Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 150Citation - Scopus: 7Correlation of Tunneling Spectra in Bi2sr2cacu2o8+? With the Resonance Spin Excitation(American Physical Society, 2001) Zasadzinski, John F.; Özyüzer, Lütfi; Miyakawa, Nobuaki; Gray, Kenneth E.; Hinks, David G.; Kendziora, Christopher A.New break-junction tunneling data are reported in Bi2Sr2CaCu2O8+δ over a wide range of hole concentration from underdoped (Tc=74k) to optimal doped (Tc=95k) to overdoped (Tc=48k). The conductances exhibit sharp dips at a voltage, Ω/e, measured with respect to the superconducting gap. Clear trends are found such that the dip strength is maximum at optimal doping and that Ω scales as 4.9kTc over the entire doping range. These features link the dip to the resonance spin excitation and suggest quasiparticle interactions with this mode are important for superconductivity.Article Citation - WoS: 2Citation - Scopus: 3Reply To "comment on 'counterintuitive Consequence of Heating in Strongly-Driven Intrinsic Junctions of Bi2sr2cacu 2o8+? Mesas'"(American Physical Society, 2011) Kurter, Cihan; Özyüzer, Lütfi; Proslier, Thomas; Zasadzinski, John F.; Hinks, David G.; Gray, Kenneth E.The main criticism raised in the preceding Comment concerns our suggestion that sharp conduction peaks in Bi 2Sr 2CaCu 2O 8+δ mesas, along with absent dip-hump features, may, in general, be a result of self-heating. The author points to the variety of experimental configurations, matrix-element effects, and doping dependencies that might allow a diversity of conductance spectra. We argue that numerous mesa studies (with fixed matrix elements) firmly establish the systematic development of sharp conductance peaks with increased self-heating, and thus, the issue of nonuniversality of tunneling characteristics is not relevant. The author mentions a number of studies that show that the mesa is superconducting near the conductance peak voltage. This is not in dispute and indicates a misinterpretation of our analysis that is clarified here. To address further comments on the technical details of our heating model, we reiterate that our conclusions are independent of our model but rather are based solely on experimental data that are not in dispute.Article Citation - WoS: 2Citation - Scopus: 2The Icrn Value in Intrinsic Josephson Tunnel Junctions in Bi2sr2cacu2o8+? (bi2212) Mesas(Springer Verlag, 2011) Kurter, Cihan; Özyüzer, Lütfi; Hinks, David G.; Gray, Kenneth E.; Zasadzinski, John F.The c-axis current-voltage I(V) characteristics have been obtained on a set of mesas of varying height sculpted on Bi2Sr2CaCu2O8+δ (Bi2212) crystals intercalated with HgB2. The intercalation, along with the small number of junctions in the mesa, N = 6–30, minimizes the degree of self-heating, leading to a consistent Josephson critical current, IC, among junctions in the mesa. The Bi2212 crystals with a bulk TC =74 K are overdoped and display negligible pseudogap effects allowing an accurate measure of the normal state resistance, RN. These properties make themesas nearlyideal for the determinationof the Josephson ICRN product.Wefind ICRN valuesconsistently ∼30% of the quasiparticle gap parameter, /e, which was measured independently using a mechanical contact, break junction technique. The latter was necessitated by higher bias heating effects in the mesas which prevented direct measurements of the superconducting gap. These values are among the highest reported and may represent the maximum intrinsic value for ICRN. The results indicate that the c-axis transport is a mixture of coherent and incoherent tunneling.Article Citation - WoS: 42Citation - Scopus: 43Counterintuitive Consequence of Heating in Strongly-Driven Intrinsic Junctions of Bi2 Sr2 Cacu2 O 8+? Mesas(American Physical Society, 2010) Kurter, Cihan; Özyüzer, Lütfi; Proslier, T.; Zasadzinski, John F.; Hinks, David G.; Gray, Kenneth E.Anomalously high and sharp peaks in the conductance of intrinsic Josephson junctions in Bi2 Sr2 CaCu2 O 8+δ (Bi2212) mesas have been commonly interpreted as superconducting energy gaps but here we show they are a result of strong self-heating. This conclusion follows directly from a comparison to the equilibrium gap measured by tunneling in single break junctions on equivalent crystals. As the number of junctions in the mesa, N, and thus heating increase, the peak voltages decrease and the peak width abruptly sharpens for N≥12. Clearly these widely variable features vs N cannot all represent the equilibrium properties. Our data imply that the sharp peaks represent a transition to the normal state. That it occurs at the same dissipated power for N=12-30 strongly implicates heating as the cause. Although peak sharpening due to heating is counterintuitive, as tunneling spectra usually broaden at higher temperatures, a lateral temperature gradient, leading to coexistence of normal hot spots and superconductive regions, qualitatively explains the behavior. However, a more uniform temperature profile cannot be ruled out. As the peak's width and voltage in our shortest mesa (N=6) are more consistent with the break junction data, we propose a figure of merit for Bi2212 mesas, the relative conductance peak width, such that small values signal a crossover into the strong self-heating regime. © 2010 The American Physical Society.Article Citation - WoS: 54Citation - Scopus: 55Persistence of Strong Electron Coupling To a Narrow Boson Spectrum in Overdoped Bi2sr2cacu2o8+î Tunneling Data(American Physical Society, 2006) Zasadzinski, John F.; Özyüzer, Lütfi; Coffey, L.; Gray, Kenneth E.; Hinks, David G.; Kendziora, Christopher A.A d-wave, Eliashberg analysis of break-junction and STM tunneling spectra on Bi2Sr2CaCu2O8+Î (Bi2212) reveals that the spectral dip feature is directly linked to strong electronic coupling to a narrow boson spectrum, evidenced by a large peak in I'2F(I). The tunneling dip feature remains robust in the overdoped regime of Bi2212 with bulk Tc values of 56Â Ka62Â K. This is contrary to recent optical conductivity measurements of the self-energy that suggest the narrow boson spectrum disappears in overdoped Bi2212 and therefore cannot be essential for the pairing mechanism. The discrepancy is resolved by considering the way each technique probes the electron self-energy, in particular, the unique sensitivity of tunneling to the off-diagonal or pairing part of the self-energy.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.