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Ultrasonic experiments on the high-Tc-superconductor Y2−xBaxCuO4−δ
Physica 148B (1987) 296-297 North-Holland, Amsterdam
ULTRASONIC EXPERIMENTS ON THE HIGH-To-SUPERCONDUCTOR
Y2_xBaxCuO4_8
V. MISLLER, K. DE G R O O T , D. M A U R E R , CH. R O T H and K.H. R I E D E R lnstitut fiir Atom- und Festk6rperphysik, Freie Universitdt Berlin, Arnimallee 14, D-IO00 Berlin 33, Fed. Rep. Germany Received 10 August 1987
Sound velocity measurements on a mixed-phase sample YI 2BaosCuO4_~ in high magnetic fields are reported. Like in normal metals, but in contrast to single-phase samples, the material is found to harden on cooling. In the vicinity of 95 K an elastic instability is observed which is referred to a semimetal-metal transition. For B0 = 6 T an anomalously large softening of--460ppm is found at T~(Bo). The importance of internal strains for the elastic behaviour and for the appearance of pronounced hysteresis effects in single-phase samples is discussed.
The great advantage of ultrasonic techniques is that they probe the interior, i.e. the bulk properties, of the investigated samples. At present, however, only very few ultrasonic measurements on high-T c superconductors have been reported [1-3] because, due to the porous nature of the ceramic samples, the ultrasonic background attenuation is relatively large and superconducting single crystals of adequate size are not available so far. It is the main purpose of this paper to show that the change in the ultrasonic phase-velocity at Tc turns out to be at least one order of magnitude larger in Y - B a - C u - O compounds than the respective change in ordinary superconductors. Furthermore, when applying a strong dc magnetic field in order to shift T c to lower temperatures, there is some experimental evidence for a structural or electronic instability at about 95 K which in zero magnetic field is hidden from the view by the superconducting transition. The measurements were performed on a rectangular (12.1 x 5.8 x 1.83 mm 3) multi-phase YB a - C u - O sample with nominal composition Y1.2Ba0.sCuO4_~. The small sintered rod was prepared through solid-state reaction in a similar manner as reported in ref. [4]. The sample was characterized by resistivity, susceptibility and Xray diffraction measurements and the mass density was determined to be p = 5 . 6 g / c m 3. The resistivity shows a superconducting transition at 0378-4363/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Yamada Science Foundation
about 9 2 K with a transition width of - 3 K . After polishing and subsequent annealing at about 770 K in oxygen, one side of the sample was gold-plated with a 0.5 p.m Au-layer. As the measurements were performed at high dc magnetic fields the longitudinal ultrasonic waves were generated and detected by means of an rf coil (direct electromagnetic phonon generation and detection technique). In order to provide high sensitivity the "sampled continuous wave" method of Miller and Bolef [5] was applied. Fig. 1 shows at reduced spectrometer sensitivity the temperature dependence of an acoustic standing wave resonance frequency v , ( - 5 . 6 MHz) in a dc magnetic field of 8 T. As Pn ~ Ca/l (where c a is the sound velocity and l the length of the sample) it follows from fig. 1 that, in contrast to our previous findings [3] in the single-phase material YBa2Cu30 7, the principal behaviour of the mixed-phase sample is characterized by a monotonic hardening on cooling. Furthermore, the multi-phase specimen does not show elastic hysteresis effects. As it is reasonable to assume that in multiphase compounds internal strains are present due to lattice mismatch, it is tempting to speculate that in single-phase samples the hysteresis effects may originate from the formation (or release) of internal strains. This speculation is further corroborated by the observation that shock-cooling of a single-phase specimen to 77 K
5,6-4 I
--
~
F]
5,60
c o o l i n g a n d after an initial h a r d e n i n g which is i n t e r r u p t e d at a b o u t 95 K, the s a m p l e a g a i n hard e n s b e l o w 90 K a n d starts to soften at T c ( B 0 ) = 7 2 K by an a m o u n t of a b o u t 2 . 6 K H z (i.e. --460 p p m ) . This m a g n i t u d e is a b o u t 1 5 - 3 0 times larger t h a n the respective zero-field v a l u e observed in c o n v e n t i o n a l s u p e r c o n d u c t o r s a n d with [(Un)
I
BOo.sYL2CuQ
0
100
56F-
J
X
\
I
200
T/K
300
Fig. 1. Temperature dependence of the 5.6 MHz longitudinal acoustic standing w a v e resonance frequency in Y1.2Ba04CuO4_~ at a dc magnetic field of 8 T and reduced spectrometer sensitivity.
(which give rise to internal strains) smears out the elastic hysteresis effects [6]. The temperature dependence of vn --5.6 MHz at increased spectrometer sensitivity is presented in fig. 2 for a dc magnetic field of B 0 = 6 T, On 1'4 "l-
1
N -- (Un)S]/(b'n)
N ~- (K S -- KN)/K
N
(where
N
refers to the normal and S to the superconducting state) reflects an unusual large relative change in the compressibility K (i.e. in the volume fluctuations) at T c. Another interesting feature which will be discussed in more detail in a forthcoming paper [6] is the elastic anomaly in the vicinity of 95 K which is indicative for an electronic instability because (at B 0 = 6T) the resistivity shows a step-like decrease in the same temperature range. This interpretation is also corroborated by the behaviour of the ultrasonic attenuation which, in the temperature range investigated here, also shows pronounced anomalies [6]. Summarizing our results we have shown that ultrasonic measurements provide new insight into the complex physics behind high-T~ superconductors.
I
~r ._
>o
References
5.63
.~"
L
Bao.8¥~.2 CuO~ " ~ 5,62
\ °°
0
297
V. Mailer et al. / Ultrasound in Y2_xBaxCu04 ~
I
50
I
100
T in K
Fig. 2. Temperature dependence of the 5.6 MHz longitudinal acoustic standing wave resonance frequency at 6T and increased spectrometer sensitivity.
[1] B. L/ithi, B. Wolf, T. Kim, W. Grill and B. Renker, preprint to appear in Proc. LT-18 (1987). [2] V. Miiller, K. de Groot, D. Maurer, Ch. Roth, K.H. Rieder, E. Eichenbusch and R. Sch611horn, preprint, to appear in Proc. LT-18 (1987). [3] D.J. Bishop, P.L. Gammel, A.P. Ramirez, R.J. Cava, B. Batlogg and E.A. Rietman, Phys. Rev. B 35 (1987) 8788. [4] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Lett. 58 (1987) 908. [5] J.G. Miller and D.I. Bolef, Rev. Sci. Instrum. 40 (1969) 915. [6] K. de Groot, D. Maurer, Ch. Roth and V. M/iller, to be published.
ULTRASONIC EXPERIMENTS ON THE HIGH-To-SUPERCONDUCTOR
Y2_xBaxCuO4_8
V. MISLLER, K. DE G R O O T , D. M A U R E R , CH. R O T H and K.H. R I E D E R lnstitut fiir Atom- und Festk6rperphysik, Freie Universitdt Berlin, Arnimallee 14, D-IO00 Berlin 33, Fed. Rep. Germany Received 10 August 1987
Sound velocity measurements on a mixed-phase sample YI 2BaosCuO4_~ in high magnetic fields are reported. Like in normal metals, but in contrast to single-phase samples, the material is found to harden on cooling. In the vicinity of 95 K an elastic instability is observed which is referred to a semimetal-metal transition. For B0 = 6 T an anomalously large softening of--460ppm is found at T~(Bo). The importance of internal strains for the elastic behaviour and for the appearance of pronounced hysteresis effects in single-phase samples is discussed.
The great advantage of ultrasonic techniques is that they probe the interior, i.e. the bulk properties, of the investigated samples. At present, however, only very few ultrasonic measurements on high-T c superconductors have been reported [1-3] because, due to the porous nature of the ceramic samples, the ultrasonic background attenuation is relatively large and superconducting single crystals of adequate size are not available so far. It is the main purpose of this paper to show that the change in the ultrasonic phase-velocity at Tc turns out to be at least one order of magnitude larger in Y - B a - C u - O compounds than the respective change in ordinary superconductors. Furthermore, when applying a strong dc magnetic field in order to shift T c to lower temperatures, there is some experimental evidence for a structural or electronic instability at about 95 K which in zero magnetic field is hidden from the view by the superconducting transition. The measurements were performed on a rectangular (12.1 x 5.8 x 1.83 mm 3) multi-phase YB a - C u - O sample with nominal composition Y1.2Ba0.sCuO4_~. The small sintered rod was prepared through solid-state reaction in a similar manner as reported in ref. [4]. The sample was characterized by resistivity, susceptibility and Xray diffraction measurements and the mass density was determined to be p = 5 . 6 g / c m 3. The resistivity shows a superconducting transition at 0378-4363/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Yamada Science Foundation
about 9 2 K with a transition width of - 3 K . After polishing and subsequent annealing at about 770 K in oxygen, one side of the sample was gold-plated with a 0.5 p.m Au-layer. As the measurements were performed at high dc magnetic fields the longitudinal ultrasonic waves were generated and detected by means of an rf coil (direct electromagnetic phonon generation and detection technique). In order to provide high sensitivity the "sampled continuous wave" method of Miller and Bolef [5] was applied. Fig. 1 shows at reduced spectrometer sensitivity the temperature dependence of an acoustic standing wave resonance frequency v , ( - 5 . 6 MHz) in a dc magnetic field of 8 T. As Pn ~ Ca/l (where c a is the sound velocity and l the length of the sample) it follows from fig. 1 that, in contrast to our previous findings [3] in the single-phase material YBa2Cu30 7, the principal behaviour of the mixed-phase sample is characterized by a monotonic hardening on cooling. Furthermore, the multi-phase specimen does not show elastic hysteresis effects. As it is reasonable to assume that in multiphase compounds internal strains are present due to lattice mismatch, it is tempting to speculate that in single-phase samples the hysteresis effects may originate from the formation (or release) of internal strains. This speculation is further corroborated by the observation that shock-cooling of a single-phase specimen to 77 K
5,6-4 I
--
~
F]
5,60
c o o l i n g a n d after an initial h a r d e n i n g which is i n t e r r u p t e d at a b o u t 95 K, the s a m p l e a g a i n hard e n s b e l o w 90 K a n d starts to soften at T c ( B 0 ) = 7 2 K by an a m o u n t of a b o u t 2 . 6 K H z (i.e. --460 p p m ) . This m a g n i t u d e is a b o u t 1 5 - 3 0 times larger t h a n the respective zero-field v a l u e observed in c o n v e n t i o n a l s u p e r c o n d u c t o r s a n d with [(Un)
I
BOo.sYL2CuQ
0
100
56F-
J
X
\
I
200
T/K
300
Fig. 1. Temperature dependence of the 5.6 MHz longitudinal acoustic standing w a v e resonance frequency in Y1.2Ba04CuO4_~ at a dc magnetic field of 8 T and reduced spectrometer sensitivity.
(which give rise to internal strains) smears out the elastic hysteresis effects [6]. The temperature dependence of vn --5.6 MHz at increased spectrometer sensitivity is presented in fig. 2 for a dc magnetic field of B 0 = 6 T, On 1'4 "l-
1
N -- (Un)S]/(b'n)
N ~- (K S -- KN)/K
N
(where
N
refers to the normal and S to the superconducting state) reflects an unusual large relative change in the compressibility K (i.e. in the volume fluctuations) at T c. Another interesting feature which will be discussed in more detail in a forthcoming paper [6] is the elastic anomaly in the vicinity of 95 K which is indicative for an electronic instability because (at B 0 = 6T) the resistivity shows a step-like decrease in the same temperature range. This interpretation is also corroborated by the behaviour of the ultrasonic attenuation which, in the temperature range investigated here, also shows pronounced anomalies [6]. Summarizing our results we have shown that ultrasonic measurements provide new insight into the complex physics behind high-T~ superconductors.
I
~r ._
>o
References
5.63
.~"
L
Bao.8¥~.2 CuO~ " ~ 5,62
\ °°
0
297
V. Mailer et al. / Ultrasound in Y2_xBaxCu04 ~
I
50
I
100
T in K
Fig. 2. Temperature dependence of the 5.6 MHz longitudinal acoustic standing wave resonance frequency at 6T and increased spectrometer sensitivity.
[1] B. L/ithi, B. Wolf, T. Kim, W. Grill and B. Renker, preprint to appear in Proc. LT-18 (1987). [2] V. Miiller, K. de Groot, D. Maurer, Ch. Roth, K.H. Rieder, E. Eichenbusch and R. Sch611horn, preprint, to appear in Proc. LT-18 (1987). [3] D.J. Bishop, P.L. Gammel, A.P. Ramirez, R.J. Cava, B. Batlogg and E.A. Rietman, Phys. Rev. B 35 (1987) 8788. [4] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Lett. 58 (1987) 908. [5] J.G. Miller and D.I. Bolef, Rev. Sci. Instrum. 40 (1969) 915. [6] K. de Groot, D. Maurer, Ch. Roth and V. M/iller, to be published.