Probing the Phase Diagram of Bi2Sr2CaCu2O8+d with Tunneling Spectroscopy

Tunneling measurements are performed on Ca-rich single crystals of Bi2Sr2CaCu2O8+d (Bi2212), with various oxygen doping levels, using a novel point contact method. At 4.2 K, SIN and SIS tunnel junctions are obtained with well-defined quasiparticle peaks, robust dip and hump features and in some cases Josephson currents. The doping dependence of tunneling conductances of Ca-rich Bi2212 are analyzed and compared to stoichiometric Bi2212. A similar profile of energy gap vs. doping concentration is found although the Ca-rich samples have a slighly smaller optimum Tc and therefore smaller gap values for any doping level. The evolution of tunneling conductance peak height to background ratios with hole concentration are compared. For a given doping level, the Ca-rich spectra showed more broadened features compared to the stoichiometric counterparts, most likely due to increased disorder from the excess Ca. Comparison of the dip and hump features has provided some potential insights into their origins.Comment: 4 pages, 4 figures; presented at the Applied Superconductivity Conference (August 4-9, 2002) in Houston, TX; to be published in IEEE Trans. Appl. Supercon

Single Junction and Intrinsic Josephson Junction Tunneling Spectroscopies of Bi2Sr2CaCu2O8+d

Tunneling spectroscopy measurements are reported on optimally-doped and overdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ 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 $\Delta$ 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.Comment: 4 pages, 4 figures; presented at the Applied Superconductivity Conference (October 3-8, 2004) in Jacksonville, FL; to be published in IEEE Trans. Appl. Supercon

Rebuttal to "Comment by V.M. Krasnov on 'Counterintuitive consequence of heating in strongly-driven intrinsic junctions of Bi2Sr2CaCu2O8+d Mesas' "

In our article [1], we found that with increasing dissipation there is a clear, systematic shift and sharpening of the conductance peak along with the disappearance of the higher-bias dip/hump features (DHF), for a stack of intrinsic Josephson junctions (IJJs) of intercalated Bi2Sr2CaCu2O8+{\delta} (Bi2212). Our work agrees with Zhu et al [2] on unintercalated, pristine Bi2212, as both studies show the same systematic changes with dissipation. The broader peaks found with reduced dissipation [1,2] are consistent with broad peaks in the density-of-states (DOS) found among scanning tunneling spectroscopy [3] (STS), mechanical contact tunneling [4] (MCT) and inferred from angle (momentum) resolved photoemission spectroscopy [5] (ARPES); results that could not be ignored. Thus, sharp peaks are extrinsic and cannot correspond to the superconducting DOS. We suggested that the commonality of the sharp peaks in our conductance data, which is demonstrably shown to be heating-dominated, and the peaks of previous intrinsic tunneling spectroscopy (ITS) data implies that these ITS reports might need reinterpretation.Comment: Rebuttal to Comment of Krasnov arXiv:1007.451

Full Bulk Spin Polarization and Intrinsic Tunnel Barriers at the Surface of Layered Manganites

Transmission of information using the spin of the electron as well as its charge requires a high degree of spin polarization at surfaces. At surfaces however this degree of polarization can be quenched by competing interactions. Using a combination of surface sensitive x-ray and tunneling probes, we show for the quasi-two-dimensional bilayer manganites that the outermost Mn-O bilayer, alone, is affected: it is a 1-nm thick insulator that exhibits no long-range ferromagnetic order while the next bilayer displays the full spin polarization of the bulk. Such an abrupt localization of the surface effects is due to the two-dimensional nature of the layered manganite while the loss of ferromagnetism is attributed to weakened double exchange in the reconstructed surface bilayer and a resultant antiferromagnetic phase. The creation of a well-defined surface insulator demonstrates the ability to naturally self-assemble two of the most demanding components of an ideal magnetic tunnel junction.Comment: 19 pages, 5 figure

Characteristic Energy of the Coulomb Interactions and the Pileup of States

Tunneling data on $\mathrm{La_{1.28}Sr_{1.72}Mn_2O_7}$ crystals confirm Coulomb interaction effects through the $\sqrt{\mathrm{E}}$ dependence of the density of states. Importantly, the data and analysis at high energy, E, show a pileup of states: most of the states removed from near the Fermi level are found between ~40 and 130 meV, from which we infer the possibility of universal behavior. The agreement of our tunneling data with recent photoemission results further confirms our analysis.Comment: 4 pages, 4 figures, submitted to PR

Effect of bilayer coupling on tunneling conductance of double-layer high T_c cuprates

Physical effects of bilayer coupling on the tunneling spectroscopy of high T$_{c}$ cuprates are investigated. The bilayer coupling separates the bonding and antibonding bands and leads to a splitting of the coherence peaks in the tunneling differential conductance. However, the coherence peak of the bonding band is strongly suppressed and broadened by the particle-hole asymmetry in the density of states and finite quasiparticle life-time, and is difficult to resolve by experiments. This gives a qualitative account why the bilayer splitting of the coherence peaks was not clearly observed in tunneling measurements of double-layer high-T$_c$ oxides.Comment: 4 pages, 3 figures, to be published in PR

Hypothesis of two-dimensional stripe arrangement and its implications for the superconductivity in high-Tc cuprates

The hypothesis that holes doped into high-Tc cuprate superconductors organize themselves in two-dimensional (2D) array of diagonal stripes is discussed, and, on the basis of this hypothesis, a new microscopic model of superconductivity is proposed and solved. The model describes two kinds of hole states localized either inside the stripes or in the antiferromagnetic domains between the stripes. The characteristic energy difference between these two kinds of states is identified with the pseudogap. The superconducting (SC) order parameter predicted by the model has two components, whose phases exhibit a complex dependence on the the center-of-mass coordinate. The model predictions for the tunneling characteristics and for the dependence of the critical temperature on the superfluid density show good quantitative agreement with a number of experiments. The model, in particular, predicts that the SC peaks in the tunneling spectra are asymmetric, only when the ratio of the SC gap to the critical temperature is greater than 4. It is also proposed that, at least in some high-Tc cuprates, there exist two different superconducting states corresponding to the same doping concentration and the same critical temperature. Finally, the checkerboard pattern in the local density of states observed by scanning tunneling microscopy in Bi-2212 is interpreted as coming from the states localized around the centers of stripe elements forming the 2D superstructure.Comment: Text close to the published version. This version is 10 per cent shorter than the previous one. All revisions are mino

On the correct formula for the lifetime broadened superconducting density of states

We argue that the well known Dynes formula [Dynes R C {\it et al.} 1978 {\it Phys. Rev. Lett.} {\bf 41} 1509] for the superconducting quasiparticle density of states, which tries to incorporate the lifetime broadening in an approximate way, cannot be justified microscopically for conventional superconductors. Instead, we propose a new simple formula in which the energy gap has a finite imaginary part $-\Delta_2$ and the quasiparticle energy is real. We prove that in the quasiparticle approximation 2$\Delta_2$ gives the quasiparticle decay rate at the gap edge for conventional superconductors. This conclusion does not depend on the nature of interactions that cause the quasiparticle decay. The new formula is tested on the case of a strong coupling superconductor Pb$_{0.9}$Bi$_{0.1}$ and an excellent agreement with theoretical predictions is obtained. While both the Dynes formula and the one proposed in this work give good fits and fit parameters for Pb$_{0.9}$Bi$_{0.1}$, only the latter formula can be justified microscopically.Comment: 6 pages, 4 figure

Modeling Study of the Dip/Hump Feature in Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} Tunneling Spectroscopy

The tunneling spectra of high temperature superconductors on Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi-2212) reproducibly show a high bias structure in the form of a dip-hump at voltages higher than the gap voltage. Of central concern is whether this feature originates from the normal state background or is intrinsic to the superconducting mechanism. We address this issue by generating a set of model conductance curves - a ''normal state'' conductance that takes into account effects such as the band structure and a possible pseudogap, and a pure superconducting state conductance. When combined, the result shows that the dip-hump feature present in the experimental conductance curves cannot be naively attributed to a normal state effect. In particular, strong dip features found in superconductor-insulator-superconductor data on optimally-doped Bi-2212, including negative dI/dV, cannot be a consequence of an extrinsic pseudogap. However, such features can easily arise from states-conserving deviations in the superconducting density of states, e.g., from strong-coupling effects.Comment: 4 pages, 4 figure