Quantum critical point in heavy fermions

The concept that heavy fermions are close to a quantum critical point and that this proximity determines their physical behavior, has opened new perspectives in the study of these systems. It has provided a new paradigm for understanding and probing the properties of these strongly correlated materials. Scaling ideas were important to establish this approach. We give below a brief and personal account of the genesis of some of these ideas 15 years ago, their implications and the future prospects for this exciting field.Comment: 7 pages, 3 figures, to be published in Brazilian Journal of Physic

Wilson and Kadowaki-Woods Ratios in Heavy Fermions

Recently we have shown that a one-parameter scaling, $T_{coh}$, describes the physical behavior of several heavy fermions in a region of their phase diagram. In this paper we fully characterize this region, obtaining the uniform susceptibility, the resistivity and the specific heat in terms of the coherence temperature $T_{coh}$. This allows for an explicit evaluation of the Wilson and the Kadowaki-Woods ratios in this regime. These quantities turn out to be independent of the distance $|\delta|$ to the quantum critical point (QCP). The theory of the one-parameter scaling corresponds to a local interacting model. Although spatial correlations are irrelevant in this case, time fluctuations are critically correlated as a consequence of the quantum character of the transition.Comment: 6 pages, 1 figure, to be published in Eur.Phys.J.

Dynamics of amorphous ferromagnets

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Dimensional Crossover in Heavy Fermions

Recently we have shown that a one-parameter scaling, the Coherence Temperature, describes the physical behavior of several heavy fermions in a region of their phase diagram. In this paper we fully characterize this region, obtaining the uniform susceptibility, the resistivity and the specific heat. This allows for an explicit evaluation of the Wilson and the Kadowaki-Woods ratios in this regime. These quantities turn out to be independent of the distance to the critical point. The theory of the one-parameter scaling corresponds to a zero dimensional approach. Although spatial correlations are irrelevant in this case, time fluctuations are critically correlated and the quantum hyperscaling relation is satisfied for $d=0$. The crossover from $d=0$ to $d=3$ is smooth. It occurs at a lenght scale which is inversely related to the stiffness of the lifetime of the spin fluctuations.Comment: 4 pages, revtex, no figures, submitted to Physical Review