Short Notes K85

phys. stat. sol. (a) 113, K85 (1989) Subject classification: 62.20 and 74.70; S10.1

Institute of Solid State Physics , Nanjing University1) Elastic Behavior Related to the Superconductivity in YBa2Q3Qx

XIAOHUA CHEN, YENING WANG, and HUIMING SHEN BY

Introduction Internal friction measurement is sensitive on an atomic scale to the mobility of atoms and various defects. Particularly, internal friction can sensitively reflect subtle changes of lattice structure which cannot be detected even by X-ray diffraction, electron microscope, and other experimental methods. So it is a powerful tool for the study of structure instabilities or phase transitions.

It was discovered that most of the A-15 alloys known as conventional high-Tc superconductor, such as NbZSn, V3Si, Nb3Ge, undergo martensitic tranformation and all show elastic instability above Tc. It was generally recognized that the high superconducting transition temperature of A-15 alloys is closely connected with the structure instability above Tc I l l .

Since the discovery of oxide superconductors, centering on the problem about whether o r not similar structure instability takes place and whether this instability is related to the superconductivity, many researches have been made by many kinds of methods, such as internal friction and elastic modulus I 2 to 51, X-ray diffraction 161,

ultrasonic attenuation and wave velocity 171, Mossbauer spectrum 181, specific heat 191, acoustic emission 1101, and so on. All results show that there exist some phase transition-like anomalies at some temperatures above Tc. Wang et al. considered that the anomaly is a certain kind of martensite-like transformation, the two phases have the same symmetry but slightly different lattice parameters, and the phase transition is of first order. In our previous paper 141, we reported the relationship between 90 K superconductivity and the phase-transition-like anomaly at about 120 K , a little higher than Tc. In the present note further results are reported.

Experiment The YBa2CujOx samples were prepared by standard powder metallurgy from high purity Y203, BaC03, and CuO powder. These samples were annealed in vacuum at about 380 to 420 O C for about 20 h in order to obtain homogeneous spegimens with suitable oxygen content x.

The Marx three-component oscillator method (longitudinal vibration) was used for measuring the internal friction and the Young modulus at about 100 kHz. All the measurements were carried out on heating at rates ranging from 0.5 to 1.5 Klmin. The resistivity was measured by the four-probe configuration. The lower temperature limit of our system is about 55 K. It was achieved by pumping nitrogen gas from a

Dewar flask continuously.

') Nanjing, People's Republic of China.

K 86 physica status solid (a) 113

1 1

1 0 ’ - & I 1

c 4

Fig. 1. Internal friction Q-l and relative change of Young’s modulus AY/Y versus temperature T for a ) sample 1 (the peak P1 is a relaxation peak. P2 and P3 are caused by martensite-like transition, and the former is related to the 90 K superconductivity, but the latter is not); b) sample 2 (the inset is the resistivity in mRcm as a function of temperature); c) sample 3 (the inset is the resistivity in mRcm as a function of temperature. The coexistence of superconductive phases with T equal to 60 K and 90 K , respectively, results in the coexistence of P4 and PZ); dy sample 4 (the inset is the resistivity in mRcm as a function of temperature. The peak P4 is absent, but the softening of the Young modulus is visible above Tc)

Short Notes K87

Results and discussion The as-grown sample 1 exhibits a strong Meissner effect in liquid nitrogen. Although the resistivity was not measured, it is sure that the oxygen content x is about 6.98 to 7.0, and Tc is about 90 K. Fig. la reveals the internal friction Q-l and the relative change of the Young modulus AY/Y as a function of temperature. As pointed out in 141, peak P1 is a relaxation peak, peaks P2 and P3 are attributed to the stress induced movement of interfaces of two phases with the same symmetry but slightly different lattice parameters, and furthermore, the peak P2 is related to the 90 K superconductivity but peak P3 is not /41. Similar to the case of A-15 alloys, for the oxider superconductor YBa2Cu30x, the phase- transition-like anomaly above 90 K (about 120 K ) may be favourable to the superconducting transition followed.

The effective method of confirming the above comment is checking whether or not the peak P2 shifts toward lower temperature when the oxygen content x locates on the plateau with the lower Tc (about 60 K ) I l l / . For this reason, sample 2 was outgassed in vacuum at about 400 OC for 1 2 h. The Tc for this oxygen outgassed specimen is about 63 K as shown in the inset of Fig. lb. From Fig. lb it can be seen that P1 and P3 remain, but P2 is restrained. Peak P4, instead of P2, accompanied with the evident minima of Young’s modulus appears at about 80 K. The occurrence of minima of Young’s modulus suggests that P4 is a phase transition peak. As the sample is annealed insufficiently, the oxygen content is inhomogeneous outside and inside the crystal grain, which leads to a broadening of the superconducting transition width and even another superconducting phase with higher Tc (90 K ) . So, peak P2 does not disappear perfectly, but superposes on the high temperature side of peak P1, and makes it broader.

The phenomenon of coexistence of two superconductive phases with Tc equal to 60

K and 90 K , respectively, results in the coexistence of P2 and P4, as can be seen more clearly in sample 3. The resistivity from room temperature to LN2 temperature is shown in the inset of Fig. lc. The resistivity drops at about 90 K, but does not achieve zero up to LN2 temperature, which indicates the coexistence of superconductive phases with 90 K - Tc and lower Tc (may be 60 K) /11/. As shown

in Fig. lc , both P2 and P4 appear. It should be pointed out that the elastic behavior below 90 K is more complicated

than that a t higher temperature. For the 60 K superconductive phase, the peak temperature and height of P4, even whether the peak appears or not, are related to the thermal history as well as to the samples. It is also similar to some ,4-15 alloys, such as V3Si, where elastic softening or structure instability above Tc always occurs, but a martensitic tranformation does not take place in every case because the superconducting transition restrains it when Ms is lower than Tc.

As an example, the Tc of sample 4 is 60 K. As shown in Fig. Id , the peak 4 is absent, which means that the martensite-like transition above Tc does not occur, but

K88 physica status solidi ( a ) 113

the elastic so f t en ing o r structure instability near 80 K is obvious. It is this instability that f a v o u r s the superconducting transition near 60 K.

In summary, the oxide s u p e r c o n d u c t o r YBa2Cu30x with 60 K - Tc shows elastic instability a n d / o r undergoes martensite-l ike transition near 80 K. This phase transition may not take place, but more o r less elastic so f t en ing always occur s . The re fo re , it could be considered that, similar to the case in A-15 alloys, in the oxide s u p e r c o n d u c t o r YBa2Cu30x, there always exists structure instability in form of elastic so f t en ing and/or martensite-l ike transition; this instability is favourab le t o the superconducting transition followed. From this point it may be considered that the phonons may also play an impor t an t ro l e in the superconductivity in YBa2Cu30x although the BCS theory would be modified.

We would like to thank Yu Zhen , Yang Zhen, and Jin Xin f o r help in p repa ra t ion of samples and experiments .

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