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Magneto-microwave absorption study of flux depinning in Hg0.7Pb0.3Ba2Ca2Cu3O8 superconductor
PHYSICA ELqEVIER
Physica C 233 (1994) 30-34
Magneto-microwave absorption study of flux depinning in Hgo.7Pbo.3BazCazCu308 superconductor F r a n k J. O w e n s a,b,., A . G . R i n z l e r a,l, Z . I q b a l c a ArmyArmament Research andDevelopment Center, Picatinny, NJ 07806, USA b Department of Physics, Hunter College, City University of New York, New York, NY 10021, USA c Allied Signal Inc., Research and Technology, Morristown, NJ 07962, USA
Received 4 August 1994; revised manuscript received 8 September 1994
Abstract The microwave absorption in the superconducting state of single-phase Hgo.vPbo.3Ba2Ca2Cu3Os+x having a T¢ of 133 K is shown to depend linearly on the square root of the DC magnetic field strength with a change of slope occurring at a DC magnetic field B* which corresponds to an onset of an irreversibility in the field dependence of the microwave absorption. A measurement of the temperature dependence of B* enables a determination of an H - T irreversibility line at 9.2 GHz. A comparison of the magnetic-field-induceddissipation with that the Y-Ba-Cu-O indicates that the Hg-Pb-Ba-Ca-Cu-O material has a larger pinning barrier to flux detrapping at 77 K.
1. Introduction W h e n a D C and an R F magnetic field are a p p l i e d p e r p e n d i c u l a r to the surface o f a type-II superconductor below Tc, there is a t i m e - d e p e n d e n t force, J ( t ) X B , on the vortices resulting in an enhanced DCmagnetic-field-dependent absorption o f microwave energy by the sample. The d y n a m i c a l response o f the vortices to the oscillating force d e p e n d s on whether the vortices are p i n n e d or in the fluid phase. Recent experimental studies in B i - S r - C a - C u - O and T 1 - B a C a - C u - O superconductors show that it is possible to observe the pinning to d e p i n n i n g transition by a change in the functional dependence o f the microwave absorption on the m a g n i t u d e o f the D C magnetic field [ 1-3 ]. The experimental results are largely * Corresponding author. Present address: Department of Chemistry, Rice University, Houston, TX, USA.
in agreement with the theoretical treatments o f vortex-induced electromagnetic absorption by Coffey a n d Clem [4]. Both theory and experiment d e m o n strate that DC-magnetic-field-dependent microwave a b s o r p t i o n is a useful m e t h o d to study vortex dissipation and characterize the flux pinning b e h a v i o r in the copper oxide superconductors. F o r example, a measurement o f the magnetic field dependence o f the microwave absorption at different temperatures below Tc allows a d e t e r m i n a t i o n o f the H - T irreversibility line o f the superconducting material. Recently a material HgBa2CaECU308 +x (Hg- 1223 ) has been synthesized having three copper oxide layers per unit cell and a Tc o f 130 K [ 5 ]. In collaboration with others we have developed a Pb-substituted version o f the (Hg-1223 ) having an average formula unit Hgo.7Pbo.aBa2CaECU308 +x (Hg, Pb- 1223 ) and an onset to the superconducting state above 135 K in some samples. This material can be synthesized reproducibly with 100% purity using conventional
0921-4534/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI0921-4534(94)00588-5
F.J. Owens et aL / Physica C 233 (1994) 30-34
31
163 006 104 005,103
¢-
81 .~
105
0
200
0
003 OLI I 10
20
|
30
40
50
angle ( 0 ) Fig. 1. X-ray diffraction of Hg-Pb-Ba-Ca-Cu-O showingthe material is almost 100%N= 3 phase. sealed tube methods [6]. Preliminary SQUID magnetometry measurements indicate that this material may have enhanced ability to trap flux at 77 K relative to other copper oxide superconductors. The irreversibility DC magnetic field at 77 K was estimated from SQUID data to be in excess of 1 T [6 ]. A more recent determination in Hg-1223 indicated the irreversibility field at 77 K to be as high as 8 T [ 7 ]. However, this later result needs confirmation. The purpose of this paper is to report studies of the DC magnetic field dependence of the microwave absorption in single-phase Hg, Pb-1223 material in order to obtain further information on the nature of flux pinning and dynamical behavior of flux in the material. The flux trapping behavior of the Hg, Pb- 1223 is compared to that in highly pure YBaCuO at 77 K. The irreversibility line at 9.2 G H z is determined and compared with that obtained from SQUID data.
2. Experimental The microwave energy directly absorbed in the sample is measured by the change in the reflection coefficient of a microwave cavity in which the sample is placed. The cavity occupies one arm of a microwave bridge system. The klystron in the other arm is tuned to the resonant frequency of the cavity which is 9.2 GHz. The absorption of microwave energy by the sample changes the reflection coefficient of the cavity and therefore the intensity of the microwave
radiation propagating to the arm of the bridge containing a diode detector. The samples were contained in and cooled using a double walled quartz finger Dewar through which cold N 2 o r He gas flows. This system enables control of the temperature, monitored by a gold-chromel thermocouple in contact with the sample, to within + 1 K. For DC magnetic field measurements the cavity is located between the poles of an electromagnet. The DC magnetic field strength is monitored by a Hall probe which is part of a Lake Shore 450 Gaussmeter. The method of synthesis and the characterization of the samples is similar to that reported previously [6]. Fig. 1 shows the X-ray diffraction data on a pressed sample on which the microwave measurements were made indicating the sample is essentially 100% pure Hg, Pb-1223 phase. The X-ray diffraction data on the pressed sample shows considerable enhancement of the [00N] reflections from the tetragonal unit cell compared to the X-ray data on an assynthesised sample indicating partial alignment of crystallites with the [001 ] direction parallel to the axis of compression. Four-probe bulk resistance measurements on the sample show a resistance drop starting at 140 K, a center point at 135 K and zero resistance at 130 K.
3. Results and analysis Fig. 2 shows the temperature dependence of the surface resistance in arbitrary units in the highly pure
F . J . Owens et
32
al. / Physica C 233 (1994) 30-34
75
12.
70l
10-
65
/
6O 8-
52! .6 ~,
: 50:
t¢ 45!
40i
4-
35i 2-
3O:
11o i ~ 5 i ~ 6 i ~
i ~ 6 i~,~ i40
TEMPERATURE(K)
,0
Temperature dependence of surface resistance through the transition temperature in zero magnetic field and a field o f 0 . 4 T in pressed samples showing an anomalous peak before decreasing into the superconducting state. F i g . 2.
18 16"
,41 ~ #
10~
86~
2~ 0 ¸ • .......
0
, .........
, .........
, .........
1000 2000 3000 MAGNETICFIELD (G)
4000
Increase of microwave absorption vs. DC magnetic field at 1 2 2 K . F i g . 3.
pressed sample of HgPb-1223 in zero DC magnetic field and a field of 0.4 T. Generally the onset of superconductivity is marked by a drastic and relatively sharp drop in the surface resistance which is primarily due to the temperature-dependent decrease in the
. . . .
10
~
. . . .
20
i
. . . .
i
. . . .
i
. . . .
30 40 50 SQRT (Gauss)
i
. . . .
60
70
Change in the surface resistance vs. the square root of the magnetic field strength at 7 7 K for increasing ( i ) and decreasing ( O ) magnetic field showing that the onset of irreversibility occurs at the magnetic field where the slope changes. F i g . 4.
DC
microwave penetration depth. The increase in the absorption above the normal-state absorption prior to the sharper drop is not typical. Notice that this enhancement before the drop increases with application o f a DC magnetic field. When the same measurement is made on a sample that was not pressed after synthesis this enhancement above the normal-state absorption is not observed. A similar increase in absorption prior to the rapid drop at Tc was observed by Manheimer et al. [ 8 ] in pressed samples of Y - B a - C u - O and was interpreted as arising from a distribution of Tcs in the sample. When all the grains are in the superconducting state application of a DC magnetic field increases the surface resistance of the sample wa shown in Fig. 2. This increased absorption is a result of dissipation due to vortex oscillation. The application of a time-dependent microwave field H(t) and a DC magnetic field perpendicular to the surface of a type-II superconductor and the resulting B×J~F force on the vortices causes the vortices to oscillate and thus increase the surface resistance of the sample. The dynamical response of the vortices to the force will depend on whether the vor-
F.J. Owens et al. I Physica C 233 (1994) 30-34
tices are pinned or in the fluid phase. Recent theoretical treatments of the DC-field-enhanced surface resistance due to vortex oscillation shows that the surface resistance depends linearly on B ~/2 in the strong and weak pinning regimes but will have a different slope in each region [ 4 ]. Measurements of the DC field dependence of the microwave absorption at 9.2 GHz in T1-Ba-Ca-Cu-O and Bi-Sr-Ca-Cu-O confirm these predictions [ 1-3 ]. Fig. 3 shows a plot of the change in the microwave absorption at 122 K vs. applied DC magnetic field in HgPb-1223. In Fig. 4 the microwave absorption is plotted vs. B 1/2 at 77 K both an increasing and decreasing DC magnetic field. The data shows that the microwave absorption does depend linearly on B I/2 and there there is a change of slope at a DC magnetic field B*. On the down sweep of the DC magnetic field there is an on-
0.1-
0.090.08-
[] [] [] 0 [] ~I~I 0
"~ O.07-
t
O.06-
0.05. --0.o4¢¢t
[]
[] a
0
33
set of hysteresis in the data starting at B* clearly demonstrating that B* corresponds to the irreversibility transition. Below B* flux is trapped in the sample and above it is not. Fig. 5 is a comparison of the fractional change in surface resistance as a function of DC magnetic field in Y - B a - C u - O and H g - P b - C a - C u - O at 77 K for up and down sweeps of the DC magnetic field. The measurements were made on the same quantity of both materials and at the same operating conditions of the microwave bridge. The Y-Ba-Cu-O material used in the measurement was highly pure powders obtained by a synthesis from citrates by a method previously described [ 9 ]. The data shows that the dissipation is significantly less in the Hg, Pb-1223 material compared to Y - B a - C u - O at 77 K. Further a hysteresis is observed in the Hg, Pb-1223 material and not in the Y-Ba-Cu-O. These results suggest that the Hg, Pb1223 material may more strongly trap flux at 77 K. In the strong pinning regime Coffey and Clem [4 ] have shown that the surface resistance due to vortex dissipation is proportional to B 1 / 2 / K 3/2 where K is the pinning force constant. From this we can estimate that at 0.2 T the pinning force constant is approximately 3 times larger at 77 K in Hg, Pb-1223 compared to Y-Ba-Cu-O.
0 0 0.021 0.010~ .O.
14 i
0
.
.
.
.
i
,
1000 2000 MAGNETIC FIELD (Gauss)
3000
0.7
12 10 == 8
0.,i 0.31
o
i~0
o
4
b
2
'16o6
2o'o6
'
'
3do6
4000
MAGNETIC FIELD (Gauss) Fig. 5. A comparison of the fractional change in the surface resistance vs. DC magnetic field at 77 K in Hg-Ba-Ca-Cu-O (a) and Y-Ba-Cu-O (b) for up and down sweeps of the DC magnetic field.
i~
. . . .
10
i
20
. . . .
i
. . . .
i
. . . .
30 40 SQRT (B)fG
i
50
. . . .
60
Fig. 6. Change in surface resistance vs. square root of DC magnetic field strength at 97 K (ll) and 91 K ( O ) showing that the field at which the slope changes is temperature dependent.
34
F.J. Owens et al. / Physica C 233 (1994) 30-34
2500-
Acknowledgement
2000
This work was carried out while one of the authors ( A G R ) was a National Research Council Post Doctoral Fellow at ARDEC.
15oo
References
.
0
0.1
.
.
0.2
.
0.3
0.4
0.5
(I -T/Tc)
Fig. 7. Irreversibility line determined by measuring the temperature dependence of the DC field at whichthe slope changes.The line through the data is fit to Eq. ( 1).
Fig. 6 is a plot of the surface resistance vs. B 1/2 at two different temperatures showing that the DC magnetic field at which the slope changes shifts to lower values at higher temperatures. A m e a s u r e m e n t of the field dependence of the microwave absorption at a n u m b e r of different temperatures allows a determ i n a t i o n of the temperature dependence of B* and thus the H - T irreversibility line of the sample at 9.2 GHz. Fig. 7 is a plot of B* vs. ( 1 - T / T o ) . The line through the data is a fit to [ 10 ] B=A(1-T/Tc) q , f o r A = 3 . 6 7 X 103 ( G ) and q = 0 . 7 2 4 .
(1)
[ 1] F.J. Owens, Physica C 178 ( 1991 ) 456. [2] F.J. Owens, Physica C 195 (1992) 225. [3] F.J. Owens, J. Phys. Con& Matter 4 (1992) 8091. [4] M.W. Coffey and J.R. Clem, Phys. Rev. Lett. 67 (1991) 386. [5] A. Schilling,M. Cantoni, J.D. Guo and H.R. Ott, Nature 363 (1993) 565. [6] Z. Iqbal, T. Datta, D. Kirven, A. Lungu, J.C. Barry, F.J. Owens, A.G. Rinzler, D. Yangand F. Reidinger, Phys. Rev. B49 (1994) 12322. [7] O. Laborde, B. Souletie, J.L. Thoulence and J.J. Capponi, Solid State Commun. 90 (1994) 443. [8] M.A. Manheimer, S. Lofland, A. Gould, S.M. Bhagat, B. Halsey and S.M. Green, Physica C 183 ( 1991 ) 324. [9] Z. Iqbal, F. Reidinger, A. Bose, N. Cipollin, T.J. Taylor, H. Eckhardt, B.L. Ramakrishna and E.W. Ong, Nature 331 (1988) 326. [ 10] K.A. Mfiller, M. Takashige and J.G. Bednorz, Phys. Rev. Lett. 58 (1987) 408.
Physica C 233 (1994) 30-34
Magneto-microwave absorption study of flux depinning in Hgo.7Pbo.3BazCazCu308 superconductor F r a n k J. O w e n s a,b,., A . G . R i n z l e r a,l, Z . I q b a l c a ArmyArmament Research andDevelopment Center, Picatinny, NJ 07806, USA b Department of Physics, Hunter College, City University of New York, New York, NY 10021, USA c Allied Signal Inc., Research and Technology, Morristown, NJ 07962, USA
Received 4 August 1994; revised manuscript received 8 September 1994
Abstract The microwave absorption in the superconducting state of single-phase Hgo.vPbo.3Ba2Ca2Cu3Os+x having a T¢ of 133 K is shown to depend linearly on the square root of the DC magnetic field strength with a change of slope occurring at a DC magnetic field B* which corresponds to an onset of an irreversibility in the field dependence of the microwave absorption. A measurement of the temperature dependence of B* enables a determination of an H - T irreversibility line at 9.2 GHz. A comparison of the magnetic-field-induceddissipation with that the Y-Ba-Cu-O indicates that the Hg-Pb-Ba-Ca-Cu-O material has a larger pinning barrier to flux detrapping at 77 K.
1. Introduction W h e n a D C and an R F magnetic field are a p p l i e d p e r p e n d i c u l a r to the surface o f a type-II superconductor below Tc, there is a t i m e - d e p e n d e n t force, J ( t ) X B , on the vortices resulting in an enhanced DCmagnetic-field-dependent absorption o f microwave energy by the sample. The d y n a m i c a l response o f the vortices to the oscillating force d e p e n d s on whether the vortices are p i n n e d or in the fluid phase. Recent experimental studies in B i - S r - C a - C u - O and T 1 - B a C a - C u - O superconductors show that it is possible to observe the pinning to d e p i n n i n g transition by a change in the functional dependence o f the microwave absorption on the m a g n i t u d e o f the D C magnetic field [ 1-3 ]. The experimental results are largely * Corresponding author. Present address: Department of Chemistry, Rice University, Houston, TX, USA.
in agreement with the theoretical treatments o f vortex-induced electromagnetic absorption by Coffey a n d Clem [4]. Both theory and experiment d e m o n strate that DC-magnetic-field-dependent microwave a b s o r p t i o n is a useful m e t h o d to study vortex dissipation and characterize the flux pinning b e h a v i o r in the copper oxide superconductors. F o r example, a measurement o f the magnetic field dependence o f the microwave absorption at different temperatures below Tc allows a d e t e r m i n a t i o n o f the H - T irreversibility line o f the superconducting material. Recently a material HgBa2CaECU308 +x (Hg- 1223 ) has been synthesized having three copper oxide layers per unit cell and a Tc o f 130 K [ 5 ]. In collaboration with others we have developed a Pb-substituted version o f the (Hg-1223 ) having an average formula unit Hgo.7Pbo.aBa2CaECU308 +x (Hg, Pb- 1223 ) and an onset to the superconducting state above 135 K in some samples. This material can be synthesized reproducibly with 100% purity using conventional
0921-4534/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI0921-4534(94)00588-5
F.J. Owens et aL / Physica C 233 (1994) 30-34
31
163 006 104 005,103
¢-
81 .~
105
0
200
0
003 OLI I 10
20
|
30
40
50
angle ( 0 ) Fig. 1. X-ray diffraction of Hg-Pb-Ba-Ca-Cu-O showingthe material is almost 100%N= 3 phase. sealed tube methods [6]. Preliminary SQUID magnetometry measurements indicate that this material may have enhanced ability to trap flux at 77 K relative to other copper oxide superconductors. The irreversibility DC magnetic field at 77 K was estimated from SQUID data to be in excess of 1 T [6 ]. A more recent determination in Hg-1223 indicated the irreversibility field at 77 K to be as high as 8 T [ 7 ]. However, this later result needs confirmation. The purpose of this paper is to report studies of the DC magnetic field dependence of the microwave absorption in single-phase Hg, Pb-1223 material in order to obtain further information on the nature of flux pinning and dynamical behavior of flux in the material. The flux trapping behavior of the Hg, Pb- 1223 is compared to that in highly pure YBaCuO at 77 K. The irreversibility line at 9.2 G H z is determined and compared with that obtained from SQUID data.
2. Experimental The microwave energy directly absorbed in the sample is measured by the change in the reflection coefficient of a microwave cavity in which the sample is placed. The cavity occupies one arm of a microwave bridge system. The klystron in the other arm is tuned to the resonant frequency of the cavity which is 9.2 GHz. The absorption of microwave energy by the sample changes the reflection coefficient of the cavity and therefore the intensity of the microwave
radiation propagating to the arm of the bridge containing a diode detector. The samples were contained in and cooled using a double walled quartz finger Dewar through which cold N 2 o r He gas flows. This system enables control of the temperature, monitored by a gold-chromel thermocouple in contact with the sample, to within + 1 K. For DC magnetic field measurements the cavity is located between the poles of an electromagnet. The DC magnetic field strength is monitored by a Hall probe which is part of a Lake Shore 450 Gaussmeter. The method of synthesis and the characterization of the samples is similar to that reported previously [6]. Fig. 1 shows the X-ray diffraction data on a pressed sample on which the microwave measurements were made indicating the sample is essentially 100% pure Hg, Pb-1223 phase. The X-ray diffraction data on the pressed sample shows considerable enhancement of the [00N] reflections from the tetragonal unit cell compared to the X-ray data on an assynthesised sample indicating partial alignment of crystallites with the [001 ] direction parallel to the axis of compression. Four-probe bulk resistance measurements on the sample show a resistance drop starting at 140 K, a center point at 135 K and zero resistance at 130 K.
3. Results and analysis Fig. 2 shows the temperature dependence of the surface resistance in arbitrary units in the highly pure
F . J . Owens et
32
al. / Physica C 233 (1994) 30-34
75
12.
70l
10-
65
/
6O 8-
52! .6 ~,
: 50:
t¢ 45!
40i
4-
35i 2-
3O:
11o i ~ 5 i ~ 6 i ~
i ~ 6 i~,~ i40
TEMPERATURE(K)
,0
Temperature dependence of surface resistance through the transition temperature in zero magnetic field and a field o f 0 . 4 T in pressed samples showing an anomalous peak before decreasing into the superconducting state. F i g . 2.
18 16"
,41 ~ #
10~
86~
2~ 0 ¸ • .......
0
, .........
, .........
, .........
1000 2000 3000 MAGNETICFIELD (G)
4000
Increase of microwave absorption vs. DC magnetic field at 1 2 2 K . F i g . 3.
pressed sample of HgPb-1223 in zero DC magnetic field and a field of 0.4 T. Generally the onset of superconductivity is marked by a drastic and relatively sharp drop in the surface resistance which is primarily due to the temperature-dependent decrease in the
. . . .
10
~
. . . .
20
i
. . . .
i
. . . .
i
. . . .
30 40 50 SQRT (Gauss)
i
. . . .
60
70
Change in the surface resistance vs. the square root of the magnetic field strength at 7 7 K for increasing ( i ) and decreasing ( O ) magnetic field showing that the onset of irreversibility occurs at the magnetic field where the slope changes. F i g . 4.
DC
microwave penetration depth. The increase in the absorption above the normal-state absorption prior to the sharper drop is not typical. Notice that this enhancement before the drop increases with application o f a DC magnetic field. When the same measurement is made on a sample that was not pressed after synthesis this enhancement above the normal-state absorption is not observed. A similar increase in absorption prior to the rapid drop at Tc was observed by Manheimer et al. [ 8 ] in pressed samples of Y - B a - C u - O and was interpreted as arising from a distribution of Tcs in the sample. When all the grains are in the superconducting state application of a DC magnetic field increases the surface resistance of the sample wa shown in Fig. 2. This increased absorption is a result of dissipation due to vortex oscillation. The application of a time-dependent microwave field H(t) and a DC magnetic field perpendicular to the surface of a type-II superconductor and the resulting B×J~F force on the vortices causes the vortices to oscillate and thus increase the surface resistance of the sample. The dynamical response of the vortices to the force will depend on whether the vor-
F.J. Owens et al. I Physica C 233 (1994) 30-34
tices are pinned or in the fluid phase. Recent theoretical treatments of the DC-field-enhanced surface resistance due to vortex oscillation shows that the surface resistance depends linearly on B ~/2 in the strong and weak pinning regimes but will have a different slope in each region [ 4 ]. Measurements of the DC field dependence of the microwave absorption at 9.2 GHz in T1-Ba-Ca-Cu-O and Bi-Sr-Ca-Cu-O confirm these predictions [ 1-3 ]. Fig. 3 shows a plot of the change in the microwave absorption at 122 K vs. applied DC magnetic field in HgPb-1223. In Fig. 4 the microwave absorption is plotted vs. B 1/2 at 77 K both an increasing and decreasing DC magnetic field. The data shows that the microwave absorption does depend linearly on B I/2 and there there is a change of slope at a DC magnetic field B*. On the down sweep of the DC magnetic field there is an on-
0.1-
0.090.08-
[] [] [] 0 [] ~I~I 0
"~ O.07-
t
O.06-
0.05. --0.o4¢¢t
[]
[] a
0
33
set of hysteresis in the data starting at B* clearly demonstrating that B* corresponds to the irreversibility transition. Below B* flux is trapped in the sample and above it is not. Fig. 5 is a comparison of the fractional change in surface resistance as a function of DC magnetic field in Y - B a - C u - O and H g - P b - C a - C u - O at 77 K for up and down sweeps of the DC magnetic field. The measurements were made on the same quantity of both materials and at the same operating conditions of the microwave bridge. The Y-Ba-Cu-O material used in the measurement was highly pure powders obtained by a synthesis from citrates by a method previously described [ 9 ]. The data shows that the dissipation is significantly less in the Hg, Pb-1223 material compared to Y - B a - C u - O at 77 K. Further a hysteresis is observed in the Hg, Pb-1223 material and not in the Y-Ba-Cu-O. These results suggest that the Hg, Pb1223 material may more strongly trap flux at 77 K. In the strong pinning regime Coffey and Clem [4 ] have shown that the surface resistance due to vortex dissipation is proportional to B 1 / 2 / K 3/2 where K is the pinning force constant. From this we can estimate that at 0.2 T the pinning force constant is approximately 3 times larger at 77 K in Hg, Pb-1223 compared to Y-Ba-Cu-O.
0 0 0.021 0.010~ .O.
14 i
0
.
.
.
.
i
,
1000 2000 MAGNETIC FIELD (Gauss)
3000
0.7
12 10 == 8
0.,i 0.31
o
i~0
o
4
b
2
'16o6
2o'o6
'
'
3do6
4000
MAGNETIC FIELD (Gauss) Fig. 5. A comparison of the fractional change in the surface resistance vs. DC magnetic field at 77 K in Hg-Ba-Ca-Cu-O (a) and Y-Ba-Cu-O (b) for up and down sweeps of the DC magnetic field.
i~
. . . .
10
i
20
. . . .
i
. . . .
i
. . . .
30 40 SQRT (B)fG
i
50
. . . .
60
Fig. 6. Change in surface resistance vs. square root of DC magnetic field strength at 97 K (ll) and 91 K ( O ) showing that the field at which the slope changes is temperature dependent.
34
F.J. Owens et al. / Physica C 233 (1994) 30-34
2500-
Acknowledgement
2000
This work was carried out while one of the authors ( A G R ) was a National Research Council Post Doctoral Fellow at ARDEC.
15oo
References
.
0
0.1
.
.
0.2
.
0.3
0.4
0.5
(I -T/Tc)
Fig. 7. Irreversibility line determined by measuring the temperature dependence of the DC field at whichthe slope changes.The line through the data is fit to Eq. ( 1).
Fig. 6 is a plot of the surface resistance vs. B 1/2 at two different temperatures showing that the DC magnetic field at which the slope changes shifts to lower values at higher temperatures. A m e a s u r e m e n t of the field dependence of the microwave absorption at a n u m b e r of different temperatures allows a determ i n a t i o n of the temperature dependence of B* and thus the H - T irreversibility line of the sample at 9.2 GHz. Fig. 7 is a plot of B* vs. ( 1 - T / T o ) . The line through the data is a fit to [ 10 ] B=A(1-T/Tc) q , f o r A = 3 . 6 7 X 103 ( G ) and q = 0 . 7 2 4 .
(1)
[ 1] F.J. Owens, Physica C 178 ( 1991 ) 456. [2] F.J. Owens, Physica C 195 (1992) 225. [3] F.J. Owens, J. Phys. Con& Matter 4 (1992) 8091. [4] M.W. Coffey and J.R. Clem, Phys. Rev. Lett. 67 (1991) 386. [5] A. Schilling,M. Cantoni, J.D. Guo and H.R. Ott, Nature 363 (1993) 565. [6] Z. Iqbal, T. Datta, D. Kirven, A. Lungu, J.C. Barry, F.J. Owens, A.G. Rinzler, D. Yangand F. Reidinger, Phys. Rev. B49 (1994) 12322. [7] O. Laborde, B. Souletie, J.L. Thoulence and J.J. Capponi, Solid State Commun. 90 (1994) 443. [8] M.A. Manheimer, S. Lofland, A. Gould, S.M. Bhagat, B. Halsey and S.M. Green, Physica C 183 ( 1991 ) 324. [9] Z. Iqbal, F. Reidinger, A. Bose, N. Cipollin, T.J. Taylor, H. Eckhardt, B.L. Ramakrishna and E.W. Ong, Nature 331 (1988) 326. [ 10] K.A. Mfiller, M. Takashige and J.G. Bednorz, Phys. Rev. Lett. 58 (1987) 408.