Spin dynamics of the low-carrier heavy-electron system Yb4As3

Journal of Magnetism and Magnetic Materials 177 181 (1998) 307-308

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Spin dynamics of the low-carrier heavy-electron system Yb4As3 M. KohgP'*, K. Iwasa a, J.-M. Mignot b, A. Ochiai c, T. Suzuki d Department of Physics, Tokyo Metropolitan University, Minamiosawa 1-1, Hachioji, Tokyo 192-03, Japan bLaboratoire Lkon Brillouin, CEA/Saclay, 91191 Gif sur Yvette, France CDepartment of Material Science and Technology, Niigata University, Niigata, 950-21, Japan aDepartment of Physics, Tohoku University, Sendai 980-77, Japan

Abstract Inelastic neutron-scattering experiments on Yb4As3 revealed the existence of low-energy spin excitations that are characteristic of a one-dimensional antiferromagnetic coupling at low temperatures. The results indicate that the one-dimensional arrangement of Yb 3+ ions caused by the charge ordering plays an important role for the heavy-electron behavior in Yb4Ass. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Heavy electron; Heavy fermion; Low-carrier system; Charge ordering; One-dimensional system; Neutron scattering

Yb4As3 has the anti-ThsP4-type cubic crystal structure above about 290 K. Below this temperature, it shrinks slightly along one of (1 1 1) directions giving a trigonal structure with a = 90.8 °. It exhibits no longrange magnetic ordering down to 0.045 K [ 11, and shows typical heavy-electron anomalies such as a large T-linear term in specific heat (y = 205 mJ/K2/mol) or a strong T-square dependence followed by a - log T dependence in the electrical resistivity (A = 0.75 ~tf~cm/K 2) in the trigonal phase [2]. The susceptibility of the compound shows the Curie-Weiss behavior at high temperatures, however, it exhibits a saturation below about 20 K, although further up-turn of the susceptibility is seen below about 7 K [21. The interesting point is that the carrier density is extremely low ( ~ 10- 3 per formula) at the low temperatures [21. Clearly, it is difficult to attribute the origin of the heavy-electron behavior in Yb4As3 to the usual dense Kondo effect. On the other hand, the results from susceptibility [21, Mfssbauer 1-11 and perturbed angular correlation 1-3] measurements suggested that the structural phase transition is accompanied with a charge ordering from a mixed valence state between Yb 2÷ and Yb 3 + in the cubic phase to a state in which one-fourth of the Yb ions aligned along the 1-1 1 1] direction (Yb0 are

*Corresponding author. Tel.: + 81 426 77 2506; fax: + 81 426 77 2483; e-mail: [email protected].

tri-valent whereas the rest of the Yb ions (Ybn) become di-valent, where Ybl and Ybn are equivelent in the cubic phase. Our recent polarized neutron experiment proved directly the existence of that type of charge ordering though it has some imperfection (Ybn still shows some amount of tri-valency) [4]. In this paper we report the results of inelastic neutron scattering experiments which indicate that the heavy-electron properties in Yb4As3 are strongly related with the charge ordering. The low-energy excitations below about 7 meV were measured on the triple-axis spectrometers 5G and 6G in JAERI, Tokai, and 4F2 in LLB, Saclay using single-crystalline samples of about 2 g. In order to get a single-domain sample at low temperatures, a strain-cool technique was used. Well-defined and strongly dispersive inelastic peaks were observed at various reciprocal lattice points below about 20 K in spite that there is no magnetic long-range order. Fig. 1 shows typical spectra observed at 1.4 K. It was found that the observed magnetic peaks can be well organized if the wave vectors of the excitations are taken as those in the one-dimensional arrays of Ybl ions aligned along the [-t 1 1] axis instead of taking three-dimensional ones. In Fig. 2 the peak positions of the observed spectra at 1.4 K are plotted against the wave vector q in the one-dimensional representation (in the unit of ~/d, where d is the atomic distance in the Ybi ion chain). Although the data are taken at various positions in the

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M. Kohgi et al. /Journal of Magnetism and Magnetic Materials 177-181 (1998) 307-308

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Fig. 1. Low-energy neutron spectra observed for Yb4As3 at 1.4 K. The dotted lines denote the calculated intensity based on the 1D-HAF model. The broken lines centered at E = 0 show the incoherent elastic peaks. The curve with hatch indicates the resolution of the spectrometer. 6

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Fig. 2. Dispersion relation of inelastic peaks of Yb4As3 in the one-dimensional representation. Different symbols indicate different directions from the (2 0 0) reciprocal lattice point (see legend). The broken line represents the peak position calculated by the 1D-HAF model. three-dimensional reciprocal lattice space, their peak positions lie on a single curve. The result clearly indicates that the Yb a÷ chains along the [1 1 1] direction caused by the charge ordering behave as one-dimensional antiferromagnets. Since the observed dispersion curve is quite close to a sine-function except near q = 0, the system may be modeled by a one-dimensional spin ½ Heisenberg system with a nearest-neighbor antiferromagnetic interaction (1D-HAF) whose Hamiltonian is H = - 2 J ~ S j • S~+ 1. Thus, the observed spectra were compared with the dynamical spin correlation function calculated by using the MOiler ansaz for 1 D - H A F [5]. Here, the density of the

spin excitations has a sharp lower b o u n d at the energy of the so-called des Croiseaux-Peason spin-wave mode: E(q) = nJ sin(dq). The dotted lines in Fig. 1 show the theoretical spectra convoluted with the resolution function. Here, n J was taken to be 3.5 meV, and the calculated intensities are normalized to the data at q = 0.8. The dotted line in Fig. 2 indicates the calculated peak position. The agreement between the observation and calculation is quite well not only for the dispersion relation but also for the spectral shape except near q = 0. It is especially noted that the asymmetric spectra observed for the wave vectors near q = 1 (see, for example, Fig. ld) are well accounted for by the existence of the c o n t i n u u m of the spin excitations characteristic of the 1 D - H A F model. This fact clearly indicates that the one-dimensional Yb 3 + chains formed by the charge ordering behave as a 1DH A F system. The response around q = 0 (see, for example, Fig. la), which is not compatible with the 1DH A F model, may be due to some other origin since the excitation energy as well as the spectral density vanish at q = 0 in the Heisenberg system. It is also noted that the model gives the C / T value of 0.19 J / K e / m o l at a lowtemperature limit and the temperature of susceptibility m a x i m u m of 17 K [6]. These values are quite close to the observed 7 value and the temperature at which a saturation of susceptibility is seen, respectively. Thus, it is quite likely that the heavy-electron-like anomalies seen in these physical quantities is actually due to the 1 D - H A F properties of the Yb 3+ chains caused by the charge ordering. However, the other unusual properties such as the heavy-electron-like behavior of the electrical resistivity or the rather steep up-turn of susceptibility below about 7 K [2] cannot be explained by the 1 D - H A F model. Actually, an unusual magnetic response is observed around q = 0 superimposed on the one-dimensional response. The imperfection of the charge ordering [4] may be also related with these facts. Further n e u t r o n scattering work to check these points is in progress.

References

[1] P. Bonville, A. Ochiai, T. Suzuki, E. Vencent, J. Phys. I 4 (1994) 595. 1-23 A. Ochiai, T. Suzuki, T. Kasuya, J. Phys. Soc. Japan 59 (1990) 4129. 1-33 M. Rams, K. Kr61as, K. Tomala, A. Ochiai, T. Suzuki, Hyperfine Interactions 97-98 (1996) 125. I-4] M. Kohgi, K. Iwasa, A. Ochiai, T. Suzuki, J.-M. Mignot, B. Gillon, A. Gukasov, J. Schweizer, K. Kakurai, M. Nishi, A. D6nni, T. Osakabe, Physica B 230 232 (1997) 638. 1-53 G. Miiller, H. Thomas, H. Beck, J.C. Bonner, Phys. Rev. B 24 (1981) 1429. 1-6] J.C. Bonner, M.E. Fisher, Phys. Rev. 135 (1964) A640.