Experimental determination of superconducting parameters for the intermetallic perovskite superconductorMgCNi3

Abstract

We have measured upper-critical-field H c2 , specific heat C, and tunneling spectra of the intermetallic perovskite superconductor MgCNi 3 with a superconducting transition temperature T c Ϸ7.6 K. Based on these measurements and relevant theoretical relations, we have evaluated various superconducting parameters for this material, including the thermodynamic critical field H c (0), coherence length (0), penetration depth (0), lower-critical-field H c1 (0), and Ginzburg-Landau parameter (0). From the specific heat, we obtain the Debye temperature ⌰ D Ϸ284 K. We find a jump of ⌬C/␥T c ϭ2.1 at T c ͑where ␥ is the normal-state electronic specific coefficient͒, which is larger than the weak-coupling BCS value of 1.43, suggesting that MgCNi 3 may be a strong-coupling superconductor. In addition, we observed a pronounced zero-bias conductance peak ͑ZBCP͒ in the tunneling spectra. We discuss the possible physical origins of the observed ZBCP.

Puzzling phonon dispersion curves and vibrational mode instability in superconducting MgCNi3

“…Thus, in the recent past several attempts have been made to investigate the structural, electronic, vibrational and superconducting properties of MgCNi 3 . 1,[4][5][6][7][8][9][10][11][12][13][14] Experimental results 9,[15][16][17] have provided evidence for a medium to strong-coupling superconductivity driven by electron-phonon interactions. Further evidence for the phonon-mediated pairing mechanism is provided by theoretical estimates ranging between 0.8 to 1.5.…”

Competition between BCS superconductivity and ferromagnetic spin fluctuations inMgCNi3

“…However, up to now, there is still not a consensus on this issue. The measured penetration depth [6], critical current behavior [7] and earlier tunneling spectra [8] suggested an unconventional superconductivity, the later tunneling data [9] supported the s-wave pairing symmetry and gave a reasonable interpretation on the contradiction to the result in Ref [8]. The s-wave pairing has also been demonstrated by the 13 C NMR experiments [10] and the specific heat measurements [1,8,11,12,13,14].…”

“…The s-wave pairing has also been demonstrated by the 13 C NMR experiments [10] and the specific heat measurements [1,8,11,12,13,14]. To our knowledge, all the previous reports on the specific heat of MgCNi 3 [1,8,11,12,13,14] were characterized in the framework of a conventional phonon-mediated pairing. However, there is an obvious deviation of the experimental data from the prediction of BCS theory in the low temperature [8,15], i.e., the entropy conservation rule is not satisfied.…”

“…However, there is an obvious deviation of the experimental data from the prediction of BCS theory in the low temperature [8,15], i.e., the entropy conservation rule is not satisfied. Such deviation has been interpreted by the presence of unreacted Ni impurities in Refs [8,15], whereas it is still prominent in the samples without Ni impurities [14]. On the other hand, strong spin fluctuations have been observed in MgCNi 3 by NMR experiment [10], which is suggested to be able to severely affect the superconductivity in MgCNi 3 [2,5,10,11,16] or even induce some exotic paring mechanism [2].…”

“…The measured penetration depth [6], critical current behavior [7] and earlier tunneling spectra [8] suggested an unconventional superconductivity, the later tunneling data [9] supported the s-wave pairing symmetry and gave a reasonable interpretation on the contradiction to the result in Ref [8]. The s-wave pairing has also been demonstrated by the 13 C NMR experiments [10] and the specific heat measurements [1,8,11,12,13,14]. To our knowledge, all the previous reports on the specific heat of MgCNi 3 [1,8,11,12,13,14] were characterized in the framework of a conventional phonon-mediated pairing.…”