YxPr1−xBa2Cu3Oy superlattices under magnetic fields using an off-axis rf magnetron sputtering technique

PHYSICA ELSEVIER

Physica C 281 (1997)325-334

Characterization and transport properties of YBa2fu3Oy/YxPrl_xBa2Cu30ysuperlattices under magnetic fields using an off-axis rf magnetron sputtering technique H.C. Yang a,,, L.M. Wang a, H.E. Horng b Department of Physics, National Taiwan University, Taipei, Taiwan b Department of Physics National Taiwan Normal University, Taipei, Taiwan

Received 26 September 1996; revised 19 May 1997; accepted 28 May 1997

Abstract We report here on the growth and transport properties of YBa2Cu3Ov/YxPrj_xBazCu30 V superlattices. The YBa2Cu3Oy/YxPr I _xBa2Cu3Oy [(YBCO/PBCO) for x = 0 and (YBCO/YPBCO) for x = 0.5] superlattices were grown

in situ in an off-axis radio-frequency magnetron sputtering system. The superlattice structure of the samples was characterized by powder X-ray diffraction. Appearance of satellite diffraction peaks near the main (001) peaks confirms the superlattice structure. Resistivity measurements carried out under magnetic fields reveal thermal activated behaviour. Hall coefficients, Rn, in mixed state show sign reversal and a diminishing of R n is observed when the thickness of the YPBCO layer is increased or the thickness of the YBCO layer is decreased. Angular dependence of the critical current density, Jc, in YBCO/PBCO superlattices shows a decoupled behaviour in the flux line dynamics as the coupling strength of the YBCO layer weakens. © 1997 Elsevier Science B.V.

1. Introduction YBazCu3Ov/YxPrl_xBa2Cu3Oy ( x = 1 and 0.5) superlattices have attracted considerable interest in recent times. Two major methods that have widely been used to grow Y B C O / Y P B C O superlattices are the laser ablation [1] and the DC magnetron sputtering [2] technique. In this paper we report on the in situ growth of YBa2Cu3Oy/YxPr 1 x B a 2 C u 3 0 y su-

* Corresponding author.

perlattices by off-axis radio-frequency (rf) magnetron sputtering techniques [3]. We report on the growth and transport properties of YBazCu30~./Y ~ Prl_xBa2Cu3Oy superlattices with x = 0 ( Y B C O / P B C O ) and x = 0.5 ( Y B C O / Y P B C O ) . The resistivity under magnetic fields shows thermal activated behaviour. Hall coefficients R H in mixed state show sign reversal and a diminishing of R H is observed when the thickness of the YPBCO layer is increased or the thickness of the YBCO layer is decreased. Angular dependence of the critical current density, Jc, shows a dimensional crossover as the coupling strength of the YBCO layer weakens.

0921-4534/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0921-4534(97)01466-4

326

H. C. Yang et al. / Physica C 281 (1997) 325-334

2. Experimental

YBa2Cu3Oy/YxPr I _xBazCu3Oy superlattices were grown in situ using a radio-frequency (rf) magnetron sputtering system. Two sputtering guns (US guns II) were mounted face to face in the vacuum chamber as shown in Fig. 1. The magnetic structure of the sputtering guns was similar. The sputtering targets were YBazCu3Oy and YxPrl xBazCu30 ~, oxide compounds. The substrates were SrTiO3(001), which were rotated during film deposition. The samples were heated to about 720°C using a radiation heating method. A detailed description of the optimum sputtering conditions for deposition of YBCO films has been reported in [3]. To our knowledge this is the first preparation of high-Tc superlattices with an off-axis radio-frequency magnetron sputtering technique. During the growth of YBa 2CU 3 0 y / g x Pr I _ ~Ba 2Cu 30.,, superlattices, a personal computer was used to automatically control the output of the rf power supply (RFX-600 generator and ATX-600 tuner, Advanced Energy Industries) and the shutters of the sputtering guns, so that the YBCO and Y~Pr 1_xBa2Cu3Oy layers can be alternately deposited onto a rotated SrTiO 3 substrate. It has been shown [4] that a buffer PBCO layer is useful in improving the superconductivity o f the YBCO films. A buffer PBCO film of 960 A in thickness was therefore grown onto the SrTiO3(001) substrate prior to the growth of Y B C O / P B C O superlattices. The growth rate of the YBa2Cu3Os and YxPrl_~Ba2Cu30 s layers was calibrated by a Dektak (Model 3050 ST) surface roughness profile. The structure of

Y B ~ P B C O

T~get

0.01 cm

\,

'\

gold dots Fig. 2. Geometry for Hall coefficient and resistivity measurements. the superlattice was characterized by the X-ray diffraction peaks. For the measurements of the transport properties, YBa2Cu3Oy/YxPr ~ xBa2Cu30 s superlattices were patterned to a 2-ram long, 100-1xm wide bridge containing Hall and resistivity terminals as shown in Fig. 2. Six gold-pads were evaporated onto the contact areas to ensure good electrical contact for the transverse Hall and longitudinal resistivity measurements using the standard direct current methods. Hall measurements were taken under magnetic fields up to 5 T parallel to the crystal c-axis. The current density in the Hall and resistivity measurements was 1 × l0 4 A / c m 2. For the measurements of angular dependence of the critical current density, the samples were mounted on a sample holder which was rotated by a computer-controlled step motor. The angle the magnetic field subtends with respect to the crystal c-axis of the sample can thus be rotated from 0 to 360 ° through the step motor.

3. Results and discussion

3.1. X-ray diffraction

gun

--~

water-cooled

.7

~

stainless-steelshield

Ilk

/

Ll ~

shutter rotatable sample bolder

computer-controlled

Fig. 1. Schematicdiagramof the off-axisradio-frequencysputtering systemusedfor synthesizingYBCO/PBCOsuperlattices.

Fig. 3 shows the typical power X-ray diffraction patterns for an YBCO film and an Y B C O / P B C O (96 ,~/60 A)10 superlattice. The subscript 10 refers to the number of modulation layers in the superlattice. In the X-ray 0-2 0 scan for YBCO films only (00L) peaks were observed, where L = 1, 2, 3 . . . . indicating an orientation of the YBCO films parallel

H.C. Yang et el. / Physica C 281 (1997) 325-334

(a)

Y B C O film

2

o.9. r

I

1

10

Q

8 I' "" ~

20

'1'

J

30

A

1

50

40

3.2. Transport properties I

o

E (b)

1 I

'

# ' 20

I

'

l

30 40 2 O (degree)

i tO

YBCO/PBCO( 96 ~,/60 ~) d = 156~

10

similar to those observed by Fullerton et el. [5]. In Fig. 4 we show the surface morphology of a typical Y B a 2 C u 3 O y / Y P B C O (120 A / 1 2 0 A) 6 superlattice probed by the scanning force microscope. The scanned area was 1.0 × 1.0 Ixm. These films have root-mean-square surface roughness of about 13.3 in the scanned area; this is close to the root-meansquare surface roughness of the SrTiO3(001) substrate.

60

2 e (degree)

0

327

'

I 50

' 60

Fig. 3. Powder X-ray diffraction pattern of (a) YBCO film and (b)

YBCO/PBCO (96 .~/60 A)10 superlattice.

to the c-axis. The satellite peaks due to the Y B C O / P B C O superlattice structure are apparent around the (00L) peaks as indicated by the arrows. This can be clearly seen in the inset which shows satellite diffraction peaks near the main (001) peak. The presence of the satellite peaks on both sides of the main peaks confirms that the periodic structure of Y B C O / P B C O films was achieved. The modulation length, d = d I + d2, where d I and d 2 are the thicknesses of the YBCO and PBCO layers, can be calculated from separation of the two successive peaks, i and i + 1, using the relation d = ( A / 2 ) [ 1 / ( s i n O i - sin0j)], where A is the wavelength of the X-ray (A = 1.542 A). The modulation length thus obtained was consistent with the calibrated thickness obtained from the Dektak surface profile. Fullerton et al. [5] have refined the structure of Y B a 2 C u 3 O y / Y x P r l _ x B a 2 C u 3 0 y and YBa2Cu3Oy/ G d B a 2 C u 3 0 ~, high-T~ superlattices and they have shown that the layer thickness fluctuations, interdiffusion and strains are present in superlattices. The powder X-ray data show features in satellite peaks

Fig. 5 shows the temperature dependence of resistivity for two series of Y B C O / P B C O superlattices. One series consists of samples with fixed thickness of the PBCO layer (48 A thick) and a varying thickness of the YBCO layer, whereas the other series consists of samples with a YBCO layer of fixed thickness (36 A thick) and a varying thickness of the PBCO layer. A linear temperature dependence of resistivity was observed for YBCO films, which exhibit a resistivity of 308 txl~ cm at 300 K and was extrapolated to zero at zero temperature. The resistivity ratio R(300 K ) / R ( 1 0 0 K ) ~ 3. The resistance curves with high temperature 'turnovers' were observed in some of the samples (samples with a thicker PBCO layer shown in Fig. 5b). As noted by Fullerton et el. [5] the layer thickness fluctuations, interdiffusion, and strains were present in superlat-

200 A

'

'

1000

A

Fig. 4. The surface morphology of a typical YBa2Cu 3O,/YPBCO (120 A/120 A)6 superlattice. The scanned area was 1.5 × 1.5 p~m.

328

H.C. Yang et aL /Physica C 281 (1997) 325-334

I

I

I

YBCO / PBCO a. (120 A/48 A)

6

tices. These factors affected the transport properties of the samples. However, the 'turnover' features in resistance occur only in superlattices with a thicker PBCO layer. We believe the 'turnover' features are mainly due to the effects of layer thickness fluctuations. The transport parameters for the YBCO/PBCO superlattices are presented in Table 1. These parameters were calculated based on the total thickness of YBCO layoers. For a fixed thickness of the PBCO layer (60 A thick), a systematic variation in resistivity, Hall coefficient, carrier concentration, To(50%), and mean free path, 1(100 K), is observed within the experimental errors as the thickness of the YBCO layer is decreased. On the other hand, for a fixed thickness of the YBCO layer (36 ~, thick), there is a systematic increase of the resistivity, Hall coefficient and a systematic decrease of carrier concentration and To(50%), as the thickness of the PBCO layer is increased. Fig. 6a,b show the Arrhenius plots of resistivity as a function of 1/T for a YBCO/PBCO (60 ,~/48 •~)16 superlattice in magnetic fields of different magnitudes, parallel to the crystal c-axis and ab-plane respectively. The resistive transition reveals a linear behaviour in the Arrhenius plot and follows a relation given by p(T) = P0exp(- Uo/KBT). The linear region extends over four orders of magnitude (10 -9 to 10 -5 l'l cm), indicating that the dissipation is a thermally activated behaviour which results from the activation of the flux units. Such thermally activated

I n

b. (96 A/48 ,~)

--

0

T

_...._.e

e. (60 A/48 ~. )

~

d. (48 A 148 .~)

d

J ~

e. (36 ~/48~ , ~ j

o

i /]]F 0 , 10

(a)

[

I

,

,

I ,

B=0T

YBCO / PBCO a. ( 3 6 8

A

124 ~. )

b. ( 3 6 . ~ / 4 8 A ) C. ( 3 6 ~ / 7 2 A) d. ( 3 6

6

~

~

d

e

~x 4 "~

o_ 2-

0

a YBCO

"

b)

i

100

,

i

200

'

i

300

T (K) Fig. 5. Temperature dependence of resistivity for two series of YBCO/PBCO superlattices. (a)(120 A/48 A)s, (96 A/48 A)m, (60 ~,/48 '~),6 ' (48 A/48 A)20 and (36o A/48 A)~ 7' ando (b) (36 o o o o o A/24 A)27, (36 A/48 A)27, (36 A/60 A)27, (36 A/72 A)27 and (36 A/96 A)27.

Table 1 Resistivity, /9, Hall coefficients, RH, carrier concentration, n H, Tc(50%) and mean free path, l, for a series of (YBCO/PBCO), superlattices Sample

YBCO film (96/60)~0 (60/60)t6 (48/60)20 (36/60)27 (36/24)27 (36/48)27 (36/60)27 (36/72)27 (36/96)27

p(200 K)

p(200 K)

Rn(200 K)

R. (200K)

nil(200 K)

nn(200K)

l(T~)

(Ixf/cm)

p(100 K)

(10 -3 cm3/C)

RH(100K)

(l/cell)

nH(100K)

(A)

213.65 256.14 439 421 673.8 294.6 460.47 673.79 634.46 665.27

2.09 1.96 1.71 1.59 1.62 1.72 1.64 1.62 1.54 1.59

1.107 0.963 1.595 1.825 1.927 1.622 1.845 1.927 2.612 2.968

0.573 0.544 0.769 0.550 0.563 0.558 0.572 0.563 0.587 0.617

0.977 1.123 0.677 0.594 0.561 0.666 0.585 0.561 0.414 0.364

1.729 1.838 1.697 1.822 1.77 1.79 1.746 1.77 1.703 1.618

59.3 32.89 19.34 22.49 11.28 38.43 20.95 11.28 15.04 15.54

The subscript n refers to the number of modulation layers.

l/Co

4.23 2.46 1.45 1.69 0.84 2.89 1.69 0.84 1.13 1.17

329

H.C. Yang et al. / Physica C 281 (1997) 325-334 10-3

i

'

I

I

r

I

YBCO/PBCO (60 .~/48 ~)

~

1 0 -4

?

I

i

~

10 4

(a)

is

,~x 10# o o Oo%0°0°0o0

.....

0.ST 1 0 -e

o

10 4

°o°°°~°°o o

o

mo

o

oo

I

I

I

I

I

I

0.012

0.014

0.016

0.018

0.020

0.022

,

]

YBCO/PBCO (60 ,~/48 ~) H 1 c-axis

lO-S

1 0 -r

magnitude of the exponent /3 is 0.82, 0.59, 0.59, 0.58 and 0.55 respectively for (120 A/48 A)12, (96 ~ / 4 8 ~)~0, (60 A-/48 A)16, (48 A/48 '~)20 and (36 A / 4 8 A)27 superlattices. The present observation of thermally activated behaviour in mixed state is consistent with the reported data [8]. Fig. 8a shows the thickness dependence of the pinning energy for a series of YBCO/PBCO superlattices with a fixed PBCO thickness (60 A thick) and increasing thickness of the YBCO layer. The applied magnetic fields were 0.5, 1, 2, 3, 4 and 5 T. The pinning energy can be seen to increase linearly as the thickness of the YBCO layer is increased. Fig. 8b shows variations in the pinning energy as a function of thickness of the YBCO/PBCO superlattices with a fixed thickness of the YBCO layer and

106

IL

(3 (')

(a)

0

t0 4

~"

t 1 0 "g

~'

(b)

,

,

,

,

0.012

0.014

0.016

0.018

,

0.020

g

,

0.022

1/I" (g "1)

Fig. 6. Arrhenius plot of resistivity as a function of 1/T for an YBCO/PBCO (60 i / 4 8 i)16 superlattice in magnetic fields parallel to (a) the crystal c-axis and (b) the ab-plane. The increment in the magnetic field for each curve is 0.5 T.

O

~'

•

0

O

~

0 • •

•

2

10 3

0 •

YBCO 1 (120 A/48 A)I

o

( m .~/48 ~.)

©o

t

OQO

°

• o

°

;.;..

°00

•

•

0 &A

|

o (soM48 ~) |

L'. (ss ~,'4e~.)J t02

I

t

I

H II c - a x i s

I I

i

i

i

i

1o 0

lO"l

101

It if) I0 4

flux motion was proposed by Anderson [6] and Kim [7]. The slopes of the curves in Fig. 6 can be used to calculate the activation energy. The activation energy Uab under a magnetic field of 1 T parallel to the ab-plane was thus found to be 1 X 105 K, much greater than Uc = 6 × 103 K under the same magnitude of magnetic field parallel to the crystal c-axis. The magnetic field dependence of the activation energy for a series of YBCO/PBCO samples is shown in Fig. 7. The magnitude of the superlattices was found to be magnetic field dependent with U(H) proportional to H e. The magnitude of the exponent fl is 0.74, 0.75, 0.73, 0.67 and 0.59 respectively for (120 A/048 A)8, (96 ,~/48 "~)~0, (60 ,~/48 A)16, (48 A / 4 8 A)20 and (36 A / 4 8 A)27 superlattices and 0.70 for a YBa2Cu3Oy film (960 A thick). The

(b)

H//c-axis G

2

~

10-1

t

t

t

~

I

too

o°%

t

I

t

1111

n if) Fig. 7. The magnetic field dependence of the activation energies Uc (Uc is the pinning energy for a field parallel to the crystal c-axis) for a series of YBCO/PBCO superlattices. (a) (120 i / 4 8 i ) s, (96 1/48 1),o, (60 i / 4 8 1),6, (48 i / 4 8 i)2o and (36 .~/48 i)27, and (b) (36 i / 2 4 i)27, (36 i / 4 8 i)27, (36 i / 6 0 •~)27 and (36 1/96 1)27.

330

14.C. Yang et al. / Physica C 281 (1997) 325-334

increasing thickness of the PBCO layer. The pinning energy can be seen to decrease monotonically with increasing thickness of the PBCO layer. Fig. 9a shows variations in the normalized Hall coefficients RH(T)/RH(IO0 K) as a function of temperature for a series of Y B C O / P B C O superlatotices with a fixed thickness of the YBCO layer (48 A thick) and a varying thickness of the PBCO layer. Negative Hall coefficients were observed and the negative Hall coefficients decreased as the thickness of the PBCO layer increased. Variations in normalized Hall coefficients RH(T)/RH(IO0 K) as a function of temperature for a series of YBCO~/PBCO superlattices with a fixed PBCO layer (60 A thick) and a decreasing thickness of the YBCO layer are shown in Fig. 9b. A systematic reduction of the negative Hall coefficient was observed as the thickness of the YBCO layer decreased. For the Y B C O / P B C O superlattices with a fixed thickness of the YBCO layer (48 A thick) and a varying thickness of the PBCO layer, the resistivity increases with increasing thickness of the PBCO layer (thereby increasing the disorder of the film), whereas the pinning energy reduces as the thickness of the PBCO layer increases. However, for Y B C O / P B C O superlattices with a fixed PBCO layer (60 A thick) and decreasing thickness of the YBCO layer, the mixed state Hall coefficients show similar features, i.e. the magnitude of the negative Hall coefficient diminishes for superlattices with weaker pinning energy and higher disorder. Budhani et al. [9] have investigated the effects of Ag + irradiation on flux pinning energy and negative Hall coefficients in T1BaCaCuOy thin films and they have reported that Ag + irradiation enhances the flux pinning energy in the low temperature range. A reduction in the magnitude of the negative Hall coefficient was also observed when the dose of Ag + irradiation was increased. In the region where the negative Hall coefficient diminished, the resistivity was observed to increase. For the Y B C O / P B C O superlattices either with a YBCO layer of fixed thickness and an increasing thickness of the PBCO layer or with a fixed thickness of the PBCO layer and a decreasing thickness of the YBCO layer, the flux pinning force was observed to decrease along with a systematic diminishing of the negative Hall coefficient.

35

i

,

I

,

dpBCO

30

I

I

60 ~

o

H II c - a x i s ,'"

--, ,¢ 25

.,,

(a)

% 20 T'0 "

15 O

D

10

I

,

I

40

-

'

60

I

I

80

100

d Y B C 0 (A)

10

i

F

, d y B C O = 4 8 ~,

(b)

' H//c-axis

8 'v"

[~]'

%6 '

x o4

", Q

6,

2

"'.[3

": : z-?-_-o5- . . . . . . ,, ~ I

I

20

40

I

I

60 80 dpBCO (•)

I

100 120

Fig. 8. (a) Thickness dependence of the pinning energy for Y B C O / P B C O superlattices with a fixed PBCO thickness of 60 ,~ and increasing thickness of the YBCO layer. The applied magnetic fields were 0.5, 1, 2, 3, 4 and 5 T. (b) Variations in the pinning energy as a function of thickness of the Y B C O / P B C O superlattices with a YBCO layer of fixed thickness and increasing thickness of the PBCO layer. The applied magnetic fields were l, 2, 3, 4 and 5 T.

It has been suggested [10,11 ] that the amplitude of the sign reversal depends on the sample microstructure through the ratio, I/Co, where l is the mean free path and SCo is the BCS coherence length. Recently, Martin et al. [12] have measured the longitudinal and transverse resistivities of RBazCu30 ~, (R = Y, Ho, Eu) to study the effects of the l/Co on the Hall anomaly. They found that the Hall anomaly depends on the microstructure factor l/Eo. For samples with l/Co = 1.7 and 2.9, which falls in the region 0.5 < l/Eo < 4 . 5 where the Hall anomaly should have enough amplitude to be observed, the Hall anomaly was observed and a scaling behaviour exits between Pxx and p~y. The effects of pinning on the Hall

H.C. Yang et al. /Physica C 281 (1997) 325-334

anomaly and scaling behaviours in YBCO/PBCO superlattices have been reported and discussed by Wang et al. [13]. A systematic reduction of the negative Hall coefficient was observed as the thickness of the YBCO layer decreased for a series of YBCO/PBCO superlattices with a fixed PBCO layer (60 A thick) (Fig. 9). These superlattices have a ratio l / ~ o which varied from 4.23, 2.46, 1.45, 1.69 and 0.84 for YBCO, (96 ~,/60 ,~)~o, (60 ,~/60 "~)16' (48 ,~/60 '~)2o and (36 ,~/60 A)27 superlattices, respectively (Table 1). The YBCO film which has the largest value of l / ~ o = 4.23, has the largest sign reversal amplitude, while the (36 ,~/60 ,~)27 superlattice which has the smallest value of 1 / ~ o = 0.84, the sign

331

anomaly diminishes. The amplitude of sign reversal amplitude clearly depends on the value of l / ~ o. At the present stage, various kinds of theoretical interpretations have been proposed to explain the anomaly of sign reversal of the Hall coefficient. The anomalous Hall coefficients probably result from the following effects: the thermomagnetic effect in mixed state by Freimuth et al. [14], the back flow current effect due to pinning centres by Wang and Ding [15], the unusual Seeback effect by Chen and Yang [16], the accumulation of charge driven by induced electromagnetic force by Horng et al. [17], the flux flow model proposed by Hagen et al. [6], the contribution of the vortices that lies parallel to the CuO 2 layer proposed by Hams et al. [18], a consequence of the

))

(a) 2

i

i

i

i

,

,

0 Q

,T.

~: O-

o-

YBCO/PBCO (3B A/BOA)

YBCO/PBCO (48 A/96 A)

E: -2

+

8 I == d

Z

YBCO/PBCO (48 A/60 k)

YBCO/PBCO (48 AABOA)

-II

2

I

I

YBCO/PBCO (4B A/48 A)

@

t

~

I

[

_

A)@~~

YBCO/PBCO

)

(60 M60

~: o-

o-

&,

?

l

l

I

I

== -2

@

E:

2

1

YBCO/PBCO (48 At24 .~)

..

@ ~g

Z,

8

-1 I

1-

1

YBCO/PBCO (96 A,'60 A)

I

I

I

-~

-1.

d

-i -~ I

I

I

1

I

I

I

-

YBCO

YBCO -1

-1-

-3 -

-3I

50

~

I

60

'

I

I

70 80 T (K)

I

'

90

1O0

70

80

90

100

T (K)

Fig. 9. Variation in normalized RH(T)/RH(IO0 K) as a function of temperature for a series of YBCO/PBCO superlattices. (a) The thickness of the YBCO layer is fixed (48 ,~) and the thickness of the PBCO layer is varied, and (b) a fixed PBCO layer (60 A) and a varying thickness of the YBCO layer. The applied magnetic field is 0.5, 1, 2, 3, 4 and 5 T (from right to left) respectively.

332

H.C. Yang et al./ Physica C 281 (1997) 325-334

time-dependent Ginzburg-Landau theory proposed by Kopnin et al. [19], the Andreev reflection at the interface between the normal core and the superconducting periphery suggested by Meilikhov and Farzetdinova [20], and recently, a unified theory considering the back flow current and the thermofluctuation effect by Wang et al. [21]. Although some mechanisms of negative Hall coefficients of high-Tc superconductors are still controversial, it is generally believed that the negative Hall coefficient is related to the vortex motion and depends on the values of 1/~ o.

-.-.

6

YBCO/PBCO (48A/24A~-

E

= - - Tachiki (o=O0) j % - - Tachiki (0=40 °

T=80 K

,;i/ /{~.

H:o.osT

b ~ 2 v

'9"'~

~

0

~.. "

L

J

J

4

%

T=75 K

--

H=0 ~ T

,

.v

-

Tachiki (o=0' Tachiki (0=40'

-

~ ....... Kes

~2

/- 4.

3.3. Flux line dynamics

,4'

':~

0The motion of flux lines in YBCO/PBCO superlattices depends on the coupling strength between the YBCO layers. If the direction of the magnetic field makes an angle, 0, with respect to the crystal c-axis and if the PBCO layer completely decouples the coupling of the YBCO layers, then the component of the magnetic field parallel to the layers can not induce strong shielding current and therefore has no influence on the transport properties. The magnetic field dependence of the transport current should therefore scale with the component of the magnetic field perpendicular to the layers which was pointed out by Kes [22]. For the decoupled superlattices, one can expect the Jc(0) behaviour as

Jc( O) = J~( UcosO), where H is the applied magnetic field. For coupled superconductors, Tachiki and Takahashi [23] have considered the effects of intrinsic and extrinsic pinning centres (defects etc.) and obtained the angular dependence of the critical current density, J¢(O). Jc(O) equals the minimum value among Jc.ab and J~x/IcosOI 1/2 i.e., Jc( 0 ) = minimum[ Jc,a,5,( Jc.c/lc°s 011/2 )], where Jc,,b and Jc,,. are the critical current densities when the magnetic field is parallel to the ab-plane and to the crystal c-axis respectively. We measured J~(O) as a function of 0 in a fixed magnetic field and Jc(H) as a function of magnetic fields at elevated temperatures with the magnetic field perpendicular to the plane. 0 is the angle that the magnetic field subtends with respect to the crys-

8 T=70 K

E"

--

H=0.5 T

~6

~,

~4

/' y

~2

O II [1 !

Tachiki (0=0'

T;schiki(0=40'

-

'. k

",

--3

o

l

o

30

T___

60

l

I

90 120 0 (deg)

150

180

Fig. 10. Angular dependence of the critical current density Jc for an YBCO/PBCO (48 ,~/24 A)20 superlattice with T~,zero= 83 K

(the subscript20 refersto the numberof modulationlayers)at (a) T=80K, H = 0.05 T, (b) T=75 K, H=0.5T, and(c) T=70 K, H = 0.5 T. Also shown are the predicteddata by Tachiki and Takahashi [23] which are normalizedto the Jc(O)at 0 = 0 and 40°, and the predicteddata by Kes [22]. tal c-axis. Fig. 10 shows the angular dependence of Jc(0) for an YBCO/PBCO (48 A / 2 4 A)20 superlattice in fixed magnetic fields near Tc. The dashed curve predicted by Kes [22] was obtained from Jc(H) with the magnetic field perpendicular to the plane of the film. The theoretically obtained angular dependence of Jc(0) predicted by Tachiki and Takahashi [23] was also plotted. The predicted data by Tachiki and Takahashi [23] were normalized to the Jc(O) at 0 = 0 and 40 °. Fittings normalized to other angles do not improve the theoretical prediction. The present data appear to fit much better with J c ( 0 ) = Jc(HCosO). This data suggest that the PBCO layer decouples the coupling between YBCO layers. Only the perpendicular component of the magnetic field contributes to the Lorentz force, and therefore the

H.C. Yang et al./Physica C 281 (1997) 325-334

angular dependence of critical current density, J¢(O) is described by the equation, Jc(O)=Jc(HcosO). The dependence of Jc on the angle 0 is governed by the magnetic field component parallel to the c-axis. In the present work, the (48 A / 2 4 '~)20 superlattice exhibited a decoupled behaviour in a magnetic flux line near T~ (To,zero= 83 K for the (48 A / 2 4 A)20 superlattice). Li et al. [24] used the YBa2Cu3Oy/Pr0.4Y0.6Ba 2Cu3Oy superlattices as a model system to study the effects of the layer coupling on the vortex pinning in high-Tc superconductors. They found that when CuO 2 planes are changed from coupled to decoupled upon increasing the temperature from below to above To, characteristics changes in the Jc(0) were observed. This result demonstrates the significance of the layer coupling to the flux pinning in YBCO/YPBCO superlattices. On the other hand, Jakob et al. [25] reported the general scaling behaviour, Jc(O) = Jc(HcosO), of the dependence of the J~(H,T,O) in layered materials with different strength of interlayer coupling. They found that the PBCO layer can decouple the layer coupling in YBa2Cu30~./PrBa2Cu 302. superlattices. Our present results in the angular dependence of the critical current density Jc in YBCO/PBCO superlattices are consistent with the reported data by Jakob et al. [25].

4. Conclusion The growth and transport properties of YBa2Cu 3Oy/Yx Prl- xBa2Cu 3Oy (YBCO/YPBCO) superlattices investigated are characterized in this study. The satellite diffraction peaks appearing around the main (00I) peaks confirm the a superlattices structure. The resistivity of these superlattices shows thermal activated behaviour under magnetic fields. Hall coefficients R H in mixed state show sign reversal and diminishing of R H occurs as the thickness of the YPBCO layer is increased or the thickness of the YBCO layer is decreased. Further, angular dependence of the critical current density Jc for the YBCO/PBCO (48 ,~/24 "~)20 superlattice shows a decoupled behaviour and the dependence of Jr on the angle 0 is governed only by the magnetic field component parallel to c-axis.

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Acknowledgements The authors thank the National Council of the Republic of China for financial support under grant No. NSC85-2112-M002-027pH and NSC85-2112M003-014pH.

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