Unusual properties of a new superconductor La 3Ni

Physica B 206 & 207 (1995) 565-567

ELSEVIER

Unusual properties of a new superconductor La3Ni N. Sato*, N. Koga, K. Imamura, T. Sakon, T. Komatsubara Department of Physics, Faculty of Science, Tohoku University, Sendai 980-77, Japan

Abstract We report unusual properties of a new superconductor La3Ni, and discuss the correlation between them. The electronic specific heat coefficient in the normal state is estimated to be about 24 mJ/K 2 per mole of formula unit. The electronic specific heat in the superconducting state reveals the T 3 power-law. This result indicates that La3Ni is an unconventional superconductor with Tc ~ 1.45 K. We also present the weakly temperature-dependent magnetic susceptibility and the T 2 dependence of the electrical resistivity at low temperatures following a saturation at higher temperatures.

1. Introduction Superconductivity remains one of the more important subjects, because the properties of some unusual superconductors are not defined by the usual Bardeen, Cooper and Schrieffer (BCS) theory. For example, heavy-fermion superconductors have been studied extensively [1], because the temperature dependence of the thermodynamic quantities shows the power-law, indicating that the gap function has nodes. Recently, we reported that La3Ni indicates the power-law behavior and the double-peaked structure in the temperature dependence of the specific heat [2]. From this power-law behavior, we concluded that La3Ni is a new example of unconventional superconductors. We attributed the double-peaked structure to two possibilities, i.e. the intrinsic phase transition in the superconducting state as in UPt 3 [3] or a mixture of two superconducting phases. This double-peaked structure varied from sample to sample, and thus it may suggest the latter possibility. In order to test these two possibilities and extract the intrinsic properties, the specific heat of several compounds denoted as

* Corresponding author.

La3+xNi (x = 0.1, 0, - 0 . 3 and - 0 . 7 ) was studied. The temperature dependence of the electrical resistivity and the magnetic susceptibility are also presented.

2. Experimental Samples were polycrystals and prepared by a conventional arc-melted technique. The starting materials were 99.9% pure La and 99.97% pure Ni. The obtained ingots were turned over several times to ensure homogeneity. According to the phase diagram of the L a - N i system [4], compounds of La3Ni and LavNi 3 (hereafter, referred to as La2.3Ni) exist, and the X-ray diffraction experiment of the powdered samples of La3Ni and La2.3Ni confirms the orthorhombic Fe3C and the hexagonal Fe3Th 7 type crystal structure, respectively. The phase diagram shows that La3 1Ni decomposes to La3Ni and La metal, but no trace of La metal phase was detected by the X-ray diffraction experiment within the experimental accuracy. The sample denoted by the formula Laz.TNi seems to be a mixture of these two compound phases. Note that the composition defined by the formula La3+xNi is nominal. The specific heat was measured by a conventional heat pulse method in a 3He cryostat and a 3He-4He

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N. Sato et al. / Physica B 206 & 207 (1995) 565-567

dilution refrigerator (down to about 260 mK). The magnetization measurement was made by a commercial S Q U I D magnetometer in a 4He cryostat. The electrical resistivity was measured by a standard DC 4probe method.

Fig. 1 illustrates the temperature dependence of the magnetic susceptibility of La3Ni together with La3Co, both of which show the Curie-Weiss-like law. The increase in the magnetic susceptibility below about 50 K is probably due to the magnetic impurities. In fact, electron probe micro analysis (EPMA) reveals the existence of Gd impurities, and the analysis of this upturn of the magnetic susceptibility yields the Gd impurity concentration of about 20ppm. The temperature-dependent magnetic susceptibility above 50 K is not likely ascribed to the localized moment residing on the Ni sites, because the slope of the 1 / x - T curve of La3Ni, i.e. the Curie constant, is nearly the same as that of LaBCo. This temperature-dependent magnetic susceptibility was considered to be attributed to a fine-structure in the density-of-states (DOS) near the Fermi energy. Next, the zero temperature value of the magnetic susceptibility g(0) was estimated as 0.94 × 10-3 emu/mol f.u. by extrapolating the data above 50 K to zero temperature. The temperature dependence of the electrical resistivity of La3Ni is plotted in Fig. 2. It was noted that the electrical resistivity tends to saturate at higher temperatures, which is similar to that of compounds of A15 structure. This saturation can be attributed to the fine-structure of the DOS, consistent with the tem-

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T(K) Fig. 2. Temperature dependence of the electrical resistivity of La3Ni. Note that the resistivity intends to saturate at higher temperatures. Inset illustrates the T 2 temperature dependence at lower temperatures. perature-dependent magnetic susceptibility as mentioned above. We also note the T 2 temperature dependence of the electrical resistivity at low temperatures, as shown in the inset of Fig. 2. This behavior is also found in A15. The coefficient A of the T 2 term is obtained to be A = 0.016 t~lLcm/K 2. The temperature dependence of the specific heat of La 3 ~Ni, La3Ni and Laz.3Ni (=La7Ni3) is given in Fig. 3 in the plot of C / T versus T. The double-peaked structure appears in LaaNi, while a single anomaly is observed around 1.45 K and 2.05K for La3.1Ni and La2 3Ni, respectively. The double-peaked structure is L

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Fig. 3. Temperature dependence of the measured specific heat C plotted as C / T versus T. The broken line indicates the fits to the specific heat of La31Ni (Laz.3Ni) in the normal state (i.e. C = 3,T + IST 3 + 6T~), where y = 2 3 . 9 m J / K 2 tool f.u. (16.4mJ/K 2 tool f.u.), /3=l.05mJ/K 4 tool f.u. (1.51mJ/K 4 mol f.u.) and 6=0.033mJ/K 6 mol f.u. (0.0019 mJ/K 6 tool f.u.).

567

N Sato et al I Physica B 206 & 207 (1995) 565-567

also found in La2.7Ni (not shown here), which was found to consist of a mixture of both La3Ni and La7Ni 3 (=La2.3Ni) phases. Therefore, it is probable that the double-peaked structure in La3Ni is due to a mixture of these two phases. The electronic specific heat of La3Ni nearly shows the T 3 temperature dependence (see Ref. [2]). As indicated in Fig. 4, the sample of La3ANi retains this power-law in the temperature dependence of the electronic specific heat, demonstrating that La3Ni is an unconventional superconductor. (As mentioned above, La3.1Ni is considered to decompose to La3Ni and La metal, but no trace of (superconducting) La metal phase was found in the temperature dependence of the specific heat.) The linear coefficient of the specific heat, 3', of Laa.tNi in the normal state was recorded as 24 mJ/K 2 mol f.u. (The 3,-value of LaTNi 3 was also noted to be 4 9 m J / K 2 mol f.u.) The ratio of A/3, 2 was then estimated as 2.8 × 10 -5 in the above units. This value was comparable to that of heavy-fermions and A15 (A/3, 2 ~ 1 × 10-5), but it deviates from that of transition metals such as Pd and Pt (A /3, 2 = 0.04 × 10-5). Miyake et al. pointed out that A15 should be classified into the heavy-fermions rather than into the ordinary transition metals category from the discussions about the A/3, 2 ratio [5]. In other words, the sharp peak structure in DOS near the Fermi level may be due to a resonance level arising from the many-body correlations between conduction electrons and some other i

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when the units for X(0) are emu/mol and those for 3' are mJ/mol K 2. This Wilson ratio is used as a measure to distinguish the contribution to X due to the spin fluctuations from that due to the mass enhancement. The spin fluctuation as a precursor of band ferromagnetism may lead to considerable depression of the superconducting transition temperature T c. In fact, heavy-fermion superconductors and compounds of La3In and La3TI have a low Wilson ratio of R < 0.52 and R ~0.65, respectively [1,6]. In contrast to these superconductors, the Wilson ratio of La3Ni is much larger, i.e. R = 2.9, although the used value of X(0) includes the orbital contribution. In this respect, La3Ni seems to be a characteristic superconductor and further investigations are needed to study the origin of the attractive force.

Acknowledgement N. Sato would like to thank Y. Shibata for the electron micro probe analysis.

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Fig. 4. Electronic specific heat Co of La3.1Ni in the superconducting state plotted as Ce/ T versus T 2. Ce is obtained by the subtraction of the phonon contribution (i.e. Ct =/3T 3 + 8T 5) from the measured specific heat C. Note that Ce is proportional to T 3 with the coefficient a = 37.2 mJ/K 4 mol f.u.

References [1] See, for example, G.R. Stewart, Rev. Mod. Phys. 56 (1984) 755. [2] N, Sato, K. Imamura, T. Sakon, T. Komatsubara, I. Umehara and K. Sato, J. Phys. Soc, Japan 63 (1994) 2061. [3] R.A. Fisher, S. Kim, B.F. Woodfield, N.E. Phillips, L. Taillefer, K. Hasselbach, J. Flouquet, A.L. Giorgi and J.L. Smith, Phys. Rev. Lett. 62 (1989) 1411. [4] Y.Y. Pan and P. Nash, in: Binary Alloy Phase Diagrams, Vol. 3, 2nd edition, ed. T.B. Massalski (ASM International, 1990) p. 2407. [5] K. Miyake, T. Matsuura and C.M. Varma, Solid State Commun. 71 (1989) 1149. [6] F. Heiniger, E. Bucher, J.P. Maita and P. Descouts, Phys. Rev. (B) 8 (1973) 3194.