Terahertz electronics on the atomic scale using intrinsic Josephson junctions in cuprate superconductors
Final Report Abstract
When a voltage V is applied to a Josephson junction the supercurrents flowing across the barrier begin to oscillate at a frequency f = 2eV/h, where e is the electron charge and h is Planck's constant (2e/h ≈ 483.3 GHz/mV). Highly attractive frequencies are in the range from about 1 Terahertz (THz) up to some THz, where only few oscillators exist. However, for a single junction the emitted power typically is only in the pW range. Also, junctions based on superconductors like Nb are limited by the superconducting energy gap to frequencies well below 1 THz. Both problems can be overcome by "intrinsic Josephson junctions" that are naturally formed within the unit cell in the most anisotropic cuprate superconductors such as Bi2Sr2CaCu2O8. A single crystal of, say, 1 µm thickness consists of a stack of about 670 of these junctions. A breakthrough in this field was achieved in 2007 at Argonne where about 700 junctions were shown to oscillate in-phase, with emitting microwave powers around 1 µW at frequencies around 1 THz. It was proposed that a standing wave oscillating inside the stack provides in-phase synchronization. This work provided the starting point of the present project which was a cooperation of the Tübingen group and H. B. Wang, National Institute of Material Science (NIMS), Tsukuba, within the Strategic Japanese-German Cooperative Program "Nanoelectronics". Both partners have a long standing expertise in the physics of intrinsic junctions. In addition the NIMS group as has an excellent record in sample fabrication. The Tübingen group has the expertise to image supercurrent and electric field distributions at low temperatures (Low Temperature Scanning Laser Microscopy, LTSLM). In preworks we found that indeed a standing wave forms in the stacks when the dc power into the sample is low enough to not cause strong overheating ("low-bias regime"). At large input power ("high bias regime") structures reminiscent to "hot spots", i. e. regions heated to temperatures above Tc coexisting with regions with temperatures still below Tc were found. Unexpectedly, structures reminiscent of standing waves were also found at high-bias, making this regime highly interesting for THz emission. The main aims of the present project were (A): Investigation of THz generation of single intrinsic junction stacks of various geometry; identification of THz generation mechanism and tailoring size of stack structures and (B): Investigation of THz generation in arrays of intrinsic junction stacks, optimization of such structures in terms of power output, tunability, functionality etc. The main results obtained were: - Identification of hot spots and waves in the high bias regime, using rectangular stacks of various lateral size. - Measurement of THz emission together with LTSLM imaging in the high bias regime on the same samples. Proof that comparatively large power emission (some µW) can be observed in the presence of hot spots and standing wave patterns. Discovery of broad tunability of THz radiation by bias current and bath temperature. - Study of THz emission, hot spot and electromagnetic wave formation in stacks of various geometry, identification of in-phase synchronization. - Observation that the hot spot position can be varied continuously by injecting bias currents from different position on the stack. Standing waves and THz emission can be turned on and off controllably with this technique. - Fabrication of intrinsic Josephson stacks with all-superconducting electrodes and observation of THz emission from these structures with emission powers comparable to mesas. The Tübingen/Tsukuba collaboration continued also after the end of the DFG-JST project. Using heat diffusion equations we have investigated the thermal properties of our structures in detail and find very good agreement with experiment. Thus, the mechanism of hotspot formation can be considered as understood. We have investigated the linewidth ∆f of radiation using an all-superconducting receiver (Collaboration NIMS/Tübingen and V. E. Koshelets, Kotel'nikov Institute of Radio Engineering and Electronics in Moscow). While in the low-bias regime ∆f turned out to be in the range 0.5 to several GHz, in the high-bias regime ∆f was as low as 23 MHz, which is by far too low to be explained by a exclusively cavity based synchronization mechanism. We believe that the synchronization is actually due to ac currents flowing through the normal conducting part of the hot spot which thus effectively acts as a shunting network. Linewidths in the 20 MHz range allow for the implementation of additional external phase-locking schemes which would decrease ∆f by orders of magnitude. If successful, intrinsic junction stacks could be highly interesting for spectroscopy applications.
Publications
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Hot Spots and Waves in Bi2Sr2CaCu2O8 Intrinsic Josephson Junction Stacks - a Study by Low Temperature Scanning Laser Microscopy, Phys. Rev. Lett. 102, 017006 (2009)
H. B. Wang, S. Guénon, J. Yuan, A. Iishi, S. Arisawa, T. Hatano, T. Yamashita, D. Koelle, and R. Kleiner
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Coherent terahertz emission of intrinsic Josephson junction stacks in the hot spot regime, Phys. Rev. Lett. 105, 057002 (2010)
H. B. Wang, S. Guénon, B. Gross, J. Yuan, Z. G. Jiang, Y. Y. Zhong, M. Gruenzweig, A. Iishi, P. H. Wu, T. Hatano, D. Koelle, R. Kleiner
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Interaction of hot spots and terahertz waves in Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks of various geometry, Phys. Rev. B 82, 214506 (2010)
S. Guénon, M. Grünzweig, B. Gross, J. Yuan, Z. G. Jiang, Y. Y. Zhong, A. Iishi, P. H. Wu, T. Hatano, D. Koelle, H. B. Wang, R. Kleiner
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Terahertz emission from large intrinsic Josephson junctions singled out from Bi2Sr2CaCu2O8+δ single crystals, Supercond. Sci. Technol. 25, 075015 (2012)
J. Yuan, M. Y. Li, B. Gross, A. Iishi, K. Yamaura, T. Hatano, K. Hirata, E. Takayama-Muromachi, P. H. Wu, D. Koelle, R. Kleiner, H. B. Wang
