T. Takasaki

Superconducting gap in polycrystalline MgB2 studied by electron tunneling

Abstract Electron-tunneling measurements on the new superconductor MgB 2 with T c =39 K have been carried out using a break-junction technique. The measurements reveal the largest gap value of 2 Δ =17–19 meV at 4.2 K. This value leads to the ratio 2 Δ / k B T c =5–5.6, indicating the strong-coupling superconductivity. We have also observed the reproducible gap values of 2 Δ ≅4–4.5 meV and 7–9 meV. The smallest gap exists even beyond 30 K. We will discuss the implication of the data.

Tunneling spectroscopy of the electron-doped layered superconductor Li0.48(THF)0.3HfNCl

Abstract Tunneling measurements on the electron-doped layered superconductor Li 0.48 (THF) 0.3 HfNCl(THF;C 4 H 8 O) with T c =25.5 K have been carried out. Since the surface of this compound is very reactive, we have employed in situ break junction to obtain unaffected interface. The result shows the gap value of 2 Δ =9–12 meV, which leads to the strong-coupling gap ratio 2 Δ / k B T c ≃5. This is the largest value among non-cuprate superconductors.

Evidence of the Fine Pseudogap at the Fermi Level in Al-based Quasicrystals

Theoretical studies have predicted that quasicrystals (QCs) have conspicuously fine electronic structures near the Fermi level ( E F ); the density of states (DOS) consists of sharp peaks and deep valleys with several 10 meV intervals. These fine electronic structures are believed to explain the unusual sensitivity of the electrical conductivity to slight changes in the chemical composition of QCs. We have investigated the DOS around the E F on single grains of icosahedral ( i -)AlCuFe, i -AlPdMn, and decagonal ( d -)AlCuCo QCs by break-junction tunneling spectroscopy at 4.2 K. Well-defined and reproducible pseudogaps at the E F are observed for these alloys. This is a direct experimental result that confirms the fine electronic structure existing in the vicinity of the E F .

Tunneling spectroscopy of deintercalated layered nitride superconductor ZrNCl0.7

Break junction tunneling spectroscopy measurements have been carried out on the novel layered nitride superconductor ZrNCl 1- x (\(x\cong 0.3\)) with T c =12–14 K. Two kinds of gap structures are reproducibly observed, both of which can be described by the BCS density of states. Values of the gap parameters at 4 K are obtained to be Δ=1.6–2.3 meV and 4.0–5.1 meV. Upon warming, these gap structures disappear at T c , thereby giving the ratio 2Δ/ k B T c =2.7–3.8 and 6.6–8.4, respectively. The former is within the value of conventional superconductors, while the latter is extremely large in comparison with that of the conventional strong-coupling superconductors. The larger ratio is similar to the value of high- T c cuprates. The origins of such two kinds of gaps are discussed.

Multiple-gap structure of the binary superconductor MgB2

Abstract Electron-tunneling measurements on the superconductor MgB 2 with T c =39 K have been carried out using a break-junction technique. We have observed the double-gap structures with several combinations of the gap sizes. The largest Δ =9 meV gap can be observed as a single gap, which gives very strong-coupling ratio 2 Δ / k B T c =5.3–5.7.

Observation of a pseudogap in α-ALMnSi by break-junction tunneling spectroscopy

Abstract Tunneling spectroscopy studies of an α-Al9Mn1.8Si2 single crystal were made at 4.2 K with the break-junction technique. A narrow dip at zero bias (V=0) with peaks on both sides was observed in the spectrum, which corresponds to a pseudogap of 25 meV in the density of states. The dense set of sharp spikes predicted by the band calculations for the crystal was not observed.

Scanning tunnelling microscopy and spectroscopy of MgB2

Experiments of the scanning tunnelling microscopy and spectroscopy (STM/STS) have been carried out on the layered superconductor MgB2 with Tc = 39 K. The measurements were done at 5 K using a Pt-Ir tip and the single-crystal grain. The STM images show characteristic hexagonal patterns with the distances of nearest neighbour atoms to be 0.3 - 0.35 nm and 0.15 - 0.2 nm, respectively, which are consistent with the Mg and B lattice sizes. The STS measurements clarify the superconducting gap 2Δ = 20 - 24 meV, which leads to the strong-coupling ratio 2Δ/kBTc = 5 - 6. The tunnelling spectra can be described by the correlated 2-gap model. These features are consistent with our previous break-junction measurements. The gap structure is homogeneous at least within the range of 200 nm × 200 nm.