arXiv:1005.0081v1 [cond-mat.supr-con] 1 May 2010 Coherent THz emission of intrinsic Josephson junction stacks in the hot spot regime H.B. Wang, 1 S. Gu´ enon, 2 B. Gross, 2 J. Yuan, 1 Z.G. Jiang, 3 Y.Y. Zhong, 3 M. Gruenzweig, 2 A. Iishi, 1 P.H. Wu, 3 T. Hatano, 1 D. Koelle, 2 and R. Kleiner 2 1 National Institute for Materials Science, Tsukuba 3050047, Japan 2 Physikalisches Institut – Experimentalphysik II and Center for Collective Quantum Phenomena, Universit¨ at T¨ ubingen, Auf der Morgenstelle 14, D-72076 T¨ ubingen, Germany 3 Research Institute of Superconductor Electronics, Nanjing University, Nanjing 210093, China (Dated: May 4, 2010) We report on THz emission measurements and low temperature scanning laser imaging of Bi2Sr2CaCu2 O8 intrinsic Josephson junction stacks. Coherent emission is observed at large dc input power, where a hot spot and a standing wave, formed in the “cold” part of the stack, coexist. By varying the hot spot size the cavity resonance frequency and the emitted radiation can be tuned. The linewidth of radiation is much smaller than expected from the quality factor of the cavity mode excited. Thus, an additional mechanism of synchronization seems to play a role, possibly arising from nonequilibrium processes at the hot spot edge. PACS numbers: 74.50.+r, 74.72.-h, 85.25.Cp Phase synchronization is one of the prerequisites to use Josephson junction arrays as tunable high frequency sources [1]. While Nb based junctions are limited to fre- quencies well below 1 THz, intrinsic Josephson junctions (IJJs)[2, 3] in Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) are, at least in principle, able to operate up to several THz. Stacks of many junctions can be made e.g. by patterning mesa structures on top of single crystals. For many years, investigations focused on small structures consisting of some 10 IJJs, with lateral mesa sizes of a few μm. Here, with few exceptions [4, 5], the IJJs in the stack tended to oscillate out-of-phase or were not synchronized at all. Experimental and theoretical studies included the gen- eration of synchronous Josephson oscillations by moving Josephson vortices [4–10], the use of shunting elements [11–14], the excitation of Josephson plasma oscillations via heavy quasiparticle injection [15, 16] or the investiga- tion of stimulated emission due to quantum cascade pro- cesses [17]. High-frequency emission of unsynchronized intrinsic junctions has been observed up to 0.5 THz [18]. Recently, coherent off-chip THz radiation with an ex- trapolated output power of some μW was observed from stacks of more than 600 IJJs, with lateral dimensions in the 100 μm range [19–23]. Phase synchronization in- volved a cavity resonance oscillating along the short side of the mesa. This radiation was studied theoretically in a series of recent papers, either based on vortex-type or plasmonic excitations [24–33] , or on nonequilibrium ef- fects caused by quasiparticle injection [34]. While in experiments [19–23] THz emission was ob- tained at relatively low bias currents and moderate dc power input, using low temperature scanning laser mi- croscopy (LTSLM) we have shown that standing wave patterns, presumably associated with THz radiation, can be obtained at high input power where, in addition, a hot spot (i.e. a region heated to above the critical temper- ature T c ) forms within the mesa structure[35]. Hot spot and waves seem to be correlated. The purpose of the present work is to investigate THz emission in this high power regime in detail, combining THz emission measure- ments and LTSLM. Apart from further clarifying the role of the hotspot, we are specifically interested in the ques- tion whether coherent radiation can be achieved in spite of the high temperatures involved. For the experiments BSCCO single crystals were grown by the floating zone technique in a four lamp arc-imaging furnace. Below we discuss results from two samples. Sample 1 was patterned on a crystal that was annealed in vacuum at 650 ◦ C for 65 hours. It had a T c of 86.6 K and a transition width ΔT c of 1.5 K. Sample 2 was made on a crystal (T c = 87.6 K, ΔT c =1.5 K) annealed in vac- uum at 600 ◦ C for 72 hours. To provide good electrical contact the single crystals were cleaved in vacuum and a 30nm Au layer was evaporated. Then, conventional photolithography was used to define the mesa size in the a-b plane (length 330 μm, width 50 μm for both samples). Ar ion milling yielded mesas with measured thicknesses of, respectively 1 μm (sample 1) and 0.7 μm (sample 2) along the c-axis (corresponding to, respectively, stacks of 670 and 470 IJJs). Insulating polyimide was used to sur- round the mesa edge at which a Au wire was attached to the mesa by silver paste. Other Au wires were connected to the big single crystal pedestal as grounds. In order to provide a load line for stable operation, the mesas were biased using a current source and variable resistor in parallel to the mesa, cf. Fig. 1 in Ref. [35]. The volt- age measured across the mesa includes the resistance of the contacting Au wires and the resistance between these wires and the mesa (4Ω for sample 1, 6.5Ω for sample 2). In the data discussed below this resistance is subtracted. THz emission measurements were performed in Tsukuba; the samples were subsequently shipped to T¨ ubingen for LTSLM. In total we detected THz emis- sion from 7 (out of a total of 12) mesas on 5 different crystals. The LTSLM setup is described in Ref.[35]. In brief, the beam of a diode laser (modulated at 10-80 kHz, spot size 1-2 μm) is deflected by a scanning unit and fo- cused onto the sample surface. Local heating by 2–3 K in