Effective Coherence Length Research Articles

Effects of the structure of the Fermi surface on the normal and superconducting parameters of the high-Tc oxide superconductors

We have described the major features of the Fermi surfaces of the high-Tc cuprates. The description in momentum space and use of Fermiology allows us to analyze the large anisotropy and multiband structure of the new materials. These properties are crucial because of the extremely small values of the coherence length. The Fermi surface can be reconstructed with the use of experimental data. Based on this method, we have evaluated the parameters such as effective mass, Fermi energy, Fermi velocity, and coherence length and estimated them for La1.8Sr0.2CuO4 and Y1Ba2Cu3O7 with the use of heat capacity and Hall effect measurements.

Spin-orbit scattering in dirty superconductors

Spin-orbit scattering and the resulting spin contamination of the pure spin-up and spin-down states of a singlet Cooper pair lead to a finite exchange term in the pairing interaction, besides the direct term of BCS theory. Both of these terms are evaluated as functions of the spin lifetime ${\ensuremath{\tau}}_{\mathrm{so}}$ and the effective coherence length $\ensuremath{\xi}$ of a dirty superconductor. We show that, as ${\ensuremath{\tau}}_{\mathrm{so}}$ decreases, the weakening of the direct term caused by spin rotation of the pair within $\ensuremath{\xi}$ is exactly compensated by intervention of the exchange term. Both effects are proportional to $\frac{\ensuremath{\xi}}{{v}_{F}{\ensuremath{\tau}}_{\mathrm{so}}}$ (where ${v}_{F}$ is the Fermi velocity). The physical conclusion is that the transition temperature ${T}_{c}$ remains unaffected, a result first noted by Anderson.

Feedback-induced line broadening in cw channel-substrate planar laser diodes

The effect of optical feedback on the spectral characteristics of channel-substrate planar single-mode laser diodes operating at room temperature is reported. The impact on the performance of interferometric sensor systems using such sources is discussed. The linewidth for the free-running laser at 10-mW output power was determined to be less than 5 MHz at room temperature. Broadening on the order of 40 times the intrinsic linewidth was observed for 0.1% feedback and increased with increasing feedback. The presence of self-oscillation modes was observed at 0.04% feedback. Satellite modes symmetrically located with respect to the primary mode appeared in the spectrum for feedback greater than 0.04%. These satellite modes are attributed to self-oscillation in the laser induced by feedback. As the feedback was increased, the satellite mode spectrum began to overlap that of the primary mode, reducing the effective coherence length from 60 m (for the single-mode linewidth) to less than a few centimeters.

Superconducting contacts to p-InAs

Since no insulating barrier exists between electroplated Pb and the naturally inverted surface channel of p-InAs, this is an attractive system for developing superconductor-semiconductor devices. The electrical and induced superconducting properties of this surface channel have been investigated. A gate-controlled planar Josephson junction further requires that superconductors be separated less than some effective coherence length within the junction barrier. The differential resistance between superconducting Pb contacts on p-InAs has been measured for devices with contact separations down to one μm on substrates with tailored n-type surface conductance. The differential resistance is dependent on the voltage between contacts, magnetic field amplitude and orientation, substrate bias, and temperature. Results show that the Pb superconductivity influences the semiconductor surface transport in a manner which suggests the potential for gate controlled Josephson coupling at realizable contact spacings.

Turbulence-degraded beam quality: improvement obtained with a tilt-correcting aperture

In this paper, we have calculated the degradation in the focal-plane irradiance distribution due to atmospheric turbulence and the potential improvement realizable by employing a wavefront filt-correcting aperture. It has been shown that, when the aperture diameter is of the order of the outer scale of turbulence, virtually no improvement is realized relative to the uncompensated case. For the case when the long-term coherence length is small compared with the diameter, there is a several-decibel improvement over the longterm case; however, for a full phase-compensating aperture (e.g., COAT), peak intensity can be increased an additional several decibels. When the coherence length is not much smaller than the diameter, close to diffraction- limited performance can be expected. Comparisons are also made of the reduction of on-axis intensity with no compensation, tilt-correction, and a full phase-compensating aperture. It is shown that the effective coherence length of the compensated aperture due to the residual amplitude fluctuations is greater than the long-term coherence length by a factor proportional to the square root of the Fresnel number of the aperture. Approximate formulas are also presented for the tilt-corrected MTF for arbitrary aperture irradiance distributions.

Radiation of whistlers by helical electron and proton beams

The feasibility of generating whistlers by means of an array of helical electron and proton beams is investigated. It is found that quasi-static beam-plasma instabilities will play a vital role in limiting the effective coherent length of the beam. The total radiated power for the whistlers is consequently decreased by 2 orders of magnitude below previously predicted levels (Dowden,1973). A proton beam is shown to be somewhat less efficient than an electron beam for generating whistlers.

Effect of impurities on the Josephson current through SNS junctions

The effect of impurities on the steady-state Josephson current through a superconductor—normal metal—superconductor junction is analyzed. It is shown that, when ξ0≪l∼d (l denoting mean free path length and d denoting the thickness of the normal metal layer), impurities cause an appreciable decrease in the current and a change in the form of its dependence on the phase. The dependence of the critical current for weak-superconduction current on the thickness of the normal metal layer is found to be of the form jc∼j0e−d/ζ with ζ denoting the effective coherence length and defined as the reciprocal of ζ−1=2πT/ℏvo/+1/l.

Bremsstrahlung and pair production at ultrahigh energies in condensed amorphous matter

The article gives a review of the current state of the theory of bremsstrahlung by electrons of ultrahigh energy and pair production by photons of ultrahigh energy in condensed matter in the case in which the effective coherence length of the processes discussed is so large that the effect of competing processes is possible. Qualitative reasoning is considered which permits correct estimation of the effects, and the results of a quantitative discussion are presented.

Transition Temperature and Tunneling Characteristics of Ferrimagnetic-Insulator-Superconductor Sandwiches

The depression of the transition temperature of In films (thinner than the effective coherence length ${\ensuremath{\xi}}_{\mathrm{eff}}$) on a single ferrite film grown in a parallel magnetic field is inversely proportional to the In film thickness. This result agrees with that already reported for an indium film sandwiched between two ferrites magnetized in parallel field; the transition temperature for the single-ferrite case can be obtained from the doubleferrite case by multiplying the ratio $\frac{{\ensuremath{\xi}}_{\mathrm{eff}}}{d}$ by 2 (which, except for a small mean-free-path correction, means dividing the indium film thickness $d$ by 2). The same results are obtained if indium is replaced by tin. Tunneling junctions (Al-${\mathrm{Al}}_{2}$${\mathrm{O}}_{4}$-In-${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$) were investigated as a function of temperature and In film thickness. The In films were found to display gapless or quasigapless superconductivity. For a fixed reduced transition temperature, the degree of gapless superconductivity is the same as that obtained by the proximity effect of a conducting ferromagnet.

Superconductivity in Cu and Pt by Means of Superimposed Films with Lead

The study of normal-superconducting sandwiches provides information on the type of electron-electron interaction taking place in the normal metal. The magnitude and sign of the interaction is obtained by comparing a characteristic length which measures the depth of penetration of electron pairs in the normal metal with the effective coherence length. In order to study the electron-electron interaction in copper and platinum, superimposed films of lead-platinum and of lead-copper were deposited on glass substrates kept at 77\ifmmode^\circ\else\textdegree\fi{}K. It has been shown that the effective coherence length should be calculated from the coefficient of normal electronic specific heat and the residual resistivity. The data were then plotted in such a way as to exhibit all the temperature dependence in one term, and the slope of such a plot is the depth of penetration of electron pairs in the normal metal. With the experimental accuracy presently available, the electron-electron interaction in platinum can be bracketed between -0.25 (repulsive interaction) and an attractive interaction of $+0.09({T}_{c}=2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\ifmmode^\circ\else\textdegree\fi{}\mathrm{K})$. In the case of copper, the data correspond to a transition temperature of about 6 mdeg.