Ultrasonic Study of Superconducting Mixed State

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When an electric current is flowed in the state that quantized flux lines go through a type II superconductor (mixed state), flux lines are forced to move by a Lorentz force, and a voltage is induced in the sample. However, if impurities or deficiencies are included in the crystal, flux lines are "pinned" by them, and the induction of voltage is suppressed by the pinning. Such impurities or deficiencies are called as "pinning centers".

In high temperature superconductors, the effect of thermal excitation on the flux pinning is large, while the superconductivity appears at high temperatures. In particular in Bi-compounds, a resistance appears even at the temperature much lower than the superconducting transition temperature (Tc). This is a serious problem in view of the application of high temperature superconductors. Therefore, it is valuable to know what kinds of deficiencies are effective for the pinning centers in the utilization at high temperatures and high magnetic fields.

So far, the flux pinning properties have been examined by the measurements of critical current and magnetization. However, we are using an ultrasonic method in this study, as will be mentioned below.

Fig.1. Interaction between ultrasound and magnetic flux lines: If flux lines are pinned, the flux line lattice is vibrated by the ultrasound propagating in the superconductor due to the coupling with pinning centers. (The figure shows the case of transverse ultrasound)

[Feature]

In Fig.1 the interaction between transverse ultrasound and flux line lattice is shown schematically. The ultrasound propagating in the crystal vibrates pinning centers on the crystal. When the flux lines are pinned, they are vibrated together with ultrasound. Since the flux lines form triangular lattice because of the repulsive force between them, ultrasound vibrates the whole lattice of flux lines. Therefore, ultrasound propagating in the superconductor can detect the stiffness of the flux line lattice.

The ultrasonic measurements have some advantages as follows:(A) The stiffness of flux line lattice can be measured directly. (B) There is few effect of weak link at crystal grain boundaries, since the effect is averaged over whole crystal in the method. (C) The detail of the pinning properties is examined by changing the frequency and the amplitude of ultrasound.

In Fig. 2, the results of the temperature dependence of the excess sound velocity due to magnetic field are shown for Bi2Sr2Ca2Cu3Oy (Tc~110 K) and Bi2Sr2CaCu2Oy (Tc~80 K) [1]. Here, the magnetic field was applied parallel and normal to the direction of the longitudinal ultrasonic propagation which is along the c-axis of the sample. With decreasing temperature, there found an effect of the flux pinning from the increase of the excess sound velocity. However, the increase of the excess sound velocity occurred at a temperature much lower than Tc. Moreover, the temperature was found to be different with the direction of the magnetic field. We explained the anisotropic behavior in terms of the "intrinsic pinning model" [2].

Fig.2. Temperature dependence of the excess sound velocity due to magnetic field.: In Bi-oxide superconductors, it is found that flux lines are released from the pinning state(depinning) at a temperature much lower than the superconducting transition temperature (80-110 K) from the increase of excess sound velocity.

[Prospects]

As mentioned above, the ultrasound is found to be one of the probes of flux pinning state. However, it is hard to say that it showed its ability sufficiently. In particular, because of the difficulty of getting big single crystals for oxide superconductors, the anisotropic properties of flux pinning are averaged in the method. Recently, we have developed ultrasonic method using single-crystalline thin films[3]. It is expected to get more detailed informations with respect to the pinning state in high Tc superconductors.

[References]

[1]Horie,Y.,Hamamoto,T.,Youssef, A., Ichikawa, F., Miyazaki,T.,Fukami,T.and Aomine,T.: Physica B, Vol.194-196, 1994, pp.1581.

[2] Horie,Y.,Miyazaki,T.,Fukami,T. and Youssef,A.: Physica C, Vol.176, Nos.4-6,1991,pp.521.

[3] Horie,Y., Youssef,A., Oku, T., Maneki, J., Tsutsui, Y., Miyazaki, T., Ichikawa, F., Fukami,T. and Aomine, T.: Jpn. J. Appl. Phys., Vol.33, Nos.11A,1994, pp.L1511.

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by　Y. Horie