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arXiv:0802.2091

Zero-crossing Shapiro steps in focused-ion-beam-tailored high-$T_c$ superconducting microstructures

Myung‐Ho Bae, R. C. Dinsmore, Mitrabhanu Sahu, Hu-Jong Lee, Alexey Bezryadin

Published Feb 14, 2008• 0 citations
arXivHTMLPDF

Abstract

Microwave response of S-shaped Bi$_2$Sr$_2$CaCu2O$_{8+x}$ (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 $f_p$$\sim$5 GHz at $T$=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 $f_p$. 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.

DOI
10.48550/arxiv.0802.2091

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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 f_p$$\sim5 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.
We have investigated the RF-induced steps in intrinsic Josephson junctions in mesa-shaped Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub y/ single crystals. By applying RF signal in the frequency range 2-20 GHz to the mesa, two types of responses were observed depending on the frequency of the applied microwave. One is the Shapiro step response observed at the frequencies above the Josephson plasma frequency for surface junction. Another is the microwave-induced vortex flow response for surface and inner junctions. We found that the junction size is also the key condition for the observation of Shapiro steps together with the frequency of applied microwave.
We propose a new method for fabricating resistively shunted intrinsic Josephson junctions (RS-IJJs). Two mesas, measuring 6 × 6μm 2 and 100 × 100 μm 2 respectively in the a - b plane, have been fabricated in a Bi 2 Sr 2 CaCu 2 O 8+ x single crystal with a SiO layer evaporated in-between as an insulator. A gold stripe was sputtered onto the insulator to connect the mesas, thus effectively shunting the smaller one. The current–voltage ( I – V ) curves of such RS-IJJs have shown the typical behaviour of a resistively shunted junction with greatly reduced hysteresis and voltage jump. Under microwave irradiation at a 3 mm waveband, we have registered three types of Shapiro steps: conventional, sub-harmonic and zero-crossing. In this rapid communication, we discuss the fabrication details, as well as the observations of the Shapiro steps.
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.
The microwave response properties of Bi 2 Sr 2 CaCu 2 O 8+x long intrinsic Josephson junctions have been experimentally and theoretically investigated. We fabricated a mesa structure consisting of 14 junctions, 50 μm long and 4 μm wide, and observed the microwave-induced steps in the I-V characteristics under irradiation with microwaves in the range 5–8 GHz. The step voltages were proportional to the square root of the microwave power, implying that the steps were produced by the motion of fluxons created by the microwaves. The Josephson frequency corresponding to the maximum step voltage was estimated to be 570 GHz. The step voltage decreased with increasing temperature, which can be explained by the decrease of the Swihart velocity. We performed a numerical simulation using coupled sine-Gordon equations and obtained a power dependence of the step voltage similar to the experimental result, and demonstrated standing waves of electric fields with in-phase plasma oscillation in all the junctions. These facts suggest that the steps observed in the experiment were produced by plasma resonance excited by the external microwaves.
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.