Regensburg 2007 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 29: Superconductivity - Mechanisms, Phase Diagram, Competing Order

TT 29.2: Talk

Thursday, March 29, 2007, 16:15–16:30, H18

Tunneling spectroscopy of the electron doped high-temperature superconductor La_2−xCe_xCuO₄ — •Michael Wagenknecht¹, Markus Turad¹, Dieter Koelle¹, Reinhold Kleiner¹, Siegfried Graser², Nils Schopohl², Akio Tsukada³, and Michio Naito⁴ — ¹Universität Tübingen, Experimentalphysik II, Auf der Morgenstelle 14, D-72076 Tübingen, Germany — ²Universität Tübingen, Lehrstuhl für Theoretische Festkörperphysik, Auf der Morgenstelle 14, 72076 Tübingen, Germany — ³NTT Basic Research Laboratories, Atsugi-shi, Japan — ⁴Department of Applied Physics, Tokyo University of Agriculture and Technology, Tokyo, Japan

We show tunnel spectroscopy data of quasiparticle transport in electron doped high-temperature superconductor (HTS) La_2−xCe_xCuO₄ bicrystal thin films with a 24^∘ symmetric [001]-tilt grain boundary patterned as Josephson junction SQUIDs (x ∼ 0.08, slightly underdoped with T_c ∼ 28 K). The differential conductance in the superconducting state shows a zero bias conductance anomaly (ZBCA), which is attributed to surface Andreev bound states at zero excitation energy due to d-wave symmetry of the order parameter in this electron doped HTS. As the ZBCA is related to the macroscopic phase coherence of the superconducting state it vanishes at T_c in zero field. However, with applied field we observe a persistent ZBCA up to at least 10 T. This is in contradiction to previously reported values of B_c2(0) ∼ 7 T based on ρ(T) measurements. Supported by calculations taking into account the effect of the vortex lattice we suggest a modified B(T) phase diagram with a higher B_c2 > 10 T.