We investigated macroscopic quantum tunneling (MQT) of Bi2Sr2CaCu2Oy intrinsic Josephson junctions (IJJs) for two device structures. One is a small mesa, which is a few nanometers thick with only two or three IJJs, and the other is a stack of a few hundred IJJs in a narrow bridge structure. The experimental results regarding the switching current distribution for the first switch from the zero-voltage state were in good agreement with the conventional theory for a single Josephson junction, indicating that the crossover temperature from thermal activation to the MQT regime for the former device structure was similar to that for the latter device structure. Together with the observation of multiphoton transitions between quantized energy levels in the MQT regime, these results strongly suggest that the observed MQT behavior is intrinsic to a single IJJ in high-Tc cuprates and is independent of the device structure. The switching current distribution for the second switch from the first resistive state, which was carefully distinguished from the first switch, was also compared with respect to the two device structures. In spite of the differences between the heat transfer environments, the second switch exhibited a similar temperature-independent behavior for both devices up to a much higher temperature than the crossover temperature for the first switch. We argue that this cannot be explained in terms of self-heating caused by dissipative currents after the first switch. As possible candidates for this phenomenon, the MQT process for the second switch and the effective increase of the electronic temperature due to the quasiparticle injection are discussed.

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We have realized a new method for the fabrication of a small mesa structure made of a few Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions by using polyimide as an adhesive to glue a crystal on a substrate. It is demonstrated that this method provides a small mesa structure having only five junctions and exhibiting uniform tunneling characteristics. We describe the details of the fabrication process and the current–voltage characteristics for a typical mesa obtained, together with the short pulse tunneling spectroscopy results.

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Short-pulse tunneling spectroscopy on a time scale of 300 ns has been conducted using intrinsic Josephson junctions naturally built in the crystal structure of a slightly underdoped Bi2Sr2CaCu2O8+δ (Bi2212) by fabricating a very small and thin mesa of 6 nm in thickness and less than 5 µm in square width. The results are characterized by a pronounced superconducting peak at 79 meV accompanied by a broad pseudgap at a much higher energy of 125 meV, indicating discrete nature of both energy structures. The temperature rise due to self-heating in a small mesa is numerically calculated based on temperature-dependent thermal conductivity and specific heat for each constituent material. It is found that the temperature rise is less than 2 K for the present experiment, which reinforces that the tunneling spectra obtained represent the genuine superconducting properties of Bi2212.