Depression of quasiparticle density of states at zero energy in La1.9Sr0.1Cu1-xZnxO4

We have measured low-temperature specific heat C(T, H) of La1.9Sr0.1Cu1-xZnxO4 (x=0, 0.01, and 0.02) both in zero and applied magnetic fields. A pronounced dip of C/T below 2 K was first observed in Zn-doped samples, which is absent in the nominally clean one. If the origin of the dip in C/T is electronic, the quasiparticle density of states N(E) in Zn-doped samples may be depressed below a small energy scale E0. The present data can be well described by the model N(E)=N(0)+alphaE^1/2, with a non-zero N(0) and positive alpha. Magnetic fields depress N(0) and lead to an increase in E0, while leaving the energy dependence of N(E) unchanged. This novel depression of N(E) below E0 in impurity-doped cuprates can not be reconciled with the semi-classical self-consistent approximation model. Discussions in the framework based on the non-linear sigma model field theory and other possible explanations are presented in this Letter.