Normal Metal Insulator Research Articles

Tunneling spectroscopy of Tl 2Ba 2CuO 6

New results from tunneling spectroscopies on near optimally doped single crystals of Tl 2Ba 2CuO 6 (Tl-2201) junctions are presented. The superconductor–insulator–normal metal (SIN) tunnel junctions are obtained using the point-contact technique with a Au tip. The tunneling conductances reproducibly show a sharp cusp-like subgap, prominent quasiparticle peaks with a consistent asymmetry, and weakly decreasing backgrounds. A rigorous analysis of the SIN tunneling data is performed using two different models for the d x 2– y 2 (d-wave) density of states (DOS). Based on these and earlier results, the tunneling DOS of Tl-2201 has exhibited the most reproducible data that are consistent with a d-wave gap symmetry. We show that the dip feature at 2 Δ that is clearly seen in SIN tunneling data of Bi 2Sr 2CaCu 2O 8+ δ is also present in Tl-2201, but at a weaker level. The gap values for crystals with a bulk T c=86 K are in the range of 19–25 meV.

Point contact tunnelling apparatus with temperature and magnetic field control

The design and testing of a new device for point contact tunnelling measurements in superconductors are described. The insert is designed for use with a continuous flow cryostat which allows for a large range of sample temperatures from 1.5 K to room temperature. The use of nonmagnetic parts allows tunnelling measurements to be performed in high magnetic fields. Testing was carried out on a conventional superconductor, Nb, in fields up to 6 T using Nb and Au tips to obtain superconductor–insulator–superconductor and superconductor–insulator–normal metal junctions. In addition, a moderate- T c oxide superconductor, Ba 1−xK xBiO 3, was used to obtain temperature and magnetic field dependencies of the energy gap.

Quantum levels and quasi-local states of SINIS structures

Quantum states of a superconductor–insulator–normal metal–insulator–superconductor sandwich (the SINIS structure) are investigated on the basis of the Bogoliubov–de Gennes equations. The dispersion equation is obtained for the quasiparticle spectrum for energies E<Δ (Δ is the energy gap in the superconductor) taking into account the Andreev scattering as well as conventional electron reflection at the interfaces of the SINIS structure. The spectrum makes it possible to calculate the Josephson current in the system. The transparency coefficient of the system for electrons with a continuous energy spectrum is calculated, and quasi-local states (“resonance levels” of transparency) are determined for the structure under investigation.

Fabrication and properties of all-refractory Nb/Al–AlOx–Ti junctions for microbolometers and microrefrigerators

Fabrication of all-refractory Nb/Al–AlOx–Ti superconducting tunnel junctions using selective titanium etching process (STEP) is described. Results including anodization properties of Ti, and junction’s I–V characteristics and subgap currents measured in the temperature range of 0.4–9.2 K are presented. The junctions show fairly high quality with respect to their stability and reproducibility. Possible utilization of these junctions as superconductor–insulator–normal metal type devices operating around 100 mK and above for ultrasensitive microbolometer and electronic microrefrigerator applications is discussed.

The current–voltage characteristic in an ultrasmall junction

The current–voltage characteristics of ultrasmall superconductor–insulator–normal metal (S–I–N), and a superconductor–insulator–superconductor (S–I–S) junctions are computed in the presence of a dissipative transmission line. The amplitude of the discontinuous jump at the energy gap of a single-particle current is greatly influenced by the size of the capacitance and the impedance of the external transmission line in the small junction. The results agree with Ambegaokar–Baratoff in the limit of vanishing impedance of a transmission line or large junction capacitance.

Differential conductance of normal metal-insulator- d-wave superconductor junctions with interface roughness

Within the framework of Blonder-Tinkham-Klapwijk (BTK) model and taking into account the interface roughness, we calculate the differential conductance of normal metal-insulator- d-wave superconductor junctions. We find that the tunneling spectrum depends strongly on both the incident angle of electrons and the crystalline axis orientation of the superconductor, and exhibits the zero-bias anomaly under suitable arrangements. Interestingly, even in the absence of the insulating layer, the tunneling spectrum of normal metal- d-wave superconductor junctions differs significantly from that for normal metal- s-wave superconductor due to the interface roughness. Moreover, both the tunnel conductance peak at the energy gap and the zero-bias conductance peak are suppressed by the interface roughness. Our results can explain many experimental measurements on the tunneling spectra of high- T c superconductors.

Coherence effects in a normal-metal-insulator-superconductor junction.

Coherence effects arising due to Andreev reflections in a superconductor--insulator--normal-metal junction are considered. It is shown that in the absence of electron-electron interaction in the metal at low temperatures, the electrical potential drop on the insulator ${\mathit{I}}_{0}$${\mathit{R}}_{\mathit{I}}$, caused by the current density ${\mathit{I}}_{0}$ through the junction, vanishes, although the resistance of the device ${\mathit{R}}_{\mathrm{\ensuremath{\infty}}}$ measured by the two-probe method can be large. The electron-electron interaction determines the zero-temperature value of ${\mathit{R}}_{\mathit{I}}$. The implications of these effects for the theory of the superconductor--normal-metal--superconductor junction are discussed.

Fabrication and properties of superconductor–insulator–normal metal tunnel junctions

This article first describes the preparation and properties of small area (∼1 μm2)Pb/Bi/In-oxide–Ag junctions made by evaporation through metal masks. These junctions have successfully been used in a 230 GHz receiver. We then describe the preparation and properties of two slightly different types of Nb/Al-based superconductor–insulator–normal conductor (SIN) junctions applying a ‘‘trilayer technique’’ similar to that used in the fabrication of Nb–Al-oxide–Nb superconductor–insulator–superconductor junctions. For one type of the Nb/Al-based SIN junctions the base layer consists of Nb, for the other type, the base layer is made of Al. In practical applications these devices will operate at temperatures where the Al is in the normal state. Junctions with areas down to about 0.5 μm2 have been made.

Numerical study of optical absorption in two-dimensional metal-insulator and normal-superconductor composites

We analyze a random RLC lattice model for the optical properties of a two-dimensional normal metal-insulator composite, using the Y − Δ transformation algorithm developed by Frank and Lobb. Within such a model, the surface plasmon resonances of a Drude metal-insulator composite are modeled by the ac resonances of a random resistor-inductor-capacitor (RLC) network. The real part of the effective conductance is found to show a broad surface plasmon resonance peak below and above the threshold. An effective-medium (EM) calculation is in excellent agreement with the results of the simulations. We also calculate the far-infrared absorption in a model composite of normal metal and superconductor, using a lattice model. The absorption shows a strong absorption below the superconducting energy gap. An approximate calculation based on the EMA is again in excellent agreement with these results.

Microwave dielectric properties of Cu(tatbp)I

Cu(tatbp)I (tatbp = triazatetrabenzporphyrinato) is a quasi-one-dimensional molecular conductor that incorporates in its structure a dense array of S = 1 2 moments localized on a metal spine Cu +2 sites and interacting strongly with charge carriers on the p-π molecular orbitals of the tatbp aromatic ring. The microwave dielectric properties have been measured at 13 GHz between 2 and 300K. The a.c. conductivity is in agreement with the d.c. measurement, and its magnitude is similar to that found in other good molecular conductors. The conductivity increases from a room temperature value of σ ∼ 3 × 10 2Ω −1cm −1 to a maximum of ca. 7 × 10 2Ω −1cm −1 at T ∼ 90K; it decreases by almost three orders of magnitude at lower temperatures, but remains practically constant below 6K at σ ∼ 1Ω −1cm −1 . In the same range the dielectric constant decreases with temperature, the slope being less pronounced below 7K. This temperature behaviour of both properties is not believed to be associated with a normal metal-insulator or semiconductor transition but to be related to the presence of local moments on the metal spine.

Superconductive bolometers for submillimeter wavelengths

Three types of composite superconductive bolometers are described in which the temperature-sensitive element is a superconducting film at the transition temperature, a superconductor–normal metal–superconductor Josephson junction, or a superconductor–insulator–normal metal quasiparticle tunneling junction. The temperature-sensitive element is evaporated onto a sapphire substrate on the reverse side of which is a bismuth film to absorb the submillimeter radiation. The noise limitations of each type of bolometer are calculated. The fabrication and measured performance of the transition-edge bolometer and the Josephson-junction bolometer are described. The best electrical (noise-equivalent power) NEP obtained with a transition-edge bolometer fabricated on a 4×4×0.005-mm sapphire substrate is (1.7±0.1) ×10−15 W Hz−1/2 at 2 Hz at an operating temperature of 1.27 K. This NEP is within a factor of 2 of the thermal noise limit. The effective absorptivity of the bismuth film is measured to be 0.47±0.05, and the corresponding detectivity D* is calculated to be (1.1±0.1) ×1014 cm W−1 Hz1/2. Suggestions are made for further improvements in sensitivity.

Effects of fluctuational pairing on electron tunneling

A theoretical study has been made of superconductor–insulator–normal metal contacts under conditions such that the normal metal is in the fluctuation state, which is close to the state of surface superconductivity. The current and its derivatives have been calculated as functions of temperature and of the magnetic field parallel to the surface of the insulator.