K. Gloos

Electronic Correlation Effects and the Coulomb Gap at Finite Temperature

We have investigated the effect of the long-range Coulomb interaction on the one-particle excitation spectrum of n-type germanium, using tunneling spectroscopy on mechanically controllable break junctions. At low temperatures, the tunnel conductance shows a minimum at zero bias voltage due to the Coulomb gap. Above 1 K, the gap is filled by thermal excitations. This behavior is reflected in the variable-range hopping resistivity measured on the same samples: up to a few degrees Kelvin the Efros-Shklovskii lnR infinity T(-1/2) law is obeyed, whereas at higher temperatures deviations from this law occur. The type of crossover differs from that considered previously in the literature.

Energy gap of intermediate-valentSmB6studied by point-contact spectroscopy

We have investigated the intermediate valence narrow-gap semiconductor SmB6 at low temperatures using both conventional spear-anvil type point contacts as well as mechanically controllable break junctions. The zero-bias conductance varied between less than 0.01 mikrosiemens and up to 1 mS. The position of the spectral anomalies, which are related to the different activation energies and band gaps of SmB6, did not depend on the the contact size. Two different regimes of charge transport could be distinguished: Contacts with large zero - bias conductance are in the diffusive Maxwell regime. They had spectra with only small non-linearities. Contacts with small zero - bias conductance are in the tunnelling regime. They had larger anomalies, but still indicating a finite 45 % residual quasiparticle density of states at the Fermi level at low temperatures of T = 0.1 K. The density of states derived from the tunelling spectra can be decomposed into two energy-dependent parts with Eg = 21 meV and Ed = 4.5 meV wide gaps, respectively.

Phase Diffusion and Suppression of the Supercurrent by Quantum-Mechanical Fluctuations of the Josephson Plasma at Small Superconducting Point Contacts

The RCSJ model of resistively and capacitively shunted Josephson junctions is used to describe superconducting point contacts over a wide range of resistances up to the metallic–tunneling transition. Their small dynamic capacitance of order C = 0.1 fF due to the point-contact geometry results in a huge plasma frequency. The critical current is then strongly suppressed and the contact resistance becomes finite because of quantum-mechanical zero-point fluctuations of the Josephson plasma and the rather large escape rate out of the zero-voltage state due to quantum tunneling. We test the predictions of the RCSJ model on the classical superconductors lead, indium, aluminum, and cadmium.

Lateral-Size Effects and Supercurrent Blockade at Mechanical-Controllable Break Junctions of the Heavy-Fermion Superconductors

Break junctions of the heavy-fermion superconductors CeCu2Si2, UBe13, UPt3, URu2Si2, UPd2Al3, and UNi2Al3have been investigated. Josephson-like superconducting anomalies could be found only for low-ohmic contacts, but without the oscillatory Fraunhofer pattern of the critical current in a magnetic field. A systematic study on the contact radius demonstrates that these anomalies are mainly due to Maxwells resistance being suppressed in the superconducting heavy-fermion phase rather than due to Josephson effect and Andreev reflection. We could not find any superconducting features by vacuum-tunneling spectroscopy.

Josephson effect with heavy-fermion superconductors

Abstract Metallic junctions of the heavy-fermion superconductors strongly suppress the Josephson effect, possibly indicating an unconventional type of superconductivity. Junctions with classical superconductors like cadmium can also have a reduced critical current Ic, a result of quantum fluctuations of the Josephson plasma due to a small dynamic capacitance. Applying this scenario to heavy-fermion superconductors reveals an intrinsic Ic underestimated by an order of magnitude.

Is CeNiSn a Kondo Semiconductor

Cerium intermetallic compounds can have different ground states, depending on the hybridization between f-electrons and conduction electrons. CeNiSn is usually classified as a Kondo semiconductor, originally because of the enhanced electrical resistivity at low temperatures [1]. But when high quality samples became available, the low-temperature resistivity turned out to be metallic [2]. Tunnel spectroscopy provides a direct access to the electronic density of states (EDOS) [3]. Using mechanically controllable break junctions (MCBJ), Ekino et al. [4] observed dI/dV spectra with ∼ 10 meV broad zero-bias (ZB) minima. They assumed — without further experimental evidence — that their junctions were in the tunnel regime, and interpreted the ZB minima as being due to a gap in the EDOS. These ZB minima were found to be suppressed in magnetic fields B ≥ 14 T only along the a-axis, indicating as a crossover from a pseudogap to a metallic heavy-fermion state [5]. Our investigation of MCBJs of CeNiSn, both in the metallic (direct contact) and in the vacuum-tunneling regime, is based on three CeNiSn single crystals with long sides in the a, b, and c direction of the orthorhombic crystal lattice, respectively. Magnetic fields up to 8 T could be applied perpendicular to the long side of the sample (perpendicular to current flow). For further details see Refs. [6, 7].

The horizon of Josephson point contacts

Abstract Josephson point contacts can have rather small dynamic capacitances of order 0.1 fF and huge plasma frequencies ω p ⩾1 ps −1 . The quantum-mechanical zero-point fluctuations of the Josephson plasma interact with the electromagnetic field only inside the horizon ∼c/ωp of the junction, and the dynamic capacitance turns out to be C≈κc/ωp at a lead capacitance per length κ. Those contacts can easily resist external high-frequency noise.

Regimes of charge transport across semiconductor junctions

Abstract Charge transport across break junctions of n-doped Ge has been investigated. We have found several different regimes of transport. Although all junctions reflect the density of states of the samples, to extract the bulk density of states only few junctions in a rather restricted range of parameters turned out to be useful.