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Neutron scattering study of the vibrational density of states of high Tc superconductors
Physica C153-155 (1988) 268-269 North-Holland, Amsterdam
NEUTRON
SCATTERING
STUDY
OF
THE
VIBRATIONAL
DENSITY
OF
STATES
OF
HIGH
T
c
SUPERCONDUCTORS
L.Rosta*, Gy. Hutiray Central Research Institute for Physics, H-1525 Budapest, ROB 48. Hungary R.Bellissent, A.Nenelle *Laboratoire L@on Brillouin (CEA-CNRS), CEN Saclay F - 9 1 1 9 1 G i f sur Yvette Cedex, France F.Hezei Hahn-Meitner-Institute, 0-1000 Berlin 39, Pf. 3B0128 A.O.Taylor, R.Osborn, Z.A.Bowden Rutherford Appleton Laboratory, Chilton, Oidcot, Oxon 0Xli OOX, Great Britain
Neutron scattering measurements were performed on polycrystallina Y-Be-Cu-O ceramics to explore the relationship between structure, vibrational density of states and superconductivity. The experiment was made on three powder samples of YBa~Cu~O . Neutron diffraction data were used to dez J y termine the structure and oxygen content by Rietveld refinement. The inelastic scattering spectra were obtained on the MET spectrometer at the ISIS neutron source. The vibrational density of states spectra were recorded for each sample at different temperatures. A V3Ge A-15-type superconductor was used for comparison.
1. INTROOUCTION A very large scale of experimental methods has been involved to explore the origin of pairing mechanism in the copper oxide superconductors and various theoretical concepts have been suggested to explain the high T o in these materials. Neither the well-Known strong electron-phonon coupling nor the unconventional (magnetic interaction based, electron-exciton, etc.) mechanisms were proved so far. Neutron scattering is a powerful tool to investigate microsoopic interaction mechanisms, although not too much experiments were made in this field [I] . Pintschovius et al {2] pointed out that strong electron-phonon coupling effects are very marked in the LaSr based compounds, however, such effects are less pronounced in the YBa family. They claim that even mechanisms of different origin may be responsible for the high T c. Very limited information are available on phonon spectra of the Y-Ba-Cu-O superconductors so far. Problems of growing large enough single crystals make it difficult to study the phonon dispersion relation and only the vibrational density of states [VOOS) can be provided from powder experiments (3,4]. Analysis of only the VDOS reveal poor information on superconducting properties (2). Therefor we have rather undertaken a study of YOa2CU3Oy samples by comparing the VDOS on variing different parameters like temperature and oxygen concentration as a most sensitive behaviour of the T c [3). We report in this paper high incident energy neutron inelastic scattering measurements, completed by neutron
0 9 2 1 - 4 5 3 4 / 8 8 / $ 0 3 . 5 0 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
diffraction determination of the structure and the oxygen concentration. 2. EXPERIMENTAL CONOITIONS The samples were prepared as described in (5) in a standard way from a batch of nominally single phase YBa2CU30y when special care was made to be free of water and other impurities. Three samples with different y were provided, 4B g of oxygen rich and 50 g of the oxygen reduced samples.The material was ground and ~illed in He-atmosphere into ~ 25 mm A~ - b o x for the diffraction measurement. The G8-I cold neutron diffractometer equipped with a banan-detector at the Saclay Orph@e reactor was used to record the diffraction curves. The 0.4 - 2.5 ~-I range was measured at 4.F2 incident wave-length. A Rietveld refinement yields the following data for the three samples with different y oxygen contents: Lattice parameters (in ~) 1.
y
=
6.34
a ~
b
2.
y
=
6.59
a
=
3.876,
=
3.853 b
=
3.817,
c
=
11.73 c
=
11.882
3.
y
=
6.90
a
=
3.880,
b
=
3.818,
c
=
11.655
The superconducting transition temperatures were deduced from dc conductivity measurements. While the y = 8.90 sample showed a sharp superconducting transition at 87 K, the y = 6.59 one gives a transition at around 47 K with a drop of resistivity to zero within 5 degree. The y = 6.34 sample remains semiconductor in the whole temperature range above 10 K (as measured). For the inelastic scattering measurements the
L. Rosta et aL /Neutron scattering study o f the vibrational density o.t'smtes
samples were translated under He atmosphere into rectangular AL cans (40 x 40 x 8 mm 3] and they were fixed on the cold finger of a closed-cycle refrigerator. Experiments were undertaken on the High Energy Transfer (HET) spectrometer at the ISIS neutron source of the Rutherford Appleton Laboratory. In order to record t i m e - o f - f l i g h t spectra up to 120 meV energy transfer, incident neutron energy of 150 meV was set end neutron energy loss data were used. The temperature was varied between 15 and 110 K for the Y-Ba-Cu-O samples. A reference measurement was carried out to provide the VOOS on a 80 g V3Ge A-15 type-II s u p e r c o n d u c t o r in a He-cryostat at 5 and 110 K. Spectre were collected in three detector banks at scattering angles of 5, 10 - 30 and 136 degrees, respectively. G(w) VOOS was deduced from the TOF spectra using the incoherent approximation as described in (4) end our results ere presented as
G[w)
Z(~) n(w]
k K'
2 h. X z m. i,S l
-2W. I e ~
S
(~_~) s
where I(w] is the scattering intensity, n(w] the population factor, K and k' - incident and scattered neutron wave-vectors, b i m i end exp(-2W i] - nuclear scattering length, mass, end O e b y e - W a l l e r factor of the i-th atom, the sum is over all s normal modes. Our date ere corrected for the empty can background and d e t e c t o r efficiency. In order to adjust the background level for presenting the "neutron" weighted G(w) function we used the V3Ge VOOS from (5) and compared it with the HET experiment data. In consequence a linaarely decreasing background was substracted. 3. RESULTS ANO OISCUSSION Fig. I. represents the comparison of the raw date at different o x y g e n concentration and temperatures. The main features of the vibrational spectra can be already seen from these curves where peeks are centered a t the 20, 40 and 80 meV regions. In Fig. 2. the VOOS curve is shown for the y = 6.59 sample. The comparative analysis of the spectra suggests that e strong sharpening of the 20 meV group occurs when the oxygen concentration is increased. This is a general feature else for some h~gher energy peaks, end the same effect is noticed when the temperature is increased. The r e d i s t r i b u t i o n of modes in the 75 meV range with the oxygen variation is in agreement with the observed Roman date (5). Concerning the temperature dependence it is important to notice that there is no difference between the 40 and 60 K spectra in the y = 8.59 sample (Fig. 2P), that is no change can be oh-
Fig.
269
I. TOF spectre in the low angle detector bank. a - y = 6 . 8 0 , b - y = 6 . 5 9 , c - y=6.34.
served in the VDOS on the s u p e r c o n d u c t i n g transition. However, there is e marked change at the 32, 42 and 75 meV energies in this sample between the 80 end 110 K spectra. At the same time no difference can be seen in the lower and higher concentration samples between the 15 and 110 k data. That's why it is supposed that for the frequency shifts in the y = 6.5B sample (which is near to the o r t h c r o m b i c - t a t r a g e n a l phase transition) structural i n s t a b i l i t i e s could be the responsible. Further analysis of these date is in progress to make link between the observed anomalies end s u p e r c o n d u c t i n g properties.
Energy t r a n s f e r
El-E2 (meV)
Fig. 2. VOOS curve for the m e d i u m angle detector (b-ratio of the 40 and 80 K spectre). REFERENCES (I) L.Pintschovius et al.,Europhys.Lett.~, 247 (1988) ( 2 ) L.Pintschovius et al.,preprint (submitted to Revue des Heutes Temperatures et Refract.) (3) L . M i h 4 1 y , L . R o s t a et e l . P h y s . R e v . 8 3 6 , B I B T B S S 7 ] (4] J.J.Rhyne et al.,Phys.Rev. B38, 2294 (1887] (5] H.Kuzmany et el., Solid State Comm. (accepted for publication] (6] E . S c h n e i d e r et e l . , P r o e e e d i n g of the Conf. on Neutron Scatt. G a t t l i n b u r g 1875 pp.223-229
NEUTRON
SCATTERING
STUDY
OF
THE
VIBRATIONAL
DENSITY
OF
STATES
OF
HIGH
T
c
SUPERCONDUCTORS
L.Rosta*, Gy. Hutiray Central Research Institute for Physics, H-1525 Budapest, ROB 48. Hungary R.Bellissent, A.Nenelle *Laboratoire L@on Brillouin (CEA-CNRS), CEN Saclay F - 9 1 1 9 1 G i f sur Yvette Cedex, France F.Hezei Hahn-Meitner-Institute, 0-1000 Berlin 39, Pf. 3B0128 A.O.Taylor, R.Osborn, Z.A.Bowden Rutherford Appleton Laboratory, Chilton, Oidcot, Oxon 0Xli OOX, Great Britain
Neutron scattering measurements were performed on polycrystallina Y-Be-Cu-O ceramics to explore the relationship between structure, vibrational density of states and superconductivity. The experiment was made on three powder samples of YBa~Cu~O . Neutron diffraction data were used to dez J y termine the structure and oxygen content by Rietveld refinement. The inelastic scattering spectra were obtained on the MET spectrometer at the ISIS neutron source. The vibrational density of states spectra were recorded for each sample at different temperatures. A V3Ge A-15-type superconductor was used for comparison.
1. INTROOUCTION A very large scale of experimental methods has been involved to explore the origin of pairing mechanism in the copper oxide superconductors and various theoretical concepts have been suggested to explain the high T o in these materials. Neither the well-Known strong electron-phonon coupling nor the unconventional (magnetic interaction based, electron-exciton, etc.) mechanisms were proved so far. Neutron scattering is a powerful tool to investigate microsoopic interaction mechanisms, although not too much experiments were made in this field [I] . Pintschovius et al {2] pointed out that strong electron-phonon coupling effects are very marked in the LaSr based compounds, however, such effects are less pronounced in the YBa family. They claim that even mechanisms of different origin may be responsible for the high T c. Very limited information are available on phonon spectra of the Y-Ba-Cu-O superconductors so far. Problems of growing large enough single crystals make it difficult to study the phonon dispersion relation and only the vibrational density of states [VOOS) can be provided from powder experiments (3,4]. Analysis of only the VDOS reveal poor information on superconducting properties (2). Therefor we have rather undertaken a study of YOa2CU3Oy samples by comparing the VDOS on variing different parameters like temperature and oxygen concentration as a most sensitive behaviour of the T c [3). We report in this paper high incident energy neutron inelastic scattering measurements, completed by neutron
0 9 2 1 - 4 5 3 4 / 8 8 / $ 0 3 . 5 0 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
diffraction determination of the structure and the oxygen concentration. 2. EXPERIMENTAL CONOITIONS The samples were prepared as described in (5) in a standard way from a batch of nominally single phase YBa2CU30y when special care was made to be free of water and other impurities. Three samples with different y were provided, 4B g of oxygen rich and 50 g of the oxygen reduced samples.The material was ground and ~illed in He-atmosphere into ~ 25 mm A~ - b o x for the diffraction measurement. The G8-I cold neutron diffractometer equipped with a banan-detector at the Saclay Orph@e reactor was used to record the diffraction curves. The 0.4 - 2.5 ~-I range was measured at 4.F2 incident wave-length. A Rietveld refinement yields the following data for the three samples with different y oxygen contents: Lattice parameters (in ~) 1.
y
=
6.34
a ~
b
2.
y
=
6.59
a
=
3.876,
=
3.853 b
=
3.817,
c
=
11.73 c
=
11.882
3.
y
=
6.90
a
=
3.880,
b
=
3.818,
c
=
11.655
The superconducting transition temperatures were deduced from dc conductivity measurements. While the y = 8.90 sample showed a sharp superconducting transition at 87 K, the y = 6.59 one gives a transition at around 47 K with a drop of resistivity to zero within 5 degree. The y = 6.34 sample remains semiconductor in the whole temperature range above 10 K (as measured). For the inelastic scattering measurements the
L. Rosta et aL /Neutron scattering study o f the vibrational density o.t'smtes
samples were translated under He atmosphere into rectangular AL cans (40 x 40 x 8 mm 3] and they were fixed on the cold finger of a closed-cycle refrigerator. Experiments were undertaken on the High Energy Transfer (HET) spectrometer at the ISIS neutron source of the Rutherford Appleton Laboratory. In order to record t i m e - o f - f l i g h t spectra up to 120 meV energy transfer, incident neutron energy of 150 meV was set end neutron energy loss data were used. The temperature was varied between 15 and 110 K for the Y-Ba-Cu-O samples. A reference measurement was carried out to provide the VOOS on a 80 g V3Ge A-15 type-II s u p e r c o n d u c t o r in a He-cryostat at 5 and 110 K. Spectre were collected in three detector banks at scattering angles of 5, 10 - 30 and 136 degrees, respectively. G(w) VOOS was deduced from the TOF spectra using the incoherent approximation as described in (4) end our results ere presented as
G[w)
Z(~) n(w]
k K'
2 h. X z m. i,S l
-2W. I e ~
S
(~_~) s
where I(w] is the scattering intensity, n(w] the population factor, K and k' - incident and scattered neutron wave-vectors, b i m i end exp(-2W i] - nuclear scattering length, mass, end O e b y e - W a l l e r factor of the i-th atom, the sum is over all s normal modes. Our date ere corrected for the empty can background and d e t e c t o r efficiency. In order to adjust the background level for presenting the "neutron" weighted G(w) function we used the V3Ge VOOS from (5) and compared it with the HET experiment data. In consequence a linaarely decreasing background was substracted. 3. RESULTS ANO OISCUSSION Fig. I. represents the comparison of the raw date at different o x y g e n concentration and temperatures. The main features of the vibrational spectra can be already seen from these curves where peeks are centered a t the 20, 40 and 80 meV regions. In Fig. 2. the VOOS curve is shown for the y = 6.59 sample. The comparative analysis of the spectra suggests that e strong sharpening of the 20 meV group occurs when the oxygen concentration is increased. This is a general feature else for some h~gher energy peaks, end the same effect is noticed when the temperature is increased. The r e d i s t r i b u t i o n of modes in the 75 meV range with the oxygen variation is in agreement with the observed Roman date (5). Concerning the temperature dependence it is important to notice that there is no difference between the 40 and 60 K spectra in the y = 8.59 sample (Fig. 2P), that is no change can be oh-
Fig.
269
I. TOF spectre in the low angle detector bank. a - y = 6 . 8 0 , b - y = 6 . 5 9 , c - y=6.34.
served in the VDOS on the s u p e r c o n d u c t i n g transition. However, there is e marked change at the 32, 42 and 75 meV energies in this sample between the 80 end 110 K spectra. At the same time no difference can be seen in the lower and higher concentration samples between the 15 and 110 k data. That's why it is supposed that for the frequency shifts in the y = 6.5B sample (which is near to the o r t h c r o m b i c - t a t r a g e n a l phase transition) structural i n s t a b i l i t i e s could be the responsible. Further analysis of these date is in progress to make link between the observed anomalies end s u p e r c o n d u c t i n g properties.
Energy t r a n s f e r
El-E2 (meV)
Fig. 2. VOOS curve for the m e d i u m angle detector (b-ratio of the 40 and 80 K spectre). REFERENCES (I) L.Pintschovius et al.,Europhys.Lett.~, 247 (1988) ( 2 ) L.Pintschovius et al.,preprint (submitted to Revue des Heutes Temperatures et Refract.) (3) L . M i h 4 1 y , L . R o s t a et e l . P h y s . R e v . 8 3 6 , B I B T B S S 7 ] (4] J.J.Rhyne et al.,Phys.Rev. B38, 2294 (1887] (5] H.Kuzmany et el., Solid State Comm. (accepted for publication] (6] E . S c h n e i d e r et e l . , P r o e e e d i n g of the Conf. on Neutron Scatt. G a t t l i n b u r g 1875 pp.223-229