Improved film synthesis has allowed us to prepare N13Ge/SiO /Pb tunnel junctions on samples with high T, (up to 21.2 K) and large energy gap (ANb 0, up to 3.85 meV). These junctions have satisfactory features for taking derivative measurements.The data were reduced by the modified McMillan-Rowell proximity gap inversion analysis developed by Arnold and Wolf to generate a I" (co) and related microscopic parameters. The trend previously seen of a movement of the lowest phonon branch to lower energies as the T, and gap increase is continued, resulting in the lowestenergy phonon mode being well defined and enhanced in strength. Theoretical functional derivatives for T, (by Bergmann and Rainer) and the energy gap (by Mitrovic et aL) qualitatively explain the rise in T"energy gap, and 2h/k~T, . Heat-capacity measurements have been performed on various samples to give bulk Nb3Ge properties, including one sample which was analyzed by tunneling a I" (u) and heat capacity. y =34+ 1.5 rn J/mole K2 and the bare density of states, X(O)=1.5+0.1 states/eVatom, suggests that a high density of states is inadequate to explain the high Tc 1n Nb3Ge. Values for 25/kgTc =4.2+0.1 and AC/QTc~1.9 1ndlcate a strong-couPIed superconductor, in agreement with tunneling results on this sample. INTRGDUCTIONThe high-T, A15 compounds are of continuing interest both for their technological applications as well as for information they can provide into a fundamental understanding of superconductivity As th. e material with the highest known T"Nb3Gc is particularly interesting. The most direct probe of microscopic superconductivity is tun-Qcl1ng.KihlstI'oID and Gcballc produced tunneling ct F(co) on NbGe samples with a wide range of compositions (17 -23 at. % Ge), gaps (ENb, o, -1.1 -3.7 meV), and T, 's (7 -20 K). The one limitation of the study was the difficulty in producing good quality high-T, samples.Since that time wc have had gI'catcI' success 1Q pI'oduc1ng more homogeneous, high-T,~3GC samples. The high 7I, has allowed us to extend the tunneling study to make meaningful comparisons with bulk properties via heatcapacity measurements. 24 -48 h before the junction area was defined by a dielectric (photoresist). A Pb counterelectrode was then deposited.The Nb3Ge thin films were prepared by electron-beam codcposlt1on 1Q thc pI'cscnce of oxygen. ' Thc substrate temperature was 900'C; the deposition rate was -30 A/sec. The oxygen partial pressure was 1.3X10 Torr. The film samples while still under vacuum were cooled to 100'C and the barrier of 25 -30 A of Si (Ref. 7) was deposited. The samples were then removed from vacuum, and the amorphous Si was allowed to oxidize in air for 4 5 vouwGE (mv) FIG. 1. Current-voltage characteristics for the largest-gap sample in this study.
%'e have made absolute specific-heat measurements through the metal-insulator transition in thin-film Mo Ge~-, . We report results for y and P and infer the thermodynamic electronic density of states. The density of states shows no critical behavior at the metal-insulator transition.There is also evidence of anomalous excess specific heat belo~the metal-insulator transition. %e compare our results to current theories on localization and electron interactions.In this paper we present results of specific-heat measurements performed on a thin-film amorphous system, Mo"Gei " through its metal-insulator transition. Trans-
We have studied the effects of transition-metal (TM) and metalloid (M) substitution in amorphous molybdenum-based TM-M alloys. Absolute-specific-heat measurements of thin-film (-1 pm thick) samples of Mo-Ge, Mo-Si, and Mo-Ti-Si alloys prepared by magnetron sputtering were made using the relaxation-time-constant method. A comparison of the superconducting transition width made both thermally and electrically indicates that the sample homogeneity is better than 0.1% for length scales greater than the coherence length. Both the electronic density of states and T, with increasing metalloid concentration and [Ti]/[Mo] ratio, in agreement with a rigid-band model. We compare our results to current theories relating the electron-phonon coupling constant A, , ph to the electronic density of states.
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