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The effect of variable microwave power on the low-field absorption in high-Tc superconducting cuprates
Physica C 167 (1990) North-Holland
520-524
THE EFFECT OF VARIABLE MICROWAVE POWER ON THE LOW-FIELD HIGH-Z’, SUPERCONDUCTING CUPRATES
ABSORPTION
IN
,R. JANES, R.S. LIU and P.P. EDWARDS IRC in Superconductivity, University of Cambridge, West Cambridge Site, Madingley Road, Cambridge, CB3 OHE, UK
J.L. TALLON Physics and Engineering Laboratory, D.S.I.R.. PO Box 31313. Lower Hutt, New Zealand Received
26 February
1990
The well established low-field microwave absorption below T, has been investigated using the ESR technique at 9 GHz, as a function of incident microwave power, in a range of superconducting cuprates, namely Er2Ba4Cu70,5_-x, T12Ba2CeCu20, and YBa&&O,_, In all cases, the signal was “saturated” above a critical microwave power level; we observed that this effect was sample-dependent.
1. Introduction It is now well established that the transition to the superconducting state in high-T, copper oxide based materials is characterized by the observation of an intense microwave absorption at low magnetic fields [ 11. Rettori et al. [ 21 interpreted this effect as arising from changes in the diamagnetic susceptibility, on moving from the Meissner state into the mixed state. This response has been studied by a large number of groups, and an extensive literature has accumulated [ 3,4] showing amongst other things that ESR is a useful diagnostic tool for screening superconducting samples. However, a controversy still exists as to the precise nature of the signal. For example, one explanation invokes the circulation of Josephson currents (induced by the oscillating microwave field impinging on the sample) in these granular materials [ 51, the shape of the absorption being a reflection of the distribution of domain sizes. Furthermore, closely spaced fluctuations, which appear superimposed on the broad absorption, have been assigned to Josephson oscillations [ 61. A number of recent measurements on single crystals of YBaZCu307_-x have revealed an anisotropic multiple line structure which is sensitive to both microwave 0921-4534/90/$03.50 (North-Holland )
0 Elsevier Science Publishers
B.V.
power and sample temperature [ 7,8] ; this has been attributed to flux quantisation effects. Signals of this nature have also been reported in pelletised samples, and indeed we have detected reproducible substructure in the ESR response, up to high field values, ca. 6000 G at 9.1 GHz [9]. The non-resonant nature of the signal was demonstrated by Lin and co workers [ lo] who obtained essentially identical spectra at S-, X- and Q-band frequencies, both for the microwave field parallel and perpendicular to the static magnetic field. In this regard we have also studied the signal using an ESR spectrometer operating at a frequency of 300 MHz [ 111 in a range of samples, which again yielded the same signal as was detected using an X-band spectrometer for the same material. Interestingly, Sastry and co-workers [ 3,121 have also reported a field independent component, together with an absorptionlike signal close to T,, and in addition proposed that a resonant absorption also takes place, the latter becoming swamped by the predominant non-resonant signal as the sample temperature is lowered below T,. Furthermore, Siu [ 13 ] has recently discussed the low-field microwave absorption in high-T, superconductors as being a convolution of both non-resonant and spin-resonant components, the latter arising from copper d-electron triplet states.
R. Janes et al. / Low-jieldmicrowaveabsorptionin superconductingcuprates
In this work we report the observation of a lowfield microwave absorption in superconducting Er2Ba4Cu7015_-x, T12Ba2CeCu20, and YBa2Cu307_x, and demonstrate the effect of raising the incident microwave power thereon. In all cases saturation occurred at a sample dependent threshold power level.
521
(well below the T, of the materials) employing a quartz insert Dewar and liquid nitrogen. Measurements in the 85-293 K range were obtained using liquid nitrogen and a variable temperature accesory, the temperature being measured most accurately by placing a thermocouple inside the ESR tube just above the sample position.
2. Experimental 3. Results and discussion ErzBa&u,O,,_, was synthesised in oxygen in one atmosphere, as described previously [ 141. Briefly, Erz03, Ba(N03)2 and submicron powders of CuO were reacted in stoichiometric proportions at 860 OC in flowing oxygen. The precursors were mixed with 0.2 mol NaN03 per formula unit to enhance the reaction rate. During the first 24 h the reacted material was ground, milled and die-pressed into pellets, tinally being sintered for 3-4 days. All samples were rapidly air-quenched, followed by annealing in flowing oxygen at 300-350°C to produce the maximum T, for this material [ 15 1. Samples of TlzBazCeCuzO, were prepared from high purity BaC03, CeOz and CuO powders, which were initially calcined in air at 900-930°C. The precursors were then ground and mixed with the appropriate amount of T1203 to produce a nominal composition of T12Ba2CeCu20,. The mixture was then pressed into a cylindrical pellet and wrapped in gold foil to prevent loss of thallium during heating. The samples were then sintered at 880°C for 10 min in an oxygen atmosphere, followed by quenching in air. Samples of YBa2Cu307_-x were prepared by well documented routes. X-ray diffraction (XRD) studies were carried out with CuKa radiation using a Spectrolab CPS-120 diffractometer. A standard four-probe method was used for electrical resistance measurements. AC susceptibility was determined using the mutual inductive technique at a frequency of 823 Hz. Field-cooled magnetization (at 100 G) was studied using a SQUID magnetometer (Quantum Design). Microwave measurements were carried out using a conventional X-band ESR spectrometer (Bruker ER200D) employing either 12.5 or 100 KHz field modulation and a standard rectangular TEio2 mode cavity. Most measurements were carried out at 77 K
Most of the ESR/microwave absorption measurements were carried out at 77 K, at which temperature all the samples were superconducting, evidenced by the appearance of an intense magnetic field dependent microwave absorption beginning at lowfield. In all cases, we found that on raising the microwave power level incident on the sample, the lowfield signal initially increased in intensity and then saturated above a sample dependent threshold level. Significantly, the latter was always accompanied by a sudden degradation of the cavity Quality factor (detuning) as evidenced by a shift in the detector diode current. We appeared therefore, to be effectively driving the material back into the normal (i.e. metallic) state by increasing the intensity of the microwave field (Hi) at the sample position. “Lossy” or conducting samples appreciably perturb the field patterns in the resonator - resulting in large shifts in resonant frequency and reduction of Q. Hi is related to the cavity Q (loaded) by the following equation: 2(H1 ),a, =a
(Qf’o)“’ >
where cr is a constant, and PO is the incident power level. For a conventional ESR transition, this will determine its intensity (in the absence of saturation). Certainly, the term saturation in the accepted sense of a conventional ESR experiment, is something of a misnomer here, as we are clearly dealing with what is essentially a non-resonant response (i.e. there is no upper spin level to populate). These cavity perturbations have been studied in detail by Rettori et al. [ 2 1, who noted a dramatic enhancement of the Q on cooling a sample of YBa2Cu30,_, below T,; this was accompanied by a large increase in the cavity resonant frequency, an effect which forms the ba-
R. Janes et al. /Low-field microwave absorption in superconducting cuprates
522
sis of the “leakage-current” measurements of Sastry et al. [3,12]. Considering each system individually, fig. 1 shows the temperature dependence of the normalised resistance and AC susceptibility for the Er2Ba4Cu7015_-x sample. The superconducting transition temperatures were as follows: T, (onset)=100 K, T, (mid-point)=98 K and T, (zero) =92 K. The temperature dependence of the AC susceptibility is also shown, the diamagnetic signal indicating T, (onset) = 93 K, consistent with the resistivity measurements. The powder XRD pattern for this sample was indicative of nearly monophasic ERZBa4Cu,015_-x, this phase being responsible for the superconductivity [ 141. Figure 2 illustrates the low-field microwave absorption for this phase (measured at 77 K) which grew in at T,,and exhibited a saturation-type effect on decreasing the attenuation. The signal loss occurred between 30-40 mW at 77 K, a process which was completely reversible. We also studied this effect in the recently synthesized superconducting T1,Ba,CeCuzO, phase [ 16 1. Figure 3 shows the normalized resistance versus temperature curve for a sample of this material, showing T, (onset) = 100 K, T, (mid-point) = 87 K and T, (zero) = 82 K. The temperature dependence of the mass diamagnetic susceptibility in the field cooled ( 100 G) situation is also included. The Meissner signal shows T, (onset) to be around 85 K, consistent with the resistance measurement. The powder XRD pattern was indicative of a Ce-doped T1,BazCuOe phase, which is responsible for the su-
0.8 -
5
2 a’
0.6 -
t
0.4 -
2
r
0.2 0 0
IIIIIIII 20
40
60
80
100
120
140
Temperature(K) Fig. 1. The temperature dependence of the normalised resistance and the AC susceptibility for an Er2Ba4Cu7015_-x sample.
1
ZOOOG SOOG
I
I_
30mW
t 40mW
Fig. 2. Low-field microwave absorption (derivative display) for the Er2Ba4Cu7015_-x sample as a function of microwave power, sample temperature= 77 K, v= 9.1 GHz. The narrow signal at ca. 3200 G, is due to a DPPH (diphenyl-picryl-hydrazyl, g= 2.0036 ) marker, mixed with the sample.
perconducting behaviour [ 161. Figure 4 shows the saturation of the low-field signal in this sample, where a much lower power level was required to effect this response. Furthermore, the same detuning effect was observed and the signal returned to its original intensity on attenuation. The broad singlet at g= 2.150 300 G) is assigned to Cu2+ in impurity or (A&,contaminant phases on the basis of previous studies [ 17-191. The intensity of this signal increased on cooling consistent with a Curie-type susceptibility, this also occurred on cooling through T,,suggesting the impurity phase is located at the surface or grain boundaries. The intensity of the localised Cu*+ signal, and indeed the DPPH signal in rig. 2, was approximately the same at both power levels, indicat-
R. Janes et al. / Low-field microwave absorption in superconducting cuprates
\\ \\ ‘\\\ \ \ \ ‘\ \ \
0.2 -
0
I 0
1 20
1
I 40
I
1 60
1
-1 \ \ I SO
100
120
140
Temperature(K)
Fig. 3. The temperature dependence of the normalised resistance and the magnetisation for the T1,BazCeCu,O, sample.
5OOOG
\
523
single crystals. In this study individual components of the spectrum were observed to increase intensity at different rates, and high power levels caused linebroadening and eventual saturation. Conversely, Blazey et al. [ 5 ] have reported a completely new series of absorptions to grow in as the incident power was increased, the variance being attributed by Dultic et al. [ 7 ] to sample homogeneities. Certainly, the results of our investigations point towards the microwave response being sample dependent with the signal disappearing at different values of microwave magnetic field strength, for the different materials examined. As discussed earlier, one explanation for the origin of the absorption has been proposed to be microwave induced supercurrents [ 5 1, and we may speculate that as the microwave power is increased, we reach a level sufficient to exceed the critical current (J,) of the weak links in the material concerned. Indeed Dulcic et al. [ 71 have also discussed this possibility. Therefore, it is possible that the saturation effect reported here is a function of the ceramic microstructure, rather than being a phase dependent response, and if so, may prove to be a useful probe of microstructure and of the effects of microstructural engineering.
References
Fig. 4. Microwave absorption derivative spectrum for the TI,Ba2CeCu20, sample (T= 77 K, ~~9.1 GHz) showing the effect of varying the incident microwave power level.
ing the lossy nature of the material in the normal state does not significantly reduce spectrometer sensitivity. We extended our studies to the well-known YBa2Cu307_-x phase, and again the same signal-loss occurred as the incident microwave power level was raised. However, the sample studied required significantly higher power levels (circa 50-60 mW) to facilitate this response. The actual origin of the saturation effect reported here is somewhat unclear, and indeed Dulcic and coworkers [ 71 have recently reported an extensive study of the effect of microwave power on the lowfield multiline spectrum detected in YBa2Cu307_-x
[ 1 ] S.V. Bhat, P. Ganguly and C.N.R. Rao, Pramana 28 ( 1987) 425. [2 C. Rettori, D. Davidov, I. Belaish and I. Felner, Phys. Rev. B 36 (1987) 4028. 13 M.D. Sastry, Studies of High Temperature Superconductors chapt. 10 (Nova Science Publ., New York, 1989). H.A. Farach, Copper Oxide [4 C.P. Poole, T. Datta, Superconductors (Wiley-Interscience, 1988) p. 163. K.N. Shrivastava, Solid State Commun. 68 ( 1988) 259. t: 11J. Stankowski, P.K. Kahol, N.S. Dalal and J.S. Moodera, Phys. Rev. B 36 (1987) 7126. [7 ‘1K.W. Blazey, A.M. Portis, K.A. Mu&r and F.H. Holtzberg, Physica C 153 (1988) 56. 18 H. Vichery, F. Beuneu and P. Lejay, Physica C 159 ( 1989) 823. [9 A. Dulcic, R.H. Crepeau and J.H. Freed, Physica C 160 (1989) 223. [‘O ‘1R. Janes, R.S. Liu, P.P. Edwards, A.D. Stevens and M.C.R. Symons, Mater. Res. Bull., submitted. [I1 I T.S. Lin, L.C. Sobotka and W. Froncisz, Nature 333 ( 1988) 21.
524
R. Janes et al. /Low-field microwave absorption in superconducting cuprates
[ 121 M.D. Sastry, Y. Babu, R.M. Kadam, A.G. Idalvi, I.K.
Gopalakrishnan, J.V. Yakhmi and R.M. Iyer, Solid State Commun. 66 (1988) 1219. [ 131 G.G. Siu, Solid State Commun. 72 (1989) 291. [ 141 D.M. Pooke, R.G. Buckley, M.R. Presland and J.L. Tallon, Phys. Rev. B, in press. [ 15 ] J.L. Tallon, D.M. Pooke, R.G. Buckley, M.R. Presland and F.J. Blunt, Phys. Rev. B, in press.
[ 161 J.H. Wang, Z.Z. Sheng, C. Dong, X. Fei, L. Sheng, A.M. Hermann and Z.X. Zhao, Physica C 158 ( 1989) 507. [ 171 F.H. Mehran and P.W. Anderson, Solid State Commun. 7 1 (1989) 29. [ 181 W.R. McKinnon, J.R. Morton and G. Pleizer, Solid State Commun. 66 (1988) 1093. [ 191 R. Jones, R. Janes, A.R. Armstrong, N.C. Pyper, D.J. Keeble and P.P. Edwards, J. Chem. Sot., in press.
520-524
THE EFFECT OF VARIABLE MICROWAVE POWER ON THE LOW-FIELD HIGH-Z’, SUPERCONDUCTING CUPRATES
ABSORPTION
IN
,R. JANES, R.S. LIU and P.P. EDWARDS IRC in Superconductivity, University of Cambridge, West Cambridge Site, Madingley Road, Cambridge, CB3 OHE, UK
J.L. TALLON Physics and Engineering Laboratory, D.S.I.R.. PO Box 31313. Lower Hutt, New Zealand Received
26 February
1990
The well established low-field microwave absorption below T, has been investigated using the ESR technique at 9 GHz, as a function of incident microwave power, in a range of superconducting cuprates, namely Er2Ba4Cu70,5_-x, T12Ba2CeCu20, and YBa&&O,_, In all cases, the signal was “saturated” above a critical microwave power level; we observed that this effect was sample-dependent.
1. Introduction It is now well established that the transition to the superconducting state in high-T, copper oxide based materials is characterized by the observation of an intense microwave absorption at low magnetic fields [ 11. Rettori et al. [ 21 interpreted this effect as arising from changes in the diamagnetic susceptibility, on moving from the Meissner state into the mixed state. This response has been studied by a large number of groups, and an extensive literature has accumulated [ 3,4] showing amongst other things that ESR is a useful diagnostic tool for screening superconducting samples. However, a controversy still exists as to the precise nature of the signal. For example, one explanation invokes the circulation of Josephson currents (induced by the oscillating microwave field impinging on the sample) in these granular materials [ 51, the shape of the absorption being a reflection of the distribution of domain sizes. Furthermore, closely spaced fluctuations, which appear superimposed on the broad absorption, have been assigned to Josephson oscillations [ 61. A number of recent measurements on single crystals of YBaZCu307_-x have revealed an anisotropic multiple line structure which is sensitive to both microwave 0921-4534/90/$03.50 (North-Holland )
0 Elsevier Science Publishers
B.V.
power and sample temperature [ 7,8] ; this has been attributed to flux quantisation effects. Signals of this nature have also been reported in pelletised samples, and indeed we have detected reproducible substructure in the ESR response, up to high field values, ca. 6000 G at 9.1 GHz [9]. The non-resonant nature of the signal was demonstrated by Lin and co workers [ lo] who obtained essentially identical spectra at S-, X- and Q-band frequencies, both for the microwave field parallel and perpendicular to the static magnetic field. In this regard we have also studied the signal using an ESR spectrometer operating at a frequency of 300 MHz [ 111 in a range of samples, which again yielded the same signal as was detected using an X-band spectrometer for the same material. Interestingly, Sastry and co-workers [ 3,121 have also reported a field independent component, together with an absorptionlike signal close to T,, and in addition proposed that a resonant absorption also takes place, the latter becoming swamped by the predominant non-resonant signal as the sample temperature is lowered below T,. Furthermore, Siu [ 13 ] has recently discussed the low-field microwave absorption in high-T, superconductors as being a convolution of both non-resonant and spin-resonant components, the latter arising from copper d-electron triplet states.
R. Janes et al. / Low-jieldmicrowaveabsorptionin superconductingcuprates
In this work we report the observation of a lowfield microwave absorption in superconducting Er2Ba4Cu7015_-x, T12Ba2CeCu20, and YBa2Cu307_x, and demonstrate the effect of raising the incident microwave power thereon. In all cases saturation occurred at a sample dependent threshold power level.
521
(well below the T, of the materials) employing a quartz insert Dewar and liquid nitrogen. Measurements in the 85-293 K range were obtained using liquid nitrogen and a variable temperature accesory, the temperature being measured most accurately by placing a thermocouple inside the ESR tube just above the sample position.
2. Experimental 3. Results and discussion ErzBa&u,O,,_, was synthesised in oxygen in one atmosphere, as described previously [ 141. Briefly, Erz03, Ba(N03)2 and submicron powders of CuO were reacted in stoichiometric proportions at 860 OC in flowing oxygen. The precursors were mixed with 0.2 mol NaN03 per formula unit to enhance the reaction rate. During the first 24 h the reacted material was ground, milled and die-pressed into pellets, tinally being sintered for 3-4 days. All samples were rapidly air-quenched, followed by annealing in flowing oxygen at 300-350°C to produce the maximum T, for this material [ 15 1. Samples of TlzBazCeCuzO, were prepared from high purity BaC03, CeOz and CuO powders, which were initially calcined in air at 900-930°C. The precursors were then ground and mixed with the appropriate amount of T1203 to produce a nominal composition of T12Ba2CeCu20,. The mixture was then pressed into a cylindrical pellet and wrapped in gold foil to prevent loss of thallium during heating. The samples were then sintered at 880°C for 10 min in an oxygen atmosphere, followed by quenching in air. Samples of YBa2Cu307_-x were prepared by well documented routes. X-ray diffraction (XRD) studies were carried out with CuKa radiation using a Spectrolab CPS-120 diffractometer. A standard four-probe method was used for electrical resistance measurements. AC susceptibility was determined using the mutual inductive technique at a frequency of 823 Hz. Field-cooled magnetization (at 100 G) was studied using a SQUID magnetometer (Quantum Design). Microwave measurements were carried out using a conventional X-band ESR spectrometer (Bruker ER200D) employing either 12.5 or 100 KHz field modulation and a standard rectangular TEio2 mode cavity. Most measurements were carried out at 77 K
Most of the ESR/microwave absorption measurements were carried out at 77 K, at which temperature all the samples were superconducting, evidenced by the appearance of an intense magnetic field dependent microwave absorption beginning at lowfield. In all cases, we found that on raising the microwave power level incident on the sample, the lowfield signal initially increased in intensity and then saturated above a sample dependent threshold level. Significantly, the latter was always accompanied by a sudden degradation of the cavity Quality factor (detuning) as evidenced by a shift in the detector diode current. We appeared therefore, to be effectively driving the material back into the normal (i.e. metallic) state by increasing the intensity of the microwave field (Hi) at the sample position. “Lossy” or conducting samples appreciably perturb the field patterns in the resonator - resulting in large shifts in resonant frequency and reduction of Q. Hi is related to the cavity Q (loaded) by the following equation: 2(H1 ),a, =a
(Qf’o)“’ >
where cr is a constant, and PO is the incident power level. For a conventional ESR transition, this will determine its intensity (in the absence of saturation). Certainly, the term saturation in the accepted sense of a conventional ESR experiment, is something of a misnomer here, as we are clearly dealing with what is essentially a non-resonant response (i.e. there is no upper spin level to populate). These cavity perturbations have been studied in detail by Rettori et al. [ 2 1, who noted a dramatic enhancement of the Q on cooling a sample of YBa2Cu30,_, below T,; this was accompanied by a large increase in the cavity resonant frequency, an effect which forms the ba-
R. Janes et al. /Low-field microwave absorption in superconducting cuprates
522
sis of the “leakage-current” measurements of Sastry et al. [3,12]. Considering each system individually, fig. 1 shows the temperature dependence of the normalised resistance and AC susceptibility for the Er2Ba4Cu7015_-x sample. The superconducting transition temperatures were as follows: T, (onset)=100 K, T, (mid-point)=98 K and T, (zero) =92 K. The temperature dependence of the AC susceptibility is also shown, the diamagnetic signal indicating T, (onset) = 93 K, consistent with the resistivity measurements. The powder XRD pattern for this sample was indicative of nearly monophasic ERZBa4Cu,015_-x, this phase being responsible for the superconductivity [ 141. Figure 2 illustrates the low-field microwave absorption for this phase (measured at 77 K) which grew in at T,,and exhibited a saturation-type effect on decreasing the attenuation. The signal loss occurred between 30-40 mW at 77 K, a process which was completely reversible. We also studied this effect in the recently synthesized superconducting T1,Ba,CeCuzO, phase [ 16 1. Figure 3 shows the normalized resistance versus temperature curve for a sample of this material, showing T, (onset) = 100 K, T, (mid-point) = 87 K and T, (zero) = 82 K. The temperature dependence of the mass diamagnetic susceptibility in the field cooled ( 100 G) situation is also included. The Meissner signal shows T, (onset) to be around 85 K, consistent with the resistance measurement. The powder XRD pattern was indicative of a Ce-doped T1,BazCuOe phase, which is responsible for the su-
0.8 -
5
2 a’
0.6 -
t
0.4 -
2
r
0.2 0 0
IIIIIIII 20
40
60
80
100
120
140
Temperature(K) Fig. 1. The temperature dependence of the normalised resistance and the AC susceptibility for an Er2Ba4Cu7015_-x sample.
1
ZOOOG SOOG
I
I_
30mW
t 40mW
Fig. 2. Low-field microwave absorption (derivative display) for the Er2Ba4Cu7015_-x sample as a function of microwave power, sample temperature= 77 K, v= 9.1 GHz. The narrow signal at ca. 3200 G, is due to a DPPH (diphenyl-picryl-hydrazyl, g= 2.0036 ) marker, mixed with the sample.
perconducting behaviour [ 161. Figure 4 shows the saturation of the low-field signal in this sample, where a much lower power level was required to effect this response. Furthermore, the same detuning effect was observed and the signal returned to its original intensity on attenuation. The broad singlet at g= 2.150 300 G) is assigned to Cu2+ in impurity or (A&,contaminant phases on the basis of previous studies [ 17-191. The intensity of this signal increased on cooling consistent with a Curie-type susceptibility, this also occurred on cooling through T,,suggesting the impurity phase is located at the surface or grain boundaries. The intensity of the localised Cu*+ signal, and indeed the DPPH signal in rig. 2, was approximately the same at both power levels, indicat-
R. Janes et al. / Low-field microwave absorption in superconducting cuprates
\\ \\ ‘\\\ \ \ \ ‘\ \ \
0.2 -
0
I 0
1 20
1
I 40
I
1 60
1
-1 \ \ I SO
100
120
140
Temperature(K)
Fig. 3. The temperature dependence of the normalised resistance and the magnetisation for the T1,BazCeCu,O, sample.
5OOOG
\
523
single crystals. In this study individual components of the spectrum were observed to increase intensity at different rates, and high power levels caused linebroadening and eventual saturation. Conversely, Blazey et al. [ 5 ] have reported a completely new series of absorptions to grow in as the incident power was increased, the variance being attributed by Dultic et al. [ 7 ] to sample homogeneities. Certainly, the results of our investigations point towards the microwave response being sample dependent with the signal disappearing at different values of microwave magnetic field strength, for the different materials examined. As discussed earlier, one explanation for the origin of the absorption has been proposed to be microwave induced supercurrents [ 5 1, and we may speculate that as the microwave power is increased, we reach a level sufficient to exceed the critical current (J,) of the weak links in the material concerned. Indeed Dulcic et al. [ 71 have also discussed this possibility. Therefore, it is possible that the saturation effect reported here is a function of the ceramic microstructure, rather than being a phase dependent response, and if so, may prove to be a useful probe of microstructure and of the effects of microstructural engineering.
References
Fig. 4. Microwave absorption derivative spectrum for the TI,Ba2CeCu20, sample (T= 77 K, ~~9.1 GHz) showing the effect of varying the incident microwave power level.
ing the lossy nature of the material in the normal state does not significantly reduce spectrometer sensitivity. We extended our studies to the well-known YBa2Cu307_-x phase, and again the same signal-loss occurred as the incident microwave power level was raised. However, the sample studied required significantly higher power levels (circa 50-60 mW) to facilitate this response. The actual origin of the saturation effect reported here is somewhat unclear, and indeed Dulcic and coworkers [ 71 have recently reported an extensive study of the effect of microwave power on the lowfield multiline spectrum detected in YBa2Cu307_-x
[ 1 ] S.V. Bhat, P. Ganguly and C.N.R. Rao, Pramana 28 ( 1987) 425. [2 C. Rettori, D. Davidov, I. Belaish and I. Felner, Phys. Rev. B 36 (1987) 4028. 13 M.D. Sastry, Studies of High Temperature Superconductors chapt. 10 (Nova Science Publ., New York, 1989). H.A. Farach, Copper Oxide [4 C.P. Poole, T. Datta, Superconductors (Wiley-Interscience, 1988) p. 163. K.N. Shrivastava, Solid State Commun. 68 ( 1988) 259. t: 11J. Stankowski, P.K. Kahol, N.S. Dalal and J.S. Moodera, Phys. Rev. B 36 (1987) 7126. [7 ‘1K.W. Blazey, A.M. Portis, K.A. Mu&r and F.H. Holtzberg, Physica C 153 (1988) 56. 18 H. Vichery, F. Beuneu and P. Lejay, Physica C 159 ( 1989) 823. [9 A. Dulcic, R.H. Crepeau and J.H. Freed, Physica C 160 (1989) 223. [‘O ‘1R. Janes, R.S. Liu, P.P. Edwards, A.D. Stevens and M.C.R. Symons, Mater. Res. Bull., submitted. [I1 I T.S. Lin, L.C. Sobotka and W. Froncisz, Nature 333 ( 1988) 21.
524
R. Janes et al. /Low-field microwave absorption in superconducting cuprates
[ 121 M.D. Sastry, Y. Babu, R.M. Kadam, A.G. Idalvi, I.K.
Gopalakrishnan, J.V. Yakhmi and R.M. Iyer, Solid State Commun. 66 (1988) 1219. [ 131 G.G. Siu, Solid State Commun. 72 (1989) 291. [ 141 D.M. Pooke, R.G. Buckley, M.R. Presland and J.L. Tallon, Phys. Rev. B, in press. [ 15 ] J.L. Tallon, D.M. Pooke, R.G. Buckley, M.R. Presland and F.J. Blunt, Phys. Rev. B, in press.
[ 161 J.H. Wang, Z.Z. Sheng, C. Dong, X. Fei, L. Sheng, A.M. Hermann and Z.X. Zhao, Physica C 158 ( 1989) 507. [ 171 F.H. Mehran and P.W. Anderson, Solid State Commun. 7 1 (1989) 29. [ 181 W.R. McKinnon, J.R. Morton and G. Pleizer, Solid State Commun. 66 (1988) 1093. [ 191 R. Jones, R. Janes, A.R. Armstrong, N.C. Pyper, D.J. Keeble and P.P. Edwards, J. Chem. Sot., in press.