Some open question and perspectives in the physics of solids at very low temperatures

Abstract

Temperature measures the average energy of a material above its ground state. In Nature, temperature ranges from millions of Kelvin at the centre of stars to 2.7 K, the characteristic temperature of the “background radiation” arising from the remains of the Big Bang. This very wide range, perhaps comparable only to scales of time, has been substantially enlarged by human science. Experimental physics has explored the behaviour of matter at temperature only a few nanoKelvin above absolute zero. Experiments at very low temperatures have enabled to explore the ground state of materials and the emergence of phenomena such as superfluidity, superconductivity and exotic magnetic phases of electronic as well as nuclear spins. In this talk, I review examples that show how neutron scattering techniques contributed to the current understanding of some of these phenomena and illustrate, with examples, a few of the open questions and perspectives in this field.