Role of correlations in determining the Van Hove strain in Sr₂RuO₄

Uniaxial pressure applied along a Ru-O-Ru bond direction induces an elliptical distortion of the largest Fermi surface of Sr2RuO4, eventually causing a Fermi surface topological transition, also known as a Lifshitz transition, into an open Fermi surface. There are various anomalies in low-temperature properties associated with this transition, including maxima in the superconducting critical temperature and in resistivity. In the present paper, we report refined measurements of the strain at which this transition occurs, employing apparatus in which the stress on the sample is measured, and resonant ultrasound measurement of the low-temperature elastic moduli. The Lifshitz transition is found to occur at a longitudinal strain epsilon(xx) of (-0.44 +/- 0.06) x 10(-2), which corresponds to a B-1g strain epsilon(xx) - epsilon(yy) of (-0.66 +/- 0.09) x 10(-2). This is considerably smaller than the strain corresponding to a Lifshitz transition in density functional theory calculations, even if the spin-orbit coupling is taken into account. Using dynamical mean-field theory, we show that electronic correlations reduce the critical strain. It turns out that the orbital anisotropy of the local Coulomb interaction on the Ru site is, furthermore, important to bring this critical strain close to the experimental number and thus well into the experimentally accessible range of strains