Doping effects on the folded phonon mode in the spin-Peierls systems CuGeO3 and α′-NaV2O5

Journal of Magnetism and Magnetic Materials 177 181 (1998) 679-680

,~

Journal of

mnatnetlsm magnetic , i ~ materials

ELSEVIER

Doping effects on the folded phonon mode in the spin-Peierls systems CuGeO3 and a'-NaV205 H. Kuroe a'*, H. Seto a, J. Sasaki a, T. Sekine a, N. Koide b, Y. Sasago b, K. Uchinokura b, M. Has&, M. Isobe d, Y. Ueda d aDepartment of Physics, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102, Japan bDepartment of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan cNational Research Institute for Metals (NRIM), 1-2-1 Sengen, Tsukuba, Ibaraki 305, Japan dInstitute for Solid State Physics, The University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan

Abstract We have studied the Zn- and Si-doping effects on the 368 cm- ~ folded phonon in CuGeO3 by means of Raman scattering. With increasing dopant concentrations, the transition temperature decreases to a constant value, and the lattice distortion at T = 0 also decreases. In the doped samples, a broad Raman peak of the folded phonon still remains observable even above the transition temperature, indicating the increase of the short-range fluctuations of lattice dimerization. In c(-NaV205, we have observed a folded phonon at 948 cm- ~ in the spin-Peierls phase. The defect of the Na ÷ ions also causes the decrease of the intensity and the increase of the half-width of the folded phonon. © 1998 Elsevier Science B.V. All rights reserved.

Keywords: Spin-Peierls transition; Raman scattering; Phonons - folded

After the discovery of the inorganic spin-Peierls (SP) system by Hase et al. [1], the SP transition in this system has been intensively studied. Below the SP transition temperature Tsp, the SP gap and the lattice distortion were observed by many kinds of experiments, including Raman scattering (RS). In the SP phase of CuGeO3, the folded phonons are additionally observed at 368 and 818 cm-1 by RS. The second-order Raman band below 250cm -1 reflected the three-dimensional density of states of the magnetic excitation [2], and the asymmetric peak at 30 cm 1 was assigned to the two-magnetic-excitation bound state [2]. Upon impurity doping, the Tsp(i) and the lattice distortion 6i(T) decrease [3], and a new phase with an antiferromagnetic order of the spins appears at low temperatures [4]. We perform RS in Cul-xZnxGel-rSirO3 (x or y = 0) to study the phonon property. Our results are compared with those of at'-NazV2Os, in which the SP transition at 34 K has been confirmed when z = 1 [5]. * Corresponding author. Tel.: + 81-3-3238-3350;fax: + 813-3238-3341; e-mail: [email protected].

The Cul_xZnxGel_rSiyO3 and a'-Na=V2Os single crystals were prepared by the floating-zone method and the solid-state reaction, respectively. The Zn or Si concentrations i, where i = x or y, determined from EPMA and Tsp obtained from the magnetic susceptibility, TSp(i), were listed in Table 1. In sample # 3, the antiferromagnetic order was confirmed by the magnetic susceptibility below 4.3 K. Homogeneous substitution is checked carefully. Fig. la shows the Raman spectra at 6 K. Since the samples have the different scattering volumes from each other, all the spectra were normalized so that the 330 cm-1 phonon in each sample has the same Raman intensity. The intensity of the 368 c m - ~ folded phonon decreased with increasing dopant concentrations. Fig. 1b shows the temperature dependence of the integrated Raman intensity Ii(T) of the 368 cm 1 folded phonon. The solid curves denote 6{(T) determined from the data of 6o(T), which was obtained by neutron diffraction [6]. We obtained the dopant concentration dependence of the TsRp(i)from RS and that of li(O)/Io(O) as listed in Table 1. The TRsp(i)decreases to about 9 K and the 6i(0) decreases with increasing dopant concentrations.

0304-8853/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 88 5 3 ( 9 7 ) 0 0 9 4 0 - 2

680

H. Kuroe et al. /Journal qf Magnetism and Magnetic Materials 177--181 (1998) 679 680

Table 1 Tsp obtained from the susceptibility, and the RS, and the relative intensity of the folded phonon in pure and doped CuGeO3

Zn

Si

Sample no.

i (i = x, y)

TSp(i) (K)

T~p(i) I,(0)/Io(0) (K)

0

0

14.0

15.0

1

I 2 3

0.007 0.017 0.021

12.2 10.4 -

11.5 9.0 9.0

0.69 0.56 0.18

4 5

0.004 0.011

13.2 11.0

13.2 13.0

0.88 0.50

i

ot'-NaV20

8 E g0

i

s

a

i

i

i

500

i

000

Flaman Shift (em -t)

40 K z = 0.90

'~_ ..WTT~_,lff,..,-~a2 K ~'¢v"v"v~v f l t ' w ~ ' ~ 2 e K '1

a

~

'

"~" #o -

"~

rr

z = 0.99

i , r , r

:

' o o

' ~

"

"

#0

~ "-a-'/p',-,~.d 20 K E ~ 1 5 K n . .

b

. . . .

920 960 1000 Rarnan Shift (cm -1)

#2

#2

320

Raman

'

360

'

400

S h i f t ( c m -1)

, , .......

z = 1.00

940 960 980 Raman Shift (cm -1)

Fig. 2. Raman spectra of a'-NaV2Os at RT, 40 K, and 15 K (a) and the temperature (b) and Na+-defect (c) dependence of the folded phonon. 0

t

I

i

I

I

10 Temperature

t

I

I

20 (K)

Fig. 1. Raman spectra at 6 K (a) and the temperature dependence of the normalized intensities of the folded phonon (b) in pure and doped CuGeO3. In the doped samples, the Raman intensity of the folded phonon still remains finite even above TRp(i), indicating the increase of the short-range fluctuations of lattice dimerization. As reported before [7], the folded phonon was observed even in the sample which shows no indication of the SP transition in the magnetic susceptibility. We observed the folded phonon even below 4.3 K in sample # 3, i.e., in the antiferromagnetic phase, indicating the survival of the lattice distortion in this phase. This result agrees with the recent theory by F u k u y a m a et al. [8]. Fig. 2a shows Raman spectra in the (a, a) geometry in u ' - N a V e O s at r o o m temperature (RT), 40 K, and 15 K. At 15 K, the additional mode appeared at 9 4 4 c m 1. This peak was sharp and symmetric. Fig. 2b shows the detailed temperature dependence of the 944 c m - 1 peak, and it disappears around Tse. Therefore, the 944 c m - 1 peak is assigned to the folded phonon, indicating the formation of the superlattice. Fig. 2c shows the Na+-defect dependence of the 944 c m - ~ peak at 15 K. This peak decreases in intensity and becomes broad in ~(-Nao.99V2Os, and disappears in

0(-Nao.9oV2Os, suggesting the decrease of the lattice distortion, which was also observed in C u G e O > We confirmed that the defect of the Na + ions in c('-NaV205 works as a nonmagnetic impurity. The continuous band around 500 c m - 1 was observed. The 531 cm 1 peak has a tail at the higher-energy side, indicating the Fano effect. As the defect of Na + ions is increased, the 531 cm -~ peak tends to be symmetric. Further study is needed to clarify the origin of the broad band strongly interacting with the phonon at 531 cm i In conclusion, we studied the doping effects in inorganic spin-Peierls systems by means of R a m a n scattering, and found the decrease of Tsp(i) and 6i(0), and the shortrange fluctuations of lattice dimerization near Tsp(i) in the doped CuGeO3. In the antiferromagnetic phase, the lattice distortion still persisted. We compared our result of the folded phonon in the doped C u G e O 3 to that of the 944 c m - 1 folded phonon in 0t'-Na~V20 5. References

[1] [2] [3] [4] [5] [6] [7] [8]

M. Hase et al., Phys. Rev. Lett. B 70 (1993) 3651. H. Kuroe et al., Phys. Rev. B 50 (1994) 16468. Y. Sasago et al., J. Phys. Soc. Japan 63 (1994) 365. M. Hase et aL, Physica B 215 (1995) 164. M. Isobe et al., J. Phys. Soc. Japan 65 (1996) 1178. M.C. Martin et al., Phys. Rev. B 53 (1996) 14713. H. Kuroe et al., Physica B 219&220 (1996) 104. H. Fukuyama et al., J. Phys. Soc. Japan 65 (1996) 1182.