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Ac losses and submillimeter absorption in single crystals La2CuO4
Physica B 165&166 (1990) 1269-1270 North-Holland
AC LOSSES AND SUBMILLIMETER ABSORPTION IN SINGLE CRYSTALS La CuO 2
+
4
+
++S. V. Var~ukhin,+l' A. Zakharov,++ ++ E.M.Gershenzon, G.N.Gol tsman, N.G.Ptitsyna, G.M.Chulkova +
++Kurchatov Atomic Energy Institute, 123182, Moscow, USSR Moscow State Pedagogical Institute, 119435, Moscow, USSR The La2Cu04 single crystals were used to carry out the measurements of transmission spectra within the submillimeter range of wavelengths, as well as the capacitance C and conductivity G in the region of acoustic frequencies of the metal-dielectric-La2Cu04 system at low temperatures. The optical spectra display a threshold character. There takes place a sharp decreasing of transmission signal in the energy range of hv>1.5meV. The C(w,T) and G(w,T) dependences have a universal form characteristic of relaxation processes of the Debye type. The relaxat ion time dependence displays a thermoact i vation character T(T)-exp(E/T) with a gap value of E~2meV,coinciding with the optical one. It is assumed that there exist excitations with a characteristic energy - 2meV in La2Cu04.A possible nature of the revealed low-energy excitations is discussed.
1. I NTRODUCTI ON Despite the intensive experimental and theoret ical studies of the high temperature superconductors, the reasons for the high Te values in them remain so far not clear. The experimental investigation of the electronic structure, phonon spectra does not reveal any principal features distinguishing these compounds from the known superconductors. This suggests that some specific features, the energy scale of which is hard to reach for a great number of experimental methods, may be responsible for the superconductivity of cuprates. For example, the energy gap of the 1-2meV value was discovered in the magnet ic excitation spectrum of La2Cu04 [l,21.The dispersion of the La2Cu04 dielectric permeability in the frequency region of v-40cm -1, discussed in ref. [3], also gives evidence of the presence of low-energy excitation in the system. In this situation, the investigation methods allowing such low-energy singularities to be studied acquire particular importance. Among the experiments of this type are investigations of the electromagnetic radiation interaction with a sample in the corresponding wavelength range and low-frequency impedance at low temperature. 2. EXPERIMENT The experiments have been performed on samples cut off from the same single crystal block.To measure the low frequency impedance, a plate was cut off in the ab plane w~ th characteristic dimensions of 6x5xO.5mm ,on both the sides of which a dielectric film of Al203 (-.02Mm) was sputtered. The In films
0921-4526/90/$03.50
© 1990 -
-0.3Mm were evaporated onto the dielectric.By using the obtained sandwich, being a plane capacitor, the C and G measurements have been taken within the frequency interval of 200HzlOcm -1, C
o
..
10 , - - - - - - - - - - - - - - - -
.
.' .
o
.iii UJ
.a
.\....
..to"'.to ..
E UJ
C
...." a
-2
-..t"
~10
.'.
f-
.
.....
-4 10 -I----~~----++-----~
3
10
30
Wove number, cm- 1
Fig. 1. Dependence of the transmission factor on the wave number.
Elsevier Science Publishers B.V. (North-Holland)
S. V. Varyukhin et aI.
1270
there takes place a sharp decrease of transmission signal. The threshold character of the spectrum suggests the presence of' a gap of the scale 15K in the energy spectrum. Fig.2 presents the results of' measuring the temperature dependences for the capacitance C and conductivity G at three frequencies for a parallel equivalent circuit of C-G connection. In the temperature range T<10K, an appreciable decrease of the capacitance C is observed the value of which depends on the frequency. The conductivity G rises considerably with increasing frequency. At that,in the G(T) dependences one observes the temperature maximum,the position of which is shifted to the range of' higher temperatures. In this case, the medium is characterized by the complex dielectric permeability C=C1-iC2 connected with the measured C and G values by the relations: c1=Cd/coS, c2=Gd/coSw, where d is the thickness, S is the area of plates. In our case, the complex dielectric permeability C=C1-;C2 is described by the Debye f'ormulae c1=co:I-~c/[1+(WT1], C2=WT~c/[1+(WTr], where ~c=cs-ccq cs is the static permeability, £oois the permeability at high frequencies. According to the Debye formulae, the £2 maximum is observed at wT=1. The maximum appearance is assigned to the T(T). The T(T) dependence can be determined f'rom the wT(T)=1 condition at the G(T) maximum. In f'ig.3 logw=f'(1/Tmax) 1 10 u.. c
10
a
u -1
10
-2
10
10
30
10
3 5 y","" "'"
0
00
0
0
8366 Hz
",,2
3
0>
o
0
'" ",
"-~
"-
-- --..,
-2 + - - - - - - - + - - - - - - + - " ' - - - ' 0.1 0.2 0.3 liT, K- 1
Fig.3.The position of the maximum G(T) versus the magnitude of' the measurement frequency W in the Logw coordinates on 1/Tmax. is plotted where Tmax is the G(T) maximum temperature. One can see that all the points fall within a straight line. Hence,we obtain for the characteristic relaxation time: T_(l) =Toexp (~/T) where To=5.6·10 s, ~=24K. The obtained ~ value is close to the threshold value of' hv, determined f'rom the optical experiments, which, apparent ly, gives evidence of' the influence of one and the same mechanism on the processes of electromagnetic radiation absorption and polarization in the quasistationary electric f'ield. In ref'. [2],the excitations in the spin system of' La2Cu04 were discovered wi th the energy gap characteristic energies of 1-2meV. This coincides with the perf'ormed calculation of ant i f'erromagnet i c resonance frequenc i es [4]. On condition of considerable electronmagnon interaction, the magnetic excitations may give rise to the singularities discovered in the present work. In studying the low-frequency impedance, the system response to the applied quasistatic electric field is investigated. At that, the electric f'ield interaction with the characteristic magnetic excitations will have an indirect character, which should result in large values of' To.
0
(f)
c
0
4
o..
10
0
'g:acDDDtltltltlcCa
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3
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10
.. 0
0
°
1635
D
0
CJ
o
0
00
..
214
,,"11:1 Co
2
10
a
l
0 0
3
10
T. K
Fig.2. Dependences of the C(T) and G(T) f'or the three measurement frequencies.
30
REFERENCES 1. Collins R.T. ,Schlesinger Z. ,Shafer M.W., McGuir T.R. Phys. Rev. B37, 1988,p.5817 . 2. Peters C.J. ,Birgeneau R.J. ,Kastner M.A., Yoshizawa H. ,Endoh Y. ,Tranquada J. ,Shirane G. Hidaka Y. ,ada M. ,Suzuki M. ,Murakami T. Phys. Rev. B37, 1988, no. 16, p.9761. 3. Reagor D. ,Ahrens E. ,Cheong S.-W. ,Migliori A. ,Fisk Z. Phys. Rev. Lett. 62, 1989, p.2048. 4. Zvezdin A.K. ,Zvezdin S.A. ,Mukhin A.A, Sinitsky A. V. Preprint of' IGP of the AS no.44, Moscow, 1989 (in Russian).
AC LOSSES AND SUBMILLIMETER ABSORPTION IN SINGLE CRYSTALS La CuO 2
+
4
+
++S. V. Var~ukhin,+l' A. Zakharov,++ ++ E.M.Gershenzon, G.N.Gol tsman, N.G.Ptitsyna, G.M.Chulkova +
++Kurchatov Atomic Energy Institute, 123182, Moscow, USSR Moscow State Pedagogical Institute, 119435, Moscow, USSR The La2Cu04 single crystals were used to carry out the measurements of transmission spectra within the submillimeter range of wavelengths, as well as the capacitance C and conductivity G in the region of acoustic frequencies of the metal-dielectric-La2Cu04 system at low temperatures. The optical spectra display a threshold character. There takes place a sharp decreasing of transmission signal in the energy range of hv>1.5meV. The C(w,T) and G(w,T) dependences have a universal form characteristic of relaxation processes of the Debye type. The relaxat ion time dependence displays a thermoact i vation character T(T)-exp(E/T) with a gap value of E~2meV,coinciding with the optical one. It is assumed that there exist excitations with a characteristic energy - 2meV in La2Cu04.A possible nature of the revealed low-energy excitations is discussed.
1. I NTRODUCTI ON Despite the intensive experimental and theoret ical studies of the high temperature superconductors, the reasons for the high Te values in them remain so far not clear. The experimental investigation of the electronic structure, phonon spectra does not reveal any principal features distinguishing these compounds from the known superconductors. This suggests that some specific features, the energy scale of which is hard to reach for a great number of experimental methods, may be responsible for the superconductivity of cuprates. For example, the energy gap of the 1-2meV value was discovered in the magnet ic excitation spectrum of La2Cu04 [l,21.The dispersion of the La2Cu04 dielectric permeability in the frequency region of v-40cm -1, discussed in ref. [3], also gives evidence of the presence of low-energy excitation in the system. In this situation, the investigation methods allowing such low-energy singularities to be studied acquire particular importance. Among the experiments of this type are investigations of the electromagnetic radiation interaction with a sample in the corresponding wavelength range and low-frequency impedance at low temperature. 2. EXPERIMENT The experiments have been performed on samples cut off from the same single crystal block.To measure the low frequency impedance, a plate was cut off in the ab plane w~ th characteristic dimensions of 6x5xO.5mm ,on both the sides of which a dielectric film of Al203 (-.02Mm) was sputtered. The In films
0921-4526/90/$03.50
© 1990 -
-0.3Mm were evaporated onto the dielectric.By using the obtained sandwich, being a plane capacitor, the C and G measurements have been taken within the frequency interval of 200Hz
o
..
10 , - - - - - - - - - - - - - - - -
.
.' .
o
.iii UJ
.a
.\....
..to"'.to ..
E UJ
C
...." a
-2
-..t"
~10
.'.
f-
.
.....
-4 10 -I----~~----++-----~
3
10
30
Wove number, cm- 1
Fig. 1. Dependence of the transmission factor on the wave number.
Elsevier Science Publishers B.V. (North-Holland)
S. V. Varyukhin et aI.
1270
there takes place a sharp decrease of transmission signal. The threshold character of the spectrum suggests the presence of' a gap of the scale 15K in the energy spectrum. Fig.2 presents the results of' measuring the temperature dependences for the capacitance C and conductivity G at three frequencies for a parallel equivalent circuit of C-G connection. In the temperature range T<10K, an appreciable decrease of the capacitance C is observed the value of which depends on the frequency. The conductivity G rises considerably with increasing frequency. At that,in the G(T) dependences one observes the temperature maximum,the position of which is shifted to the range of' higher temperatures. In this case, the medium is characterized by the complex dielectric permeability C=C1-iC2 connected with the measured C and G values by the relations: c1=Cd/coS, c2=Gd/coSw, where d is the thickness, S is the area of plates. In our case, the complex dielectric permeability C=C1-;C2 is described by the Debye f'ormulae c1=co:I-~c/[1+(WT1], C2=WT~c/[1+(WTr], where ~c=cs-ccq cs is the static permeability, £oois the permeability at high frequencies. According to the Debye formulae, the £2 maximum is observed at wT=1. The maximum appearance is assigned to the T(T). The T(T) dependence can be determined f'rom the wT(T)=1 condition at the G(T) maximum. In f'ig.3 logw=f'(1/Tmax) 1 10 u.. c
10
a
u -1
10
-2
10
10
30
10
3 5 y","" "'"
0
00
0
0
8366 Hz
",,2
3
0>
o
0
'" ",
"-~
"-
-- --..,
-2 + - - - - - - - + - - - - - - + - " ' - - - ' 0.1 0.2 0.3 liT, K- 1
Fig.3.The position of the maximum G(T) versus the magnitude of' the measurement frequency W in the Logw coordinates on 1/Tmax. is plotted where Tmax is the G(T) maximum temperature. One can see that all the points fall within a straight line. Hence,we obtain for the characteristic relaxation time: T_(l) =Toexp (~/T) where To=5.6·10 s, ~=24K. The obtained ~ value is close to the threshold value of' hv, determined f'rom the optical experiments, which, apparent ly, gives evidence of' the influence of one and the same mechanism on the processes of electromagnetic radiation absorption and polarization in the quasistationary electric f'ield. In ref'. [2],the excitations in the spin system of' La2Cu04 were discovered wi th the energy gap characteristic energies of 1-2meV. This coincides with the perf'ormed calculation of ant i f'erromagnet i c resonance frequenc i es [4]. On condition of considerable electronmagnon interaction, the magnetic excitations may give rise to the singularities discovered in the present work. In studying the low-frequency impedance, the system response to the applied quasistatic electric field is investigated. At that, the electric f'ield interaction with the characteristic magnetic excitations will have an indirect character, which should result in large values of' To.
0
(f)
c
0
4
o..
10
0
'g:acDDDtltltltlcCa
"DO
3
.. aD ocPCJCao00"o .. DO °00
10
.. 0
0
°
1635
D
0
CJ
o
0
00
..
214
,,"11:1 Co
2
10
a
l
0 0
3
10
T. K
Fig.2. Dependences of the C(T) and G(T) f'or the three measurement frequencies.
30
REFERENCES 1. Collins R.T. ,Schlesinger Z. ,Shafer M.W., McGuir T.R. Phys. Rev. B37, 1988,p.5817 . 2. Peters C.J. ,Birgeneau R.J. ,Kastner M.A., Yoshizawa H. ,Endoh Y. ,Tranquada J. ,Shirane G. Hidaka Y. ,ada M. ,Suzuki M. ,Murakami T. Phys. Rev. B37, 1988, no. 16, p.9761. 3. Reagor D. ,Ahrens E. ,Cheong S.-W. ,Migliori A. ,Fisk Z. Phys. Rev. Lett. 62, 1989, p.2048. 4. Zvezdin A.K. ,Zvezdin S.A. ,Mukhin A.A, Sinitsky A. V. Preprint of' IGP of the AS no.44, Moscow, 1989 (in Russian).