- Email: [email protected]
Possible evidence for d + s pairing symmetry in YBCO using tricrystal microbridges
PHYSICA Physica C 282-287 (1997)1515-1516
ELSEVIER
Possible evidence for d + s pairing microbridges J. H. Miller, Jr.,
symmetry
in
YBCO
using
tricrystal
Z. G. Zou, J. Liu, Z.-S. Zheng, and W. K Chu
Texas Center for Superconductivity, Texas 77204-5932 USA
University of Houston,
4800 Calhoun Road,
Houston,
We have measured the field-modulated critical currents of YBCO thin film microbridges on tricrystal substrates. Two central peaks of equal height are observed, consistent with predominantly d-wave pairing symmetry. However, a nonvanishing critical current at zero field suggests the existence of a real s-wave component. We have also characterized an ionirradiated YBCO tricrystal microbridge, which has been found to exhibit more ideal dx2.y2-1ike behavior with increasing ion irradiation dosage. 1. INTRODUCTION A determination of the pairing state symmetries of cuprate superconductors is important to help elucidate the m e c h a n i s m responsible for high-temperature superconductivity. Phase-sensitive experiments that directly probe the pairing state symmetries of YBCO and T1-2201 [1] are consistent with dx2.y2-symmetric pairing, with little or no imaginary s-wave component. However, some experiments on Ph/YBCO tunnel junctions [2] indicate the existence of an s-wave component, suggesting the possibility of a real mixture of large d-wave and small s-wave components in YBCO.
provide additional information besides the phase shift, and are capable of detecting either a complex or a real mixture of s- and d-wave components of the order parameter. T w o main peaks, of approximately equal height, are observed in the magnetic field-dependent critical currents of nominally pure Y B C O tricrystal junctions operating in the short junction limit, consistent with predominantly d-wave pairing symmetry. However, the m i n i m u m critical current at zero field is nonvanishing, consistent with a real mixture of large d-wave and small s-wave components. Such a pairing state is equivalent to an orthorhombically distorted d-wave order parameter.
2. EXPERIMENT Our method of probing pairing symmetry [3] is to measure the fieldmodulated critical current of a cuprate thin film microbridge on a SrTi03 tricrystal substrate, as illustrated in Fig. 1. The critical current is limited by the two shortest of the three grain boundaries crossing the microbridge. This device is analogous to an s-d corner junction, since one of the two short boundaries acts as a ~-junction, while the other acts as a 0-junction, if the p a i r i n g symmetry is d-wave. Our m e a s u r e m e n t s 0921-4534/97/$17.00 © Elsevier Science B.V. All rights reserved. Pll S0921-4534(97)00840-X
Fig. 1. Tricrystal microbridge misorientation angles.
with
30°
"1516
JH. Miller Jr. et al./Physica C 282-287 (1997) 1515-1516
Table L Experimentally determined parameters for a 5-~tm wide YBCO tricrystal irradiated with 150 keV He ~ion s. Ion Dosage (xl014 cm"2)
/¢ (B=0)/Icmax
0 1.67 4.17 6.67 8.33 10.0 11.7
0.88 0.86 0.84 0.77 0.90 0.77 O.58
~2.5
---
Table I shows several experimentallydetermined parameters for a YBCO tricrystal microbridge that was irradiated with increasing dosages of 150 keV He + ions. RSJ-like current-voltage characteristics were observed below T¢ (R = 0) in all cases except for the highest dosage indicated, where the I - V curve became quite rounded. The Josephson penetration length Aj was calculated using ~ 2 = 4~/poJ~ since the film thickness t = 500 A was less than the London penetration length. We generally observed a m i n i m u m at zero field, and two symmetric maxima, i n the field-modulated critical currents. No evidence for time-reversal symmetry breaking, which would have been manifested as an asymmetric I¢(B), was observed. The ratio of the zero-field critical current to the maximum critical c u r r e n t I¢ (B=0)/I¢ m~" can, in principle, provide information about the relative m a g n i t u d e s of the s- and d- wave components in the short junction limit (w < ~tj ). A ratio of zero would indicate pure d-wave p a i r i n g symmetry, while a ratio of one would be consistent with predominantly s-wave pairing symmetry. The behavior becomes closer to that expected for an ideal d-wave superconductor
T~(R=0) (K) 78 76 71 63 56 49 37 5
microbridge
1o~ (~A)
~j (~m)
500 375 213 135 94 47 14 .
.
4.1 4.7 6.2 7.8 9.4 13.2 24.2 .
.
.
.
as the T c and J~ are both suppressed by ionirradiation, as indicated by the reduction of Ic (B=0) /I¢ m~" (except for the anomalous increase in the ratio at 8.33 × 10u ions/cm2). Much of this behavior can be attributed to the fact that ~tj increases, becoming longer than the width of the microbridge (i.e. short junction limit), as the critical current is reduced. However, it is also likely that a n y real s-wave component becomes reduced as the number of irradiation-induced defects, which act as oxygen traps that pull oxygen away from the Cu-O chains, increases a n d the structure becomes more tetragonal. We thank Q. Y. Ying, M. F. Davis, a n d N. Tralshawala for their assistance. This work was supported by the State of T e x a s through the Texas Center for Superconductivity and the Advanced Research Program, and by the Robert A. Welch Foundation. 3. REFERENCES 1. C.C. Tsuei et al., Science 272 (1996) 329, and references cited therein. 2. A. G. Sun et al., Phys. Rev. Lett. 72 (1994) 2267. 3. J . H . Miller, Jr. et al.,Phys. Rev. Lett. 74 (1995) 2347; J . H . Miller, Jr., et a l . , J. L o w Temp. Phys. 105 (1996) 527.
ELSEVIER
Possible evidence for d + s pairing microbridges J. H. Miller, Jr.,
symmetry
in
YBCO
using
tricrystal
Z. G. Zou, J. Liu, Z.-S. Zheng, and W. K Chu
Texas Center for Superconductivity, Texas 77204-5932 USA
University of Houston,
4800 Calhoun Road,
Houston,
We have measured the field-modulated critical currents of YBCO thin film microbridges on tricrystal substrates. Two central peaks of equal height are observed, consistent with predominantly d-wave pairing symmetry. However, a nonvanishing critical current at zero field suggests the existence of a real s-wave component. We have also characterized an ionirradiated YBCO tricrystal microbridge, which has been found to exhibit more ideal dx2.y2-1ike behavior with increasing ion irradiation dosage. 1. INTRODUCTION A determination of the pairing state symmetries of cuprate superconductors is important to help elucidate the m e c h a n i s m responsible for high-temperature superconductivity. Phase-sensitive experiments that directly probe the pairing state symmetries of YBCO and T1-2201 [1] are consistent with dx2.y2-symmetric pairing, with little or no imaginary s-wave component. However, some experiments on Ph/YBCO tunnel junctions [2] indicate the existence of an s-wave component, suggesting the possibility of a real mixture of large d-wave and small s-wave components in YBCO.
provide additional information besides the phase shift, and are capable of detecting either a complex or a real mixture of s- and d-wave components of the order parameter. T w o main peaks, of approximately equal height, are observed in the magnetic field-dependent critical currents of nominally pure Y B C O tricrystal junctions operating in the short junction limit, consistent with predominantly d-wave pairing symmetry. However, the m i n i m u m critical current at zero field is nonvanishing, consistent with a real mixture of large d-wave and small s-wave components. Such a pairing state is equivalent to an orthorhombically distorted d-wave order parameter.
2. EXPERIMENT Our method of probing pairing symmetry [3] is to measure the fieldmodulated critical current of a cuprate thin film microbridge on a SrTi03 tricrystal substrate, as illustrated in Fig. 1. The critical current is limited by the two shortest of the three grain boundaries crossing the microbridge. This device is analogous to an s-d corner junction, since one of the two short boundaries acts as a ~-junction, while the other acts as a 0-junction, if the p a i r i n g symmetry is d-wave. Our m e a s u r e m e n t s 0921-4534/97/$17.00 © Elsevier Science B.V. All rights reserved. Pll S0921-4534(97)00840-X
Fig. 1. Tricrystal microbridge misorientation angles.
with
30°
"1516
JH. Miller Jr. et al./Physica C 282-287 (1997) 1515-1516
Table L Experimentally determined parameters for a 5-~tm wide YBCO tricrystal irradiated with 150 keV He ~ion s. Ion Dosage (xl014 cm"2)
/¢ (B=0)/Icmax
0 1.67 4.17 6.67 8.33 10.0 11.7
0.88 0.86 0.84 0.77 0.90 0.77 O.58
~2.5
---
Table I shows several experimentallydetermined parameters for a YBCO tricrystal microbridge that was irradiated with increasing dosages of 150 keV He + ions. RSJ-like current-voltage characteristics were observed below T¢ (R = 0) in all cases except for the highest dosage indicated, where the I - V curve became quite rounded. The Josephson penetration length Aj was calculated using ~ 2 = 4~/poJ~ since the film thickness t = 500 A was less than the London penetration length. We generally observed a m i n i m u m at zero field, and two symmetric maxima, i n the field-modulated critical currents. No evidence for time-reversal symmetry breaking, which would have been manifested as an asymmetric I¢(B), was observed. The ratio of the zero-field critical current to the maximum critical c u r r e n t I¢ (B=0)/I¢ m~" can, in principle, provide information about the relative m a g n i t u d e s of the s- and d- wave components in the short junction limit (w < ~tj ). A ratio of zero would indicate pure d-wave p a i r i n g symmetry, while a ratio of one would be consistent with predominantly s-wave pairing symmetry. The behavior becomes closer to that expected for an ideal d-wave superconductor
T~(R=0) (K) 78 76 71 63 56 49 37 5
microbridge
1o~ (~A)
~j (~m)
500 375 213 135 94 47 14 .
.
4.1 4.7 6.2 7.8 9.4 13.2 24.2 .
.
.
.
as the T c and J~ are both suppressed by ionirradiation, as indicated by the reduction of Ic (B=0) /I¢ m~" (except for the anomalous increase in the ratio at 8.33 × 10u ions/cm2). Much of this behavior can be attributed to the fact that ~tj increases, becoming longer than the width of the microbridge (i.e. short junction limit), as the critical current is reduced. However, it is also likely that a n y real s-wave component becomes reduced as the number of irradiation-induced defects, which act as oxygen traps that pull oxygen away from the Cu-O chains, increases a n d the structure becomes more tetragonal. We thank Q. Y. Ying, M. F. Davis, a n d N. Tralshawala for their assistance. This work was supported by the State of T e x a s through the Texas Center for Superconductivity and the Advanced Research Program, and by the Robert A. Welch Foundation. 3. REFERENCES 1. C.C. Tsuei et al., Science 272 (1996) 329, and references cited therein. 2. A. G. Sun et al., Phys. Rev. Lett. 72 (1994) 2267. 3. J . H . Miller, Jr. et al.,Phys. Rev. Lett. 74 (1995) 2347; J . H . Miller, Jr., et a l . , J. L o w Temp. Phys. 105 (1996) 527.