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Current-dependent flux–flow resistance and resonant current steps in BSCCO intrinsic Josephson junctions
Journal of Physics and Chemistry of Solids 67 (2006) 438–441 www.elsevier.com/locate/jpcs
Current-dependent flux–flow resistance and resonant current steps in BSCCO intrinsic Josephson junctions Sunmi Kim a,*, Shinya Urayama a, Huabing Wang a, Shin-ichi Kawakami a, Kunihiro Inomata a, Masanori Nagao a, Kyung Sung Yun a, Yoshihiko Takano a, Kiejin Lee b, Takeshi Hatano a b
a National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan Department of Physics, Sogang University, CPO Box 1142, Seoul 121-742, South Korea
Abstract We report a current dependence of flux–flow resistance (FFR) and transport properties in intrinsic Jospehson junctions (IJJs) under magnetic fields parallel to an ab-plane. In Bi2Sr2CaCu2O8Cd IJJs with the ab-plane dimensions of 1.8!10.5 mm2, the oscillations of FFR have been observed with two apparent periods of 0.382 T in low fields and 0.765 T in high fields. The dominant period HpZ0.765 T is decided by a sample width and corresponds to the field for adding one flux quantum per layer. Under certain conditions, we also observed the mergence of two peaks on the oscillating FFR with half period 1/2Hp into one peak with the period Hp in low fields and the inversions between bottoms and peaks in high fields. We found that this current-dependent FFR implying information of vortex lattice correlates with the transport properties such as current steps on current–voltage curves. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Superconductors; D. Electric properties; D. Magnetic properties.
1. Introduction High Tc superconductors have been paid attention for highfrequency applications up to THz regime due to their large energy gaps [1]. Especially, since highly anisotropic Bi2Sr2CaCu2O8Cd (BSCCO) compound consists of naturally stacked intrinsic Josephson junctions (IJJs), high frequency generations are expected with small signal line width and high power [2]. However, for high frequency application, it is essential to synchronize all junctions in a stack to a so-called in-phase state. As reported in references [3–5], one of the ways is to use the collective motion of Josephson fluxons. In stacked IJJs ˚ is much superconducting layers with a thickness dZ3 A ˚, thinner than the London penetration depth lLZ1500–1700 A so the inductive coupling appears between neighbouring junctions [5,6], and a mutual phase locking can be expected in all junctions by collective Josephson vortex motion. Recently, in the magnetic fields parallel to superconducting layers, it is theoretically discussed that the moving Josephson * Corresponding author. Tel.:C81 298 51 3351x6674; fax: C81 298 59 2801. E-mail address: [email protected] (S. Kim).
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.10.052
vortices under a c-axis bias current form a rectangular lattice for the in-phase mode and a triangular lattice for the out-ofphase mode [7]. Experimentally, the configuration of Josephson vortex lattices in BSCCO IJJs was considered as a triangular lattice, namely the ground state by the periodic oscillation of flux–flow resistance (FFR) [8] and the oscillation of FFR is explained as a dynamical matching of Josephson vortex lattice with sample edges [9]. On the other hand, without oscillation, the field dependence of flux–flow resistivity was reported as linear field dependence in low field and quadratic dependence in high field [10,11]. It was reported the moving vortex lattice generates an electromagnetic wave when the velocity of the lattice matches the plasma wave velocity [12,13] and it has been intensively investigated about different modes of electromagnetic waves related to Fiske and flux–flow modes with respect to the vortex motion [14–17]. Although there are many researches about vortex lattice motion and Jospehson plasma wave in the IJJs individually, it is still obscure for the relation between them. In this paper we study the magnetic field dependence of FFR to characterize the Josephson vortex lattice in BSCCO IJJs. We also discuss the current-dependent FFR containing information of vortex lattice motion, and the transport properties reflecting electromagnetic wave emission.
S. Kim et al. / Journal of Physics and Chemistry of Solids 67 (2006) 438–441
2. Experiment BSCCO whiskers were grown by annealing the sintered Tedoped BSCCO precursors and a detailed growth method was reported in Ref. [18]. A whisker with clean and flat surface was etched by focused ion beam (FIB) in form of In-line junction with dimensions of 1.8!10.5 mm2. Measured by a scanning ion microscope (SIM), the thickness of the fabricated IJJs was estimated to be 0.22 mm, implying about 146 junction numbers involved in the stack. A critical temperature Tc and a critical current density Jc of BSCCO IJJs are 81 K and 1.784 kA/cm2 in zero field at 10 K. With general parameters, superconducting ˚ , and the (in-plane) London penetration depth layer dZ3 A ˚ l abZ1500 A, the Josephson penetration depth lJ z ðF0 d=4pm0 jc l2ab Þ1=2 is about 0.31 mm [19]. Therefore our sample, about 5.8 times larger than the calculated lJ, can be regarded as long Josephson junctions. Electric transport properties were measured using a Physical Property Measurement System (PPMS) of Quantum Design with a four-terminal configuration. The sample was set on a holder, which can be rotated with a resolution of 0.0018. In order to enhance the effect from the edges, which were regarded as barriers for Josepshon vortex motions [9], we applied magnetic fields parallel to the ab-plane and along the longer side (i.e., a-axis) of BSCCO IJJs as shown in the inset of Fig. 1. The in-plane alignment was precisely determined by measuring the angular dependence of FFR at a constant bias current of 1 mA and a magnetic field of 1 T at 60 K. Fig. 1 shows the misalignment effect on FFR oscillation with (a) qZ08 and (b) qZ0.358 at 40 K. Even such a small misalignment qZ0.358 can smear the oscillation of FFR at high field as shown in Fig. 1b. To our knowledge, it is caused
Fig. 1. Magnetic field dependence of flux–flow resistance (FFR) at 40 K when (a) qZ08 and (b) qZ0.358, clearly indicating misalignment effect is remarkable even q is quite small, where q orientation angle between magnetic fields and the ab-plane. The inset is a side view of BSCCO IJJs fabricated by FIB etching. Magnetic field was applied along the longer side of junction to enhance edge effect.
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by pancake vortices formed from the c-axis component of the misaligned field. 3. Result and discussion 3.1. Periodic oscillation of FFR The field dependence of the flux–flow resistance with c-axis bias current of 1 mA (about 0.52% of IcZ193 mA in zero field) at 50 K is shown in Fig. 2. Observed were two kinds of clear oscillation periods of FFR: 0.382 T in low fields of H/Hp!2.5, and 0.765 T in high fields of H/HpO2.5. The dominant oscillation period HpZ0.765 T in high fields is in good agreement with the calculated period HPZ(F0/ws)Z0.766 T ˚ , where F0, w, and s are the flux with wZ1.8 mm and sZ15 A quantum, the junction width, and the layer periodicity along the c-axis, respectively. The small discrepancy results from the FIB etching process. Since Hp corresponds to the field for adding one flux quantum per junction, formed in the IJJs can be a rectangular configuration of Josephson vortex lattice i.e.inphase motion of Josephson vortices. The observed period at low field 0.382 T is 1/2 Hp, implying triangular Josephson vortex lattice formed in the stack as described in Ref. [8]. Our results also show that the period of triangular one in low fields transforms to be the rectangular one in high fields. The detailed mechanism of this phenomenon will be published elsewhere by Hatano et al., [20]. 3.2. Current dependence of FFR In principle the c-axis current exerts Lorentz force on a Josephson vortex lattice sliding along the ab-plane. The increase of the current bias gives rise to not only an increase of the velocity of the sliding Jospehson vortex but also the configuration change of the vortex lattice. The effect of the c-axis bias current on Josephson vortex flow is
Fig. 2. FFR as a function of magnetic field with c-axis bias current of 1 mA at 50 K. Magnetic fields were normalized by an oscillation period HpZ0.765 T.
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Fig. 3. (a) FFR at various currents of 1–10 mA at 50 K, in relatively low fields (H/HpZ1–3.5), where there is peak transformation. (b) I–V characteristics at different fields; (i) H/HpZ1.75, 2.25, 2.75, (ii) H/HpZ1.5, 2.5, and (iii) H/HpZ2, 3.
shown in Figs. 3a and 4a. As shown in Fig. 3a, in low fields (H/Hp!2.5) the two peaks of FFR with half period 1/ 2Hp merge into one peak with the period Hp at high bias current. It indicates a transformation from the triangular period to the rectangular one by higher bias current and it agrees with the structural change of the Josephson vortex lattice in Ref. [21]. On the other hand, in high fields H/HpO2.5 shown in Fig. 4a, the oscillation peaks of FFR are inverted as bottoms and vice versa by the bias current. There is no change of the oscillation period. This inversion of the oscillation peaks of FFR is caused by the velocity change of the sliding Josephson vortices since the flux–flow voltage and FFR are proportional to vortex velocity [9]. In addition, we found that these FFR anomalies correlated with the transport properties such as resonant current steps on the I–V characteristics. 3.3. The relation between FFR oscillation and transport properties Shown in Fig.3b are the I–V characteristics measured at specific fields parallel to the layers. Considering the FFR
0
20
40
60
Voltage (mV) Fig. 4. (a) FFR at various currents of 1–10 mA at 50 K, in relatively high fields (H/HpZ2–6). (b) I–V characteristics and their differential conductance dI/dV at fields H/HpZ3, 4, 5, showing strong current steps.
oscillation, we classified the fields as (i) H/HpZ1.75, 2.25, 2.75, where FFR peaks are with half period (1/2Hp) corresponding to the triangular lattice, (ii) H/HpZ1.5, 2.5, and (iii) H/HpZ2, 3 where both bottoms and peaks of FFR oscillation with a period for the rectangular lattice. The increase of magnetic field decreased the hysteresis on I–V characteristics as well as the critical current then finally, the hysteresis disappeared above 2.5 H/Hp. We observed the pronounced current steps on the I–V characteristics only in rectangular period’s case as shown in (ii) and (iii) of Fig. 3b. In particular, odd steps came out at peak fields H/HpZn, and even steps revealed at bottom fields H=Hp Z nC 1=2 where n is integer number. Note that bottoms of FFR are transformed to peaks by increasing the bias current, with the oscillation period unchanged. In nonhysteretic region the current step is obvious. In the case of triangular lattice, no steps were observed. It is understood from our results that the I–V curves were modulated by the parallel magnetic fields forming Josephson vortex lattice, and especially the current steps appear in the specific fields
S. Kim et al. / Journal of Physics and Chemistry of Solids 67 (2006) 438–441
where are characterized by the oscillation of FFR corresponding to the period of the rectangular vortex lattice. Fig. 4b shows the I–V characteristics with steps and their differential conductance dI/dV at the fields H/HpZ3, 4, and 5. These steps are expected as a strong enhancement of superconducting current at fixed voltage when the Josephson frequencies match the resonant frequencies of cavity modes in parallel fields, i.e., Fiske steps [12,15,16]. The detailed discussion on Fiske steps with regarding the vortex motion will be reported elsewhere. In present experiments, we paid close attention to the strong enhancement of dI/dV at fixed voltages. As shown in Fig. 4b, maximum conductance appears at 12.1, 8.6 and 6.6 mA for H/HpZ3, 4, and 5. Compared with current-dependent FFR, the current levels of each steps in Fig. 4b actually coincide with the currents where the inversion from peak to bottom of FFR oscillation takes place, as marked in Fig. 4a. 4. Conclusion We measured a current dependence of FFR and transport properties determined by the dynamics of Josephson vortex lattice in BSCCO IJJs. We observed the transformation of FFR oscillation by increasing the bias current. In low fields H/Hp!2.5 the oscillation period corresponds to a triangular lattice and it is transformed to the period of a rectangular lattice by higher bias current. While in high fields H/HpO 2.5 we observed that the oscillation peaks showing the period of rectangular lattice are inverted to bottoms by current bias. From I–V characteristics we found clear current steps in the specific fields where exhibit the oscillation period of FFR corresponding to not triangular period but rectangular one. These steps were considered to be Fiske steps related with geometric resonance. The conductance enhancement of these current steps happened at constant voltage results in the inversion from the peaks to the bottoms of FFR oscillation. This inversion was considered to be the resonance of ac Jospephson frequency exited by the moving vortex lattice with the sample width (for rectangular lattice) and it maybe is from energy dissipation due
441
to the Josephson plasma excitation. These results supply important information for possible high-frequency application of BSCCO IJJs. Acknowledgements The authors would like to thank T. Yamashita and M. Tachiki for valuable discussion. References [1] S. Beuven, O. Harnack, L. Amatuni, H. Kohlstedt, M. Darula, IEEE Trans. Appl. Supercond. 7 (1997) 2591. [2] K.K. Likharev, Dynamics of Josephson Junctions and Circuits, Gordon & Breach, 1986. [3] M. Machida, T. Koyama, A. Tanaka, M. Tachiki, Physica C 330 (2000) 85. [4] R. Kleiner, P. Mu¨ller, H. Kohlstedt, N.F. Pedersen, S. Sakai, Phys. Rev. B 50 (1994) 3942. [5] S. Heim, M. Mo¨ßle, T. Clauß, R. Kleiner, Supercond. Sci. Technol. 15 (2002) 1226. [6] S. Sakai, A.V. Ustinov, N. Thyssen, H. Kohlstedt, Phys. Rev. B 58 (1998) 5777. [7] J.H. Kim, Phys. Rev. B 65 (2002) 100509. [8] S. Ooi, T. Mochiku, K. Hirata, Phys. Rev. Lett. 89 (2002) 247002. [9] M. Machida, Phys. Rev. Lett. 90 (2003) 037001. [10] A.E. Koshelev, Phys. Rev. B 62 (2000) R3616. [11] Yu.I. Latyshev, A.E. Koshelev, L.N. Bulavskii, Phys. Rev. B 68 (2003) 134604. [12] A.E. Koshelev, I.S. Aranson, Phys. Rev. Lett. 85 (2000) 3938. [13] T. Koyama, M. Tachiki, Solid State Commun. 96 (1995) 367. [14] V.M. Kransnov, N. Mros, A. Yurgens, D. Winkler, Phys. Rev. B 59 (1999) 8463. [15] M.D. Fiske, Rev. Mod. Phys. 36 (1964) 221. [16] I.O. Kulik, JETP Lett. 2 (1965) 84. [17] A. Irie, Y. Hirai, G. Oya, Appl. Phys. Lett. 72 (1998) 2159. [18] M. Nagao, M. Sato, H. Maeda, S. Kim, T. Yamashita, Appl. Phys. Lett. 79 (2001) 2612. [19] A. Irie, S. Heim, S. Schromm, M. Mo¨ßle, T. Nachtrab, M. Go´do, R. Kleiner, P. Mu¨ller, G. Oya, Phys. Rev. B 62 (2000) 6681. [20] T. Hatano, M.B. Wang, S.M. Kim, S. Urayama, S. Kawakami, S.-J. Kim, M. Nagao, K. Inomata, Y. Takano, T. Yamashita, M. Tachiki, IEEE Trans. Appl. Supercond. 15 (2005) 912. [21] K. Hirata, S. Ooi, E.H. Sadki, T. Mochiku, Physica B 329–333 (2003) 1332.
Current-dependent flux–flow resistance and resonant current steps in BSCCO intrinsic Josephson junctions Sunmi Kim a,*, Shinya Urayama a, Huabing Wang a, Shin-ichi Kawakami a, Kunihiro Inomata a, Masanori Nagao a, Kyung Sung Yun a, Yoshihiko Takano a, Kiejin Lee b, Takeshi Hatano a b
a National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan Department of Physics, Sogang University, CPO Box 1142, Seoul 121-742, South Korea
Abstract We report a current dependence of flux–flow resistance (FFR) and transport properties in intrinsic Jospehson junctions (IJJs) under magnetic fields parallel to an ab-plane. In Bi2Sr2CaCu2O8Cd IJJs with the ab-plane dimensions of 1.8!10.5 mm2, the oscillations of FFR have been observed with two apparent periods of 0.382 T in low fields and 0.765 T in high fields. The dominant period HpZ0.765 T is decided by a sample width and corresponds to the field for adding one flux quantum per layer. Under certain conditions, we also observed the mergence of two peaks on the oscillating FFR with half period 1/2Hp into one peak with the period Hp in low fields and the inversions between bottoms and peaks in high fields. We found that this current-dependent FFR implying information of vortex lattice correlates with the transport properties such as current steps on current–voltage curves. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Superconductors; D. Electric properties; D. Magnetic properties.
1. Introduction High Tc superconductors have been paid attention for highfrequency applications up to THz regime due to their large energy gaps [1]. Especially, since highly anisotropic Bi2Sr2CaCu2O8Cd (BSCCO) compound consists of naturally stacked intrinsic Josephson junctions (IJJs), high frequency generations are expected with small signal line width and high power [2]. However, for high frequency application, it is essential to synchronize all junctions in a stack to a so-called in-phase state. As reported in references [3–5], one of the ways is to use the collective motion of Josephson fluxons. In stacked IJJs ˚ is much superconducting layers with a thickness dZ3 A ˚, thinner than the London penetration depth lLZ1500–1700 A so the inductive coupling appears between neighbouring junctions [5,6], and a mutual phase locking can be expected in all junctions by collective Josephson vortex motion. Recently, in the magnetic fields parallel to superconducting layers, it is theoretically discussed that the moving Josephson * Corresponding author. Tel.:C81 298 51 3351x6674; fax: C81 298 59 2801. E-mail address: [email protected] (S. Kim).
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.10.052
vortices under a c-axis bias current form a rectangular lattice for the in-phase mode and a triangular lattice for the out-ofphase mode [7]. Experimentally, the configuration of Josephson vortex lattices in BSCCO IJJs was considered as a triangular lattice, namely the ground state by the periodic oscillation of flux–flow resistance (FFR) [8] and the oscillation of FFR is explained as a dynamical matching of Josephson vortex lattice with sample edges [9]. On the other hand, without oscillation, the field dependence of flux–flow resistivity was reported as linear field dependence in low field and quadratic dependence in high field [10,11]. It was reported the moving vortex lattice generates an electromagnetic wave when the velocity of the lattice matches the plasma wave velocity [12,13] and it has been intensively investigated about different modes of electromagnetic waves related to Fiske and flux–flow modes with respect to the vortex motion [14–17]. Although there are many researches about vortex lattice motion and Jospehson plasma wave in the IJJs individually, it is still obscure for the relation between them. In this paper we study the magnetic field dependence of FFR to characterize the Josephson vortex lattice in BSCCO IJJs. We also discuss the current-dependent FFR containing information of vortex lattice motion, and the transport properties reflecting electromagnetic wave emission.
S. Kim et al. / Journal of Physics and Chemistry of Solids 67 (2006) 438–441
2. Experiment BSCCO whiskers were grown by annealing the sintered Tedoped BSCCO precursors and a detailed growth method was reported in Ref. [18]. A whisker with clean and flat surface was etched by focused ion beam (FIB) in form of In-line junction with dimensions of 1.8!10.5 mm2. Measured by a scanning ion microscope (SIM), the thickness of the fabricated IJJs was estimated to be 0.22 mm, implying about 146 junction numbers involved in the stack. A critical temperature Tc and a critical current density Jc of BSCCO IJJs are 81 K and 1.784 kA/cm2 in zero field at 10 K. With general parameters, superconducting ˚ , and the (in-plane) London penetration depth layer dZ3 A ˚ l abZ1500 A, the Josephson penetration depth lJ z ðF0 d=4pm0 jc l2ab Þ1=2 is about 0.31 mm [19]. Therefore our sample, about 5.8 times larger than the calculated lJ, can be regarded as long Josephson junctions. Electric transport properties were measured using a Physical Property Measurement System (PPMS) of Quantum Design with a four-terminal configuration. The sample was set on a holder, which can be rotated with a resolution of 0.0018. In order to enhance the effect from the edges, which were regarded as barriers for Josepshon vortex motions [9], we applied magnetic fields parallel to the ab-plane and along the longer side (i.e., a-axis) of BSCCO IJJs as shown in the inset of Fig. 1. The in-plane alignment was precisely determined by measuring the angular dependence of FFR at a constant bias current of 1 mA and a magnetic field of 1 T at 60 K. Fig. 1 shows the misalignment effect on FFR oscillation with (a) qZ08 and (b) qZ0.358 at 40 K. Even such a small misalignment qZ0.358 can smear the oscillation of FFR at high field as shown in Fig. 1b. To our knowledge, it is caused
Fig. 1. Magnetic field dependence of flux–flow resistance (FFR) at 40 K when (a) qZ08 and (b) qZ0.358, clearly indicating misalignment effect is remarkable even q is quite small, where q orientation angle between magnetic fields and the ab-plane. The inset is a side view of BSCCO IJJs fabricated by FIB etching. Magnetic field was applied along the longer side of junction to enhance edge effect.
439
by pancake vortices formed from the c-axis component of the misaligned field. 3. Result and discussion 3.1. Periodic oscillation of FFR The field dependence of the flux–flow resistance with c-axis bias current of 1 mA (about 0.52% of IcZ193 mA in zero field) at 50 K is shown in Fig. 2. Observed were two kinds of clear oscillation periods of FFR: 0.382 T in low fields of H/Hp!2.5, and 0.765 T in high fields of H/HpO2.5. The dominant oscillation period HpZ0.765 T in high fields is in good agreement with the calculated period HPZ(F0/ws)Z0.766 T ˚ , where F0, w, and s are the flux with wZ1.8 mm and sZ15 A quantum, the junction width, and the layer periodicity along the c-axis, respectively. The small discrepancy results from the FIB etching process. Since Hp corresponds to the field for adding one flux quantum per junction, formed in the IJJs can be a rectangular configuration of Josephson vortex lattice i.e.inphase motion of Josephson vortices. The observed period at low field 0.382 T is 1/2 Hp, implying triangular Josephson vortex lattice formed in the stack as described in Ref. [8]. Our results also show that the period of triangular one in low fields transforms to be the rectangular one in high fields. The detailed mechanism of this phenomenon will be published elsewhere by Hatano et al., [20]. 3.2. Current dependence of FFR In principle the c-axis current exerts Lorentz force on a Josephson vortex lattice sliding along the ab-plane. The increase of the current bias gives rise to not only an increase of the velocity of the sliding Jospehson vortex but also the configuration change of the vortex lattice. The effect of the c-axis bias current on Josephson vortex flow is
Fig. 2. FFR as a function of magnetic field with c-axis bias current of 1 mA at 50 K. Magnetic fields were normalized by an oscillation period HpZ0.765 T.
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Fig. 3. (a) FFR at various currents of 1–10 mA at 50 K, in relatively low fields (H/HpZ1–3.5), where there is peak transformation. (b) I–V characteristics at different fields; (i) H/HpZ1.75, 2.25, 2.75, (ii) H/HpZ1.5, 2.5, and (iii) H/HpZ2, 3.
shown in Figs. 3a and 4a. As shown in Fig. 3a, in low fields (H/Hp!2.5) the two peaks of FFR with half period 1/ 2Hp merge into one peak with the period Hp at high bias current. It indicates a transformation from the triangular period to the rectangular one by higher bias current and it agrees with the structural change of the Josephson vortex lattice in Ref. [21]. On the other hand, in high fields H/HpO2.5 shown in Fig. 4a, the oscillation peaks of FFR are inverted as bottoms and vice versa by the bias current. There is no change of the oscillation period. This inversion of the oscillation peaks of FFR is caused by the velocity change of the sliding Josephson vortices since the flux–flow voltage and FFR are proportional to vortex velocity [9]. In addition, we found that these FFR anomalies correlated with the transport properties such as resonant current steps on the I–V characteristics. 3.3. The relation between FFR oscillation and transport properties Shown in Fig.3b are the I–V characteristics measured at specific fields parallel to the layers. Considering the FFR
0
20
40
60
Voltage (mV) Fig. 4. (a) FFR at various currents of 1–10 mA at 50 K, in relatively high fields (H/HpZ2–6). (b) I–V characteristics and their differential conductance dI/dV at fields H/HpZ3, 4, 5, showing strong current steps.
oscillation, we classified the fields as (i) H/HpZ1.75, 2.25, 2.75, where FFR peaks are with half period (1/2Hp) corresponding to the triangular lattice, (ii) H/HpZ1.5, 2.5, and (iii) H/HpZ2, 3 where both bottoms and peaks of FFR oscillation with a period for the rectangular lattice. The increase of magnetic field decreased the hysteresis on I–V characteristics as well as the critical current then finally, the hysteresis disappeared above 2.5 H/Hp. We observed the pronounced current steps on the I–V characteristics only in rectangular period’s case as shown in (ii) and (iii) of Fig. 3b. In particular, odd steps came out at peak fields H/HpZn, and even steps revealed at bottom fields H=Hp Z nC 1=2 where n is integer number. Note that bottoms of FFR are transformed to peaks by increasing the bias current, with the oscillation period unchanged. In nonhysteretic region the current step is obvious. In the case of triangular lattice, no steps were observed. It is understood from our results that the I–V curves were modulated by the parallel magnetic fields forming Josephson vortex lattice, and especially the current steps appear in the specific fields
S. Kim et al. / Journal of Physics and Chemistry of Solids 67 (2006) 438–441
where are characterized by the oscillation of FFR corresponding to the period of the rectangular vortex lattice. Fig. 4b shows the I–V characteristics with steps and their differential conductance dI/dV at the fields H/HpZ3, 4, and 5. These steps are expected as a strong enhancement of superconducting current at fixed voltage when the Josephson frequencies match the resonant frequencies of cavity modes in parallel fields, i.e., Fiske steps [12,15,16]. The detailed discussion on Fiske steps with regarding the vortex motion will be reported elsewhere. In present experiments, we paid close attention to the strong enhancement of dI/dV at fixed voltages. As shown in Fig. 4b, maximum conductance appears at 12.1, 8.6 and 6.6 mA for H/HpZ3, 4, and 5. Compared with current-dependent FFR, the current levels of each steps in Fig. 4b actually coincide with the currents where the inversion from peak to bottom of FFR oscillation takes place, as marked in Fig. 4a. 4. Conclusion We measured a current dependence of FFR and transport properties determined by the dynamics of Josephson vortex lattice in BSCCO IJJs. We observed the transformation of FFR oscillation by increasing the bias current. In low fields H/Hp!2.5 the oscillation period corresponds to a triangular lattice and it is transformed to the period of a rectangular lattice by higher bias current. While in high fields H/HpO 2.5 we observed that the oscillation peaks showing the period of rectangular lattice are inverted to bottoms by current bias. From I–V characteristics we found clear current steps in the specific fields where exhibit the oscillation period of FFR corresponding to not triangular period but rectangular one. These steps were considered to be Fiske steps related with geometric resonance. The conductance enhancement of these current steps happened at constant voltage results in the inversion from the peaks to the bottoms of FFR oscillation. This inversion was considered to be the resonance of ac Jospephson frequency exited by the moving vortex lattice with the sample width (for rectangular lattice) and it maybe is from energy dissipation due
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to the Josephson plasma excitation. These results supply important information for possible high-frequency application of BSCCO IJJs. Acknowledgements The authors would like to thank T. Yamashita and M. Tachiki for valuable discussion. References [1] S. Beuven, O. Harnack, L. Amatuni, H. Kohlstedt, M. Darula, IEEE Trans. Appl. Supercond. 7 (1997) 2591. [2] K.K. Likharev, Dynamics of Josephson Junctions and Circuits, Gordon & Breach, 1986. [3] M. Machida, T. Koyama, A. Tanaka, M. Tachiki, Physica C 330 (2000) 85. [4] R. Kleiner, P. Mu¨ller, H. Kohlstedt, N.F. Pedersen, S. Sakai, Phys. Rev. B 50 (1994) 3942. [5] S. Heim, M. Mo¨ßle, T. Clauß, R. Kleiner, Supercond. Sci. Technol. 15 (2002) 1226. [6] S. Sakai, A.V. Ustinov, N. Thyssen, H. Kohlstedt, Phys. Rev. B 58 (1998) 5777. [7] J.H. Kim, Phys. Rev. B 65 (2002) 100509. [8] S. Ooi, T. Mochiku, K. Hirata, Phys. Rev. Lett. 89 (2002) 247002. [9] M. Machida, Phys. Rev. Lett. 90 (2003) 037001. [10] A.E. Koshelev, Phys. Rev. B 62 (2000) R3616. [11] Yu.I. Latyshev, A.E. Koshelev, L.N. Bulavskii, Phys. Rev. B 68 (2003) 134604. [12] A.E. Koshelev, I.S. Aranson, Phys. Rev. Lett. 85 (2000) 3938. [13] T. Koyama, M. Tachiki, Solid State Commun. 96 (1995) 367. [14] V.M. Kransnov, N. Mros, A. Yurgens, D. Winkler, Phys. Rev. B 59 (1999) 8463. [15] M.D. Fiske, Rev. Mod. Phys. 36 (1964) 221. [16] I.O. Kulik, JETP Lett. 2 (1965) 84. [17] A. Irie, Y. Hirai, G. Oya, Appl. Phys. Lett. 72 (1998) 2159. [18] M. Nagao, M. Sato, H. Maeda, S. Kim, T. Yamashita, Appl. Phys. Lett. 79 (2001) 2612. [19] A. Irie, S. Heim, S. Schromm, M. Mo¨ßle, T. Nachtrab, M. Go´do, R. Kleiner, P. Mu¨ller, G. Oya, Phys. Rev. B 62 (2000) 6681. [20] T. Hatano, M.B. Wang, S.M. Kim, S. Urayama, S. Kawakami, S.-J. Kim, M. Nagao, K. Inomata, Y. Takano, T. Yamashita, M. Tachiki, IEEE Trans. Appl. Supercond. 15 (2005) 912. [21] K. Hirata, S. Ooi, E.H. Sadki, T. Mochiku, Physica B 329–333 (2003) 1332.