The Coldest Place in Nassau County Dr. Matthew Smylie
Department of Physics and Astronomy The coldest place in all of Nassau County is located in 205 Berliner Hall, at the bottom of a 2.5” diameter sample chamber inside of a 6’ tall cryostat. This dewar, or vacuum flask, is familiar to anyone who wants to keep coffee hot and water cold in a thermos bottle: it is double-walled with vacuum insulation between the walls to keep the insides at constant temperature. This dewar contains a 9 T superconducting electromagnet immersed in 50 L of liquid helium at 4.2 K, insulated by vacuum from a 50 L reservoir of liquid nitrogen at 77 K, insulated by vacuum from room temperature (300 K). The dewar is made of stainless steel, as this is an extremely poor conductor of thermal energy. A re-liquefier sits atop the cryostat, capturing and re-liquefying all helium and nitrogen that boils out. Using this device, we are able to achieve sample temperatures below 1.5 K, 200 times colder than room temperature and 1.85 times colder than the empty vacuum in deep space.
How to get colder than cold? At atmospheric pressure, liquid helium boils at 4.2 K. It has the lowest boiling point of any element; no colder coolants exist in the universe. How do we make the liquid helium get colder? The same way your body regulates its temperature. When you sweat, the water evaporates off your skin. Evaporating the water costs energy, which comes from your body temperature— evaporation causes your body to cool down. We force liquid helium to evaporate with a vacuum pump. As the helium evaporates, the remaining liquid cools down. The evaporated helium gas is captured and reliquefied using a compressor, such that no valuable helium (~$25/liter) is lost. To cool a sample, a small chamber is filled with liquid helium and forced to evaporate. This cold gas flows upward through an annulus formed by the inner and outer walls of the double-walled sample chamber, cooling the sample chamber. The sample itself sits at the bottom of a 1.63 m stainless steel tube in low-pressure helium gas; the cold chamber walls cool this exchange gas, which then cools the sample. This allows us to reach temperatures of ~1.46 K. Going lower requires pumping harder which means evaporating helium faster: you can’t pump it in as fast as you need to pump it out.
Superconductivity: a low temperature magic trick At low enough temperatures, the electrical resistance abruptly decreases to zero in certain materials. This phenomenon is known as superconductivity. Electrons have a quantum spin of ½; as a result, the Pauli exclusion principle forces all electrons in a solid to occupy different energy states. In a superconductor, conduction electrons with opposite momentum and quantum spin pair to create a quasi-particle which has zero net momentum and zero net spin. As a result, the quasiparticle no longer obeys the Pauli exclusion principle, and the paired electrons all occupy one energy state. The electrons scatter off of the lattice still, but this no longer creates electrical resistance as the electrons scatter into equally superconducting states. Above a particular temperature TC, random thermal motion overcomes the pairing interaction between electrons, and the superconducting state is lost. Magnetic fields can be used to suppress superconductivity; above a critical field, the spins of the electrons both align with the applied field, and the superconducting state is lost. The TC vs applied field diagram maps out the thermodynamics of this interaction and provides insight into why the electrons partner up in the first place.
Photosphere of Sun
Steel melts
Chamber
Liq. N2
Day on Mercury Oven on high Dallol, Ethiopia Room temperature Surface of Mars Antarctica Night on the moon O2 boils N2 boils N2 melts Surface of Pluto
H2 boils
Liq. He Magnet
Normal
Helium boils
Superconducting
Temperature of outer space Sample stage
Sample