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Observation and polaronic nature of superconductivity in new metallic oxides
Physica C 153-155 (1988) 1517-1518 North-Holland, Amsterdam
OBSERVATION AND POLARONIC NATURE OF SUPERCONDUCTIVITY A.S.Alexandrov,
IN NEW METALLIC OXIDES
E.A. Protasov
Moscow Physical Engineering Institute, Moscow, U.S.S.R. The results of synthesis, measurements of the magnetic susceptibility, the upper critical field, X-Ray photoelectron spectroscopy, and the intrinsic Jcsephson tunneling in Sr-La-Cu-O, and Y-La-Cu-O are presented. An explanation of some of the anomalous properties of LBSO, LSCO and YBCO superconductors on the basis of the bipolaron theory of superconductivity proposed by Alexandrov and Ranninger /AR,1981/ is proposed. La2_xSrxCuO 4, YBa2Cu307, Y2_xBaxCuO 4 superconducting ceramics as well as Y2CuO 4 have been prepared as prescribed in literature /I/. Thin (0.1-1rm) YBCO and LSCO films on SrTiOa-substrates have been also obtained~by the laser evaporation, on which sharp (2K wide) transitions with Tc above 90 K(YBCO) and ~igh Gritical currents of over 5-10~A/cm e (at 77K) have been achieved. The value and temperature behavior of the normal magnetic susceptibility ~(T) of YBCO is similar to those of A-15 compounds. On the other hand, LSCO has a much lower value of ~, which is temperature independent. All samples of LSCO show anomalous non-liner temperature dependence of the upper critical field Hc2(T) with d2Hc2/dT 2 > 0 near Tc. T h e ~rRay photoelectron spectroscopy shows Cu +, Cu e+ and Cu ~+ ions on the grains' surface as well as the presence of OH groups, causing a shift in the La3d, Y3d and Cu2p lines. Etching by At+ and 0 + ions with the subsequent heating up to 300°C in a vacuum permited to determine valence states inside ceramic grains. The shape of the Cu2P3/2 line in ceramic with good superconducting properties corresponds to the composition of the lines, ch~-racteristic for Cu2+(E=932.6eV) and Cu~+(E=933.6eV). The maximum E=917eV pecular for Cu+ is not observed in the spectrum. The critical current measurements of the ceramic specimens with high resistivity demonstrate the existance of the nonregular system of Josephson contacts on the grains' boundaries. It is well known that in d- and f-bands metallic oxides with sufficiently narrow electronic bands - D ~ IeV,D is a half bandwidth, the polarcn narrowing 0921-4534/88/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
of the band and corresponding mass enhancement occur: W = Dexp(-g2), m*/m = exp(g 2)
(I)
where g is a dimensionless electron-phonon coupling constant of the Frolich hamiltonian. As the many-polaron theory shows the taking into account of the polaron effect qualitatively changes the nature of the superconducting state: in the intermediate coupling region ~ = I the ordinary BSC superconductivity is replaced by polaron /2/ and bipolaron superconductivity in the strong coupling limit ~ I /3/. Here = 2z~g 2 D is a characteristic phonon frequency, and z is the nearest neighbours' numbe r. As a result instead of a monotonous rise of Tc with the increase of the constant of the electron-phonon coupling, predicted by the BCS theory, generalized for the case of strong coupling, the many-polaron theory predicts a rather narrow maximum of the dependence of T c ( ~ ) /2/.(Fig. l). The following anomalous properties of LSCO, LBCO, YBCO are easily explained by AR-thecry /4/: I. Assuming the bipolaron mass m** = 100m, their concentration n=1022 cm -3 and using Tc=3.3n2/3/m ** /3/ we have Tc = 130K. 2. With the same values of m** and n we obtain an enormously large magnetic field penetration depth ~ = 3 0 0 0 A °. 3. The high values of the normal resistivity ( p ~ 1 0 0 ~ . c m ) , of the thermoelectric power ( ~ = 20 ~v/K),
1518
A.S. Alexandrov and E.A. Protasov
and of the normal electron heat capacity ( ~/~>30 mJ ) are also commolK 2 patible with the picture of the heavy small polarcns (m*>~3Om) or small bipolarons /3/. 4. The temperature dependences of ~ ( T ) and of Hc2(T) suggest that lanthanium ceramics are bipolaronic supermonductors 6k>_2) while yttrium ones are polaronic (~.~-I). It is worth while to notice that in the polaronic superconductors the isotope effect tends to zero near the maximum on the curve of the dependence of Tc( ~ ). 5. As was shown in /5/ the anomalous pressure dependence of Tc of the lanthanium and yttrium ceramics can be explained in the framework of the bipolaronic superconductivity /3/. 6. The heat capacity Jump per one currier in YBCO
KB
,C, .v_ n
(2)
where K B is a Boltzmann constant. Eq(2) indicates the nonadiabatic character of the electrons (holes) motion (E=<60 ) and seems to present a direct proof of the exlstance of the extremely narrow electron band Eq(1). 7. As was mentioned by N.Mott /6/ the observation of the Anderson localization effect, which is unlikely unless the effective mass of the carriers is enhanced, a layer structure as well as mixed valence states of Cu also favour the bipolaron picture of the new high-temperature superconductors.
"1'~.K
Yiii0
9O
60
3O
t
2
a
k
/ Observation and polaronic nature FIGURE 1 Dependence of the critical temperature on the electron-phonon coupling constant. The shading shows the area of pclaron and bipolaron superconductivity. The dotted line corresponds to the BCS theory, generalized for the strong coupling case. REFERENCES (I) J.G.Bednorz and K.A.Muller~ Z. Phys. B64, 189(1986); M.K.Wu et al. Phys.Rev.Lett. 58, 908 (1987). (2) A.S.Alexandrov, Journal of Physical Chemistry, v.57, p.273 (1983). (3) A.Alexandrov, J.Ranninger, Phys. Rev. B 23, p. 1796, 1981; A.S. Alexandrov, J.Ranninger, S.Robaszkiewicz, Bhys.Rev. B 33, P. 4526 (1986). (4) A.S.Alexandrov~ JETP Letters, v.46, addendum (1987) (5) A.Driessen, R. Griessen, N.Koeman, E.Salomons, R.Bronwer, D.G.de Groot, K.Heeck and J.Rector, preprint, (1987). (6) N.Mott, Nature v.327, p. 185(1987).
OBSERVATION AND POLARONIC NATURE OF SUPERCONDUCTIVITY A.S.Alexandrov,
IN NEW METALLIC OXIDES
E.A. Protasov
Moscow Physical Engineering Institute, Moscow, U.S.S.R. The results of synthesis, measurements of the magnetic susceptibility, the upper critical field, X-Ray photoelectron spectroscopy, and the intrinsic Jcsephson tunneling in Sr-La-Cu-O, and Y-La-Cu-O are presented. An explanation of some of the anomalous properties of LBSO, LSCO and YBCO superconductors on the basis of the bipolaron theory of superconductivity proposed by Alexandrov and Ranninger /AR,1981/ is proposed. La2_xSrxCuO 4, YBa2Cu307, Y2_xBaxCuO 4 superconducting ceramics as well as Y2CuO 4 have been prepared as prescribed in literature /I/. Thin (0.1-1rm) YBCO and LSCO films on SrTiOa-substrates have been also obtained~by the laser evaporation, on which sharp (2K wide) transitions with Tc above 90 K(YBCO) and ~igh Gritical currents of over 5-10~A/cm e (at 77K) have been achieved. The value and temperature behavior of the normal magnetic susceptibility ~(T) of YBCO is similar to those of A-15 compounds. On the other hand, LSCO has a much lower value of ~, which is temperature independent. All samples of LSCO show anomalous non-liner temperature dependence of the upper critical field Hc2(T) with d2Hc2/dT 2 > 0 near Tc. T h e ~rRay photoelectron spectroscopy shows Cu +, Cu e+ and Cu ~+ ions on the grains' surface as well as the presence of OH groups, causing a shift in the La3d, Y3d and Cu2p lines. Etching by At+ and 0 + ions with the subsequent heating up to 300°C in a vacuum permited to determine valence states inside ceramic grains. The shape of the Cu2P3/2 line in ceramic with good superconducting properties corresponds to the composition of the lines, ch~-racteristic for Cu2+(E=932.6eV) and Cu~+(E=933.6eV). The maximum E=917eV pecular for Cu+ is not observed in the spectrum. The critical current measurements of the ceramic specimens with high resistivity demonstrate the existance of the nonregular system of Josephson contacts on the grains' boundaries. It is well known that in d- and f-bands metallic oxides with sufficiently narrow electronic bands - D ~ IeV,D is a half bandwidth, the polarcn narrowing 0921-4534/88/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
of the band and corresponding mass enhancement occur: W = Dexp(-g2), m*/m = exp(g 2)
(I)
where g is a dimensionless electron-phonon coupling constant of the Frolich hamiltonian. As the many-polaron theory shows the taking into account of the polaron effect qualitatively changes the nature of the superconducting state: in the intermediate coupling region ~ = I the ordinary BSC superconductivity is replaced by polaron /2/ and bipolaron superconductivity in the strong coupling limit ~ I /3/. Here = 2z~g 2 D is a characteristic phonon frequency, and z is the nearest neighbours' numbe r. As a result instead of a monotonous rise of Tc with the increase of the constant of the electron-phonon coupling, predicted by the BCS theory, generalized for the case of strong coupling, the many-polaron theory predicts a rather narrow maximum of the dependence of T c ( ~ ) /2/.(Fig. l). The following anomalous properties of LSCO, LBCO, YBCO are easily explained by AR-thecry /4/: I. Assuming the bipolaron mass m** = 100m, their concentration n=1022 cm -3 and using Tc=3.3n2/3/m ** /3/ we have Tc = 130K. 2. With the same values of m** and n we obtain an enormously large magnetic field penetration depth ~ = 3 0 0 0 A °. 3. The high values of the normal resistivity ( p ~ 1 0 0 ~ . c m ) , of the thermoelectric power ( ~ = 20 ~v/K),
1518
A.S. Alexandrov and E.A. Protasov
and of the normal electron heat capacity ( ~/~>30 mJ ) are also commolK 2 patible with the picture of the heavy small polarcns (m*>~3Om) or small bipolarons /3/. 4. The temperature dependences of ~ ( T ) and of Hc2(T) suggest that lanthanium ceramics are bipolaronic supermonductors 6k>_2) while yttrium ones are polaronic (~.~-I). It is worth while to notice that in the polaronic superconductors the isotope effect tends to zero near the maximum on the curve of the dependence of Tc( ~ ). 5. As was shown in /5/ the anomalous pressure dependence of Tc of the lanthanium and yttrium ceramics can be explained in the framework of the bipolaronic superconductivity /3/. 6. The heat capacity Jump per one currier in YBCO
KB
,C, .v_ n
(2)
where K B is a Boltzmann constant. Eq(2) indicates the nonadiabatic character of the electrons (holes) motion (E=<60 ) and seems to present a direct proof of the exlstance of the extremely narrow electron band Eq(1). 7. As was mentioned by N.Mott /6/ the observation of the Anderson localization effect, which is unlikely unless the effective mass of the carriers is enhanced, a layer structure as well as mixed valence states of Cu also favour the bipolaron picture of the new high-temperature superconductors.
"1'~.K
Yiii0
9O
60
3O
t
2
a
k
/ Observation and polaronic nature FIGURE 1 Dependence of the critical temperature on the electron-phonon coupling constant. The shading shows the area of pclaron and bipolaron superconductivity. The dotted line corresponds to the BCS theory, generalized for the strong coupling case. REFERENCES (I) J.G.Bednorz and K.A.Muller~ Z. Phys. B64, 189(1986); M.K.Wu et al. Phys.Rev.Lett. 58, 908 (1987). (2) A.S.Alexandrov, Journal of Physical Chemistry, v.57, p.273 (1983). (3) A.Alexandrov, J.Ranninger, Phys. Rev. B 23, p. 1796, 1981; A.S. Alexandrov, J.Ranninger, S.Robaszkiewicz, Bhys.Rev. B 33, P. 4526 (1986). (4) A.S.Alexandrov~ JETP Letters, v.46, addendum (1987) (5) A.Driessen, R. Griessen, N.Koeman, E.Salomons, R.Bronwer, D.G.de Groot, K.Heeck and J.Rector, preprint, (1987). (6) N.Mott, Nature v.327, p. 185(1987).