Introduction

Abstract

The study of matter at very low temperature is fascinating because the phonon activity dies down at very low temperatures and one can look into the electronic behaviour minutely. Cryogenic baths of liquefied gases provide excellent medium to cool down samples. Liquefaction of a gas is a combination of an isothermal compression followed by an adiabatic expansion. Cascade process were adopted in liquefying oxygen by Cailletet and Pictet independently in 1877. The final cooling stage has always been a Joule-Thomsen (J-T) valve. Another important breakthrough came in 1898 when James Dewar succeeded in liquefying hydrogen making a temperature range of 20–14 K accessible. The moment of triumph came in July, 1908 when years of hard work by Kamerlingh Onnes at Leiden ultimately resulted in the liquefaction of helium. A temperature range of 4.2–0.8 K thus became accessible in the laboratory. A cascade process using Lair, LO2, LN2 and LH2 and the J-T expansion valve was employed. Within 3 years of this discovery came the defining moment of the discovery of superconductivity in April, 1911 in pure Hg at just below 4.2 K.