PHYSICAL REVIEW
8
VOLUME 47, NUMBER 2
1
JANUARY 1993-II
NQR study of copper in Lup 9Pro &Ba2Cu307
G. Markandeyulu,
K. V. Gopalakrishnan,
A. K. Rajarajan, L. C. Gupta, R. Vijayaraghavan,
Tata Institute of Fundamental
and A. S. Tamhane
Research, Bombay 400 005, India
K. I. Gnanasekar
Indian Institute
of Technology,
Potoai, Bombay 400 005, India
R. Pinto
Tata Institute of Fundamental Research, Bombay 400 005, India
(Received 13 July 1992)
Nuclear quadrupole resonance (NQR) of copper in Lu09Pro, BazCu, O, at 4.2 K has been observed.
The resonances of Cu(II) and 'Cu(II) have been observed at 31.50 and 29. 15 MHz, respectively. These
frequencies are the same as those in YBa2Cu30, but not in accordance with the expectations from the
decrease of resonance frequency in RBa2Cu307 compounds. v&'/v&' is 1.08 which is the ratio of the
eg( 'Cu)/eg( 'Cu), implying that there is uo static magnetic field at the Cu(II) site. The relaxation
times T& and T2 at 31.50 MHz have been determined and are found to be 42 ms and 140 ps, respectively.
s) in YBa2Cu307 and EuBa2Cu30„ indicating that fluctuating moments
T& is much less than that
are present in the material. These could be either on Cu ions that are close to Pr ions or on Pr ions
themselves. The contrasting behavior of resonance frequency and T& in Luo 9PIO 'jBa2Cu30& with respect
to other 1:2:3compounds is commented upon.
(-1
num.
I.
to
for
24 h. The sample was then cooled to room temperature.
A very fine powder of the material was prepared for the
x-ray-diffraction experiments.
x-ray-diffraction
of
The
(XRD)
pattern
Luo 9Pro, Ba2Cu307, obtained using Cu K radiation, is
shown in Fig. 1. XRD pattern of the multiphase composite of nominal composition LuBa2Cu307 is also shown in
Fig. 1 for the sake of easy comparison. The XRD pattern
clearly shows that Luo 9Pro, Ba2Cu307 forms with orthorhombic structure; traces of some impurity phases are
present, indicated by the lines marked with asterisks.
The lattice parameters calculated from the XRD pattern
are a =3. 806 A, b = 3. 8747 A, and c = 11.6014 A which
are to be compared with those of PrBa2Cu307 (a =3. 86
A, b =3. 899 A, and c =11.775 A) and of, for example,
YBa2Cu307 (a =3.816 A, b =3. 887 A, and c =11.664
24 h. Finally the sample temperature
INTRODUCTION
at -90 K has been observed in
Superconductivity
RBa2Cu307 (R = Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er,
These compounds crystallize in orthoTm, and Yb). '
The comrhombic structure with space group Pm
pounds with Ce and Tb do not form in this structure.
PrBa2Cu307 does form in the same structure, but is semiconducting and antiferromagnetic with T~ of Cu(II) sublattice -285 K and T& of Pr sublattice= 17 K.
Recently, Tamhane et a/. have synthesized materials
Lu, Pr„Ba2Cu307 forming in the orthorhombic 1:2:3
structure for x ~0. 15. The samples with x =0.07 and
0. 1 also form
in the same 1:2:3 phase. However, they
Pr BazCu307
contain traces of impurity phases. Lu
system superconducts up to a value of x =0. 5. ' As a
part of our program of NMR and nuclear quadrupole
resonance (NQR) studies in high-T, materials, we have
carried out NQR of Cu(II) in Luo 9Pro, Ba2Cu307 at 4.2
K. This paper presents the results of these measurements.
&
was reduced
450'C and the sample was kept at that temperature
A).
The sample was further characterized by measuring its
resistance R (T) as a function of temperature.
Figure 2
shows the plot of R ( T) vs T. It is to be noted t'hat R ( T)
varies linearly with T, for 300& T 90 K. This behavior
[namely, linear temperature dependence of R ( T) with T]
is usually observed in high-T, materials. Diamagnetic
response of the sample was observed using ac y(T) measurements. Figure 3 shows the ac y (313 Hz) of the material as a function of temperature.
Superconducting
of the material as determined
transition temperature
from these two measurements is 85 K.
NQR experiments were carried out, at a fixed temperature 4.2 K, in the frequency interval 28 —32 MHz using a
pulsed NMR spectrometer. Spin-echo spectra of Cu(II)
)
II. EXPERIMENTS
The material was synthesized starting from 99.99%
pure Luz03 and Pr60&& and 99.999% pure BaCO3 and
CuO, as described in Ref. 10. The thoroughly ground
mixture of the constituents was sintered at 900'C for 24
h, reground, and sintered again at 900 C for 24 h. The
resulting material was thoroughly ground, compacted in
the form of a pellet and heated, in a Aowing stream of oxygen, at 800'C for 1 h; the temperature was then raised
to 890 C and the sample was left at that temperature for
47
1123
O~1993 The American Physical Society
BRIEF REPORTS
1124
04-
M
—1
M
0
I
U
o
6 -Luo. o
O. t
a
"a
7
(b)
C9
n o
O O
I
150
800
300
250
(K)
4
35
5
50
FIG. 1. X-ray-diffraction patterns of (a) a sample of nominal
composition LuBa~Cu30„and (b) Lup 9Prp ~Ba2Cu, O7. The pattern of LuBa2Cu30„suggests that the material has not formed
in the 1:2:3 phase. The sample LuQ 9Prp &Ba2Cu307 consists
largely of 1:2:3 phase. The h, k, l values of the reAections are
shown over the peaks. There are impurity phases indicated by
peaks marked with +.
[as is well known, there are two crystallographic Cu sites
in 1:2:3 structure, designated as Cu(I), or the chain sites,
and the Cu(II), or the plane sites ] were recorded using
the standard vr/2 7. npulse se-qu-e. nce. Figure 4 shows the
intensity of the Fourier transform of the echo as a function of frequency, measured point by point at an interval
of 10 kHz in the frequency ranges 29. 10—29.25 MHz, and
31.38—31.59 MHz, and at intervals varying from 20 to
Each spin-echo spectrum
100 kHz outside these ranges.
was averaged over 1000 shots.
Spin-lattice relaxation time T, of the Cu(II) nuclei, at
was measured using the pulse sequence
r m'/2 ro ~ ke-ep-ing r-o fixed (=40 p, s) and varying w.
31.50 MHz,
1.2
The relaxation time obtained by fitting the magnetization
recovery to a single exponential is 42 ms. Spin-spin relaxation time was measured (using pulse sequence n/2 r m.
to be 140 ps.
)--.
III. RESULTS AND DISCUSSION
It is seen from Fig. 4 that there are two NQR peaks,
one occurring at 31.50 and the other at 29. 15 MHz.
These arise due to the two isotopes Cu and Cu occupying the Cu(II) sites. The ratio of the frequencies of the
two lines is 1.08, which is very close to the ratio of the
moments of the two isotopes ( eQ/ eQ
quadrupole
=1.082). Also, their intensities are roughly in the ratio
of the natural abundances of the two isotopes. It is to be
noted that the two resonance frequencies are the same as
The width of the
measured in the case of YBa2Cu307.
resonance is -330 kHz which compares well with that
usually reported in well-formed YBa2Cu307. This provides a microscopic test as to the good quality of the material. We observed Cu(I) resonance as well, in the neighborhood of 22 MHz. The signal was rather weak and
therefore was not measured extensively in this study.
As mentioned above, the two resonance frequencies are
in the ratio of the quadrupole moments of the two isotopes Cu and Cu. This clearly implies that there is no
"
Lu0 gPr0
25
S
Q
4~
20
~
15-
g 0.6
~
I
I
100
50
FIG. 3. AC g of LuQ 9PIQ, Ba2Cu307 as a function of temperature. The superconducting transition is a fairly sharp one.
OO
28 (Degrees)
vr
I
0
Temperature
0RO
C9
2-OO
—3
O
O
O
0
Lu0 9Pr0 &Ba2Cu30&
—2-
N
Lu0. 9Pr0. 1Ba2Cu3 7
Cu(II) NQR at 4.8K
63 Cu
W
~
0.3
%
~
0.0
j~
I
0
50
I
100
I
I
I
150
200
250
Temperature
FIG. 2. Resistance of a
300
(K)
sample Lup 9Prp &Ba2Cu307 as a function of temperature. The resistance varies linearly with temperature before dropping to zero at the superconducting transition
temperature.
~
&065C
A
O
w
I.
29
~
~
C4
0
28
~
~
~ ~ ~
.
~
.
~
~
~
~ ~ ~
~
I
30
Frequency (MHz)
FICJ. 4. NQR spectrum
4.2 K.
of Cu(II) in Lu09Pro, Ba2Cui07 at
BRIEF REPORTS
47
1125
static magnetic field at the Cu(II) sites. However, spinlattice relaxation time is rather short as compared with
that reported for YBa2Cu307 or EuBa2Cu307 at 4.2 K.
In the two latter compounds there is no magnetic moment occupying the rare-earth site (Eu is present in 3+
state which, in the ground state, has =0). Ti at 4.2 K
in both these materials has been reported'
to be —1 s.
This shows that there is a fluctuating field present at the
Cu(II) sites rendering the relaxation much faster than in
YBa2Cu307 or EuBa2Cu307. As mentioned earlier, it is
known that in PrBa2Cu30~, Cu(II) atoms carry magnetic
moment which order antiferromagnetically
at -285 K.
Thus one may suggest, in the present case, that the Cu(II)
atoms in the immediate neighborhood of Pr atoms have a
moment. Further, Pr atoms themselves have a magnetic
moment. Therefore, the enhanced relaxation rate in this
material, as compared with that in YBa2Cu307, could be
due to the fluctuating moments associated with Pr atoms
and/or the Cu(II) atoms in their vicinity. Further work
is being carried out to clarify this aspect.
The frequency of Cu(II) resonance also deserves to be
commented upon when compared with that in other
RBazCu307. ' It has been established that the NQR frequency of Cu(II) in the R Ba2Cu307 system decreases systematically as R goes from Nd to Yb; it is 33.5 MHz for
NdBa&Cu307 and 30.5 MHz for YbBa2Cu307. If the material under study is regarded to be LuBa2Cu307, one
would have expected the frequency to be less than 30.4
MHz. That the frequency is 31.5 MHz in the present
case suggests strongly the inhuence of Pr atoms which
modify the quadrupole interaction at the Cu(II) site. This
M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Hor, R. L. Meng,
L. Ciao, Z. J. Huang, Y. Q. Wang, and C. W. Chu, Phys. Rev.
Lett. 58, 908 (1987).
K. N. Yang, Y. Dalichaouch, J. M. Ferreira, B. W. Lee, J. J.
Neumeier, M. S. Torikachvili, H. Zhou, and M. B. Maple,
Solid State Commun. 63, 515 (1987).
J. M. Tarascon, W. R. McKinnon, L. H. Greene, G. W. Hull,
and E. M. Vogel, Phys. Rev. B 36, 226 (1987).
4M. A. Beno, L. Soderholm, D. W. Capone II, D. G. Hinks, J.
R. H. Heffner, J. D. Thompson, J. E. Crow, A. Kabade, T.
Mihalisin, and J. Schwegler, Phys. Rev. B 42, 2688 (1990).
~W. H. Li, J. W. Lynn, S. Skanthakumar,
T. W. Clinton, A.
Kebede, C. S. Jee, J. E. Crow, and T. Mihalisin, Phys. Rev. B
J
D. Jorgensen, Ivan K. Schuller, C. Segre, K. Zhang, and J. D.
Grace, Appl. Phys. Lett. 51, 57 (1987).
5J. L. Peng, P. Klavius, R. N. Shelton, H. B. Radonsk, and L.
Bernardez, Phys. Rev. B 40, 4517 (1989).
D. W. Cooke, R. S. Kwok, R. L. Lichti, T. R. Adams, C.
Boekema, W. K. Dawson, A. Kebede, J. Schwegler, J. E.
Crow, and T. Mihalisin, Phys. Rev. B 41, 4801 (1990).
7A. P. Reyes, D. E. MacLaughlin, M. Takigawa, P. C. Hammel,
is an important
point that needs further investigation.
IV. CONCLUSION
To conclude, we have reported here observation of
NQR at 4. 2 K of copper isotopes occupying the Cu(II)
sites in superconducting
Luo9Pro, Ba2Cu307. The contrasting behavior of the resonance frequency and relaxation rate in this material, with reference to YBa2Cu307
and other R Ba2Cu307, is presented.
40, 5300 (1989).
A. S. Tamhane, R. Nagarajan, R. Pinto, L. C. Gupta, R. Vijayaraghavan, V. Badri, and U. V. Varadaraju, Mater. Lett.
14, 185 (1992).
R. Pinto, S. P. Pai, A. S. Tamhane, P. R. Apte, L. C. Gupta,
R. Vijayaraghavan, K. I. Gnanasekar, and H. V. Keer (unpublished).
M. Mali, D. Brinkmann, L. Pauli, J. Roos, and H. Zimmermann, Phys. Lett. A 124, 112 {1987).
M. Itoh, K. Karashima, M. Kyogoku, and I. Aoki, Physica C
160, 177 (1989), and references therein.
A. K. Rajarajan, L. C. Gupta, and R. Vijayaraghavan, Physica C 193, 413 (1992).