Unconventional current-phase relations in YBCO dc-SQUIDs

1
Unconventional current-phase relations in YBCO
dc-SQUIDs

• S.A. Charleboisa, T. Lindströma, A.Ya.
  Tzalenchukb, Z. Ivanova, T. Claesona
• aDep. of Microtechnology and Nanoscience -
  Quantum Device Physics Laboratory, Chalmers
  University of Technology, SE-412 96 Göteborg,
  Sweden
• bNational Physical Laboratory, Teddington,
  Middlesex, TW11 0LW, UK

2
The presence of a second harmonic in the current
phase relation

• p-loops have been extensively studied in d-wave
  HTS 1.
• Aside from those, high misorientation bicrystal
  Josephson junctions in HTS are expected to have
  unconventional CPR 2.
• Observation of a second harmonic in the CPR of
  HTS bicrystals has been reported previously
  3,4,6.
• We discuss expected features of the critical
  current versus magnetic field curves in such
  cases.
• We report the observation of these features in
  0-45 bicrystal YBCO junctions.

3
Excellent correspondence
2µmjunctions
Theory
Experiment
Critical current The theoretical curve (red in
the right figure) fits the measurement very well
SQUID response The theoretical curve (left) fits
the measurements (left) show good qualitative
agreement
4
Different behavior in submicron junctions

• Junctions have lower Ic then expected for their
  size
• Width ? 10 but Ic ? 100
• Indicates reduced transparency
• The SQUIDs with submicron junctions do not show
  doubling in Ic(?) curves
• Thermal averaging (EJ kBT) could hide the
  effect of the 2nd harmonic on the Ic(?) curves
• A small shift between the positive and negative
  current bias is observed
• approx. 0.1Fo

The critical current vs. applied magnetic field
for two SQUIDs with the same loop size
(1515) µm2. SQUID A 0.3/0.2 µm wide junctions
(values multiplied by 10 for clarity). SQUID B
2/2 µm junctions. All curves measured at 4 K.
5
Symmetric SQUID
Position of the minima and maxima of Ic(?)
Ic(?) for various values of ?

• Complex secondary maxima develop at (n1)? for
  ?gt½
• for ?gt½, the potential is double well like
• No shift between and current bias
• Modulation is not complete even though the
  junctions are identical

6
Asymmetric SQUID
Position of the minima and maxima of Ic(?)
Ic(?) for various values of ?

• Secondary maxima develop for ?gt½
• for ?gt½, the potential is double well like
• the position is parameter dependant
• Shift between and current bias
• Shift present for ?lt½ where the potential is not
  double well like

7
Conclusion

• We show that a 2nd harmonic,
• Even below the double well formation threshold
  (?lt½ )
• Leads to observable shift in the Ic(?) extrema
• Reduces significantly the amplitude of SQUID
  modulations
• We have observed unconventional CPR in submicron
  0-45 bicrystal Josephson junctions
• shift with bias polarity inversion
• It confirms the high quality of the junctions is
  preserved by the fabrication technology

• Note on fabricationThe YBCO films were grown by
  pulsed laser deposition on 0-45 SrTiO3
  bicrystal substrates. The films thickness is
  250 nm. The SQUIDs were fabricated by Ar ion
  milling through an amorphous carbon mask
  patterned by electron beam lithography 5.
• References
• 1 C.C. Tsuei and J.R. Kirtley,Rev. Mod. Phys.
  72 (2000) 969.
• 2 A.M. Zagoskin cond-mat/9903170 T. Löfwander,
  V.S. Shumeiko, G. Wendin, Supercond. Sci.
  Technol. 14 (2001) R53.
• 3 E. Ilichev et al., Phys. Rev. Lett. 86
  (2001) 5369 I.V. Borisenko et al., Physica C 368
  (2002) 328.
• 4 T. Lindström et al., Phys. Rev. Lett. 90
  (2003) 11702.
• 5 A.Yu. Tzalenchuk et al., submitted to Phys.
  Rev. B.