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Far-infrared studies of the metal-insulator transition in PrNiO3 and NdNiO3
Physica C 235-240 (1994) ! 289-1290 North-Holland
PHYStGA
Far-infrared studies of the metal-insulator transition in PrNiO3 aii
*
~High Field Magnet Laboratory, University of Nijmegen Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands. blnstitut de Ci~ncia de Materials, Consell Superior d'Iovestigations Cientifiques Campus de la UAB, 08193-Bellaterra, Espafia. We present infrared reflectivity m e a s u r e m e n t s of the rare-earth nickelates PrNiOa and NdNiOa between 30 and 5000 cm -1 at t e m p e r a t u r e s between 15 and 300 K. We observe a dramntic change of the s p e c t r a from metallic-like to nearly insulating in a narrow range of t e m p e r a t u r e s below the metM-insulator transition.
A general electronic band framework for oxides postulated by Zaanen, Sawatzky and Allen, in addition to the Mott-Hubbard gap systems, for ' l a t e " transition metal oxides introduces a novel type of metal-insulator (MI) transition related to the closing of the charge-transfer (CT) gap. [1] Such MI transition has been proposed recently for the lqNiO3 ( R = Pr. Nd, Sm, Eu) family. [2] In this work we report the t e m p e r a t u r e evolution (t0 300K)of the optical conductivity in the far infrared (FIR) (30 - 5000 cm -1) of PrNiOa and NdNi()s. T h e PrNiOa and NdNiO3 samples were prepared by J. Torrance at the IBM Ahnaden Laboratory in San Jose. Fig. 1 (a) shows the reflectivity of NdNiOa in the range 40-3000 cm -x between 15-300K and Fig. 1 (b) .~how.~ t.ho reflectivity of PrNi()a. The most pron-unced feature df these spectra is a rapid cilange of their character over a narrow I'aii~,' tJ[" t~,nil)erat.ures, close to t h e M I ~ r a n s i l ion, ~,xl~,lidilig ovc,r all OllPrgy i'allg~ " up to al~l,rox. 31)1)1) t i l l - 1 ill NdNi()a (s~'¢' illsot in t h e l.-'lg. I (a)) all(I iil) Io a l i p r o x , l()/)l) ciil - i in l'i'Ni():~. B~,low TM # t h e effect ot' t e l ] l p e r a t u r e on t h e p h o n o n
sl)ectrum is fairly minor. A metallic conductivity is present at low frequencies, already at approximately 30 K below the MI transition. We analyze the data using a model dielectric function with *Fiaancial support from the EEC (SCI-0389M(A). mid 0036-F), the Spanish CICY-I--MIDAS (mat:91-0742) mad DGICY]- (PB92-0849) programme are acknowledged.
Drude term, phonon oscillators and mid-infrared term. In Fig. 2 we show results of the fit for PrNiOa at 125 and at 155 K . At125 K we used wp = '1000 cm - i and F = 2500 cm -1 Above the MI transition, at 155 K we use ~p = 11000 cm -1 1-' - 16000 c n l - t , w : l , I - - 110 cm -1 , ,¢;~I. = 500 and 3M = 10 cm -1 T h e plasma frequency extracted from the fit was used to estimate the n / m " ratio. For PrNiOa at T = 135 K ~'r ~ 11000 cin -1 and n/m" ~ 1.34 1021 cm -a. For n -~ 1.8 10 .2.-, cm -a, m" ~ 13, in agreement with transport. measurements [3] . This enhanced electron mass reflects the strong correlation in the conduction band. For PrNiOa also the RT specific resistivity from FIR data p ~ 1.3 m f / c m compares favorable with the DC 2.0 m~Zcm.[3] In the inset in the Fig. '2 we ;)lot. a finite-energy f-sum as a function of the temperature. Near TMS, for a frequency range belweon 30 and 3(101") cn1-1 the cJJ~cltvc nuillb~,r of carriers, prol)ortional to the partial suiilrul,' shows a dramatic change, alt.llough ,'xlelltliIlg ow~r a l)road~r r a n g e of t,elllperalur~s i llall ill,' I ) ( ' l'~:sist":',"'~2, ', al.~,_, .-,,,,.,,,' ......,, in Fig. 2). A l l
el)ileal sun] rul~~ requires that the iniegrat~d al'~a under o l ( a ) must be constant. This means that across the 3II transition the missing conductivi'y from the low frequency part of the spectrum (~' <_ 2000 cm -1 ) should be compensated by contributions extending up to very high t'requencies, above our ,lpper limit. In a conventional semiconduc-
0921-4534/941507.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)01208-3
A. Wittlin et al./Physica C 235-240 (1994) 1289-1290
1290
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Figure t. The reflectance of NdNiOa (a) and PrNiO3 (b) vs frequency at several temperatures between 300 and 15 K. Below 225 K (a) and below 155 K (b) the spectra were measured with 10 K intervals. Inset: the reflectivity of NdNiO3 between 50 and 3000 cm -1
0.2
R
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(b) 155 K
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500
1000
1500
2000
Frequency(cm "1) Figure 2. Measured (dasheo lines) and fitted (solid nne) reflectance of PrNiOa at 125 (a) and at 155 K (b). Inset in (a) displays the temperature dependence of the resistivity after Ref. [3], and the inset in (b) shows the temperature dependence of partial f-sum for NdNiO3 and PrNiOa. Both the resistivity and integrated conductivity are shown in arbitrary units.
REFERENCES tor this shit'tit~g, of Sl),,-t ral w,.i~l~t occurs within a frequency rang,, ot" tiw order of several tinms the gal), In the I)rt,sent case, the activatioll gap E q deduced from the transport clara is approx. S0 cm -1 and one then would expe:t that all spectral weight should be recovered within the explored frequency range. Our data indicate that it is not the case. The physical origin o( this behavior, recently also found in Kondo-insulator systems [4] is by now unclear, however, it could possibly be associated wit.la the empty sta*,es in the 3d band.
1. .I. Zaane,, (;.A. Sawatzky and .I.W. Alien, Phys. Rev. L,,lt..55, ! 18 (15)85). 2. J.B. "Ibrrance ct al., Phys. R(,v. B 45, 8209 (1992). 3. X. (~ranados et al, Phys. Rev. B 48, 11666 (1993) and references herein. 4. Z. Schlesinger et at., Phys. Rex,. Lett. 71, 1748 (1993).
PHYStGA
Far-infrared studies of the metal-insulator transition in PrNiO3 aii
*
~High Field Magnet Laboratory, University of Nijmegen Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands. blnstitut de Ci~ncia de Materials, Consell Superior d'Iovestigations Cientifiques Campus de la UAB, 08193-Bellaterra, Espafia. We present infrared reflectivity m e a s u r e m e n t s of the rare-earth nickelates PrNiOa and NdNiOa between 30 and 5000 cm -1 at t e m p e r a t u r e s between 15 and 300 K. We observe a dramntic change of the s p e c t r a from metallic-like to nearly insulating in a narrow range of t e m p e r a t u r e s below the metM-insulator transition.
A general electronic band framework for oxides postulated by Zaanen, Sawatzky and Allen, in addition to the Mott-Hubbard gap systems, for ' l a t e " transition metal oxides introduces a novel type of metal-insulator (MI) transition related to the closing of the charge-transfer (CT) gap. [1] Such MI transition has been proposed recently for the lqNiO3 ( R = Pr. Nd, Sm, Eu) family. [2] In this work we report the t e m p e r a t u r e evolution (t0 300K)of the optical conductivity in the far infrared (FIR) (30 - 5000 cm -1) of PrNiOa and NdNi()s. T h e PrNiOa and NdNiO3 samples were prepared by J. Torrance at the IBM Ahnaden Laboratory in San Jose. Fig. 1 (a) shows the reflectivity of NdNiOa in the range 40-3000 cm -x between 15-300K and Fig. 1 (b) .~how.~ t.ho reflectivity of PrNi()a. The most pron-unced feature df these spectra is a rapid cilange of their character over a narrow I'aii~,' tJ[" t~,nil)erat.ures, close to t h e M I ~ r a n s i l ion, ~,xl~,lidilig ovc,r all OllPrgy i'allg~ " up to al~l,rox. 31)1)1) t i l l - 1 ill NdNi()a (s~'¢' illsot in t h e l.-'lg. I (a)) all(I iil) Io a l i p r o x , l()/)l) ciil - i in l'i'Ni():~. B~,low TM # t h e effect ot' t e l ] l p e r a t u r e on t h e p h o n o n
sl)ectrum is fairly minor. A metallic conductivity is present at low frequencies, already at approximately 30 K below the MI transition. We analyze the data using a model dielectric function with *Fiaancial support from the EEC (SCI-0389M(A). mid 0036-F), the Spanish CICY-I--MIDAS (mat:91-0742) mad DGICY]- (PB92-0849) programme are acknowledged.
Drude term, phonon oscillators and mid-infrared term. In Fig. 2 we show results of the fit for PrNiOa at 125 and at 155 K . At125 K we used wp = '1000 cm - i and F = 2500 cm -1 Above the MI transition, at 155 K we use ~p = 11000 cm -1 1-' - 16000 c n l - t , w : l , I - - 110 cm -1 , ,¢;~I. = 500 and 3M = 10 cm -1 T h e plasma frequency extracted from the fit was used to estimate the n / m " ratio. For PrNiOa at T = 135 K ~'r ~ 11000 cin -1 and n/m" ~ 1.34 1021 cm -a. For n -~ 1.8 10 .2.-, cm -a, m" ~ 13, in agreement with transport. measurements [3] . This enhanced electron mass reflects the strong correlation in the conduction band. For PrNiOa also the RT specific resistivity from FIR data p ~ 1.3 m f / c m compares favorable with the DC 2.0 m~Zcm.[3] In the inset in the Fig. '2 we ;)lot. a finite-energy f-sum as a function of the temperature. Near TMS, for a frequency range belweon 30 and 3(101") cn1-1 the cJJ~cltvc nuillb~,r of carriers, prol)ortional to the partial suiilrul,' shows a dramatic change, alt.llough ,'xlelltliIlg ow~r a l)road~r r a n g e of t,elllperalur~s i llall ill,' I ) ( ' l'~:sist":',"'~2, ', al.~,_, .-,,,,.,,,' ......,, in Fig. 2). A l l
el)ileal sun] rul~~ requires that the iniegrat~d al'~a under o l ( a ) must be constant. This means that across the 3II transition the missing conductivi'y from the low frequency part of the spectrum (~' <_ 2000 cm -1 ) should be compensated by contributions extending up to very high t'requencies, above our ,lpper limit. In a conventional semiconduc-
0921-4534/941507.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)01208-3
A. Wittlin et al./Physica C 235-240 (1994) 1289-1290
1290
10
1.0
" "'i .......
i
.....
~'
•
~
1.0
0.8
'
~
,
O.fi
.'
0.4
i
"
-
0.8[ 0.6 0.4
02
0,6
......
0.4
o 0.2 "
0.2
.....
, ,
!,
:
i
........
. . . .
i
i .........
'
4
I
0.6
0.6
Ii
X
4
~ ( I N
4 ~..
(b) P r N i O 3 ,
.
,
.
r i
I(D
200
300
,
,;
0 .(1 40~11
500
....
v..,o,1 "-....1 I~
I~
.J
0.4
0.4
'
/
(a) 125 K
t~ 0.8
i
15 K
....
,..
I
0.8
0.2
i
2t, , , .... ~. . . . . . . %,.,
t
..
600
F r e q u e n c y (cl}n "l)
Figure t. The reflectance of NdNiOa (a) and PrNiO3 (b) vs frequency at several temperatures between 300 and 15 K. Below 225 K (a) and below 155 K (b) the spectra were measured with 10 K intervals. Inset: the reflectivity of NdNiO3 between 50 and 3000 cm -1
0.2
R
iOs
"""
12
16
10(wr (ILK)
(b) 155 K
0.0
500
1000
1500
2000
Frequency(cm "1) Figure 2. Measured (dasheo lines) and fitted (solid nne) reflectance of PrNiOa at 125 (a) and at 155 K (b). Inset in (a) displays the temperature dependence of the resistivity after Ref. [3], and the inset in (b) shows the temperature dependence of partial f-sum for NdNiO3 and PrNiOa. Both the resistivity and integrated conductivity are shown in arbitrary units.
REFERENCES tor this shit'tit~g, of Sl),,-t ral w,.i~l~t occurs within a frequency rang,, ot" tiw order of several tinms the gal), In the I)rt,sent case, the activatioll gap E q deduced from the transport clara is approx. S0 cm -1 and one then would expe:t that all spectral weight should be recovered within the explored frequency range. Our data indicate that it is not the case. The physical origin o( this behavior, recently also found in Kondo-insulator systems [4] is by now unclear, however, it could possibly be associated wit.la the empty sta*,es in the 3d band.
1. .I. Zaane,, (;.A. Sawatzky and .I.W. Alien, Phys. Rev. L,,lt..55, ! 18 (15)85). 2. J.B. "Ibrrance ct al., Phys. R(,v. B 45, 8209 (1992). 3. X. (~ranados et al, Phys. Rev. B 48, 11666 (1993) and references herein. 4. Z. Schlesinger et at., Phys. Rex,. Lett. 71, 1748 (1993).