Achim Marx

ANOMALOUS LOW TEMPERATURE BEHAVIOR OF SUPERCONDUCTING ND1.85CE0.15CUO4-Y

We have measured the temperature dependence of the in-plane London penetration depth lambda(T) and the maximum Josephson current Ic(T) using bicrystal grain boundary Josephson junctions of the electron-doped cuprate superconductor Nd(1.85)Ce(0.15)CuO(4-y). Both quantities reveal an anomalous temperature dependence below about 4 K. In contrast to the usual monotonous decrease (increase) of lambda(T) (Ic(T)) with decreasing temperature, lambda(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 Pr(1.85)Ce(0.15)CuO(4-y). From this we conclude that the anomalous behavior of Nd(1.85)Ce(0.15)CuO(4-y) is caused by the presence of the Nd3+ paramagnetic moments. Correcting the measured lambda(T) dependence of Nd(1.85)Ce(0.15)CuO(4-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 lambda(T) dependence measured for Pr(1.85)Ce(0.15)CuO(4-y).

Spin-dependent transport in the double-perovskite Sr2CrWO6

We report on the preparation and characterization of the double-perovskite compound Sr2CrWO6 with a Curie temperature of 390 K. We have fabricated both Sr2CrWO6 bulk sintered polycrystalline bars and high-quality epitaxial thin films on SrTiO3 substrates by pulsed-laser deposition. The samples were characterized by thermogravimetric analysis, x-ray diffraction, electrical transport, and magnetization measurements. Polycrystalline samples containing a large number of grain boundaries show a large low-field magnetoresistance of up to 100% at 5 K. At room temperature, this effect is reduced to a few percent. Our results show that Sr2CrWO6 is an interesting candidate for room-temperature magnetoelectronic materials.

Possible pseudogap behavior of electron-doped high-temperature superconductors

We have measured the low-energy quasiparticle excitation spectrum of the electron doped high-temperature superconductors (HTS) Nd(1.85)Ce(0.15)CuO(4-y) and Pr(1.85)Ce(0.15)CuO(4-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.

Andreev bound states in high temperature superconductors

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 (YBCO), Bi2Sr2CaCu2O(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.

Heteroepitaxial growth of transition metal oxides using UHV laser molecular beam epitaxy

We have grown heteroepitaxial thin film structures composed of various transition metal oxides such as the colossal magnetoresistance manganites, superconducting cuprates, ruthenates as well as insulating titanates on SrTiO3, NdGaO3 and LaAlO3 substrates using a UHV laser molecular beam epitaxy (laser-MBE) system. The film growth was controlled in-situ using high pressure RHEED as well as scanning probe techniques (AFM/STM). The fabricated films were analyzed by x-ray diffraction, transmission electron microscopy and the measurement of the transport properties. The manganite, ruthenate and titanate thin film structures show good epitaxy with small mosaic spread. The observation of RHEED oscillations during the film deposition indicates a layer by layer growth mode. This is further supported by the observed small surface roughness of typically less than 3 nm rms for a 100 nm thick film. We also could find a clear correlation between the observed RHEED pattern and the surface morphology measured by AFM/STM. Our analysis shows that UHV laser MBE is well suited for the fabrication of complicated heteroepitaxial thin film structures required for oxide electronics.

Low-frequency 1/f noise in doped manganite grain-boundary junctions

We have performed a systematic analysis of the low-frequency

Superconducting Nd1.85Ce0.15CuO4−y bicrystal grain boundary Josephson junctions

1/f

Low frequency voltage noise in high temperature superconductor Josephson junctions

noise in single grain-boundary junctions in the colossal magnetoresistance material

Characterization of individual noise sources in high-temperature superconductor Josephson junctions

{mathrm{La}}_{2/3}{mathrm{Ca}}_{1/3}{mathrm{MnO}}_{3ensuremath{-}ensuremath{delta}}.

Nd/sub 1.85/Ce/sub 0.15/CuO/sub 4-y/ bicrystal grain boundary Josephson junctions

The grain-boundary junctions were formed in epitaxial