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Plenarvorträge - DPG-Tagungen

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Tiefe Temperaturen Mittwoch<br />

TT 18.2 Mi 14:45 H20<br />

Experimental Realization of all High-Tc dc π/2-SQUIDs —<br />

•Christof Schneider, German Hammerl, Rainer Held, Klaus<br />

Wiedenmann, and Jochen Mannhart — Experimentalphysik VI,<br />

Universität Augsburg<br />

In the presentation, we will report on the realization of all high-Tc<br />

π/2-SQUIDs. The characteristics of the π/2-SQUIDs, which were fabricated<br />

using YBa2Cu3O7−δ thin films grown on tetracrystalline substrates<br />

will be presented and compared with the properties of standard high-Tc<br />

SQUIDs.<br />

TT 18.3 Mi 15:00 H20<br />

Imaging of Josephson Current Distribution in YBa2Cu3O7-Nb<br />

Zigzag Junctions — •D. Dönitz 1 , E. Goldobin 1 , R. Straub 1 ,<br />

R. Kleiner 1 , D. Koelle 1 , H.J.H. Smilde 2 , Ariando 2 , H.<br />

Hilgenkamp 2 , and H. Rogalla 2 — 1 Universität Tübingen, Physikalisches<br />

Institut – Experimentalphysik II, Auf der Morgenstelle 14,<br />

D-72076 Tübingen, Germany — 2 Low Temperature Division, Faculty of<br />

Science and Technolgoy and MESA+ Research Institute, University of<br />

Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands<br />

We used low temperature scanning electron microscopy (LTSEM) to<br />

image the supercurrent distribution in zigzag YBa2Cu3O7 (YBCO)–Nb<br />

ramp edge Josephson junctions. These junctions consist of several artificial<br />

facets of length a ≤ λJ and alternating orientation along the in-plane<br />

crystallographic axes of the YBCO film. At zero applied magnetic field<br />

we clearly observed the reversal of the direction of the Josephson current<br />

flowing through neighboring facets. This is a direct visualization of<br />

the opposite sign of the critical current in neighboring facets, i.e. they<br />

can be considered as 0 and π junctions. This constitutes direct imaging<br />

of the sign change in the phase of the d x 2 −y 2 order parameter symmetry<br />

in YBCO. We performed numerical simulations of the sine–Gordon<br />

model which showed that for our particular geometry the phase profile<br />

along the zigzag junctions corresponds to a compressed chain of antiferromagnetically<br />

ordered semifluxons. At the matching field, for which<br />

the critical current is maximum, the simulations predict a supercurrent<br />

distribution which has the same sign and an arc–like profile in all facets.<br />

This prediction is directly confirmed by LTSEM.<br />

TT 18.4 Mi 15:15 H20<br />

Doping dependence of pseudogap and superconductivity in<br />

electron doped high-temperature-superconductors: results from<br />

tunneling spectroscopy — •B. Welter 1 , L. Alff 1 , Y. Krockenberger<br />

1 , M. Naito 2 , and R. Gross 1 — 1 Walther-Meissner-Institut,<br />

Bayerische Akademie der Wissenschaften, Walther-Meissner-Str. 8, 85748<br />

Garching, Germany — 2 NTT Basic Research Laboratories, Atsugi, Japan<br />

Studying the influence of doping on the characteristics of oxide superconductors<br />

is essential to both the understanding of the underlying<br />

physics and the development of new applications. Here, we report<br />

on tunneling spectroscopy measurements on grain boundary junctions<br />

of electron doped high-temperature superconductors La2−xCexCuO4,<br />

Pr2−xCexCuO4 and Nd2−xCexCuO4 with different Ce concentrations<br />

around optimal doping. We focus on the pronounced depletion in the<br />

density of states near EF observed in the normal state above Bc2 and its<br />

evolution with temperature, magnetic field and doping [1]. Considerations<br />

concerning the conservation of states rule even indicate a coexistence of<br />

this pseudogap regime and the superconducting state.[2]<br />

[1] L. Alff, Y. Krockenberger, B. Welter, M. Schonecke, R. Gross, D.<br />

Manske and M. Naito, Nature 422, 698 (2003)<br />

[2] B. Welter, Y. Krockenberger, L. Alff, M. Naito und R. Gross, to appear<br />

in Physica C<br />

TT 18.5 Mi 15:30 H20<br />

STM structure analysis of bismuth high-TC crystals depending<br />

on the bias voltage — •Hendrik Glowatzki, Torsten Stemmler,<br />

Johannes Zeggel, Alicia Krapf, Christoph Janowitz, and Recardo<br />

Manzke — Institut f. Physik, Humboldt-Universität zu Berlin,<br />

Newtonstr. 15, 12489 Berlin<br />

A detailed structural analysis of single and double layered bismuth<br />

single crystals (BSCCO) has been performed by Scanning Tunneling<br />

Microscopy (STM). Because of the special layered configuration of these<br />

superconductors, containing semiconducting, insulating and the superconducting<br />

metallic layers, we obtain different atomic images depending<br />

on the applied tunneling voltage. In particular, semiconducting layers<br />

will be invisible for bias voltages below its gap energy /1/. So it has<br />

become possible to observe not only the surface layer but also deeper<br />

CuO2-layers which cause high-TC superconductivity. The results can be<br />

understood from the particular crystal structure of BSCCO.<br />

[1] Oda, M. et al. Phys. Rev. B 53, 2253 (1995)<br />

TT 18.6 Mi 15:45 H20<br />

Cavity resonances in Bi2Sr2CaCu2O8 intrinsic Josephson<br />

junction stacks under low frequency microwave irradiation —<br />

•Tobias Clauss, Takashi Uchida, Michael Mößle, Dieter<br />

Koelle, and Reinhold Kleiner — Universität Tübingen, Physikalisches<br />

Institut - Experimentalphysik II, Auf der Morgenstelle 14, 72076<br />

Tübingen<br />

One important problem towards the realization of THz oscillators from<br />

intrinsic Josephson junction stacks is to make the junctions in the stack<br />

to oscillate in-phase. One possibility to generate such oscillations is to<br />

excite cavity resonances by collective motion of Josephson fluxons in the<br />

stack.<br />

So far, cavity resonances have only been observed for Bi2Sr2CaCu2O8<br />

stacks at high external magnetic fields. We have investigated single crystal<br />

Bi2Sr2CaCu2O8 stacks at zero magnetic field under microwave irradiation<br />

with frequencies between 2 and 25 GHz. Measured current voltage<br />

characteristics show structures which can be attributed to cavity resonances.<br />

Using low temperature scanning electron microscopy to image<br />

the current distribution in the stack we find clear cavity resonances with<br />

wavelengths of some µm. We believe that the resonances are excited by<br />

the collective motion of vortex-antivortex pairs generated by the external<br />

microwave field.<br />

TT 18.7 Mi 16:00 H20<br />

Intrinsic Josephson Effects in RuSr2GdCu2O8 — •Timo Nachtrab<br />

1 , D. Koelle 1 , R. Kleiner 1 , C. T. Lin 2 , and C. Bernhard 2<br />

— 1 Universität Tübingen, Physikalisches Institut -Experimentalphysik<br />

II, Auf der Morgenstelle 14, 72076 Tübingen, Germany — 2 Max-Planck-<br />

Institut für Festkörperforschung, Heisenbergstrasse 1, 70569 Stuttgart,<br />

Germany<br />

We report on electrical transport measurements on single crystals of<br />

RuSr2GdCu2O8(Ru1212). The material shows coexistence of magnetic<br />

ordering (Tm ≈ 130 K) and superconductivity (Tc ≈ 55 K) at low temperatures.<br />

The crystal structure consists of alternating RuO2 layers (with<br />

a weak ferromagnetic (F) component in-plane) and superconducting (S)<br />

CuO2 layers, separated by insulating (I) layers of SrO. Ru1212 thus forms<br />

an intrinsic SIFIS multilayer structure. For transport perpendicular to<br />

the planes an intrinsic Josephson effect as, e.g., in Bi2Sr2CaCu2O8+δ<br />

(Bi2212) has been predicted. In addition, the magnetic component inside<br />

the barrier might provoke interesting phenomena like π-phases in<br />

the superconducting order parameter.<br />

We measured the interlayer current transport in micron-sized Ru1212<br />

single crystals in magnetic fields up to 7T and temperatures between<br />

4.2K and 300K. The system shows a clear intrinsic Josephson effect, e.g.<br />

the typical multi-branched current-voltage characteristics known from<br />

high-Tc cuprates. Our magnetic field measurements reveal that fluxon<br />

motion takes place in a very similar fashion as in Bi2212. So far, however,<br />

we did not observe any unconventional behavior due to the magnetism<br />

of the RuO2 layers.<br />

TT 18.8 Mi 16:15 H20<br />

Charge-imbalance relaxation rate in layered d-wave superconductors<br />

— •Joachim Keller and Dmitri Ryndyk — Institut für<br />

Theoretische Physik, Universität Regensburg, 93040 Regensburg<br />

Intrinsic (inter-layer) Josephson effect in layered d-wave superconductors<br />

is accompanied by static and oscillating charge fluctuations on the<br />

layers. In a system of weakly coupled very thin atomic layers, such charge<br />

fluctuations may lead to non-equilibrium effects described by a shift of<br />

the chemical potential of the condensate and a charge imbalance between<br />

electron- and hole-like quasiparticles. We have developed the microscopic<br />

theory based on the nonequilibrium Green function method and calculated<br />

static and high-frequency charge imbalance relaxation rates in dwave<br />

superconductors (which are determined at low temperatures by<br />

elastic scattering).<br />

16:30 Pause

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