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Surface acoustic wave investigations of Y1-xPrxBa2Cu3Oy films
I IBD ELSEVIER
Physica B 219&220 (1996) 179-181
Surface acoustic wave investigations of Y l_xPrxBa2Cu3Oyfilms M. Yoshizawa a'*, K. Shiga a, N. Yoshimoto a, N. Oki b, H. Iwasaki c' 1, S. Kenmochi c, N. Kobayashi c "Department of Materials Science and Technology, Faculty of Engineering, lwate University, Morioka, Japan bBasic Technology R&D center, Kayaba lndustry Co., Ltd., Sagamihara, Japan ¢IMR, Tohoku University, Sendal, Japan
Abstract Temperature and magnetic field dependence of surface acoustic wave (SAW) velocity has been measured for Yx-xPrxBa2Cu3Oy films with x = 0.3 and x = 1. SAW velocity shows a step-like anomaly at the superconducting transition To, and a remarkable softening at low temperatures. The amount of the anomaly at Tc is 4.5 x 10-5, which is the same order as bulk samples. SAW velocity below Tc is enhanced by applying the magnetic field, which is caused by interaction between the vortices and SAW.
1. Introduction The superconducting state of high-T¢ materials shows interesting behaviors in sound velocity change in the magnetic field. In particular, vortex state in superconductors has been intensively studied by ultrasonic method. Lemmens et al. reported the elastic hardening in the magnetic field for YBCO [1]. Similar phenomena have been observed in BSCCO [2] and (Lat -xSrx)2CuO4 systems [3]. The previous ultrasonic investigations were made for conventional measure frequency region. Higher-frequency study has been desired for getting dynamic properties of the system. High-frequency measurement, however, is strongly affected by the sample quality. One of the methods for the high-frequency measurement is use of a surface acoustic wave (SAW), where Rayleigh wave propagates in the vicinity of the surface effectively. In this paper, we report the first SAW measurements of
* Corresponding author. 1 Present address: Japan Advanced Institute of Science and Technology, Hokuriku.
Yo.TPro.sBa2Cu3Oy and PrBa2Cu3Oy as functions of temperature and magnetic field.
2. Experimental 2.1. Sample preparation Yo.7Pro.3Ba2Cu3Oy and PrBa2Cu3Oy are prepared by RF sputtering method in the mixed gas atmosphere of argon and oxygen. The substrate is MgO with (100) surface. The sample was made at 750°C and annealed with the oxygen gas of 10mTorr. The atomic composition of the grown film was checked by ICP analysis. The film thickness is about 500 nm for both samples. 2.2. Surface acoustic wave device A piezoelectric ZnO film was made on the MgO substrate by the sputtering method. The thickness of the film with c-axis orientation is about 61am. Interdigital-type SAW device with ten-finger-pairs was fabricated by
0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 6 8 8 - 5
180
M. Yoshizawa et al. / Physica B 219&220 (1996) 179 181
a photolithography on the ZnO film. The wavelength of SAW is 80 gm, which corresponds to the resonant frequency of 92.37 MHz. Experimental set-up was described in Ref. [4]. SAW velocity was measured by a phase comparison method. The SAW velocity is 2.65 x 105 cms -1 and 2.60 x 105 cm s 1 at room temperature for Y0.TPr0.3BazCu3Oy and PrBa2Cu3Or, respectively. These values are the average of the sample and the MgO substrate, because SAW propagates not only on the grown film, but penetrates deeply (approximately 1 wavelength) in the substrate.
tural phase transition in the vicinity of the superconducting transition temperature. This possibility would not be denied at this moment. A large elastic softening is seen at low temperatures. Since SAW velocity of both samples shows analogous linear temperature dependence in the logarithmic temperature scale, this softening is considered to be caused by the structural instability, namely two-level system (TLS), introduced during the sputtering, rather than caused by the superconducting properties. Temperature variation of SAW propagating on the thin film containing TLS with the thickness t is described as
3. Results and discussion
=', -
3.1. Temperature dependence
Fig. 1 shows the temperature dependence of the SAW velocity of Yo.vPr0.3BazCu30~,. The superconducting transition temperature T¢ is reduced by Pr substitution [5]. The value of Tc is 47 K determined by the ultrasonic measurements, which is slightly lower than 51 K obtained by the resistivity measurements carried out for the same sample. A step-like anomaly with the relative change o f A v / v = 4 . 5 x l 0 5 is seen at Tc as shown in the inset of Fig. 1. This value is compared to the bulk experiment [6], Av/v = 3 x 10 5, and is considered to be extremely large. Because the thickness of the film is much thinner than the SAW length, the contribution to the SAW velocity from the sample is roughly estimated to be only a few percent. If the anomaly at 47 K is actually associated with superconductivity, the elastic anomaly would be enhanced for some reasons. The fact that 47 K is somewhat different from To determined by the resistivity measurement, however, may suggest a possible struc-
i)1,o=
where no M2 and 2 are the coupling constant and the SAW length, respectively [7]. To is the reference temperature, and K the constant determined by the elastic properties of the substrate and the sample, p is the effective density, which is mainly determined by the MgO substrate, and v the SAW velocity. The reference temperature To is 50 and 35 K for Yo.7Pro.3 BazCu30~. and PrBazCu3Oy, respectively. We adopted K = 1, p = 6 g c m 3 and v = 2 . 6 x l 0 S c m s -1. The value of no M2 for Yo.TPo.3BazCu3Oy is 5.2 x 109 and 9.8 × 109ergcm -3 for the temperature ranges of 10K
1.5
0.5 0.0
-0.5
~_. 0.5
-1.0 o°°°c° -I.5 0.2
f E
B=I22~/B:
~f
-0.2
-3-2" .0S [i:i: ~a~Ttc V ~ Y ° ' T P -3.s ~ I
~
~
r° 3BaCzU3Oy
-05 i 10
, 20
i 30
I
40
50
60
T (K) 10
T (K)
Fig. 1. Temperature dependence of SAW velocity of Yo.TPro.3Ba2Cu3Oy and PrBa2Cu3Oy. The inset shows the anomaly in the vicinity of the superconducting transition.
Fig. 2. Magnetic contribution in the temperature dependence of SAW velocity with several magnetic fields. The data in the absence of the magnetic field is subtracted as a background. The line indicates the theoretical one at B = 2 T [9]. The adjusted parameters are shown in the text.
M. Yoshizawa et al. / Physica B 219&220 (1996) 179 181 0.6
0.4 O v
0.2 !
o •
~"
0
-0.2
-0.4
0
2
4
6
8
10
181
To, the sound velocity increases steeply in the weak field, and shows a shoulder-type anomaly at 0.5 T, which is the boundary to the normal state. The increase of the sound velocity with the magnetic field is due to the destruction of superconductivity. At low temperatures, however, one finds an additional minimum in the weak field region. SAW velocity minimum is shown at about 1.2 and 0.6 T for 4.5 and 10 K, respectively. Organic superconductor •-(BEDT-TTF)2Cu(NCS)2 shows a similar complicated magnetic field dependence in the elastic constant in the superconducting phase [11]. Although the origin of the anomaly has not been fully understood, the change in the vortex state would be a possible origin.
B (T)
Fig. 3. Magnetic field dependence of SAW velocity in Yo.TPr0.3BazCu30~. and PrBa2Cu30~.. significant below 30 K. Such elastic hardening has been already reported for YBCO, BSCCO and LSCO systems, and interpreted by vortex pinning effect. Recently, similar phenomena has been observed in CeRu2 [8], which is a possible candidate of Fulde-Ferrell LarkinOvchinnikov state. The observed anomaly here was analyzed by the theory of Pankert [9]. The theoretically curve for B = 2 T is also displayed in Fig. 2. The theory predicts that the maximum amount of the elastic hardening is B2/4~, which is the energy gain due to the magnetic field. Our data on Yo.:Pro.3Ba2Cu3Oy do not follow B 2, but B 1/2, while CeRu 2 shows a clear B 2 one. The analysis gives the value of the pinning potential U = 125 K at 2 T, if the resistivity in the magnetic field is assumed to be the activation-~ype temperature (T) dependence described as poexp( - U(B)/T) with Po = 5 × 10 -4 ~cm. The value of the pinning potential from this experiment is consistent with U = 200 K obtained by the resistivity measurement [10]. The SAW velocity shows a remarkable decrease below a certain temperature T* in the superconducting phase. The value of T* is 23 K in the case of 2 T, and T* slightly decreases with increasing field. 3.2. Magnetic .field dependence Fig. 3 shows the magnetic field dependence of the sound velocity at several temperatures. At 37 K below
Acknowledgements The authors thank Mr. M. Saito and Mr. M. Nakamura for their help in the experiments and Mr. N. Mizuno of Kayaba Industry Co., Ltd. for the hospitality during the fabrication of the SAW device. The experiments were partly done at High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University.
References [1] P. Lemmens, P. Fr6ning, S. Ewert, J. Pankert, H. Passing and A. Comberg, Physica B 165&166(1990) 1275. [21 J. Pankert, G. Marbach, A. Comberg, P. Lemmens, P. Fr6ning and S. Ewert, Phys. Rev. Lett. 65 (1990) 3052. [3] T. Hanaguri, T. Fukase, I. Tanaka and H. Kojima, Phys. Rev. B 48 (1993) 9772. [4] M. Yoshizawa, H. Sugawara, Y. Nakamura and N. Oki, Solid State Commun. 83 (1992) 571. [5] J.J. Neumeier and M.B. Maple, Physica C 191 (1992) 158. [6] B. Golding, N.O. Birge, W.H. Haemmerle, R.J. Cava and E. Rietman, Phys. Rev. B 36 (1987) 5606. [7] A. Tate, S. Tamura and T. Sakuma, Solid State Commun. 30 (1979) 517. [8] M. Yoshizawa, M. Ozawa, H. Sugawara, Y. Onuki and T. Sasaki, to be published. [9] J. Pankert, Physica B 165&166 (1990) 1273. [10] H. lwasaki, O. Taniguchi, S. Kenmochi and N. Kobayashi, Physica B 194 196 (1994) 2117. [11] M. Yoshizawa, Y. Nakamura, T. Sasaki and N. Toyota, Solid State Commun. 89 (1994) 701.
Physica B 219&220 (1996) 179-181
Surface acoustic wave investigations of Y l_xPrxBa2Cu3Oyfilms M. Yoshizawa a'*, K. Shiga a, N. Yoshimoto a, N. Oki b, H. Iwasaki c' 1, S. Kenmochi c, N. Kobayashi c "Department of Materials Science and Technology, Faculty of Engineering, lwate University, Morioka, Japan bBasic Technology R&D center, Kayaba lndustry Co., Ltd., Sagamihara, Japan ¢IMR, Tohoku University, Sendal, Japan
Abstract Temperature and magnetic field dependence of surface acoustic wave (SAW) velocity has been measured for Yx-xPrxBa2Cu3Oy films with x = 0.3 and x = 1. SAW velocity shows a step-like anomaly at the superconducting transition To, and a remarkable softening at low temperatures. The amount of the anomaly at Tc is 4.5 x 10-5, which is the same order as bulk samples. SAW velocity below Tc is enhanced by applying the magnetic field, which is caused by interaction between the vortices and SAW.
1. Introduction The superconducting state of high-T¢ materials shows interesting behaviors in sound velocity change in the magnetic field. In particular, vortex state in superconductors has been intensively studied by ultrasonic method. Lemmens et al. reported the elastic hardening in the magnetic field for YBCO [1]. Similar phenomena have been observed in BSCCO [2] and (Lat -xSrx)2CuO4 systems [3]. The previous ultrasonic investigations were made for conventional measure frequency region. Higher-frequency study has been desired for getting dynamic properties of the system. High-frequency measurement, however, is strongly affected by the sample quality. One of the methods for the high-frequency measurement is use of a surface acoustic wave (SAW), where Rayleigh wave propagates in the vicinity of the surface effectively. In this paper, we report the first SAW measurements of
* Corresponding author. 1 Present address: Japan Advanced Institute of Science and Technology, Hokuriku.
Yo.TPro.sBa2Cu3Oy and PrBa2Cu3Oy as functions of temperature and magnetic field.
2. Experimental 2.1. Sample preparation Yo.7Pro.3Ba2Cu3Oy and PrBa2Cu3Oy are prepared by RF sputtering method in the mixed gas atmosphere of argon and oxygen. The substrate is MgO with (100) surface. The sample was made at 750°C and annealed with the oxygen gas of 10mTorr. The atomic composition of the grown film was checked by ICP analysis. The film thickness is about 500 nm for both samples. 2.2. Surface acoustic wave device A piezoelectric ZnO film was made on the MgO substrate by the sputtering method. The thickness of the film with c-axis orientation is about 61am. Interdigital-type SAW device with ten-finger-pairs was fabricated by
0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 6 8 8 - 5
180
M. Yoshizawa et al. / Physica B 219&220 (1996) 179 181
a photolithography on the ZnO film. The wavelength of SAW is 80 gm, which corresponds to the resonant frequency of 92.37 MHz. Experimental set-up was described in Ref. [4]. SAW velocity was measured by a phase comparison method. The SAW velocity is 2.65 x 105 cms -1 and 2.60 x 105 cm s 1 at room temperature for Y0.TPr0.3BazCu3Oy and PrBa2Cu3Or, respectively. These values are the average of the sample and the MgO substrate, because SAW propagates not only on the grown film, but penetrates deeply (approximately 1 wavelength) in the substrate.
tural phase transition in the vicinity of the superconducting transition temperature. This possibility would not be denied at this moment. A large elastic softening is seen at low temperatures. Since SAW velocity of both samples shows analogous linear temperature dependence in the logarithmic temperature scale, this softening is considered to be caused by the structural instability, namely two-level system (TLS), introduced during the sputtering, rather than caused by the superconducting properties. Temperature variation of SAW propagating on the thin film containing TLS with the thickness t is described as
3. Results and discussion
=', -
3.1. Temperature dependence
Fig. 1 shows the temperature dependence of the SAW velocity of Yo.vPr0.3BazCu30~,. The superconducting transition temperature T¢ is reduced by Pr substitution [5]. The value of Tc is 47 K determined by the ultrasonic measurements, which is slightly lower than 51 K obtained by the resistivity measurements carried out for the same sample. A step-like anomaly with the relative change o f A v / v = 4 . 5 x l 0 5 is seen at Tc as shown in the inset of Fig. 1. This value is compared to the bulk experiment [6], Av/v = 3 x 10 5, and is considered to be extremely large. Because the thickness of the film is much thinner than the SAW length, the contribution to the SAW velocity from the sample is roughly estimated to be only a few percent. If the anomaly at 47 K is actually associated with superconductivity, the elastic anomaly would be enhanced for some reasons. The fact that 47 K is somewhat different from To determined by the resistivity measurement, however, may suggest a possible struc-
i)1,o=
where no M2 and 2 are the coupling constant and the SAW length, respectively [7]. To is the reference temperature, and K the constant determined by the elastic properties of the substrate and the sample, p is the effective density, which is mainly determined by the MgO substrate, and v the SAW velocity. The reference temperature To is 50 and 35 K for Yo.7Pro.3 BazCu30~. and PrBazCu3Oy, respectively. We adopted K = 1, p = 6 g c m 3 and v = 2 . 6 x l 0 S c m s -1. The value of no M2 for Yo.TPo.3BazCu3Oy is 5.2 x 109 and 9.8 × 109ergcm -3 for the temperature ranges of 10K
1.5
0.5 0.0
-0.5
~_. 0.5
-1.0 o°°°c° -I.5 0.2
f E
B=I22~/B:
~f
-0.2
-3-2" .0S [i:i: ~a~Ttc V ~ Y ° ' T P -3.s ~ I
~
~
r° 3BaCzU3Oy
-05 i 10
, 20
i 30
I
40
50
60
T (K) 10
T (K)
Fig. 1. Temperature dependence of SAW velocity of Yo.TPro.3Ba2Cu3Oy and PrBa2Cu3Oy. The inset shows the anomaly in the vicinity of the superconducting transition.
Fig. 2. Magnetic contribution in the temperature dependence of SAW velocity with several magnetic fields. The data in the absence of the magnetic field is subtracted as a background. The line indicates the theoretical one at B = 2 T [9]. The adjusted parameters are shown in the text.
M. Yoshizawa et al. / Physica B 219&220 (1996) 179 181 0.6
0.4 O v
0.2 !
o •
~"
0
-0.2
-0.4
0
2
4
6
8
10
181
To, the sound velocity increases steeply in the weak field, and shows a shoulder-type anomaly at 0.5 T, which is the boundary to the normal state. The increase of the sound velocity with the magnetic field is due to the destruction of superconductivity. At low temperatures, however, one finds an additional minimum in the weak field region. SAW velocity minimum is shown at about 1.2 and 0.6 T for 4.5 and 10 K, respectively. Organic superconductor •-(BEDT-TTF)2Cu(NCS)2 shows a similar complicated magnetic field dependence in the elastic constant in the superconducting phase [11]. Although the origin of the anomaly has not been fully understood, the change in the vortex state would be a possible origin.
B (T)
Fig. 3. Magnetic field dependence of SAW velocity in Yo.TPr0.3BazCu30~. and PrBa2Cu30~.. significant below 30 K. Such elastic hardening has been already reported for YBCO, BSCCO and LSCO systems, and interpreted by vortex pinning effect. Recently, similar phenomena has been observed in CeRu2 [8], which is a possible candidate of Fulde-Ferrell LarkinOvchinnikov state. The observed anomaly here was analyzed by the theory of Pankert [9]. The theoretically curve for B = 2 T is also displayed in Fig. 2. The theory predicts that the maximum amount of the elastic hardening is B2/4~, which is the energy gain due to the magnetic field. Our data on Yo.:Pro.3Ba2Cu3Oy do not follow B 2, but B 1/2, while CeRu 2 shows a clear B 2 one. The analysis gives the value of the pinning potential U = 125 K at 2 T, if the resistivity in the magnetic field is assumed to be the activation-~ype temperature (T) dependence described as poexp( - U(B)/T) with Po = 5 × 10 -4 ~cm. The value of the pinning potential from this experiment is consistent with U = 200 K obtained by the resistivity measurement [10]. The SAW velocity shows a remarkable decrease below a certain temperature T* in the superconducting phase. The value of T* is 23 K in the case of 2 T, and T* slightly decreases with increasing field. 3.2. Magnetic .field dependence Fig. 3 shows the magnetic field dependence of the sound velocity at several temperatures. At 37 K below
Acknowledgements The authors thank Mr. M. Saito and Mr. M. Nakamura for their help in the experiments and Mr. N. Mizuno of Kayaba Industry Co., Ltd. for the hospitality during the fabrication of the SAW device. The experiments were partly done at High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University.
References [1] P. Lemmens, P. Fr6ning, S. Ewert, J. Pankert, H. Passing and A. Comberg, Physica B 165&166(1990) 1275. [21 J. Pankert, G. Marbach, A. Comberg, P. Lemmens, P. Fr6ning and S. Ewert, Phys. Rev. Lett. 65 (1990) 3052. [3] T. Hanaguri, T. Fukase, I. Tanaka and H. Kojima, Phys. Rev. B 48 (1993) 9772. [4] M. Yoshizawa, H. Sugawara, Y. Nakamura and N. Oki, Solid State Commun. 83 (1992) 571. [5] J.J. Neumeier and M.B. Maple, Physica C 191 (1992) 158. [6] B. Golding, N.O. Birge, W.H. Haemmerle, R.J. Cava and E. Rietman, Phys. Rev. B 36 (1987) 5606. [7] A. Tate, S. Tamura and T. Sakuma, Solid State Commun. 30 (1979) 517. [8] M. Yoshizawa, M. Ozawa, H. Sugawara, Y. Onuki and T. Sasaki, to be published. [9] J. Pankert, Physica B 165&166 (1990) 1273. [10] H. lwasaki, O. Taniguchi, S. Kenmochi and N. Kobayashi, Physica B 194 196 (1994) 2117. [11] M. Yoshizawa, Y. Nakamura, T. Sasaki and N. Toyota, Solid State Commun. 89 (1994) 701.