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Dielectric permittivity of Y1Ba2Cu3O7−δ in the microwave region
DIELECTRIC
November 1989
MATERIALS LETTERS
Volume 8, number 11,12
PERMITTIVITY
OF Y1Ba2Cu307_6 IN THE MICROWAVE
REGION
D.C. DUBE, SC. MATHUR Physics Department, Indian Institute
of Technology, New Delhi 110016, India
S.J. JANG and A.S. BHALLA Materials Research Laboratory, The Pennsylvania State University, l_IniversttyPark, PA 16802, USA
Received 3 1 August 1989
Complex electrical permittivity of Y ,Ba2Cu307_-6superconductor ( TCz 90 K) has been measured bation
resonance technique. The dielectric constant and microwave parameters rise again below the transition temperature
1. Introduction
at 9.2 GHz using a perturexhibit a sharp fall in the vicinity of 90 K, both
Specimen
The recent discovery of high-T, superconductors has generated overwhelming interest among scientists and technologists all over the world. Microwave studies on high-T, superconductors are expected to help a great deal in understanding the charge transport processes in these materials. Several investigations have recently appeared on microwave absorption [ 1.21, surface impedance [ 3-5 1, and microwave conductivity [ 61 of superconductors. We report here our results on complex permittivity of Y IBa2Cu307_-6 in the microwave region.
2. Experimental
absorption
---_
H
TOP VIEW Fig. 1. Section of microwave resonator with superconducting sample. Electric (---) and magnetic field (---) profiles in the resonator shown.
details
The experiments were conducted using a perturbation resonance technique [ 7 1. A rectangular cavity x 7.0 cm in length was constructed from a standard WR-90 copper waveguide. At the center of the broad wall there was a small hole to put the specimen in the cavity. Conducting plates with a small aperture (2.5 mm wide) on either side of the guide provided inductive coupling to the cavity. The cavity resonated in TE,03 mode at 9.2 GHz. The Q of the cavity was z 6000. Fig. 1 shows a section of the resonator with electric and magnetic field profiles. The specimen lies in the maximum electric field region 0 167-577x/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
E
in the cavity. The shift in resonance frequency and Q values with and without the specimen were measured by connecting the cavity to a Hewlett-Packard network analyzer (model 85 10T). The cavity was attached to a liquid-nitrogen cryostat for measurements at low temperatures. Y,Ba2Cu30,_s specimens were prepared from Yz03, CuO, and BaOz powders by the usual ceramic processes. The samples were cut as thin rectangular rod having typical dimensions 4.4x0.75 x 1.2 mm3 from a high-density pressed sintered pellet. The diB.V.
451
Volume 8, number
11,12
MATERIALS
LETTERS
November
1989
electric constant E’ and loss factor E” were calculated using the following expressions
Here, fs and fc are resonance frequencies with and without the specimen, respectively, and Q, and Q, are corresponding loaded Q values of the cavity. k’, and V, are cavity and sample volumes, respectively. Dimensions L and h are depicted in fig. 1. Fig. 3. Dielectric loss factor t” for Y,BaZCu307_-d as a function of temperature at 9.2 GHz.
3. Results and discussion Placing a small sample of YLBa2Cu307_-6 in the cavity, it is possible to see cavity perturbations using HP network analyzer 8510T. Thus it is possible to study the temperature dependence of the complex permittivity E*= E’ -it” in this material using the cavity perturbation technique [ 71. Due to the high Q of the cavity and the high detection efficiency this technique is more accurate and sensitive than other microwave techniques available. The variation of E’ and E” with temperature at 9.2 GHz is shown in figs. 2 and 3. The value of E’ at room temperature is 130 and falls slowly and linearly to a value of 123.8 at 150 K. Below this temperature it shows slight curvature, touching a minimum at 137 K. On further cooling it starts rising, reaching a maximum value of 125.3 at 104 K. Below 104 K it falls again. This fall is much sharper than the previous one. This second
Fig. 2. Dielectric constant temperature at 9.2 GHz.
452
t’ for Y ,BaZCu307_-6 as a function
of
minimum is around 91 K. Below 91 K the E’ value shows a very sharp increase. The E” value also follows a linear temperature dependence as we cool below room temperature. This behavior continues up to 120 K, below which the fall becomes more steep around 95 K turning into a very sharp fall around 9 1 K. Like E’, t” also exhibits a sharp rise below 91 K. The structure observed for t’ between 9 1 and 140 K is either missing or highly subdued in the E” versus temperature plot. The characteristic linear decrease of E’ and E” from room temperature down to around 104 K is consistent with that reported by Trybula et al. [ 81. However, the numerical values reported by other authors are much smaller than obtained by us. This may be either due to the high density ( x 92%) of our samples or due to the high accuracy of the technique and the sensitivity of our HP85 1OT network analyzer. The presence of maxima in the E’ plot (fig. 2) at z 105 K could be due to some kind of ordering effects in the YBC structure and observable in the microwave region. The dc resistivity measurements on Y,Ba2Cu@_6 sample gives a T, of 9 1 K. In the present microwave studies a sharp minimum is recorded for both 6’ and E” at 9 1 K but their magnitudes are still finite. These results, at best, are again suggestive of the ordering occurring in the material in the vicinity of the T,. The sharp increase in E’ and E” values below 91 K is unusual. To check if it is a genuine observation and not an experimental artifact, measurements were conducted on superconducting tape (roughly 70 ~01%
Volume 8, number
11,12
MATERIALS
1
Fig. 4. Temperature dependence conducting tape at 9.2 GHz.
of dielectric
constant
for super-
polymer) also. Results shown in figs. 4 and 5 are similar in profile to pure superconductor results. It is interesting to note that E’ (and E” ) values for tape are reduced to -25% of values for pure supercon-
Fig. 5. Temperature dependence ducting tape at 9.2 GHz.
of dielectric
loss for supercon-
LETTERS
November
1989
ductor, the reduction being in almost the same proportion as superconducting powder in the tape. The dc resistivity measurements on the same pure-phase superconducting sample do not show any increase in resistance below the critical temperature. It may be mentioned here that Poirier et al. [ 61 also observed a rise in microwave absorption and a frequency shift below the critical temperature in La,,&,.,CuO~ superconductor at 16.8 GHz. This typical behavior of high-T, superconductors seems to stem from non-superconducting phases present in the material though no definite mechanism has yet evolved or inferred in our studies.
References [ 1 ] A. Dulcic, R.H. Crepean and J.H. Freed, Phys. Rev. B 38 (1988) 5002. [ 21 A.K. Ganguli, K.S. Nanjunda Swamy, G.N. Subbanna, A.M. Umarji, S.V. Bhat and C.N.R. Rao, Solid State Commun. 67 (1988) 39. [ 31 S. Sridhar and W.L. Kennedy, Rev. Sci. Instr. 59 ( 1988) 53 1. [4] J.S. Martens, J.B. Beyer and D.S. Ginley, Appl. Phys. Letters 52 (1988) 1822. [ 51 A. Fathy, D. Kalokitis and E. Belohoubek, Phys. Rev. B 38 (1988) 7023. [6] M. Poirier, G. Quirion, K.R. Poeppelmeier and J.P. Thiel. Phys. Rev. B 36 (1987) 3906. [7] D.C. Dube, M.T. Lanagan, J.H. Kim and S.J. Jang, J. Appl. Phys. 63 (1988) 2466. [ 81 Z. Trybula, J. Stankowski and J. Baszynski, Physica C 156 (1988) 485.
453
November 1989
MATERIALS LETTERS
Volume 8, number 11,12
PERMITTIVITY
OF Y1Ba2Cu307_6 IN THE MICROWAVE
REGION
D.C. DUBE, SC. MATHUR Physics Department, Indian Institute
of Technology, New Delhi 110016, India
S.J. JANG and A.S. BHALLA Materials Research Laboratory, The Pennsylvania State University, l_IniversttyPark, PA 16802, USA
Received 3 1 August 1989
Complex electrical permittivity of Y ,Ba2Cu307_-6superconductor ( TCz 90 K) has been measured bation
resonance technique. The dielectric constant and microwave parameters rise again below the transition temperature
1. Introduction
at 9.2 GHz using a perturexhibit a sharp fall in the vicinity of 90 K, both
Specimen
The recent discovery of high-T, superconductors has generated overwhelming interest among scientists and technologists all over the world. Microwave studies on high-T, superconductors are expected to help a great deal in understanding the charge transport processes in these materials. Several investigations have recently appeared on microwave absorption [ 1.21, surface impedance [ 3-5 1, and microwave conductivity [ 61 of superconductors. We report here our results on complex permittivity of Y IBa2Cu307_-6 in the microwave region.
2. Experimental
absorption
---_
H
TOP VIEW Fig. 1. Section of microwave resonator with superconducting sample. Electric (---) and magnetic field (---) profiles in the resonator shown.
details
The experiments were conducted using a perturbation resonance technique [ 7 1. A rectangular cavity x 7.0 cm in length was constructed from a standard WR-90 copper waveguide. At the center of the broad wall there was a small hole to put the specimen in the cavity. Conducting plates with a small aperture (2.5 mm wide) on either side of the guide provided inductive coupling to the cavity. The cavity resonated in TE,03 mode at 9.2 GHz. The Q of the cavity was z 6000. Fig. 1 shows a section of the resonator with electric and magnetic field profiles. The specimen lies in the maximum electric field region 0 167-577x/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
E
in the cavity. The shift in resonance frequency and Q values with and without the specimen were measured by connecting the cavity to a Hewlett-Packard network analyzer (model 85 10T). The cavity was attached to a liquid-nitrogen cryostat for measurements at low temperatures. Y,Ba2Cu30,_s specimens were prepared from Yz03, CuO, and BaOz powders by the usual ceramic processes. The samples were cut as thin rectangular rod having typical dimensions 4.4x0.75 x 1.2 mm3 from a high-density pressed sintered pellet. The diB.V.
451
Volume 8, number
11,12
MATERIALS
LETTERS
November
1989
electric constant E’ and loss factor E” were calculated using the following expressions
Here, fs and fc are resonance frequencies with and without the specimen, respectively, and Q, and Q, are corresponding loaded Q values of the cavity. k’, and V, are cavity and sample volumes, respectively. Dimensions L and h are depicted in fig. 1. Fig. 3. Dielectric loss factor t” for Y,BaZCu307_-d as a function of temperature at 9.2 GHz.
3. Results and discussion Placing a small sample of YLBa2Cu307_-6 in the cavity, it is possible to see cavity perturbations using HP network analyzer 8510T. Thus it is possible to study the temperature dependence of the complex permittivity E*= E’ -it” in this material using the cavity perturbation technique [ 71. Due to the high Q of the cavity and the high detection efficiency this technique is more accurate and sensitive than other microwave techniques available. The variation of E’ and E” with temperature at 9.2 GHz is shown in figs. 2 and 3. The value of E’ at room temperature is 130 and falls slowly and linearly to a value of 123.8 at 150 K. Below this temperature it shows slight curvature, touching a minimum at 137 K. On further cooling it starts rising, reaching a maximum value of 125.3 at 104 K. Below 104 K it falls again. This fall is much sharper than the previous one. This second
Fig. 2. Dielectric constant temperature at 9.2 GHz.
452
t’ for Y ,BaZCu307_-6 as a function
of
minimum is around 91 K. Below 91 K the E’ value shows a very sharp increase. The E” value also follows a linear temperature dependence as we cool below room temperature. This behavior continues up to 120 K, below which the fall becomes more steep around 95 K turning into a very sharp fall around 9 1 K. Like E’, t” also exhibits a sharp rise below 91 K. The structure observed for t’ between 9 1 and 140 K is either missing or highly subdued in the E” versus temperature plot. The characteristic linear decrease of E’ and E” from room temperature down to around 104 K is consistent with that reported by Trybula et al. [ 81. However, the numerical values reported by other authors are much smaller than obtained by us. This may be either due to the high density ( x 92%) of our samples or due to the high accuracy of the technique and the sensitivity of our HP85 1OT network analyzer. The presence of maxima in the E’ plot (fig. 2) at z 105 K could be due to some kind of ordering effects in the YBC structure and observable in the microwave region. The dc resistivity measurements on Y,Ba2Cu@_6 sample gives a T, of 9 1 K. In the present microwave studies a sharp minimum is recorded for both 6’ and E” at 9 1 K but their magnitudes are still finite. These results, at best, are again suggestive of the ordering occurring in the material in the vicinity of the T,. The sharp increase in E’ and E” values below 91 K is unusual. To check if it is a genuine observation and not an experimental artifact, measurements were conducted on superconducting tape (roughly 70 ~01%
Volume 8, number
11,12
MATERIALS
1
Fig. 4. Temperature dependence conducting tape at 9.2 GHz.
of dielectric
constant
for super-
polymer) also. Results shown in figs. 4 and 5 are similar in profile to pure superconductor results. It is interesting to note that E’ (and E” ) values for tape are reduced to -25% of values for pure supercon-
Fig. 5. Temperature dependence ducting tape at 9.2 GHz.
of dielectric
loss for supercon-
LETTERS
November
1989
ductor, the reduction being in almost the same proportion as superconducting powder in the tape. The dc resistivity measurements on the same pure-phase superconducting sample do not show any increase in resistance below the critical temperature. It may be mentioned here that Poirier et al. [ 61 also observed a rise in microwave absorption and a frequency shift below the critical temperature in La,,&,.,CuO~ superconductor at 16.8 GHz. This typical behavior of high-T, superconductors seems to stem from non-superconducting phases present in the material though no definite mechanism has yet evolved or inferred in our studies.
References [ 1 ] A. Dulcic, R.H. Crepean and J.H. Freed, Phys. Rev. B 38 (1988) 5002. [ 21 A.K. Ganguli, K.S. Nanjunda Swamy, G.N. Subbanna, A.M. Umarji, S.V. Bhat and C.N.R. Rao, Solid State Commun. 67 (1988) 39. [ 31 S. Sridhar and W.L. Kennedy, Rev. Sci. Instr. 59 ( 1988) 53 1. [4] J.S. Martens, J.B. Beyer and D.S. Ginley, Appl. Phys. Letters 52 (1988) 1822. [ 51 A. Fathy, D. Kalokitis and E. Belohoubek, Phys. Rev. B 38 (1988) 7023. [6] M. Poirier, G. Quirion, K.R. Poeppelmeier and J.P. Thiel. Phys. Rev. B 36 (1987) 3906. [7] D.C. Dube, M.T. Lanagan, J.H. Kim and S.J. Jang, J. Appl. Phys. 63 (1988) 2466. [ 81 Z. Trybula, J. Stankowski and J. Baszynski, Physica C 156 (1988) 485.
453