Incommensurate CDW in Rb0.3MoO3: 87Rb NMR

Solid State Communications, Vol. 57, No. 8, pp. 611-613, 1986. Printed in Great Britain.

0038-1098/86 $3.00 + .00 Pergamon Press Ltd.

INCOMMENSURATE CDW IN Rbo.3MoOa: STRb NMR K. Nomura* and K. Kume Department of Physics, Tokyo Metropolitan University, Setagaya-ku, Tokyo 158, Japan and M. Sato Institute for Molecular Science, Myodaiji, Okazaki 444, Japan

(Received 14 October 1985 by W. Sasaki) The STRb NMR measurement was carried out in a single-crystal Rbo.3MoO3. Below 180 K the NMR line well characterized the existence of incommensurate CDW. The absence of lock-in transition was confirmed from the temperature dependence of line shape. The CDW held the incommensurate phase even at 77 K, in spite of the nearly commensurate value of CDW wave vector observed in the neutron diffraction experiment. The CDW amplitude obtained from NMR was consistent with the BCS curve. THE SO-CALLED blue bronzes Ao.3oMoO3 (A = K or Rb), which are ternary molybdenum oxides, reveal a quasi-one-dimensional conduction and undergo a metalsemiconductor phase transition of Peierls-type at about 180K. Below 180K X-ray diffuse scattering [1] has found an incommensurate CDW along the monoclinic b-axis. As in other classes of CDW systems (the transition metal trichalcogenides and the halogened transition metal tetrachalcogenides), the non-linear conductivity has been observed and attributed to the depinning of CDW [2]. The narrow band noise was also observed and discussed in connection with the sliding of CDW [3]. The temperature dependence of CDW wave vector q has been studied by neutron and X-ray diffraction experiments [ 4 - 7 ] . All authors have reported that the wave vector component along b-axis approaches the commensurate value of 0.75 with de creasing temperature. Although they have obtained the nearly commensurate value below 100K, they have not found the abrupt change of CDW wave vector due to the lock-in transition. The conclusions of above authors have not been consistent with each other and there have been a controversy about the existence of lock-in incommensurate-commensurate transition. Recent NMR experiment in Rb0.3oMoO3 [8] has reported the incommensurate-commensurate transition at about 100 K by the change ofRb NMRline. But their conclusion seems to be still ambiguous. In this view point, we will report a Rb NMR experiment in Rbo.3oMoO3 single crystal together with a result of neutron diffraction. By a detailed temperature dependence

* Present address: Department of Physics, Hokkaido University, Sapporo 060, Japan. 611

experiment of Rb NMR line shape, we will show that there is no lock4n transition. The single crystal sample Rbo.3MoO3 was grown by electrolysis described by Wold et aL [9]. NMR measurements were made by the conventional pulse NMR apparatus which operated at 13.8MHz. The rf pulse field H~ was applied along b-axis to avoid an eddy current loss and the static magnetic field H0 lay in the plane perpendicular to b-axis. The line shapes were obtained by the Fourier transform (FT) of a free induction decay after a 7r/2 rf pulse. The FT was carried out using Iwatsu SM2100 after averaging repeated transients. The temperature dependence measurements were made by use of a gas flow temperature controller between 77 and 300K. The neutron diffraction measurement was also carried out at the interesting temperature range around 100 K. As a 87Rb NMR spectrum we obtained a single line with a relatively narrow width at room temperature and the line position varied against the static magnetic field direction. The nucleus of STRb has spin of 3/2 and has an electric quadrupole moment. Then each transition line is shifted by the electric-quadrupole interaction with the electric field gradient in a crystal [10]. The angular dependence of line position in our sample well characterized a second-order quadrupolar shift. The observed line was understood to correspond to the central line (-- 1/2, 1/2) at Rb(1) site [11]. The line width could be explained by a R b - R b nuclear dipolar interaction and a small additive broadening due to inhomogeneous quadrupolar coupling. We observed the temperature dependence of NMR spectrum by keeping H0 along a + 2c. The variation of the NMR spectrum is shown in Fig. 1 at several typical

612

INCOMMENSURATE CDW IN Rbo.3MoO3 : STRb NMR

Vol. 57, No. 8

(kHz) 15

10

I

100

w

~

~

~

.-d

200

o

6 ~

300

T (K)

120 K Fig. 2. Temperature dependence of the CDW amplitude obtained from the NMR line. The solid line represents the BCS order parameter.

77K

I

0

10

20

30

4O

(kHz)

Fig. 1. STRb NMR line (-- 1/2, 1/2) in Rb0.aMoO3 singlecrystal at several typical temperatures. temperatures. Below 180K, the NMR line broadened with decreasing temperature and showed a characteristic double-horned shape. This line resembled the one observed by Butaud et al. [8], but our line was asymmetrically shaped due to the second order quadrupole coupling. With decreasing temperature the broadening of line became more pronounced and the asymmetry also increased. However, the line shape showed no qualitative change down to the lowest temperature. These behaviors of the NMR line were well understood by considering the incommensurate CDW. The NMR central line (--1/2, 1/2) is shifted by the second order quadrupolar interaction with the electric field gradient at nuclear site and the local resonance frequency shift 6f(r) at site r is spatially modulated under the existence of CDW with a wave vector q, as follows

6f(r) = fQ(1 + e cosqr) 2, where fQ is a quadrupolar shift at the absence of CDW

and e is the amplitude of spatial modulation of electric field gradient. The value of e is reasonably assumed to be proportional to the CDW amplitude. If the CDW has a commensurate value of q, several separated lines are expected to be observed for the NMR line. But our spectrum is not like this. In the case of the incommensurate value of q, the resulting line is spread between fQ(1 + e) 2 and f o ( 1 - e ) 2. This line well explained both the double-horned shape and the asymmetry. Therefore the existence of the incommensurate CDW was understood at temperatures down to 77 K and the absence of the lock-in transition was also confirmed. The picture of the discommensuration, where the most part of the CDW held the commensurate phase, was also discarded on the same consideration. Then we can discuss the amplitude of the CDW. The distance ~ between double horns just equals to 4foe. In Fig. 2, we show the temperature dependence of the value (~, which corresponded to the CDW amplitude by the above mentioned reason. The BCS order parameter, which is represented by the solid curve in Fig. 2, reproduces the result within the experimental temperature region. Thus, the temperature dependence of the order parameter is consistent with the mean field theory of Peierls system. In Fig. 3 we show the magnitude of b* component of the CDW wave vector obtained by the neutron diffraction using the sample which belonged to the same batch as NMR. At about 100K, the wave vector was very close to the commensurate value of 0.75 and the temperature dependent behavior was consistent with previous results [4-7] in Ko.aMoOa. Fleming et al. [7] have insisted that the CDW is commensurate at the temperature below 100 K by the absolute value of CDW

Vol. 5"7, No. 8

INCOMMENSURATE CDW IN Rbo.aMoOa: aTRb NMR

qb

double-horned shape line was observed for the Rb central transition and was understood through the electric quadrupole interaction under the existence of the incommensurate CDW. The NMR line showed no qualitative change down to 77 K and this result insisted the absence of the lock-in transition at about 100K. The CDW held the incommensurate phase even at 77 K, although the neutron diffraction experiment observed the nearly commensurate value of the CDW wave vector. We obtained the CDW amplitude from the NMR line shape and its temperature dependence was consistent with the BCS curve.

(b 1)

0.750

0.745

80

I

90

613

I

100 T(K)

Fig. 3. Temperature dependence of the magnitude of b* component of the CDW wave vector measured by neutron diffraction. wave vector. But, as we discussed above, our spectrum apparently showed the existence of incommensurate CDW even below 100 K. The NMR line shape is more conclusive for the commensurability of CDW than the wave vector itself. Therefore, the earlier reports of the observation of commensurate CDW are questionable at present. Butaud et al. have reported the I - C transition by their earlier observation [8]. But their refined spectrum has suggested that the CDW state kept the incommensurate phase even below 100K and have been consistent with our result. Butaud et al. have discussed that the time dependent impurity effect covered the incommensurate-commensurate transition in their subsequent experiments. But the resolution of their earlier spectrum itself seems to be not so high to conclude the existence of commensurate CDW and it may be explained by a signal saturation or a phase mixture. In summary, we carried out the STRb NMR measurement in single crystal Rbo.aoMoO3. Below 180 K, the

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