Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Montag
TT 8.18 Mo 14:30 Poster A
Proximity-Effekt in supraleitenden Nb/Ag- und Nb/Ag/Fe-
Schichtpaketen — •H. Stalzer 1 , A. Cosceev 1 , C. Sürgers 1 ,
H. v. Löhneysen 1,2 , J.-M. Brosi 3 , G.-A. Chakam 3 und W.
Freude 3 — 1 Physikalisches Institut und DFG Center for Functional
Nanostructures (CFN), Universität Karlsruhe, D-76128 Karlsruhe —
2 Forschungszentrum Karlsruhe, Institut für Festkörperphysik, D-76021
Karlsruhe — 3 Institut für Hochfrequenztechnik und Quantenelektronik
und DFG Center for Functional Nanostructures (CFN), Universität
Karlsruhe, D-76128 Karlsruhe
Der Proximity-Effekt in supraleitenden Nb/Ag- und Nb/Ag/Fe-
Schichten (Schichtdicken dNb0 nm, dAgU0−800 nm, dFe = 2−20 nm) wird
durch Messung der Magnetisierung M in einem parallelen Magnetfeld B
im Temperaturbereich T = 0.07 − 10 K untersucht. Für die Nb/Ag-
Doppelschichten zeigt M(T) unterhalb von T Nb
c = 9.1 K einen weiteren
diamagnetischen Übergang bei T ∗ durch den Einsatz der Supraleitung
in der Ag-Schicht. Die Verläufe des Breakdown“-Feldes Bc(T, dAg) und
”
der charakteristischen Temperatur T ∗ (dAg) zeigen Abweichungen vom
” schmutzigen“ Grenzfall aufgrund der großen mittleren freien Weglängen
in der Ag-Schicht. In den Nb/Ag/Fe-Schichtpaketen wird der Übergang
bei T ∗ durch die Fe-Schicht unterdrückt. Ausserdem beobachten wir eine
Abhängigkeit des Signals von der Position der Probe zwischen den Pickup-Spulen
des Magnetometers durch Entmagnetisierungseffekte. Diese
ist für den Übergang der Ag-Schicht verschieden vom Übergang in der
Nb-Schicht aufgrund unterschiedlicher Abschirmströme.
TT 8.19 Mo 14:30 Poster A
Tunneling spectra of high-temperature superconductor - manganite
junctions — •Mitja Schonecke, Bettina Welter, Lambert
Alff, and Rudolf Gross — Walther-Meißner-Institut, Walther-
Meißner-Str. 8, 85748 Garching
We use YBa2Cu3O7−δ ramp-type junctions to investigate the
conductance spectra of superconductor-normal metal (S/N) and
superconductor-ferromagnet (S/F) interfaces. The geometry of ramptype
junctions takes advantage of the larger coherence length of the
superconductor in the ab-plane by coupling the two electrodes in this
direction. With our fabrication process it is possible to obtain samples
either in the tunneling limit or in the highly transparent Andreev-regime.
While the S/N samples in the tunneling limit show splitting of the zero
bias conductance peak at zero magnetic fields, the Andreev-spectra of
S/F junctions are dominated by multiple sharp features. The origin of
theses features has not yet been clarified.
TT 8.20 Mo 14:30 Poster A
Andreev reflection and nonequilibrium Josephson effect in
(quasi)ballistic S-N-S junctions — •Dmitri Ryndyk — Institut
für Theoretische Physik, Universität Regensburg, 93040 Regensburg
Josephson effect in mesoscopic S-N-S junctions is possible due to the
phase coherent Andreev reflection even in the absence of a pairing interaction
inside normal region. Recently both stationary and nonstationary
Josephson effects were investigated experimentally in ballistic (or close
to ballistic) structures, and a number of expected as well as unexpected
features were observed. We consider the theory in the case of ballistic
and quasiballistic normal regions. Both stationary and nonstationary
Josephson effects are very sensitive to nonequilibrium effects in a normal
channel.
TT 8.21 Mo 14:30 Poster A
Andreev reflection at superconducting contacts to InGaAs/InP
heterostructures — •I. E. Batov 1 , Th. Schäpers 2 , A. A. Golubov
3 , and A. V. Ustinov 1 — 1 Physikalisches Institut III, Universität
Erlangen-Nürnberg, 91058 Erlangen — 2 Institut für Schichten und Grenzflächen,
Forschungszentrum Jülich, 52425 Jülich — 3 Faculty of Applied
Physics, University of Twente, The Netherlands
Transport in hybrid devices consisting of superconductors (S) in contact
with a two-dimensional electron gas (2DEG) is a subject of increasing
interest in recent years. Novel transport phenomena arising due to Andreev
reflection at the S/2DEG interface were predicted recently in such
devices, such as magnetoconductance oscillations in the Quantum Hall
regime, suppression of the Hall resistance near the S/2DEG interface, etc.
To access the novel transport regime of interest, in most cases the high
transparency of the S/2DEG contacts is essential. In this work, we report
on the fabrication of highly transmissive contacts to the two-dimensional
electron gas in InGaAs/InP heterostructures. As a superconductor NbN
is used. High junction transmissivity was achieved by exploiting a pro-
cess of sputtering a thin (10nm) Au interlayer followed by the deposition
of the NbN electrode and rapid annealing at 400 ◦ C. We observed
a decrease in the differential resistance of the junction with pronounced
double-dip structure within superconducting energy gap. The effect of a
magnetic field perpendicular to the plane of the 2DEG on the differential
resistance of the interface was studied. Experimental data are analyzed
within a model of ballistic SN-2DEG contact, based on the quasiclassical
Green-function approach.
TT 8.22 Mo 14:30 Poster A
Supraleiter-Normalleiter-Supraleiter-Kontakte als Schalter für
die Anwendung in supraleitenden Multiplexern — •Oliver
Lindner 1 , Margret Peters 2 , Jörn Beyer 2 und Thomas
Schurig 2 — 1 FHTW Berlin — 2 PTB Berlin
In komplexen kryoelektronischen Schaltungen, wie z.B. in mehrkanaligen
Tieftemperaturdetektor-Anordnungen mit SQUID-basierter Auslese,
können zur Begrenzung der Schaltungskomplexität Multiplexer eingesetzt
werden. In diesem Beitrag wird ein neues Konzept eines supraleitenden
Multiplexers vorgestellt, das auf Supraleiter-Normalleiter-
Supraleiter-(SNS)-Schaltern ähnlich SNS-Josephson-Transistoren [1] beruht.
Hierbei wird durch Erhöhung der Elektronentemperatur im N-
Bereich des Kontakts eine Beeinflussung des Suprastroms durch den SNS-
Kontakt erreicht. Die SNS-Schalter werden in supraleitende Transformatorstrukturen
integriert und vom supraleitenden Zustand in den normalleitenden
geschaltet. Dadurch lassen sich Signalmodulations- und Multiplexfunktionen
realisieren. Es wird die Herstellung von lateralen Nb-Au-
Nb-Kontakten mit Abständen der supraleitenden Elektroden von unter
1 Mikrometer präsentiert. Die Charakterisierung der supraleitenden und
normalleitenden Eigenschaften der SNS-Kontakte und ihr Schaltverhalten
bei Arbeitstemperaturen von 4.2 K und darunter werden dargestellt
und Rückschlüsse auf die Integration der SNS-Schalter in die Multiplexerschaltung
diskutiert.
[1] A.F.Morpurgo, T.M.Klapwijk, B.J.van Wees, Appl.Phys.Lett. 72, 966
(1998)
TT 8.23 Mo 14:30 Poster A
Pulse driven arrays of HTS bicrystal Josephson junctions —
•Andriy Rebikov 1,2 , Alexander Klushin 1 , and Norbert Klein 1
— 1 Forschungszentrum Juelich GmbH, Institut fuer Schichten und Grenzflaechen,
52425, Juelich, Germany — 2 Department of Radiophysics,
Kiev Taras Shevchenko University, Kiev, the Ukraine
For appropriate pulse driven arrays of shunted HTS bicrystal Josephson
junctions parameters of junctions such as critical current Ic, normal
resistance Rn and characteristic voltage were optimised to guarantee a
large first voltage step comparable with Ic. A new shunting concepts were
developed and tested. With a critical current of 1 mA, the product IcRn
of 20 uV matches a convenient input frequency of approximately 12 GHz.
HTS arrays with 100 junctions were driven with both a broadband twolevel
digital code and sine wave and have shown a full frequency locking
at liquid nitrogen temperature. Pulse-driven Josephson arrays are being
developed for ac quantum voltage standards, for a new temperature
standards and generation of arbitrary waveforms.
TT 8.24 Mo 14:30 Poster A
Design considerations for large area magnetic calorimeters and
detector arrays — •T. Daniyarov 1 , A. Burck 1 , M. Linck 1 , H.
Rotzinger 1 , A. Fleischmann 1 , and C. Enss 2 — 1 Kirchhoff-Institut
für Physik, Universität Heidelberg, INF 227, 69120 Heidelberg —
2 Department of Physics, Brown University, Providence, RI 02912, USA
Only recently it was demonstrated that energy dispersive x-ray detectors
based on the concept of metallic magnetic calorimeters can achieve
resolving powers close to E/δE = 2000. Due to the calorimetric detection
scheme magnetic calorimeters are not limited to ionizing radiation and
do not suffer from dead layers at the surface. This combination of properties
is attractive for a large number of applications, provided that the
active area of the detector is large enough. We present sensor geometries,
which allow for the fabrication of large area magnetic calorimeters and
discuss the optimization of flux coupling to the dc-SQUID magnetometer
used for the inductive readout of such detectors. Additionally, an array of
detectors can be used to increase the total cross section, but this option
also adds complexity to the device. We address the problem of thermal
and magnetic crosstalk between the sensors of a detector array and propose
promising sensor and pickup loop designs to meet the requirements
for high resolution particle detection.