Cyril Proust, LNCMI Toulouse

Mercredi 19 Decembre, 16h
Amphi Howleck, Esc C, 1ème etage

Thermodynamic phase diagram of static charge
order in underdoped YBa2Cu3Oy

Cyril Proust
LNCMI Toulouse

25 years after the discovery of high temperature cuprate superconductors, the observation of quantum oscillations has deeply changed the theoretical landscape relevant to these materials. The measurements of quantum oscillations on both sides of the phase diagram of cuprates confirm the existence of a Fermi surface with sharply defined excitations on the overdoped side but also show that the Fermi surface has suffered a drastic modification on the other side. The small Fermi pocket inferred from quantum oscillations in the underdoped regime combined with the negative Hall and Seebeck coefficients pointing to an electron pocket, greatly strengthens the case that the Fermi surface of YBa2Cu3Oy undergoes a reconstruction because the translational symmetry of its lattice is broken at low temperature.
Many studies such as Hall and Seebeck coefficients point to a reconstruction of the Fermi surface due to charge stripe order. Indeed, NMR measurements discover a field-induced, most likely uni-axial, modulation of the charge density in the CuO2 planes that has the same the 4a-periodic charge-stripe modulation. More recently, long range charge fluctuations in YBCO were recently observed with resonant soft x-ray and hard x-ray scattering at high temperature and in zero magnetic field. These measurements identify charge fluctuations at two wavevectors. In this talk, I will present sound velocity measurements, a thermodynamic probe, in magnetic fields large enough to suppress superconductivity at low temperature. We provide the first thermodynamic signature of the competing charge order phase transition in YBa2Cu3Oy allowing construction of a field-temperature phase diagram. The comparison of different acoustic modes indicates that the charge modulation is biaxial.

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