Surface impedance of $k$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br crystals

Precision measurements of the real and imaginary parts of the microwave surface impedance $Z_{ac}(T)=R_{ac}(T)+iX_{ac}(T)$ of the conducting ac layers of the $k$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br crystals in the temperature interval of $0.5 < T < 100$ K have demonstrated a series of features: (i) the temperature course of the field penetration depth is close to linear $\Delta\lambda_{ac}(T)\infty\Delta X_{ac}(T)$ in the superconducting state at $T\ll T_c\approx11.5$ K; (ii) the curves $R_{ac}(T)=X_{ac}(T)$ coincide at $T_c<T<40$ K; (iii) the $X_{ac}(T)$ value at $T > 40$ K increases in comparison with $R_{ac}(T)$; (iv) the dependence $R_{ac}(T)$ K is nonmonotonic in thin crystals. These features of the impedance $Z_{ac}(T)$ with increasing T are interpreted in terms of (i) the $d$-type symmetry of the superconducting order parameter, (ii) normal skin effect, (iii) manifestations of the antiferromagnetic fluctuations, and (iv) the size effect. The electrodynamic parameters of $k$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br have been determined.