Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Conference Object Citation - WoS: 1Citation - Scopus: 1Self-Heating Effect in Intrinsic Tunneling Spectroscopy of Hgbr2 Intercalated Bi2.1sr1.4ca1.5cu 2o8+? Single Crystals(IEEE, 2007) Kurter, Cihan; Özyüzer, Lütfi; Zasadzinski, John F.; Hinks, David G.; Gray, Kenneth E.We report tunneling results in intrinsic Josephson junction (IJJ) stacks fabricated in the form of square micromesas on HgBr2 intercalated Bi2.1Sr1.4Ca1.5Cu2O 8+δ (Bi2212) single crystals using photolithography and Ar ion milling techniques. Self-heating is the most common problem encountered in interlayer tunneling and it is likely to reduce the reliability of IJJ data. Although intercalation reduces heating a hundredfold, it still needs to be minimized substantially in order to approach the authentic superconducting energy gap observed by tunneling using more conventional junctions. We report tunneling characteristics of two mesas with the same height but different sizes (5 × 5 μm2 and 10 × 10 μm2) to show that heating effects are strongly related to IJJ stack size. For the smaller mesa, we observed an energy gap close to that seen in single SIN (S: superconductor, I: insulator, N: normal metal) and SIS break junctions as well as the dip and hump structures at high bias. The subgap data of 5 × 5 μm2 mesa were successfully fit with a momentum averaged d-wave model using convenient parameters. Thus our data is consistent with the predominant pairing symmetry suggested by point contact tunneling, break junction, scanning tunneling microscopy/spectroscopy and angle resolved photoemission measurements in Bi2Sr2CaCu2O8+δ.Conference Object Citation - WoS: 2Citation - Scopus: 2Spin Polarized Current Injection Through Hgbr2 Intercalated Bi2212 Intrinsic Josephson Junctions(Institute of Electrical and Electronics Engineers Inc., 2007) Özyüzer, Lütfi; Kurter, Cihan; Özdemir, Mustafa; Zasadzinski, John F.; Gray, Kenneth E.; Hinks, David G.To investigate the effect of polarized current on tunneling characteristics of intrinsic Josephson junctions (IJJs), spin-polarized and spin-degenerate current have been injected through the c-axis of HgBr2 intercalated Bi2.1Sr1.5Ca1.4Cu2O 8+δ (Bi2212) single crystals on which 10 × 10 μm 2 mesas have been fabricated. These two spin conditions are achieved by depositing either Au (15 nm)/Co (80 nm)/Au (156 nm) multilayers or single Au film on HgBr2 intercalated Bi2212 with Tc = 74 K followed by photolithography and Ar ion beam etching. The I-V characteristics have been measured with and without a magnetic field parallel to c-axis at 4.2 K. A fine, soft Au wire is used to make a gentle mechanical contact on the top of a particular mesa in the array. Tunneling conductance characteristics were obtained and the magnetic field dependence of sumgap voltage peaks was investigated. These peaks do not change in position with increasing magnetic field for both contact configurations. In addition, the temperature dependence of tunneling characteristics of the IJJs are obtained and existence of pseudogap feature is observed above Tc for HgBr2 intercalated Bi2212.Article Citation - WoS: 1Effect of Magnetic Field on Quasiparticle Branches of Intrinsic Josephson Junctions With Ferromagnetic Layer(Elsevier Ltd., 2007) Özyüzer, Lütfi; Özdemir, Mustafa; Kurter, Cihan; Hinks, David G.; Gray, Kenneth E.The interlayer tunneling spectroscopy has been performed on micron-sized mesa arrays of HgBr2 intercalated superconducting Bi2212 single crystals. A ferromagnetic multilayer (Au/Co/Au) is deposited on top of the mesas. The spin-polarized current is driven along the c-axis of the mesas through a ferromagnetic Co layer and the hysteretic quasiparticle branches are observed at 4.2 K. Magnetic field evolution of hysteretic quasiparticle branches is obtained to examine the effect of injected spin-polarized current on intrinsic Josephson junction characteristics. It is observed that there is a gradual distribution in quasiparticle branches with the application of magnetic field and increasing field reduces the switching current progressively.Article Investigation of the Tunneling Spectra in Hgbr2-Intercalated Bi-2212 Single Crystals Below and Above Tc(Elsevier Ltd., 2007) Kurter, Cihan; Mazur, Daniel; Özyüzer, Lütfi; Hinks, David G.; Gray, Kenneth E.Interlayer tunneling spectroscopy measurements were performed on mesa arrays of Bi-2212 single crystals, intercalated with HgBr2. Tunneling conductances were obtained over a wide temperature range to examine the spectral features, especially the behavior of the quasiparticle peaks corresponding to superconducting energy gaps (SGs). Experimental spectra showed that gap-like features are still present even for the temperatures far above the transition temperature, Tc. This evidence is consistent with the idea that the SG evolves into a pseudogap above Tc for HgBr2-intercalated Bi-2212 single crystals.Article Citation - WoS: 19Citation - Scopus: 25Large Energy Gaps in Ca C6 From Tunneling Spectroscopy: Possible Evidence of Strong-Coupling Superconductivity(American Physical Society, 2007) Kurter, Cihan; Özyüzer, Lütfi; Mazur, Daniel; Zasadzinski, John F.; Rosenmann, Daniel; Claus, Helmut; Hinks, David G.; Gray, Kenneth E.Tunneling in Ca C6 crystals reproducibly reveals superconducting gaps Δ of 2.3±0.2 meV that are ∼40% larger than reported earlier. In an isotropic s -wave scenario, that puts Ca C6 into the class of very strongly coupled superconductors, since 2Δ k Tc ∼4.6, implying that soft Ca phonons are primarily involved in the superconductivity. This conclusion explains the relatively large Ca isotope effect found recently for Ca C6, but it could also signal a strong anisotropy in the electron-phonon interaction.Conference Object Citation - WoS: 4Citation - Scopus: 5Comparison of Intrinsic Josephson and Sis Tunneling Spectroscopy of Bi 2sr2cacu2o8+?(IEEE, 2005) Özyüzer, Lütfi; Kurter, Cihan; Zasadzinski, John F.; Gray, Kenneth E.; Hinks, David G.; Miyakawa, NobuakiTunneling spectroscopy measurements are reported on optimally-doped and overdoped Bi2Sr2Ca2Cu2O 8+δ single crystals. A novel point contact method is used to obtain superconductor-insulator-normal metal (SIN) and SIS break junctions as well as intrinsic Josephson junctions (IJJ) from nanoscale crystals. Three junction types are obtained on the same crystal to compare the quasiparticle peaks and higher bias dip/hump structures which have also been found in other surface probes such as scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. However, our IJJ quasiparticle spectra consistently reveal very sharp conductance peaks and no higher bias dip structures. The IJJ conductance peak voltage divided by the number of junctions in the stack consistently leads to a significant underestimate of Δ when compared to the single junction values. The comparison of the three methods suggests that the markedly different characteristics of IJJ are a consequence of nonequilibrium effects and are not intrinsic quasiparticle features.Article Citation - WoS: 3Citation - Scopus: 3Tunneling Spectroscopy of Heavily Underdoped Crystals of Bi2sr2cacu2o8-?(Elsevier Ltd., 2000) Özyüzer, Lütfi; Zasadzinski, John F.; Miyakawa, Nobuaki; Kendziora, Christopher A.; Sha, J.; Hinks, David G.; Gray, Kenneth E.Crystals of Bi2Sr2CaCu2O8+δ with optimal Tc=95 K have been underdoped using two different methods and the superconducting gaps have been obtained by tunneling. In some cases, three different tunneling geometries have been utilized: point contact, STM and break junctions. The first doping method involves control of the oxygen content by annealing in various partial pressures of oxygen. These crystals exhibit a narrow spread of gap values over a wide doping range from overdoped (Tc=56 K) to underdoped with Tc=70 K. However, for underdoped crystals with Tc midpoints in the range 25 K - 63 K, there is a dramatic increase in the spread of gap values which may signal the development of static phase separation of either chemical or electronic origin. To avoid possible chemical phase separation, we have explored another doping procedure which incorporates Dy substitution on the Ca site. These crystals exhibit a relatively narrow superconducting transition width and some preliminary tunneling spectra will be presented.Article Citation - WoS: 18Citation - Scopus: 17Tunneling Spectroscopy of Tl2ba2cuo6(Elsevier Ltd., 1999) Özyüzer, Lütfi; Yusof, Zikri; Zasadzinski, John F.; Li, Ting-Wei; Hinks, David G.; Gray, Kenneth E.New results from tunneling spectroscopies on near optimally doped single crystals of Tl2Ba2CuO6 (Tl-2201) junctions are presented. The superconductor-insulator-normal metal (SIN) tunnel junctions are obtained using the point-contact technique with a Au tip. The tunneling conductances reproducibly show a sharp cusp-like subgap, prominent quasiparticle peaks with a consistent asymmetry, and weakly decreasing backgrounds. A rigorous analysis of the SIN tunneling data is performed using two different models for the dx(2)-y(2) (d-wave) density of states (DOS). Based on these and earlier results, the tunneling DOS of Tl-2201 has exhibited the most reproducible data that are consistent with a d-wave gap symmetry. We show that the dip feature at 2 Δ that is, clearly seen in SIN tunneling data of Bi2Sr2CaCu2O8+δ is also present in Tl-2201, but at a weaker level. The gap values for crystals with a bulk Tc = 86 K are in the range of 19-25 meV.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.