Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 40Citation - Scopus: 45Terahertz Wave Emission From Intrinsic Josephson Junctions in High- Tc Superconductors(IOP Publishing Ltd., 2009) Özyüzer, Lütfi; Şimşek, Yılmaz; Köseoğlu, Hasan; Türkoğlu, Fulya; Kurter, Cihan; Welp, U.; Koshelev, A. E.; Gray, Kenneth E.; Kwok, W. K.; Yamamoto, T.; Kadowaki, K.; Koval, Yu I.; Wang, Huabing; Müller, Paul H.Recently, we experimentally demonstrated that rectangular mesa structures of intrinsic Josephson junctions (IJJ) in Bi2Sr2CaCu 2O8+d (Bi2212) can be used as a compact solid-state generator of continuous, coherent and polarized terahertz (THz) radiation. In the present work, we will exhibit tall mesas (over 600 junctions) which were fabricated using UV lithography, e-beam lithography with photoresist and e-beam lithography with a Ti selective etching technique. We will present measurements of the c-axis resistance as a function of temperature and of current-voltage characteristics of THz emitting mesas with lateral sizes ranging from 30 × 300 to 100 × 300νm2. Furthermore, we will discuss the dependence of the characteristics of the mesa structures on the oxygen doping level of the Bi2212 crystals. We will also experimentally show that the voltage-frequency relation of the ac Josephson effect has to match the cavity resonance for successful emission.Article Citation - WoS: 53Quasiparticle and Josephson Tunneling of Overdoped Bi2sr2cacu2o8+? Single Crystals(American Physical Society, 2000) Özyüzer, Lütfi; Zasadzinski, John F.; Kendziora, Christopher A.; Gray, Kenneth E.The point contact tunneling technique is used to examine quasiparticle and Josephson currents in overdoped Bi2Sr2CaCu2O8+δ (Bi-2212) single crystals with bulk Tc values ranging from 82 K down to 62 K. Superconductor-insulator-normal-metal (SIN) tunnel junctions are formed between Bi-2212 crystals and a Au tip, which display well-resolved quasiparticle gap features including sharp conductance peaks. Reproducible superconductor-insulator-superconductor (SIS) tunnel junctions are also obtained between two pieces of the Bi-2212 crystals, resulting in simultaneous quasiparticle and Josephson currents. The dynamic conductances of both SIN and SIS junctions are qualitatively similar to those found on optimally doped Bi-2212, but with reduced gap values, e.g., Δ = 15-20 meV for Tc = 62 K. Fits to the conductance data in the gap region are obtained using a model with dx2-y2 symmetry, and it is shown that this provides a better fit than s-wave symmetry. Both SIN and SIS tunneling conductances also display dip and hump features at high bias voltages similar to those found on optimal and underdoped crystals, indicating that these are intrinsic properties of the quasiparticles. The SIS data indicate that these features appear to be part of a larger spectrum that extends out to 300-400 mV. The Josephson current has been measured for 13 SIS junctions on the 62 K crystals with resistances varying over two decades. It is found that the maximum value depends on junction resistance in a manner consistent with Ambegaokar-Baratoff theory, but with a reduced IcRn product.Article Citation - WoS: 200Citation - Scopus: 212Predominantly Superconducting Origin of Large Energy Gaps in Underdoped Bi2sr2cacu2o8+? From Tunneling Spectroscopy(American Physical Society, 1999) Miyakawa, Nobuaki; Zasadzinski, John F.; Özyüzer, Lütfi; Guptasarma, Prasenjit; Hinks, David G.; Kendziora, Christopher A.; Gray, Kenneth E.New tunneling data are reported in Bi2Sr2CaCu2O8+δ which show quasiparticle excitation gaps, Δ, reaching values as high as 60 meV for underdoped crystals with Tc = 70 K. These energy gaps are nearly 3 times larger than those of overdoped crystals with similar Tc. Despite the large differences in gap magnitude, the tunneling spectra display qualitatively similar characteristics over the entire doping range. Detailed examination of the spectra, including the Josephson IcRn product measured in break junctions, indicates that these energy gaps are predominantly of superconducting origin.Article Citation - WoS: 4Citation - Scopus: 4Simultaneous Quasiparticle and Josephson Tunneling in Bscco-2212 Break Junctions(Institute of Electrical and Electronics Engineers Inc., 1999) Özyüzer, Lütfi; Miyakawa, Nobuaki; Zasadzinski, John F.; Yusof, Zikri M.; Romano, Pierom; Kendziora, Christopher A.; Hinks, David G.; Gray, Kenneth E.Tunneling measurements are reported for superconductor-insulator-superconductor (SIS) break junctions on underdoped, optimally-doped, and overdoped single crystals of tSrsCaCiOs-).,! (Bi2212). The junction I -V characteristics exhibit welldefined quasiparticle current jumps at eV = 2A as well as hysteretic Josephson currents. The quasiparticle branch has been analyzed in the framework of dxa_y2 (d-wave) superconductivity and indicates that there is preferential tunneling along the lobe directions of the d-wave gap. For overdoped Bi-2212 with TC-62 K, the Josephson current is measured as a function of junction resistance, Rn, which varied by two orders of magnitude (1 kO to 100 kO). IcRn product is proportional to the 0.47 power of /c and displays a maximum of 7.0 mV. When the hole doping is decreased from overdoped (Tc=62 K) to the underdoped regime (Tc=70 K), the average IcRn product increases as does the quasiparticle gap. The maximum IcRn is ~ 40% of the A/e at each doping level, with a value as high as 25 mV in underdoped Bi-2212.