W. Sacks

Nanometer scale mapping of the density of states in an inhomogeneous superconductor

Using high-speed scanning tunneling spectroscopy (STS), we perform a full mapping of the quasiparticle density of states (DOS) in single crystals of Bi2 − xPbxSr2CaCu208 + δ. The measurements carried out at 5 K showed a complex spatial pattern of important variations of the local DOS on the nanometer scale. Superconducting areas, characterized by a well-pronounced superconducting gap, are coexisting with regions of a peakless gap structure which we attribute to the pseudogap. Within the large superconducting regions, the spectra reveal strong peak/dip/hump signatures, identical to the pristine Bi2Sr2CaCu208 + δ case. On the contrary, in very small superconducting regions this fine structure is attenuated. Such a behavior of the local DOS suggests that the peak/dip/hump fine structure is not only a consequence of the superconducting state, but is also directly related to the scale of the phase coherence. The role of Bi-Pb substitutional disorder is discussed.

Scanning tunneling spectroscopy on the novel superconductor CaC6.

We present scanning tunneling microscopy and spectroscopy of the newly discovered superconductor CaC6. The tunneling conductance spectra, measured between 3 and 15 K, show a clear superconducting gap in the quasiparticle density of states. The gap function extracted from the spectra is in good agreement with the conventional BCS theory with Delta0=1.6+/-0.2 meV. The possibility of gap anisotropy and two-gap superconductivity is also discussed. In a magnetic field, direct imaging of the vortices allows us to deduce a coherence length in the ab plane xiab approximately 33 nm.

Strong coupling and double-gap density of states in superconducting MgB2

Using scanning tunneling spectroscopy at T = 4.2 K, we perform simultaneously the topographic imaging and the quasiparticle density of states (DOS) mapping in granular MgB2. We observe a new type of spectrum, showing a pronounced double gap, with the magnitudes of ΔS = 3.9 meV and ΔL = 7.5 meV, i.e. well below and well above the BCS limit. The largest gap value gives the ratio 2ΔL/kBTC = 4.5, which implies strong electron-phonon coupling. Other superconducting regions are found to have a characteristic BCS-shaped DOS. However, the variation of the spectral shape and lower gap widths, from 2.0 meV to 6.5 meV, indicate the importance of surface inhomogeneity and proximity effects in previously published tunneling data. Our finding gives no evidence for any important gap anisotropy. Instead, it strongly supports the multiple-gap scenario in MgB2 in the clean limit, and the single-gap scenario in the dirty limit.

Constraints on the quasiparticle density of states in high-Tc superconductors

In this letter we present new tunneling data on YBa2Cu3O7 thin films by low-temperature scanning tunneling spectroscopy. Unusual peak-dip-hump features, previously reported in Bi2Sr2CaCu2O8 + δ, are also found in YBa2Cu3O7. To analyse these common signatures, we propose a new heuristic model in which, in addition to the d-wave symmetry, the gap function is energy dependent. A simple expression for the quasiparticle density of states is derived, giving an excellent agreement with the experiment. The dynamics of the quasiparticle states and the energy scales involved in the superconducting transition are discussed.

Probing the superconducting condensate on a nanometer scale

Superconductivity is a rare example of a quantum system in which the wave function has a macroscopic quantum effect, due to the unique condensate of electron pairs. The amplitude of the wave function is directly related to the pair density, but both amplitude and phase enter the Josephson current: the coherent tunneling of pairs between superconductors. Very sensitive devices exploit the superconducting state, however properties of the condensate on the local scale are largely unknown, for instance, in unconventional high-Tc cuprate, multiple gap, and gapless superconductors. The technique of choice would be Josephson STS, based on Scanning Tunneling Spectroscopy (STS), where the condensate is directly probed by measuring the local Josephson current (JC) between a superconducting tip and sample. However, Josephson STS is an experimental challenge since it requires stable superconducting tips, and tunneling conditions close to atomic contact. We demonstrate how these difficulties can be overcome and present the first spatial mapping of the JC on the nanometer scale. The case of an MgB2 film, subject to a normal magnetic field, is considered.

Fabrication and characterization of scanning tunneling microscopy superconducting Nb tips having highly enhanced critical fields

We report a simple method for the fabrication of Niobium superconducting (SC) tips for scanning tunneling microscopy which allow atomic resolution. The tips, formed in situ by the mechanical breaking of a niobium wire, reveal a clear SC gap of 1.5 meV and a critical temperature Tc = 9.2 ± 0.3 K, as deduced from Superconductor Insulator Normal metal (SIN) and Superconductor Insulator Superconductor (SIS) spectra. These match the values of bulk Nb samples. We systematically find an enhanced value of the critical magnetic field in which superconductivity in the tip is destroyed (around 1 T for some tips) up to five times larger than the critical field of bulk Nb (0.21 T). Such enhancement is attributed to a size effect at the tip apex

Local tunneling spectroscopy of the electron-doped cuprate superconductor Sm1.85Ce0.15CuO4

We present local tunneling spectroscopy in the optimally electron-doped cuprate Sm2-xCexCuO4 x=0.15. A clear signature of the superconducting gap is observed with an amplitude ranging from place to place and from sample to sample (Delta~3.5-6meV). Another spectroscopic feature is simultaneously observed at high energy above \pm 50meV. Its energy scale and temperature evolution is found to be compatible with previous photoemission and optical experiments. If interpreted as the signature of antiferromagnetic order in the samples, these results could suggest the coexistence on the local scale of antiferromagnetism and superconductivity on the electron-doped side of cuprate superconductors.

Quasiparticle spectrum of the cuprate Bi 2 Sr 2 Ca Cu 2 O 8 + δ : Possible connection to the phase diagram

We previously introduced [T. Cren et al., Europhys. Lett. 52, 203 (2000)] an energy-dependant gap function,

Quasiparticle state density on the surface of superconducting thin films of MgB2

\ensuremath{\Delta}(E)

Vacuum tunneling spectroscopy of high-temperature superconductors: A critical study.

, that fits the unusual shape of the quasiparticle (QP) spectrum for both BiSrCaCuO and YBaCuO. A simple anti-resonance in