Electron-phonon Enhancement Factor Research Articles

Large electron-phonon interaction but low-temperature superconductivity in lab6

Combined experimental and theoretical studies are reported of the Fermi surface, band structure, generalized magnetic susceptibility, electron-phonon enhancement factor λ and superconducting transition temperature TT of LaB6. Whereas the unusually large λ values, ranging from 1.0 to 2.5, are expected to result in high TT values, TT is observed to be only 0.122°K. These results further emphasize the need for appropriate theoretical formulations for these systems.

A specific heat and magnetization study of highT c ceramic superconductors

From the temperature dependence of the specific heat of the semiconductor La2CuO4 and the high temperature superconductors La1.8Sr0.2CuO4 (T c =37.2 K) and YBa1.9K0.1Cu3O6.9 (T c =91.5 K) in the range 1.5–30 K, a strong similarity of the lowfrequency part of their phonon density of states with a peak around 10 meV could be inferred. In the case of La1.8Sr0.2CuO4 the thermodynamical critical field belowT c has been determined and using the Rutger's formula and the BCS model, a Sommerfeld coefficient γ=9 mJ·mol−1 K−1 was obtained, which, taking into account recent results of band structure calculations leads to an electron-phonon enhancement factor γ=1.3, value compatible withT c =36 K when using McMillan's formula forT c . A systematic study of the magnetization offered evidence for strong flux trapping effects at higher fields and for Meissner shielding by superconducting Josephson currents in fields below 6 mT at 4.2 K.

The thermopower of metallic glasses

The authors present experimental results for the thermopowers of a number of amorphous transition-metal alloys in the temperature range 2-300K. They show that the observed temperature dependences arise from the temperature-dependent electron-phonon enhancement factor lambda (T). The form of lambda (T) obtained implies that the electron-phonon coupling factor, alpha 2, does not vary as omega -1 as is the case for amorphous simple metals but in fact increases as omega increases. The magnitudes of the observed enhancements are generally in good agreement with those obtained from superconductivity data and the inclusion of spin-fluctuation and electron-electron interaction effects does not alter this agreement. They do, however, find significant disagreements for a number of alloys, which may be an indication of further renormalisation contributions. They also briefly discuss the observed signs of the thermopowers, considering the predictions of the Mott s-d scattering model and the possibility of significant d-band conductivity.

Electronic Structure of BaPb1-xBixO3

The results of linear augmented-plane-wave electronic structure calculations for the perovskite-type compounds BaPb1-xxBixO3 are reviewed in the light of recent experimental measurements of the x-dependent electronic properties near EF(x). In the metallic regime x<0.35, satisfactory agreement is found between the calculated density of states N(EF,x) and specific-heat data. The experimental-theoretical ratio suggests an electron-phonon enhancement factor (1+λ) ≈2. A similar comparison between the calculated and observed free-carrier plasma energies \\varOmegap(x) yields ∼ 10% agreement over the entire alloy series.

Superconductivity and the Fermi surface of Tc

Results of bandstructure calculations of Tc, which has a HCP lattice structure, are used to calculate the superconducting transition temperature Tc and the Fermi surface of the metal. The strong-scattering theory of Gaspari and Gyorffy (1972) is used in conjunction with McMillan's formula (1968) to calculate Tc in the spherical band approximation. As the bandstructure calculations were made for potentials constructed with and without correlation, the superconducting transition temperature is calculated for all potentials. The theoretical values of Tc are found to be very sensitive to changes in the electron-phonon enhancement factor lambda and the Coulomb pseudopotential mu *. However the theoretical value of Tc calculated for a potential which includes correlation agrees well with the experimental value for a particular choice of mu *. The Fermi surface cross sections are obtained for all the potentials and a qualitative comparison is made with the available preliminary experimental results.

AN EXPERIMENTAL TEST OF THE RIGID-MUFFIN-TIN APPROXIMATION USED IN THE THEORY OF ELECTRON-PHONON INTERACTION

AN EXPERIMENTAL TEST OF THE RIGID-MUFFIN-TIN APPROXIMATION USED IN THE THEORY OF ELECTRON-PHONON INTERACTION

Precision measurements of cyclotron masses and Fermi velocities in the noble metals by the de Haas-van Alphen effect

A total of 54 cyclotron masses have been determined with great precision from the temperature dependence of the de Haas-van Alphen (dHvA) amplitudes in the noble metals Cu, Ag, and Au. The accuracy of the data is \ifmmode\pm\else\textpm\fi{}0.3%. Special care was given to the temperature and amplitude measurements. In particular the influence of magnetic interaction and of the incomplete penetration of the modulation field into the high-purity samples, which were among the most serious sources of systematic errors, were eliminated. The mass data were deconvoluted to get values of the electron velocities ${v}^{*}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}})$ for each point on the Fermi surfaces. The qualitative behavior of ${v}^{*}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}})$ agrees with that found in previous works, but the absolute values differ by as much as 10%. Local values of the electron-phonon enhancement factor, $1+\ensuremath{\lambda}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}})$, were obtained in Cu by comparing velocities deduced from dHvA cyclotron masses which include electron-phonon renormalization with band velocities deduced from a band-structure calculation. Cyclotron masses were calculated for about 40 orbits not measured in each of the three metals. The coefficient of electronic specific heat was calculated from the experimental data. The agreement with the data obtained from specific-heat measurements is excellent.

Heat capacity of technetium

The heat capacity of technetium has been measured in zero field between 3 and 15 K. A superconducting transition temperature T/subc/ = 7.86 K is found. The electronic heat-capacity coefficient and zero-degree Debye temperature are 4.30 mJ/mole K$sup 2$ and 454 K, respectively. An electron-phonon enhancement factor of 0.65 is found from the McMillan equation. The thermodynamic properties of the superconducting state also indicate that technetium is an intermediate- coupling superconductor. (AIP)

Experimental study of the anisotropic electron-phonon interaction in A1

From analysis of surface-Landau-level resonance in the (110) sample plane of A1, we have determined the Fermi velocity and electron-phonon scattering rate at three characteristic locations on the Fermi surface. Using the measured velocities and calculated band velocities we extract local values of the electron-phonon enhancement factor λ. A γ T 3 temperature dependence is found for the scattering rate with γ varying by a factor 8 for the three Fermi surface locations. The experimental results are compared with recent theoretical calculations.

Upper Critical Field and the Density of States in Amorphous Strong-Coupling Superconductors

The upper critical field of the amorphous superconductors ${\mathrm{Bi}}_{0.85}$${\mathrm{Tl}}_{0.15}$, Ga, ${\mathrm{Sn}}_{0.86}$${\mathrm{Cu}}_{0.14}$, and ${\mathrm{Pb}}_{0.75}$${\mathrm{Bi}}_{0.25}$ is measured in the temperature range from 1.5 \ifmmode^\circ\else\textdegree\fi{}K to the zero-field transition temperature. Films of the amorphous metals about 1500 \AA{} thick are obtained by quenched condensation onto a substrate at He temperature. The initial slopes of the ${B}_{c2}(T)$ curves are used to determine the electronic density of states at the Fermi surface. For all the investigated materials these values were enhanced compared with the density of states which one obtains from the free electron model. For Ga, ${\mathrm{Sn}}_{0.86}$${\mathrm{Cu}}_{0.14}$, and ${\mathrm{Pb}}_{0.75}$${\mathrm{Bi}}_{0.25}$ the experimental enhancement factor agrees well with the electron-phonon enhancement factor $1+\ensuremath{\lambda}$, where $\ensuremath{\lambda}$ is taken from superconducting-tunneling experiments. The temperature dependence of ${B}_{c2}$ deviates from that predicted by Werthamer, Helfand, and Hohenberg, showing ${B}_{c2}$ values too large in the low-temperature region. It is suggested that the strong-coupling behavior of the amorphous superconductors is responsible for the deviation at low temperature.

Determination of the Electron-Phonon Enhancement Factor from Specific-Heat Data

The electron-phonon enhancement factor is determined for scandium, yttrium, vanadium, platinum, and palladium solely from specific-heat data. The key to the method is that at high temperatures the electron-phonon enhancement factor is zero. Therefore, if the electronic specific-heat coefficient at high temperatures is calculated and compared with the low-temperature value, both the band-structure density of states and the electron-phonon enhancement factor can be determined.

Specific Heat of Gold-Zinc Alloys below 3 °K

The electronic specific heat is found to initially increase on adding zinc to gold, in agreement with recent work of Clune and Green who relate this behavior to changes in the electron-phonon enhancement factor superimposed on a rigid-band model. The results for a 7.5-at.%-Zn alloy were stable against drastic changes in heat treatment, whereas those for a 10-at.% alloy were very dependent on heat treatment. It is therefore postulated that the $\ensuremath{\alpha}$-phase region extends only to \ensuremath{\sim} 9 at.% at room temperature, contrary to recent phase diagrams. The results at 10 at.% Zn also suggest that very slow cooling retains a large part of the $\ensuremath{\alpha}$ phase, but quenching produces the ordered phase. A possible explanation is that the high lattice-vacancy concentration immediately after quenching facilitates atomic movement.

Rigid-Band Behavior in the Specific Heats of PbTl and PbBi Alloys: Electron-Phonon Enhancement Effects

The normal-state specific heats of pure lead and four dilute PbTl and PbBi alloys have been measured in the temperature range 1.0-2.3 K. The fractional rate of change of the electronic specific-heat coefficient $\ensuremath{\gamma}$ with respect to the number of conduction electrons per atom was found to be 0.25\ifmmode\pm\else\textpm\fi{}0.06 for PbTl alloys and 0.51\ifmmode\pm\else\textpm\fi{}0.04 for PbBi alloys. When changes in the electron-phonon enhancement factor as determined from tunneling measurements are included, good agreement is found between the measured changes in $\ensuremath{\gamma}$ and those expected from the rigid-band model. Semiquantitative calculations indicate that changes in the electron-phonon enhancement factor may also account for the apparent discrepancy between previous experimental results and the rigid-band model in noble-metal alloys. The Debye temperature was found to decrease for both lead alloy series.