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Investigation of microwave responses in BSCCO-2212 intrinsic Josephson junctions up to short millimeter wavebands
ELSEVIER
Physica C 3 4 1 - 3 4 8 (2000) 2 7 3 7 - 2 7 3 8 w ww.el sevier.nl/Iocate/physc
I n v e s t i g a t i o n of m i c r o w a v e r e s p o n s e s in BSCCO-2212 i n t r i n s i c J o s e p h s o n j u n c t i o n s up to short m i l l i m e t e r w a v e b a n d s a,b
a,b
a
.
.a
.
.a,b
..
ab
H. B. Wang , J. C h e n , Y. Aruga, T. Taehikl, Y. Mlzugakl , K. Nakapma ' , • be d T. Yamashlta ', and P. H. Wu aResearch Institute of Electrical Communication, Tohoku University, Sendal 9808577, Japan bCREST, Japan Science & Technology Corporation (JST), Japan CNew Industry Creation Hatchery Center, Tohoku University, Sendal, Japan dDepartment of Electronic Science & Engineering, University of Nanjing, Nanjing 210093, China With an overlap structure for improving coupling, intrinsic Josephson junctions (IJJs) with an a-b plane area down to 1 ~tm2 were fabricated on argon-annealed Bi~SrzCaCu208÷" single crystals. Responses at frequencies from a few to 200 gigahertz were carefully investigated by measuring the samples' current-voltage (I-V) curves. The experimental results indicate that high frequency applications of IJJs over a very wide frequency range are potentially possible• 1. INTRODUCTION Intrinsic Josephson junctions (IJJs) in high-T c superconductors are good candidates for very high frequency applications [1,2] due to the large energy gap and characteristic voltage, as well as junction array's capability to modify resistance and thus to improve the microwave coupling. So far responses of IJJs in Bi~Sr2CaCu2Os~~ (BSCCO) have been studied mainly at frequencies below 20 GHz, with a few experiments reported for much higher frequencies [3,4]. Using an overlap structure effective to improve microwave coupling and to reduce junction's a-b plane area, we report in this paper our recent results of microwave responses at frequencies up to 200 GHz.
2. EXPERIMENTAL In order to reduce contact resistance and modify oxygen doping, we started our experiments using annealed BSCCO single crystals covered with a 1000/~ gold layer• To get low critical current density and low junction plasma frequency, argon gas was used in the annealing, resulting in a critical current density of a few hundred Amperes per square centimeter, much lower than what could be 0921-4534/00/$ - see front matter ~, 2000 Elsevier Science BA( PII S0921-4534(00)01472-6
obtained from oxygen annealed samples. As reported by us previously [5], the fabrication procedures for overlap structure could be briefly summarized as (i) the gold-covered single crystals were etched into a stop-like structure by At-ion milling; (ii) a SiO layer was evaporated on top of the sample as an insulator; (iii) a second gold layer was sputtered onto the top of the sample and a photoresist strip was photolithographically patterned at the edge of single crystals; and (iv) the samples were controllably milled to a certain depth, typically producing an array made of a few tens of junctions. All the samples were measured with two or three probes• The patterned gold strip, usually integrated with an antenna, was used as top electrode. With the above-mentioned process, we were able to fabricate IJJs with a-b plane sizes down to 1 ~tm2 and the number of junctions involved was also controllable (down to 3). Shown in Fig. 1 (a) and its close-up are the typical dc I-V curves for a sample with an a-b plane area of 4 ~tm2 and containing about 20 junctions as shown by the number of the resistive branches. Due to the tiny a-b plane sizes and small junction number, nonequilibrium injection of quasiparticles and ohmic All rights reserved.
2738
H.B. Wang et aL /Physica C 341-348 (2000) 2737-2738
heating were not serious in the sample, therefore, the back-bending of I-V curves was not visibly observed and the energy gap 2A was up to 50 meV. In order to observe microwave responses, we mounted the sample at the end of a 3 mm waveguide, making the c-axis of the former parallel to the narrow side of the latter for best coupling. The microwave power was transferred to the sample from a Gunn oscillator (for measurements at 200 GHz a doubler was inserted) via a dielectric waveguide. Thanks to the present structure and the tiny sizes, the IJJs were more sensitive to microwave irradiation than the IJJs in conventional mesa structure were. But, at any power level, no individual Shapiro steps were observed. Instead, we observed remarkable suppression of critical currents, and disappearance of multi-resistive branches with unchanged last resistive branch at much higher power level, as shown in Fig.1 (b) to (d). Further increasing the power, we observed a typical power-dependent current upturns on current-voltage curves, which
was very similar to what we reported for YBa2Cu307_8 IJJs. One may also notice that there was an uneven distribution of suppressed critical currents, which were observed in most of our samples. It indicated that the coupling between the microwaves and the junctions could be improved by properly biasing the junction array. Even at 200 GHz, suppression of critical currents was still clearly observed. 3. DISCUSSIONS AND CONCLUSIONS In short, using an overlap structure, we were able to effectively couple signals at short millimeter wavebands to intrinsic Josephson junctions. The individual Shapiro steps were not seen, probably because the radiation frequency was not high enough compared with plasma frequency of a single junction (72 GHz, given the dielectric constant of 10) [6], or the phase differences of all junctions were not stable or in-phase. However, upturns did appear on the IoV curves which was explained as the envelope of high order Shapiro steps [7], indicating that applications over a wide frequency range are potentially possible. Obviously more experimental and theoretical work should be done for further understanding of this phenomenon. Authors would like to thank Alex I. Braginski for fruitful discussions. REFERENCES
Fig. 1. Typical current-voltage curves, of a mesa on BSCCO single crystal with an a-b plane area of 4 ~m 2, without (a) and with increasing microwave powers at 100 GHz (b)-(d).
1R. Kleiner, F. Steinmeyer, G. K,mkel, P. Miiller, Phys. Rev. Lett., 68(1992)2394. 2 G. Oya, N. Aoyama, A. Irie, et al., Jpn. J. Appl. Phys., 31(1992)L829. 3W. Walkenhorst, G. Hechtfischer, S. Schlotzer, et al.,Phys. Rev. B, 56(1997)8396. ' H. B. Wang, Y. Aruga, T. Tachiki, et al., Appl. Phys. Lett., 75(1999)2310. H. B. Wang, J. Chen, T. Tachiki, and T. Yamashita, IEEE Trans. on Appl. Supercond., 9(1999)3008. R. L. Kautz, J. Appl. Phys. 52(1981)3528. 7 H. B. Wang, J. Chen, T. Tachiki, et al., J. of Appl. Phys., 85(1999)3740.
Physica C 3 4 1 - 3 4 8 (2000) 2 7 3 7 - 2 7 3 8 w ww.el sevier.nl/Iocate/physc
I n v e s t i g a t i o n of m i c r o w a v e r e s p o n s e s in BSCCO-2212 i n t r i n s i c J o s e p h s o n j u n c t i o n s up to short m i l l i m e t e r w a v e b a n d s a,b
a,b
a
.
.a
.
.a,b
..
ab
H. B. Wang , J. C h e n , Y. Aruga, T. Taehikl, Y. Mlzugakl , K. Nakapma ' , • be d T. Yamashlta ', and P. H. Wu aResearch Institute of Electrical Communication, Tohoku University, Sendal 9808577, Japan bCREST, Japan Science & Technology Corporation (JST), Japan CNew Industry Creation Hatchery Center, Tohoku University, Sendal, Japan dDepartment of Electronic Science & Engineering, University of Nanjing, Nanjing 210093, China With an overlap structure for improving coupling, intrinsic Josephson junctions (IJJs) with an a-b plane area down to 1 ~tm2 were fabricated on argon-annealed Bi~SrzCaCu208÷" single crystals. Responses at frequencies from a few to 200 gigahertz were carefully investigated by measuring the samples' current-voltage (I-V) curves. The experimental results indicate that high frequency applications of IJJs over a very wide frequency range are potentially possible• 1. INTRODUCTION Intrinsic Josephson junctions (IJJs) in high-T c superconductors are good candidates for very high frequency applications [1,2] due to the large energy gap and characteristic voltage, as well as junction array's capability to modify resistance and thus to improve the microwave coupling. So far responses of IJJs in Bi~Sr2CaCu2Os~~ (BSCCO) have been studied mainly at frequencies below 20 GHz, with a few experiments reported for much higher frequencies [3,4]. Using an overlap structure effective to improve microwave coupling and to reduce junction's a-b plane area, we report in this paper our recent results of microwave responses at frequencies up to 200 GHz.
2. EXPERIMENTAL In order to reduce contact resistance and modify oxygen doping, we started our experiments using annealed BSCCO single crystals covered with a 1000/~ gold layer• To get low critical current density and low junction plasma frequency, argon gas was used in the annealing, resulting in a critical current density of a few hundred Amperes per square centimeter, much lower than what could be 0921-4534/00/$ - see front matter ~, 2000 Elsevier Science BA( PII S0921-4534(00)01472-6
obtained from oxygen annealed samples. As reported by us previously [5], the fabrication procedures for overlap structure could be briefly summarized as (i) the gold-covered single crystals were etched into a stop-like structure by At-ion milling; (ii) a SiO layer was evaporated on top of the sample as an insulator; (iii) a second gold layer was sputtered onto the top of the sample and a photoresist strip was photolithographically patterned at the edge of single crystals; and (iv) the samples were controllably milled to a certain depth, typically producing an array made of a few tens of junctions. All the samples were measured with two or three probes• The patterned gold strip, usually integrated with an antenna, was used as top electrode. With the above-mentioned process, we were able to fabricate IJJs with a-b plane sizes down to 1 ~tm2 and the number of junctions involved was also controllable (down to 3). Shown in Fig. 1 (a) and its close-up are the typical dc I-V curves for a sample with an a-b plane area of 4 ~tm2 and containing about 20 junctions as shown by the number of the resistive branches. Due to the tiny a-b plane sizes and small junction number, nonequilibrium injection of quasiparticles and ohmic All rights reserved.
2738
H.B. Wang et aL /Physica C 341-348 (2000) 2737-2738
heating were not serious in the sample, therefore, the back-bending of I-V curves was not visibly observed and the energy gap 2A was up to 50 meV. In order to observe microwave responses, we mounted the sample at the end of a 3 mm waveguide, making the c-axis of the former parallel to the narrow side of the latter for best coupling. The microwave power was transferred to the sample from a Gunn oscillator (for measurements at 200 GHz a doubler was inserted) via a dielectric waveguide. Thanks to the present structure and the tiny sizes, the IJJs were more sensitive to microwave irradiation than the IJJs in conventional mesa structure were. But, at any power level, no individual Shapiro steps were observed. Instead, we observed remarkable suppression of critical currents, and disappearance of multi-resistive branches with unchanged last resistive branch at much higher power level, as shown in Fig.1 (b) to (d). Further increasing the power, we observed a typical power-dependent current upturns on current-voltage curves, which
was very similar to what we reported for YBa2Cu307_8 IJJs. One may also notice that there was an uneven distribution of suppressed critical currents, which were observed in most of our samples. It indicated that the coupling between the microwaves and the junctions could be improved by properly biasing the junction array. Even at 200 GHz, suppression of critical currents was still clearly observed. 3. DISCUSSIONS AND CONCLUSIONS In short, using an overlap structure, we were able to effectively couple signals at short millimeter wavebands to intrinsic Josephson junctions. The individual Shapiro steps were not seen, probably because the radiation frequency was not high enough compared with plasma frequency of a single junction (72 GHz, given the dielectric constant of 10) [6], or the phase differences of all junctions were not stable or in-phase. However, upturns did appear on the IoV curves which was explained as the envelope of high order Shapiro steps [7], indicating that applications over a wide frequency range are potentially possible. Obviously more experimental and theoretical work should be done for further understanding of this phenomenon. Authors would like to thank Alex I. Braginski for fruitful discussions. REFERENCES
Fig. 1. Typical current-voltage curves, of a mesa on BSCCO single crystal with an a-b plane area of 4 ~m 2, without (a) and with increasing microwave powers at 100 GHz (b)-(d).
1R. Kleiner, F. Steinmeyer, G. K,mkel, P. Miiller, Phys. Rev. Lett., 68(1992)2394. 2 G. Oya, N. Aoyama, A. Irie, et al., Jpn. J. Appl. Phys., 31(1992)L829. 3W. Walkenhorst, G. Hechtfischer, S. Schlotzer, et al.,Phys. Rev. B, 56(1997)8396. ' H. B. Wang, Y. Aruga, T. Tachiki, et al., Appl. Phys. Lett., 75(1999)2310. H. B. Wang, J. Chen, T. Tachiki, and T. Yamashita, IEEE Trans. on Appl. Supercond., 9(1999)3008. R. L. Kautz, J. Appl. Phys. 52(1981)3528. 7 H. B. Wang, J. Chen, T. Tachiki, et al., J. of Appl. Phys., 85(1999)3740.