Competition between BCS superconductivity and ferromagnetic spin fluctuations inMgCNi3

Abstract

The low temperature specific heat of the superconductor MgCNi3 and a non-superconductor MgC0.85Ni3 is investigated in detail. An additional contribution is observed from the data of MgCNi3 but absent in MgC0.85Ni3, which is demonstrated to be insensitive to the applied magnetic field even up to 12 Tesla. A detailed discussion on its origin is then presented. By subtracting this additional contribution, the zero field specific heat of MgCNi3 can be well described by the BCS theory with the gap ratio (∆/kBTc ) determined by the previous tunneling measurements. The conventional s-wave pairing state is further proved by the magnetic field dependence of the specific heat at low temperatures and the behavior of the upper critical field.PACS numbers: 74.25. Bt, 74.20.Rp, 74.70.Ad Since the discovery of the new intermetalic perovskite superconductor MgCNi 3 [1], plenty of efforts have been focused on the superconducting pairing symmetry in this material because its conduction electrons are derived predominantly from Ni which is itself a ferromagnet [2,3,4,5]. 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]. 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. 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]. Consequently, the behavior of the specific heat will inevitably be changed by the spin fluctuations. Therefore, before a real pairing mechanism being concluded from the specific heat data, we have to carefully investigate how the ferromagnetic spin fluctuations contribute to the specific heat of MgCNi 3 .In this work, we elaborate on the specific heat (C) of MgC x Ni 3 system both in normal state and supercon- * Electronic address: hhwen@aphy.iphy.ac.cn ducting state. A low temperature upturn is clearly distinguished in the C/T vs T 2 curves and found to be insensitive to the applied magnetic field. By doing some quantitative analysis, we present the evidence of most possible mechanisms responsible for this upturn. After s...

Superconducting energy gap in MgCNi3single crystals: Point-contact spectroscopy and specific-heat measurements

“…8 and 10, a strong low-temperature upturn of C/T (T ) is observed, indicating the presence of a Schottky anomaly probably due to magnetic impurities. In the case of the polycrystalline samples of Wälte et al 12 and Shan et al, 13 the deviations are much smaller and could be described either by a higher phonon term (∼ T 5 )or by additional electron-paramagnon interaction. In our case, C n could also be fitted with C n = aT + bT 3 + cT 5 .O u rC n comprises inevitably also the addenda, but since all known specific-heat measurements on MgCNi 3 so far, independently of the form of the sample (polycrystals or single crystals) and the method of measurements, show the presence of a low-temperature upturn, we believe it is intrinsic.…”

“…The latter effect was attributed either to the Schottky contribution and/or the paramagnetism of unreacted impurities, 8,10 or to an electron-paramagnon interaction in MgCNi 3 itself. 12, 13 Wälte et al 12 have proposed a two-band-two-gap model to account for different sizes of superconducting gaps found by different techniques. The anomalous point-contact Andreev-reflection spectra obtained by Shan et al 14 were interpreted using a model in which point contact made on a BCS superconductor is in series with the Josephson junction due to the polycrystalline character of the samples.…”

Growth of Single Crystals of MgCNi3

“…Moreover, the superconducting origin of this material has not yet been clearly identified, and the experimental results are still conflicting. For example, both s [1,[6][7][8][9][10] and non-s wave [11][12][13] gap symmetry were observed by using various experimental tools. Also, the electron-phonon coupling constants in the conventional phonon-mediated pairing mechanism extracted from a number of specific heat measurements were different.…”

Specific Heat and Upper Critical Field of Sc5Ir4Si10Superconductor

“…18) As for a conventional s-wave superconductor, the specific heat in the vortex state is dominated by the contribution from the localized quasiparticles in the vortex core, and the specific heat coefficient ( ) H γ is in a linear relationship with the magnetic field. On the other hand, for a superconductor with nodes in the gap, for example d-wave superconductors with line nodes, Volovik et al 19) pointed out that supercurrents In our case, the slope of ( ) H γ Δ at low field is larger than that at higher field, which has been witnessed in two-gap superconductors such as MgB 2 20) and 23) quite different from the similar compound Lu 2 Fe 3 Si 5 , which is already proved to be a two-gap superconductor, 9,24) and T c is rapidly suppressed by nonmagnetic impurities.…”