Anomalous Low Temperature Behavior of SuperconductingNd1.85Ce0.15CuO4−

Abstract

We have measured the temperature dependence of the in-plane London penetration depth λ ab (T ) and the maximum Josephson current Ic(T ) using bicrystal grain boundary Josephson junctions of the electron-doped cuprate superconductor Nd1.85Ce0.15CuO4−y. Both quantities reveal an anomalous temperature dependence below about 4 K. In contrast to the usual monotonous decrease (increase) of λ ab (T ) (Ic(T )) with decreasing temperature, λ ab (T ) and Ic(T ) are found to increase and decrease, respectively, with decreasing temperature below 4 K resulting in a non-monotonous overall temperature dependence. This anomalous behavior was found to be absent in analogous measurements performed on Pr1.85Ce0.15CuO4−y. From this we conclude that the anomalous behavior of Nd1.85Ce0.15CuO4−y is caused by the presence of the Nd 3+ paramagnetic moments. Correcting the measured λ ab (T ) dependence of Nd1.85Ce0.15CuO4−y for the temperature dependent susceptibility due to the Nd moments, an exponential dependence is obtained indicating isotropic s-wave pairing. This result is fully consistent with the λ ab (T ) dependence measured for Pr1.85Ce0.15CuO4−y. The vast majority of experiments on the cuprate superconductors are performed on hole-doped materials. Much less attention has been paid to the system Ln 2−x Ce x CuO 4−y (with Ln = Pr, Nd, Sm, Eu) [1] which represents an electron-doped material. Both hole-and electron-doped cuprates have in common the copper oxygen planes as the central building blocks of the high-temperature superconductors (HTS) suggesting similar superconducting properties. However, as can already be seen from the differences of the generic phase diagram on the electron-and hole-doped side, the physics of electron-and hole-doped HTS is different. In particular, the order parameter (OP) symmetry of the electron-doped cuprates is most likely of s-wave type [2][3][4][5], in contrast to the d-wave OP symmetry in the hole-doped HTS. To clarify the specific differences and similarities between the electron-and hole-doped HTS a more detailed experimental study of the electron-doped HTS is required.

Magnetic penetration depth in unconventional superconductors

“…In 1935 F. and H. London [134] introduced a phenomenological model of superconductivity in which the magnetic field inside a superconductor B obeys the equation, 2 2…”

“…This issue is important in YBCO where the response can be significantly different for currents along the a and b axes. From the reflectance one obtains the conductivity and the frequency dependent penetration depth, ( ) ( ) 2 4 c λ ω π ωσ ω = . Since the data begins well above 0 ω = one must either extrapolate backward or use a sum rule argument to obtain…”

“…The problem was resolved by Hirschfeld and Goldenfeld [91,165] who showed that resonant (unitary-limit) scattering generates a nonzero density of quasiparticle states near E = 0. These states lead to a 2 T variation of the penetration depth below a crossover temperature * T . The unitary limit was required since Born limit scatterers would lead to a rapid suppression of T c , which was not observed.…”

“…[148,195], somewhat similar to the situation in the copper oxides. We discuss two of the most widely studied compounds, κ-(ET) 2 Cu(NCS) 2 Despite this very strong evidence for nodal quasiparticles, the superfluid density is never purely linear and in fact exhibits an interesting regularity. Figure 13 shows the penetration depth itself plotted versus 1.5 T .…”

“…Higher angular momentum states have been proposed. For example an f-wave has been suggested for Sr 2 RuO 4 . [212,155] It is also possible to have mixed symmetries such as s id + , s d + and for lattices that do not possess inversion symmetry, one can in principle even have singlet and triplet combinations.…”