Evidence for phonon-mediated coupling in superconducting Ba1−xKxBiO3

Physica C 162-164 (1989) 1405-1408 North-Holland

EVIDENCE FOR PHONON-MEDIATED COUPLING IN SUPERCONDUCTING Ba 1.xKxBiO3* D.G. HINKS, B. DABROWSKI, D.R. RICHARDS, J.D. JORGENSEN, SHIYOU PEI AND J.F. ZASADZINSKI** Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 Bal.,xK~BiO3, with a T c of 30K, shows a large 180 isotope effect which indicates that S • P n meepaJdng r meclianism. c o Superconducting n d u energy c gapt measurements i n g from IR nons areo involveo reflectivity and tunneling are consistent with moderate coupling (2Z~Tc = 3.5 + 0.5). A characteristic pnonon enemy of abou! 40 m.eV would be required to obtain the high Tc. Neutron scattering measurements snow a large aensity ot pnonons in the range 40 to 80 meV and strong coupling of electrons to these modes is indicated in tunneling spectroscopy. Additional results are reported, including the structural phase diagram, which suggest that superconductivity is phonon mediated. INTRODUCTION The discovery of superconductivity near 30K in Ba 1_xKxBi03 has raised important questions about the mechanism responsible for the high transition temperature. 1-3 While these compounds share a number of properties with the copper oxides, they are dissimilar in that the superconducting phase is cubic rather than layered in structure and no magnetism is found. The low carder density as well as early indications of w e a k - c o u p l e d superconductivity have led to the speculation that a non-phononic mechanism is responsible for the high Tc.4 We report here the results of a comprehensive study of Ba 1.xKxBi03 including the structural phase diagram, isotope effect, IR reflectance and tunneling spectroscopy. The results are consistent with phonon-mediated superconductivity with high-frequency optical phonons playing an important role. 1. EXPERIMENTAL RESULTS 1.1 Structure There is clearly a relationship between superconductivity and structure in Ba 1.xKxBiO3.5 It was shown initially by Cava et al and confirmed by others that the superconducting phase (x=0.37 to

0.5) is simple cubic, with the ideal perovskite structure. As the K concentration is reduced, electrons are added to the valence band where half-filling occurs for the parent compound BaBiO3. This material would be metallic according to band theory, but a strong electron-phonon interaction (involving Bi-O stretching modes) causes a Peierls distortion of the lattice, doubling the unit cell and opening an energy gap in the electronic density of states. 6 This results in insulating behavior. Details of the structures for the entire composition range (x=0.0 to 0.5) have been investigated by neutron powder diffraction and the phase diagram is shown in figure 1. The end member phase BaBiO 3 has a body-centered monoclinic structure with space group symmetry, 12/m, at 300K. This structure is derived from the cubic perovskite by a BiO6 rigid octahedral tilt (along the cubic [110] axis) and a frozen-out, symmetric breathing mode distortion of the oxygen atoms. The breathing mode distortion creates two distinct Bi sites which can be thought of as a bismuth charge disproportionation or alternately as a commensurate charge density wave (CDW). The commensurate CDW and the insulating behavior associated with the CDW gap persists up to x=0.t

* Work supported by the U.S. Department of Energy, BES-Materials Sciences, under contract #W-31-109-ENG-38. ** Illinois Institute of Technology, Physics Department, Chicago, IL 60616 092 I--4534/89/$03.50 © Elsevier Science Publishers B.V. ( North-Holland )

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For concentrations between 0.1 and 0.37 there is no evidence of the breathing-mode distortion but the B i d 6 tilt remains leading to an orthorhombic structure with symmetry group, Ibmm. The loss of superconductivity for x<0.37 is therefore not due to the commensurate CDW associated with breathing mode distortions. However, electronic transport measurements in the range x=0.1 to 0.37 display semiconducting behavior and optical reflectance measurements show evidence of an energy gap at the Fermi level as well as free carrier effects. 7 In addition to the commensurate structural modulation associated with the octahedral rotation, an incommensurate modulation has been observed in electron diffraction8 which may be the same breathing mode distortion as in BaBiO 3 but on a local scale, i.e., a local CDW. Hewat et a19 confirmed the existence of the modulation but concluded that it resulted from electron-beam heating during the measurement. We have measured the modulation wavelength as a function of K concentration and this is shown in the inset of figure 1. The systematic composition dependence strongly suggests that the incommensurate modulation is an intrinsic property. The structural distortions of the oxygen octahedra as x is reduced below 0.37 indicates that the electronic system in

the cubic phase is strongly coupled to particular oxygen phonon modes. This suggests that these same oxygen modes are responsible for the superconductivity. 1.2 Isotope Effect For a conventional monatomic (BCS) superconductor, T c = M -c¢ , where M is the mass of the atom and 0¢ = 0.5. For a multicomponent material, T c = Mi-O.i, with an a i defined for each ion with mass M i, all of which sum to 0.5. Coulomb repulsion can reduce the value of (z, but enhancements are not observed. Thus, a value of o¢ less than but close to the BCS value indicates conventional superconductivity. Measurement of the isotope effect in this material is made more difficult by the fact that K loss during the exchange procedure can also affect T c. For this reason we used concentrations near 0.375 (maximum Tc) so that any K loss would not change T c but only the volume fraction of the superconducting phase. 10 Figure 2 shows the resistive transition for a series of samples with composition x=0.375. Details of the processing steps are given in reference 10. Sample A was processed with O 16 while samples B and C were initially processed with 016 and then with 1 and 2 cycles respecively of 018. Sample D 1.2 1.0 0.8

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D.G. Hinks et al. / Evidence for phonon-mediated coupling was processed initially with 0 1 6 followed by two cycles of 0 1 8 and finally two cycles of 0 1 6 demonstrating that the initial T c was recovered. The fraction of 018 exchange was measured by TGA and found to be 96 ± 3%. With our observed ATc of 1.35 + 0.05K, ¢ is found to be 0.41 :L0.03 which strongly indicates phonon mediated paidng. Calculations of the isotope effect using a molecular dynamics simulation 11 of the phonon density of states have led to a value of ~ = 0.41 in the weak coupled limit. For moderate coupling ( ;L = 1.0) and an ~2F(¢o) which is weighted toward the high frequency end as suggested by tunneling spectroscopy (discussed later), a value of cz = 0.37 is obtained. Two other measurements of the isotope effect have been reported: Cava et al 4. ~ =0.22 + 0.03 and Kondoh at a112. 0¢= 0.35 + 0.05 1.3 Superconducting Gap Measurements Unlike the superconducting CuO materials, the superconducting gap parameter, A, should be reasonably isotropic owing to the cubic structure. Once the. energy gap is found, the so-called strong coupling ratio 2~dkTc can be determined. This ratio, which is 3.53 for a conventional BCS superconductor, is a measure of the coupling strength. Schlesinger et a113. have recently measured the energy gap using infrared reflectivity and have obtained the following values: & = 4.35 meV for T c = 29K and A = 3.85 for T c = 26K. Both results lead to the ratio 2A/kTc = 3.5 + 0.5 indicating a weak to moderate coupling strength. We note that the upper bound estimate from this experiment is similar to that of niobium which has a coupling strength, ~. = 1.0. Tunneling measurements have been performed on samples with x = 0.375, 0.4 and 0.5. The tunneling density of states is shown for x = 0.375 by the solid curve in figure 3. The fitted BCS density of states E/(E2-A2) 1/2 (dashed line) has been smeared by introducing an imaginary part, F, to the energy to account for lifetime broadening. The origin of [" is not known at the present time but it clearly becomes larger as the metal to

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Figure 3 Normalized tunneling conductance for x--0.375 (solid line). Dashed line is smeared BCS fit. semiconductor transition is approached at x =0.37. For example, the x= 0.4 sample had a T c of 22K and A = 3.5 meV, T' = 1.5 meV and this value of the gap parameter is consistent with the optical measurements. However, the values for the x=0.375 sample of figure 3 (Tc = 28K) are A = 6.5 tO 7.0 meV and F" = 5.0 meV leading to a strong coupling ratio of about 5.5. It is not clear at the present time whether B a l . x K x B i O 3 is indeed becoming more strong coupling near the phase boundary or whether the broadening is simply causing a large uncertainty in the determination of A. The energy gap measurements together suggest a BCS -like superconductor with weak or moderate coupling strength such that ;L is about 1.0. 1.4 Phonons in Neutron Scattering and Tunneling The above measurements suggest weak or moderate electron-phonon coupling and thus the high T c of 30K requires phonons with a characteristic energy of about 40 meV. Measurements of the phonon density of states G((o) have been determined using inelastic neutron scattering I land these are shown in figure 4 for

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consequence of a high characteristic phonon energy, not a large value of ~.. REFERENCES 1. L.R. Mattheiss, E.M. Gyorgy and D.W. Johnson Jr., Phys.Rev. B 37 (1988)3745

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2. R.J. Cava, B. Batlogg, J.J. Krajewski, R. Farrow, L.W. Rupp Jr., A.E. White, K. Short, W.F. Peck and T. Kometani, Nature 3322 (1988) 814

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3. D.G. Hinks, B. Dabrowski, J.K. Jorgensen, A.W. Mitchell, D.R. Rlchards, Shiyou Peh and Donglu Shi, Nature 333 (1988) 836

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4. B. Batlogq, R.J. Cava, L.W. Rupp, Jr., A.M. Mujsce, J.J. Krajewski, J.P. Remeika, W.F. Peck, Jr., A.S. Cooper and G.P. Espinosa, Phys.Rev.Letters 61 (1988) 1670

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E (meV) Figure 4 A) Tunneling second derivative. B) and C) are phonon density of states from neutron scattering for Ba1-xKxBiO3 (x=0.04) and BaBiO3 respectively.

5. Shiyou Pei, J.D. Jorgensen, B. Dabrowski, D.G. HinKs, D.R. Richards, A.W. Mitchell, J.M. Newsam, S.K. Sinha, D. Vaknin and A.J. Jacobsen. (To be submitted for publication) 6. M.J. Rice and Y.R. Wang, Physica C 157 (1989) 192 7.

x=0.4 (B) and for the parent compound BaBiO 3 (C). It is clear that high energy phonon modes are present in the range 40 to 80 meV. Molecular dynamics calculations of the phonon density of states shows that these high energy modes are due to optical vibrations of the oxygen atoms. High bias conductance and second derivative tunneling spectroscopy measurements show features in the energy range 10 to 65 meV which are characteristic of phonon effects as seen in conventional superconductors14. These structures which appear as dips in figure 4A correspond well to the peaks in G(o}) and to peaks in F(o)) inferred from Raman scattering 15. More importantly the tunneling measurement is scaled by a factor lIE 2 which means that the structures observed near 60 meV indicate much stronger electron phonon coupling than for the phonons near 25 meV. Thus ec2F(o))will have a higher spectral weight at the high frequencies which provides a natural explanation for the superconducting properties of B a l . x K x B i O 3 . It appears that the high T c is a

H. Sato, S. Tajima, H. Takagi and S. Uchida, Nature 338 (1989) 241

8. Shiyou Pei, N.J. Zaluzec, J.D. Jorgensen, B.

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