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Dielectric measurements in the submillimeter wavelength region
Infrared Phys. Vol. 25. No. 1/2, pp. 369 373, 1985 Printed in Great Britain
0020 0891/85 $3.00 + 0.00 Pergamon Press Ltd
DIELECTRIC M E A S U R E M E N T S IN THE S U B M I L L I M E T E R WAVELENGTH REGION A. A. VOLKOV, YU. G. GONCHAROV, G. V. KOZLOV, S. P. LEBEDEV and A. M. PROKHOROV General Physics Institute, U.S.S.R. Academy of Sciences, Vavilov 38, Moscow 117942, U.S.S.R.
(Receil~ed 27 July 1984) Abstract The paper describes a method for dielectric measurements in the submillimeter range based on the application of a backward-wave-oscillator technique. The application of submillimeter dielectric spectroscopy for solving real problems of solid-state physics is discussed.
Reliable dielectric measurements in the submillimeter region have been practically unavailable for a long time because of the absence of a suitable radiation source. Now the situation in this field is completely different due to the development of special submillimeter spectrometers using continuously tunable monochromatic radiation from sources of the backward-wave-oscillator type (BWO). The submillimeter BWO spectrometers have been found to be extremely efficient instruments for dielectric measurements, combining the most significant advantages of the IR spectroscopy and the microwave technique. They make it possible to measure precisely and independently the real E' and imaginary E" parts of the permittivity at separate frequency points and at the same time to register these quantities in the form of spectra recorded in a wide frequency range on a real time scale. Transition from measuring the transmission or reflection spectra of the samples to dielectric spectroscopy, i.e. to measuring ~'(v) and E"(v) spectra, in the framework of any spectral approach implies a qualitatively new experimental level. An indispensable condition of dielectric measurements is the use of measuring schemes permitting a rigorous mathematical description, as only in this case can one expect to obtain qualitative data with sufficient accuracy. Such a base scheme in quasioptical submillimeter techniques is that of a plain wave passing through a dielectric layer. In the routinely used case of normal wave incidence, the complex transmission coefficient t = x/-Texp(iq~) of the isotropic layer is determined by the well-known relationships: Ill
(1
T=E
-
R) 2 + 4RsinZ~p
(1 - RE) 2 + 4REsin2(N + ~9)'
ER sin 2(N + ~9) b q) = N + arctan 1 - E R c o s 2 ( N + t)) + arctan a2 + b 2 + a E=exp
(4~kd) - ~
N-
(a -- l )2 ~_ b 2
R - (a + 1) 2 + b 2'
n + ik = x/Eft,
2~nd
b arctan a ~ l - ' (1)
)o ,
2b @ = arctan a2 + b 2 - 1'
a + ib = x ~ ,
where )~ is the radiation wavelength, d is the sample thickness and E and # are the complex dielectric constant and magnetic permeability of the substance, respectively. As is seen, in these relationships Eand # are equal in rights and, therefore, for magnetic substances in the problem of measuring dielectric spectra may be solved only in parallel with determining the magnetic spectra. When using the technique of a plain layer one may measure such parameters as T, R, q) and ~. They form the hierarchy T, ~o, R, ~ by their importance and applicability in dielectric measurements on BWO spectrometers. As is seen, the situation here is completely different from that in FIR 369
370
A.A. VOLKO',et al.
spectroscopy, where the basic experimental technique is the reflection spectra measurements. This difference is associated with the fact that the transmission spectra of substances opaque for FIR spectrometers can. as a rule, be measured on BWO spectrometers having a higher signal-to-noise level. When measuring the spectra of non-magnetic substances, in order to calculate e' and ( ' it is necessary to measure at each frequency two parameters characterizing the wave interaction with the sample. As experience shows, an optimum pair for transparent samples ( T > 10 *l is 7" and {f~ for opaque samples, R and ~. The accuracies, 1 5",, over e' and 5 10",, over ~" may be considered as standard for BWO spectrometers. ~2~ In the case of magnetic materials (It ¢ 1) dielectric measurements become considerably complicated. For determining the four unknown values (, (', I( and IE', generally speaking, it is necessary to measure all four parameters of the wave interaction with the sample, i.e. T, ¢>, R and ~/J {unlike the case where ll = 1. now all these parameters are independent). However, in many practically important cases one can manage to simplify the problem. The fact is that at submillimeter wavelengths the magnetic permeabilities of the substances are, as a rule, very small, except for the narrow parts of the spectrum situated near the resonant frequencies of the magnon modes. In this case, the dielectric spectra (iv) and ~"(v) of the substance outside the magnetic absorption lines can be determined by the usual method, for instance by Tlv) and {plv), and then the dependences/(iv) and l('{v) can be calculated from the data T(vt and q)(v) inside the line using the values of e' and e" determined outside the lines.* One more problem exists in measuring the spectra of magnetic materials, due to the presence of non-diagonal elements in the magnetic permeability tensor. In particular, in Faraday geometry linearly polarized waves used for dielectric measurements are not proper waves of the system and change their polarization at the crystal output. The problem of wave propagation in such systems with dielecteic and magnetic anisotropy is extremely complicated even for theoretical consideration. Therefore, when measuring the spectra of magnetic materials it is appropriate to choose a geometry of the experiment in which the Faraday effect has no influence. A scheme of the quasioptical part of the submillimeter BWO spectrometer "Epsilon'" intended for measuring dielectric spectra of solids and liquids by the T a n d ¢) method is presented in Fig. 1.(3~ It is based on polarization double-beam interferometry. The T(v) measurements are performed by a single-channel scheme, one arm of the interferometer being closed. The recording time of T(/) by 100 points in the range of one BWO is about 10 s. The ¢)(vt spectra are measured using phase modulation
1
I ~o
~l" i
I
s
I
I
, 1
--~
2 3 4
5
I
6 L__I
i a
9
Fig. 1. Measm'ing channel of the spectrometer "Epsilon". 1, Radiation generator IBWO); 2, Tcllon lenses: 3, absorbing diaphragms; 4, modulator; 5, network attenuator; 6, polarizer: 7. dividing grids; 8, thermostat with the sample; 9, mirror (phase modulator}; 10, phase-shift compensator; l l,analyzer; 12, radiation detector. The sections isolated by the dashed lines are not used in TIv) measurements.
*In this method association of magnclic and dielectric subsystems of tile crystal is not taken iW.oaccount and. thcrcfore, the method is limited by weak magnon modes such as antiferromagnets.
Dielectric measurements in the submillimeterregion
371
of the radiation in the interferometer. This process takes 30s. About 1 rain after the T and ~p measurements, the spectrometer provides the dielectric spectra d(v) and d'(v) of the sample. The basic parameters of the spectrometer are as follows: frequency range = 2 35 c m - 1 frequency resolution = 104 105 dynamic range = 40 50dB the range of the parameters measured = - 103< E'< 104, 10 4 10-3 < E"< 103 temperature region = 4.2 700 K characteristic transverse dimensions of the samples = 10 x 10mm 2. The advantages of dielectric measurements on BWO spectrometers can be illustrated by the results of studying ferroelectrics, superionic conductors and antiferromagnets the substances of most interest for the submillimeter spectroscopy of solids.
Ferroelectrics The main problem in the dielectric spectroscopy of ferroelectrics is the revealing and investigation of soft modes of phase transitions. Ferroelectric soft modes feature high damping and abnormally high dielectric contributions (AE ~ 104). The radiation absorption coefficient inside the soft-mode line amounts to 103 c m - 1. Investigation of such complicated substances became possible at submillimeter wavelengths only with the development of BWO spectroscopy. Ferroelectrics became the first targets of BWO spectroscopy and they have been studied recently in detail. 14-1°~ Figure 2 presents submillimeter dielectric spectra of the T1GaSe2 crystal. The ferroelectric properties of this semiconductor crystal and isomorphic compounds have been revealed by just such dielectric measurements at submillimeter wavelengths. {11,12) We have shown that dielectric dispersion in this substance is connected with the presence of temperature-unstable lattice vibration soft mode. By the character of the soft-mode parameter variation with temperature we predicted two previously unknown phase transitions in T1GaSe2 crystals at temperatures of 120 and 107 K into incommensurate and ferroelectric phases, respectively.I1~ Both of these predictions were confirmed by the results of subsequent investigations.
Superionic conductors In the spectra of these substances the submillimeter range is in the region where the interaction of the lattice vibration and the diffusion motion of mobile ions, inherent in superionic phase transitions, should manifest itself. 100 --
50--
4 0-,,#
100
3 2 4
\
50
I
J
I
10
I
30
20
FREQUENCY
(cm -~)
Fig. 2. Submillimeter dielectric spectra of a TlOaSez crystal, (1) T = 298 K, (2) T = 201 K, (3) 7"= 123 K, (4) T = 108 K, (5) T = 100K.
372
A . A . VOI.KOV el dl.
IE ¢o
7
l
...... •...........~ ...............
10
30
2O
FREQUENCY
( cm -~]
Fig. 3. Submillimeter conductivity spectra of the superionic conductor RbAg415at (a)298 K and (b~4.2 K. Figure 3 presents submillimeter spectra of one of the most interesting superionic conductors the RbAg41s crystal. This crystal has two phase transitions "/'1 - 122 K and T2 = 208 K. Superionic conductivity is inherent in both high-temperature phases. Our investigations have shown that at high temperature there are three modes in the RbAg,,Is submillimeter spectrum, each of which is subject to variations at phase transitions. However, a most interesting phenomenon was observed at low temperaturesJ ~3) As it turned out, the spectrum of the crystal dynamic conductivity split iolo a series of
45
...:'" ...:"
X:}
• ,..'-' " ....
S<, 4O
1 0 - 2 1 ................................."'""..
Y'"""'"" '"""'"............"....
k
10 , I
30
..
v2(TmFeO 3)
7
v2( {FeC, ~1
E(o 2o
>X -X X - X-XX (..) L, '/1 ( OyFeOs 1 x'" ×" z LU X/ ~*'6{~x-X~x--xx-~_x-x-x-X~x-x~xKzx-~ x x--x-x xgx.x-.., x "x :D O' | ~×...-.-UJ rr i, 10
09
'a.
O2 O.'1 OO
I
"W,~xXX
17 3
175
'177
179 O
• t" u (TmFe o ~)
[
I
IOO
2O0
_--J
F R E Q U E N C Y (cm I) TEMPERATURE Fig. 4. Results of measurements on the "'Epsilon'" spectrometer of T(v) and ~o(v) spectra of a 0.818 m m thick YFeO3 plate and tire IF(V) and I('(v) spectra calculated by them.
[K]
Fig. 5. T e m p e r a t u r e dependences of A FM R frcqucntics in T m F c O 3 , DyFeO,~ and Y F e O ~ (vL is thc lm~,frequency branch, v2 is high-frcquency branch I.
5OO
Dielectric measurements in the submillimeter region
373
high-quality absorption lines (Fig. 3). Such a rich spectrum of lattice vibrations at submillimeter wavelengths has not been observed previously in the crystals. We assume that the results on RbAg415 submillimeter spectra are of principal importance in understanding the nature of superionic conductivity in a solid.
Ant!]erromagnets. In these substances, as well as in ferroelectrics, there are soft modes. But in the given case they are connected with oscillations of not electric, but magnetic moments ofsublattices. For this reason, their manifestation should be expected in the #(v) spectra. Figure 4 presents the spectra of the YFeO3 antiferromagnet in the vicinity of a high-frequency branch of antiferromagnetic resonance (AFMR). ~1~ A distinguishing feature of the lines is their high quality, reaching in some cases the value of 103. At present, the observation of such narrow lines at submillimeter wavelengths is not possible by either FIR techniques or Raman spectroscopy. The YFeO3 magnetic spectra presented in Fig. 4 were calculated by the T(v) and ~p(v) spectra using the procedure described above. Figure 5 presents the temperature dependences obtained for soft-mode parameters in some orthoferrites. The possibility of reordering of magnetic moments in the lattices of these crystals results in a variety of phase transformations being observed in them. As a result, instabilities are observed in both the lower and the upper branches of AFMR (see Fig. 5). The examples presented do not cover all the advantages of dielectric measurements on BWO spectrometers but, we hope, give a general idea of them. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1 I. 12. 13. 14.
Brekhovskih L. M., Waves in Stratified Media. Izd. Akad. Nauk SSSR, Moscow (1957). Volkov A. A., Kozlov G. V. and Lebedev S. P., Radiotekh. Electron. 24, 1405 (19791. Volkov A. A., Goncharov Yu. G., Kozlov G. V., Lebedev S. P. and Maltsev V. 1., Prib. tekh. eksp. 2, 236 (1984~. Kozlov G. V., Lebedev S. P., Prokhorov A. M. and Volkov A. A., Ferroelectrics 25, 531 (1980). Volkov A. A., lshibashi I., Kozlov G. V., Lebedev S. P., Petzelt J. and Prokhorov A. M., J. phys. Soc. Japan (Suppl. B) 49, 78 (1980). Petzelt J., Kozlov G. V., Volkov A. A. and lshibashi Y., Z. Phys. B33, 369 (1979). Voklov A. A., Kozlov G. V., Lebedev S. P., Petzelt J. and Ishibashi Y., Ferroelectric.s 45, 157 (1982). Kozlov G. V., Volkov A. A. and Lebedev S. P., Usp. fiz. Nauk 135, 515 (1981). Kozlov G. V., Volkov A. A., Scott J. F., Feldkamp G. E. and Petzelt J., Phys. Rev. B28, 255 (1983). Kozlov G. V., lzr. Akad. Nauk SSSR, Ser. fiz. 47, 587 (1983). Volkov A. A., Goncharov Yu. G., Kozlov G. V., Lebedev S. P., Prokhorov A. M., Aliev R. A. and Allakhverdiev K. R., J E T P Lett. 37, 517 (1983). Volkov A. A., Goncharov Yu. G., Kozlov G. V., Allakhverdiev K. R. and Sardarly R. M., Fizika trerd. Tela 25, 3583 ( 1983 ). Volkov A. A., Kozlov G. V., Mirzoev G. I. and Goffman V. G., J E T P Lett. 38, 182 (1983). Volkov A. A., Goncharov Yu. G., Kozlov G. V., Kocharyan K. N., Lebedev S. P., Prokhorov A. S. and Prokhorov A. M., J E T P Lett. 39, 140 (1984).
0020 0891/85 $3.00 + 0.00 Pergamon Press Ltd
DIELECTRIC M E A S U R E M E N T S IN THE S U B M I L L I M E T E R WAVELENGTH REGION A. A. VOLKOV, YU. G. GONCHAROV, G. V. KOZLOV, S. P. LEBEDEV and A. M. PROKHOROV General Physics Institute, U.S.S.R. Academy of Sciences, Vavilov 38, Moscow 117942, U.S.S.R.
(Receil~ed 27 July 1984) Abstract The paper describes a method for dielectric measurements in the submillimeter range based on the application of a backward-wave-oscillator technique. The application of submillimeter dielectric spectroscopy for solving real problems of solid-state physics is discussed.
Reliable dielectric measurements in the submillimeter region have been practically unavailable for a long time because of the absence of a suitable radiation source. Now the situation in this field is completely different due to the development of special submillimeter spectrometers using continuously tunable monochromatic radiation from sources of the backward-wave-oscillator type (BWO). The submillimeter BWO spectrometers have been found to be extremely efficient instruments for dielectric measurements, combining the most significant advantages of the IR spectroscopy and the microwave technique. They make it possible to measure precisely and independently the real E' and imaginary E" parts of the permittivity at separate frequency points and at the same time to register these quantities in the form of spectra recorded in a wide frequency range on a real time scale. Transition from measuring the transmission or reflection spectra of the samples to dielectric spectroscopy, i.e. to measuring ~'(v) and E"(v) spectra, in the framework of any spectral approach implies a qualitatively new experimental level. An indispensable condition of dielectric measurements is the use of measuring schemes permitting a rigorous mathematical description, as only in this case can one expect to obtain qualitative data with sufficient accuracy. Such a base scheme in quasioptical submillimeter techniques is that of a plain wave passing through a dielectric layer. In the routinely used case of normal wave incidence, the complex transmission coefficient t = x/-Texp(iq~) of the isotropic layer is determined by the well-known relationships: Ill
(1
T=E
-
R) 2 + 4RsinZ~p
(1 - RE) 2 + 4REsin2(N + ~9)'
ER sin 2(N + ~9) b q) = N + arctan 1 - E R c o s 2 ( N + t)) + arctan a2 + b 2 + a E=exp
(4~kd) - ~
N-
(a -- l )2 ~_ b 2
R - (a + 1) 2 + b 2'
n + ik = x/Eft,
2~nd
b arctan a ~ l - ' (1)
)o ,
2b @ = arctan a2 + b 2 - 1'
a + ib = x ~ ,
where )~ is the radiation wavelength, d is the sample thickness and E and # are the complex dielectric constant and magnetic permeability of the substance, respectively. As is seen, in these relationships Eand # are equal in rights and, therefore, for magnetic substances in the problem of measuring dielectric spectra may be solved only in parallel with determining the magnetic spectra. When using the technique of a plain layer one may measure such parameters as T, R, q) and ~. They form the hierarchy T, ~o, R, ~ by their importance and applicability in dielectric measurements on BWO spectrometers. As is seen, the situation here is completely different from that in FIR 369
370
A.A. VOLKO',et al.
spectroscopy, where the basic experimental technique is the reflection spectra measurements. This difference is associated with the fact that the transmission spectra of substances opaque for FIR spectrometers can. as a rule, be measured on BWO spectrometers having a higher signal-to-noise level. When measuring the spectra of non-magnetic substances, in order to calculate e' and ( ' it is necessary to measure at each frequency two parameters characterizing the wave interaction with the sample. As experience shows, an optimum pair for transparent samples ( T > 10 *l is 7" and {f~ for opaque samples, R and ~. The accuracies, 1 5",, over e' and 5 10",, over ~" may be considered as standard for BWO spectrometers. ~2~ In the case of magnetic materials (It ¢ 1) dielectric measurements become considerably complicated. For determining the four unknown values (, (', I( and IE', generally speaking, it is necessary to measure all four parameters of the wave interaction with the sample, i.e. T, ¢>, R and ~/J {unlike the case where ll = 1. now all these parameters are independent). However, in many practically important cases one can manage to simplify the problem. The fact is that at submillimeter wavelengths the magnetic permeabilities of the substances are, as a rule, very small, except for the narrow parts of the spectrum situated near the resonant frequencies of the magnon modes. In this case, the dielectric spectra (iv) and ~"(v) of the substance outside the magnetic absorption lines can be determined by the usual method, for instance by Tlv) and {plv), and then the dependences/(iv) and l('{v) can be calculated from the data T(vt and q)(v) inside the line using the values of e' and e" determined outside the lines.* One more problem exists in measuring the spectra of magnetic materials, due to the presence of non-diagonal elements in the magnetic permeability tensor. In particular, in Faraday geometry linearly polarized waves used for dielectric measurements are not proper waves of the system and change their polarization at the crystal output. The problem of wave propagation in such systems with dielecteic and magnetic anisotropy is extremely complicated even for theoretical consideration. Therefore, when measuring the spectra of magnetic materials it is appropriate to choose a geometry of the experiment in which the Faraday effect has no influence. A scheme of the quasioptical part of the submillimeter BWO spectrometer "Epsilon'" intended for measuring dielectric spectra of solids and liquids by the T a n d ¢) method is presented in Fig. 1.(3~ It is based on polarization double-beam interferometry. The T(v) measurements are performed by a single-channel scheme, one arm of the interferometer being closed. The recording time of T(/) by 100 points in the range of one BWO is about 10 s. The ¢)(vt spectra are measured using phase modulation
1
I ~o
~l" i
I
s
I
I
, 1
--~
2 3 4
5
I
6 L__I
i a
9
Fig. 1. Measm'ing channel of the spectrometer "Epsilon". 1, Radiation generator IBWO); 2, Tcllon lenses: 3, absorbing diaphragms; 4, modulator; 5, network attenuator; 6, polarizer: 7. dividing grids; 8, thermostat with the sample; 9, mirror (phase modulator}; 10, phase-shift compensator; l l,analyzer; 12, radiation detector. The sections isolated by the dashed lines are not used in TIv) measurements.
*In this method association of magnclic and dielectric subsystems of tile crystal is not taken iW.oaccount and. thcrcfore, the method is limited by weak magnon modes such as antiferromagnets.
Dielectric measurements in the submillimeterregion
371
of the radiation in the interferometer. This process takes 30s. About 1 rain after the T and ~p measurements, the spectrometer provides the dielectric spectra d(v) and d'(v) of the sample. The basic parameters of the spectrometer are as follows: frequency range = 2 35 c m - 1 frequency resolution = 104 105 dynamic range = 40 50dB the range of the parameters measured = - 103< E'< 104, 10 4 10-3 < E"< 103 temperature region = 4.2 700 K characteristic transverse dimensions of the samples = 10 x 10mm 2. The advantages of dielectric measurements on BWO spectrometers can be illustrated by the results of studying ferroelectrics, superionic conductors and antiferromagnets the substances of most interest for the submillimeter spectroscopy of solids.
Ferroelectrics The main problem in the dielectric spectroscopy of ferroelectrics is the revealing and investigation of soft modes of phase transitions. Ferroelectric soft modes feature high damping and abnormally high dielectric contributions (AE ~ 104). The radiation absorption coefficient inside the soft-mode line amounts to 103 c m - 1. Investigation of such complicated substances became possible at submillimeter wavelengths only with the development of BWO spectroscopy. Ferroelectrics became the first targets of BWO spectroscopy and they have been studied recently in detail. 14-1°~ Figure 2 presents submillimeter dielectric spectra of the T1GaSe2 crystal. The ferroelectric properties of this semiconductor crystal and isomorphic compounds have been revealed by just such dielectric measurements at submillimeter wavelengths. {11,12) We have shown that dielectric dispersion in this substance is connected with the presence of temperature-unstable lattice vibration soft mode. By the character of the soft-mode parameter variation with temperature we predicted two previously unknown phase transitions in T1GaSe2 crystals at temperatures of 120 and 107 K into incommensurate and ferroelectric phases, respectively.I1~ Both of these predictions were confirmed by the results of subsequent investigations.
Superionic conductors In the spectra of these substances the submillimeter range is in the region where the interaction of the lattice vibration and the diffusion motion of mobile ions, inherent in superionic phase transitions, should manifest itself. 100 --
50--
4 0-,,#
100
3 2 4
\
50
I
J
I
10
I
30
20
FREQUENCY
(cm -~)
Fig. 2. Submillimeter dielectric spectra of a TlOaSez crystal, (1) T = 298 K, (2) T = 201 K, (3) 7"= 123 K, (4) T = 108 K, (5) T = 100K.
372
A . A . VOI.KOV el dl.
IE ¢o
7
l
...... •...........~ ...............
10
30
2O
FREQUENCY
( cm -~]
Fig. 3. Submillimeter conductivity spectra of the superionic conductor RbAg415at (a)298 K and (b~4.2 K. Figure 3 presents submillimeter spectra of one of the most interesting superionic conductors the RbAg41s crystal. This crystal has two phase transitions "/'1 - 122 K and T2 = 208 K. Superionic conductivity is inherent in both high-temperature phases. Our investigations have shown that at high temperature there are three modes in the RbAg,,Is submillimeter spectrum, each of which is subject to variations at phase transitions. However, a most interesting phenomenon was observed at low temperaturesJ ~3) As it turned out, the spectrum of the crystal dynamic conductivity split iolo a series of
45
...:'" ...:"
X:}
• ,..'-' " ....
S<, 4O
1 0 - 2 1 ................................."'""..
Y'"""'"" '"""'"............"....
k
10 , I
30
..
v2(TmFeO 3)
7
v2( {FeC, ~1
E(o 2o
>X -X X - X-XX (..) L, '/1 ( OyFeOs 1 x'" ×" z LU X/ ~*'6{~x-X~x--xx-~_x-x-x-X~x-x~xKzx-~ x x--x-x xgx.x-.., x "x :D O' | ~×...-.-UJ rr i, 10
09
'a.
O2 O.'1 OO
I
"W,~xXX
17 3
175
'177
179 O
• t" u (TmFe o ~)
[
I
IOO
2O0
_--J
F R E Q U E N C Y (cm I) TEMPERATURE Fig. 4. Results of measurements on the "'Epsilon'" spectrometer of T(v) and ~o(v) spectra of a 0.818 m m thick YFeO3 plate and tire IF(V) and I('(v) spectra calculated by them.
[K]
Fig. 5. T e m p e r a t u r e dependences of A FM R frcqucntics in T m F c O 3 , DyFeO,~ and Y F e O ~ (vL is thc lm~,frequency branch, v2 is high-frcquency branch I.
5OO
Dielectric measurements in the submillimeter region
373
high-quality absorption lines (Fig. 3). Such a rich spectrum of lattice vibrations at submillimeter wavelengths has not been observed previously in the crystals. We assume that the results on RbAg415 submillimeter spectra are of principal importance in understanding the nature of superionic conductivity in a solid.
Ant!]erromagnets. In these substances, as well as in ferroelectrics, there are soft modes. But in the given case they are connected with oscillations of not electric, but magnetic moments ofsublattices. For this reason, their manifestation should be expected in the #(v) spectra. Figure 4 presents the spectra of the YFeO3 antiferromagnet in the vicinity of a high-frequency branch of antiferromagnetic resonance (AFMR). ~1~ A distinguishing feature of the lines is their high quality, reaching in some cases the value of 103. At present, the observation of such narrow lines at submillimeter wavelengths is not possible by either FIR techniques or Raman spectroscopy. The YFeO3 magnetic spectra presented in Fig. 4 were calculated by the T(v) and ~p(v) spectra using the procedure described above. Figure 5 presents the temperature dependences obtained for soft-mode parameters in some orthoferrites. The possibility of reordering of magnetic moments in the lattices of these crystals results in a variety of phase transformations being observed in them. As a result, instabilities are observed in both the lower and the upper branches of AFMR (see Fig. 5). The examples presented do not cover all the advantages of dielectric measurements on BWO spectrometers but, we hope, give a general idea of them. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1 I. 12. 13. 14.
Brekhovskih L. M., Waves in Stratified Media. Izd. Akad. Nauk SSSR, Moscow (1957). Volkov A. A., Kozlov G. V. and Lebedev S. P., Radiotekh. Electron. 24, 1405 (19791. Volkov A. A., Goncharov Yu. G., Kozlov G. V., Lebedev S. P. and Maltsev V. 1., Prib. tekh. eksp. 2, 236 (1984~. Kozlov G. V., Lebedev S. P., Prokhorov A. M. and Volkov A. A., Ferroelectrics 25, 531 (1980). Volkov A. A., lshibashi I., Kozlov G. V., Lebedev S. P., Petzelt J. and Prokhorov A. M., J. phys. Soc. Japan (Suppl. B) 49, 78 (1980). Petzelt J., Kozlov G. V., Volkov A. A. and lshibashi Y., Z. Phys. B33, 369 (1979). Voklov A. A., Kozlov G. V., Lebedev S. P., Petzelt J. and Ishibashi Y., Ferroelectric.s 45, 157 (1982). Kozlov G. V., Volkov A. A. and Lebedev S. P., Usp. fiz. Nauk 135, 515 (1981). Kozlov G. V., Volkov A. A., Scott J. F., Feldkamp G. E. and Petzelt J., Phys. Rev. B28, 255 (1983). Kozlov G. V., lzr. Akad. Nauk SSSR, Ser. fiz. 47, 587 (1983). Volkov A. A., Goncharov Yu. G., Kozlov G. V., Lebedev S. P., Prokhorov A. M., Aliev R. A. and Allakhverdiev K. R., J E T P Lett. 37, 517 (1983). Volkov A. A., Goncharov Yu. G., Kozlov G. V., Allakhverdiev K. R. and Sardarly R. M., Fizika trerd. Tela 25, 3583 ( 1983 ). Volkov A. A., Kozlov G. V., Mirzoev G. I. and Goffman V. G., J E T P Lett. 38, 182 (1983). Volkov A. A., Goncharov Yu. G., Kozlov G. V., Kocharyan K. N., Lebedev S. P., Prokhorov A. S. and Prokhorov A. M., J E T P Lett. 39, 140 (1984).