Phase-sensitive order parameter symmetry test experiments utilizing Nd(2-x)Ce(x)CuO(4-y)/Nb zigzag junctions

Point-Contact Spectroscopy in Bulk Samples of Electron-Doped Cuprate Superconductors

Point-contact spectroscopy was performed on bulk samples of electron-doped high temperature superconductor Nd2-xCexCuO4-δ. The samples were characterized using X-ray diffraction and scanning electron microscopy equipped with a wavelength-dispersive spectrometer and an electron backscatter diffraction detector. Samples with Ce content x = 0.15 showed the absence of spurious phases and randomly oriented grains, most of which had dimensions of approximately 220 µm2. The low-bias spectra in the tunneling regime, i.e., high-transparency interface, exhibited a gap feature at about ±5 meV and no zero-bias conductance, despite the random oriented grains investigated within our bulk samples, consistent with most of the literature data on oriented samples. High-bias conductance was also measured in order to obtain information on the properties of the barrier. A V-shape was observed in some cases, instead of the parabolic behavior expected for tunnel junctions.

Specific heat evidence for bulk s-wave gap symmetry of optimally electron-doped Pr(1.85)Ce(0.15)CuO(4-y) superconductors

We reanalyze specific heat data for optimally electron-doped Pr(1.85)Ce(0.15)CuO(4-y) superconductors. The magnetic field dependence of the electronic specific heat in the vortex state does not support d-wave gap symmetry but agrees quantitatively with an s-wave theory. Furthermore, the field dependence at a finite temperature almost coincides with that for a conventional s-wave superconductor, Vi(3)Si. The present work provides bulk evidence for a nodeless s-wave gap symmetry in optimally electron-doped cuprates.

Coexistence of the antiferromagnetic and superconducting order and its effect on spin dynamics in electron-doped high-T(c) cuprates

In the framework of the slave-boson approach to the t-t'-t''-J model, it is found that for electron-doped high- T(c) cuprates, the staggered antiferromagnetic (AF) order coexists with the superconducting (SC) order in a wide doping level ranged from underdoped to nearly optimally doped at the mean-field level. In the coexisting phase, it is revealed that the spin response is commensurate in a substantial frequency range below a crossover frequency ω(c) for all dopings considered, and it switches to the incommensurate structure when the frequency is higher than ω(c). This result is in agreement with the experimental measurements. Comparison of the spin response between the coexisting phase and the pure SC phase with a d(x(2)-y(2))-wave pairing plus a higher harmonics term (DP+HH) suggests that the inclusion of the two-band effect is important to consistently account for both the dispersion of the spin response and the non-monotonic gap behavior in the electron-doped cuprates.

Imaging of order parameter induced pi phase shifts in cuprate superconductors by low-temperature scanning electron microscopy

Low-temperature scanning electron microscopy (LTSEM) has been used to image the supercurrent distribution in ramp-type Josephson junctions between Nb and either the electron-doped cuprate Nd_{2-x}Ce_{x}CuO_{4-y} or the hole-doped cuprate YBa_{2}Cu_{3}O_{7}. For zigzag-shaped devices in the short junction limit the critical current is strongly suppressed at zero applied magnetic field. The LTSEM images show that this is due to the Josephson current counterflow in neighboring 0 and pi facets, which is induced by the d_{x;{2}-y;{2}} order parameter in the cuprates. Thus, LTSEM provides imaging of the sign change of the superconducting order parameter, which can also be applied to other types of Josephson junctions.

Dirty superconductivity in the electron-doped cuprate Pr(2-x)Ce(x)CuO(4-delta): tunneling study

We report a tunneling study between Pr(2-x)Ce(x)CuO(4-delta) and lead as a function of doping, temperature, and magnetic field. The temperature dependence of the gap follows the BCS prediction. Our data fit a nonmonotonic d-wave order parameter for the whole doping range studied. From our data we are able to conclude that the electron-doped cuprate Pr(2-x)Ce(x)CuO(4-delta) is a weak-coupling BCS dirty superconductor.

Nodeless d-wave superconducting pairing due to residual antiferromagnetism in underdoped Pr2-xCexCuO4-delta

We investigate the doping dependence of the penetration depth versus temperature in electron-doped Pr(2-x)Ce(x)CuO(4-delta) using a model which assumes the uniform coexistence of (mean-field) antiferromagnetism and superconductivity. Despite the presence of a d(x2-y2) pairing gap in the underlying spectrum, we find nodeless behavior of the low-T penetration depth in the underdoped case, in accord with experimental results. As doping increases, a linear-in-T behavior of the penetration depth, characteristic of d-wave pairing, emerges as the lower magnetic band crosses the Fermi level and creates a nodal Fermi surface pocket.

0-pi Josephson tunnel junctions with ferromagnetic barrier

We fabricated high quality Nb/Al2O3/Ni(0.6)Cu(0.4)/Nb superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Using a ferromagnetic layer with a steplike thickness, we obtain a 0-pi junction, with equal lengths and critical currents of 0 and pi parts. The ground state of our 330 microm (1.3lambda(J)) long junction corresponds to a spontaneous vortex of supercurrent pinned at the 0-pi step and carrying approximately 6.7% of the magnetic flux quantum Phi(0). The dependence of the critical current on the applied magnetic field shows a clear minimum in the vicinity of zero field.