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Phonon anomaly of under-doped YBa2Cu3O6.6 studied by neutron scattering
Physica B 219&220 (1996) 204-206
ELSEVIER
Phonon anomaly of under-doped Y B a 2 C u 3 0 6 . 6 studied by neutron scattering T. Nishijima a, M. Arai a'b'*, K. Yamaya c, Y. Okajima c, A.C. Hannon d, T. Otomo ° "Department of Physics, Faculty of Science, Kobe University, 1-1 Rokkodai, Nada, Kobe 657, Japan bResearch Development Corporation of Japan, Honcho, Kawaguchi 332, Japan cDepartment of Applied Physics, Hokkaido University, Sapporo 060, Japan aRutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX1l OQX, UK eNational Laboratoryfor High Energy Physics, Tsukuba 305, Japan
Abstract Total Pair Correlation Function (T(r)), and phonon density of states (PDOS) were measured by neutron scattering on the powder sample of YBazCu306.6 (To ~ 60 K) at temperatures between 20 and 295 K. Lattice anomalies were observed in the T(r) and PDOS much above Tc as well as T~. These results indicate that the anomaly at Tc can be related to the superconducting transition but one above T~ may be attributed to the induced lattice anomaly caused by the pseudo-spin-gap opening.
1. Introduction The anomalous behaviours of the phonon states of the High-T c superconductors have been reported in various experiments. For example, EXAFS [1], ion-channeling [2] and ultrasound [-3] measurements provided strong relation between lattice anomaly and superconductivity. It is, however, unclear whether phonon has a main role on the superconductivity. On the other hand, the extended t-J model [4] suggested an importance of spinlattice interaction, which gives an induced lattice anomaly at Tc, etc. The phase diagram introduced by this model indicates two phase boundaries in the underdoped region, one is for superconducting state (lower temperature) and another is singlet-RVB state (higher temperature). For the present case, i.e., YBazCu306. 6 (To ~ 60K), the latter phase boundary is about 150K. Actually there are experimental reports on the anomaly
* Corresponding author.
near the TRVB from NMR ]-5] and ion-channeling [-6] measurements. Hence, we were motivated to clarify the anomalous behaviours at Tc and above To, and performed neutron powder scattering experiments to obtain the total pair correlation function T(r) and the phonon density of states PDOS of under-doped YBa2CuaO6.6 powder sample.
2. Experimental Neutron powder diffraction experiments were carried out using the LAD diffractometer at the ISIS facility of the Rutherford Appleton Laboratory. The sample was sealed in a vanadium can and cooled by a closed cycle refrigerator. The measurements were performed at ten temperature points, i.e., 20, 45, 65, 100, 130, 160, 180 and 294 K. Each measurement took for about 16 h. The structure factor, S(Q), was obtained after correction on normalization for incident flux, background, detector efficiency, sample absorption, multiple scattering, and
0921-4526/96/$15.00 (~;~, 1996 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 6 9 6 - 6
Z
Nish(jima et al./Physica B 219&220 (1996) 204 206
inelasticity. Then the T(r) was taken by Fourier transformation on S(Q) by using the data up to 45 A- ]. Inelastic neutron scattering measurements were carried out on the chopper spectrometer INC, which is installed at the KENS facility in the National Laboratory for High Energy Physics. The spectrometer covers the scattering angle from 5' to 130: with 176 detectors. The available incident energy is from 20 meV to 1 eV with an energy resolution of 5% of the incident energy. The powder sample was contained 95 g in an alminium can and cooled down by the closed cycle refrigerator. The data were collected at 32, 45, 60, 80, 120, 160, 180, 200, 240 and 295 K.
First Peak of T(r)
" E 1.933 " ~
1.932
e" ~
1.931
v tO ,+-,
1.930
O
1.928
Q. .~
1.927
<
C0 (1)
205
2.15
"" ..:..i ' '~-~ 'Posi;io'n" .i-;~-:-'H'e;ght' ' ' ~ 2.1o
'~
//
2,05
"
"O CD ;x"
T
2.00 _~.
1.929
1.95 1,926
Q-
50
0
100
150
200
250
300
Temperature (K) Second Peak of T(r)
E 9
3. Results
~
Fig. 1 shows the T(r) of YBa2Cn306.6 at 20, 100 and 294 K. Some significant changes, especially peak height, were observed on a number of peaks in various spatial regions of r. However, as far as the peak position is concerned, it is doubtful whether we have sufficient spatial resolution to examine the small change of the peak position. In order to minimize the accumulation of systematic errors, which may be introduced during the data reduction procedure, each data was analysed in the same manner very carefully. After this procedure the reliability on the peak position was much improved to discuss about the systematic change with temperature. Fig. 2 shows the temperature dependence of height and position of the peaks at 1.9 and 2.4 A, which correspond to the distance of Cu(2)-O(2) in the CuO2 plane and Cu(2) 0(4) (apical site), respectively. As shown in Fig. 2, the peak height and the position have an obvious change at T~( = 57K) and also at about TRVB( ~ 150K). All of
~.~ C 0
2.398
• •. ~ . . . . . . . . . . . . . . . . . . . . . . .
2.390
~: ~;~'. ~ " ",..
1.45
1.40
+Position ---e -Height
1.35
"U CD
2.385
1.30
~"
2.380
1.25
CD (Q :=T
e,.
1,20
O t'l
t
2.375 1.15
t~ n(1)
2,370 50
100
150
200
1.10 300
250
Temperature (K) Fig. 2. Temperature dependence of the peak position and the peak height of T(r). The solid line and the dotted line is a guide for the eye.
x
O9
•
4160K 5K I 295K
100
120
X ;y: •
v 5
•
4[~
i
•
--R.T.
•
•
100K
i
•
•
•
i
,
•
I
•
i
//
•
•
,:,
YBa2Cu306.6
0
121 Q.
I
Ei=130meV t
0
20
40
60
80
Energy Transfer (meV) Fig. 3. Generalized phonon density of states (PDOS) at 45, 160 and 295 K for E~ = 130 meV.
1
0
A 2
4
6
8
10
r (Angstrom) Fig. 1. Total Pair Correlation Function of YBa2Cu306.6 at 20, 100K and ambient temperature.
these peaks correspond to the distances between oxygens contained in the O(4)-Cu(2)-O(2, 3) pyramid. Fig. 3 shows the phonon density of states, G(E), obtained by inelastic neutron scattering at 45, 160 and 295 K. Clear structure of PDOS is not seen because of the
206
T. Nishijima et aL /Physica B 219&220 (1996) 204-206 i
I
!
i
i
1.06 O~
1.04
e--..~.
-
,
:~ 1.02
~
•
I~
i
'1~
#w
'~'
'@,
0.98 r
(5 0. 96
r
-//'~~~: ~
I,"4 0.94 0
~'
----- YG(E) (36~50meV) --~-- YG(E) (54,-,65meV)
I
I
I
I
I
50
100
150
200
250
300
Temperature (K) Fig. 4. Temperature dependence of integrated intensity between 36 and 50 meV and between 54 and 65 meV. The solid and dotted lines are a guide for the eye.
of planar oxygens. This result agrees with the consequence obtained before by inelastic neutron scattering on YBa2Cu30 7 by Arai et al. [8]. In the present work on the under-doped YBa2Cu306.6, the prominent changes were newly observed at TRva( ~ 150K) in the same phonon mode. Recent theoretical work on the extended t J model expected that an anomaly on the phonon mode with d-wave symmetry has a remarkable change at TRVBdue to the spin-lattice coupling [9]. Our results are indeed consistent with this theoretical prediction, although we also found an anomaly of the phonon mode with non-d-wave symmetry, which may be explained by a spin interaction between the CuO2 planes [10]. Therefore, the present work showed a possibility that the lattice anomalies at Tc are related to the superconducting transition, but those well above T~ may be due to the opening of the pseudo spin-gap.
relaxed energy resolution of the INC spectrometer. However, we could find definite changes in the two specific energy regions, involving the peaks at 41 and 63 meV. Burns et al. [7] assigned these peaks as the Ag modes. The mode at 41 meV is related to the out-of-plane displacement of the planar oxygen 0(2) and O(3), and one at 63 meV is the 0(4) (apical oxygen) mode parallel to the c-axis. In order to make more clear the temperature dependence of the PDOS, we integrated the PDOS in a certain energy range. The results were depicted in Fig. 4. We can recognize the phonon anomaly at Tc and TRVB are similar to the behaviour of the Total Pair Correlation Function.
Acknowledgements
4. Discussion
[7] [8] [9] [10]
As described in the previous section, we found that the phonon anomaly occurred at Tc in the out-of-plane mode
This work was supported by the Sumitomo foundation and a Grant-in-Aid of the Ministry of Education, Science and Culture.
References [1] [2] [3] [4] [5] [6]
S. D. Conradson et al., Science 243 (1989) 1340. T. Haga et al., Phys. Rev. B 41 (1990) 826. M. Nohara et al., Phys. Rev. Lett. 70 (1993) 3447. T. Tanamoto et al., J. Phys. Soc. Japan 62 (1993) 717. Y. Itoh et al., J. Phys. Soc. Japan 61 (1992) 1287. K. Yamaya et al., in: Proc. ETL Workshop on High Temperature Superconductors (1993). G. Burns et al., Physica C 181 (1991) 37. M. Arai et al., Phys. Rev. Lett. 69 (1992) 359. B. Normand et al., to be published. K. Kuboki et al., to be published.
ELSEVIER
Phonon anomaly of under-doped Y B a 2 C u 3 0 6 . 6 studied by neutron scattering T. Nishijima a, M. Arai a'b'*, K. Yamaya c, Y. Okajima c, A.C. Hannon d, T. Otomo ° "Department of Physics, Faculty of Science, Kobe University, 1-1 Rokkodai, Nada, Kobe 657, Japan bResearch Development Corporation of Japan, Honcho, Kawaguchi 332, Japan cDepartment of Applied Physics, Hokkaido University, Sapporo 060, Japan aRutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX1l OQX, UK eNational Laboratoryfor High Energy Physics, Tsukuba 305, Japan
Abstract Total Pair Correlation Function (T(r)), and phonon density of states (PDOS) were measured by neutron scattering on the powder sample of YBazCu306.6 (To ~ 60 K) at temperatures between 20 and 295 K. Lattice anomalies were observed in the T(r) and PDOS much above Tc as well as T~. These results indicate that the anomaly at Tc can be related to the superconducting transition but one above T~ may be attributed to the induced lattice anomaly caused by the pseudo-spin-gap opening.
1. Introduction The anomalous behaviours of the phonon states of the High-T c superconductors have been reported in various experiments. For example, EXAFS [1], ion-channeling [2] and ultrasound [-3] measurements provided strong relation between lattice anomaly and superconductivity. It is, however, unclear whether phonon has a main role on the superconductivity. On the other hand, the extended t-J model [4] suggested an importance of spinlattice interaction, which gives an induced lattice anomaly at Tc, etc. The phase diagram introduced by this model indicates two phase boundaries in the underdoped region, one is for superconducting state (lower temperature) and another is singlet-RVB state (higher temperature). For the present case, i.e., YBazCu306. 6 (To ~ 60K), the latter phase boundary is about 150K. Actually there are experimental reports on the anomaly
* Corresponding author.
near the TRVB from NMR ]-5] and ion-channeling [-6] measurements. Hence, we were motivated to clarify the anomalous behaviours at Tc and above To, and performed neutron powder scattering experiments to obtain the total pair correlation function T(r) and the phonon density of states PDOS of under-doped YBa2CuaO6.6 powder sample.
2. Experimental Neutron powder diffraction experiments were carried out using the LAD diffractometer at the ISIS facility of the Rutherford Appleton Laboratory. The sample was sealed in a vanadium can and cooled by a closed cycle refrigerator. The measurements were performed at ten temperature points, i.e., 20, 45, 65, 100, 130, 160, 180 and 294 K. Each measurement took for about 16 h. The structure factor, S(Q), was obtained after correction on normalization for incident flux, background, detector efficiency, sample absorption, multiple scattering, and
0921-4526/96/$15.00 (~;~, 1996 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 6 9 6 - 6
Z
Nish(jima et al./Physica B 219&220 (1996) 204 206
inelasticity. Then the T(r) was taken by Fourier transformation on S(Q) by using the data up to 45 A- ]. Inelastic neutron scattering measurements were carried out on the chopper spectrometer INC, which is installed at the KENS facility in the National Laboratory for High Energy Physics. The spectrometer covers the scattering angle from 5' to 130: with 176 detectors. The available incident energy is from 20 meV to 1 eV with an energy resolution of 5% of the incident energy. The powder sample was contained 95 g in an alminium can and cooled down by the closed cycle refrigerator. The data were collected at 32, 45, 60, 80, 120, 160, 180, 200, 240 and 295 K.
First Peak of T(r)
" E 1.933 " ~
1.932
e" ~
1.931
v tO ,+-,
1.930
O
1.928
Q. .~
1.927
<
C0 (1)
205
2.15
"" ..:..i ' '~-~ 'Posi;io'n" .i-;~-:-'H'e;ght' ' ' ~ 2.1o
'~
//
2,05
"
"O CD ;x"
T
2.00 _~.
1.929
1.95 1,926
Q-
50
0
100
150
200
250
300
Temperature (K) Second Peak of T(r)
E 9
3. Results
~
Fig. 1 shows the T(r) of YBa2Cn306.6 at 20, 100 and 294 K. Some significant changes, especially peak height, were observed on a number of peaks in various spatial regions of r. However, as far as the peak position is concerned, it is doubtful whether we have sufficient spatial resolution to examine the small change of the peak position. In order to minimize the accumulation of systematic errors, which may be introduced during the data reduction procedure, each data was analysed in the same manner very carefully. After this procedure the reliability on the peak position was much improved to discuss about the systematic change with temperature. Fig. 2 shows the temperature dependence of height and position of the peaks at 1.9 and 2.4 A, which correspond to the distance of Cu(2)-O(2) in the CuO2 plane and Cu(2) 0(4) (apical site), respectively. As shown in Fig. 2, the peak height and the position have an obvious change at T~( = 57K) and also at about TRVB( ~ 150K). All of
~.~ C 0
2.398
• •. ~ . . . . . . . . . . . . . . . . . . . . . . .
2.390
~: ~;~'. ~ " ",..
1.45
1.40
+Position ---e -Height
1.35
"U CD
2.385
1.30
~"
2.380
1.25
CD (Q :=T
e,.
1,20
O t'l
t
2.375 1.15
t~ n(1)
2,370 50
100
150
200
1.10 300
250
Temperature (K) Fig. 2. Temperature dependence of the peak position and the peak height of T(r). The solid line and the dotted line is a guide for the eye.
x
O9
•
4160K 5K I 295K
100
120
X ;y: •
v 5
•
4[~
i
•
--R.T.
•
•
100K
i
•
•
•
i
,
•
I
•
i
//
•
•
,:,
YBa2Cu306.6
0
121 Q.
I
Ei=130meV t
0
20
40
60
80
Energy Transfer (meV) Fig. 3. Generalized phonon density of states (PDOS) at 45, 160 and 295 K for E~ = 130 meV.
1
0
A 2
4
6
8
10
r (Angstrom) Fig. 1. Total Pair Correlation Function of YBa2Cu306.6 at 20, 100K and ambient temperature.
these peaks correspond to the distances between oxygens contained in the O(4)-Cu(2)-O(2, 3) pyramid. Fig. 3 shows the phonon density of states, G(E), obtained by inelastic neutron scattering at 45, 160 and 295 K. Clear structure of PDOS is not seen because of the
206
T. Nishijima et aL /Physica B 219&220 (1996) 204-206 i
I
!
i
i
1.06 O~
1.04
e--..~.
-
,
:~ 1.02
~
•
I~
i
'1~
#w
'~'
'@,
0.98 r
(5 0. 96
r
-//'~~~: ~
I,"4 0.94 0
~'
----- YG(E) (36~50meV) --~-- YG(E) (54,-,65meV)
I
I
I
I
I
50
100
150
200
250
300
Temperature (K) Fig. 4. Temperature dependence of integrated intensity between 36 and 50 meV and between 54 and 65 meV. The solid and dotted lines are a guide for the eye.
of planar oxygens. This result agrees with the consequence obtained before by inelastic neutron scattering on YBa2Cu30 7 by Arai et al. [8]. In the present work on the under-doped YBa2Cu306.6, the prominent changes were newly observed at TRva( ~ 150K) in the same phonon mode. Recent theoretical work on the extended t J model expected that an anomaly on the phonon mode with d-wave symmetry has a remarkable change at TRVBdue to the spin-lattice coupling [9]. Our results are indeed consistent with this theoretical prediction, although we also found an anomaly of the phonon mode with non-d-wave symmetry, which may be explained by a spin interaction between the CuO2 planes [10]. Therefore, the present work showed a possibility that the lattice anomalies at Tc are related to the superconducting transition, but those well above T~ may be due to the opening of the pseudo spin-gap.
relaxed energy resolution of the INC spectrometer. However, we could find definite changes in the two specific energy regions, involving the peaks at 41 and 63 meV. Burns et al. [7] assigned these peaks as the Ag modes. The mode at 41 meV is related to the out-of-plane displacement of the planar oxygen 0(2) and O(3), and one at 63 meV is the 0(4) (apical oxygen) mode parallel to the c-axis. In order to make more clear the temperature dependence of the PDOS, we integrated the PDOS in a certain energy range. The results were depicted in Fig. 4. We can recognize the phonon anomaly at Tc and TRVB are similar to the behaviour of the Total Pair Correlation Function.
Acknowledgements
4. Discussion
[7] [8] [9] [10]
As described in the previous section, we found that the phonon anomaly occurred at Tc in the out-of-plane mode
This work was supported by the Sumitomo foundation and a Grant-in-Aid of the Ministry of Education, Science and Culture.
References [1] [2] [3] [4] [5] [6]
S. D. Conradson et al., Science 243 (1989) 1340. T. Haga et al., Phys. Rev. B 41 (1990) 826. M. Nohara et al., Phys. Rev. Lett. 70 (1993) 3447. T. Tanamoto et al., J. Phys. Soc. Japan 62 (1993) 717. Y. Itoh et al., J. Phys. Soc. Japan 61 (1992) 1287. K. Yamaya et al., in: Proc. ETL Workshop on High Temperature Superconductors (1993). G. Burns et al., Physica C 181 (1991) 37. M. Arai et al., Phys. Rev. Lett. 69 (1992) 359. B. Normand et al., to be published. K. Kuboki et al., to be published.