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Applied Physics Letters

Shapiro steps observed in annular intrinsic Josephson junctions at low microwave frequencies

Huabing Wang, Sunmi Kim, Shinya Urayama, Masanori Nagao, Toshihisa Hatano, Shunichi Arisawa, Takashi Yamashita, Peiheng Wu

Published Feb 6, 2006• 15 citations
Publisher

Abstract

With annular intrinsic Josephson junctions structured from a slice of Bi2Sr2CaCu2O8+δ single crystal 200nm thick, we have devised a way to inject fluxons into the junctions only by sweeping the bias current. The stacks have shown typical properties concerning Josephson fluxon conservation. These annular junctions are sensitive to microwave irradiation, and quite sharp Shapiro steps have been observed at ∼20GHz which is much lower than the plasma frequency of the junctions, indicating many applications can be possible with practically accessible microwave sources.

DOI
10.1063/1.2172010

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With annular intrinsic Josephson junctions structured from a slice of Bi2Sr2CaCu2O8+δ single crystal 200nm thick, we have devised a way to inject fluxons into the junctions only by sweeping the bias current. The stacks have shown typical properties concerning Josephson fluxon conservation. These annular junctions are sensitive to microwave irradiation, and quite sharp Shapiro steps have been observed at ∼20GHz which is much lower than the plasma frequency of the junctions, indicating many applications can be possible with practically accessible microwave sources.
With a thin gold layer, we fabricated mesa-like shunted intrinsic Josephson junction stacks on Bi2Sr2CaCu2O8+x single crystals. Instead of the multibranch structure and large voltage jumps often observed in conventional intrinsic junctions, current–voltage characteristics typical of resistively shunted junctions were obtained. Individual Shapiro steps were clearly visible with irradiation at frequencies from a few to 20 GHz. The experiments demonstrated not only the ac Josephson effects directly but also the possibility of employing intrinsic Josephson junctions in high-frequency applications.
With a 10 μm×10 μm mesa patterned on Bi2Sr2CaCu2O8 single crystals, we measure the current–voltage (I–V) curves of a stack of intrinsic Josephson junctions. Current steps are observed at an equal voltage spacing of 4 mV when the sample is subjected to microwave radiation at around 7 GHz. With increase of the microwave power, more steps occur while the spacing between neighboring steps does not seem to change. The magnitude of each step depends on the microwave power in an oscillating way. Tuning the microwave frequency causes such steps to occur over separate frequency ranges, and each range is quite narrow. A temperature rise from 4.2 to 14.3 K completely quenches the step structures. Possible explanations for the step structures, based on resonances excited by microwave or geometric resonances in the junction cavity, are discussed.
Microwave-induced steps were observed in I-V characteristics of intrinsic Josephson junctions fabricated on an overdoped BSCCO single crystal with a weak magnetic field parallel to the c-axis. Voltage intervals between the steps under the irradiation of 12.2, 13.2 and 13.9 GHz microwaves were independent of the microwave power and proportional to the frequency. From the measurement of voltage intervals, it was found that all the junctions in the fabricated junction stack simultaneously produced Shapiro steps, assuming that the steps are Shapiro steps.
Zero-crossing Shapiro steps in focused-ion-beam-tailored high-TcT_cTc​ superconducting microstructures
Microwave response of S-shaped Bi2_22​Sr2_22​CaCu2O8+x_{8+x}8+x​ (Bi-2212) micron-scale samples, in which the supercurrent was forced to flow perpendicular to the crystal layers, was investigated. A treatment with a focused ion beam allowed us to reduce the plasma frequency down to fpf_pfp​$\sim$5 GHz at TTT=0.3 K in naturally stacked Josephson junctions in a crystal. We observed Shapiro steps at frequencies as low as $\sim$5 GHz. Well-developed zero-crossing Shapiro steps were observed at frequencies as low as $\sim$10 GHz. They appeared as constant-voltage plateaus with a non-zero voltage occurring at zero bias current. We confirmed that zero-crossing Shapiro steps in the Bi-2212 stacked junctions can be observed when the irradiated frequency is sufficiently larger than fpf_pfp​. The observed high-order fractional steps in the microwave responses indicate that the interlayer-coupled Bi-2212 Josephson junctions have nonsinusoidal current-phase relation. Based on the temperature dependence of the steps we also showed that the finite slope of the steps is due to the enhancement of the phase diffusion effect.
On a slice of 200–300-nm-thick Bi2Sr2CaCu2O8+x, stacks of intrinsic Josephson junctions are patterned, connected to each other, and integrated with a bow-tie antenna and some chokes. Over an area of 150 μm by 170 μm, we have been able to fabricate up to 256 stacks with over 11 000 junctions. Such three-dimensional arrays of intrinsic Josephson junctions respond sensitively to irradiation at 760 GHz from a far-infrared laser, showing clear zero-crossing Shapiro steps up to 0.75 V. In addition, possible applications in quantum voltage standards are discussed here. The high density of integration of the Josephson junctions is also promising for many other applications.