Jean Klein

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.

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.

Recombination of charge carriers in anthracene

Abstract A simple model of radiationless transitions proposed previously is used to rationalize recent experimental results concerning the magnetic field dependence of delayed fluorescence of the anthracene crystal, induced by high-energy radiation. It is suggested that the charge transfer (CT) state responsible for the observed effects is the (1, 0, 1) state.

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

High-speed scanning tunnelling spectroscopy (STS) was used at low temperature to study the quasiparticle excitation spectrum on the surface of c-axis-oriented superconducting thin films of MgB2. The tunnelling spectra measured on as-grown films were compared with those acquired on chemically etched samples. In most cases the STS reveals only one small superconducting gap to be present in the tunnelling spectra, consistent with c-axis tunnelling and the particular electronic band structure of MgB2. We found that the etching leads to the enhancement of the gap energy by 25% from 2.2 ± 0.3 meV to 2.8 ± 0.3 meV, and to the modification of the temperature dependence of the superconducting gap which, in both cases, has clearly a non-BCS shape. We argue that the modification of the electronic structure at the surface of the material due to the etching is responsible for these changes and discuss the possible origins of the effect.

SCANNING TUNNELING SPECTROSCOPY ON THE GdSr2RuCu2O8 COMPOUND

Topographic and spectroscopic information on GdSr2RuCu2O8 sintered pellets have been obtained by a home built low temperature Scanning Tunneling Microscope (STM) operating at 4.2 K. The topographic image of the surface showed non homogeneous samples with grains of typical size of about 100 nm. In many locations studied, the Tunneling Spectroscopy reveals the presence of charging effects in the current-voltage characteristics over a voltage range up to 100 mV. Two types of charging effects are clearly distinguished: one corresponds to the reduction of the tunneling conductance around zero bias and is attributed to the Coulomb blockade, and another onw, a stepwise increasing of the current as a function of the bias voltage is identified as Coulomb staircase regime. Besides these spurious charging effects, the current-voltage characteristics often show a pronounced non-linearity around 4.0 mV. This non-linearity, disappearing above the critical temperature of the materials, is connected to the superconducting gap in the GdSr2RuCu2O8.

TWO GAP SIGNATURE IN MAGNESIUM DIBORIDE

Scanning Tunneling Spectroscopy is used at low temperature in an extensive study of the recently discovered superconducting compound MgB2. Both powder- and crystal-based samples show a very unusual double-gap structure in the tunneling conductance spectra. The spectral weights of the gaps may vary from one junction to another but their energy positions remain constant. Both gaps vanish at the critical temperature of the bulk material. These findings confirm the existence of two-gap superconductivity in MgB2, which opens a new class of multiple-gap superconductors.

Delayed Reikbination Fluorescence and Production of Triplet Charge Transfer and Exciton States in VUV-Excited Molecular Crystals

Abstract At sufficiently high excitation energies, the molecular crystals exhibit a new, slowly decaying magnetic field sensitive fluorescent component. The experimentally observed properties of this special luminescence component in anthracene and p-terphenyle are discussed, considering in particular the excitation spectra of the intensity for energies up to 40 eV. According to the characteristic magnetic modulation of the intensity, the light emission follows the recombination of long lived electron-hole pairs created initially in a triplet spin state via a deep valence hole. The nature of the photodynamic processes responsible for such recombination fluorescence precursors is suggested to be connected with the well documented mechanism of photoinduced exciton fission in triplet pair states, which involves intermolecular charge transfer transitions within a pair of neighboring molecules. It is shown that the measured excitation spectra are fairly interpreted on the whole explored energy domain; at the h...

SCANNING TUNNELING SPECTROCOPY ON MgB2 THIN FILMS

Low temperature Scanning Tunneling Microscopy/Spectroscopy (STM/STS) was used to study the superconducting properties of as grown MgB2 thin films. Two different Pulsed Laser Ablation deposition procedures were used to fabricate the samples. Depending on growth conditions, STS/STM analysis revealed either superconducting or normal metallic behavior on the sample surfaces. In the first case the tunneling spectra with well-pronounced superconducting gap of 2.6-3.2 meV were observed at T = 4.2 K. STS spectra were found to be position independent, thus showing a very high spatial homogeneity of the films. No evidence of double gap structures was found. Moreover, in contrast with a bulk TC = 35.7K of the samples, the superconductivity with a damped TC was observed on the surface. In the second case, the superconducting features were completely absent in the tunneling spectra. We associate such a behaviour of the samples of second type with the presence of a thick metallic cap layer all over the surface.