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The magnetic field and temperature dependence of the electronic specific heat$C_{\rm el}$ have been systematically investigated in $\rm La_{2-{\itx}}Sr_{\it x}Cu_{1-{\it y}}Ni_{\it y}O_4$ (LSCNO) in order to study Ni-impurityeffects on the superconducting (SC) gap. In LSCNO with $x$=0.15 and $y$=0.015,the value of $\gamma$ ($\equiv C_{\rm el}/T$) at $T$=0 K, $\gamma_0$, isenhanced under the magnetic field $H$ applied along the $\bm c$-axis. Theincrement of $\gamma_0$, $\Delta \gamma_0$, follows the Volovik relation$\Delta \gamma_0$=$A\sqrt{H}$, characteristic of the SC gap with line nodes,with prefactor $A$ similar to that of a pure sample. The $C_{\rm el}/T$ vs. $T$curve under $H$=0 shows a d-wave-like SC anomaly with an abrupt increase at$T_{\rm c}$ and $T$-linear dependence at $T$$\ll$$T_{\rm c}$, although the$\gamma_0$-value in the $C_{\rm el}/T$ vs. $T$ curve increases with increasingNi concentrations. Interestingly, as the SC part of $C_{\rm el}/T$, $C_{\rmel}/T$$-$$\gamma_0$$\equiv$$\gamma_{\rm s}$, decreases in LSCNO, $T_{\rm c}$ isreduced in proportion to the decrease of $\gamma_{\rm s}$. These findings canbe explained phenomenologically by a simple model in which Ni impurities bringabout strong pair breaking at the edges of the coherent nodal part of the Fermisurface but in the vicinity of the nodes of the SC gap. The reduction of the SCcondensation energy $U_0$ in LSCNO, evaluated from $C_{\rm el}$ at $T${0.3em}\raisebox{0.4ex}{$<$} {-0.75em}\raisebox{-.7ex}{$\sim$} {0.3em}$T_{\rmc}$, is also understood by the same model.

physical review. b, condensed matter and materials physics

Ni-impurity effects on the superconducting gap of  La$_{2-x}$Sr$_{x}$CuO$_4$ studied from the magnetic field and temperature  dependence of the electronic specific heat