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Positron states in novel superconductors
Physica C 235-240 (1994) 2473-2474 North-Holland
PHYSICA
Positron States in Novel Superconductors Shoji Ishibashi, Norio Terada, Madoka Tokumoto, Masayuki Hirabayashi, Nobumori Kinoshita and Hideo Ihara Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan Positron distributions and lifetimes in novel superconductors and related materials have been calculated. Resuits are discussed in terms of Fermiology and defect spectroscopy.
1. I N T R O D U C T I O N
Fermiology is one of the most successful fields utilizing positron annihilation phenomena. Distributions of electrons in the momentum space are obtained by measuring angular correlation of annihilation radiation (ACAR) spectra. Fermi surface structures are observed as breaks in such a spectrum if positrons sample conduction electrons. In highly anisotropic systems, positrons are distributed 1- or 2-dimensionally and, sometimes, overlaps between positrons and conduction electrons are very small. In such a case, it is difficult to detect Fermi surface structures. Knowledge of the positron distribution is essential. It is also known that positrons are sensitive probes of open volumes in condensed matters. Positrons at different states annihilate with different lifetimes. Positron lifetime spectroscopy has been utilized for studies of crystalline defects, intercalations, phase transitions and so on. In the present work, positron distributions and lifetimes in LuNi2B2C [1], SrCuO2 [2], C60 compounds [3] and B E D T - T T F salts [4] are calculated by the superposed-atom model and the numerical relaxation technique [5I. Obtained results are
iitVuS~lg&b~u
iil
view
of
lm _
:
1
.
ol,u ,,Ni oB .C
0
2
a Figure 1. Positron distribution in LuNi2B~C. The contour spacing is 1/20 of the maximum value. The positron density is minimal at the atomic positions and higher in interstitial regions.
calculated lifetime is 120 ps.
~,,~ . . . .
spectroscopy. 2. R E S U L T S A N D D I S C U S S I O N S 2.1. L u N i 2 B 2 C The positron distribution is 3-dimensional as shown in Fig. 1 and there are significant overlaps between positrons and each atom. This is a favorable condition for Fermi-surface studies• The
Positrons have significant overlaps with Cu and O atoms in this material though the density maximum of the positron distribution is in the Sr plane. The corresponding lifetime is 149 ps. Calculations for trapped states of positrons suggest that positrons are deeply trapped at the Sr vacancy with the binding energy of 2.8 eV while the O vacancy does not act as a positron trapping center. The positron lifetime at the Sr wcancy
0921-4534D4/$07.00 © 1994 - ElsevierScience B.V. All rights reserved. SSD! 0921-4534(94)01802-2
2474
S. lshibashi et al./Physica C 235-240 (1994) 2473-2474
was calculated to be 238 ps. SrCuO2 becomes superconducting by introduction of Sr vacancies or substitution of !anthanoid atoms. Although the positron distribution in the bulk itself is favorable for Fermi surface studies, the positron trapping would make the Fermi-surface structure obscure in the former case. Positrons could be utilized, in this case, as a probe of Sr vacancies which are thought to supply Cu-O planes with holes [6]. 2.3. C60 c o m p o u n d s Solid C6o has a face-centered-cubic (fcc) structure and combines with alkali or a l l , line-earth metals (M) to form M=C60 compounds. The structure varies largely with M and x [3]. Calculated distributions and lifetimes of positrons are significantly different among these compounds [7,8]. As for superconducting K3C60 and Rb3C6o, it :s .~Av. . . . d that positrons sample conduction electrons well. This is a favorable condition for Fermiology though the existence of a number of occupied bands, which do not contribute Fermi surfaces at all, is an obstacle. Fermi-surfacerelated signals are supposed to be rather small. The calculations suggest also possibilities of following applications of the positron lifetime spectroscopy: (1) determination of a high-frequency dielectric constant ¢o0 in pure and lightly doped C60 (zoo is estimated to be .,- 3.5 [9]), (2) detection of M-site vacancies, (3) monitoring the phase transition in K1C60, RblC60 and CslC60 and (4) investigation of a site selectivity of alkali-metal vacancy in K3C¢0 and Rb3C60 between octahedral and tetrahedral sites. 2.4. B E D T - T T F salts BEDT-TTF salts have an alternating structure of anion layers and BEDT-TTF donors [4]. In these materials, positrons annihilate predominantly (more than 80 %) with electrons of the anions and the ethylene groups of the BEDT-TTF molecule [6,10]. The HOMO of the BEDT-TTF molecule, which contributes to the conductivity, consists mainly of the atomic orbitals of the T T F skeleton and the outer S atoms [11]. Positrons do not sample effectively electrons of conduction bands crossing the Fermi level. Furthermore, it is another difficulty that many occupied valence
bands exist for these materials. 3. S U M M A R Y In terms of Fermiology, positrons are expected to be good probes for LuNi2B2C aald SrCuO2 if defects do not exist. The condition is less favorable for MxC60 or BEDT-TTF salts, since the existence of many occupied valence bands makes Fermi-surface signals obscure. As for BEDT-TTF salts, positron distributions themselves are not favorable. Defect spectroscopies with positrons are promising for both SrCuO2 and C60 compounds. REFERENCES 1. T. Siegrist, H.W. Zandbergen, R.J. Cava, J.J. Krajewski and W.F. Peck Jr, Nature 367 (1994) 254. 2. Z. Hiroi, M. Azuma, M. Takano and Y. Bando, J. Solid State Chem. 95 (1991) 230. 3. H.W. Kroto, J.E. Fischer and D.E. Cox (eds.), The Fullerenes, Pergamon, Oxford, 1993. 4. J.M. Williams, J.R. Ferraro, R.J. Thorn, K.D. Carlson, U. Geiser, H.H. Wang, A.M. Kini and M.H. Whangbo, Organic Superconductors, Prentice-Hall, New Jersey, 1992. 5. M.J. Puska and R.M. Nieminen, J. Phys. F: Met. Phys. 13 (1983) 333. 6. S. Ishibashi, N. Tcrada, M. Hirabayashi, M. Tokumoto, N. Kinoshita and H. Ihara, J. Phys. Chem. Solids 54 (1993) 1247. 7. S. Ishibashi, N. Terada, M. Tokumoto, N. Kinoshita and H. Ihara, Fullerene Sci. Technol. 1 (1997~ 239. 8. S. Ishibashi, N. Terada, M. Tokumoto, N. Kinoshita and H. Ihara, J. Phys. IV, Colloq.
(France) 3(C4)(1993) 153 9. S. Ishibashi, N. TeraAa, M. Tokumoto, N. Kinoshita and H. Ihara, J. Phys.: Condens. Matter 4 (1992) L169. 10. S. Ishibashi, M. Tokumoto, N. Kinoshita, N. Terada and H. ihara, Solid State Commun. 85 (1993) 397. 11. T. Mori, A. Kobayashi, Y. Sasaki, H. Kobayashi, G. Saito and H. Inokuchi, Bull. Chem. Soc. Jpn. 57 (1984) 627.
PHYSICA
Positron States in Novel Superconductors Shoji Ishibashi, Norio Terada, Madoka Tokumoto, Masayuki Hirabayashi, Nobumori Kinoshita and Hideo Ihara Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan Positron distributions and lifetimes in novel superconductors and related materials have been calculated. Resuits are discussed in terms of Fermiology and defect spectroscopy.
1. I N T R O D U C T I O N
Fermiology is one of the most successful fields utilizing positron annihilation phenomena. Distributions of electrons in the momentum space are obtained by measuring angular correlation of annihilation radiation (ACAR) spectra. Fermi surface structures are observed as breaks in such a spectrum if positrons sample conduction electrons. In highly anisotropic systems, positrons are distributed 1- or 2-dimensionally and, sometimes, overlaps between positrons and conduction electrons are very small. In such a case, it is difficult to detect Fermi surface structures. Knowledge of the positron distribution is essential. It is also known that positrons are sensitive probes of open volumes in condensed matters. Positrons at different states annihilate with different lifetimes. Positron lifetime spectroscopy has been utilized for studies of crystalline defects, intercalations, phase transitions and so on. In the present work, positron distributions and lifetimes in LuNi2B2C [1], SrCuO2 [2], C60 compounds [3] and B E D T - T T F salts [4] are calculated by the superposed-atom model and the numerical relaxation technique [5I. Obtained results are
iitVuS~lg&b~u
iil
view
of
lm _
:
1
.
ol,u ,,Ni oB .C
0
2
a Figure 1. Positron distribution in LuNi2B~C. The contour spacing is 1/20 of the maximum value. The positron density is minimal at the atomic positions and higher in interstitial regions.
calculated lifetime is 120 ps.
~,,~ . . . .
spectroscopy. 2. R E S U L T S A N D D I S C U S S I O N S 2.1. L u N i 2 B 2 C The positron distribution is 3-dimensional as shown in Fig. 1 and there are significant overlaps between positrons and each atom. This is a favorable condition for Fermi-surface studies• The
Positrons have significant overlaps with Cu and O atoms in this material though the density maximum of the positron distribution is in the Sr plane. The corresponding lifetime is 149 ps. Calculations for trapped states of positrons suggest that positrons are deeply trapped at the Sr vacancy with the binding energy of 2.8 eV while the O vacancy does not act as a positron trapping center. The positron lifetime at the Sr wcancy
0921-4534D4/$07.00 © 1994 - ElsevierScience B.V. All rights reserved. SSD! 0921-4534(94)01802-2
2474
S. lshibashi et al./Physica C 235-240 (1994) 2473-2474
was calculated to be 238 ps. SrCuO2 becomes superconducting by introduction of Sr vacancies or substitution of !anthanoid atoms. Although the positron distribution in the bulk itself is favorable for Fermi surface studies, the positron trapping would make the Fermi-surface structure obscure in the former case. Positrons could be utilized, in this case, as a probe of Sr vacancies which are thought to supply Cu-O planes with holes [6]. 2.3. C60 c o m p o u n d s Solid C6o has a face-centered-cubic (fcc) structure and combines with alkali or a l l , line-earth metals (M) to form M=C60 compounds. The structure varies largely with M and x [3]. Calculated distributions and lifetimes of positrons are significantly different among these compounds [7,8]. As for superconducting K3C60 and Rb3C6o, it :s .~Av. . . . d that positrons sample conduction electrons well. This is a favorable condition for Fermiology though the existence of a number of occupied bands, which do not contribute Fermi surfaces at all, is an obstacle. Fermi-surfacerelated signals are supposed to be rather small. The calculations suggest also possibilities of following applications of the positron lifetime spectroscopy: (1) determination of a high-frequency dielectric constant ¢o0 in pure and lightly doped C60 (zoo is estimated to be .,- 3.5 [9]), (2) detection of M-site vacancies, (3) monitoring the phase transition in K1C60, RblC60 and CslC60 and (4) investigation of a site selectivity of alkali-metal vacancy in K3C¢0 and Rb3C60 between octahedral and tetrahedral sites. 2.4. B E D T - T T F salts BEDT-TTF salts have an alternating structure of anion layers and BEDT-TTF donors [4]. In these materials, positrons annihilate predominantly (more than 80 %) with electrons of the anions and the ethylene groups of the BEDT-TTF molecule [6,10]. The HOMO of the BEDT-TTF molecule, which contributes to the conductivity, consists mainly of the atomic orbitals of the T T F skeleton and the outer S atoms [11]. Positrons do not sample effectively electrons of conduction bands crossing the Fermi level. Furthermore, it is another difficulty that many occupied valence
bands exist for these materials. 3. S U M M A R Y In terms of Fermiology, positrons are expected to be good probes for LuNi2B2C aald SrCuO2 if defects do not exist. The condition is less favorable for MxC60 or BEDT-TTF salts, since the existence of many occupied valence bands makes Fermi-surface signals obscure. As for BEDT-TTF salts, positron distributions themselves are not favorable. Defect spectroscopies with positrons are promising for both SrCuO2 and C60 compounds. REFERENCES 1. T. Siegrist, H.W. Zandbergen, R.J. Cava, J.J. Krajewski and W.F. Peck Jr, Nature 367 (1994) 254. 2. Z. Hiroi, M. Azuma, M. Takano and Y. Bando, J. Solid State Chem. 95 (1991) 230. 3. H.W. Kroto, J.E. Fischer and D.E. Cox (eds.), The Fullerenes, Pergamon, Oxford, 1993. 4. J.M. Williams, J.R. Ferraro, R.J. Thorn, K.D. Carlson, U. Geiser, H.H. Wang, A.M. Kini and M.H. Whangbo, Organic Superconductors, Prentice-Hall, New Jersey, 1992. 5. M.J. Puska and R.M. Nieminen, J. Phys. F: Met. Phys. 13 (1983) 333. 6. S. Ishibashi, N. Tcrada, M. Hirabayashi, M. Tokumoto, N. Kinoshita and H. Ihara, J. Phys. Chem. Solids 54 (1993) 1247. 7. S. Ishibashi, N. Terada, M. Tokumoto, N. Kinoshita and H. Ihara, Fullerene Sci. Technol. 1 (1997~ 239. 8. S. Ishibashi, N. Terada, M. Tokumoto, N. Kinoshita and H. Ihara, J. Phys. IV, Colloq.
(France) 3(C4)(1993) 153 9. S. Ishibashi, N. TeraAa, M. Tokumoto, N. Kinoshita and H. Ihara, J. Phys.: Condens. Matter 4 (1992) L169. 10. S. Ishibashi, M. Tokumoto, N. Kinoshita, N. Terada and H. ihara, Solid State Commun. 85 (1993) 397. 11. T. Mori, A. Kobayashi, Y. Sasaki, H. Kobayashi, G. Saito and H. Inokuchi, Bull. Chem. Soc. Jpn. 57 (1984) 627.