We have performed a detailed study of the tunneling spectra of bicrystal grain boundary junctions (GBJs) fabricated from the high temperature superconductors (HTS) YBa2Cu3O 7−δ (YBCO), Bi2Sr2CaCu2O 8+δ (BSCCO), La1.85Sr0.15CuO4 (LSCO) and Nd1.85Ce0.15CuO4−y (NCCO). In all experiments the tunneling direction was along the CuO2 planes. With the exception of NCCO, for all materials a pronounced zero bias conductance peak (ZBCP) was observed which decreases with increasing temperature and disappears at the critical temperature. These results can be explained by the presence of a dominating d-wave symmetry of the order parameter resulting in the formation of zero energy Andreev bound states at surfaces and interfaces of HTS. The absence of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair potential in this material. The observed nonlinear shift of spectral weight to finite energies by applying a magnetic field is in qualitative agreement with recent theoretical predictions.To appear in Physical Review B There is strong evidence that the superconducting order parameter (OP) in the HTS has a dominating d-wave symmetry [1,2]. For this pairing symmetry there is a π-phase shift of the OP in orthogonal k-space directions resulting in a positive and negative sign of the pair potential in those directions. This also means that there are directions with nodes of the pair potential, e. g. for a pure d x 2 −y 2 -symmetry, the nodes are along the [110] direction in the CuO 2 plane. For the tunneling spectra of junctions employing HTS electrode materials with a d-wave symmetry of the OP, a pronounced ZBCP has been predicted originating from mid-gap surface (interface) states or zero energy bound states (ZES) at the Fermi level [3][4][5][6][7][8]. The physical reason for these states originates from the fact that quasiparticles incident and reflecting from the surface propagate through different order parameter fields which leads to Andreev reflection. The constructive interference between incident and Andreev reflected quasiparticles results in bound states. Stable ZES are formed if the scattering induces a change in sign of the OP. For a d x 2 −y 2 -wave symmetry such sign change and, hence, the presence of ZES is possible for all surfaces parallel to the c-axis except for those with the lobe directions perpendicular to the surface, whereas for a s-wave symmetry no ZES are possible. The spectral weight of the ZES for a d x 2 −y 2 -wave symmetry depends on the orientation of the surface with respect to the crystal axis. The maximum spectral weigth is expected for a (110) surface and, hence, a maximum ZBCP is expected for tunneling in the direction of the nodal lines, i. e., the [110] direction. This has been observed recently using low temperature scanning tunneling spectroscopy (LTSTS) [9] and planar type junctions [10]. We note that the ZBCP is sensitive to surface roughness making it difficult to distinguish between the directions in the plane [11][12][13].Initially, the ZBCP in the tunneling spectra...

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.

Abstract. Andreev bound states at the surface of superconductors are expected for any pair potential showing a sign change in different k-directions with their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation of Andreev bound states in high temperature superconductors (HTS) employing tunneling spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YBa2Cu3O 7−δ (YBCO), Bi2Sr2CaCu2O 8+δ (BSCCO), and La1.85Sr0.15CuO4 (LSCO) show a pronounced zero bias conductance peak that can be interpreted in terms of Andreev bound states at zero energy that are expected at the surface of HTS having a d-wave symmetry of the order parameter. In contrast, for the most likely s-wave HTS Nd1.85Ce0.15CuO4−y (NCCO) no zero bias conductance peak was observed. Applying a magnetic field results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic field. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented.

We have measured the low-energy quasiparticle excitation spectrum of the electron doped hightemperature superconductors (HTS) Nd1.85Ce0.15CuO4−y and Pr1.85Ce0.15CuO4−y as a function of temperature and applied magnetic field using tunneling spectroscopy. At zero magnetic field, for these optimum doped samples no excitation gap is observed in the tunneling spectra above the transition temperature Tc. In contrast, below Tc for applied magnetic fields well above the resistively determined upper critical field, a clear excitation gap at the Fermi level is found which is comparable to the superconducting energy gap below Tc. Possible interpretations of this observation are the existence of a normal state pseudogap in the electron doped HTS or the existence of a spatially non-uniform superconducting state.

We have studied the electric transport properties of symmetrical [001] tilt Nd1.85Ce0.15CuO4−y (NCCO) bicrystal grain boundary Josephson junctions (GBJs) fabricated on SrTiO3 bicrystal substrates with misorientation angles of 24 • and 36.8 • . The superconducting properties of the NCCOGBJs are similar to those of GBJs fabricated from the hole doped high temperature superconductors (HTS). The critical current density Jc decreases strongly with increasing misorientation angle. The products of the critical current Ic and the normal resistance Rn (∼ 100 µV at 4.2 K) are small compared to the gap voltage and fit well to the universal scaling law IcRn ∝ √ Jc found for GBJs fabricated from the hole doped HTS. This suggests that the symmetry of the order parameter, which most likely is different for the electron and the hole doped HTS has little influence on the characteristic properties of symmetrical [001] tilt GBJs. PACS: 74.25.Fy, 74.50.+r Applied Physics Letters to be published June 1, 1998Bicrystal grain boundary Josephson junctions (GBJs) have been studied intensively using epitaxial thin films of the various hole doped high temperature superconductors (HTS) [1][2][3]. However, there is very limited information on GBJs fabricated from the electron doped material Nd 1.85 Ce 0.15 CuO 4−y (NCCO) [4]. For the vast majority of GBJs the transport properties can be well described within the intrinsically shunted junction (ISJ) model [2,5,6]. In this model a continuous, but spatially inhomogeneous insulating grain boundary barrier is assumed that contains a high density of localized defect states. The localized states allow for resonant tunneling of quasiparticles providing an intrinsic resistive shunt, whereas resonant tunneling of Cooper pairs is prevented by Coulomb repulsion. This results in reduced I c R n products and a scaling behavior I c R n ∝ √ J c observed in many experiments [2,[5][6][7]. Meanwhile, it is well established that most hole doped HTS have a dominant d-wave component of the order parameter (OP) [8]. The influence of the d-wave symmetry of the OP on the magnetic field dependence of I c has been shown for asymmetric 45 • tilt YBCO-GBJs [9]. So far, in the ISJ-model the likely d-wave pairing state of the hole doped HTS has not been taken into account and the relevance of the d-wave symmetry of the OP for the characteristic properties of symmetrical [001] tilt GBJs such as the small value and the scaling behavior of the I c R n product is still a point of controversy. To clarify this issue we have studied [001] tilt NCCO-GBJs. Since for the electron doped material NCCO there is convincing experimental evidence for a s-wave symmetry of the OP [10-13], NCCO-GBJs represent an interesting model system to test the influence of the OP symmetry on the transport properties of GBJs.The NCCO-GBJs were fabricated by molecular beam epitaxy (MBE) of c-axis oriented NCCO thin films on SrTiO 3 bicrystal substrates. The substrate temperature during growth was about 730 • C and ozone was used as oxidation gas. For ...

We present high-resolution measurements of the specific heat and the thermal expansion of the inorganic spin-Peierls cuprate CuGeO 3 doped with 1.4% Zn and 0.5% Si, respectively. Both compounds exhibit rather well-defined spin-Peierls transitions at about 11 K, which cause anomalies in both the specific-heat and the thermal-expansion coefficients. By comparing these anomalies we derive very large pressure dependencies of the transition temperatures. In particular, the pressure dependencies of the doped compounds are significantly larger than those in pure CuGeO 3 . From this difference we conclude that the suppression of the spin-Peierls transition in CuGeO 3 by doping with Zn or Si is significantly reduced under pressure.

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We have studied the electric transport properties of symmetrical [001] tilt Nd1.85Ce0.15CuO4−y (NCCO) bicrystal grain boundary Josephson junctions (GBJs) fabricated on SrTiO3 bicrystal sub-strates with misorientation angles of 24 • and 36.8 •. The superconducting properties of the NCCO-GBJs are similar to those of GBJs fabricated from the hole doped high temperature superconductors (HTS). The critical current density Jc decreases strongly with increasing misorientation angle. The products of the critical current Ic and the normal resistance Rn (∼ 100 µV at 4.2 K) are small compared to the gap voltage and fit well to the universal scaling law IcRn ∝ √ Jc found for GBJs fabricated from the hole doped HTS. This suggests that the symmetry of the order parameter, which most likely is different for the electron and the hole doped HTS has little influence on the characteristic properties of symmetrical [001] tilt GBJs. Bicrystal grain boundary Josephson junctions (GBJs) have been studied intensively using epitaxial thin films of the various hole doped high temperature superconductors (HTS) [1-3]. However, there is very limited information on GBJs fabricated from the electron doped material Nd 1.85 Ce 0.15 CuO 4−y (NCCO) [4]. For the vast majority of GBJs the transport properties can be well described within the intrinsically shunted junction (ISJ) model [2,5,6]. In this model a continuous, but spatially inhomogeneous insulating grain boundary barrier is assumed that contains a high density of localized defect states. The localized states allow for resonant tunneling of quasiparticles providing an intrinsic resistive shunt, whereas resonant tunneling of Cooper pairs is prevented by Coulomb repulsion. This results in reduced I c R n products and a scaling behavior I c R n ∝ √ J c observed in many experiments [2,5-7]. Meanwhile, it is well established that most hole doped HTS have a dominant d-wave component of the order parameter (OP) [8]. The influence of the d-wave symmetry of the OP on the magnetic field dependence of I c has been shown for asymmetric 45 • tilt YBCO-GBJs [9]. So far, in the ISJ-model the likely d-wave pairing state of the hole doped HTS has not been taken into account and the relevance of the d-wave symmetry of the OP for the characteristic properties of symmetrical [001] tilt GBJs such as the small value and the scaling behavior of the I c R n product is still a point of controversy. To clarify this issue we have studied [001] tilt NCCO-GBJs. Since for the electron doped material NCCO there is convincing experimental evidence for a s-wave symmetry of the OP [10-13], NCCO-GBJs represent an interesting model system to test the influence of the OP symmetry on the transport properties of GBJs. The NCCO-GBJs were fabricated by molecular beam epi-taxy (MBE) of c-axis oriented NCCO thin films on SrTiO 3 bicrystal substrates. The substrate temperature during growth was about 730 • C and ozone was used as oxidation gas. For the critical temperature T c , the resistivity ρ(250 K), and ρ(25 K) typical values of...