Magnetization study on the field-induced quantum critical point in YbRh_2Si_2

Abstract

We study the field-induced quantum critical point (QCP) in YbRh${}_2$Si${}_2$ by low-temperature magnetization, $M(T)$, and magnetic Grüneisen ratio, $\hat{I}{“}_{\mathrm{mag}}$, measurements and compare the results with previous thermal expansion, ${\hat{I}}^2(T)$, and critical Grüneisen ratio, $\hat{I}{“}^{cr}(T)$, data on YbRh${}_2$(Si${}_{0.95}$Ge${}_{0.05}$)${}_2$. In the latter case, a slightly negative chemical pressure has been used to tune the system towards its zero-field QCP. The magnetization derivative $-dM/dT$ is far more singular than thermal expansion, reflecting a strongly temperature dependent pressure derivative of the field at constant entropy, $(dH/dP{)}_S=V_m{\hat{I}}^2/(dM/dT)$ ($V_m$: molar volume), which saturates at $(0.15\pm 0.04)$ T/GPa for $T\to 0$. The line $T^{\star }(H)$, previously observed in Hall- and thermodynamic measurements, separates regimes in $T$-$H$ phase space of stronger $(\hat{I}\mu >1$) and weaker $(\hat{I}\mu <1$) divergent $\hat{I}{“}_{\mathrm{mag}}(T)\propto T^{-\hat{I}\mu }$.

4 Pages, 3 Figures, submitted to Proceedings of ICM 2009 (Karlsruhe)