Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Freitag
TT 31 Supraleitung: Vortexdynamik, Vortexphasen, Pinning
Zeit: Freitag 10:15–12:45 Raum: H20
TT 31.1 Fr 10:15 H20
Vortex radiation in long Josephson nanojunctions — •A.A. Abdumalikov
Jr. 1 , M.V. Fistul 1 , V.V. Kurin 2 , and A.V. Ustinov 1
— 1 Physikalisches Institut III, Universität Erlangen-Nürnberg, Erwin-
Rommel-str.1, 91058 Erlangen, Germany — 2 Institute for Physics of Microstructure
(RAS), Nizhniy Novgorod, Russia
We theoretically, numerically and experimentally investigate the dynamics
of vortices in long submicron-wide Josephson junctions. Using a
variational approach, the current voltage characteristics are calculated
for various junction width and number of vortices trapped in the junction.
The theoretical and numerically calculated current-voltage characteristics
show good agreement. The maximum velocity of vortices is
increasing with decreasing the junction width. Due to interaction with
stray magnetic fields outside the junction, the vortices radiate Cherenkov
waves as they move in the junction. Locking of the vortex oscillation frequency
to that of the Cherenkov radiation leads to strong resonances on
the current-voltage characteristics. We observe such resonances in experiment
in qualitative agreement with our theoretical analysis.
TT 31.2 Fr 10:30 H20
Study of semifluxons in artificial long Josephson 0-π-junctions
— •Tobias Gaber1 , Edward Goldobin1,2 , Albert Sterck1 , Dieter
Koelle1 , and Reinhold Kleiner1 — 1Physikalisches Institut,
Experimentalphysik II, Universität Tübingen, Auf der Morgenstelle 14,
72076 Tübingen — 2Institut für Mikro- und Nanoelektronische Systeme,
Universität Karlsruhe (TH), Hertzstr. 16, D-76187 Karlsruhe
In long Josephson junctions (LJJ) consisting of two parts with positive
and negative critical current (0 and π parts) half-integer vortices (semifluxons)
can spontaneously appear at the boundary between these parts.
Such a boundary formally corresponds to a π-discontinuity of the Josephson
phase. In our experiments we artificially create such a discontinuity
in conventional Nb-AlOx-Nb LJJs using a pair of current injectors. Such
JJs allow one to study arbitrary fractional vortices and their dynamics.
In the 0-π-state the Ic(H) dependence shows a minimum at H = 0 as
in natural 0-π-LJJs. We investigate the dynamics of the magnetic flux
in such junctions for various lengths and observe half-integer zero-field
steps (ZFS) with n = 1
3 5 , , 2 2 2
on the IV -characteristics which appear due
to the motion of semi-integer flux. We also show that semi-integer ZFS
quantisation persists even when arbitrary fractional vortices (other that
Φ0/2) are participating in the motion.
TT 31.3 Fr 10:45 H20
Rasche Relaxation der irreversiblen Ströme durch Vortex
Schütteln in dünnen Supraleitern — •Ernst Helmut Brandt 1
und Grigorii P. Mikitik 1,2 — 1 Max-Planck-Institut für Metallforschung,
D-705069 Stuttgart — 2 Verkin Institute for Low Temperature
Physics, Kharkov 61103, Ukraine
Beim Auftreten von Vortex-Pinning zeigen Supraleiter 2. Art stark irreversibles
Verhalten. Durch Anlegen eines schwachen magnetischen Wechselfeldes
senkrecht zum angelegten konstanten Magnetfeld relaxieren die
irreversiblen Supraströme rasch, etwa exponentiell mit der Zeit. Dieses
Vortex-Schütteln wird quantitativ erklärt zuerst für lange dünne Streifen
in senkrechtem Gleichfeld (Hdc�z) und dazu senkrechtem Wechselfeld
quer (Hac�y) [1] oder parallel (Hac�x) [2] zum Streifen (transversales und
longitudinales Schütteln) und dann fuer dünne rechteckige Platten oder
Filme, in denen beide Schüttelarten auftreten und damit zwei Relaxationszeiten
[3]. Die irreversiblen Abschirmströme kreisen in der Platte
zuerst in einer einfachen Schleife. Während der Relaxation treten dann
aber mehrere neue Stromschleifen auf, da die Stromkomponente ⊥ Hac
schneller relaxiert als die � Hac. Nach einiger Zeit bleiben nur noch die
reversiblen Ströme übrig, die so trotz Pinning gemessen werden können.
[1] E.H. Brandt and G.P. Mikitik, Phys.Rev.Lett. 89, 027002 (2002).
[2] G.P. Mikitik and E.H. Brandt, Phys.Rev. B 67, 104511 (2003).
[3] G.P. Mikitik and E.H. Brandt, Phys.Rev. B, submitted
TT 31.4 Fr 11:00 H20
Observation of Abrikosov Lattice Melting in Real Space by
Magnetic Force Microscopy — •Alexander Schwarz, Marcus
Liebmann, and Roland Wiesendanger — IAP, University of Hamburg,
Hamburg, Germany
In this study, we use low temperature magnetic force microscopy to
visualize the melting of a regular Abrikosov vortex lattice frozen into a
Bi2Sr2CaCu2O8 single crystal by field cooling. Due to the high sensitivity
and spatial resolution of our instrument (below 50 nm [1]), individual flux
lines (perpendicular bound pancake vortices) are easily resolved. Upon
increasing the temperature towards TC, the flux lines appear broader,
because (i) the penetration depth increases, and (ii) they become more
mobile within their lattice position. However, the regular hexagonal arrangement
stays intact, until a certain temperature (T ≈ 50 K) is reached
beyond which the contrast vanishes. Strongly pinned individual vortices
can remain visible and act as scattering centers for the now unbound
pancake vortices, which are in the liquid state. After recondensation,
the regular hexagonal arrangement is restored, but somewhat tilted with
respect to the original configuration.
[1] M. Liebmann et al., Rev. Sci. Instrum. 73, 3508 (2002).
TT 31.5 Fr 11:15 H20
Magnetic flux pinning in bilayers of high-temperature superconductors
and ferromagnets — •J. Albrecht, S. Soltan, and
H.–U. Habermeier — Max–Planck–Institut für Festkörperforschung,
Heisenbergstr. 1, 70569 Stuttgart
Epitaxial bilayers of YBCO and ferromagnetic perowskites such as
doped lanthanum manganites or strontium ruthenate are grown on single
crystalline substrates by pulsed laser deposition. From quantitative
magnetooptical investigations we find that the critical current density in
these bilayers strongly depend on the magnetization state of the ferromagnet
[1]. This leads to a hysteretic behaviour of the critical currents
in these structures which is absent if the ferromagnet is replaced by an
oxidic metal. The observed effect can be explained by pinning of flux
lines at the magnetic domain walls in the ferromagnet.
[1] J. Albrecht, S. Soltan and H.–U. Habermeier, Europhys. Lett. 63,
881 (2003).
TT 31.6 Fr 11:30 H20
Vortex matter phase diagram of pure and Zn-doped
YBa2Cu3O7−x in magnetic fields up to 50 T — •Yurii Skourski
1,2 , Günter Fuchs 1 , Nadezhda Kozlova 1 , Konstantin
Nenkov 1 , Gernot Krabbes 1 , and Karl-Hartmut Müller 1 —
1 Leibniz Institut für Festkörper- und Werkstoffforschung Dresden, P.O.
Box 270016, D-01171 — 2 Max-Planck-Institut für Physik komplexer
Systeme, Nöthnitzer Strasse 38, 01187 Dresden
The vortex matter phase diagram of melt textured YBa2Cu3O7−x
(YBCO) and of Zn doped YBCO was investigated by magnetization and
resistance measurements in magnetic fields up to 50 T. Data for the irreversibility
field Hirr(T) and the upper critical field Hc2(T) obtained in
pulsed and static fields show a good agreement. A strong suppression
of the superconducting transition temperature by about 15 K was observed
for a YBCO sample doped with 1.2%Zn. In addition, the shape
of Hirr(T) and Hc2(T) was found to be strongly affected by Zn doping.
Consequently, the transition from the vortex glass to the vortex liquid
phase found for undoped YBCO is shifted to lower magnetic fields for Zn
doped YBCO. The experimental data are compared with the predictions
of existing models.
TT 31.7 Fr 11:45 H20
Oberflächenbarriere und Vortex Matching in YBa2Cu3O7−δ Nanobrücken
— •Johannes Eisenmenger, Frank-Michael Kamm,
Alfred Plettl und Paul Ziemann — Abteilung Festkörperphysik,
Universität Ulm, 89069 Ulm
Dünne Typ-II Supraleiter-Schichten im parallelen Magnetfeld bilden
bei bestimmten Magnetfeldern besonders stabile Flussschlauchanordnungen,
was sich u.a. in einer erhöhten kritischen Stromdichte äußert. Der
Wert dieser sogenannten Matching-Felder hängt u.a. vom angelegten Feld
und der Dicke der Schicht ab. Eine wesentliche Rolle spielt dabei die
Oberflächenbarriere, die das Eindringen und Verlassen der Flussschläuche
aus der Schicht behindert. In unseren Messungen werden erstmalig solche
Matching-Effekte auch an supraleitenden Brücken beobachtet. Die
YBa2Cu3Oδ−7 Brücken mit der Länge von 240 nm und einer Breite und
Dicke von 100 nm wurden mittels Elektronenstrahllithograhie strukturiert
und nasschemisch geätzt. Die erhöhte Stabilität spiegelt sich in der
Magnetfeldabhängigkeit der kritischen Stromdichte wieder. Vergleichbare
Matching-Felder werden auch bei der Feldabhängigkeit der Stromstärke