We report microwave cavity perturbation measurements of the temperature dependence of the penetration depth, λ(T), and conductivity, σ(T) of Pr2−xCexCuO 4−δ (PCCO) crystals, as well as parallel-plate resonator measurements of λ(T) in PCCO thin films. Penetration depth measurements are also presented for a Nd2−xCexCuO 4−δ (NCCO) crystal. We find that ∆λ(T) has a power-law behavior for T < Tc/3, and conclude that the electron-doped cuprate superconductors have nodes in the superconducting gap. Furthermore, using the surface impedance, we have derived the real part of the conductivity, σ1(T), below Tc and found a behavior similar to that observed in hole-doped cuprates. 74.25.Nf, 74.25.Fy, 74.72.Jt Existing experimental data on the electron-doped cuprate superconductors [1] have been interpreted as being consistent with an s-wave pairing state symmetry [2]. In particular, magnetic screening length measurements versus temperature have been interpreted as showing an activated behavior consistent with s-wave symmetry [3][4][5][6][7], mainly with a large activation gap, 2∆(0)/k B T c > 4 [3][4][5][6]. This is in contrast to the predominantly d-wave behavior widely observed in the hole-doped cuprates [8], most conclusively demonstrated in phase-sensitive experiments [9]. Raman scattering supports a d-wave symmetry in the hole-doped cuprate superconductors [10] and suggests s-wave in electron-doped cuprates [11]. Tunneling spectroscopy measurements have also been interpreted in the same manner [7,12,13]. A zero-bias conductance peak is expected for tunneling into the gap nodal directions in d-wave superconductors due to the change in sign of the order parameter on the Fermi surface, allowing for the formation of Andreev bound states at the Fermi energy [14]. Such a peak has been seen in such superconductors as Y Ba 2 Cu 3 O 7−δ (YBCO) [13], which are known to have a dominant d-wave order parameter. Until recently [15], no such peak was observed in the electron-doped cuprates [7,13], consistent with an s-wave symmetry for these materials.Since there is no distinction between electron and hole doping of the Hubbard model around half-filling [16,17], one does not expect electron-and hole-doped cuprates to have different pairing mechanisms and symmetries. In this letter we present evidence for nodes in the excitation spectrum, and by implication that the pairing state symmetry in the electron-doped cuprates is predominantly d-wave. This evidence comes from temperature dependent penetration depth measurements which, although not able to singularly determine the pairing state symmetry, provide strong indications that there are an abundance of low energy excitations in these materials.
