Origin of high-temperature superconductivity

Origin of high-temperature superconductivity

Physica B 312–313 (2002) 53–55 Origin of high-temperature superconductivity John D. Dowa,*, Dale R. Harshmanb,a,1 a b Department of Physics, Arizona...

79KB Sizes 2 Downloads 49 Views

Physica B 312–313 (2002) 53–55

Origin of high-temperature superconductivity John D. Dowa,*, Dale R. Harshmanb,a,1 a b

Department of Physics, Arizona State University, Tempe, AZ 85287, USA Physikon Research Corporation, P.O. Box 1014, Lynden, WA 98264, USA

Abstract High-Tc superconductivity is shown to occur in the hole-condensate which occupies the SrO, BaO, or interstitial regions of layered perovskite superconductors. Cuprate-planes are unnecessary for high-Tc superconductivity, as demonstrated by the doped Sr2 YRuO6 system, which has no cuprate-planes. A single class of high-temperature superconductors includes cuprates, ruthenates, and certain organics (e.g., k-½BEDT-TTF2 Cu½NCS2 ), with the holes on O (or S) z-paired. r 2002 Published by Elsevier Science B.V. PACS: 74.72.h; 74.10.+v; 74.20.z Keywords: High-Tc compounds; Occurrence of high-Tc ; Theories and models

1. Doped Sr2 YRuO6 : SrO layers superconduct Many authors believe that high-temperature superconductivity originates in cuprate-planes which are common to many such superconductors, but some experiments have been overlooked which actually suggest that the main superconductivity of layered perovskite superconductors originates in the SrO, BaO, or interstitial regions, rather than in the cuprate-planes [1,2]. One primary indication of this viewpoint comes from Sr2 YRuO6 doped with Cu on Ru sites, a system that has less than 1% impurity phase of any kind, exhibits no evidence of cuprate-planes, and has too little Cu to assume a superconducting phase related to Cu (e.g., CuO2 planes). The material superconducts with an onset at E45 K (samples have been studied with 5%, 10%, *Corresponding author. 6031 East Cholla Lane, Scottsdale, AZ 85253, USA. Tel.: +1-480-423-8540; fax: +1-480-4235183. E-mail address: [email protected] (J.D. Dow). 1 Supported by the US Office of Naval Research (Contract No. N00014-94-1-0147) (J.D.D. and D.R.H.), and by Physikon Project No. PL-206 (D.R.H.).

and 15% CuRu dopants), although it has only two types of layers, ðSrOÞ2 and YRuO4 : (Dopants other than Cu should also cause Sr2 YRuO6 to superconduct.) We have studied Sr2 YRuO6 doped with Ru-site Cu using many techniques [3]: (i) resistivity, (ii) electron spin . resonance (ESR), (iii) 99 Ru Mossbauer absorption (52 to 32 transition) [4], (iv) magnetic susceptibility, and (v) muon spin rotation (mþ SR) spectroscopy. From these studies we know that: (i) the Ru ions order ferromagnetically in the a–b planes at TN E23 K [3]; (ii) a spin-glass-like state exists at TG E29:3 K [5]; (iii) the Cu, which dopes the Ru site, orders at about 86 K; (iv) above TG the fluctuating moments of Ru ions may break pairs; (v) both resistivity and ESR detect evidence for the onset of superconductivity at E45 K (in the case of ESR, vortex dissipation is detected in the form of an isothermal resistivity RS (H) that increases characteristically as H 1=2 at temperatures below Tc ; and decreases as H increases above Tc ). In addition, the 99 Ru M.ossbauer spectra show that the hyperfine field vanishes near 30 K, and that magnetism and superconductivity coexist (at least in different layers). At lower temperature, 4.2 K, 18 M.ossbauer lines are detected, indicating that Ru is in the Ruþ5 charge state. Perhaps the most conclusive data reflecting the charge character of the two types of layers are provided by

0921-4526/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 0 6 6 - 3

54

J.D. Dow, D.R. Harshman / Physica B 312–313 (2002) 53–55

muon spectroscopy. It is well known that positive muons tend to trap near the negatively charged oxygen ions in the oxide superconductors. In the case of Sr2 YRuO6 ; there are three distinct oxygen sites: one in the SrO layer (which we identify as O(3)) and two in the YRuO4 layer, with these two being indistinguishable when Y=Ru, so we identify them as one: O(1,2)=O(1)+O(2). In experiments conducted on the Cu-doped material, Sr2 YRu1u Cuu O6 (with u ¼ 0:05; 0.10, and 0.15) [6], the mþ decay spectra exhibit two distinct signals corresponding to two magnetically different muon trapping sites. One type of muon site (mOð1;2Þ ) is assigned to the oxygen ions in the Y(Ru1u Cuu ÞO4 layers (this oxygen is approximately fully charged to O2 ), and the other (mOð3Þ ) is attributed to oxygen ions in the SrO layers, which are undercharged to OZ ; with Zo2 (consequently these oxygens have holes on them). The measured stopping probabilities confirm the bond-valence sum calculation [7] and show that about 90% of the muons tend to trap at the mOð1;2Þ sites, while only about 10% stop at the comparatively less negative mOð3Þ sites. Thus, the holes carrying the superfluid density likely reside in the SrO layers near the mOð3Þ sites. The mþ SR data show little evidence of bulk superconductivity above about 30 K. This fact we attribute to the rapidly fluctuating Ru moments that are not readily observed by the muons in a transverse field above 30 K due to motional narrowing, and appear to be detrimental to pair formation. Thus, while ESR measurements indicate the presence of (at least) short-lived fluxons between E30 and E45 K, the mþ SR data show a significant relaxation rate l below about 30 K, the temperature at which the fluctuations associated with Ru begin to slow down dramatically and begin to form a few dilute moments. Recent data [8] actually show dilute spin-glass-like behavior of the Ru moments in the fast fluctuating limit (compared with the muon lifetime) at TG (for u=0.1). As the temperature is decreased further, the individual Cu-doped YRuO4 layers begin to order ferromagnetically, at a N!eel temperature of TN E23 K, with the magnetic polarization of each sheet reversing direction from one magnetic YRuO4 layer to the next, resulting in a net antiferromagnetic state and a net zero local field along the SrO layers. The mþ SR data [6] indicate the presence of vortices at low temperatures, as shown by the observed difference between fieldcooled and zero-field-cooled relaxation data taken at E2 K. For doped Sr2 YRuO6 ; the main experimental points are that the superconductivity (i) is p-type and (ii) is detected in the SrO layers. Moreover, the superconductivity appears incompatible with the YRuO4 layers which contain E3 kG magnetic fields at the muon sites, and are n-type (because the SrO layers are p-type, and each unit cell must be charged neutral).

2. Oxygen (or sulfur) high-temperature superconductivity A central thesis of our approach is that there exists only one kind of high-temperature superconductivity, and that it involves holes in the SrO, BaO, or interstitial regions of the various layered perovskite superconductors. (In the case of some organic materials, S ions may supply the holes.) An example that indicates that the BaO layers, not the cuprate-planes, are the location of the primary superconductivity, is PrBa2 Cu3 O7 ; which ceases to superconduct when Pr occupies a Ba-site on one side of a cuprate-plane, but nevertheless superconducts when Pr and Ba are both on their natural sites [9]. The cuprateplane is essentially midway between the Pr-site and the Ba-site. While the creation of a PrBa defect displaces some Ba, the displaced Ba defect created, unlike PrBa ; is non-magnetic and not a pair-breaker. Hence it is the PrBa defect on one side of the cuprate-plane that breaks pairs and destroys the superconductivity, while Pr, on the other side of the cuprate-plane, does not destroy the superconductivity. Consequently the superconductivity does not occupy the cuprate-plane layer. Similar arguments cause us to regard some organic compounds, such as k-½BEDT-TTF2 Cu½NCS2 [10], to be (S-based) high-temperature superconductors [11]. Other materials which we re-interpret as hosting superconductivity, not in their charge-reservoir layers [12] but in their BaO layers, are HgBa2 Can1 Cun O2nþ2 and Tc2 Ba2 Can1 Cun O2nþ4 : In summary, it is possible to construct a theory of high-temperature superconductivity which appears to explain all of the materials that superconduct p-type due to holes on O or S, and which relies on a new pairing model [1,2].

References [1] D.R. Harshman, J.D. Dow, Zeta-pairing of holes in hightemperature superconductors, to be published. [2] J.D. Dow, D.R. Harshman, Physics of high-temperature superconductivity, to be published. [3] H.A. Blackstead, J.D. Dow, D.R. Harshman, M.J. DeMarco, M.K. Wu, D.Y. Chen, F.Z. Chien, D.B. Pulling, W.J. Kossler, A.J. Greer, C.E. Stronach, E. Koster, B. Hitti, M. Haka, S. Toorongian, Eur. Phys. J. B 15 (2000) 649. [4] M.J. DeMarco, H.A. Blackstead, J.D. Dow, M.K. Wu, D.Y. Chen, F.Z. Chien, M. Haka, S. Toorongian, J. Fridmann, Phys. Rev. B 62 (2000) 14301. [5] D.R. Harshman, W.J. Kossler, A.J. Greer, C.E. Stronach, D.R. Noakes, E. Koster, M.K. Wu, F.Z. Chien, J.P. Franck, J.D. Dow, Spin glass behavior and spin-fluctuations in superconducting Sr2 YðRu1u Cuu ÞO6 ; to be published.

J.D. Dow, D.R. Harshman / Physica B 312–313 (2002) 53–55 [6] D.R. Harshman, W.J. Kossler, A.J. Greer, C.E. Stronach, E. Koster, B. Hitti, M.K. Wu, D.Y. Chen, F.Z. Chien, H.A. Blackstead, J.D. Dow, Phys. B 289–290 (2000) 360. [7] I.D. Brown, Structure and Bonding in Crystals, Vol. II, in: M. O’Keefe, A. Navrotsky (Eds.), Academic Press, New York, 1980, pp. 1–20. [8] D.R. Harshman, W.J. Kossler, A.J. Greer, C.E. Stronach, D.R. Noakes, E. Koster, M.K. Wu, F.Z. Chien, H.A. Blackstead, D.B. Pulling, J.D. Dow, Phys. C 364–365 (2001) 392.

55

[9] H.A. Blackstead, J.D. Dow, I. Felner, W.B. Yelon, Phys. Rev. B 63 (2001) 094517. [10] D.R. Harshman, A.T. Fiory, R.C. Haddon, M.L. Kaplan, T. Pfiz, E. Koster, I. Shinkoda, D. Ll. Williams, Phys. Rev. B 49 (1994) 12990 and references therein. [11] H. Urayama, H. Yamochi, G. Saito, S. Sato, T. Sugano, M. Kinoshita, A. Kawamoto, J.Tanaka, T. Inabe, T. Mori, Y. Maruyama, H. Inokuchi, K. Oshima, Synth. Metals 27 (1988) A393. [12] H.A. Blackstead, J.D. Dow, Solid State Commun. 95 (1995) 613.