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Phonon anomaly of La1.85Sr0.15CuO4 studied by neutron scattering
PHYSICA[ ELSEVIER
Physica B 213&214 119951 75 77
Phonon anomaly of La .8sSro.lsCuO4 studied by neutron scattering M. Arai "'b'*, K. Yamada ~, S. Hosoya a, S. Wakimoto c, K. Ubukata", M. Fujita a, T. Nishijima a, T. Otomo e, Y. Endoh c ~Department ~ Physics, Kobe University, Rokkodai, Nada, Kobe 657, Japan Research Development Corporation q[ Japan, Honcho, Kawayuchi 332, Japan ~Department oJ"Physics, Tohoku University, Aoba, Sendal 980, Japan dFaculty of Engineering, Yamanashi University, Miyamae, Kohu 400, Japan ° National Laboratory for Hiyh Energy Physics, Tsukuba 305, Japan
Abstract Inelastic neutron-scattering studies were performed on a powder and an assembly of single crystals of Lal.85Sro.15CuO4, Tc ~ 36 K. Phonon density of states and the Q-dependence of the dynamic structure factor as well, as the details of the phonon-scattering function, were investigated as a function of temperature. We found that specific phonon modes at the Brillouin zone centre and the zone boundary showed anomalous behaviour at To. Thus we suggest a strong correlation between the phonon states and the superconducting states.
1. Introduction
2. Experiment
We have reported that raising temperature and doping of Zn atoms in La~.85Sro., 5CuO4 have a similar effect on phonon states, suppressing an additional increase in the phonon density of states (PDOS, G(E)) in the superconducting states [1]. A very similar effect was found in YBa2Cu307 and it was explained that the anomaly is related to a local structural instability [2]. There are large number of reports showing evidence of a local structural instability at T, [3]. Thus, we have extended the study to observe more details of the phonon anomaly in La~.ssSro.15CUO4 (To ~ 36 K), which has the simplest crystal structure among the high-To materials. Here we briefly report a study by neutron scattering on a powder sample and large amount of high-quality single crystals, which we recently succeeded to synthesize.
The experiments were performed on the MARI spectrometer at the ISIS facility of the Rutherford Appleton Laboratory in the UK and on the INC spectrometer at the KENS facility of the National Laboratory for High Energy Physics in Japan. We used 200g of powder sample, hollow cylinder for the measurement of PDOS and the dynamical structure factor (S (Q, E)) in a wide range of Q E space. We also used an assembly of eight single crystals, totalling 80g. For the single-crystal experiment, we adopted a geometries with the c-axis parallel and perpendicular to k~.
* Corresponding author.
3. Results In Fig. 1 PDOS (G(E)) at 15 and 38K are depicted, as measured with an incident energy of 130 meV. It is in
0921-4526/95/$09.50 (" 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 1 9 5 1 0 0 0 6 6 - 6
76
M. Arai et al./Physica B 213&214 (1995) 75 77
0.03
•
-
•
i
-
-
-
i
•
-
-
i
•
•
•
!
•
•
0.3
•
•
, - i . - .
La..85Sr0.,sCuO4 El=
~,~E0.015
~
1
.
.
.
i
.
20
.
.
l
.
.
.
0.1
i
40 60 80 Energy Transfer (meV)
Z.7
0
Tc=36K
II U.I
r~ 2.6
"
0
"
"
'
20
"
"
"
'
40
"
"
"
'
60
"
"
"
'
80
"
.
2
100
°~
o ,o
/
"Z
Fig. 1. PDOS of Lal.~sSro.15CuO 4 at 15 and 38K. Ei= 130 meV.
~
i , . ., , l .~ . . .
0.2
i
0
i , , . i•
E 200meV
0 . 0 0 5
0
i , , ,
La 1.85Sro.15CuO4 O.25
1 3 0 m e V
>~ 0.02
o.ol
i . , . i . - ,
"
"
1 O0
T e m p e r a t u r e (K) Fig. 2. The temperature dependence of the PDOS integrated between 80 and 90 meV. Two different types of circle correspond to two independent experiments.
very good agreement with the previously reported results I-4]. The only major difference is the separation of the peaks at 55meV corresponding to the out-of-plane motion of the in-plane oxygen atoms, due to the better energy resolution of MAR1. The sharp peak at 85 meV is explained as an in-plane motion of the in-plane oxygen along the C u - O bond. Those were expected theoretically based on the Born von-Kfirmfin model. As seen in Fig. 1, there are very small but systematic differences between the intensities at 15 and 38K. In Fig. 2 the temperature dependence of the intensity integrated between 80 and 90meV is depicted. There is maximum at Tc similar to the results on YBa2Cu30; [2], though the statistical error is quite large. As we discussed in Ref. [2] the enhancement at T~ is more clearly observed in the Qdependence of S(Q, E). In Fig. 3, the Q-dependence of S(Q, E) of the powder sample integrated between 50
4
6
.
t
.
•
•
8 10 Q(k')
I
•
•
•
I
12
•
•
14
•
I
•
16
18
Fig. 3. Q-dependence of S(Q, E) between 50 and 90meV with Ei = 200 meV.
and 90meV at 15 and 3 8 K is depicted. A tiny but systematic enhancement occurs at T¢ at the nodal humps, which may correspond to the (rtTt0) or (000) points in the reciprocal space. Observed results on the powder sample have a behaviour similar to that observed in YBa2Cu3OT. Thus we expect that even in La,.ssSro.15CuO4 there is a local structural distortion, which was the conclusion of Refs. [3, 5]. In order to elucidate the details of the anomalous behaviour of the p h o n o n states, we carried out neutronscattering experiments on single crystals. In this experiment on MAR1, we have detected very sharp peaks from the scattering by phonons at high-symmetry points in the reciprocal space. In Fig. 4 the Q-dependence of S (Q, E) in the a b plane integrated between 70 and 75 meV at 20 and 80 K is depicted. It is now much more clearly observed that there is a considerable difference in the intensity above and below T¢ ~ 36 K. There is a sharp enhancement and decrease, at the (rtTt0) and (000) equivalent
4.5 4
La] 85Sr015CUO4
~- 3.s
•
(ooo',
'
Ei=250meV
3 t~
g 2.5 II
z~
2 1.5 (~0)
1
0.5
,
2
,
,
,
4
.
.
,
.
.
6
.
,
.
.
.
8
,
.
.
.
,
10 Q(A-1)
.
.
.
,
.
12
.
.
,
.
.
14
.
,
16
18
Fig. 4. Q-dependence of S{Q, E) in the a b plane between 70 and 75 meV of single crystal with E~ = 250meV.
M. Arai et al./Physica B 213&214 (1995) 75-77
points, respectively. We have also found anomalies at different energies, such as 40, 60, 80meV. Those anomalies also occur at high-symmetry points as summarized in Fig. 5. (The details will be published elsewhere.) Although we should delay the conclusion until further data analysis, it has been estimated that these anomalies are not attributed to a simple line shift or broadening induced by superconductivity.
v
"
(0 2r~ 0)
[ 6 0 ' 7 5 m ~ 0 m e V ;
up]
77
by taking Fourier transform of S(Q, E) as in Ref. [5]. The most interesting results in the single-crystal sample are that the anomaly occurs at the high-symmetry points in the Brillouin zone at (000) and (~n0). Around these points there is an exotic magnetic excitation from twodimensional antiferromagnetic correlations, and it is very close to the nesting position of the Fermi surface. Furthermore, the low-energy tilt phonon mode, which drives the structural-phase transition from the tetragonal to orthorhombic structure at high temperature, has a sharp dip in energy [6] and an inherent softening [7] at (n ~ 0). Therefore we expect that there should be a definite correlation between the anomaly of high-energy phonons and the low-energy phonon, which makes the buckling structure in the Cu O plane in the orthorhombic structure. Therefore, we suggest that an extended re-study on the low-energy phonon may give a direct insight of the role of the phonon for the superconductivity, although no prominent anomaly has been observed so far. The present study is supported by a Grant-in-Aid from the Ministry of Education, Science and Culture, and the Sumitomo Foundation.
(2r~ 0 0) Fig. 5. Brillouin zone diagram, in which observed phonon anomaly is summarized. "Up" stands for an increase in intensity at 20 K and "down" for a decrease.
4. Discussion The observed anomaly is a common behaviour among the oxide superconductors [1 5]. Thus, we believe that these are related to a local structural instability, which was concluded from the dynamic pair correlation function
References [1] M. Arai et al., Physica C 181 (1991) 45. [2] M. Arai et al., Phys. Rev. Lett. 69 (1992) 359. [3] T. Egami et al., Rev. Solid State Sci. 1 (1987) 247; B.H. Toby et al., Phys. Rev. Lett. 64 (1990) 2414; W. Dmowski et al., Phys. Rev. Lett. 61 (1988) 2608. [4] B. Renker et al., Z. Phys. B 67 (1987) 15. [5] M. Arai et al., J. Superconductivity 7 (1994) 415. [6] P. B6ni et al., Phys. Rev. B 38 (1988) 185. [7] T.R. Thurston et al., Phys. Rev. B 39 (1989) 4327.
Physica B 213&214 119951 75 77
Phonon anomaly of La .8sSro.lsCuO4 studied by neutron scattering M. Arai "'b'*, K. Yamada ~, S. Hosoya a, S. Wakimoto c, K. Ubukata", M. Fujita a, T. Nishijima a, T. Otomo e, Y. Endoh c ~Department ~ Physics, Kobe University, Rokkodai, Nada, Kobe 657, Japan Research Development Corporation q[ Japan, Honcho, Kawayuchi 332, Japan ~Department oJ"Physics, Tohoku University, Aoba, Sendal 980, Japan dFaculty of Engineering, Yamanashi University, Miyamae, Kohu 400, Japan ° National Laboratory for Hiyh Energy Physics, Tsukuba 305, Japan
Abstract Inelastic neutron-scattering studies were performed on a powder and an assembly of single crystals of Lal.85Sro.15CuO4, Tc ~ 36 K. Phonon density of states and the Q-dependence of the dynamic structure factor as well, as the details of the phonon-scattering function, were investigated as a function of temperature. We found that specific phonon modes at the Brillouin zone centre and the zone boundary showed anomalous behaviour at To. Thus we suggest a strong correlation between the phonon states and the superconducting states.
1. Introduction
2. Experiment
We have reported that raising temperature and doping of Zn atoms in La~.85Sro., 5CuO4 have a similar effect on phonon states, suppressing an additional increase in the phonon density of states (PDOS, G(E)) in the superconducting states [1]. A very similar effect was found in YBa2Cu307 and it was explained that the anomaly is related to a local structural instability [2]. There are large number of reports showing evidence of a local structural instability at T, [3]. Thus, we have extended the study to observe more details of the phonon anomaly in La~.ssSro.15CUO4 (To ~ 36 K), which has the simplest crystal structure among the high-To materials. Here we briefly report a study by neutron scattering on a powder sample and large amount of high-quality single crystals, which we recently succeeded to synthesize.
The experiments were performed on the MARI spectrometer at the ISIS facility of the Rutherford Appleton Laboratory in the UK and on the INC spectrometer at the KENS facility of the National Laboratory for High Energy Physics in Japan. We used 200g of powder sample, hollow cylinder for the measurement of PDOS and the dynamical structure factor (S (Q, E)) in a wide range of Q E space. We also used an assembly of eight single crystals, totalling 80g. For the single-crystal experiment, we adopted a geometries with the c-axis parallel and perpendicular to k~.
* Corresponding author.
3. Results In Fig. 1 PDOS (G(E)) at 15 and 38K are depicted, as measured with an incident energy of 130 meV. It is in
0921-4526/95/$09.50 (" 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 1 9 5 1 0 0 0 6 6 - 6
76
M. Arai et al./Physica B 213&214 (1995) 75 77
0.03
•
-
•
i
-
-
-
i
•
-
-
i
•
•
•
!
•
•
0.3
•
•
, - i . - .
La..85Sr0.,sCuO4 El=
~,~E0.015
~
1
.
.
.
i
.
20
.
.
l
.
.
.
0.1
i
40 60 80 Energy Transfer (meV)
Z.7
0
Tc=36K
II U.I
r~ 2.6
"
0
"
"
'
20
"
"
"
'
40
"
"
"
'
60
"
"
"
'
80
"
.
2
100
°~
o ,o
/
"Z
Fig. 1. PDOS of Lal.~sSro.15CuO 4 at 15 and 38K. Ei= 130 meV.
~
i , . ., , l .~ . . .
0.2
i
0
i , , . i•
E 200meV
0 . 0 0 5
0
i , , ,
La 1.85Sro.15CuO4 O.25
1 3 0 m e V
>~ 0.02
o.ol
i . , . i . - ,
"
"
1 O0
T e m p e r a t u r e (K) Fig. 2. The temperature dependence of the PDOS integrated between 80 and 90 meV. Two different types of circle correspond to two independent experiments.
very good agreement with the previously reported results I-4]. The only major difference is the separation of the peaks at 55meV corresponding to the out-of-plane motion of the in-plane oxygen atoms, due to the better energy resolution of MAR1. The sharp peak at 85 meV is explained as an in-plane motion of the in-plane oxygen along the C u - O bond. Those were expected theoretically based on the Born von-Kfirmfin model. As seen in Fig. 1, there are very small but systematic differences between the intensities at 15 and 38K. In Fig. 2 the temperature dependence of the intensity integrated between 80 and 90meV is depicted. There is maximum at Tc similar to the results on YBa2Cu30; [2], though the statistical error is quite large. As we discussed in Ref. [2] the enhancement at T~ is more clearly observed in the Qdependence of S(Q, E). In Fig. 3, the Q-dependence of S(Q, E) of the powder sample integrated between 50
4
6
.
t
.
•
•
8 10 Q(k')
I
•
•
•
I
12
•
•
14
•
I
•
16
18
Fig. 3. Q-dependence of S(Q, E) between 50 and 90meV with Ei = 200 meV.
and 90meV at 15 and 3 8 K is depicted. A tiny but systematic enhancement occurs at T¢ at the nodal humps, which may correspond to the (rtTt0) or (000) points in the reciprocal space. Observed results on the powder sample have a behaviour similar to that observed in YBa2Cu3OT. Thus we expect that even in La,.ssSro.15CuO4 there is a local structural distortion, which was the conclusion of Refs. [3, 5]. In order to elucidate the details of the anomalous behaviour of the p h o n o n states, we carried out neutronscattering experiments on single crystals. In this experiment on MAR1, we have detected very sharp peaks from the scattering by phonons at high-symmetry points in the reciprocal space. In Fig. 4 the Q-dependence of S (Q, E) in the a b plane integrated between 70 and 75 meV at 20 and 80 K is depicted. It is now much more clearly observed that there is a considerable difference in the intensity above and below T¢ ~ 36 K. There is a sharp enhancement and decrease, at the (rtTt0) and (000) equivalent
4.5 4
La] 85Sr015CUO4
~- 3.s
•
(ooo',
'
Ei=250meV
3 t~
g 2.5 II
z~
2 1.5 (~0)
1
0.5
,
2
,
,
,
4
.
.
,
.
.
6
.
,
.
.
.
8
,
.
.
.
,
10 Q(A-1)
.
.
.
,
.
12
.
.
,
.
.
14
.
,
16
18
Fig. 4. Q-dependence of S{Q, E) in the a b plane between 70 and 75 meV of single crystal with E~ = 250meV.
M. Arai et al./Physica B 213&214 (1995) 75-77
points, respectively. We have also found anomalies at different energies, such as 40, 60, 80meV. Those anomalies also occur at high-symmetry points as summarized in Fig. 5. (The details will be published elsewhere.) Although we should delay the conclusion until further data analysis, it has been estimated that these anomalies are not attributed to a simple line shift or broadening induced by superconductivity.
v
"
(0 2r~ 0)
[ 6 0 ' 7 5 m ~ 0 m e V ;
up]
77
by taking Fourier transform of S(Q, E) as in Ref. [5]. The most interesting results in the single-crystal sample are that the anomaly occurs at the high-symmetry points in the Brillouin zone at (000) and (~n0). Around these points there is an exotic magnetic excitation from twodimensional antiferromagnetic correlations, and it is very close to the nesting position of the Fermi surface. Furthermore, the low-energy tilt phonon mode, which drives the structural-phase transition from the tetragonal to orthorhombic structure at high temperature, has a sharp dip in energy [6] and an inherent softening [7] at (n ~ 0). Therefore we expect that there should be a definite correlation between the anomaly of high-energy phonons and the low-energy phonon, which makes the buckling structure in the Cu O plane in the orthorhombic structure. Therefore, we suggest that an extended re-study on the low-energy phonon may give a direct insight of the role of the phonon for the superconductivity, although no prominent anomaly has been observed so far. The present study is supported by a Grant-in-Aid from the Ministry of Education, Science and Culture, and the Sumitomo Foundation.
(2r~ 0 0) Fig. 5. Brillouin zone diagram, in which observed phonon anomaly is summarized. "Up" stands for an increase in intensity at 20 K and "down" for a decrease.
4. Discussion The observed anomaly is a common behaviour among the oxide superconductors [1 5]. Thus, we believe that these are related to a local structural instability, which was concluded from the dynamic pair correlation function
References [1] M. Arai et al., Physica C 181 (1991) 45. [2] M. Arai et al., Phys. Rev. Lett. 69 (1992) 359. [3] T. Egami et al., Rev. Solid State Sci. 1 (1987) 247; B.H. Toby et al., Phys. Rev. Lett. 64 (1990) 2414; W. Dmowski et al., Phys. Rev. Lett. 61 (1988) 2608. [4] B. Renker et al., Z. Phys. B 67 (1987) 15. [5] M. Arai et al., J. Superconductivity 7 (1994) 415. [6] P. B6ni et al., Phys. Rev. B 38 (1988) 185. [7] T.R. Thurston et al., Phys. Rev. B 39 (1989) 4327.