- Email: [email protected]
Study of the flux pinning in the YBa2Cu3Oy system
PHYSlCA
Physiea C 185-189 (1991) 332-336 North-H011and
Study o f the F l u x Pinning in t h e YBa2CusO~ S y s t e m " M. K. W u ,
M. J. W a n g , a n d C. C. Chi + , Department of Physics and Materials
Science C e n t e r , National T s i n g - - H u a U n i v e r s i t y , T a i w a n , R O C F. H o l t z b e r g , IBM T. J. W a t s o n Research C e n t e r , Yorktown H e i g h t s , N Y
W e h a v e carried out a systematic study of the YBa~Cu30~(YqBC~) single crystals ineluding those grown f r o m ( Y B C ~ ) / A g 2 0 composite. The resistive transition, and the detailed I - - V characteristic in magnetic fields behaves differently f o r crystals grown by different methods. T h e results suggest that the f l u x dynamics in this high Tc oxide is complex a n d depends strongly on the sample conditions. Detailed electrical and magnetic properties in conjunction with the microstructure will be presented and discussed in comparison with the existing theories.
INTRODUCTION Since the discovery of high T c superconductivity, the potential of applications has attracted unprecedented effort to achieve high critical current density in the cuprate superconductors. T h o u g h improvement of the critical currents in materials with specific processing technique has been m a d e , it is still below the requirement for practical application, especially in the materials with bulk form. The lack of the detailed knowledge in the dynamics of the f l u x lines hindered the progress. Mtiller e t al. [1"] first demonstrated the existence of time dependence of the r e m n a n t magnetization at low temperature in the ceramic samples. This led them to propose the importance of the glassy structure in the high T c materials. H o w e v e r , similar effect observed in the single crystal has led others to suggest the significance of the thermally activated flux creep [-2-]. Several n o n - - c o n v e n t i o n a l ol~ervations [ 3 - - 5 ] , such as the resistive transition broadening ira these high Te oxides h a v e generated
numerous attempts I-6--9-] to explain the elfeet. R e c e n t l y , b y considering the r a n d o m disorder, Fisher et al. ( F F H ) E1 0-] predicted the existence of a second order phase transition at finite temperature below which the zero freq u e n c y resistance is zero. Experimental evidence for the existence of this v o r t e x - glass transition has b e e n reported in epitaxial films [11"], in single crystal [12-] and in polycryst~lline YBa2Cu3Or-x[13-]. Nevertheless, a unified picture to satisfactorily explain the observed data is still n o t available. In this w o r k , w e report the detailed measurements o f the I V characteristic of single crystals w h i c h possess different d e f e c t structures. O u r results suggest that depends on the microstructural defects of the sanlple, there exist a phase transition which corresponds to the v o r t e x - solid ( o r glass) to v o r t e x - liquid transition. The detailed I - - V characteristic below the transition temperature strongly depends on the defect structure of the materials.
* W o r k is s u p p o r t e d b y T h e R O C N a t i o n a l S c i e n c e C o u n c i l G r a n t NSC790208M00795. -~- On leave f r o m IBM T. J. W a t s o n Research C e n t e r , Yorktown Heights, NY .0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights rcscrvcd.
M.K. Wu et aL / Study of the flux pinning in the YBa2CuaOysystem E2KP~NTAL Y B C O single crystals g r o w n b y t w o diff e r e n t t e c h n i q u e s were used in this study. One was p r e p a r e d using t h e c o n v e n t i o n a l f l u x g r o w n m e t h o d ( s a m p l e A ) E 14"]. The other was g r o w n f r o m the m i x t u r e o f stoichiometrie YBCO a n d A g O w h e r e A g s e r v e d as low melting f l u x f o r c r y s t a l g r o w t h ( s a m p l e B) E 1 5 ] . The m a j o r d i f f e r e n c e s o f these crystal were t h a t A h a s m u c h larger t w i n w h i c h can be visible u s i n g optical m i c r o s c o p e , while the t w i n spacing i n s a m p l e B is e x t r e m e l y small a n d can be o b s e r v e d o n l y by h i g h resolution electron microscopy. It w a s e s t i m a t e d t h e twin spacing of s a m p l e B is in the order o f ~'~ 50 j~. T y p i cal s a m p l e dimensions w e r e l x l x 0 . 2 rnm 3 with the shortest dimension along c axis were used. T h e electrical c o n t a c t s were m a d e by e v a p o r a t e d Ag pads a n d p l a t i n u m wire were a t t a c h e d w i t h A g - - p a s t e . A f t e r the leads were m o u n t e d , the samples w e r e glued to a thin A l u m i n u m plate. T h e samples were t h e n a n n e a l e d in a f l o w o x y g e n e n v i r o n m e n t at - ~ 500 "C f o r 1 - 2 hours. T h e contact resistances w e r e less t h a n 1 o h m . T h e a l u m i n u m plates w e r e m o u n t e d w i t h t h e r m a l grease on the sample holder m a d e b y copper containing a c a r b o n - glass t h e r m o m e t e r a n d heater in a variable t e m p e r a t u r e insert inside a 9 Testa sup e r c o n d u c t i n g magnet. T h e t e m p e r a t u r e stability w a s f o u n d to be w i t h i n 0. 1 ~ . The t r a n s p o r t m e a s u r e m e n t s w e r e carried out w i t h a Keithley 220 current source and a Keithley 181 v o l t m e t e r . F o r e a c h v o l t a g e m e a s u r e m e n t the c u r r e n t s in both directions were used. To be sure the voltage stability o f the measurem e n t s , l o n g w~tLn~ ~ m e w,as employed. RESULTS AND DISCUSSIONS F i g u r e 1 shows the t e m p e r a t u r e dependence o f t h e resistance f o r samples A and B at fields up to 8 T. T h e zero resistance ( w i t h i n the s y s t e m resolution o f o u r m e a s u r e m e n t ) t e m p e r a t u r e s at zero field are 9 3 . 5 ~ a n d 89
333
8£-005 1
Q
gE-005 • • • • • ~.l R4T
"~O
~.~
• • • •,
6.W~,&T
• --,.....
5.820T 4.6561" 3.492T
,,,,,
1.184T
0.000T
• --..
~
I:~ 2E-005"
.o~
"
, ~
! OE+O00
75
eo
e
'P"''
e5 ~o T e m p e r a t u r e (K)
.......
o5
too
4 E - 0 0 3 - - (b) o I
2 ~
2E-003
e~
IE-O03
OE+O00
70
75
BO
85
90
9,5
I00
Temperature (K)
Figure 1: T e m p e r a t u r e dependence of resistance of crystals A a n d B at fields up to 8 Tesla. " K , respectively. The transition widths are within 1 "K for both samples. Differences in the field b r o a d e n i n g of the resistive transition for the t w o samples were observed. Particul a r l y , the distinct " k n e e " in increasing field is observed in s a m p l e A but not in s a m p l e B~ On the other h a n d , a n obvious tail region which becomes m o r e e v i d e n t is observed in sample B. Figure 2 ( a ) shows the E - - J c u r v e s at 4. 656 T for s a m p l e A. Figure 2 ( b ) is the correspending ( E / J ) - - J curves. F i g u r e s 3 ( a ) a n d 3 ( b ) are the d a t a for sample B b u t at a field of 2. 328 T. It is clear that for s a m p l e A there exists a t e m p e r a t u r e (Ts) at w h i c h the slope of Log(E)--Log(J) is exactly linear. For temperature below T,, a downward curvature is evident at all c u r r e n t s , while a b o v e T s, the curvature is u p w a r d and becomes linear. The H vs. T, c u r v e , as shown in figure 4, is found to fall on the same c u r v e as H v s . T~=0
334
M.K Wu et aL I Study.of the flux pirming in the YBa2Cu30~ system
where Ta-o is the temperature at w h i c h resistance is zero ( w i t h i n the resolution of o u r measurements).
For sample B,
t h e data also
show clearly at a characteristic t e m p e r a t u r e ( T ~ ) where the curve changes from n o n linear to linear in E vs. J. Plotting H vs. T J , as shown in figure 4 , shows that this TJ coincides with the temperature at which Tg-o. A similar plot for the data o f sample A shows t O "~
,,V,;l!~
(a) 10 "="
E
•
0 ~
".
•
I0 "
o°°°
0.
o ooo
•
r~
o°.. oo°.
o0
•
10 B=4.656T L0 I0
1(1 ~
.I
(A/cm z)
! ! ! !
1o "t :
•
1"
:=sallltd •
=o•i,oP e °
• e o~olo •
10 4 t
.
•
I 0 -,
oeQl.
• ......:::: (b)
. | v , Oo° :,.: .... i |." %', ,! I o " ° ° o ° . . . . ° : : 7"..:.
B=2 328T
.. :.::
IO ° ' ~
'°!:
,o1
B=4.656T
,,..-'~,.:!i:i! .: ::....:.
~.
]o ::
>
1
; !!:. ::.:::
•
I0 ~ I0
IO
that its Ta is slightly higher than Ts, as also displayed in Figure 4. The temperature at w h i c h a " k n e e " is observed for s a m p l e A in the R - - T c u r v e , T k ~ , is found to be higher than both Ts and Ta. Analyzing the data for both samples A and B shows that the thermally activated flux f l o w picture can not describe our observations , as exemplified by sample B in Figure 5. By examin/ng the results for sample A suggests that the data can be well described in the context o f the v o r t e x - - s o l i d to v o r t e x - - l i q u i d transition picture proposed by F F H . As suggested by the F F H theory, at I"=, the E - - J data should f o l l o w a power law behavior, E e c jc=+~)/2, where z is the critical exponent. The slope of the linear l o g ( E ) - - l o g ( J ) curve, which determines the exponent z , is then measured. The results at different magnetic fields are displayed in Figure 6. Our restflts indicate t h a t the exponent z is not a universal constant
~
I0'
"'~
(c)
,o-q
]
"
,o .......
"!
0.001
0.01
":'."..
.,!:':::!
:':'"'.
O
~
.
.
i
1
10 "~qI
L ,.,
,°-I
:.." 0
. ."
L..,: .......
"., ".." ....... O.l
•
-
I0
I
tO0
(A/cln z)
" ":':':'...- ":':':'....: • :':':':..- ":':':'.... ' •
"
" • , ,,OoOo
...................
°"°
::;::::
::::::::
:::::::."
•
• . . . . . . . . . . . . . . . . . .
:::::::: o,
B=4.656T
..,
]
"
,'.-.I- :-~.~: -%"-....... .,. ...... , • ~.,.,.,....f • ; :i:i.
I
i
.."
: .....
Q
1
."
, "!;;:!::;::~.. . . :". . Y
J
• oo. o o . ~ W
.
• " / " • " • "//I " " . ". ...... . / A I
O O
....
..t::-.:."
,,,i":
•
. ........."
i::' •
.
'
"'°
-... .............
;
tO " t o*'°
t'~ '
10'
IO =
j/t
10 ~
="
Figure 2: ( a ) E - - J curve, ( b ) ( E / J ) - - J curve, ( c ) ( E / J ) / t "(=-1> vs. J / t ~ , where t ---- [ T - - T s / T.[ for sample A at H-----4. 5 6 5 T.
i
L. 1O=
I
I0 ' ..................................... O.l)(I 1 I),0| f) ]
:........ 1
I0
,_aL.. lf)O
Figure 3 : ( a ) E - - J curve, and ( b ) ( E / J ) - J curve for sample B at H = 2. 328 T.
M.K. Wu et aL / Study of the flux pinning in the YBazCujOy system as a f u n c t i o n of the magnetic field. It maintains a l m o s t constant at fields smaller than 6 Tesla a n d then increases as field is laxger than 6 T. Its v a l u e is f o u n d to be between 3 and 4. 5 , and is smaller compared with the earlier resuits [ ' 1 1 ~ 1 3 " ] . W e take this z value and plot E / J sealed b y IT--Ts/Ts I "('-D versus J sealed by I T - - T , / T s I ~'. This m a k e s t h e data f r o m all the E vs. J c u r v e s a t d i f f e r e n t t e m p e r a t u r e u n d e r c o n s t a n t field f a l l i n t o t w o b r a n c h e s , as s h o w n in F i g u r e 2 ( c ) f o r field a t 4. 656 T. I n f a c t , t h e d a t a at d i f f e r e n t fields also fall i n t o the s a m e t w o branches. T h i s sealing f o r m s t r o n g l y d e m o n s t r a t e d t h e existence of a v o r tex solid ( v o r t e x glass) to v o r t e x liquid m e l t ing b e h a v i o r . 10z
t
. . . . . . . . . . . . . . . . .
I
-I
:::::
O.O00T
::::= 2.3~8T *,*,*~,a 4 . G 5 6 T ~ G.gO4T
]
i
0.00O
0.005]
0.010
0.011
I].012
0.013
0.014
0,015
I/T
Figure 5 : I ~ , R vs. 1 / T o f s a m p l e B at different m a g n e t i c fields. 4,5
;
"1.0
3.5
•
•
\
335
•
•
3.0
. . . . : ::o
2.5
720
-I-
- -
' .......................................
It
60
65
90
7~, Temperature
B0
~5
90
(25
(KI
F i g u r e 4 : B vs. T f o r ( s a m p l e A ) T s , T ~ , T a , TR=0, a n d (sample B ) T ~ , T~=0. A surprising result is t h e value of v , w h i c h is f o u n d to be n e a r 0. 35. It is k n o w n that the m e a n field result gives the value o f v to be 0. 5. T h e o b s e r v a t i o n o f 0. 35 for v is not u n d e r stood at the p r e s e n t time. H o w e v e r , ff we c o n s i d e r the s y s t e m to be 2 - - d i m e n s i o n al i n s t e a d o f 3 - - D , v is t h e n determined w i t h a v a l u e clc¢,e to I. 6 ~ ' o - o ¢ - ' - ~ *h,~ " - - D n a t u r e of the c u p r a t e s m a y need to be considered f o r more detailed analysis. In addit i o n , t h e s u d d e n increase o f the critical e x p o n e n t z a t field near 6 T is likely due to t h e crossover f r o m a 3 - D to 2 ~ D behavior o f the f l u x m o t i o n . O n t h e other h a n d , t h e results for sample
I'l'e~la
I
Figure 6 . M a g n e t i c field dependence of the critical e x p o n e n t z of sample A. B is quite opposite co t h a t of the sample A. The d a t a s e e m t o fit to the picture proposed by. Griessen E l 6 - ] in terms o f a parallel resistor model w h i c h incorporates f l u x - - c r e e p , f l u x - flow a n d a distribution o f a c t i v a t i o n energies. A s h o r t s u m m a r y o f the a b o v e observations i n c l u d e s : 1. T h e v o r t e x solid to v o r t e x liquid transition is observed in the single c r y s t a l which has twins observable under optical microscope (~mnl@ A ) ~ h l : t n o t .in. . .t h e erv~tal wPdch has x ~ -L-- - ¢ microtwins o n l y observable in a high r ~ o l u tion electron microscope. 2. The critical e x p o n e n t z is not a universal constant b u t r a t h e r depends s t r o n g l y on the sample m i c r o s t r u c t u r e . 3. T h e r e possibly exists a 3 - - D to 2 - - D crossover o f the f l u x motion w h e n the applied
336
M.K Wu et at / Study of the flux pinning in the YBa2Cu30ysystem
field exceeds a critical value. F o r t h e present case, the critical field is about 6 T. These resttRs indicate that the detailed flux d y n a m i c in the cuprate superconductors depends on t h e detailed defect structure. I n the context o f t h e v o r t e x - glass transition, the critical e x p o n e n t z can be understood as the measure of the flux p i n n i n g strength. A larger z implies stronger pinning. By comparing the resutts in literatures w h i c h confirms the existence of a glass t r a n s i t i o n , we found that the p i n n i n g strength in our sample A is rather weak a n d in sample B is e v e n weaker. By examination of the m i c r o s t r u c t u r e , our results suggest that twin b o u n d a r y m a y not be an effective defect to pin the f l u x motion. A larger size a n d randomly distributed defect is a much m o r e efficient flux p i m d n g center. ACKNOW1 .F.13~E~[ENTS T h e authors like to t h a n k Prof. T. K. Lee and Prof. N. C. Yeh for their critical comments. REFERENCES 1. K . A . MiilIer, M. Takashige, a n d J. G. Bednorz, Phys. Rev. Left. 5 8 , 1143 ( 1 9 8 7 ) . 2. T . K . W o r t h i n g t o n , W. J. Gallagher, T. R. D i n g e r , and R. L. S a n d s t o r m , in Novel S u p e r c o n d u c t i v i t y , edited by S. A. W o l f a n d V . Z . Kresin ( p l e n u m , N e w Y o r k , 1 9 8 7 ) , p. 781. 3. Y. I y e , T. T a m e g a i , T. S a k a k i b a r a , T. G o t o , N. M i u r a , H. T a k e y a , and H. T a k e i , Physica C, 1 5 3 - - 1 5 5 , 26 (1988).
4. T . T . M . P a l s t r a , B. Battlog, R . B . v a n D o v e r , L . F . S c h n e e m e y e r a n d J. V. Waszczak, Appl. Phys. Left. 5 4 , 763 (1989). 5. P . L . G a m m e l , L . F . S c h n e e m e y e r , J. V. Waszczak a n d D. 3. Bishop, P h y s . Rev. Lett. 6 1 , 1666 ( 1 9 8 8 ) . 6. M. T i n k h a m , P h y s . Rev. L e t t . , 6 1 , 1658 ( 1 9 8 8 ) . 7. D . R . Nelson a n d H . S . S e u n g , P h y s . Rev. B 3 9 , 9 1 5 3 ( 1 9 8 9 ) . 8. S . P . O b u k h o v a n d M. R u b i n s t e i n , Phys. Rev. Lett. 6 5 , 1279 ( 1 9 9 0 ) . 9. M . V . F e i g " [ m a n , V . B . G e s h k e n b e i n , A. I. Larkin a n d V . M . V i n o k u r , P h y s . Rev. Lett. 6 2 , 2 8 5 7 ( 1 9 8 9 ) . 10. D . S . F i s h e r , M. P. A. Fisher a n d D. A. Huse, P h y s . R e v . B 4 3 , 130 ( 1 9 9 1 ) . 11. R . H . K o c h , V. Foglietti a n d M. P. A. Fisher, Phys. R e v . Lett. 6 4 , 2 5 8 6 (1990). 12. P . L . O a m m e l , L . F . S c h n e e m e y e r and D . J . Bishop, Phys. Rev. Lett. 6 6 , 13.
14.
15.
16.
953 ( 1 9 9 1 ) . T . K . W o r t h i n g t o n , E. Oisson, C . S . Nichols, T . M . S h a w and D. R. Clarke, P h y s . Rev. B 4 3 , 1 0 5 3 8 (1991). D . L . Kaiser, F. Holtzberg, M. F. Chisholm a n d T . K . W o r t h i n g t o n , J. Crystal. Growth, 85, 593 ( 1 9 8 7 ) . M. J. Wang, M. K. W u a n d Y . H u a n g , C. H. C h e n , T. L. K u o , to be published. G . R . G r i e s s e n , Physica C , 1 7 5 , 315 (1991).
Physiea C 185-189 (1991) 332-336 North-H011and
Study o f the F l u x Pinning in t h e YBa2CusO~ S y s t e m " M. K. W u ,
M. J. W a n g , a n d C. C. Chi + , Department of Physics and Materials
Science C e n t e r , National T s i n g - - H u a U n i v e r s i t y , T a i w a n , R O C F. H o l t z b e r g , IBM T. J. W a t s o n Research C e n t e r , Yorktown H e i g h t s , N Y
W e h a v e carried out a systematic study of the YBa~Cu30~(YqBC~) single crystals ineluding those grown f r o m ( Y B C ~ ) / A g 2 0 composite. The resistive transition, and the detailed I - - V characteristic in magnetic fields behaves differently f o r crystals grown by different methods. T h e results suggest that the f l u x dynamics in this high Tc oxide is complex a n d depends strongly on the sample conditions. Detailed electrical and magnetic properties in conjunction with the microstructure will be presented and discussed in comparison with the existing theories.
INTRODUCTION Since the discovery of high T c superconductivity, the potential of applications has attracted unprecedented effort to achieve high critical current density in the cuprate superconductors. T h o u g h improvement of the critical currents in materials with specific processing technique has been m a d e , it is still below the requirement for practical application, especially in the materials with bulk form. The lack of the detailed knowledge in the dynamics of the f l u x lines hindered the progress. Mtiller e t al. [1"] first demonstrated the existence of time dependence of the r e m n a n t magnetization at low temperature in the ceramic samples. This led them to propose the importance of the glassy structure in the high T c materials. H o w e v e r , similar effect observed in the single crystal has led others to suggest the significance of the thermally activated flux creep [-2-]. Several n o n - - c o n v e n t i o n a l ol~ervations [ 3 - - 5 ] , such as the resistive transition broadening ira these high Te oxides h a v e generated
numerous attempts I-6--9-] to explain the elfeet. R e c e n t l y , b y considering the r a n d o m disorder, Fisher et al. ( F F H ) E1 0-] predicted the existence of a second order phase transition at finite temperature below which the zero freq u e n c y resistance is zero. Experimental evidence for the existence of this v o r t e x - glass transition has b e e n reported in epitaxial films [11"], in single crystal [12-] and in polycryst~lline YBa2Cu3Or-x[13-]. Nevertheless, a unified picture to satisfactorily explain the observed data is still n o t available. In this w o r k , w e report the detailed measurements o f the I V characteristic of single crystals w h i c h possess different d e f e c t structures. O u r results suggest that depends on the microstructural defects of the sanlple, there exist a phase transition which corresponds to the v o r t e x - solid ( o r glass) to v o r t e x - liquid transition. The detailed I - - V characteristic below the transition temperature strongly depends on the defect structure of the materials.
* W o r k is s u p p o r t e d b y T h e R O C N a t i o n a l S c i e n c e C o u n c i l G r a n t NSC790208M00795. -~- On leave f r o m IBM T. J. W a t s o n Research C e n t e r , Yorktown Heights, NY .0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights rcscrvcd.
M.K. Wu et aL / Study of the flux pinning in the YBa2CuaOysystem E2KP~NTAL Y B C O single crystals g r o w n b y t w o diff e r e n t t e c h n i q u e s were used in this study. One was p r e p a r e d using t h e c o n v e n t i o n a l f l u x g r o w n m e t h o d ( s a m p l e A ) E 14"]. The other was g r o w n f r o m the m i x t u r e o f stoichiometrie YBCO a n d A g O w h e r e A g s e r v e d as low melting f l u x f o r c r y s t a l g r o w t h ( s a m p l e B) E 1 5 ] . The m a j o r d i f f e r e n c e s o f these crystal were t h a t A h a s m u c h larger t w i n w h i c h can be visible u s i n g optical m i c r o s c o p e , while the t w i n spacing i n s a m p l e B is e x t r e m e l y small a n d can be o b s e r v e d o n l y by h i g h resolution electron microscopy. It w a s e s t i m a t e d t h e twin spacing of s a m p l e B is in the order o f ~'~ 50 j~. T y p i cal s a m p l e dimensions w e r e l x l x 0 . 2 rnm 3 with the shortest dimension along c axis were used. T h e electrical c o n t a c t s were m a d e by e v a p o r a t e d Ag pads a n d p l a t i n u m wire were a t t a c h e d w i t h A g - - p a s t e . A f t e r the leads were m o u n t e d , the samples w e r e glued to a thin A l u m i n u m plate. T h e samples were t h e n a n n e a l e d in a f l o w o x y g e n e n v i r o n m e n t at - ~ 500 "C f o r 1 - 2 hours. T h e contact resistances w e r e less t h a n 1 o h m . T h e a l u m i n u m plates w e r e m o u n t e d w i t h t h e r m a l grease on the sample holder m a d e b y copper containing a c a r b o n - glass t h e r m o m e t e r a n d heater in a variable t e m p e r a t u r e insert inside a 9 Testa sup e r c o n d u c t i n g magnet. T h e t e m p e r a t u r e stability w a s f o u n d to be w i t h i n 0. 1 ~ . The t r a n s p o r t m e a s u r e m e n t s w e r e carried out w i t h a Keithley 220 current source and a Keithley 181 v o l t m e t e r . F o r e a c h v o l t a g e m e a s u r e m e n t the c u r r e n t s in both directions were used. To be sure the voltage stability o f the measurem e n t s , l o n g w~tLn~ ~ m e w,as employed. RESULTS AND DISCUSSIONS F i g u r e 1 shows the t e m p e r a t u r e dependence o f t h e resistance f o r samples A and B at fields up to 8 T. T h e zero resistance ( w i t h i n the s y s t e m resolution o f o u r m e a s u r e m e n t ) t e m p e r a t u r e s at zero field are 9 3 . 5 ~ a n d 89
333
8£-005 1
Q
gE-005 • • • • • ~.l R4T
"~O
~.~
• • • •,
6.W~,&T
• --,.....
5.820T 4.6561" 3.492T
,,,,,
1.184T
0.000T
• --..
~
I:~ 2E-005"
.o~
"
, ~
! OE+O00
75
eo
e
'P"''
e5 ~o T e m p e r a t u r e (K)
.......
o5
too
4 E - 0 0 3 - - (b) o I
2 ~
2E-003
e~
IE-O03
OE+O00
70
75
BO
85
90
9,5
I00
Temperature (K)
Figure 1: T e m p e r a t u r e dependence of resistance of crystals A a n d B at fields up to 8 Tesla. " K , respectively. The transition widths are within 1 "K for both samples. Differences in the field b r o a d e n i n g of the resistive transition for the t w o samples were observed. Particul a r l y , the distinct " k n e e " in increasing field is observed in s a m p l e A but not in s a m p l e B~ On the other h a n d , a n obvious tail region which becomes m o r e e v i d e n t is observed in sample B. Figure 2 ( a ) shows the E - - J c u r v e s at 4. 656 T for s a m p l e A. Figure 2 ( b ) is the correspending ( E / J ) - - J curves. F i g u r e s 3 ( a ) a n d 3 ( b ) are the d a t a for sample B b u t at a field of 2. 328 T. It is clear that for s a m p l e A there exists a t e m p e r a t u r e (Ts) at w h i c h the slope of Log(E)--Log(J) is exactly linear. For temperature below T,, a downward curvature is evident at all c u r r e n t s , while a b o v e T s, the curvature is u p w a r d and becomes linear. The H vs. T, c u r v e , as shown in figure 4, is found to fall on the same c u r v e as H v s . T~=0
334
M.K Wu et aL I Study.of the flux pirming in the YBa2Cu30~ system
where Ta-o is the temperature at w h i c h resistance is zero ( w i t h i n the resolution of o u r measurements).
For sample B,
t h e data also
show clearly at a characteristic t e m p e r a t u r e ( T ~ ) where the curve changes from n o n linear to linear in E vs. J. Plotting H vs. T J , as shown in figure 4 , shows that this TJ coincides with the temperature at which Tg-o. A similar plot for the data o f sample A shows t O "~
,,V,;l!~
(a) 10 "="
E
•
0 ~
".
•
I0 "
o°°°
0.
o ooo
•
r~
o°.. oo°.
o0
•
10 B=4.656T L0 I0
1(1 ~
.I
(A/cm z)
! ! ! !
1o "t :
•
1"
:=sallltd •
=o•i,oP e °
• e o~olo •
10 4 t
.
•
I 0 -,
oeQl.
• ......:::: (b)
. | v , Oo° :,.: .... i |." %', ,! I o " ° ° o ° . . . . ° : : 7"..:.
B=2 328T
.. :.::
IO ° ' ~
'°!:
,o1
B=4.656T
,,..-'~,.:!i:i! .: ::....:.
~.
]o ::
>
1
; !!:. ::.:::
•
I0 ~ I0
IO
that its Ta is slightly higher than Ts, as also displayed in Figure 4. The temperature at w h i c h a " k n e e " is observed for s a m p l e A in the R - - T c u r v e , T k ~ , is found to be higher than both Ts and Ta. Analyzing the data for both samples A and B shows that the thermally activated flux f l o w picture can not describe our observations , as exemplified by sample B in Figure 5. By examin/ng the results for sample A suggests that the data can be well described in the context o f the v o r t e x - - s o l i d to v o r t e x - - l i q u i d transition picture proposed by F F H . As suggested by the F F H theory, at I"=, the E - - J data should f o l l o w a power law behavior, E e c jc=+~)/2, where z is the critical exponent. The slope of the linear l o g ( E ) - - l o g ( J ) curve, which determines the exponent z , is then measured. The results at different magnetic fields are displayed in Figure 6. Our restflts indicate t h a t the exponent z is not a universal constant
~
I0'
"'~
(c)
,o-q
]
"
,o .......
"!
0.001
0.01
":'."..
.,!:':::!
:':'"'.
O
~
.
.
i
1
10 "~qI
L ,.,
,°-I
:.." 0
. ."
L..,: .......
"., ".." ....... O.l
•
-
I0
I
tO0
(A/cln z)
" ":':':'...- ":':':'....: • :':':':..- ":':':'.... ' •
"
" • , ,,OoOo
...................
°"°
::;::::
::::::::
:::::::."
•
• . . . . . . . . . . . . . . . . . .
:::::::: o,
B=4.656T
..,
]
"
,'.-.I- :-~.~: -%"-....... .,. ...... , • ~.,.,.,....f • ; :i:i.
I
i
.."
: .....
Q
1
."
, "!;;:!::;::~.. . . :". . Y
J
• oo. o o . ~ W
.
• " / " • " • "//I " " . ". ...... . / A I
O O
....
..t::-.:."
,,,i":
•
. ........."
i::' •
.
'
"'°
-... .............
;
tO " t o*'°
t'~ '
10'
IO =
j/t
10 ~
="
Figure 2: ( a ) E - - J curve, ( b ) ( E / J ) - - J curve, ( c ) ( E / J ) / t "(=-1> vs. J / t ~ , where t ---- [ T - - T s / T.[ for sample A at H-----4. 5 6 5 T.
i
L. 1O=
I
I0 ' ..................................... O.l)(I 1 I),0| f) ]
:........ 1
I0
,_aL.. lf)O
Figure 3 : ( a ) E - - J curve, and ( b ) ( E / J ) - J curve for sample B at H = 2. 328 T.
M.K. Wu et aL / Study of the flux pinning in the YBazCujOy system as a f u n c t i o n of the magnetic field. It maintains a l m o s t constant at fields smaller than 6 Tesla a n d then increases as field is laxger than 6 T. Its v a l u e is f o u n d to be between 3 and 4. 5 , and is smaller compared with the earlier resuits [ ' 1 1 ~ 1 3 " ] . W e take this z value and plot E / J sealed b y IT--Ts/Ts I "('-D versus J sealed by I T - - T , / T s I ~'. This m a k e s t h e data f r o m all the E vs. J c u r v e s a t d i f f e r e n t t e m p e r a t u r e u n d e r c o n s t a n t field f a l l i n t o t w o b r a n c h e s , as s h o w n in F i g u r e 2 ( c ) f o r field a t 4. 656 T. I n f a c t , t h e d a t a at d i f f e r e n t fields also fall i n t o the s a m e t w o branches. T h i s sealing f o r m s t r o n g l y d e m o n s t r a t e d t h e existence of a v o r tex solid ( v o r t e x glass) to v o r t e x liquid m e l t ing b e h a v i o r . 10z
t
. . . . . . . . . . . . . . . . .
I
-I
:::::
O.O00T
::::= 2.3~8T *,*,*~,a 4 . G 5 6 T ~ G.gO4T
]
i
0.00O
0.005]
0.010
0.011
I].012
0.013
0.014
0,015
I/T
Figure 5 : I ~ , R vs. 1 / T o f s a m p l e B at different m a g n e t i c fields. 4,5
;
"1.0
3.5
•
•
\
335
•
•
3.0
. . . . : ::o
2.5
720
-I-
- -
' .......................................
It
60
65
90
7~, Temperature
B0
~5
90
(25
(KI
F i g u r e 4 : B vs. T f o r ( s a m p l e A ) T s , T ~ , T a , TR=0, a n d (sample B ) T ~ , T~=0. A surprising result is t h e value of v , w h i c h is f o u n d to be n e a r 0. 35. It is k n o w n that the m e a n field result gives the value o f v to be 0. 5. T h e o b s e r v a t i o n o f 0. 35 for v is not u n d e r stood at the p r e s e n t time. H o w e v e r , ff we c o n s i d e r the s y s t e m to be 2 - - d i m e n s i o n al i n s t e a d o f 3 - - D , v is t h e n determined w i t h a v a l u e clc¢,e to I. 6 ~ ' o - o ¢ - ' - ~ *h,~ " - - D n a t u r e of the c u p r a t e s m a y need to be considered f o r more detailed analysis. In addit i o n , t h e s u d d e n increase o f the critical e x p o n e n t z a t field near 6 T is likely due to t h e crossover f r o m a 3 - D to 2 ~ D behavior o f the f l u x m o t i o n . O n t h e other h a n d , t h e results for sample
I'l'e~la
I
Figure 6 . M a g n e t i c field dependence of the critical e x p o n e n t z of sample A. B is quite opposite co t h a t of the sample A. The d a t a s e e m t o fit to the picture proposed by. Griessen E l 6 - ] in terms o f a parallel resistor model w h i c h incorporates f l u x - - c r e e p , f l u x - flow a n d a distribution o f a c t i v a t i o n energies. A s h o r t s u m m a r y o f the a b o v e observations i n c l u d e s : 1. T h e v o r t e x solid to v o r t e x liquid transition is observed in the single c r y s t a l which has twins observable under optical microscope (~mnl@ A ) ~ h l : t n o t .in. . .t h e erv~tal wPdch has x ~ -L-- - ¢ microtwins o n l y observable in a high r ~ o l u tion electron microscope. 2. The critical e x p o n e n t z is not a universal constant b u t r a t h e r depends s t r o n g l y on the sample m i c r o s t r u c t u r e . 3. T h e r e possibly exists a 3 - - D to 2 - - D crossover o f the f l u x motion w h e n the applied
336
M.K Wu et at / Study of the flux pinning in the YBa2Cu30ysystem
field exceeds a critical value. F o r t h e present case, the critical field is about 6 T. These resttRs indicate that the detailed flux d y n a m i c in the cuprate superconductors depends on t h e detailed defect structure. I n the context o f t h e v o r t e x - glass transition, the critical e x p o n e n t z can be understood as the measure of the flux p i n n i n g strength. A larger z implies stronger pinning. By comparing the resutts in literatures w h i c h confirms the existence of a glass t r a n s i t i o n , we found that the p i n n i n g strength in our sample A is rather weak a n d in sample B is e v e n weaker. By examination of the m i c r o s t r u c t u r e , our results suggest that twin b o u n d a r y m a y not be an effective defect to pin the f l u x motion. A larger size a n d randomly distributed defect is a much m o r e efficient flux p i m d n g center. ACKNOW1 .F.13~E~[ENTS T h e authors like to t h a n k Prof. T. K. Lee and Prof. N. C. Yeh for their critical comments. REFERENCES 1. K . A . MiilIer, M. Takashige, a n d J. G. Bednorz, Phys. Rev. Left. 5 8 , 1143 ( 1 9 8 7 ) . 2. T . K . W o r t h i n g t o n , W. J. Gallagher, T. R. D i n g e r , and R. L. S a n d s t o r m , in Novel S u p e r c o n d u c t i v i t y , edited by S. A. W o l f a n d V . Z . Kresin ( p l e n u m , N e w Y o r k , 1 9 8 7 ) , p. 781. 3. Y. I y e , T. T a m e g a i , T. S a k a k i b a r a , T. G o t o , N. M i u r a , H. T a k e y a , and H. T a k e i , Physica C, 1 5 3 - - 1 5 5 , 26 (1988).
4. T . T . M . P a l s t r a , B. Battlog, R . B . v a n D o v e r , L . F . S c h n e e m e y e r a n d J. V. Waszczak, Appl. Phys. Left. 5 4 , 763 (1989). 5. P . L . G a m m e l , L . F . S c h n e e m e y e r , J. V. Waszczak a n d D. 3. Bishop, P h y s . Rev. Lett. 6 1 , 1666 ( 1 9 8 8 ) . 6. M. T i n k h a m , P h y s . Rev. L e t t . , 6 1 , 1658 ( 1 9 8 8 ) . 7. D . R . Nelson a n d H . S . S e u n g , P h y s . Rev. B 3 9 , 9 1 5 3 ( 1 9 8 9 ) . 8. S . P . O b u k h o v a n d M. R u b i n s t e i n , Phys. Rev. Lett. 6 5 , 1279 ( 1 9 9 0 ) . 9. M . V . F e i g " [ m a n , V . B . G e s h k e n b e i n , A. I. Larkin a n d V . M . V i n o k u r , P h y s . Rev. Lett. 6 2 , 2 8 5 7 ( 1 9 8 9 ) . 10. D . S . F i s h e r , M. P. A. Fisher a n d D. A. Huse, P h y s . R e v . B 4 3 , 130 ( 1 9 9 1 ) . 11. R . H . K o c h , V. Foglietti a n d M. P. A. Fisher, Phys. R e v . Lett. 6 4 , 2 5 8 6 (1990). 12. P . L . O a m m e l , L . F . S c h n e e m e y e r and D . J . Bishop, Phys. Rev. Lett. 6 6 , 13.
14.
15.
16.
953 ( 1 9 9 1 ) . T . K . W o r t h i n g t o n , E. Oisson, C . S . Nichols, T . M . S h a w and D. R. Clarke, P h y s . Rev. B 4 3 , 1 0 5 3 8 (1991). D . L . Kaiser, F. Holtzberg, M. F. Chisholm a n d T . K . W o r t h i n g t o n , J. Crystal. Growth, 85, 593 ( 1 9 8 7 ) . M. J. Wang, M. K. W u a n d Y . H u a n g , C. H. C h e n , T. L. K u o , to be published. G . R . G r i e s s e n , Physica C , 1 7 5 , 315 (1991).