Superconductivity in high-Tc cuprates is particularly sensitive to disorder due to the unconventional d-wave pairing symmetry. We investigated effects of disorder on the spectral properties of Bi2Sr2CaCu2O8+x high-Tc superconductor. We found that already small defect densities suppress the characteristic spectral signature of the superconducting state. The spectral line shape clearly reflects new excitations within the gap, as expected for defect-induced pair breaking. At the lowest defect concentrations the normal state remains unaffected, while increased disorder leads to suppression of the normal quasiparticle peaks.Zbog nekonvencionalne d-valne simetrije, supravodljivost u visokotemperaturnim kupratima je posebno osjetljiva na neuređenost. Ispitivali smo utjecaj neuređenosti na spektralna svojstva Bi2Sr2CaCu2O8+x supravodiča primjenom ARPES metode. Već mala gustoća defekata smanjuje karakteristični spektralni odziv supravodljivog stanja. Oblik spektralne linije očito reflektira nova pobuđenja unutar zabranjene vrpce, kako se i očekuje u slučaju razbijanja Cooperovih parova izazvanog defektima. Za male koncentracije defekata, normalno stanje ostaje nepromijenjeno, dok viši stupanj neuređenosti smanjuje intenzitet spektralnih linija kvazičestica

Berger, E.

Forró, L.

Grioni, M.

Margaritondo, G.

Pavuna, D.

Rullier-Albenque, F.

Vobornik, I.

English

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ISSN1330–0008CODENFIZAE4LETTER TO THE EDITORDEFECT-INDUCED CHANGES IN THE SPECTRAL PROPERTIES OFHIGH-Tc CUPRATESI. VOBORNIKa1, H. BERGERa, F. RULLIER-ALBENQUEb, G. MARGARITONDOa,D. PAVUNAa, L. FORROc and M. GRIONIaaInstitut de Physique Appliquée, Ecole Polytechnique Fédérale, CH-1015 Lausanne,SwitzerlandbLaboratoire des Solides Irradiés, CEA, Ecole Polytechnique, 91128 Palaiseau Cedex,FrancecLaboratoire de Physique des Solides Semicristallins, IGA, Ecole Polytechnique Fédérale,CH-1015 Lausanne, SwitzerlandDedicated to Professor Boran Leontić on the occasion of his 70th birthdayReceived 9 November 1999; revised manuscript received 1 February 2000Accepted 14 February 2000Superconductivity in high-Tc cuprates is particularly sensitive to disorder due tothe unconventional d-wave pairing symmetry. We investigated effects of disorderon the spectral properties of Bi2Sr2CaCu2O8+x high-Tc superconductor. We foundthat already small defect densities suppress the characteristic spectral signature ofthe superconducting state. The spectral line shape clearly reflects new excitationswithin the gap, as expected for defect-induced pair breaking. At the lowest defectconcentrations the normal state remains unaffected, while increased disorder leadsto suppression of the normal quasiparticle peaks.PACS numbers: 74.25.-q, 74.62.Dh, 74.72.Hs UDC 538.945Keywords: superconductivity, high-Tc cuprates, effects of disorder, spectral properties,Bi2Sr2CaCu2O8+x, defect-induced pair breaking, suppression of the normal quasiparticlepeaksThe superconducting (SC) order parameter in high-Tc cuprates is highlyanisotropic. A number of experimental results indicate that the carriers in the1Currently at: INFM - Laboratorio TASC, Elettra, S.S. 14, Km. 163.5, Basovizza, 34012Trieste, ItalyFIZIKA A (Zagreb) 8 (1999) 3, 107–112 107vobornik et al.: defect-induced changes in the spectral . . .SC state pair with the dx2−y2 symmetry. The unconventional pairing stimulates in-terest in finding how disorder affects superconductivity in these materials. In low-Tcsuperconductors, with isotropic pairing symmetry, magnetic defects lead to a rapidsuppression of Tc and ultimately destruction of superconductivity [1,2]. If the orderparameter is anisotropic, both magnetic and nonmagnetic defects are harmful forsuperconductivity [3–7].These theoretical predictions can be tested in experiments on samples withcontrolled amounts of introduced disorder [8,9]. Chemical substitution in the CuO2planes may perturb the local antiferromagnetic order, resulting in magnetic scatter-ing centers [10–12]. Electron irradiation, on the contrary, introduces homogeneouslydistributed Frenkel-type point defects [8,9], most likely of nonmagnetic character[9].We report here the results of a systematic angle-resolved photoemission(ARPES) investigation of the effects of disorder on the spectral properties ofelectron-irradiated Bi2Sr2CaCu2O8+x. In the SC state, a residual density of statesat the Fermi level, due to the defect-induced pair breaking, is clearly observed [13].At the lowest defect concentrations, only the superconducting state is affected.At increased concentrations, however, the observed changes in the normal statequasiparticle features are similar to the ones found in highly underdoped regime[14].Our samples were grown by the flux method [15]. The electron irradiation wasperformed on a van de Graaff accelerator with the irradiation flux fixed to 2× 1014e/(cm2s) and the electron energy of 2.5 MeV. The electron irradiation in theseconditions displaces mainly Cu and O atoms, resulting in defect concentrationsconfined to the low 10−3 dpa (displacements-per-atom) range at constant carrierconcentration [8,9,13]. Depending on the irradiation time, we obtained critical tem-peratures of 82, 72 and 62 K, as opposed to 90 K before the irradiation.ARPES experiments were performed with a Scienta 300 electrostatic hemispher-ical analyzer whose energy resolution is better than 10 meV and the angular accep-tance is ±1◦. Photons of 21.2 eV were used, which gives a total k-space window of0.4 nm−1. The high symmetry directions were determined by the Laue X-ray diffrac-tion. The samples were cleaved in-situ at the base pressure better than 2.7×10−8 Pa(2× 10−10 torr). The SC-state measurements were performed at 25 K and normalstate data were taken at 95 K.Figure 1 illustrates the SC-state spectra for the three irradiated samples and forthe pristine sample (each specimen is identified by its Tc). All data were taken closeto the (0,π) point in the Brillouin zone where a dx2−y2 gap exhibits a maximum.The signature of the SC state in ARPES is a coherent peak at a binding energyapproximately corresponding to the half-magnitude of the SC gap, followed by adip and a hump features at larger binding energies [16–18]. The leading edge of thespectra is limited only by the experimental resolution.All spectra in Fig. 1a exhibit the superconductivity-related features. The coher-ent peak is at the same binding energy for all samples, consistent with the constantand optimal carrier concentration [13]. The peak intensity decreases as Tc decreases,108 FIZIKA A (Zagreb) 8 (1999) 3, 107–112vobornik et al.: defect-induced changes in the spectral . . .i.e., as the amount of disorder increases. The leading edge is no longer resolution-limited. It extends instead beyond the Fermi energy, suggesting new states withinthe gap. This is emphasized in Fig. 1b, which illustrates the near-edge portionof the spectra compared to a reference 90 K spectrum. The shaded area in Fig.1(b) corresponds to the gap states and clearly increases with the induced disorderbeyond the experimental uncertainty.Fig. 1. (a) Superconducting statespectra for the pristine (Tc = 90K) and irradiated (Tc = 82 K, 72K and 62 K) samples, normalizedat the highest binding energy; (b)Near-edge portion of the same spec-tra normalized to the coherent peakand compared to a reference pristineTc = 90 K) spectrum. The shadedarea corresponds to the new stateswithin the SC gap, created throughthe defect-induced pair breaking.The spectral feature near EF reflects normal quasiparticle (QP) states, whichobey the Fermi-Dirac distribution. They coexist with the SC condensate below Tc,as expected for the defect-induced pair-breaking. A similar spectral feature wasrecently reported in the scanning tunneling measurements of Bi2Sr2CaCu2O8+x,where it was attributed to the quasiparticle scattering resonances [19]. There isa good agreement between our results and the theoretical predictions of Ref. [7],which analyzed the effects of nonmagnetic defects on the SC state spectral func-tion. At our low defect concentrations (< 10−3 dpa), Ref. [7] argues that the ob-served spectral changes are due to the resonant scattering, previously investigatedin heavy-fermion superconductors [20–23]. The concept of the resonant scatteringis important for the cuprates, since it rules out pairing without nodes in the gapFIZIKA A (Zagreb) 8 (1999) 3, 107–112 109vobornik et al.: defect-induced changes in the spectral . . .[7,24].Figures 2 and 3 illustrate the normal state dispersion along the Γ-M (0, π)and Γ-X (π, −π) high-symmetry directions. The pristine 90 K sample exhibits allfeatures found in previous ARPES studies [16,18]. Along Γ-X, a fast dispersingstate crosses the Fermi surface. Along Γ-M-Z, no crossing occurs; the quasiparticlestates remain just below the chemical potential, forming a flat band or an extendedvan Hove singularity.     %LQGLQJ HQHUJ\ H93KRWRHOHFWURQLQWHQVLW\7F  .ΓΖ7F  . 7F  . 7F  .ΓΓΓΖΖΖ, 9RERUQLN HW DO)LJ Fig. 2. Normal state spectra along the Γ-M (0, π) high-symmetry direction for thepristine Tc = 90 K sample and the irradiated Tc = 82 K, 72 K, 62 K samples. Theinsets indicate the locations in the Brillouin zone where the spectra were taken.These normal-state characteristics remain unchanged for the 82 K sample - incontrast to the significant difference in the SC state (Fig. 1). On going from the82 K to the 72 K and to the 62 K samples, the normal-state properties changedramatically. The most pronounced is the loss of coherent spectral intensity. Thespectral features become broader and their dispersion cannot be easily traced.We found similar behaviour in the normal state of neutron-irradiatedBi2Sr2CaCu2O8+x, where the quasiparticle features were completely suppressedat the highest disorder levels [25]. The normal state behaviour [14] of underdopedBi2Sr2CaCu2O8+x also resembles that of the irradiated samples. The suppression ofthe quasiparticle peaks in the underdoped regime was initially attributed uniquelyto reduced carrier concentration and increased electronic correlations. Our datasuggest instead that disorder introduced through doping could be, at least in part,responsible for the peculiar spectral properties of underdoped cuprates. In fact,110 FIZIKA A (Zagreb) 8 (1999) 3, 107–112vobornik et al.: defect-induced changes in the spectral . . .doping is usually achieved by chemical substitution or oxygen reduction, both af-fecting the CuO2 planes and likely to increase disorder.     7F  .%LQGLQJ HQHUJ\ H93KRWRHOHFWURQLQWHQVLW\Γ;7F  . 7F  . 7F  .ΓΓΓ;;;, 9RERUQLN HW DO)LJ Fig. 3. Same as Fig. 2, but for the Γ-X (π, −π) direction.In summary, our ARPES data provide a first direct evidence for defect-inducedpair-breaking in a d-wave superconductor. The loss of the coherent-peak intensity inthe SC-state spectra is accompanied by an increase of the residual density of statesat the Fermi level, as expected for coexisting normal and superconducting carriersdue to the pair-breaking events. In the normal state, disorder causes a dramaticsuppression of the quasiparticles features, which qualitatively resembles the spectralevolution in highly-underdoped cuprates. These findings call for a reconsiderationof the effects of disorder in the cuprates, in particular at low carrier concentrations.AcknowledgementsThis work was supported by the Fond National Suisse de la Recherche Scien-tifique and by the Ecole Polytechnique Fédérale de Lausanne.References1) A. A. Abrikosov and L. P. Gor’kov, Soviet Phys. JETP 12 (1961) 1243;2) P. W. Anderson, J. Phys. Chem. Solids 11 (1959) 26;3) K. Ueda and T. M. Rice, in Theory of Heavy Fermions and Valence Fluctuations, eds.T. Kasuya and T. Saso, Springer Verlag, Berlin (1985), p. 267;FIZIKA A (Zagreb) 8 (1999) 3, 107–112 111vobornik et al.: defect-induced changes in the spectral . . .4) R. J. Radtke, K. Levin, H.-B. Schttler and M. R. Norman, Phys. Rev. B 48 (1993)R653;5) K. Ziegler, M. H. Hettler and P. J. Hirschfeld, Phys. Rev. Lett. 77 (1996) 3013;6) K. Ziegler, M. H. Hettler and P. J. Hirschfeld, Phys. Rev. B 57 (1998) 10825;7) R. Fehrenbacher, Phys. Rev. B 54 (1996) 6632;8) F. Rullier-Albenque, A. Legris, H. Berger and L. Forró, Physica 254 (1995) 88;9) J. Giapintzakis, D. M. Ginsberg, M. A. Kirk and S. Ockers, Phys. Rev. B 50 (1994)15967;10) H. Alloul, P. Mendels, H. Casalta, J. F. Marucco and J. Arabski, Phys. Rev. Lett. 67(1991) 3140;11) A. V. Mahajan, H. Alloul, G. Collin and J. F. Marucco, Phys. Rev. Lett. 72 (1994)3100;12) T. R. Chien, Z. Z. Wang and N. P. Ong, Phys. Rev. Lett. 67 (1991) 2088;13) I. Vobornik, H. Berger, D. Pavuna, M. Onellion, G. Margaritondo, F. Rullier-Albenque,L. Forró and M. Grioni, Phys. Rev. Lett. 82 (1999) 3128;14) H. Ding, M. R. Norman, T. Yokoya, T. Takeuchi, M. Randeria, J. C. Campuzano, T.Takahashi, T. Mochiku and K. Kadowaki, Phys. Rev. Lett. 78 (1997) 2628;15) D. Mandrus, L. Forr, C. Kendziora and L. Mihaly, Phys. Rev. B 45 (1992) 12640;16) Z.-X. Shen and D. S. Dessau, Physics Reports 253 (1995) 1;17) H. Ding, M. R. Norman, J. C. Campuzano, M. Randeria, A. F. Bellman, T. Yokoya,T. Takahashi, T. Mochiku and K. Kadowaki, Phys. Rev. B 54 (1996) R9678;18) M. Randeria and J. C. Campuzano, Varenna Lectures 1997, Report No. LANL cond-mat/9709107, and references therein;19) E. W. Hudson, S. H. Pan, A. K. Gupta, K.-W. Ng and J. C. Davis, Science 285 (1999)88;20) C. J. Pethick and D. Pines, Phys. Rev. Lett. 57 (1986) 118;21) S. Schmitt-Rink, K. Miyake and C. M. Varma, Phys. Rev. Lett. 57 (1986) 2575;22) P. J. Hirschfeld, P. Wölfle and D. Einzel, Phys. Rev. B 37 1988) 83;23) H. Monien, K. Scharnberg and D. Walker, Solid State Commun. 63 (1987) 263;24) R. Fehrenbacher, Phys. Rev. Lett. 77 (1996) 1849;25) I. Vobornik, C. Quitmann, M. Zacchigna, F. Zwick, M. Grioni, A. Karkin, R. J. Kelley,M. Onellion and G. Margaritondo, Surf. Sci. 402-404 (1998) 761.PROMJENE SPEKTRALNIH SVOJSTAVA KUPRATA VISOKOG TcIZAZVANE DEFEKTIMAZbog nekonvencionalne d-valne simetrije, supravodljivost u visokotemperaturnimkupratima je posebno osjetljiva na neured–enost. Ispitivali smo utjecaj neured–enostina spektralna svojstva Bi2Sr2CaCu2O8+x supravodiča primjenom ARPES metode.Već mala gustoća defekata smanjuje karakteristični spektralni odziv supravodljivogstanja. Oblik spektralne linije očito reflektira nova pobud–enja unutar zabranjenevrpce, kako se i očekuje u slučaju razbijanja Cooperovih parova izazvanog defek-tima. Za male koncentracije defekata, normalno stanje ostaje nepromijenjeno, dokviši stupanj neured–enosti smanjuje intenzitet spektralnih linija kvazičestica.112 FIZIKA A (Zagreb) 8 (1999) 3, 107–112

Promjene spektralnih svojstava kuprata visokog Tc izazvane defektima

Promjene spektralnih svojstava kuprata visokog Tc izazvane defektima

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