Low-temperature thermodynamic properties of the heavy-fermion compound YbAgGe close to the field-induced quantum critical point

We present temperature and field dependent heat capacity and magnetization data taken at temperatures down to $50\mathrm{mK}$ and in an applied magnetic field up to $11.5\mathrm{T}$ for YbAgGe, a heavy-fermion compound with a field induced quantum critical point. These data clearly indicate that the same electronic degrees of freedom are responsible for the features seen in both specific heat and magnetization data. In addition, they further refine the different boundaries suggested for the $H\text{−}T$ phase diagram of YbAgGe through previous, magneto-transport measurements, and allow for further understanding of different phases on the $H\text{−}T$ phase diagram, in particular, clearly disconnecting the field-induced quantum critical point in YbAgGe from any sort of saturation of the Yb moment in higher applied magnetic field.

Figure 1

Representative plots of the low-field temperature dependent susceptibility of YbAgGe ($H\Vert ab$, $H⩽1\mathrm{T}$) below $1\mathrm{K}$. Arrows mark data taken on increase/decrease of temperature. Inset shows break of slope near $1\mathrm{K}$ in $H=0.1\mathrm{T}$ data.

Figure 2

Temperature dependent susceptibility of YbAgGe in magnetic fields between $2\mathrm{T}$ and $11\mathrm{T}$ $(H\Vert ab)$ below $3\mathrm{K}$. Data taken on temperature increase.

Figure 3

(a) Representative plots of low-temperature field-dependent magnetization, $M\left(H\right)$, of YbAgGe $(H\Vert ab)$. Curves are shifted along the $y$ axis by multiplicative factors of $0.1{\mu }_B$ for clarity; (b) $dM∕dH$ derivatives for the representative $M\left(H\right)$ taken on increase of the magnetic field; curves for $T⩾0.25\mathrm{K}$ have an offset by a multiplicative of $0.05{\mu }_B∕\mathrm{T}$ for clarity (see text for the discussion of different features).

Figure 4

Example of hysteretic low field/low temperature transition as seen in raw $M\left(H\right)$ data and in the derivative $dM∕dH$.

Figure 5

Low temperature part of the temperature-dependent heat capacity of YbAgGe $(H\Vert ab)$ plotted as $C_p∕T$ vs $T$ on a semilog plot. Arrow marks magnetic transition in $H=4\mathrm{T}$ curve.

Figure 6

Low temperature differential susceptibility, $dM∕dH$, and Sommerfeld coefficient, $C∕T$, as a function of applied magnetic field. Data for $T⩾0.39\mathrm{K}$ are shifted vertically by a multiplicative of $0.1{\mu }_B∕\mathrm{T}$ $(dM∕dH)$ and multiplicative of $0.825\mathrm{J}∕\mathrm{mol}{\mathrm{K}}^2$ $(C∕T)$, respectively, for clarity. Asterisks and open symbols, $C∕T$ obtained using the data from Ref. 1, other symbols, this work.

Figure 7

(a) Low temperature part of the temperature-dependent heat capacity of YbAgGe $(H\Vert ab)$, with nuclear contributions subtracted, plotted as $\Delta C_p∕T$ vs $T$ on a semilog plot. Arrow marks the magnetic transition in the $H=4\mathrm{T}$ curve; (b) Low temperature differential susceptibility, $dM∕dH$, and Sommerfeld coefficient, $C∕T$, with nuclear contributions subtracted, as a function of applied magnetic field. Data for $T=0.25\mathrm{K}$ are shifted vertically by $0.1{\mu }_B∕\mathrm{T}$ $(dM∕dH)$ and $0.825\mathrm{J}∕\mathrm{mol}{\mathrm{K}}^2$ $(C∕T)$ for clarity.

Figure 8

$H\text{−}T$ phase diagram of YbAgGe $(H\Vert ab)$: solid lines: magnetic phase lines (Refs. 1, 7); dashed lines: Hall effect lines (Refs. 6, 8; symbols: data from this work; ●: from field-up $M\left(H\right)$ data; ⋆: from $M\left(T\right)$ data taken on increase of temperature; *: from field-down $M\left(H\right)$ data; ×: from $M\left(T\right)$ data taken on temperature decrease; and ▵, ▿: from heat capacity data plotted as $C_p\left(T\right)$ and $C\left(H\right)∕T$, respectively.