Multiple regions of quantum criticality in YbAgGe

Dilation and thermopower measurements on YbAgGe, a heavy-fermion antiferromagnet, clarify and refine the magnetic-field temperature ($H$-$T$) phase diagram and reveal a field-induced phase with $T$-linear resistivity. On the low-$H$ side of this phase we find evidence for a first-order transition and suggest that YbAgGe at 4.5 T may be close to a quantum critical end point. On the high-$H$ side our results are consistent with a second-order transition suppressed to a quantum critical point near 7.2 T. We discuss these results in light of global phase diagrams proposed for Kondo lattice systems.

Figure 1

The phase diagram of YbAgGe below 1 K with $H$ applied perpendicular to the $c$ axis. Panel (a) summarizes measurements from the literature; the variables carry their usual meanings. Panel (b) summarizes dilation and thermoelectric power data from this work with the same guides to the eye as panel (a) (see text).

Figure 2

Representative longitudinal dilation data with $H$ applied parallel to the $\mathit{ab}$ axis. Panel (a) shows the low-temperature thermal expansion in the fixed fields listed next to arrows denoting the appropriate axes for each data set (the data at 0 $\mathrm{T}/1.2$ T have been shifted by $+10/-9.0\times {10}^{-6}/$T for clarity). The features labeled $T_i$ correspond to data where $i$ denotes the regions or phases of Fig. 1 (see text). A semilog plot of ${\alpha }_{\mathit{ab}}/T$ vs $T$ in the vicinity of 4.5 T is shown in the inset where the direction of increasing $H$ is shown by a curved arrow, and the solid line is a guide to the eye for the low-temperature 4.7-T data. Panel (b) shows the magnetostriction at 387 mK. The features labeled $H_j$ correspond to data where $j$ denotes the guides to the eye of Fig. 1a (see text). The inset shows an expanded view of the transitions below 6 T at 17 mK.

Figure 3

Representative transverse TEP data with $H$ applied parallel to the $\mathit{ab}$ axis and the temperature gradient in the basal plane, perpendicular to $H$. Panel (a) shows the temperature dependence of $S$ across phase boundaries or crossover regions at the fields shown. The features labeled $T_i$ correspond to data where $i$ denotes the regions or phases of Fig. 1 (see text). Panel (b) shows $S/T$ vs $T$ in applied fields on a semilog plot. The solid line through the 8-T data is a guide to the eye. The field dependence of $S$ at 0.4 K is shown in the inset where the features labeled $H_j$ correspond to data where $j$ denotes the guides to the eye of Fig. 1a (see text).

Figure 4

The temperature dependence of the Grüneisen parameter ${\Gamma }_{\mathit{ab}}$ of YbAgGe in applied magnetic fields. The curved arrow represents the direction of increasing field. (The apparent, slight downturn of the 8.0-T data at the low-$T$ end is comparable to the noise in these data.) The inset shows the low-temperature end of the 4.5-T data on a log-log plot. The solid line is a guide to the eye representing a $T^{-5/2}$ temperature dependence.