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arXiv:0807.1194 · Physical Review B

Comparative study of macroscopic quantum tunneling in Bi2Sr2CaCu2Oy intrinsic Josephson junctions with different device structures

K. Ota, Kenji Hamada, Ryota Takemura, Masayuki Ohmaki, T. Machi, K. Tanabe, Minoru Suzuki, Atsutaka Maeda, Haruhisa Kitano

Published Apr 3, 2009• 41 citations
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Abstract

We investigated macroscopic quantum tunneling (MQT) of ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{y}$ 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-${T}_{c}$ 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 in the electronic temperature due to the quasiparticle injection are discussed.

DOI
10.1103/physrevb.79.134505

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We investigated macroscopic quantum tunneling (MQT) of Bi2Sr2CaCu2Oy{\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{y}Bi2​Sr2​CaCu2​Oy​ 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{T}_{c}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 in the electronic temperature due to the quasiparticle injection are discussed.
A nanometer-thick small mesa consiting of only two or three Bi 2 Sr 2 CaCu 2 O y intrinsic Josephson junctions (IJJs) is studied through the switching current distribution measurements down to 0.4 K. Experimental results clearly show that the first switching events from the zero-voltage state for 1 K < T < 4 K are successfully described by a conventional thermal activation (TA) theory for a single Josephson junction, and that they become independent of temperature below T* ∼ 0.7 K. We observe the microwave-induced peak in the switching distribution at 0.4 K, which is induced by the microwave irradiation at 55 GHz. These results strongly suggest that the system crossovers to macroscopic quantum tunneling (MQT) regime below T* , which is as high as the previously reported value for a stacked IJJs with several tens of junctions, in contrast to the recent result on a similar mesa-structured surface IJJ.
The macroscopic quantum tunneling (MQT) in the current-biased intrinsic Josephson junctions (IJJs) of high- T c cuprates has attracted much attention for decades. Although the MQT for the phase switches from the zero to the first voltage state (1st SW) in the multiple-branched I-V curves is well explained by the conventional theory, the occurrence of MQT for the higher order switches such as the switch from the 1st to 2nd voltage state (2nd SW) has been still debated. Here, we present an experimental study on the phase switches of small IJJs fabricated from underdoped Bi 2 Sr 2 (Ca,Y)Cu 2 O y . We observed the single photon transition between quantized energy levels in the 3rd phase switches at 59.15 GHz and 2 K. The comparison with the previous studies on the nearly optimal-doped Bi 2 Sr 2 CaCu 2 O y clearly suggests a possibility that the MQT rate for the higher-order phase switches is commonly enhanced by the effective suppression of the energy barrier for the higher-order phase escape due to the phase-running state after the 1st SW, in spite of the large difference in a critical current density and T c .
We report an experimental observation of the macroscopic quantum tunnelling (MQT) in a d-wave high-temperature superconductor (HTSC) Bi2Sr2CaCu2O8+δ intrinsic Josephson junction (IJJs). It is well known that the current-biased Josephson tunnel junction provides an ideal stage for studying a variety of macroscopic quantum phenomena such as MQT. They have been studied and observed in low-temperature superconductor (LTSC) Josephson junctions since the early 1980s and have been established well by now. On the other hand, in the case of HTSC the d-wave pairing symmetry allows for dissipative quasiparticle excitations within the superconducting energy gap, which is expected to severely interfere with an observation of MQT. Contrary to this naive conception, our experimental results have explained that the effects of the nodal quasiparticles are not strong enough to obscure the observation of the MQT. Furthermore, we found that the MQT in IJJs was observed at approximately 1 K, which was ten times higher than that of LTSC Josephson junctions. This higher classical-to-quantum crossover temperature is due to their high plasma frequency. We also discuss the resonant activation of IJJs in the presence of microwave radiation.
We study the microwave-induced phase switches from the finite voltage state for the underdamped intrinsic Josephson junctions (IJJs) made of Bi 2 Sr 2 CaCu 2 O y (Bi2212). We observe the resonant double-peak structure in the switching current distribution at low temperatures. This feature is successfully explained by a quantum mechanical model where the strong microwave field effectively suppresses the potential barrier for the phase escape from a potential well and the macroscopic quantum tunneling (MQT) is resonantly enhanced. The detailed analyses considering the effects of multiple phase retrapping processes after the phase escape strongly suggest that the intense microwave field suppresses the energy-level spacing in the potential well, by effectively decreasing the fluctuation-free critical current and the Josephson plasma frequency. This effect also reduces the number of photons required for the multiphoton transition between the ground and the first excited states, making it possible to observe the energy level quantization in the MQT state. The temperature dependence of the resonance peak emerging in the switching rate clearly demonstrates that the quantized energy state can be survived up to ~10 K, which is much higher than a crossover temperature predicted by the conventional Caldeira-Leggett theory.
Macroscopic quantum tunneling (MQT) has been observed in an intrinsic Josephson junction (IJJ) stack of a Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO) single crystalline whisker with high precision using a home made setup. The cross-over temperature between thermal activation and MQT was about 260 mK, and the Josephson plasma frequency was estimated to be 86 GHz. Both the thermal escape theory and the MQT theory are consistent with the experiments. These facts strongly suggest that single crystalline BSCCO whiskers are high enough quality to be used as intrinsic Josephson quantum devices such as intrinsic Josephson phase qubits. This is the first demonstration of MQT in BSCCO single crystalline whiskers.
We present a study of the first and second switches (SWs) with nearly identical switching current ISWI_{\rm SW}ISW​ in Bi2_22​Sr2_22​Ca0.85_{0.85}0.85​Y0.15_{0.15}0.15​Cu2_22​Oy_yy​ intrinsic Josephson junctions (IJJs). In contrast to early studies where a large difference between ISW1stI_{\rm SW}^{\rm 1st}ISW1st​ and ISW2ndI_{\rm SW}^{\rm 2nd}ISW2nd​ was observed, a crossover temperature TcrT_{\rm cr}Tcr​ to the quantum phase escapes for the 2nd SW is strongly reduced $(\sim$2.8~K) and agrees with that for the 1st SW. This confirms that the anomalous enhancement of Tcr2ndT_{\rm cr}^{\rm 2nd}Tcr2nd​, commonly observed in the IJJs with ISW1st<ISW2ndI_{\rm SW}^{\rm 1st} < I_{\rm SW}^{\rm 2nd}ISW1st​<ISW2nd​, originates from the phase running state after the 1st SW. Detailed analysis of the microwave-induced escapes strongly suggests that nonlinear bifurcated phenomena possibly occurs in IJJs.