High-resolution thermal expansion and magnetostriction of YBa2Cu3Ox: uniaxial pressure derivatives of Tc, oxygen ordering, superconducting fluctuations and charge density wave

Thermal expansion directly probes the pressure (volume) derivative of the entropy and is thus very useful for studying various physical properties of solids.  In this talk I will first provide a short introduction to high-resolution capacitance dilatometry.  Here, very tiny length changes  (fractions of an Angstrom) can be detected, resulting in a relative resolution up to 10_-10 for a macroscopic sample, far exceeding traditional x-ray or neutron scattering techniques by several orders of magnitude. After a short introduction to the technique, I will first present data on YBa₂Cu₃O_x , highlighting the role of superconducting fluctuations as a function of both hole doping and magnetic field.  Finally, magnetostriction data up to 30 T for an underdoped crystal (x = 6.67, T_c = 65 K) will be presented in order to study the interplay between 3D-charge-order and superconductivity, and to search for the existence of a well-defined upper critical field H_c2.