YbNiAl: A new Yb-based heavy-fermion antiferromagnet

We have investigated the compound YbNiAl, which crystallizes in the same structure as the well-known mixed-valent system YbCuAl, and have found that it is the first example of a clear cut Yb-based heavy-fermion antiferromagnet. The resistivity increases logarithmically by 25% between 100 and 10 K, C/T extrapolates below 0.4 K to a gamma value of 350 mJ/K-2 mol and the entropy at the antiferromagnetic ordering temperature T-N = 3 K reaches only 0.45R In 2. We compare the properties of this compound with those of YbPtAl, where Yb is trivalent