do not have at normal temperatures, such as superconductivity, all matter experiences the loss of thermally induced vibrations at cryogenic temperatures. Low-temperature spectroscopy takes advantage of this "freezing-out" of vibrations in cooled materials, whose optical properties are improved in that their spectral lines become sharper, or more resolved, since the spectral broadening caused by atomic or molecular vibrations, which increases with temperature, is eliminated or significantly reduced.
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