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Josephson vortex dynamics in La2−xSrxCuO4 single crystals
Physica B 284}288 (2000) 581}582
Josephson vortex dynamics in La Sr CuO single crystals \V V
Takashi Tachiki *, Kensuke Nakajima , Tsutomu Yamashita , Isao Tanaka , Hironao Kojima RIEC, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan NICHe, Tohoku University, Aramakiaza, Aoba-ku, Sendai 980-8579, Japan Faculty of Engineering, Yamanashi University, Miyamae 7, Kofu 400-8511, Japan CREST, Japan Science and Technology Corporation, Japan
Abstract We observed a multi-branch behavior in the #ux-#ow branch of current}voltage characteristics of stacks fabricated along the c axis in La Sr CuO single crystals. The voltage increases with increasing an external magnetic "eld \V V applied parallel to the ab plane. The multi-branch behavior may originate from a chaotic motion of Josephson vortices with many velocity modes. 2000 Elsevier Science B.V. All rights reserved. Keywords: Flux #ow; Josephson vortices; La Sr CuO ; Vortex motion \V V
1. Introduction In cuprate superconductors, Josephson vortices introduced between the CuO layers can move with very high velocity under currents applied along the c axis. The #ux #ow with the velocity more than 10 m/s and the collective vortex motion with many modes were observed in Bi Sr CaCu O (BSCCO) [1,2]. It was predicted that V electromagnetic waves are emitted by decaying of the Josephson plasma excited by the #ux #ow [3]. The plasma frequency in La Sr CuO (LSCO) is much \V V higher than that of BSCCO. Hence it is interesting to understand vortex dynamics in LSCO in order to generate the submillimeter waves. We report an investigation of vortex dynamics in LSCO, which results from measurements of the current}voltage (I}<) characteristics of stacks fabricated along the c axis in external magnetic "elds. 2. Experimental The single crystals of La Sr CuO (x"0.09) pre\V V pared by the traveling solvent #oating zone method were * Corresponding author. E-mail address: [email protected] (T. Tachiki)
cut into rods. Four-probe geometry was formed to put Ag electrodes on both sides of the rod. The rods were annealed at 5003C for 30 min in #owing oxygen to reduce contact resistance. Stacks of 63}239 lm area and 1 lm long were fabricated along the c axis in the rod by a focused ion beam technique [4]. The sample was mounted on a sample rotator with a precision of 0.053 when the I}< curves were measured in an external magnetic "eld.
3. Results and Discussion Fig. 1 shows the I}< characteristics of the stack in the absence of an external magnetic "eld at 4.2 K. The cross-sectional area of the stack is 63 lm. The currents were swept up and down repeatedly. We can see a peculiar nature of the I}< curves that show many branches at a high bias region. We can explain that the voltages are caused by motion of Josephson vortices which are induced by a self-"eld due to #owing the current. These branches are modi"ed by the magnetic "eld of only 5 mT and the modi"cation depends on the direction of the "eld; the voltages vanish completely in the "eld applied parallel to the c axis. We interpret this fact as pinning for Josephson vortices due to the interaction with pancake vortices that are formed by the "eld parallel to the c axis
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 2 2 0 1 - 2
582
T. Tachiki et al. / Physica B 284}288 (2000) 581}582
Fig. 1. I}< characteristics of the stack along the c axis of LSCO in the absence of an external magnetic "eld.
and easily pinned. This supports that the unstable multiple branches originate from a chaotic motion of Josephson vortices. As seen in Fig. 2(a), for the external "elds H applied parallel to the ab plane, the #ux-#ow branches are shifted to a higher voltage region with increasing the "eld strength. The cross-sectional area of the stack is 239 lm. Fig. 2(b) shows the highest #ux-#ow voltage < and the lowest one < at 27.7 mA as a function of H . The well-known relation between #ux-#ow voltage < and magnetic #ux density B is <"Bvl, where v and l represent the average vortex velocity and the length of the stack, respectively. The linear dependence of < on B indicates a constant vortex velocity. One notices that B (the upper abscissa of Fig. 2(b)), including the self-"eld, becomes 0 T at a point where ¸ attains 0 V. ¸ is the linear dependence which approximates < . On the other hand, the shape of < shows that the velocity has di!erent values, since the "eld dependence of < is not linear. ¸ was derived by considering a constant velo city determined by the highest slope of < . The area between ¸ and ¸ corresponds to the region where the velocity varies from 2.8;10 m/s to 1.1;10 m/s.
Fig. 2. I}< characteristics for k H "0 and 20 mT (a) and "eld dependence of < and < (See Fig. 2(a), (b)).
This indicates that Josephson vortices move with changing between these mode velocities.
References [1] [2] [3] [4]
J.U. Lee et al., Appl. Phys. Lett. 67 (1995) 1471. G. Hecht"scher et al., Phys. Rev. B 55 (1997) 14 638. T. Koyama et al., Solid State Commun. 96 (1995) 367. T. Tachiki et al., preprint.
Josephson vortex dynamics in La Sr CuO single crystals \V V
Takashi Tachiki *, Kensuke Nakajima , Tsutomu Yamashita , Isao Tanaka , Hironao Kojima RIEC, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan NICHe, Tohoku University, Aramakiaza, Aoba-ku, Sendai 980-8579, Japan Faculty of Engineering, Yamanashi University, Miyamae 7, Kofu 400-8511, Japan CREST, Japan Science and Technology Corporation, Japan
Abstract We observed a multi-branch behavior in the #ux-#ow branch of current}voltage characteristics of stacks fabricated along the c axis in La Sr CuO single crystals. The voltage increases with increasing an external magnetic "eld \V V applied parallel to the ab plane. The multi-branch behavior may originate from a chaotic motion of Josephson vortices with many velocity modes. 2000 Elsevier Science B.V. All rights reserved. Keywords: Flux #ow; Josephson vortices; La Sr CuO ; Vortex motion \V V
1. Introduction In cuprate superconductors, Josephson vortices introduced between the CuO layers can move with very high velocity under currents applied along the c axis. The #ux #ow with the velocity more than 10 m/s and the collective vortex motion with many modes were observed in Bi Sr CaCu O (BSCCO) [1,2]. It was predicted that V electromagnetic waves are emitted by decaying of the Josephson plasma excited by the #ux #ow [3]. The plasma frequency in La Sr CuO (LSCO) is much \V V higher than that of BSCCO. Hence it is interesting to understand vortex dynamics in LSCO in order to generate the submillimeter waves. We report an investigation of vortex dynamics in LSCO, which results from measurements of the current}voltage (I}<) characteristics of stacks fabricated along the c axis in external magnetic "elds. 2. Experimental The single crystals of La Sr CuO (x"0.09) pre\V V pared by the traveling solvent #oating zone method were * Corresponding author. E-mail address: [email protected] (T. Tachiki)
cut into rods. Four-probe geometry was formed to put Ag electrodes on both sides of the rod. The rods were annealed at 5003C for 30 min in #owing oxygen to reduce contact resistance. Stacks of 63}239 lm area and 1 lm long were fabricated along the c axis in the rod by a focused ion beam technique [4]. The sample was mounted on a sample rotator with a precision of 0.053 when the I}< curves were measured in an external magnetic "eld.
3. Results and Discussion Fig. 1 shows the I}< characteristics of the stack in the absence of an external magnetic "eld at 4.2 K. The cross-sectional area of the stack is 63 lm. The currents were swept up and down repeatedly. We can see a peculiar nature of the I}< curves that show many branches at a high bias region. We can explain that the voltages are caused by motion of Josephson vortices which are induced by a self-"eld due to #owing the current. These branches are modi"ed by the magnetic "eld of only 5 mT and the modi"cation depends on the direction of the "eld; the voltages vanish completely in the "eld applied parallel to the c axis. We interpret this fact as pinning for Josephson vortices due to the interaction with pancake vortices that are formed by the "eld parallel to the c axis
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 2 2 0 1 - 2
582
T. Tachiki et al. / Physica B 284}288 (2000) 581}582
Fig. 1. I}< characteristics of the stack along the c axis of LSCO in the absence of an external magnetic "eld.
and easily pinned. This supports that the unstable multiple branches originate from a chaotic motion of Josephson vortices. As seen in Fig. 2(a), for the external "elds H applied parallel to the ab plane, the #ux-#ow branches are shifted to a higher voltage region with increasing the "eld strength. The cross-sectional area of the stack is 239 lm. Fig. 2(b) shows the highest #ux-#ow voltage < and the lowest one < at 27.7 mA as a function of H . The well-known relation between #ux-#ow voltage < and magnetic #ux density B is <"Bvl, where v and l represent the average vortex velocity and the length of the stack, respectively. The linear dependence of < on B indicates a constant vortex velocity. One notices that B (the upper abscissa of Fig. 2(b)), including the self-"eld, becomes 0 T at a point where ¸ attains 0 V. ¸ is the linear dependence which approximates < . On the other hand, the shape of < shows that the velocity has di!erent values, since the "eld dependence of < is not linear. ¸ was derived by considering a constant velo city determined by the highest slope of < . The area between ¸ and ¸ corresponds to the region where the velocity varies from 2.8;10 m/s to 1.1;10 m/s.
Fig. 2. I}< characteristics for k H "0 and 20 mT (a) and "eld dependence of < and < (See Fig. 2(a), (b)).
This indicates that Josephson vortices move with changing between these mode velocities.
References [1] [2] [3] [4]
J.U. Lee et al., Appl. Phys. Lett. 67 (1995) 1471. G. Hecht"scher et al., Phys. Rev. B 55 (1997) 14 638. T. Koyama et al., Solid State Commun. 96 (1995) 367. T. Tachiki et al., preprint.