Journal of Low Temperature Physics, Vol 117, Nos. 3/4, 1999

NQR in La1.85Sr0.15Cu1-xLixO4 under high pressure(1.9GPa)
Naoki FUJIWARA, Nobuo MORI, Hiroshi KAGEYAMA,and Yutaka UEDA
Institute for Solid State Physics, University of Tokyo,Roppongi, Minato-ku 106-8666, Japan

Nuclear quadrupole resonance (NQR) under steady pressure (1.9GPa)
was performed on La1.85Sr0.15 Cu1-xLix O4 where Li substitution causes metal(x = 0)-insulator (x — 0.5) transition. Two kinds of Cu-NQR signals corresponding to both states have been observed at the intermediate amount ofx. Effect of the pressure was investigated for x = 0.25 and 0.3 samples.
Possible Li-distribution pattern was proposed for the x — 0.3 sample.
PACS numbers: 74.62 Fj, 76.60 Gv, 74-72 Dn

1.

INTRODUCTION

Li substitution for Cu in LSCO system is one of the interesting problems
in that the in-plane substitution causes metal-insulator transition and brings
holes in CuO2 plane like Sr substitution. Effect of the Li substitution has
been mainly investigated in the end material La2CuO4. The main interest iswhether Li-induced holes delocalize in a wide range forming the singlet withCu ions. Antiferromagnetic long-range order (AFLRO) is lost by only 3%
Li substitution, which suggests that degree of hole delocalization is high1.
The effect on crystal structure or lattice parameters has been studied andsimilarities to the Sr substitution has been pointed out 2. The susceptibility decreases with increasing Li concentration and becomes diamagnetic at
50% Li substitution1,2. The long-range ordering of Cu and Li cations has
been observed at this concentration3 and confinement of the singlet to CuO4
plaquette is suggested. However, magnetic excitation gap estimated fromnuclear spin-lattice relaxation is much smaller than the estimation between
the singlet and the lowest-lying triplet4.
4850022-2291/99/1100-0485$16.00/0 © 1999 Plenum Publishing Corporation

N. FUJIWARA et al.

486

Similar situation also holds for La1.85Sr0.15Cu1-xLixO4 , however, superconductivity is realized at lightly Li doped region instead of the AFLRO. Oneof the interesting features in this system is coexistence of two kinds of CuNQR signals at x = 0.2 ~ 0.3: One is a broad signal and the local environment is similar to that of the metallic state (x = 0), the other is a sharp one
and is the same observed in La 2 Cu 1 _ x Li x O 4 . Rykov et. al reported that theformer signal is hardly observed in x = 0.25 sample whereas both signals areobserved in x — 0.2 sample1. Only 5% excess Li substitution induces qualitatively discontinuity in the NQR spectra. La1.85Sr0.15Cu1_xLixO4 is expected
to locate on very critical position in the microscopic viewpoint. Thus small
external action would give rise to qualitative variation in the NQR spectra.
In the present work, we measured NQR spectra in La1.85Srg0.15Cu1-xLixO4(x = 0.25 and 0.3) under steady high pressure (1.9GPa) and compared withthose at O.lGPa.

2.

Experimental Equipment

Steady high pressure was retained by mounting a cryostat equippedwith a piston and a cylinder on a 100tpress and controlling pressure during
the experiments. Powder samples plunged into liquid medium were closedby 012.7 Teflon cap and Cu-Be plug. The schematic figure is shown in Fig.
1.

Fig. 1. Schematic figure of the cryostat mounted on a 100t press.

NQR in Li doped LSCO system under high pressure (1.9GPa) 487

3.

Experimental Results and Discussion

NQR spectra were measured by using a conventional spectrometer. The
NQR spectra for x = 0.25 sample at 0.1 GPa are shown in Fig. 2(a). A
broad signal around 37MHz corresponds to that of the superconductive state(x = 0) as is mentioned in the introduction. Although the material is aninsulator as a bulk since the Li concentration is high, there still exists theenvironment of electric field gradient similar to the superconductive state in
the microscopic viewpoint. The broad signal observed even for the x — 0.25
sample completely disappears for x = 0.3 sample as is shown in Fig. 3(a).
The concentration where the broad signal disappears is quantitatively 5%larger than Rykov et. al.'s results1. In any case, the broad signal completely
disappears at x — 0.3.

Fig. 2. NQR spectra for La1.85Sr0.15Cu1_xLixO4 (x = 0.25) at 4.2KFig. 3. NQR spectra for La1.85Sr0.15Cu1-xLixO4 (x = 0.3) at 4.2K

488

N. FUJIWARA et al.

NQR spectra at 1.9 GPa for the x = 0.25 and x = 0.3 samples areshown in Figs. 2(b) and 3(b), respectively. No drastic change was observed
by applying pressure for the x = 0.3 sample except for the broadening ofthe linewidth. The linewidth of 63Cu at 1.9GPa is 1.0MHz and is 0.2MHz
broader than that at 0.1 GPa. However, relative intensity of the broad signal
to the sharp one changed 35% by applying pressure for the x = 0.25 sample.
Hole doping gives the same effect on lattice parameters irrespective of
the Li substitution or the Sr substitution2. The lattice parameters of theCuO2 plane shrink 4.38 x 10 -3 A for 5% hole doping, whereas the parameterfor c-axis increases by the Li substitution and the rate depends on the Srconcentration. Effect of pressure on lattice is estimated from the elastic
constant, K = — 1(dL) where £ is a lattice parameter. The parameters
have been estimated to be 2.29 x 10-3 and -2.21 x 10-3 ( GPa -1 ) for theCuO2 plane and c-axis, respectively5. Then, the lattice is expected to shrink
0.435% under the pressure of 1.9GPa for both a- and b-axes. The shrink
corresponds to 5% hole doping 3. If the shrink by the Li substitution isequivalent to that by applying pressure, the NQR spectra for the x = 0.25
sample at 1.9GPa should be the same with those for the x = 0.3 sample
at 01GPa. However, the broad peak for the x — 0.25 sample does notdisappear by applying pressure. Therefore, such a assumption based solelyon the lattice parameters is not realized.
The present experiments suggest that the local distance of Cu-O or Li0 is rather important than the lattice parameters. Li ions occupy the Cusites in the Cu02 plane and break the network of Cu-O. It is known thatthe position of oxygen between Li and Cu ions is close to Cu ions comparedwith Li ions 3. It is concluded from this fact that Li ions avoid to occupy thenearest neighbor positions each other. The fact leads to the simplest picturesof Li distribution pattern in the CuO2 plane for x = 1/3 as is shown in Figs.
4(a) and (b).
Figure 4(a) represents a Li-stripe pattern where the local environmentof all Cu ions is the same, namely, four nearest neighbor sites of Cu ionsare occupied by two Cu ions and two Li ions. Variation of the Li-stripe
pattern is also likely to realize as is shown in Fig. 4(b), however the local
environment is rather close to that of Fig. 4(a) in the meaning that oneof the nearest neighbor sites is Li ion. These pictures for x = 1/3 save toexplain the drastic variation of NQR spectra at x = 0.3. All Cu ions possess
almost the same local environment and therefore yield only one kind of NQR
signals at x = 0.3, whereas at the concentration below x = 0.3 Cu ions areinevitably included whose nearest neighbor sites are occupied by four Cuions, which gives rise to the broad signal corresponding to LSCO system.

NQR in Li doped LSCO system under high pressure (1.9GPa) 489

Fig. 4. Pattern of Li distribution in Cu02 plane for 33% Li concentration.
The cross represents Cu ion and open circle represents Li ion, respectively.
Oxygen is not shown explicitly although it locates on the lines, (a) A Listripe pattern, (b) Modified pattern of figure 4(a).

4. CONCLUSION
Nuclear quadrupole resonance under steady pressure (1.9GPa) was performed on La1.85Sr0.15Cu1-xLixO4. Effect of the pressure was investigatedfor x = 0.25 and 0.3 samples. Prom the experimental results, possibility ofLi stripelike pattern at x — 0.3 was suggested.

ACKNOWLEDGMENTS
This research is supported by Grant-in-Aid for the Ministry of Education, Science and Culture, Japan and by grant of the Ogasawara foundation
for the Promotion of Science and Engineering, Japan.

REFERENCES
1. A. L. Rykov, H. Yasuokaand Y. Ueda, Physica. (7247, 327 (1995).
2. J. L. Sarrao, D. P. Young, Z. Fisk, E. G. Moshopoulou, J. D. Thompson, B. C.
Chakoumakos and S. E. Nagler, Phys. Rev. B 54, 12014 (1996).
3. J. P. Attfield,and G. Ferey, J. Solid State chem. 80, 112 (1989).
4. Y. Yoshinari, P. C. Hammel, J. A. Martindale, E. Moshopoulou, J. D. Thompson,
J. L. Sarrao and Z. Fisk Phys. Rev. Lett. 77, 2069 (1996).
5. "Physical Properties of High Temperature Superconductors III" edited by D. M.
Ginsberg, World Science Publishing Co. Pte. Ltd. (1996)