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Title: Large Ca isotope effect in the CaC{sub 6} superconductor.

Abstract

We have measured the Ca isotope effect coefficient, {alpha}(Ca), in the newly discovered superconductor CaC{sub 6} and find a value of 0.53(2). This result shows that the superconductivity is dominated by coupling of the electrons by Ca phonon modes. The C phonons contribute very little, assuming that this material is a conventional electron-phonon coupled superconductor. Thus, in contrast to another layered material MgB{sub 2}, where high-energy phonons in the B layers are responsible for the superconductivity, in layered CaC{sub 6} the phonons responsible for superconductivity are primarily low-energy modes of the intercalated Ca.

Authors:
; ; ; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
957384
Report Number(s):
ANL/MSD/JA-56361
Journal ID: ISSN 0031-9007; PRLTAO; TRN: US1000690
DOE Contract Number:
DE-AC02-06CH11357
Resource Type:
Journal Article
Resource Relation:
Journal Name: Phys. Rev. Lett.; Journal Volume: 75; Journal Issue: 014509 ; 2007
Country of Publication:
United States
Language:
ENGLISH
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; ELECTRONS; ISOTOPE EFFECTS; PHONONS; SUPERCONDUCTIVITY; SUPERCONDUCTORS

Citation Formats

Hinks, D. G., Rosenmann, D., Claus, H., Bailey, M. S., Jorgensen, J. D., and Materials Science Division. Large Ca isotope effect in the CaC{sub 6} superconductor.. United States: N. p., 2007. Web. doi:10.1103/PhysRevB.75.014509.
Hinks, D. G., Rosenmann, D., Claus, H., Bailey, M. S., Jorgensen, J. D., & Materials Science Division. Large Ca isotope effect in the CaC{sub 6} superconductor.. United States. doi:10.1103/PhysRevB.75.014509.
Hinks, D. G., Rosenmann, D., Claus, H., Bailey, M. S., Jorgensen, J. D., and Materials Science Division. Mon . "Large Ca isotope effect in the CaC{sub 6} superconductor.". United States. doi:10.1103/PhysRevB.75.014509.
@article{osti_957384,
title = {Large Ca isotope effect in the CaC{sub 6} superconductor.},
author = {Hinks, D. G. and Rosenmann, D. and Claus, H. and Bailey, M. S. and Jorgensen, J. D. and Materials Science Division},
abstractNote = {We have measured the Ca isotope effect coefficient, {alpha}(Ca), in the newly discovered superconductor CaC{sub 6} and find a value of 0.53(2). This result shows that the superconductivity is dominated by coupling of the electrons by Ca phonon modes. The C phonons contribute very little, assuming that this material is a conventional electron-phonon coupled superconductor. Thus, in contrast to another layered material MgB{sub 2}, where high-energy phonons in the B layers are responsible for the superconductivity, in layered CaC{sub 6} the phonons responsible for superconductivity are primarily low-energy modes of the intercalated Ca.},
doi = {10.1103/PhysRevB.75.014509},
journal = {Phys. Rev. Lett.},
number = 014509 ; 2007,
volume = 75,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • We report temperature- and magnetic field-dependent bulk muon spin rotation measurements in a c-axis-oriented superconductor CaC{sub 6} in the mixed state. Using both a simple second-moment analysis and the more precise analytical Ginzburg-Landau model, we obtained a field-independent in-plane magnetic penetration depth {lambda}{sub ab}(0)=72(3) nm. The temperature dependencies of the normalized muon spin relaxation rate and of the normalized superfluid density result to be identical and both are well represented by the clean limit BCS model with 2{Delta}/k{sub B}T{sub c} = 3.6(1), suggesting that CaC{sub 6} is a fully gapped BCS superconductor in the clean limit regime.
  • Tunneling in CaC{sub 6} crystals reproducibly reveals superconducting gaps {Delta} of 2.3 {+-} 0.2 meV that are {approx}40% larger than reported earlier. In an isotropic s-wave scenario, that puts CaC{sub 6} into the class of very strongly coupled superconductors, since 2{Delta}/kT{sub c}-4.6, implying that soft Ca phonons are primarily involved in the superconductivity. This conclusion explains the relatively large Ca isotope effect found recently for CaC{sub 6}, but it could also signal a strong anisotropy in the electron-phonon interaction.
  • Joining YBa/sub 2/Cu/sub 3/O/sub 6.5 + delta/ (123 phase) and Bi/sub 4/Sr/sub 4/Ca/sub 2/Cu/sub 4/O/sub 16 + delta/ (4424 phase) as structurally characterized high-T/sub c/ superconductors, the thallium-containing superconductor (Tl/sub .75/Bi/sub .25/)/sub 1.33/Sr/sub 1.33/Ca/sub 1.33/Cu/sub 2/O/sub 6.67 + delta/ with the ideal stoichiometry (Tl, Bi)/sub 1/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub 6.5 + delta/ (1212 phase) is reported here. As prepared from the component oxides, 1212 has an initial deviation from resistance linearity at 120 K, a superconducting onset temperature of 92 K, and zero resistance at 75 K. The tetragonal unit cell (P4/mmm, a = 3.800 /angstrom/; c = 12.072 /angstrom/,more » deduced from powder data) contains double copper oxygen sheets (like 4424 and 123) that alternate with single thallium-bismuth oxygen sheets (in contrast to 4424, which contains double bismuth oxygen sheets), resulting in a total of three stacked perovskite-like cells (as in 123). The copper oxide sheets (with intersheet spacing 3.38 /angstrom/) are separated by Ca/sup 2 +/ and the Cu oxide sheets and (Tl, Bi) oxide sheets (with spacing 4.35 /angstrom/) are separated by Sr/sup 2 +/, Ca/sup 2 +/, and excess (Tl, Bi)/sup 2 +/. The 1212 cell constitutes the building block for the centered, more complex 4424 cell. The 1212 structure persists to Bi contents as low as 1% and can also be stabilized by Pb instead of Bi; Tl cuprates also from other superconductors with lower T/sub c/.« less
  • The primary yield data in the reaction of Hg 6(/sup 3/P/sub 1/) atoms with propane and the quenching cross section yields ( sigma Q/sup 2/) were combined to determine the deuterium isotope effect on the primary reaction. The magnitudes of the effects are reported for the propanes. It was found that the secondary bond effect was very large, 18-18 times in favor of C-H over C-D. For primary C--H bond cleavage the deuterium isotope effect was much smaller than for the secondary positions, yet still abnormally high for deuterium substitution. (P.C.H.)