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Optical properties of V2O3 doped with chromium
Solid State Communications Vol. 8, pp. 1521—1524, 1970. Pergamon Press.
Printed in Great Britain
OPTICAL PROPERTIES OF V
203 DOPED WITH CHROMIUM
A. S. Barker, Jr. and J. P. Remeika Bell Telephone Laboratories Incorporated, Murray Hill, New Jersey
(Received 14 July 1970 L~yN. B. Hannay)
The optical conductivity of chromium doped V2O3 has been measured in the metallic, insulating and antiferromagnetic phases in the range 0.04 to 3 eV. The conductivity shows unusual feat~iresin all three phases. In particular the fundamental electronic absorption edge in the antiferromagnetic phase is very soft and shifts to lower energies when the spins become randomly oriented on entering the insulating phase.
PURE V2O~shows an abrupt metal-insulator transition at about 150 °K.1,2 At this transition the electrical conductivity a ‘ jumps a factor of a valuephase of approximately 2000 mho/ cm in 106 the to metallic just above 150 °K. McWhan,
polished faces normal to the c-axis of the Al phase. The optical techniques and methods of data analysis very to thoseindescribed 9 andare will notsimilar be discussed detail by others. here. In Fig. 1 we show the infrared reflectivity of a (V 0988Cr0012)2O3 sample as a function of temperature. From the phase diagram we note that cooling from 300 to 100 °Kwill cause the sample to pass from the I through the Al phase to the AFI phase. The reflectivity shows these three phases quite The the the metallic stateclearly. is rather lowreflectivity suggestingofthat
Rice and Remeika have reported a second type of transition in V203 which can occur when samples are doped with chromium. ~ These authors interpret this transition as a Mott transition between an insulating-phase with localized electrons41n and Fig.1(top) a metallic phase with we show itinerant electrons. schematically the phase diagram established by
designation ‘metallic’ must be used with some reservation. This Al phase was investigated more fully by studying a pure V 203 sample at 300 °K. Figure 2 shows the reflectivity spectrum.
McWhan ef a!. The three phases are now established to be metallic (M), insulating (I), 5~ and antiferromagnetic There is considerable insulating interest in (AFI). studying the optical properties of the three phases. In particular in the I phase (Mott insulating state) there must be strong correlations of the electrons5 which cause a breakdown of one— electron band theory and give rise to an energy gap. In the present paper we report on the optical properties in all three phases and in particular identify the gap in the / phase.
The spectrum has the general shape characteristic of a metal with a plasma edge near 1eV. A Kramers — Kronig analysis9 of the data shows, however, great differences from metallic behavior. The edge near 1eV is not associated with the real part of the dielectric function e’ passing through zero. ‘ is positive over the entire range measured and the edge results from structure in the real part of the conductivity (a’). The spectrum of a’ is shown in the lower part of Fig. 2. Also shown to the left is the d.c. value 00 for comparison. The Drude theory ‘°of the a ‘ spectrum would give a curve always
Samples have been prepared by special techniques which are described elsewhere. 6 Reflectivity measurements were made on
1521
1522
OPTICAL PROPERTIES OF V
203 DOPED WITH CHROMIUM
spectrum shows therfore the existence of considerable absorption above that predicted for free charge carriers.
03 ‘Cr 400
Vol. 8, No. 19
-
0.8
M
-
>.
200-
w U 0.4>
V20
UIs)
a:
0 0
I
5
.—Pr.,iur.
S Cr 300K
_________________________
0.8
0.6
0.4
v203+I.2%Cr
I0~
X.e~
~
v20~
i02.
Cr203~j 0.01
0
AFT
:~GOAs
IC’
0.2
____________________________________ 100 200 I 300 I T
K
FIG. 1.Phase diagram and infrared reflectivity of doped V2O~.The phase boundaries are somewhat schematic being drawn smoothly through the points measured by McWhan, Rice and Remeika with chromium doping plotted to the right and the application of pressure plotted to the left. The lower graph shows the reflectivity of a chromium doped sample which passes through all three phases on cooling.
.0
FIG.2. spectra of 1ev) pure V203 in the Al and 300°K the AFIOptical phases. reflectivity Kramers—Kronig (top) gives analysis the of conductivity shown below. Similar analysis of 80 °Kdata gives the conductivity of the AFI phase. The absorption which rises slowly above 0.1eV may be compared with the abrupt edge in GaAs. Frequencies markers indicate the optical phonons in Cr203 for comparison with the phonon absorption in the 80 °Kconductivity below 0.1eV. The lower part of Fig. 2 shows a’ when the is cooled into the AFI phase. Feinleib and Paul have previously investigated
v2o3 sample
this phase in the region 0.1—0.4eV by infrared 2 The transmission through thin range samples. shape of our a ‘ curvevery in this is similar below 00 which turns and falls as 1/ct~2above some characteristic collision frequency. The measured
to their results but the values of a ‘ are a factor of 2—4 higher. The reason for this
Vol. 8, No. 19
OPTICAL PROPERTIES OF V
203 DOPED WITH CHROMIUM
1523
discrepancy is not understood. It is apparent from the slowly rising absorption above 0.1eV that we cannot quote an unambiguous optical energy gap in the AFI phase. We assign a value of approx2 For imately 0.1eV have Feinleib and Paul. comparison, theasdashed curve shows the gap structure of a typical semiconductor (GaAs). For this latter material a’ drops steeply at low energies and continues to drop 3 more decades below the lowest ordinate plotted. Since the electrical conductivity of V~O 3(AFI)has a fairly well defined activation energy of 0.1eV, the optical data suggest that the concept of a sharp mobility gap lying in a slowly varying density of states may be valid. The peaks near 0.05eV (Fig. 2) are transverse infrared active phonons. V203 in the .~l phase has the corundum structure 8 which distorts We show at to monoclinic in the AFI phase~’ the bottom of the figure the energies of the two highest frequency phonons in a- Cr2O~which has the corundum structure. From the close similarities we conclude that the V-O~modes are similar to the a—Cr203 modes but additional splittings occur because of the new monoclinic unit cell. Figure 3 shows the a ‘ spectra of a sample of ( V0988Cr0012)203. In the AFI phase., the spectrum is very similar to V20,(AFI). Near 200°K this sample showed a jump in d.c. conductivity characteristic of the 11 phase, however o~,did not rise as high as for the ‘l phase of pure V204.This may be a result of inhomogenities in the Cr doping. The a’ spectrum lies somewhat lower than that of \~2O~(\i)and the infrared phonons are now observed. The main feature to be emphasized however is the strong absorption near 0.1eV which removes the edge observed in the AFI phase.
V203+I.2%Cr
M
2Oo~_....
—
~
i0~
\
,•/
f’. ~
\,~
/ 3~IOK
,./
b
O~0I2O0K) 02
-
. ~
AFI
~
I.,’
~o’
I
(cv) FIG. 3. Optical conductivity of V 0.0)
0.1
1.0
2O~plus 1.2 per cent chromium in the three phases. Phonon structure shows in all~spectra just below 0.1eV. The edge which rises above 0.1 eV shifts to lower energy on passing from the antiferromagnetic to the insulating phase. somewhat premature to make detailed comparisons with theory, calculations based on the atomic limit of the Mott—Hubbard model predict some of the behavior observed here. 11 On passing from AFI (ordered spin arrangement) to / (disordered spins) the calculated band widens so that excitations are possible for smaller photon energy. The shift calculated is quite small compared with the shift shown in Fig. 3. The model calculation does give a slowly rising absorption rather than a sharp edge. In the Mott—Hubbard model the gap in the
I phase is associated with the localization of At 300 °Kthe chromium doped sample is in 3 The the I phase discussed by McWhan ef al. structure is corundum and as expected we see two phonon peaks without the additional splitting of the AFT phase. The most significant structure is the new edge which appears near 0.1eV. The entire edge lies lower in energy than the AFI edge even though the activation energy is larger here than in the AFT phase. While it is
electrons and the required break the order andattendant raise an energy electron to an to excited state.~~ The absorption in Fig. 3 shows that the gap is approximately 0.1—0.5eV. As might be expected for a highly correlated state the gap structure is not sharp. We find a’ c.’~ near the edge in contrast to the very steep rise characteristic of semiconductors described by conventional band theory.
1524
OPTICAL PROPERTIES OF V
203 DOPED WITH CHROMIUM
Vol. 8, No. 19
Acknowledgements — The authors are indebted to T.M. Rice and D.B. McWhan for several helpful discussions and to H.C. Montgomery for measurements of the Hall effect and conductivity of some of the samples studied. We also thank E.M. Kelly for assistance in crystal growth.
REFERENCES 1.
ADLER, D., Solid State Physics, Vol. 21 (Edited by SEITZ F. and TURNBALL D), Academic Press, New York (1969).
2.
FEINLEIB
3.
McWHAN D.B., RICE T.M., and REMEIKA J.P., Phys. Rev. Leti. 23, 1384 (1969).
4.
MOTT N.F., Proc. Phys. Soc. London 62, 416 (1949). Also Phil Mag. 6, 287 (1961).
5.
RICE T.M. and MCVIHAN D. B., IB’1 J. Res. Dev. 14, 251 (1970),
6.
McWHAN D.B. and REMEIKA J.P., (to be published).
7.
JAYARAMAN A. McWHAND.B., REMEIKAJ.P. and DERNIERP.D., (tobe published).
8.
DERNIER P. D. and MAREZIO M., (to be published).
9.
SPITZER W. G. and KLEINMAN A., Plus. Rei. 121, 1324, (1961).
J.
and PAUL W., Phys. Rex’. 155, 841 (1967).
10.
MOTT N. F., and JONES H., The Theor2,- of the Properlies of ‘dejais and .4llovs. Ch. III. Dover Publications Inc., New Yorlc (1958).
11.
BRINKMAN V1. F. and RICE T.M., (to be published).
La conductivité optique de V2O~ dope au chrome est mesurée dans les phases metallique. isolante et antiferrom.ignetique a des energies comprises entre 0,04 et 3eV. La conductivité presente des caractéristiques particulieres dans toutes les trois phases. En particulier, le bord de l’absorption électronique fondamentale est tres mou dens la phase antiferromagnetique et se deplace vers les energies infërieures lorsque les spins prennent une orientation aleatoire au passage a la phase isolante.
Printed in Great Britain
OPTICAL PROPERTIES OF V
203 DOPED WITH CHROMIUM
A. S. Barker, Jr. and J. P. Remeika Bell Telephone Laboratories Incorporated, Murray Hill, New Jersey
(Received 14 July 1970 L~yN. B. Hannay)
The optical conductivity of chromium doped V2O3 has been measured in the metallic, insulating and antiferromagnetic phases in the range 0.04 to 3 eV. The conductivity shows unusual feat~iresin all three phases. In particular the fundamental electronic absorption edge in the antiferromagnetic phase is very soft and shifts to lower energies when the spins become randomly oriented on entering the insulating phase.
PURE V2O~shows an abrupt metal-insulator transition at about 150 °K.1,2 At this transition the electrical conductivity a ‘ jumps a factor of a valuephase of approximately 2000 mho/ cm in 106 the to metallic just above 150 °K. McWhan,
polished faces normal to the c-axis of the Al phase. The optical techniques and methods of data analysis very to thoseindescribed 9 andare will notsimilar be discussed detail by others. here. In Fig. 1 we show the infrared reflectivity of a (V 0988Cr0012)2O3 sample as a function of temperature. From the phase diagram we note that cooling from 300 to 100 °Kwill cause the sample to pass from the I through the Al phase to the AFI phase. The reflectivity shows these three phases quite The the the metallic stateclearly. is rather lowreflectivity suggestingofthat
Rice and Remeika have reported a second type of transition in V203 which can occur when samples are doped with chromium. ~ These authors interpret this transition as a Mott transition between an insulating-phase with localized electrons41n and Fig.1(top) a metallic phase with we show itinerant electrons. schematically the phase diagram established by
designation ‘metallic’ must be used with some reservation. This Al phase was investigated more fully by studying a pure V 203 sample at 300 °K. Figure 2 shows the reflectivity spectrum.
McWhan ef a!. The three phases are now established to be metallic (M), insulating (I), 5~ and antiferromagnetic There is considerable insulating interest in (AFI). studying the optical properties of the three phases. In particular in the I phase (Mott insulating state) there must be strong correlations of the electrons5 which cause a breakdown of one— electron band theory and give rise to an energy gap. In the present paper we report on the optical properties in all three phases and in particular identify the gap in the / phase.
The spectrum has the general shape characteristic of a metal with a plasma edge near 1eV. A Kramers — Kronig analysis9 of the data shows, however, great differences from metallic behavior. The edge near 1eV is not associated with the real part of the dielectric function e’ passing through zero. ‘ is positive over the entire range measured and the edge results from structure in the real part of the conductivity (a’). The spectrum of a’ is shown in the lower part of Fig. 2. Also shown to the left is the d.c. value 00 for comparison. The Drude theory ‘°of the a ‘ spectrum would give a curve always
Samples have been prepared by special techniques which are described elsewhere. 6 Reflectivity measurements were made on
1521
1522
OPTICAL PROPERTIES OF V
203 DOPED WITH CHROMIUM
spectrum shows therfore the existence of considerable absorption above that predicted for free charge carriers.
03 ‘Cr 400
Vol. 8, No. 19
-
0.8
M
-
>.
200-
w U 0.4>
V20
UIs)
a:
0 0
I
5
.—Pr.,iur.
S Cr 300K
_________________________
0.8
0.6
0.4
v203+I.2%Cr
I0~
X.e~
~
v20~
i02.
Cr203~j 0.01
0
AFT
:~GOAs
IC’
0.2
____________________________________ 100 200 I 300 I T
K
FIG. 1.Phase diagram and infrared reflectivity of doped V2O~.The phase boundaries are somewhat schematic being drawn smoothly through the points measured by McWhan, Rice and Remeika with chromium doping plotted to the right and the application of pressure plotted to the left. The lower graph shows the reflectivity of a chromium doped sample which passes through all three phases on cooling.
.0
FIG.2. spectra of 1ev) pure V203 in the Al and 300°K the AFIOptical phases. reflectivity Kramers—Kronig (top) gives analysis the of conductivity shown below. Similar analysis of 80 °Kdata gives the conductivity of the AFI phase. The absorption which rises slowly above 0.1eV may be compared with the abrupt edge in GaAs. Frequencies markers indicate the optical phonons in Cr203 for comparison with the phonon absorption in the 80 °Kconductivity below 0.1eV. The lower part of Fig. 2 shows a’ when the is cooled into the AFI phase. Feinleib and Paul have previously investigated
v2o3 sample
this phase in the region 0.1—0.4eV by infrared 2 The transmission through thin range samples. shape of our a ‘ curvevery in this is similar below 00 which turns and falls as 1/ct~2above some characteristic collision frequency. The measured
to their results but the values of a ‘ are a factor of 2—4 higher. The reason for this
Vol. 8, No. 19
OPTICAL PROPERTIES OF V
203 DOPED WITH CHROMIUM
1523
discrepancy is not understood. It is apparent from the slowly rising absorption above 0.1eV that we cannot quote an unambiguous optical energy gap in the AFI phase. We assign a value of approx2 For imately 0.1eV have Feinleib and Paul. comparison, theasdashed curve shows the gap structure of a typical semiconductor (GaAs). For this latter material a’ drops steeply at low energies and continues to drop 3 more decades below the lowest ordinate plotted. Since the electrical conductivity of V~O 3(AFI)has a fairly well defined activation energy of 0.1eV, the optical data suggest that the concept of a sharp mobility gap lying in a slowly varying density of states may be valid. The peaks near 0.05eV (Fig. 2) are transverse infrared active phonons. V203 in the .~l phase has the corundum structure 8 which distorts We show at to monoclinic in the AFI phase~’ the bottom of the figure the energies of the two highest frequency phonons in a- Cr2O~which has the corundum structure. From the close similarities we conclude that the V-O~modes are similar to the a—Cr203 modes but additional splittings occur because of the new monoclinic unit cell. Figure 3 shows the a ‘ spectra of a sample of ( V0988Cr0012)203. In the AFI phase., the spectrum is very similar to V20,(AFI). Near 200°K this sample showed a jump in d.c. conductivity characteristic of the 11 phase, however o~,did not rise as high as for the ‘l phase of pure V204.This may be a result of inhomogenities in the Cr doping. The a’ spectrum lies somewhat lower than that of \~2O~(\i)and the infrared phonons are now observed. The main feature to be emphasized however is the strong absorption near 0.1eV which removes the edge observed in the AFI phase.
V203+I.2%Cr
M
2Oo~_....
—
~
i0~
\
,•/
f’. ~
\,~
/ 3~IOK
,./
b
O~0I2O0K) 02
-
. ~
AFI
~
I.,’
~o’
I
(cv) FIG. 3. Optical conductivity of V 0.0)
0.1
1.0
2O~plus 1.2 per cent chromium in the three phases. Phonon structure shows in all~spectra just below 0.1eV. The edge which rises above 0.1 eV shifts to lower energy on passing from the antiferromagnetic to the insulating phase. somewhat premature to make detailed comparisons with theory, calculations based on the atomic limit of the Mott—Hubbard model predict some of the behavior observed here. 11 On passing from AFI (ordered spin arrangement) to / (disordered spins) the calculated band widens so that excitations are possible for smaller photon energy. The shift calculated is quite small compared with the shift shown in Fig. 3. The model calculation does give a slowly rising absorption rather than a sharp edge. In the Mott—Hubbard model the gap in the
I phase is associated with the localization of At 300 °Kthe chromium doped sample is in 3 The the I phase discussed by McWhan ef al. structure is corundum and as expected we see two phonon peaks without the additional splitting of the AFT phase. The most significant structure is the new edge which appears near 0.1eV. The entire edge lies lower in energy than the AFI edge even though the activation energy is larger here than in the AFT phase. While it is
electrons and the required break the order andattendant raise an energy electron to an to excited state.~~ The absorption in Fig. 3 shows that the gap is approximately 0.1—0.5eV. As might be expected for a highly correlated state the gap structure is not sharp. We find a’ c.’~ near the edge in contrast to the very steep rise characteristic of semiconductors described by conventional band theory.
1524
OPTICAL PROPERTIES OF V
203 DOPED WITH CHROMIUM
Vol. 8, No. 19
Acknowledgements — The authors are indebted to T.M. Rice and D.B. McWhan for several helpful discussions and to H.C. Montgomery for measurements of the Hall effect and conductivity of some of the samples studied. We also thank E.M. Kelly for assistance in crystal growth.
REFERENCES 1.
ADLER, D., Solid State Physics, Vol. 21 (Edited by SEITZ F. and TURNBALL D), Academic Press, New York (1969).
2.
FEINLEIB
3.
McWHAN D.B., RICE T.M., and REMEIKA J.P., Phys. Rev. Leti. 23, 1384 (1969).
4.
MOTT N.F., Proc. Phys. Soc. London 62, 416 (1949). Also Phil Mag. 6, 287 (1961).
5.
RICE T.M. and MCVIHAN D. B., IB’1 J. Res. Dev. 14, 251 (1970),
6.
McWHAN D.B. and REMEIKA J.P., (to be published).
7.
JAYARAMAN A. McWHAND.B., REMEIKAJ.P. and DERNIERP.D., (tobe published).
8.
DERNIER P. D. and MAREZIO M., (to be published).
9.
SPITZER W. G. and KLEINMAN A., Plus. Rei. 121, 1324, (1961).
J.
and PAUL W., Phys. Rex’. 155, 841 (1967).
10.
MOTT N. F., and JONES H., The Theor2,- of the Properlies of ‘dejais and .4llovs. Ch. III. Dover Publications Inc., New Yorlc (1958).
11.
BRINKMAN V1. F. and RICE T.M., (to be published).
La conductivité optique de V2O~ dope au chrome est mesurée dans les phases metallique. isolante et antiferrom.ignetique a des energies comprises entre 0,04 et 3eV. La conductivité presente des caractéristiques particulieres dans toutes les trois phases. En particulier, le bord de l’absorption électronique fondamentale est tres mou dens la phase antiferromagnetique et se deplace vers les energies infërieures lorsque les spins prennent une orientation aleatoire au passage a la phase isolante.