Borocarbide superconductors: Materials and physical properties

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Physica B 237-238 (1997)292-295

Borocarbide superconductors: Materials and physical properties H . T a k a g i a'*, M . N o h a r a a, R.J. C a v a b alnstitute for Solid State Physics, University of Tokyo, Tokyo 106, Japan b Bell Laboratories, Lucent technologies, Murray Hill 07974, USA

Abstract

An overview is given on materials and physical properties of the borocarbide intermetallic superconductors, RETM2B/C (RE = rare-earth, TM = Ni, Pd, Pt). Extensive studies on Lu(Y)Ni2BzC have revealed that the compound is a three-dimensional and clean limit type II superconductor with a moderately high density of states at the Fermi level. The high density of states originates from the presence of a narrow d band near the Fermi level. Intermediate coupling with phonons and the high density of states are likely to account for the observed high transition temperature. The presence of the narrow d band, on the other hand, results in substantial electron correlation effects. Keywords: Borocarbide; Superconductivity

In the past decade, superconductivity research has been focusing mainly on the physics and chemistry of copper oxides, due to their extremely high transition temperatures and exotic properties associated with strong electron correlation. In contrast, research on intermetallic superconductors had been relatively inactive until recently, due to the long hiatus in the discovery of new high T¢ intermetallics. Recently, motivated by the observation of filamental superconductivity in mixed phase Y - N i - B by Mazumder et al. [1], a family of superconducting quaternary borocarbide intermetallics with high superconducting transition temperatures were established [2, 3]. The highest T¢ observed in the family reaches 23 K, a temperature equalling the previous longstanding record of Nb3Ge. The new family of borocarbide superconductors is expressed by the chemical formula RETMzB2C with RE = rare earth elements and T M -- Ni, Pd, Pt. The crystal structure of RETMzB2C can be viewed as a filled version of ThCr2Si2 structure, consisting of alternating TMzB2 and rocksalt REC * Corresponding author.

layers. Single-phase samples can be easily formed by arc melting and subsequent annealing for TM =Ni and Pt. Single-phase LuNizB2C, YNi2BzC and L a P t / B / C show very sharp superconducting transitions at 16.6, 15.6 and 11 K, respectively, with full diamagnetic shielding below T¢ [3]. The coexistence of superconductivity and antiferromagnetism is observed in RENi2B/C with the magnetic rare earth elements, Tm, Er, Ho and Dy, on the RE site [4]. Among these magnetic superconductors, of particular interest is the Ho compound. This compound shows re-entrant superconductivity at around 5 K due to the sequential magnetic ordering of the Ho moment [4, 5]. Nonmagnetic LaNi2B/C crystallizes in the RETMzB2C structure but does not show superconductivity at least down to 1.3 K, which may give a clue for clarifying key factors for the superconductivity [3]. Single-phase YPd2B2C has not yet been obtained, likely due to a phase instability at low temperature. However, mixed phase Y - P d - B - C , with a visible amount of YPd/BzC phase in the X-ray powder diffraction pattern, shows bulk superconductivity at 23 K [2,6].

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H. Takagi et al. / Physica B 237-238 (1997) 292-295 Table 1 Superconducting parameters for YNi2B2C estimated from the upper and lower critical fields

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110 750

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63

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Superconductivity at a relatively high transition temperature has been observed also in ThTM2B2C (TM --- Ni, Pd, Pt) [7]. Among the borocarbide superconductors, nonmagnetic YNi2B2C and LuNi2B2C have been studied most extensively. Superconducting parameters for YNi2B2C, estimated from upper and lower critical fields measurements on single crystals, are summarized in Table 1. We find several important messages on the nature of the superconductivity in the table. First of all, the material is a clean limit type II superconductor. The upper critical field, approximately ~ 10 T, is by a factor of 3 smaller than A-15 superconductors with comparable To. The resultant coherence length is 50 A, which is much smaller than the electron mean free path estimated from the normal state resistivity [8]. Despite its layered crystal structure, the material is electronically three-dimensional. Comparing the upper critical fields for magnetic fields perpendicular and parallel to the Ni2B2 plane, we find an anisotropy of approximately 1.3, which results in an effective mass anisotropy of mc/mab * * ~ 1.7. The observation here is consistent with the anisotropic magnetization measurements

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Band calculations on superconducting Ni borocarbides h a v e been done by several groups [10, 11]. In the calculated band structure, three branches with different characters - a very narrow band primarily of Ni 3d character, a very broad B-C s-p manifold band, and a Lu5d(Y4d) band are seen near the Fermi level. Recently, taking the advantage of the clean metal behavior of the borocarbides, the de-Haas-van Alphen oscillations have been successfully observed [12-14]. The observed frequencies are consistent with the band calculations, indicating that band theory provides a good description for the ground state of

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temperature. As pointed out previously [16], the experimentally determined 3' is a factor of 1.6 larger than that estimated from the band calculation. If we ascribe this discrepancy to effective mass enhancement due to electron-phonon interaction, an electron-phonon coupling constant 2 is estimated as 0.6, implying intermediate coupling. Indeed, the specific heat jump at Tc in Fig. 1, A C / 3 ' T c ~ 1.8, is substantially larger than the BCS weak coupling limit value of 1.43. These features appear to imply that both moderate coupling with phonons and the high density of states are responsible for the high transition temperature. Mattheiss et al. pointed out that the broad s-p manifold band is a key in the realization of the high-temperature superconductivity [17]. This broad band is extremely sensitive to NiB+ tetrahedral geometry, giving rise to a large deformation potential against Ni-B bond bending. The high-T~ can therefore result from strong coupling with high-frequency optical phonons. Recently, soft Xray emission spectroscopy (SXES) has been done on superconducting YNi2B2C and nonsuperconducting LaNi2B2C [18]. This technique allows one to determine the B 2p partial density of states experimentally. The results indicate that the B partial density of states peak, located near the Fermi

level in superconducting YNi2B2C, shifts to 0.5-1.0 eV above the Fermi level in nonsuperconducting LaNi2B2C. Comparing with the band calculation by Mattheiss [17], the density of states peak can be attributed to the broad sp band. Superconductivity therefore shows up only when the broad sp band is located at the Fermi level, consistent with the picture proposed by Mattheiss et al. The presence of the narrow d band, on the other hand, implies that electron correlation is important as well. A number of experimental observations indicate that correlation effects play a significant role in the electronic properties of the borocarbides. Photoemission spectroscopy on YNi2B2C clearly demonstrates a substantial contribution from Ni 3d states at the Fermi level [19]. The observed bandwidth is approximately 20% narrower than the results of band calculations, likely due to the correlation effects. The normal state resistivity of Lu(Y)Ni2B2C is dominated by a T2-dependence at low temperatures, with the T 2 coefficient A roughly satisfying the Kadowaki-Woods relationship [20]. Recent 11B and 61Ni NMR studies on LuNi2B2C revealed strong enhancement of (T1T)- t with decreasing temperature, in contrast to the weakly temperature-dependent Knight shift [21]. This indicates that antiferromagnetic spin fluctuations contribute considerably to (T1 T)-1. The borocarbide is therefore a superconductor with antiferromagnetic spin fluctuations [22]. In the context of antiferromagnetic spin fluctuation, it is interesting to note that the specific heat data in Fig. 1, as well as those reported previously [23], show x/~-dependence of linear specific heat coefficient 3,* in the superconducting state, rather than H-linear dependence as seen in conventional superconductors. It has been pointed out theoretically that dependence of 3,* in the superconducting state is expected for d-wave paring 1-24]. Whether or not the observation here is compatible with conventional electron-phonon mediated superconductivity is worthy of further exploration. In summary, the new family of quaternary borocarbides are three-dimensional and clean type II superconductors, with moderately high density of states at the Fermi level. Intermediate electron-phonon coupling as well as the high density of states appear to account for the observed high

H. Takagi et al. /Physica B 237-238 (1997) 292-295

transition temperature. On the other hand, due to the presence of a narrow d-band at the Fermi level, electron correlation effect cannot be neglected in the borocarbides. The possible interplay between the electron correlation and superconductivity has not yet been clarified. This work has been done in collaboration with many colleagues at University of Tokyo and Bell laboratories. We would like to thank particularly J.J. Krajewski, W.F. Peck, Jr., H. Eisaki, S. Uchida, M. Issiki, K. Ikushima, H. Yasuoka, M. Tokunaga, N. Miura, S. Shin, A. Fujimori, B. Batlogg and S.A. Carter for their collaboration and useful discussion. References I-1] C. Mazumder, R. Nagarajan, C. Godart, L.C. Gupta, M. Latroche, S.K. Dhar, C. Levy-Clement, B.D. Padalia and R. Vijayaraghavan, Solid State Commun. 87 (1993) 413. 1-2] R.J. Cava, H. Takagi, B. Batlogg, H.W. Zandbergen, J.J. Krajewski, W.F. Peck, Jr., R.B. van Dover, R.J. Felder, T. Siegrist, K. Mizuhashi, J.O. Lee, H. Eisaki, S.A. Carter and S. Uchida, Nature 367 (1994) 146. [3] R.J. Cava, H. Takagi, H.W. Zandbergen, J.J. Krajewski, W.F. Peck, Jr., T. Siegrist, B. Baflogg, R.B. van Dover, R.J. Felder, K. Mizuhashi, J.O. Lee, H. Eisaki and S. Uchida, Nature 367 (1994) 252. [4] H. Eisaki, H. Takagi, R.J. Cava, B. Batlogg, J.J. Krajewski, W.F. Peck, Jr., K. Mizuhashi, J.O. Lee and S. Uchida, Phys. Rev. B 50 (1994) 647. I-5] A.I. Goldman, C. Stassis, P.C. Canfield, J. Zarestky, P. Dervenagas, B.K. Cho, D.C. Johnston and B. Sternlieb, Phys. Rev. B 50 (1994) 9668. I-6] H. Fujii, S. Ikeda, S. Arisawa, K. Hirata, H. Kumakura, K. Kadowaki and K. Togano, Jpn. J. Appl. Phys. 33 (1994) L590.

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