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Microwave surface impedance of patterned YBa2Cu3O7 thin films
PHYSICA!]!
Physica B 194-196 (1994) 1605-1606 North-Holland
Microwave Surface Impedance
of Patterned
YBa2Cua07
Thin Films
Adrian Porch, Michael J. Lancaster and +Richard G. Humphreys Superconductivity Research Group, University of Birmingham, Birmingham B15 2TT, UK + Defence Research Agency, Malvern, Worcestershire WR14 3PS, UK We have used microwave coplanar resonators to measure the surface impedance below Tc of very high quality patter:ned thin films of YBa2Cu307 on MgO substrates. We obtain surface resistances at 8GHz of 23#f~ and 105p12 at 15K and 77K, respectively. Our technique allows us to estimate the absolute values of the ab-plane penetration depth AL(T) and we obtain a value of AL(0) for the best film around 150nm. At low T we find that ;~L(T)/,~L(0) = 1 + c~(T/T¢) 2, with ~ _ 0.65 for all of our films. This behaviour differs from that of conventional s-wave superconductors. We measure the ab-plane microwave surface impedance Z~ = R~ + iwtt0)~L of patterned thin films of YBa2CuaO7 (YBCO) using the coplanar resonator technique, which we have described in detail elsewhere 1. This technique has the advantage of requiring only single sided film deposition. The films are deposited by coevaporation on polished (001)-oriented single crystal MgO substrates with a thin buffer layer of in situ deposited MgO. The films are then patterned using a combination of argon ion b e a m milling and EDTA wet etching. After subsequent oxygen annealing the films have.. T~ between 88-91K, with J~(77K) in excess of 2 x 106Acm -2. Our standard linear coplanar line has a central line width of 200pro, with gaps either side of 73tim; a line length of 8 m m results in a fundamental resonant frequency of around 8GHz. We patterned two parallel resonators on the same film, one with the standard gap width of 7 3 p m and another with a gap width of 12pm. Figure 1 shows the fractional resonant frequency shift (relative to 12K) for a pair of resonators patterned on a 350nm thick film, and a pair patterned on a 700nm thick film. We find that the microwave response is independent of microwave fields approaching Hcl, implying that the edge material is unaffected by the patterning. We selfconsistently calculate the current distribution of the resonator cross section which allows us to calculate the resonant frequency as a function of )~L;
from this we estimate )~L(T) absolutely I for each of the two films of figure 1. The largest uncertainty is due to inaccuracies in measuring the gap widths, giving a +10% error in the absolute values of IL; however, relative values are in error by around -t-1%. We plot the d a t a for the supercurrent conductivity cr2(T) (x 1 / ) ~ ( T ) in figure 2. We note that or2 lies well below t h a t predicted by the clean local limit of the BCS theory, particularly at low temperatures. Taking the d a t a for T < 30K, we find that AL(T) = AL(0)(1 + c~(T/Tc) 2) with a = 0.65-40.03. We find this same power law behaviour in all of our films, even though the values of AL(0) vary between 150-220nm, depending on film quality. This behaviour is similar to that observed by others for high-To thin films 2. The Rs data are more variable, and in figure 3 we plot Rs(T) for four different films of variable sample quality. Surprisingly, we find that AL and Rs are only weakly correlated (if the effects are intrinsic then Rs c( A~ at low T). The behaviour of R~ is likely to be extrinsic, although the apparent universal form for AL(T)/AL(0) is of interest. A contentious issue is whether high-To superconductors are s-wave paired. Microwave measurements of the real part of the conductivity cr1 have not yielded conclusive proof of a coherence peak 3 of the kind predicted by BCS theory just below To. This peak m a y be suppressed by the effects of strong coupling, but this would cause
(/921-4526/94/'$07.00 © 1994 - Elsevier Science B.V. All rights reserved
SSDI 0921-4526(93) 1383-W
1606
0.025
124
I
thin=648 thick=781
0.020 I--
1
~i'
0.015
F-
ture dependent electronic mean free path g(T) at low T will reduce cr2(T) by the additional factor A final possibility is that YBCO is a d-wave superconductor with line nodes in the energy gap on portions of the Fermi surface. This would give rise to ,~L ac T at low T, which can change to T 2 as a result of impurity scattering 4. A recent report s indicates AL oc T in high quality YBCO crystals at low T; this may represent the first observation of the intrinsic behaviour of AL in YBCO.
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wide,thin
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0.000
i
i
-
-
10 20
-
_
~
.
50
T
40
I
50
.
i
,
60
I
,
10000
I
70
80
90
Temperature/K Figure 1. The percentage frequency shift (defined as a positive quantity) for four resonators of different cross sectional geometries. The wide and narrow results refer to gap widths of 73/~m and 12#m, repectively; the thin and thick films have thicknesses of 350nm and 700am, respectively. 1.0
100
10
20 50 40 50
60 70 80 90
Temperature/K
1
Figure 3. Rs(T) plotted for four different films of the same thickness (350nm) at 8GHz. The best film has values of Rs at 15K and 77K of 23 + 5/~f~ • and 105 + 25tt~, respectively.
0.6 1 o~
C12
10
0.8
~1
lOO0
0.4
-J
170nm
0.2
" QN\.
\
60
70 80
),(0)[781]= 176nm 0.0
0
10 20 30
40 50
90
Temperature/K Figure 2. The normalised supercurrent conductivity o'2(T)/o'2(0) = )~(O)/)~(T) calculated from the data of figure 1. The BCS result is for the London limit.
~2(T)/0"2(0) t o rise above the weakly coupled BCS prediction (we find it lies below the BCS prediction). Another possibility is that the coherence peak may be removed by inelastic scattering, and within the BCS framework a highly tempera-
1. A. Porch et al., to be published in IEEE Trans. Appl. Supercond. (1993); A. Porch, M. J. Lancaster and R. G. Humphreys, submitted to IEEE Trans. Microwave Theory and Tech. (1993). 2. S. M. Anlage and D.-H. Wu, J. Supercond. 5 (1992) 395; J. M. Pond et al., Appl. Phys. Lett. 59 (1991) 303 3. For example, J. R. Waldram, A. Porch, H. M. Cheah and S.-F. Lee, submitted to this conference 4. J. Annett, N. Goldenfeld and S. R. Renn, Phys. Rev. B 43 (1991) 2778 5. W. N. Hardy, D. A. Bonn, D. C. Morgan, R. Liang and K. Zhang, preprint (1993)
Physica B 194-196 (1994) 1605-1606 North-Holland
Microwave Surface Impedance
of Patterned
YBa2Cua07
Thin Films
Adrian Porch, Michael J. Lancaster and +Richard G. Humphreys Superconductivity Research Group, University of Birmingham, Birmingham B15 2TT, UK + Defence Research Agency, Malvern, Worcestershire WR14 3PS, UK We have used microwave coplanar resonators to measure the surface impedance below Tc of very high quality patter:ned thin films of YBa2Cu307 on MgO substrates. We obtain surface resistances at 8GHz of 23#f~ and 105p12 at 15K and 77K, respectively. Our technique allows us to estimate the absolute values of the ab-plane penetration depth AL(T) and we obtain a value of AL(0) for the best film around 150nm. At low T we find that ;~L(T)/,~L(0) = 1 + c~(T/T¢) 2, with ~ _ 0.65 for all of our films. This behaviour differs from that of conventional s-wave superconductors. We measure the ab-plane microwave surface impedance Z~ = R~ + iwtt0)~L of patterned thin films of YBa2CuaO7 (YBCO) using the coplanar resonator technique, which we have described in detail elsewhere 1. This technique has the advantage of requiring only single sided film deposition. The films are deposited by coevaporation on polished (001)-oriented single crystal MgO substrates with a thin buffer layer of in situ deposited MgO. The films are then patterned using a combination of argon ion b e a m milling and EDTA wet etching. After subsequent oxygen annealing the films have.. T~ between 88-91K, with J~(77K) in excess of 2 x 106Acm -2. Our standard linear coplanar line has a central line width of 200pro, with gaps either side of 73tim; a line length of 8 m m results in a fundamental resonant frequency of around 8GHz. We patterned two parallel resonators on the same film, one with the standard gap width of 7 3 p m and another with a gap width of 12pm. Figure 1 shows the fractional resonant frequency shift (relative to 12K) for a pair of resonators patterned on a 350nm thick film, and a pair patterned on a 700nm thick film. We find that the microwave response is independent of microwave fields approaching Hcl, implying that the edge material is unaffected by the patterning. We selfconsistently calculate the current distribution of the resonator cross section which allows us to calculate the resonant frequency as a function of )~L;
from this we estimate )~L(T) absolutely I for each of the two films of figure 1. The largest uncertainty is due to inaccuracies in measuring the gap widths, giving a +10% error in the absolute values of IL; however, relative values are in error by around -t-1%. We plot the d a t a for the supercurrent conductivity cr2(T) (x 1 / ) ~ ( T ) in figure 2. We note that or2 lies well below t h a t predicted by the clean local limit of the BCS theory, particularly at low temperatures. Taking the d a t a for T < 30K, we find that AL(T) = AL(0)(1 + c~(T/Tc) 2) with a = 0.65-40.03. We find this same power law behaviour in all of our films, even though the values of AL(0) vary between 150-220nm, depending on film quality. This behaviour is similar to that observed by others for high-To thin films 2. The Rs data are more variable, and in figure 3 we plot Rs(T) for four different films of variable sample quality. Surprisingly, we find that AL and Rs are only weakly correlated (if the effects are intrinsic then Rs c( A~ at low T). The behaviour of R~ is likely to be extrinsic, although the apparent universal form for AL(T)/AL(0) is of interest. A contentious issue is whether high-To superconductors are s-wave paired. Microwave measurements of the real part of the conductivity cr1 have not yielded conclusive proof of a coherence peak 3 of the kind predicted by BCS theory just below To. This peak m a y be suppressed by the effects of strong coupling, but this would cause
(/921-4526/94/'$07.00 © 1994 - Elsevier Science B.V. All rights reserved
SSDI 0921-4526(93) 1383-W
1606
0.025
124
I
thin=648 thick=781
0.020 I--
1
~i'
0.015
F-
ture dependent electronic mean free path g(T) at low T will reduce cr2(T) by the additional factor A final possibility is that YBCO is a d-wave superconductor with line nodes in the energy gap on portions of the Fermi surface. This would give rise to ,~L ac T at low T, which can change to T 2 as a result of impurity scattering 4. A recent report s indicates AL oc T in high quality YBCO crystals at low T; this may represent the first observation of the intrinsic behaviour of AL in YBCO.
norrow,thin ~_~/lt norrow,thick ' ~ i
I 0.010 "% p~-
wide,thin
0.005 i
0.000
i
i
-
-
10 20
-
_
~
.
50
T
40
I
50
.
i
,
60
I
,
10000
I
70
80
90
Temperature/K Figure 1. The percentage frequency shift (defined as a positive quantity) for four resonators of different cross sectional geometries. The wide and narrow results refer to gap widths of 73/~m and 12#m, repectively; the thin and thick films have thicknesses of 350nm and 700am, respectively. 1.0
100
10
20 50 40 50
60 70 80 90
Temperature/K
1
Figure 3. Rs(T) plotted for four different films of the same thickness (350nm) at 8GHz. The best film has values of Rs at 15K and 77K of 23 + 5/~f~ • and 105 + 25tt~, respectively.
0.6 1 o~
C12
10
0.8
~1
lOO0
0.4
-J
170nm
0.2
" QN\.
\
60
70 80
),(0)[781]= 176nm 0.0
0
10 20 30
40 50
90
Temperature/K Figure 2. The normalised supercurrent conductivity o'2(T)/o'2(0) = )~(O)/)~(T) calculated from the data of figure 1. The BCS result is for the London limit.
~2(T)/0"2(0) t o rise above the weakly coupled BCS prediction (we find it lies below the BCS prediction). Another possibility is that the coherence peak may be removed by inelastic scattering, and within the BCS framework a highly tempera-
1. A. Porch et al., to be published in IEEE Trans. Appl. Supercond. (1993); A. Porch, M. J. Lancaster and R. G. Humphreys, submitted to IEEE Trans. Microwave Theory and Tech. (1993). 2. S. M. Anlage and D.-H. Wu, J. Supercond. 5 (1992) 395; J. M. Pond et al., Appl. Phys. Lett. 59 (1991) 303 3. For example, J. R. Waldram, A. Porch, H. M. Cheah and S.-F. Lee, submitted to this conference 4. J. Annett, N. Goldenfeld and S. R. Renn, Phys. Rev. B 43 (1991) 2778 5. W. N. Hardy, D. A. Bonn, D. C. Morgan, R. Liang and K. Zhang, preprint (1993)