Charge fluctuation in Yb4(As1−xSbx)3

Charge fluctuation in Yb4(As1−xSbx)3

Physica B 259—261 (1999) 275—276 Charge fluctuation in Yb (As Sb )  \V V  Y. Nemoto *, H. Aoki , T. Goto , A. Ochiai, T. Suzuki Graduate School...

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Physica B 259—261 (1999) 275—276

Charge fluctuation in Yb (As Sb )  \V V  Y. Nemoto *, H. Aoki , T. Goto , A. Ochiai, T. Suzuki Graduate School of Science and Technology, Niigata University, Ikarashi Ninocho 8050, Niigata 950-2181, Japan Material Science and Technology, Niigata University, Niigata 950-2181, Japan The Institute for Solid State Physics, The University of Tokyo, Tokyo 106-8666, Japan

Abstract We present ultrasonic velocity and attenuation measurements of the Sb-substituted compounds Yb (As Sb ) ,  \V V  x"0, 0.12, 0.24, 0.29. The results of our experiments reveal that the one-dimensional charge ordering of Yb> along [1 1 1] direction in Yb As changes into a charge glass order in Yb (As Sb ) .  1999 Elsevier Science B.V. All         rights reserved. Keywords: Yb (As Sb ) ; Elastic constant; Charge ordering; Charge glass  \V V 

Mixed valence compound Yb As with very low car  rier density undergoes a structural phase transition at ¹ "292 K from a cubic lattice with the space group  ¹ to a trigonal lattice with C associated with the   charge ordering of Yb> and Yb> ions [1]. Ultrasonic measurements [2] showed that the transverse elastic constant C with ! symmetry exhibits a large softening of   18% below 400 K down to ¹ "292 K, whereas no  softening has been found near ¹ in the bulk modulus  C "(C #2C )/3 with ! symmetry and the trans   verse mode (C !C )/2 with ! symmetry. Thus, it is    reasonable to assume that the order parameter Q , Q , WX XV Q with ! symmetry corresponding to the charge flucVW  tuation mode gives rise to the one-dimensional linear chain of Yb> ions along the body diagonal [1 1 1] direction in the trigonal phase. In order to examine the charge ordering in the Sbsubstituted compounds Yb (As Sb ) , the electric  \V V  resistivity, the magnetic susceptibility and the Hall coefficient have been measured [3]. The charge ordering temperature ¹ decreases with increasing Sb concentration  x. In this work, we present ultrasonic velocity and attenuation measurements on the compounds Yb (As Sb ) ,  \V V  x"0, 0.12, 0.24, 0.29. * Corresponding author. Tel.: #81-25-262-6136; fax: #8125-262-6135; e-mail: [email protected]

Fig. 1 displays the temperature dependence of the transverse elastic constant C for Yb (As Sb ) ,   \V V  x"0, 0.12, 0.24, 0.29. The elastic constant C"ov is easily obtained from the density o. Below 400 K the C mode of Yb As exhibits a remarkable softening of    18% towards ¹ "292 K. The ultrasonic echo suddenly  vanishes due to discontinuous increase of the ultrasonic attenuation indicating the first-order phase transition [2]. The charge ordering of Yb (As Sb ) shifts to       lower temperature side at ¹ "231 K and C shows   more pronounced softening of 34% below 350 K down to ¹ "231 K. For both compounds of Yb (As Sb )   \V V  with x"0, 0.12, the temperature dependence of C above ¹ can be well fitted by the Curie-Wiess law   ¹!¹  , C "C (1)   ¹!H





obtained from the free energy F assuming a bilinear coupling of the order parameter Q , Q , Q and the WX XV VW elastic strain e , e , e . In the case of Yb (As Sb ) WX XV VW       the charge ordering temperature shifts to lower side at ¹ "120 K. The softening of C below 340 K is reduced   to 25%. The temperature dependence of C in Yb (As     Sb ) above ¹ deviates from the Curie—Weiss law of     Eq. (1). In Yb (As Sb ) the transition temperature       ¹ becomes unclear and the softening of C below   350 K has a tendency to further reduce. In this sample it

0921-4526/99/$ — see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 9 2 5 - 9

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Y. Nemoto et al. / Physica B 259—261 (1999) 275—276

elastic constant C for Yb (As Sb ) . The observed        pronounced ultrasonic dispersion indicates that the relaxation of the charge fluctuation in Yb (As Sb ) is       of thermal activation type. Assuming the charge fluctuation with a single relaxation time q(E), attenuation a reads S



G(E)uq(E) *C dE . a " S 2ov 1#uq(E)

Fig. 1. Temperature dependence of the elastic constant C for  Yb (As Sb ) , x"0, 0.12, 0.24, 0.29.  \V V 

Fig. 2. Temperature dependence of the ultrasonic attenuation up to 102 MHz and of the longitudinal elastic constant C for  Yb (As Sb ) . The inset shows different kind of fits of the       relaxation time q.

is remarkable that an additional weak softening between 200 and 90 K appears in the longitudinal mode C and  in the transverse mode (C !C )/2 which is not ob  served for the compounds with x"0, 0.12 and 0.24. Furthermore, the difference of the temperature variation among elastic modes changes to be small. Fig. 2 shows the temperature dependence of the ultrasonic attenuation up to 102 MHz and of the longitudinal

(2)

Here, *C is a variation of the elastic constant between, before and after a chrage relaxation, u is a frequency of the sound wave. In the present analysis we adopt a Gaussian type as a distribution function G(E) for the activation energy E. As shown in the inset of Fig. 2 fits of the relaxation time q"q exp(E/k¹) yields a characteristic  time q with 10\—10\ s and a center of the activation  energy distribution E with 0.037—0.039 eV. At low tem peratures the charge fluctuation time gets as slow as 10\ s. In the case of mixed valence compounds of Sm X (X"S, Se, Te) with Th P structure, there is no     charge ordering of Sm> and Sm>. At low temperatures specific heats of Sm Se and Sm Te show a broad     maximum and the magnetization curve does a pronounced hysteresis of the field-cooling and zero-fieldcooling process reflecting the metastable state for spin glass ordering in Sm Te [4]. We measured the ultra  sonic properties for Sm Se [5] and Sm Te , the elastic     constants C , (C —C )/2 and C exhibit the same     temperature dependence and the remarkable ultrasonic dispersion around 150 K indicating the random distribution of Sm> and Sm>. These ultrasonic results for Sm Se and Sm Te resembles well the one for the sub    stituted compound of Yb (As Sb ) . Thus, the       present results of our experiments reveal that the onedimensional charge ordering of Yb> linear chains in Yb As changes into a charge glass order in   Yb (As Sb ) .       References [1] A. Ochiai, T. Suzuki, T. Kasuya, J. Phys. Soc. Japan 59 (1990) 4129. [2] T. Goto, Y. Nemoto, S. Nakamura, A. Ochiai, T. Suzuki, Physica B 230—232 (1997) 702. [3] H. Aoki, A. Ochiai, T. Suzuki, R. Helfrich, F. Steglich, Physica B 230—232 (1997) 698. [4] T. Tayama, K. Tenya, H. Amitsuka, T. Sakakibara, A. Ochiai, T. Suzuki, J. Phys. Soc. Japan 65 (1996) 3467. [5] A. Tamaki, T. Goto, S. Kunii, T. Suzuki, T. Fujimura, T. Kasuya, J. Phys. C 18 (1985) 5849.