Intrinsic Josephson junctions under microwave irradiation

ELSEVIER

Physica C 293 (1997) 25-30

Intrinsic Josephson junctions under microwave irradiation W. Prusseit a,,, M. Rapp ~, K. Hiram b, T. Mochiku b a Physik Dept. EIO, Technical University Munich, 85747 Garching, Germany b National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, 305 Japan

Abstract Small mesa-type junctions around 10 ~m lateral size have been realized by argon ion milling on top of high quality Bi2Sr2CaCu208 single crystals and studied in external microwave fields. Although the junction dimensions are much smaller than the magnetic penetration depth they have to be regarded as long junctions since phase variations occur on a smaller scale. A couple of unexpected features in the current-voltage characteristics, including zero current crossings, are most likely understood by coherent vortex flow. © 1997 Elsevier Science B.V.

1. Introduction Since the discovery of the intrinsic Josephson properties of the layered high temperature superconductors (HTS) by Kleiner et al. [1,2], this natural arrangement of stacked junctions has been in the discussion for high frequency applications. Phase locking in a series array of Josephson junctions results in narrow band, tunable rf radiation where the bandwidth is reduced ( A f c t 1/N) with increasing number N of participating junctions. In long stacked junctions vortices are coupled and will arrange in stable configurations moving collectively and giving rise to resonances dependent on the junction geometry [3]. Actually, coupled fluxon modes have already been observed in artificially grown stacks of long, conventional N b / A I O x / N b junctions which can be regarded as model systems for the layered cuprates

* Corresponding author. Fax: +49 089 2891 2414; e-mail: prusseit @physik.tu-muenchen.de.

[4]. Such flux flow oscillators could be used as high power rf sources as well [5]. Moreover, rf irradiation of junctions can lead to photon assisted Cooper pair and quasiparticle tunneling. Close to the resulting step features in the current-voltage characteristics heterodyne mixing is even expected to produce a conversion gain [6-8]. Another intriguing application of the inverse Josephson effect is the voltage standard which is based on hysteretic Shapiro steps crossing the voltage axis at zero current [9]. Hence, microwave irradiation gives rise to a frequency defined dc voltage drop across the junctions. For a realistic device, however, thousands of junctions in series are needed to produce a reasonable output voltage - - a fact that seems to render the HTS a gift of nature. Within this paper we study the response of hysteretic, intrinsic Josephson junctions to an external microwave field. It will turn out, that a profound understanding of the rf driven vortex flow is the key to explain the observations.

09214534/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 ( 9 7 ) 0 1 5 0 2 - 5

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W. Prusseit et al. / Physica C 293 (1997) 25-30 16

2. Experimental For the present study we employed high quality Bi~SrzCaCu20 8 (BSCCO) single crystals grown by the travelling solvent floating zone technique [10]. The boules were cut into several pieces of quadratic 5 × 5 × 1 1111113 large single crystals. Prior to the patterning process these crystals were cleaved into several hundred micrometer thick platelets and sputter deposited with gold contact layers on both sides. On top of each crystal plate some hundred mesa structures ranging from 10 × 10 ixm 2 down to 6 X 6 ~ m 2 were patterned using grids for electron microscopy as masks. The mesa height between 50 and 300 nm was controlled via ion milling time. No degradation of the superconducting properties could be observed after this treatment. To enhance the anisotropy some of the samples were annealed at 400°C at an oxygen pressure of 0.1 mbar for 10 h. The crystals were glued onto a gold plated silicon substrate with silver epoxy paste which defined the bottom electrode. This chip was mounted next to an X-band waveguide (WR75). The dc top contact to single mesas could be realized by a 25 Ixm A u / N i wire with a tip of less than 2 Izm in diameter. Since this wire runs through holes across the waveguide it simultaneously acts as an antenna for coupling in the microwave signal. It should be noted that this arrangement resulted in an asymmetric feed current from the edge of the mesa and contact resistivities between 5 X 10 -6 and 5 × 10 -5 12 cm 2. The contact resistance of this two-point configuration has been subtracted in the data below. On each single crystal several mesas were measured in succession and gave identical results.

3. Results and discussion In Fig. 1 the resistive transitions across such mesas are shown for two different samples. Upon cooling the resistance of the as-grown crystal (JP2) is decreasing before it turns up into a semiconducting behavior below 180 K. Above the sharp transition around 88 K the resistivity reaches its maximum of 5.5 1"~ cm. Obviously, the as-grown crystals are slightly overdoped. Annealing at 0.1 mbar (JP3) results in an underdoped state characterized by a

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150 200 Temperature

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Fig. 1. Resistive transitions of mesa structures on top of as-grown (JP2) and annealed (JP3) BSCCO single crystals.

negative slope up to room temperature. The maximum resistivity increases up to 15.7 f~ cm while Tc nearly remains unchanged around 86 K. All the data which will be discussed below were taken at 4.2 K. The only difference between annealed and unannealed samples was a reduction of the critical current density by a factor of three. The features seen in the I - V curves, however, were sample independent besides this scaling factor. The current-voltage characteristic of a 6 X 6 txm 2 mesa (JP2) is depicted in Fig. 2 where only a few of the resistive branches have been traced out. From the mesa height of 250 nm we expect a stack of 160 junctions. The minimum voltage steps between adjacent branches, which can be attributed to the switching of single junctions, are 10 mV summing up to a

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W. Prusseit et al. / Physica C 293 (1997) 25-30

total voltage drop of 1.8 V in the resistive state. These values are considerably smaller than expected from a 2A gap voltage of 35-50 mV which is usually seen in tunneling spectroscopy. As demonstrated by Tanabe et al., this strong gap suppression can be understood as a direct consequence of the strong quasiparticle injection into the thin superconducting CuO 2 double layers [11,12]. Taking into account the d-wave character of the order parameter they deduced a cubic voltage dependence of the quasiparticle tunneling which is in reasonably good agreement with our data. In the light of this reasoning the sharp voltage step (g.s.) around 0.6 mA can be explained by a positive feedback of the quasiparticle current on the gap voltage leading to a collapsed, nonuniform current flow - - a n effect that is well known from conventional LTS tunnel junctions

[131. 4. Intrinsic Josephson junctions under microwave irradiation When a microwave field is irradiated onto the stacked junctions the I - V curves change drastically. Fig. 3 gives the current-voltage characteristics of the same mesa (JF2) as above at different power levels of the applied 12 GHz field. It is worth mentioning, however, that the power just represents the clystron generator and not the field with is actually coupled into the junction. The latter depends on the dynamic 1.2

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impedance of the mesa which changes with bias. To avoid overlap the curves are vertically offset by 0.2 mA. At these power levels the dc Josephson current is completely suppressed. While the overall appearance of the outermost resistive trace hardly changes the maximum critical current and the dynamic conductivity of the branches is decreased resulting in a bending down towards the quasiparticle tunneling curve. Due to the reduction of the critical current densities and the smearing of the gap by photon assisted tunneling the hysteresis becomes smaller and vanishes in the high power limit. Moreover, closer inspection reveals that the maximum gap voltage is reduced with increasing rf amplitude due to additional ac currents which further enhance the quasiparticle injection. Hence, this rf induced gap suppression gives a more realistic estimate of the power actually coupled to the junctions. The onset of switching of single junctions is marked by a pronounced step structure shifting to higher currents and voltages with increasing rf power. The most interesting structures are observed below that voltage. An exploded view of this regime (rectangular window) is depicted in Fig. 4. We can distinguish four characteristic features (denoted A D). At low bias close to zero the dynamic resistance is very high (A). There are two possible mechanisms to explain this behavior. The first is photon assisted Cooper pair tunneling. In the quantum limit this process leads to Shapiro steps. However, even at very much lower rf power, we never observed dis-

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W. Prusseit et al. / Physica C 293 (1997) 25-30

tinct Shapiro step structures. Obviously, this would require phase locking of many junctions to the external frequency which is hard to achieve. In the classical limit (eVrf > hf) or without phase locking the steps overlap and give rise to a continuously increasing voltage as observed here [14]. The second mechanism is vortex flow in the ab-direction. Although the mesas are much smaller than the magnetic screening length (50-100 ~m) they are still larger than the Josephson penetration depth ( ~ 1 I~m) which sets the length scale of phase variations along the junctions [2]. Hence, Josephson vortices can be generated and driven across the mesa. In fact, the I - V characteristic is nonlinear in this regime and exhibits inflection points. As we will see below a lot of structure develops here with increasing microwave power. A kink leads to the low resistance voltage step (B) mentioned above. This structure is most probably due to vortex flow across the mesa and resembles fluxon modes seen in long stacked N d / A I O x / N b tunnel junctions in external magnetic fields [4]. In our case, however, vortices are more likely injected by the asymmetric feed current lead. When the current is further enhanced, the first junctions of the stack start to switch into the resistive state. However, before the first branches become stable at higher voltages (D) we observe a fast, chaotic switching in regime (C). Actually, within the minimum integration period of 2 ms of the dc voltmeter no branches can be resolved and at a certain bias current every voltage value within (C) is realized at random in striking contrast to the distinct branches seen without irradiation. This new phenomenon of random switching could be understood as a dynamic feedback between the mesa and the radiation field. The changing impedance modifies the coupling strength to the radiation field which in turn shifts the bias point. The evolution for the low bias regime (A,B) with increasing power level is depicted in Fig. 5. As a consequence of irradiation inflection points appear and smoothly evolve into hysteretic voltage jumps. Up to around 4.5 dB m these features are continuously shifted to higher voltage and lower bias current values (see Fig. 5(a)). The bottom part of the curve even intersects the voltage axis at zero current crossings (ZCC). Above 5.5 dB m the situation changes

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completely: features become retrograde, zero current crossings vanish and new resonances appear at higher bias (see Fig. 5(b)). Obviously, these structures cannot be understood in terms of photon assisted tunneling and Shapiro steps because the voltage is much higher than expected from the Josephson frequency-voltage relation and the position strongly depends on power. Very similar steps have been reported by Irie and Oya in much larger junctions [ 15,16]. However, there are marked differences. First, the mesa junctions used in the present study are by a factor of 100 smaller in their lateral dimensions and the current densities at which resonances occur are orders of magnitude higher. Second, the extremely high voltage of some hundred millivolts has to be compared with some few millivolts found previously. Further-

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W. Prusseitet al. / Physica C 293 (1997) 25-30

more, the steps are neither equally spaced, nor do they simply correlate with the upturn (B). The only reasonable explanation for these structures is in terms of coherent vortex motion resonating within the junction cavity where the steps represent different modes. The detailed structure of the resonances should strongly depend on the junction geometry which might provide an explanation for the differences mentioned above. A detailed view of the zero current crossings is given in Fig. 6 for two slightly different power levels of the 12 GHz signal. Around 2.5 dB m two symmetric but not equally spaced crossings can be stabilized whereas at 3 dB m the low voltage crossing has become unstable and only the higher voltage crossing is accessible. As already indicated in Fig. 5(a), with increasing power the voltage of the ZCC reaches a maximum before it drops to zero. Fig. 7 shows the ZCC at the maximum voltage value for three different frequencies of the radiation field. Again, there is an additional mode at 10 GHz which is completely absent at higher frequencies. The power and frequency dependence of the ZCC is depicted in Fig. 8. At 12 GHz the absolute maximum occurs around 2.75 mW. However, a closer inspection reveals an oscillatory behavior with at least one side maximum at 5 mW. This strange power dependence rules out a simple rectification effect, e.g. by the point contact to the mesa. Instead, it rather supports the picture of coherent vortex motion and the sharp cutoff at 3 mW suggests that the mode responsible for this high voltage ZCC has become unstable and given way to

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Fig. 7. Maximum voltage zero current crossings at 10, 12 and 15 GHz. The additional zero voltage step is only accessible at 10 GHz. another vortex configuration. Comparing the maximum dc voltage at three different frequencies the inset shows a linear dependence. If vortices with a density n per length are driven across the junction of length L at the external beating frequency f , a voltage drop of V0 = nCboLf can be expected in agreement with the observed linear frequency dependence. However, the measured voltage values can only be explained by an extremely high vortex density which seems unphysical. Hence, although the idea of coherently moving vortices could account for a couple of experimental findings, there are still open questions. A study of the junction size dependence of the phenomenon will greatly help to test the vortex flow working hypothesis. 160 140

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W. Prusseit et aL // Physica C 293 (1997) 25-30

5. Summary In summary, we have studied the effect of microwave irradiation on hysteretic, intrinsic Josephson junctions in BSCCO single crystals. Besides gap suppression and critical current reduction a variety of complex and unexpected structure shows up in the current-voltage characteristics at low bias including zero current crossings. However, none of these features could be assigned to Shapiro steps and simultaneous phase locking of all junctions to an external frequency seems extremely hard to achieve. It should be emphasized that the zero current crossings depend on the power level of the irradiated microwave field and cannot be used e.g. as a voltage standard since they do not obey a simple frequency-voltage relation. The most likely explanation for these features is coherent vortex motion within a cavity given by the junction dimensions. Although the employed mesa structures are small with respect to the magnetic screening length, they are still larger than the Josephson penetration depth which determines the scale of phase variations. Consequently, they have to be regarded as long junctions. A systematic study of junction size effects on the resonance structures should help to verify the coherent vortex flow picture.

Acknowledgements We would like to thank H. Kinder for his support and helpful discussions. The financial support of the

Bavarian Research Foundation via FORSUPRA is gratefully acknowledged.

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