Normal-state Sheet Resistance Research Articles

Superconducting Tc enhancement in weakly disordered Ge-covered tin films.

We report on the variation of the superconducting transition temperature with the electrical resistance ratio in weakly disordered Ge-covered tin films deposited at room temperature. The normal-state sheet resistance R/sub N//sup D'Alembertian/ varied between 0.2

Effect of Precoating on the Fluctuation-Induced Conductivity in Superconducting Metal Films

The effect of the lithium fluoride (LiF) precoating on the fluctuation-induced conductivity (FIC) of superconducting indium and vanadium films was investigated. Several kinds of samples were used. The measured FIC is well explained by the traditional superconducting fluctuation theory with two fitting parameters, the transition temperature T c and the pair breaking parameter δ. The experimental relation δ= a + b R sq n holds for each type of sample, where R sq n is the normal state sheet resistance. The coefficient b for the unprecoated samples is much smaller than that for the LiF precoated samples in contrast to the lead case. The coefficient b is not so much influenced by the LiF overcoating.

Pair-breaking model for disorder in two-dimensional superconductors

Measurements of resistive transitions, critical fields, and magnetoconductance as a function of increasing normal-state sheet resistance ${R}_{N}$ reveal the systematics of a pronounced change from superconducting to insulating behavior in thin $\frac{\mathrm{In}}{\mathrm{In}{\mathrm{O}}_{x}}$ composite films. With increasing ${R}_{N}$, a rapid suppression of the transition temperature together with a decreasing critical-field slope parameter are observed. These results are modeled using a pair-breaking formalism in which the pair-breaking rate is found to be proportional to ${R}_{N}$ and independent of temperature.

Upward Curvature of the Temperature Dependence of Upper Critical Magnetic Field Perpendicular to Superconducting Aluminum Films

The temperature dependence of the electrical resistance of thin granular aluminum films shows the maximum far above the superconducting transition temperature due to both effects of thermodynamic fluctuations and electron localization, for the film with large normal-state sheet resistance R N . The temperature dependence of the upper critical field or the film with R N ≃10 3 (\(\varOmega\square\)) shows the upward curvature. The results on the upper critical field are explained by a current theory of dirty superconductors including electron localization and electron-electron interactions.

Paraconductivity of Ge-covered tin films, II

The temperature dependence if the excess electrical conductivity of thin superconducting tin films (a) covered by a protective germanium layer and (b) sandwiched between two germanium layers above the transition temperature has been measured. These thin films have been deposited on glass substrates held at room temperature. It was found that the pair- breaking parameter δ for germanium covered tin filns varies exponentially with normal state sheet resistance R □ N and could be represented by δ = 3.77×10 -4 exp (1.76 R □ N ) in the range 0.28≲ R □ N <1.6ohm/▭. Higher R ▭ N samples show close agreement with Aslamazov-Larkin expression. Information obtained from type (b) samples showed that although protective germanium layer increases the δ values, sandwich type structure lowers it significantly. Results show that such a strong dependence of δ could be related to changes in the Cooper pair relaxation time and the proximity effect.

Critical-Exponent Measurements of a Two-Dimensional Superconductor

Voltage-current measurements of an amorphous indium/indium-oxide thin-film composite have yielded values for the exponent $\ensuremath{\eta}(T)$ describing the power-law decay of the order-parameter correlation function appropriate to the Kosterlitz-Thouless description of phase transitions in two-dimensional systems. A pronounced crossover in the characteristics at the phase-transition temperature is obtained. Theoretical expressions both above and below ${T}_{c}$ are verified, and a value of $\ensuremath{\eta}({T}_{c})=0.23$ is measured for a film with normal-state sheet resistance of 3560 \ensuremath{\Omega}/\ensuremath{\square} measured at 8 K.

Renormalization effects near the vortex-unbinding transition of two-dimensional superconductors

The theory of topological phase transitions predicts a universal relation between the vortex-unbinding temperature ${T}_{c}$ and the areal superelectron density ${n}_{s}({T}_{c})$ in two-dimensional superconductors. Evaluating ${n}_{s}({T}_{c})$ with the theory of dirty superconductors leads to a relation between ${T}_{c}$ and the normal-state sheet resistance ${R}_{N}^{\ensuremath{\square}}$. This latter relation, however, must be modified to account for the distinction between the unrenormalized ${{n}_{s}}^{0}$ and the renormalized value ${{n}_{s}}^{R}$, in the critical region near ${T}_{c}$. This can be expressed via a nonuniversal parameter ${\ensuremath{\epsilon}}_{c}=\frac{{{n}_{s}}^{0}({T}_{c})}{{{n}_{s}}^{R}({T}_{c})}$, which depends in turn for homogeneous films on ${N}_{0}$, the density of statistically independent vortices within an area ${\ensuremath{\xi}}^{2}$. For the Hg-Xe thin films studied earlier, we find that ${\ensuremath{\epsilon}}_{c}=1.2\ifmmode\pm\else\textpm\fi{}0.1$ and ${N}_{0}=0.05\ifmmode\pm\else\textpm\fi{}0.03$.

Evidence for the Kosterlitz-Thouless Transition in Thin Superconducting Aluminum Films

The complex impedance of an aluminum film with normal-state sheet resistance of 4130 \ensuremath{\Omega}/sq has been measured as a function of temperature and frequency. The experiment provides convincing evidence that there is a vortex-antivortex dissociation transition of the type predicted by the theory of Kosterlitz and Thouless for topological long-range order in two dimensions.

Role of Clusters in the Approach to Localization of Josephson-Coupled Granular Lead Films

The superconducting properties of thin granular lead films provide evidence for the existence of clusters which mitigate electrostatic charging effects and allow Josephson coupling to remain favorable in samples with normal-state sheet resistance near the maximum metallic resistivity of 30 000 ..cap omega../sq. The disappearance of zero-resistance transitions together with the precipitous onset of temperature broadening in this range is therefore most likely associated with the localization of electronic states.

Fluctuation-Induced Conductivity above the Superconducting Transition Temperature in Indium Films Containing Gadolinium Impurities

Fluctuation-induced conductivity (FIC) in indium films containing paramagnetic impurities (gadolinium) is investigated. The FIC is well described as the sum of the Aslamazov-Larkin (AL) term, the Maki-Thompson (MT) term and the third term. The sum of the AL term and the MT term is insufficient to express the FIC. The pair-breaking parameter δ depends not only on the normal state sheet resistance but also on the concentration of gadolinium impurities in the manner predicted by Thompson. Experimental data are analysed also by the use of the formulae proposed by one of the present authors (K.O) in which the magnetic impurity effect is explicitly taken into account.

Excess conductivity of thin superconducting films of tin above the transition temperature

The excess conductivity of thin superconducting Sn films has been measured. These films have been grown at room temperature and ∼ 77 K on glass substrates. The results show that the pair breaking parameter is not simply and solely related to the normal state sheet resistance but depends also on the substrate temperature.

Comparative studies of noise limitations in superconducting thin-film radiation detectors

We report measurements on the noise limitations of superconducting thin films operating as thermal detectors (bolometers) and as current-dependent detectors. By making measurements in each mode on the same film it is possible to make a comparative study of the noise limitations for both detection mechanisms in the same device. Thin tin films with normal state sheet resistances on the order of 100 Ω per square have been studied. The devices were scribed to produce total normal state resistances on the order of 10 kΩ and were mounted in the vacuum chamber of a demountable tail Dewar so that they were not in direct contact with liquid helium. The resulting bolometers have response times and sensitivities similar to those of Martin and Bloor. The dominant noise mechanisms we have observed appear to be Johnson noise in the thermal mode and 1/f noise in the current mode. These results confirm previous conclusions regarding the sensitivity of these films as radiation detectors.