Role of carbon for superconductivity inMgCxNi3from specific heat

Abstract

The influence of carbon deficiency on superconductivity of MgCNi 3 is investigated by specific heat measurements in the normal and superconducting state. In order to perform a detailed analysis of the normal state specific heat, a computer code is developed which allows for an instantaneous estimate of the main features of the lattice dynamics. By analyzing the evolution of the lattice vibrations within the series and simultaneously considering the visible mass enhancement, the loss in the electron-phonon coupling can be attributed to significant changes of the prominent Ni vibrations. The present data well supports the recently established picture of strong electron-phonon coupling and ferromagnetic spin fluctuations in this compound. The discovery of superconductivity in MgCNi 3 has caused much attention, 1 since the large Ni content suggests a magnetic state rather than superconductivity. Indeed, up to now a lot of experimental and theoretical publications point to a ferromagnetic instability at temperatures near and below the superconducting transition temperature T c ≈ 7 K.2,3,4,5,6 So far there is much discussion on the carbon content in MgCNi 3 , since only samples with carbon excess of ≈ 50 % show large T c values, whereas for the stoichiometric composition a strongly reduced T c is found, 7,8 which may be triggered by enhanced pair-breaking due to increasing spin fluctuations.9 However specific heat measurements indicate that this is not the case but instead the electron-phonon coupling is significantly reduced, 5 accompanied by a considerable hardening of low-energy vibrations.10 Band structure calculations predict a constant 9 or decreasing 5 electron density of states at the Fermi level (EDOS), leaving the interesting prospect of significantly changing lattice dynamics. Recent measurements of the carbon isotope effect support this picture.

s-wave superconductivity probed by measuring magnetic penetration depth and lower critical field ofMgCNi3single crystals

“…On the one hand, ab-initio calculations rapidly showed that the electronphonon coupling constant (λ e−ph ∼ 0.2) is far too low to account for the high critical temperatures observed in oxypnictides (up to ∼ 55 K) and an unconventional mechanism mediated by the SF associated with a sign reversal of the (s-wave) order parameter between electrons and holes sheets of the Fermi surface has been proposed [10]. On the other hand, it has been suggested that the narrow van Hove singularity could be responsible for a nearly unstable phonon mode in MgCNi 3 inducing a large, although reduced by SF, λ e−ph [11,12] in agreement with experiments which yield an average electronphonon coupling constant in the order of 1.7 [11,13,14]. The interplay between electron-phonon coupling and SF is further emphasized in this system by the existence of a large isotopic effect [15] which has been suggested to be enhanced by the strong SF [16].…”

“…On the other hand, the Hall probe has been placed close to the center of the sample in the HPM measurements and is hence sensitive to the bulk of the sample. In the case of MgCNi 3 , it is known that the critical temperature has a surprising high sensitivity to a very small change in the C or Ni stoechiometry [4,20] and also surface stress [14,38]. Assuming that the critical temperature of the surface is 20 % smaller than the bulk value, very good fit to the data could be obtained for ρ T DO S using Eq.1 for T < 3 4 T c (still taking ∆(0) = 2k B T c , see solid lines in Fig.…”

Superconductivity in the vicinity of ferromagnetism in oxygen free perovskite MgCNi3: An experimental and density functional theory study

“…They considered small m sp = 0.1, and obtained much reduced T c = 11.5 K even in the case of Θ D = 300 K. Spin-fluctuation is responsible for that higher value (3.02) of λ, due to excess amount of Carbon [38]. The difference in experimental and fitted electronic specific heat data (C es /γT) [see Fig.…”

Search forSc3XB(X=In,Tl,Wiendlocha1, Toboła2, Kaprzyk3 2006Phys. Rev. B370270View full textAdd to dashboardCite

“…Quite recently, two important investigations were carried for this system. It was deduced from the specific heat measurements [60] that in a carbon-deficient sample low-energy nickel phonon modes (which are probably the most important in superconductivity) were shifted to higher energies. Furthermore, electronic structure calculations performed for MgC x Ni 3 in the range 0.80 < x < 1 exhibited the decrease of n(E F ) with the increasing x [61], but less drastic than in our Sc 3 XB x systems.…”