Intrinsic tunneling spectra of Bi_2(Sr_{2-x}La_x)CuO_6

We have measured intrinsic-tunneling spectra of a single CuO-layer La-doped Bi_2Sr_{2-x}La_xCuO_{6+\delta} (Bi2201-La_x). Despite a difference of a factor of three in the optimal superconducting critical temperatures for Bi2201-La_{0.4} and Bi2212 (32 and 95 K, respectively) and different spectral energy scales, we find that the pseudogap vanishes at a similar characteristic temperature T*\approx 230-300K for both compounds. We find also that in Bi2201-La_x, PG humps are seen as sharp peaks and, in fact, even dominate the intrinsic spectra.Comment: Submitted to Phys. Rev. Let

Discrimination between the superconducting gap and the pseudo-gap in Bi2212 from intrinsic tunneling spectroscopy in magnetic field

Intrinsic tunneling spectroscopy in high magnetic field ($H$) is used for a direct test of superconducting features in a quasiparticle density of states of high-$T_c$ superconductors. We were able to distinguish with a great clarity two co-existing gaps: (i) the superconducting gap, which closes as $H \to H_{c2}(T)$ and $T\to T_c(H)$, and (ii) the $c$-axis pseudo-gap, which does not change neither with $H$, nor $T$. Strikingly different magnetic field dependencies, together with previously observed different temperature dependencies of the two gaps ~\cite{Krasnov}, speak against the superconducting origin of the pseudo-gap.Comment: 4 pages, 4 eps figure

Local Strong Coupling Pairing in DD-Wave Superconductor with Inhomogeneous Bosonic Modes

Recent local tunneling data indicate strong nanoscale inhomogeneity of superconducting gap in high temperature superconductors. Strong local nanoscale inhomogeneity in the bosonic scattering mode has also been observed in the same samples. We argue that these two inhomogeneities directly related to each other. To address local boson scattering effects, we develop a local strong coupling model of superconducting pairing in a coarse grained superconducting state. Each patch is characterized by local coupling to the bosonic mode as well as by local mode energy. We find that local gap value on each patch grows with the local strength of electron-boson interaction. At the same time local gap value decreases with the local boson mode energy, an observation consistent with the tunneling experiments. We argue that features in the tunneling spectrum due to boson scattering are consistent with experimentally observed spectra. We also address the $^{16}O$ to $^{18}O$ isotope substitution. Since both coupling constant and boson energy could change upon isotope substitution, we prove that interplay between these two effects can produce results that are very different from conventional BCS model.Comment: 16 pages latex file, 15 eps and ps fig files. See more details at http://theory.lanl.go

Theory of Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} Cross-Whisker Josephson Junctions

Takano {\it et al.} [Phys. Rev. B {\bf 65}, 140513 (2002) and unpublished] made Josephson junctions from single crystal whiskers of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ crossed an angle $\phi_0$ about the $c$ axis. From the mesa structures that formed at the cross-whisker interface, they inferred a critical current density $J_c(\phi_0)$. Like the single crystal results of Li {\it et al.} [Phys. Rev. Lett. {\bf 83}, 4160 (1999)], we show that the whisker data are unlikely to result from a predominantly d-wave order parameter. However, unlike the single crystals, these results, if correct, require the whisker c-axis transport to be coherent.Comment: 5 pages, 4 figures, accepted for publication in Physical Review

Fourier-Transformed Local Density of States and Tunneling into a DD-Wave Superconductor with Bosonic Modes

We analyze the effects of the electronic coupling to bosonic modes in a d-wave superconductor. The role of the scattering due to boson on the momentum transfer between electronic states in the Brilloine zone is addressed. We consider specific examples of $B_{1g}$ phonon, breathing mode phonon and spin resonance at $(\pi,\pi)$. The Fourier spectrum of the energy derivative local density of states (LDOS) is calculated. To properly calibrate the effects of different modes we fix the quasipartilce renormalization at specific momentum points. It is found that the $B_{1g}$ mode with highly anisotropic momentum-dependent coupling matrix element gives rise to well definded features in the Fourier spectrum, at the energy of mode plus gap, with a momentum transfer along the Cu-O bond direction of cuprates. This result is in a striking contrast to the cases of the coupling to other modes and also to the case of no mode coupling. The origin of this difference is explored in detail. A comparison with the recent STM experiments is briefly discussed.Comment: 9 pages, 4 eps figures include

Interlayer Quasiparticle Transport in the Vortex State of Josephson Coupled Superconductors

We calculate the dependence of the interlayer quasiparticle conductivity, $\sigma_q$, in a Josephson coupled d-wave superconductor on the magnetic field B||c and the temperature T. We consider a clean superconductor with resonant impurity scattering and a dominant coherent interlayer tunneling. When pancake vortices in adjacent layers are weakly correlated at low T the conductivity increases sharply with B before reaching an extended region of slow linear growth, while at high T it initially decreases and then reaches the same linear regime. For correlated pancakes $\sigma_q$ increases much more strongly with the applied field.Comment: 4 pages, 3 figure

Coupling between phonons and intrinsic Josephson oscillations in cuprate superconductors

The recently reported subgap structures observed in the current-voltage characteristic of intrinsic Josephson junctions in the high-T_c superconductors Tl_2Ba_2Ca_2Cu_3O_{10+\delta} and Bi_2Sr_2CaCu_2O_{8+\delta} are explained by the coupling between c-axis phonons and Josephson oscillations. A model is developed where c-axis lattice vibrations between adjacent superconducting multilayers are excited by the Josephson oscillations in a resistive junction. The voltages of the lowest structures correspond well to the frequencies of longitudinal c-axis phonons with large oscillator strength in the two materials, providing a new measurement technique for this quantity.Comment: 4 pages, 3 figures, revtex, aps, epsf, psfig. submitted to Physical Review Letters, second version improved in detai

Spectrally stable nitrogen-vacancy centers in diamond formed by carbon implantation into thin microstructures

The nitrogen-vacancy center (NV) in diamond, with its exceptional spin coherence and convenience in optical spin initialization and readout, is increasingly used both as a quantum sensor and as a building block for quantum networks. Employing photonic structures for maximizing the photon collection efficiency in these applications typically leads to broadened optical linewidths for the emitters, which are commonly created via nitrogen ion implantation. With studies showing that only native nitrogen atoms contribute to optically coherent NVs, a natural conclusion is to either avoid implantation completely, or substitute nitrogen implantation by an alternative approach to vacancy creation. Here, we demonstrate that implantation of carbon ions yields a similar yield of NVs as implantation of nitrogen ions, and that it results in NV populations with narrow optical linewidths and low charge-noise levels even in thin diamond microstructures. We measure a median NV linewidth of 150 MHz for structures thinner than 5 $\mu$m, with no trend of increasing linewidths down to the thinnest measured structure of 1.9 $\mu$m. We propose a modified NV creation procedure in which the implantation is carried out after instead of before the diamond fabrication processes, and confirm our results in multiple samples implanted with different ion energies and fluences