\end{array}
$

$%
\begin{array}{cc}
\epsfxsize=8cm \epsffile{Cvsralpha1.eps} & \epsfxsize=8cm %
\epsffile{Hvsralpha1.eps}%
\end{array}
$

$%
\begin{array}{cc}
\epsfxsize=8cm \epsffile{Cvsbetar1.eps} & \epsfxsize=8cm %
\epsffile{Hvsbetar1.eps}%
\end{array}
$

$%
\begin{array}{cc}
\epsfxsize=8cm \epsffile{Cvsalphar1.eps} & \epsfxsize=8cm %
\epsffile{Hvsalphar1.eps}%
\end{array}
$

Thermal stability\[Stab\]
-------------------------

In order to discuss the thermal stability conditions, one may use both canonical and grand canonical ensembles. The positivity of the heat capacity and determinant of the Hessian matrix are the requirements usually referred to stability criterion in canonical and grand canonical ensembles, respectively. Since $M$ is a function of $S$ and $Q$, we can write the heat capacity and determinant of the Hessian matrix with the following explicit forms $$\begin{aligned}
C_{Q} &=&\frac{T}{\left( \frac{\partial ^{2}M}{\partial S^{2}}\right) _{Q}},
\label{CQ} \\
H &=&det\left[
\begin{array}{cc}
\frac{\partial ^{2}M}{\partial S^{2}} & \frac{\partial ^{2}M}{\partial
S\partial Q} \\
\frac{\partial ^{2}M}{\partial Q\partial S} & \frac{\partial ^{2}M}{\partial
Q^{2}}%
\end{array}%
\right] .  \label{H}\end{aligned}$$

Analytical calculations of the heat capacity and determinant of the Hessian matrix are too large and therefore we leave out the analytical result for reasons of economy. We plot some figures to discuss the stability conditions. Numerical calculations show that although large black holes have positive definite temperature, there is a lower limit for the horizon radius of physical small black holes, 