Nanoscale electronic structure of the layered nitride superconductors α-KxTiNCl and β-HfNCly observed by scanning tunneling microscopy and spectroscopy

Scanning tunneling microscopy and spectroscopy (STM-STS) measurements have been carried out on the α (FeOCl)–type KxTiNCl (x∼0.5, Tc=16 K) and β (SmSI)–type HfNCly (y∼0.7, Tc=24 K) layered nitride superconductors. The STM images at 5 K showed clear atomic arrangements for both the compounds, namely, the rectangular lattice on α-KxTiNCl and the triangular lattice on β-HfNCly. The tunneling spectra in the superconducting states at low temperatures demonstrate qualitatively different features between these superconductors. For α-KxTiNCl, the spatial distributions of the density of states and the superconducting gap structures are very inhomogeneous, while those on β-HfNCly are found to be almost homogeneous. The nanoscale electronic features between these compounds correlate with the different lattice structures of the M (=Ti or Hf) N conducting layers, which are caused by the lattice symmetry difference itself or induced by the difference in the local doping distributions in these chemically reactive compounds. The averaged gap magnitudes in the superconducting states, Δ̅ ≃10.2 meV and 7.5 meV for α-KxTiNCl and β-HfNCly, corresponding to the gap ratios 2Δ̅ /kBTc≃ 15 and 7.2, respectively, indicate the unusually strong coupling effects of the superconductivity

Tunneling spectra of break junctions involving Nb₃Sn

The electronic gap structure of Nb3Sn was measured by the break-junction (BJ) tunneling technique. The superconducting gap values are estimated to be in the range 2∆ = 4–5.5 meV at T = 4.2 K as follows from the observed distinct conductance peaks. In addition to the superconducting gap structure, we observed reproducible hump-like structures at the biases of about ± 20 and ± 50 mV. Such a coexistence of gap and hump structures resembles the situation found in the high-Tc copper-oxide superconductors. Above the superconducting critical temperature Tc ~ 18 K, the humps appear as the only gap-like structures. Their possible origin is discussed in connection to the structural phase transition occurring in Nb₃Sn

Tunneling spectroscopy of single-crystal clathrate Ba8Ga16Sn30

Tunneling measurements have been carried out on the Ba8Ga16Sn30. The break-junction was employed to obtain reliable tunneling spectra. The pseudo-gap like feature is obtained as 2Δ≈175 meV, at 4.2 K, which agrees with thermoelectric power data at low temperature. We have also obtained a small gap 2Δ≈30–40 meV

Tunneling spectroscopy of RTe2 (R=La, Ce) and possible coexistence between charge-density waves and magnetic order

Tunneling spectra are measured on the charge-density wave (CDW) compound LaTe2 and the isostructural antiferromagnet CeTe2 with TN = 4.3 K by a break-junction method. In contrast to the well-developed CDW gap 2δ = 0.9 eV wide for LaTe2, the spectrum of polycrystal CeTe2 is V shaped with 2δ = 1.2 eV. These gap structures agree with the optical-conductivity spectra for both compounds. The V-shaped spectrum for CeTe2 suggests that the CDW gap is modified by the presence of electronic states hybridized with the 4f electron states. A smaller gap 2δ* = 0.5 eV was observed for the bulk tunneling current in the c plane of single crystals of CeTe2. On cooling below TC = 6.1 K, where a short-range ferromagnetic order sets in, 2δ* exhibits a sharp minimum. Furthermore, subgap anomalies appear below TC and suddenly increase at TN. The normalized zero-bias conductance shows a minimum at TC and a sudden increase below TN. These facts suggest possible interplay of CDW's and magnetic order in CeTe2

Tunneling measurements of CeRhAs single crystal

Kondo-semiconducting gap of CeRhAs single crystal has been measured by break-junction tunneling. The observed largest gap at 4.2 K is 2Δ~0.5 eV, while smaller sizes below 0.3–0.4 eV are also reproducible. We present the temperature evolution of the gap and discuss the gap size in comparison with the other Kondo semiconductors

Electron tunneling experiments on La-substituted Kondo-semiconductor CeRhAs

Polycrystalline Ce1-xLaxRhAs is investigated by means of break-junction tunneling. On Ce substituted by La (x=0.01), a pronounced hump structure is developed at the bias ±0.25 V with a shallow dip on it. The hump emergence is consistent with a drastic reduction in the resistivity. These facts give direct evidence for the appearance of mid-gap states near the Fermi energy by a small La substitution for Ce in CeRhAs