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Critical current density and upper critical field for epitaxial Y2Ba4Cu8O16 films
Solid State Communicatione, Printad in Great Britain.
Vol.
73,
No.
5,
pp.
337-340,
1990.
0038-1098/90$3.00+.00 Pergamon Preee
CRITICAL CURRENT DENSITY AND UPPER CRITICAL FIELD FOR EPITAKIAL Y,Ba,Cu,O,, R. C.
Budhani,
M. W. Ruckman, R. L. Sabatini, H. Suenaga Division of Materials Science Brookhaven National Laboratory Upton, NY 11973 (Date
Received
17 October,
1989
by J.
end D. 0.
plc
FILMS
Welch
Taut)
Temperature and field dependence of the critical current density (J,.) and upper critical field (&a) in epitaxial Y,Ba,Cu,O,, films are reported. J, (T,H) data are in excellent agreement with a model of thermally activated flux motion in type-II superconductors. The pinning potential varies from 283 to 125 meV for eero to 40 KG fields and the pre-exponential frequency is in the GHe range. A critical comparison of indicates that the flux line lattice is our data with reports on YBa,Cu,O, more dynamic in Y,Ba,Cu,O,,. The enisotropy in the Ginsberg-Landau coherence length deduced from the elope of the Her phase boundary is comparable to that in YBe&u,O,.
YaBe,Cu,0,,(2-4-8) is a relatively new superconducting compound in the phase diagram of Cu, Ba, Y and 0. Thie structure, initially identified as a defect form of YBe,Cu,O, with a CuG double layer intercalation,i-2 has been synthesized as thin filme3-4 and recently in the bulk form.5 Electron microecopy/diffraction and x-ray diffraction analysis of the films identifies the structure as an orthorhombic unit cell with apace group Aemea. This new phase differs from YBaaCu,O, in that the single CuO chain parallel to b axis is replaced by a double Cu-0 chain to form an edge-sharing, square-planar network with oxygen atoms bonded to copper along both y and a axes. Unlike in YBaaCu,O,, where the oxygen atoms in the chain have two copper neighbors along b, the three-fold co-ordination of 0 in the double chains makes this phase more stable (i.e. lees likely to lose oxygen). Furthermore, the “a” and “b” lattice parameters of the 2-4-8 phase are the same, which make the The structure lees susceptible to twinning. normal-state transport properties end structure of the 2-4-8 films deposited on SrTiO, eubetratee have been etudied.3-4,6-7 However, no detailed studies of its superconducting properties have been reported. In spite of the lower T, (the zero resistance state is still above the boiling temperature of liquid nitrogen) its stability against the lose of oxygen, resistance to planar defect formation, higher normal-state conductivity end the larger separation between the superconducting planes, make the study of superconducting transport in the 2-4-8 phase very interesting. In this paper we report the first detailed measurements of the magnetic field dependence of the superconducting transport and critical current density in highly-oriented thin films of YaBa.Cu.0,. deposited on SrTiO,(lOO) eubetratee. The critical current density (Jc) of the films in zero field is >lOa A/cm’ et 77 K. Horever, the current density decreaeee sharply in maB-
netic fields directed parallel to the c-axis of the structure. The current density is lees sensitive to fields parallel to the a-b plane. The temperature and field dependence of J, has been analyzed in terms of thermally activated flux creep. The elopes of Her phase boundary, as determined by the low-current resistive transition at 1/2pN, where pN is the extra polated value of the normal state resietivity, is -1.7~10’ G/K and -0.27~10’ G/K for the parallel and perpendicular fields, respectively. Films, typically 1000-2000 h thick, were prepared by thermal coevaporation of Y, BaF,, and Cu onto (100) SrTiO, substrates. The chamber pressure during evaporation was 5x10-’ Torr and the rate of growth was 7 Aieec. The post-deposition annealing protocol included a temperature ramp of 500 C/hr to 815’C in flowing dry oxygen, 40 mine at 815’C in wet O,, followed by 20 mins at the same temperature in dry 0,. The sample was then cooled to room temperature at a rate of 100 C/hr in flowing 0,. For reeistivity measurements, the films were patterned in the form of a 300 pm x 1 sea bridge using a negative photoresist lithography process and dilute hydrochloric acid etch. After etching, the remaining photoresist was removed by annealing the films in flowing 0, at 6OO’C. This procedure burns off all carbonaceous materials from the surface of the films. An x-ray diffraction pattern of a 2000 A film is shown in Fig. 1. All major diffraction peaks in the pattern correspond to (001) and (hG0) reflections of Y,Ba,Cu,O,, and the SrTiO, substrate respectively. A weak line at 28-38.75 degree corresponds to (005) reflection of YBa,Cu,O,. A comparison of the inteneities of (005) and (0012) with results of Ref. (3) and (a), SuBBeet that the l-2-3 phase is Present as an impurity with en absolute concentration of lees than 5 percent. The resistivity of a 2000 A film at a 10 pA excitation current is shown in Fig. 2. Experi337
Vol. 73, No. 5
UPPER CRITICAL FIELD FOR EPITAKIAL Y2Ba4Cus0,6 FILMS
lh 1.
Thrta (Degree)
X-ray diffraction pattern of a 2000 h thick Y,Ba,Cu,O,, film deposited on (100) SrTiO, substrate. Inset of the figure shows unit cell of the 2-4-g phase, oxygen atoma have been omitted for clarity.
55
60
TEMPERATURE ( K ) 65 70 75 go -
85
go .
1
orthogonal to the field in both cases. The inset in Fig. 2(a) shows the zero field resistivity over a 4.2 to 300 K range. The room temperature resistivity of the material is 94 @cm, and its temperature dependence shows a negative zero-temperature intercept. The onset of superconductivity is at 62.5 K, and the zero resistance state occurs at 79.0 K. These observations are consistent with revious transport measurements on 2-4-8 films.s 87 The magnetic field dependence of the resistive transition shows significant broadening with increasing field. For a 40 KG field, this effect is larger by a factor of -3 for the perpendicular orientation. Like earlier observations on other cuprate superconductors, the magnetic field does not change the onset of the transition, significantly. The variation of Ii,,for the two field orientations is shown in Fig. 3. The temperature in the figure corresponds to the value at which the resietivity has dropped by 50 percent of its normal state value.The normal state behavior was obtained by extrapolating the resiativity data above 90 K to lower temperatures. Her varies linearly with temperature up to the maximum field used in this study yielding dHca/dT - -0.27~10' G/K. For parallel fields, if we assume a linear behavior between the data points, the resulting slope is --1.7x10* G/K. Thus, the anisotropy between the two field directions is -6. A WerthamerHelfand-Hohenberg-typeextrapolation8 to lower temperature yields 160 and 960 KG as Herl(O) and H,,11(0)respectively [since the breadth of the resistive transition in high T, materials is enhanced by both critical fluctuations and flux creep, the values of Hca deduced here may be lower than that obtained by magnetization measurements]. It is worthwhile to make a comparison of the observed anisotropy with data on single crystal and thin films of YBa,Cu,O,
I
75
2.
80 05 TEMPERATURE ( K )
78 ’
Temperature (K) 79
80
I
I
90
Temperature dependence of electrical resistivity; (a) field parallel to "c" axis, and (b) field on the a-b plane. Inset of (a) shows zero field transition over O-300 K temperature range.
mental resolution in these measurements ie -lo-* @cm. The upper part of the figure shows data taken with the field directed perpendicular to the plane of the film (perpendicular to the "a-b" plane of the lattice), while data for the field parallel are shown in the lower part of the figure. The transport current was
Temperature (K) 3.
Phase boundary of the upper critical field (H,,) for the parallel and perpendicular fields.
I
Vol. 73, No. 5
UPPER CRITICAL FIELD FOR EPITAKIAL Y~B~~CU~O,~ FILMS
deduced from realstlvlty measurements. The anlaotro y ranges from 3 to 6 for single crystals1 where as re orts on oriented films vary from 1.8 to 4.4.po111 The s-b plane and c axis coherence lengths can be estimated using the anlsotroplc Ginsberg-Lsndsu relstlone:
Eab - (oo/2xHc,l)~f' and EC = (so/2xHc#"
with these expressions, our data yields: Esb(0) - 44.0 A and E,(O) - 7.3 A. Results of critical current density messurementa carried out by sweeping the tempersture at a given field are shown in Fig. 4. (The criteria for the value of J, was 1 uV/asn.) The zero-field current density rises by five orders of magnitude over s temperature lntervsl The J, msxlmum in the figure corof -2 K. responds to 100 mA current through a 300 pm wide bridge. However, J, drops sharply with the application of the field. We explain the J, data of Fig. 4 in te'rmeof thermally actlvsted flux creep. The drift velocity of a flux bundle orthogonal to the transport current J can be written a*:12*13
lO*2 40
TEMPERATURE ( K ) 60 I. 70 00
-I- 50
90
101 IO4 103 102 101 loo 106
) (b) L
'- 66
68
70
72
74
76
70
I
00
TEMPERATURE ( K ) 4.
Temperature dependence of critical current density (J,) for perpendicular and parallel field orientation [Figs. (a) and (b) respectively). Solid lines in the figure are fit to Eq. (3). The parameters used for fitting are listed in the Table.
"d - dQ exp[--& B
(U,t BJVs )]
where B is the magnetic induction; V, the volume of the flux bundle over which the Lorentz force acts; U,, the pinning potential; a, the width of the potential well; G the attempt frequency of the hopping process and d the hopping dlstance. [In this analysis we ignore non-linear effects in the dependence of activation energy on current denslty.14] The electric field generated due to flux motion la:
E -
2Gd exp(-" -k--~) slnh (E) B B
Defining EC as the minimum critical electric field chosen for the msssurement of critical current, we hsvez
Kit J = slnh" BVs
[k
exe(
(3)
In order to analyze our Jc data to determine U,(O), we etsrt with the moat general form Uo(t)gHa(t)En(t). The temperature dependence of H, and E in the parabolic approximation yields Uo(t)a(l-ta)a[l+ta/l-ta]n~~. Here, n-0,1,2, or 3 depending on the relevant length scale for the energy of the flux lines. Based on arguments concerning correlated flux motion in high K materials, Yeshurun and Halozemoffl4 and subsequently Tlnkham15 have proposed that K(B)g(t) where K(B) varies approximately UO as l/B and g(t) - (l-ta)(l-t*)l/r. Here U, has been assumed to be independent of J which may not be true at high transport currents due to distortion of the potential well.16 Furthermore, Yeshurun and Malozemoff14 have argued that for fields greater than 0.2 Hcl, the correlation length perpendicular to field is equal to the flux line lattice spacing. In that case, U, depends on E only along the field direction, leading to n-1. This assumption also predicts l/B dependence for U,(O). Hagan and Grlesaen,l' on the other hand, have taken n=2. We have fitted the J, data to Eq. (3) with the temperature dependent U, for dlfferent values of n. The best fit to the data is obtained if, apart from assuming a temperature and field dependent U,, an explicit temperature dependence is given to the activation volume V appearing in the praexponential term of Eq. (3). The temperature dependence of U, and V used for the best fits are, (l+ta)r/r(l-t')l/r and (l+t')/(l-tr) respectively,which correspond to U,(t)aHE(t)E'(t) and V(t)aE*(t). This form of U,(t) does not predict an explicit dependence on the field. It should be noted that for measurements at zero field, the value of magnetic induction used in Eq. (3) is 0.5 Gauss, which roughly corresponds to the stray field. The parameters used to obtain the best fits are listed in the Table. Assuming that the distance of thermally activated flux motion is equal to
UPPER CRITICAL FIELD FOR EPITAXIAL Y2Ba4Cu80,6 FILMS
340
Table I. Fitting Parameters used in Fig. 4(a) and (b).
(iG)
Uo (meV)
(Sear)
B.V.o (G.cm4)
40-L
sol
125
1.6~10'
3.05x10-"
144
1.4x10'
2.2x10-r'
201 101
156
1.2x10'
1.9x10_"
207
8.3x10'
1.8x10-"
51
234
5.9x10'
1.6x10-"
5x10-4
237
5.9x10.
2.7x10-=*
4011
272
1.6~10'
3x10"'
2011
283
1.2x10'
2.3x10-r'
the nearest neighbor separation in a triangular flux line lattice, [1.075(9,/B)'/'],the attempt frequency ranges from 5.6x10' to 1.6x10' as the field increases from B x 0 to 40 kG. The pinning potential Uo(0) varies from 237 meV to 125 meV with the increasing field and doea not follow a l/B dependence. Recent measurements of the thermally activated resiativity of YBa,Cu,O,18-20 have shown a wide spectrum of activation energies. We have also measured the current-dependent,thermallyactivated, resistivty of Y,Ba,Cu,Olr films. The activation energy, U,(O), deduced from these measurements is dependent on the choice of g(t). With the present form of g(t), the pinning potential calculated from the current-dependent
vol. 73, No. 5
thermally-activated resistivity are in excellent agreement vith data presented here. Details of these meesurements will be published elsewhere. It is instructive to compare the pinning potential for the Y,Ba,Cu,O,, phase vith the available data on YBarCu.0,. Results of Zeldov et al.,2o with the same form of g(t) as used here and of Hettinger et al.19 using g(t) (l-ta)r(l-t)l/r [aa8uming F.(t)a(l-t)''1 shows that the pinning potential in l-2-3 is higher by a factor of 3-4 as compared to its value for 2-4-8. A plausible explanation for the ease of flux motion in 2-4-8 films is the increased separation betveen the superconducting Cu-0 planes which enhances the 2D neture of superconductivity in this material. In conclusion, we have measured the field and temperature dependence of electrical resiativity and critical current density in highly oriented thin films of Y,Ba,Cu.O,,. At zero field, the films nhow superconducting onset at 82.5 K and J, > 10' A/cm' at 77 K. In magnetic fields, the transition shows the characteristic broadening observed in other cuprate superconductors. For parallel and perpendicular orientations, the anisotropy in the critical field is 6 and the in-plane and c axis coherence length 44.8 h and 7.3 A respectively. Analysis of the transport critical current shows dissipation occurring due to thermally activated flux creep. Acknowledgment - This research was performed under the auspices of the U.S. Department of Energy, Division of Materials Sciences, Office of Basic Energy Sciences under Contract No. DEAC02-76CH00016.
References 1. l-l. W. Zandbergen, R. Gronsky, K. Wang, and G. Thomas, Nature 331, 596 (1988). 2. J. Narayan, V. N. Shukla, S. J. Lukaviewicx, N. Biunno, R. Singh, A. F. 11. Schreiner, and S. J. Pennycook, Appl. Phys. Lett. 5l, 940 (1987). 3. A. F. Marshall, R. W. Barton, K. Char, A. Kapitulnick, B. Oh, R. H. Harmaond,and S. S. Ladermann, Phys. Rev. B 21, 9353 (1988). 4. P. Marsch, R. M. Fleurning,M. L. Mandich, 12. A. N. Deaantolo, J. Kwo, M. Hong, and L. J. Martinez-Miranda, Nature z, 141 13. (1988). 5. R. J. Cava, J. J. Krajewski, W. F. Peck, Jr., B. Batlogg, L. W. Rupp, Jr., R. M. 14. Flemming, A. C. W. P. James, and P. Marsh, Nature 33, 328 (1989). 15. 6. M. L. Mondich, A. M. DeSantolo, R. H. Flemming, P. Marsh, S. Nakahara, S. 16. Sunshine, J. Kwo, M. Hong, T. Boone, T. Y. Komehani, and L. Martinez-Miranda, Phys. 17. Rev. B 38, 5031 (1988). 7. K. Char, Mark Lee, R. W. Barton, A. F. 18. Marshall, I. Boxovic, R. H. Hammond, Y. R. Beaaley, T. H. Geballe, A. Kapitulnik, and S. S. Laderman, Phys. Rev. B 2, 834 19. (1988). 8. N. R. Werthamer, B. Helfand, and P. C. Hohenberg, Phya. Rev. 147, 295 (1966). 9. J. S. Moodera, R. Meaurvy, J. E. Tkaczyk, 20. C. X. Hao, G. A. Gibson, and P. H. Tedron, Phya. Rev. B 21, 619 (1988). 10. P. Chaudhari, R. T. Collins, P. Freitas, R. J. Gambino, J. R. Kirkley, R. H. Koch,
R. B. Laibowitr.,F. K. LeGones, T. R. McGuire, T. Penny, 2. Schlesinger, A. P. Segmellar, S. loner, and E. J. Mcnniff, Jr., Phys. Rev. B 36, 8903 (1988). A. Kaputulnik, B. Oh, M. Naito, K. Char, A. D. Kent, M. Nieaaert, E. Hellman, S. Arnason, J. W. P. Hau, M. R. Hahn, P. Rosenthal, R. Barton, M. R. Beasley, T. H. Geballe, and R. H. Haxuaond,in Proc. of High Temperature Superconductors,World Scientific, Singapore, 1987. P. W. Anderson, Phys. Rev. Lett. 2, 309 (1962). A. M. Campbell and J. E. Evetts, Adv. Phys: 2l, 199 (1972). Y. Yeshurum and A. P. Malozemoff, Phys. Rev. Lett. 60, 2202 (1988). M. Tinkham, Phys. Rev. Lett. 6l, 1658 (1988). Y. Xu, M. Suenaga, A. R. Moodenbaugh, and D. 0. Welch, Phys. Rev. B, in press. C. W. Hagen and R. Griessen, Phys. Rev. Lett. 62, 2857 (1989). T. T. M. Palatra, B. Batlogg, R. B. van Dover, L. F. Schneemeyer, and J. V. Waszczak, Appl. Phys. Lett. 54, 763 (1989). J. D. Hettinger, A. G. Swanson, W. J. Mankiewich, R. E. Howard, B. L. Straughn, and E. G. Burkhardt, Phys. Rev. Lett. g, 2044 (1989). E. Zeldov, N. M. Amer, G. Koren, A. Gupta, R. J. Gambino, and M. W. McElfresh, Phys. Rev. Lett. 62, 3093 (1989).
Vol.
73,
No.
5,
pp.
337-340,
1990.
0038-1098/90$3.00+.00 Pergamon Preee
CRITICAL CURRENT DENSITY AND UPPER CRITICAL FIELD FOR EPITAKIAL Y,Ba,Cu,O,, R. C.
Budhani,
M. W. Ruckman, R. L. Sabatini, H. Suenaga Division of Materials Science Brookhaven National Laboratory Upton, NY 11973 (Date
Received
17 October,
1989
by J.
end D. 0.
plc
FILMS
Welch
Taut)
Temperature and field dependence of the critical current density (J,.) and upper critical field (&a) in epitaxial Y,Ba,Cu,O,, films are reported. J, (T,H) data are in excellent agreement with a model of thermally activated flux motion in type-II superconductors. The pinning potential varies from 283 to 125 meV for eero to 40 KG fields and the pre-exponential frequency is in the GHe range. A critical comparison of indicates that the flux line lattice is our data with reports on YBa,Cu,O, more dynamic in Y,Ba,Cu,O,,. The enisotropy in the Ginsberg-Landau coherence length deduced from the elope of the Her phase boundary is comparable to that in YBe&u,O,.
YaBe,Cu,0,,(2-4-8) is a relatively new superconducting compound in the phase diagram of Cu, Ba, Y and 0. Thie structure, initially identified as a defect form of YBe,Cu,O, with a CuG double layer intercalation,i-2 has been synthesized as thin filme3-4 and recently in the bulk form.5 Electron microecopy/diffraction and x-ray diffraction analysis of the films identifies the structure as an orthorhombic unit cell with apace group Aemea. This new phase differs from YBaaCu,O, in that the single CuO chain parallel to b axis is replaced by a double Cu-0 chain to form an edge-sharing, square-planar network with oxygen atoms bonded to copper along both y and a axes. Unlike in YBaaCu,O,, where the oxygen atoms in the chain have two copper neighbors along b, the three-fold co-ordination of 0 in the double chains makes this phase more stable (i.e. lees likely to lose oxygen). Furthermore, the “a” and “b” lattice parameters of the 2-4-8 phase are the same, which make the The structure lees susceptible to twinning. normal-state transport properties end structure of the 2-4-8 films deposited on SrTiO, eubetratee have been etudied.3-4,6-7 However, no detailed studies of its superconducting properties have been reported. In spite of the lower T, (the zero resistance state is still above the boiling temperature of liquid nitrogen) its stability against the lose of oxygen, resistance to planar defect formation, higher normal-state conductivity end the larger separation between the superconducting planes, make the study of superconducting transport in the 2-4-8 phase very interesting. In this paper we report the first detailed measurements of the magnetic field dependence of the superconducting transport and critical current density in highly-oriented thin films of YaBa.Cu.0,. deposited on SrTiO,(lOO) eubetratee. The critical current density (Jc) of the films in zero field is >lOa A/cm’ et 77 K. Horever, the current density decreaeee sharply in maB-
netic fields directed parallel to the c-axis of the structure. The current density is lees sensitive to fields parallel to the a-b plane. The temperature and field dependence of J, has been analyzed in terms of thermally activated flux creep. The elopes of Her phase boundary, as determined by the low-current resistive transition at 1/2pN, where pN is the extra polated value of the normal state resietivity, is -1.7~10’ G/K and -0.27~10’ G/K for the parallel and perpendicular fields, respectively. Films, typically 1000-2000 h thick, were prepared by thermal coevaporation of Y, BaF,, and Cu onto (100) SrTiO, substrates. The chamber pressure during evaporation was 5x10-’ Torr and the rate of growth was 7 Aieec. The post-deposition annealing protocol included a temperature ramp of 500 C/hr to 815’C in flowing dry oxygen, 40 mine at 815’C in wet O,, followed by 20 mins at the same temperature in dry 0,. The sample was then cooled to room temperature at a rate of 100 C/hr in flowing 0,. For reeistivity measurements, the films were patterned in the form of a 300 pm x 1 sea bridge using a negative photoresist lithography process and dilute hydrochloric acid etch. After etching, the remaining photoresist was removed by annealing the films in flowing 0, at 6OO’C. This procedure burns off all carbonaceous materials from the surface of the films. An x-ray diffraction pattern of a 2000 A film is shown in Fig. 1. All major diffraction peaks in the pattern correspond to (001) and (hG0) reflections of Y,Ba,Cu,O,, and the SrTiO, substrate respectively. A weak line at 28-38.75 degree corresponds to (005) reflection of YBa,Cu,O,. A comparison of the inteneities of (005) and (0012) with results of Ref. (3) and (a), SuBBeet that the l-2-3 phase is Present as an impurity with en absolute concentration of lees than 5 percent. The resistivity of a 2000 A film at a 10 pA excitation current is shown in Fig. 2. Experi337
Vol. 73, No. 5
UPPER CRITICAL FIELD FOR EPITAKIAL Y2Ba4Cus0,6 FILMS
lh 1.
Thrta (Degree)
X-ray diffraction pattern of a 2000 h thick Y,Ba,Cu,O,, film deposited on (100) SrTiO, substrate. Inset of the figure shows unit cell of the 2-4-g phase, oxygen atoma have been omitted for clarity.
55
60
TEMPERATURE ( K ) 65 70 75 go -
85
go .
1
orthogonal to the field in both cases. The inset in Fig. 2(a) shows the zero field resistivity over a 4.2 to 300 K range. The room temperature resistivity of the material is 94 @cm, and its temperature dependence shows a negative zero-temperature intercept. The onset of superconductivity is at 62.5 K, and the zero resistance state occurs at 79.0 K. These observations are consistent with revious transport measurements on 2-4-8 films.s 87 The magnetic field dependence of the resistive transition shows significant broadening with increasing field. For a 40 KG field, this effect is larger by a factor of -3 for the perpendicular orientation. Like earlier observations on other cuprate superconductors, the magnetic field does not change the onset of the transition, significantly. The variation of Ii,,for the two field orientations is shown in Fig. 3. The temperature in the figure corresponds to the value at which the resietivity has dropped by 50 percent of its normal state value.The normal state behavior was obtained by extrapolating the resiativity data above 90 K to lower temperatures. Her varies linearly with temperature up to the maximum field used in this study yielding dHca/dT - -0.27~10' G/K. For parallel fields, if we assume a linear behavior between the data points, the resulting slope is --1.7x10* G/K. Thus, the anisotropy between the two field directions is -6. A WerthamerHelfand-Hohenberg-typeextrapolation8 to lower temperature yields 160 and 960 KG as Herl(O) and H,,11(0)respectively [since the breadth of the resistive transition in high T, materials is enhanced by both critical fluctuations and flux creep, the values of Hca deduced here may be lower than that obtained by magnetization measurements]. It is worthwhile to make a comparison of the observed anisotropy with data on single crystal and thin films of YBa,Cu,O,
I
75
2.
80 05 TEMPERATURE ( K )
78 ’
Temperature (K) 79
80
I
I
90
Temperature dependence of electrical resistivity; (a) field parallel to "c" axis, and (b) field on the a-b plane. Inset of (a) shows zero field transition over O-300 K temperature range.
mental resolution in these measurements ie -lo-* @cm. The upper part of the figure shows data taken with the field directed perpendicular to the plane of the film (perpendicular to the "a-b" plane of the lattice), while data for the field parallel are shown in the lower part of the figure. The transport current was
Temperature (K) 3.
Phase boundary of the upper critical field (H,,) for the parallel and perpendicular fields.
I
Vol. 73, No. 5
UPPER CRITICAL FIELD FOR EPITAKIAL Y~B~~CU~O,~ FILMS
deduced from realstlvlty measurements. The anlaotro y ranges from 3 to 6 for single crystals1 where as re orts on oriented films vary from 1.8 to 4.4.po111 The s-b plane and c axis coherence lengths can be estimated using the anlsotroplc Ginsberg-Lsndsu relstlone:
Eab - (oo/2xHc,l)~f' and EC = (so/2xHc#"
with these expressions, our data yields: Esb(0) - 44.0 A and E,(O) - 7.3 A. Results of critical current density messurementa carried out by sweeping the tempersture at a given field are shown in Fig. 4. (The criteria for the value of J, was 1 uV/asn.) The zero-field current density rises by five orders of magnitude over s temperature lntervsl The J, msxlmum in the figure corof -2 K. responds to 100 mA current through a 300 pm wide bridge. However, J, drops sharply with the application of the field. We explain the J, data of Fig. 4 in te'rmeof thermally actlvsted flux creep. The drift velocity of a flux bundle orthogonal to the transport current J can be written a*:12*13
lO*2 40
TEMPERATURE ( K ) 60 I. 70 00
-I- 50
90
101 IO4 103 102 101 loo 106
) (b) L
'- 66
68
70
72
74
76
70
I
00
TEMPERATURE ( K ) 4.
Temperature dependence of critical current density (J,) for perpendicular and parallel field orientation [Figs. (a) and (b) respectively). Solid lines in the figure are fit to Eq. (3). The parameters used for fitting are listed in the Table.
"d - dQ exp[--& B
(U,t BJVs )]
where B is the magnetic induction; V, the volume of the flux bundle over which the Lorentz force acts; U,, the pinning potential; a, the width of the potential well; G the attempt frequency of the hopping process and d the hopping dlstance. [In this analysis we ignore non-linear effects in the dependence of activation energy on current denslty.14] The electric field generated due to flux motion la:
E -
2Gd exp(-" -k--~) slnh (E) B B
Defining EC as the minimum critical electric field chosen for the msssurement of critical current, we hsvez
Kit J = slnh" BVs
[k
exe(
(3)
In order to analyze our Jc data to determine U,(O), we etsrt with the moat general form Uo(t)gHa(t)En(t). The temperature dependence of H, and E in the parabolic approximation yields Uo(t)a(l-ta)a[l+ta/l-ta]n~~. Here, n-0,1,2, or 3 depending on the relevant length scale for the energy of the flux lines. Based on arguments concerning correlated flux motion in high K materials, Yeshurun and Halozemoffl4 and subsequently Tlnkham15 have proposed that K(B)g(t) where K(B) varies approximately UO as l/B and g(t) - (l-ta)(l-t*)l/r. Here U, has been assumed to be independent of J which may not be true at high transport currents due to distortion of the potential well.16 Furthermore, Yeshurun and Malozemoff14 have argued that for fields greater than 0.2 Hcl, the correlation length perpendicular to field is equal to the flux line lattice spacing. In that case, U, depends on E only along the field direction, leading to n-1. This assumption also predicts l/B dependence for U,(O). Hagan and Grlesaen,l' on the other hand, have taken n=2. We have fitted the J, data to Eq. (3) with the temperature dependent U, for dlfferent values of n. The best fit to the data is obtained if, apart from assuming a temperature and field dependent U,, an explicit temperature dependence is given to the activation volume V appearing in the praexponential term of Eq. (3). The temperature dependence of U, and V used for the best fits are, (l+ta)r/r(l-t')l/r and (l+t')/(l-tr) respectively,which correspond to U,(t)aHE(t)E'(t) and V(t)aE*(t). This form of U,(t) does not predict an explicit dependence on the field. It should be noted that for measurements at zero field, the value of magnetic induction used in Eq. (3) is 0.5 Gauss, which roughly corresponds to the stray field. The parameters used to obtain the best fits are listed in the Table. Assuming that the distance of thermally activated flux motion is equal to
UPPER CRITICAL FIELD FOR EPITAXIAL Y2Ba4Cu80,6 FILMS
340
Table I. Fitting Parameters used in Fig. 4(a) and (b).
(iG)
Uo (meV)
(Sear)
B.V.o (G.cm4)
40-L
sol
125
1.6~10'
3.05x10-"
144
1.4x10'
2.2x10-r'
201 101
156
1.2x10'
1.9x10_"
207
8.3x10'
1.8x10-"
51
234
5.9x10'
1.6x10-"
5x10-4
237
5.9x10.
2.7x10-=*
4011
272
1.6~10'
3x10"'
2011
283
1.2x10'
2.3x10-r'
the nearest neighbor separation in a triangular flux line lattice, [1.075(9,/B)'/'],the attempt frequency ranges from 5.6x10' to 1.6x10' as the field increases from B x 0 to 40 kG. The pinning potential Uo(0) varies from 237 meV to 125 meV with the increasing field and doea not follow a l/B dependence. Recent measurements of the thermally activated resiativity of YBa,Cu,O,18-20 have shown a wide spectrum of activation energies. We have also measured the current-dependent,thermallyactivated, resistivty of Y,Ba,Cu,Olr films. The activation energy, U,(O), deduced from these measurements is dependent on the choice of g(t). With the present form of g(t), the pinning potential calculated from the current-dependent
vol. 73, No. 5
thermally-activated resistivity are in excellent agreement vith data presented here. Details of these meesurements will be published elsewhere. It is instructive to compare the pinning potential for the Y,Ba,Cu,O,, phase vith the available data on YBarCu.0,. Results of Zeldov et al.,2o with the same form of g(t) as used here and of Hettinger et al.19 using g(t) (l-ta)r(l-t)l/r [aa8uming F.(t)a(l-t)''1 shows that the pinning potential in l-2-3 is higher by a factor of 3-4 as compared to its value for 2-4-8. A plausible explanation for the ease of flux motion in 2-4-8 films is the increased separation betveen the superconducting Cu-0 planes which enhances the 2D neture of superconductivity in this material. In conclusion, we have measured the field and temperature dependence of electrical resiativity and critical current density in highly oriented thin films of Y,Ba,Cu.O,,. At zero field, the films nhow superconducting onset at 82.5 K and J, > 10' A/cm' at 77 K. In magnetic fields, the transition shows the characteristic broadening observed in other cuprate superconductors. For parallel and perpendicular orientations, the anisotropy in the critical field is 6 and the in-plane and c axis coherence length 44.8 h and 7.3 A respectively. Analysis of the transport critical current shows dissipation occurring due to thermally activated flux creep. Acknowledgment - This research was performed under the auspices of the U.S. Department of Energy, Division of Materials Sciences, Office of Basic Energy Sciences under Contract No. DEAC02-76CH00016.
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