Neutron scattering study of the correlation of magnetism and superconductivity in heavy-fermion superconductor UPd2Al3

Journal of Magnetism and Magnetic Materials 177-181 (1998) 449 450

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Neutron scattering study of the correlation of magnetism and superconductivity in heavy-fermion superconductor UPdzA13 N. Metoki a'*, Y. Haga a, Y. Koike a, N. Aso a'b, Y. O n u k i a'c aAdvanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Naka, lbaraki 319-11, Japan bPhysics Department, Graduate School of Science, Tohoku University Aramaki, Aoba, Sendal 980-77, Japan Graduate School of Science, Osaka University, Machikaneyama, Osaka 560, Japan

Abstract Neutron scattering experiments have been carried out in order to study the interplay between magnetism and superconductivity in the heavy-fermion superconductor UPd2A13. We have observed 1% suppression of the (0 0 0.5) magnetic peak intensity below the critical temperature T~. It is direct evidence for the coupling of the magnetic order with the superconducting order parameters. Spin excitation gap associated with superconductivity is observed for the first time in a heavy-fermion superconductor. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Heavy-fermion superconductor; Spin excitation gap; Neutron scattering

Interplay between magnetism and superconductivity is one of the main topics in physics of strongly correlated electron systems [1]. The coupling of the magnetic and superconducting order parameters is observed in UPt3 [2]. Power-law behavior of specific heat and nuclear spin relaxation time are understood in terms of anisotropic superconducting gap. However, no trace of the gap related to superconductivity has been observed so far by means of inelastic neutron scattering. In this paper we describe evidence for the interplay between magnetism and superconductivity in UPd2A13. It exhibits antiferromagnetic (AFM) ordering (0.85/~B/U) below the N6el temperature TN = 14.5 K, and superconductivity below the critical temperature T¢ = 2 K. The magnetism and superconductivity in UPdzA13 have been believed to be rather independent, because no evidence for the coupling was found [3 6]. Neutron scattering experiments were carried out using a cold-neutron triple-axis spectrometer LTAS installed at the research reactor JRR-3M in Japan Atomic Energy Research Institute. The neutron beam was monochromatized by a PG monochrometer (fixed incident energy at 4.4 meV) with a Be filter cooled down to 10 K.

*Corresponding author. Fax: + 81 29 282 6716; e-mail: [email protected].

The collimation was 26'-70'-72'-72' which allowed us the resolution of 0.18 meV at AE = 0 meV. Single-crystal samples were grown by the Cochralskii pulling method in a tetra-arc furnace [7]. T¢ and the residual resistivity ratio are 1.9 and 60 K, respectively. Fig. 1 shows the temperature (T)-dependence of the (0 0 0.5) AFM peak intensity 1. For H = 0, I increases continuously from TN to To. Below Tc I turns to decrease. For H = 1 T, I increases down to 1.5 K, which is Tc at H = 1 T. For H = 3 T, I increases monotonically. It corresponds to the fact that the sample exhibits no superconductivity for this condition. This result is understood that the superconducting order parameter is coupled to the magnetic one and suppresses it. Fig. 2 shows the inelastic neutron scattering profile at constant Q = (0 0 0.5) for various temperatures. At 4.2 K, the profile can be described by a quasi-elastic and an inelastic Lorentzian peak at the excitation energy AE = 1.5 meV, as well as a sharp Bragg peak and an incoherent scattering. The inelastic peak would be attributed to a spin-wave excitation [5]. The quasi-elastic scattering would be due to thermal fluctuation, since it decreases with decreasing temperature from 4.2 to 2 K. Below Tc we observed that the quasi-elastic peak shifts towards high energy and becomes an inelastic peak at AE about 0.4 meV. This change indicates that the spin-excitation gap appears below T~. It is confirmed that the spin

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Fig. 1. The T-dependence of the (0 0 0.5) magnetic Bragg peak intensity. Data are shifted along the vertical axis. excitation gap is accompanied by superconductivity by the field dependence of the profile. The energy gap decreases with applying field, and disappears for H > Hc2. The peak positions at the lowest temperature 0.4 K is 0.36 meV. Roughly speaking, AE of about 0.4 meV is in the same order but smaller than the superconducting gap expected from the BCS theory, 2A = 3.5kBT~ = 0.8 meV. AE exhibits a clear T-dependence comparable to the weak-coupling BCS order parameters shown in Fig. 3a. The peak intensity shows a continuous increase as shown in Fig. 3b. This T-dependence is clearly demonstrated again by measuring the scattering intensity at AE = 0.4 meV (inset of Fig. 3). It looks like the superconducting order parameter. An inelastic neutron scattering profile has been measured as a function of the m o m e n t u m transfer Q along the (0 0 l) and (h 0 0.5) directions. We found a remarkable dispersion relation of the gap with a minimum at Q = (0 0 0.5). The existence of dispersion indicates the collective excitation of the spin excitation gap. In conclusion, we have observed the coupling of the magnetic and superconducting order parameters. Spin excitation gap in a heavy-fermion superconductor is observed for the first time. These results are indicative of the strong interplay between magnetism and superconductivity in UPd2A13. Authors would like to thank Profs. M. Tachiki, T. Komatsubara and Dr. N. Sato for helpful discussions.

References

[1] G. Aeppli, C. Broholm, in: K.A. Gschneider, Jr., L. Eyring, G.H. Lander, G.R. Choppin (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 19, Ch. 131, Elsevier, Amsterdam, 1994, p. 123. [2] G. Aeppli, D. Bishop, C. Broholm, E. Bucher, K. Siemensmeyer, M. Steiner, N. Stiisser, Phys. Rev. Lett. 63 (1989) 676.

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Fig. 3. (a) The T-dependence of the excitation energy, and (b) the T-dependence of the peak intensity of the inelastic peak due to spin excitation gap. [33 H. Kita, A. D6nni, Y. Endoh, K. Kakurai, N. Sato, T. Komatsubara, J. Phys. Soc. Japan 63 (1994) 726. [4] B.D. Gaulin, D. Gibbs, E.D. Isaacs, J.G. Lussier, J.N. Reimers, A. Schr6der, L. Taillefer, P. Zschack, Phys. Rev. Lett. 73 (1994) 890. [5] T.E. Mason, T. Petersen, G. Aeppli, A.P. Ramirez, E. Bucher, J. Hufnagl, Rise Report R-660 (1993) 37. [6] T. Petersen, T.E. Mason, G. Aeppli, A.P. Ramirez, E. Bucher, R.N. Kleiman, Physica B 199&200 (1994) 151. [73 Y. Haga, Y. Yamamoto, Y. Inada, D. Aoki, K. Tenya, M. Ikeda, T. Sakakibara, Y. Onuki, J. Phys. Soc. Japan 65 (1996) 3646.