Entropy Accumulation near Itinerant Magnetic Quantum Critical Points

Quantum critical point (QCP) occurs at a continuous phase transition at zero temperature. It follows from general hyperscaling argument that, near a QCP, the Gr\"uneisen ratio (ratio of thermal expansion coefficient to specific heat) diverges and entropy accumulates [1]. The enhanced entropy has been observed near the field-induced metamagnetic QCP in Sr3Ru2O7[2]. Here we present a detailed theoretical study of entropy across itinerant-magnetic QCPs, with a focus on the ferromagnetic cases. We propose a regularization scheme for the effect of a dangerously irrelevant quartic coupling on the free energy [3], and calculate the entropy using this scheme. While the entropy accumulation near the QCP basically follows the hyperscaling arguments, the correction to scaling is sizeable especially for the two-dimensional case. We compare the theoretical results with the experimental data for Sr3Ru2O7 [2], providing an entropic characterization of the degree to which the metamagnetic QCP in this system is described by the itinerant-magnetic quantum criticality. \\[4pt] [1] L. Zhu et al, PRL 91, 066404 (2003).\\[0pt] [2] A.W. Rost et al, Science 325, 1360 (2009).\\[0pt] [3] J. Wu, L. Zhu, and Q. Si, arXiv:1010.4593, to appear in J. Phys.: Conf. Series.