HgBa2CaCu2O6+δ (Hg -1212) thin films were fabricated by exchanging the T1 cations in TlBa2CaCu2O7-δ (Tl-1212) thin films with Hg cations, causing a 30-K increase in Tc. To determine how this exchange effects such a Tc increase, the irreversibility lines, temperature dependence of critical current density, and temperature dependence of Hall angle ofHg-1212 and TI-1212 thin films were measured and then compared. The results strongly suggest that the Tc shift is caused by a doubling of charge carrier density

Albert Gapud

Byeongwon Kang

Don Overmyer

Judy Wu

Michael Siega

Shaolin Yan

Yi-Yuan Xie

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Comparative Transport Studies of '1212' Superconductors

HgBa{sub 2}CaCu{sub 2}O{sub 6+{delta}} (Hg-1212) thin films were fabricated by exchanging the TI cations in TlBa{sub 2}CaCu{sub 2}O{sub 7-{delta}} (Tl-1212) thin films with Hg cations, causing a 30-K increase in Tc. To determine how this exchange effects such a Tc increase, the irreversibility lines, temperature dependence of critical current density, and temperature dependence of Hall angle of Hg-1212 and T1-1212 thin films were measured and then compared. The results strongly suggest that the Tc shift is caused by a doubling of charge carrier density

Gapud, Albert

Wu, Judy

Yan, Shaolin

Xie, Yi-Yuan

Kang, Byeongwon

Siegal, Michael P.

Overmyer, Donald L.

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1, ,. s%wx?i290-@5-5rComparative Transport Studies of’ 1212’ SuperconductorsAlbert Gapud’*YJudy WU2,Shaolin Yanz,Yi-YuanXi&,Byeongwon Kangz,Michael Siega3, and Don L. OvermyeP‘Instituteof Mathematical Sciences and, Physics “’)&..University of the Philippines Los Baiios “’’”’?@4031 College, Lagun& Philippines ~~fb ~l!!(<&A,aag@physics.uplb. edu.ph‘Department of Physics and Astronomy os$~eti ‘. .“University of Kansas, Lawrence, Kansas, USA3Sandia National Laboratories, Albuquerque, New Mexico, USA7’JABSTRACTHgBa,CaCu,Oti (Hg-12 12) thin films were fabricated by exchanging the TI cations in TlBazCaCu20,J (Tl-1212) thin films with Hg cations, causing a 30-K increase in Tc. To determine how this exchange effectssuch a Tc increase, the irreversibility lines, temperature dependence of critical current density, and tem-perature dependence of Hall angleofHg-1212 and T1-1212 thin films were measured and then compared.The results strongly suggest that the Tc shift is caused by a doubling of charge carrier density.Keywords: Hg-1212, T1-1212, transport, Hall effect, irreversibility.INTRODUCTIONThe purpose of this study (Gapud, 1999a) is to ascertainthe nature of the 30-K shift in the critical temperature(T,) in exchanging the T1 cation in TIBa,CaCu,O,a (Tl-1212, TC - 90 K) with the Hg cation to produceHgBa,CaCu,06+& (Hg-12 12, T, -120 K). This rareavailability of high-quality samples of both ‘ 1212’species makes this study possible, and could shed newlight on the still-unexplained mechanism of high-temperature superconductivity. Since the Hg- 1212 filmswere fabricated from T1-1212 by replacing TI cationsby Hg cations (Wu et al., 1999) the superconducting‘1212’ layered structured is preserved under this cationexchange; therefore the TC shift is not due to theanisotropic layered structure prevalent in high-temperature superconductors, but more likely due to*Corresponding Author8differences in the electronic structure and charge earnerdensity. It will be shown that the TCshift is due primarilyto a shift in charge earner density.EXPERIMENTALDETAILST’heHg-12 12 thin films in this study were fabricatedusing a recently patented cation exchange process (Wuet al., 1999) in which T1-1212 precursor films areconverted into Hg-1212 films by surgically replacing Tlcations with Hg cations. The T1-1212 films werefabricated using a modified crucible process (Siegal etal., 1998) in which DC-sputtered superconducting filmswere &ther improved by annealing in TI vapor.Two major studies were conducted. In the first (Gapudet aL, 1999b,Gapud et al., 1999c), the irreversibilitylines (IL’s) of the’12 12’ species were rneas~ed usingtwo independent methods. The first method usedmagnetic measurements, in which the applied field atScience Diliman (July-December 1999) 11:2, 8-10DISCLAIMERThis report was prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, .or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument..which magnetization becomes irreversible (Hti) wasinterpolated from magnetization hysteresis loopsmeasured at different temperatures using a QuantumDesign superconducting quantum interference device(SQUID) magnetometer system with a maximumapplied field of 5.5 Tesla. The second method usedtransport measurements, in which the irreversibilityfields H“ were associated with the critical (zero-resistivity) temperature determined from resistivity-versus-temperature curves measured at differentmagnetic fields in the same SQUIDsystem. In additimthe “magnetic” critical current density (JC)for various‘1212’ films was calculated fkommagnetic hysteresisloops at diffkrent temperatures (Gapud et al., 1999c).Lnthe second part, Hall-effect measurements (Gapudet al., 1999d) W* conducted in order to ‘pro% thescattering mechtiismY as well as the nature of thecharge c@!@~~~ese were done ii ~ 8-Tesla Januss+di&ghk-@iii* Wihgiidiidb=jand transverse (p ) resistivities were measured under..... ...V .-.>.q$:q<..- .::-cycled curreits ~d magnetic field. From thk data wascalculated the Hall angle, cotf3~=“p=/j?V.DISCUSSION OF RESULTSThe results of the IL study showed for the first timethat the IL’s of two species of like anisotropy whenplotted against normalized temperature T/TC will101Ex0.10.01H“(T) “-‘~3H “T)‘“’/+’,.,,..,---&.,>.-., +TI-1212 *1--- -11-1212*2..---- +H9-1212 #l-Hg-1212S2+Hg-i212#3t. 9 t , 1 1 i0.0 , 0.2 0.4 0.6 0.8.- .,....1 -T/TC .,,.j~.$ .Fig. 1. Semi-log plot of &nspo?&me&uredirreversibil{wlines(IL’s) W(T) and magnetically measured IL’s H*(T) for Hg-1212and m-12 12 thin films.Comparative Transport Studies of ‘1212’ Superconducto~overlap. This is shown in Fig. 1,plotted in semilog scaleto show that both transport and magnetic data alsofollow the same exponential decay law, exp[-tz (l-T/T)], where a Xl and a is the same for both species,as well as for different data sets of each species.Another overlap occurs in a different part of themagnetic phase diagram, namely the critical currentdensity J=(T)curves of the two species (i.e., below thein-eversl%ilityline) which also overlap at low fieldswhensimilarly normalized (Gapud, 1999c). These results arereminiscent of the well-known fact that intrinsicproperties of BCS (isotropic) superconductors overlapwhen plotted against T/TC.This occurs because the onlydifference between such materials is in their orderparameter. It is similarly argued that the IL overlap of‘1212’ species is due significantly to a difference inorder parameter.Because the exchange of Hg and Tl cations causes asignificant change ‘mthe distribution of oxygen in thelattice, it was suspected ihat this shift in the orderparameter may be due to a shift in the density ofchargecarriers in the superconducting CU02 planes. Toinvestigate this possl%ility, Halkffect measurementswere conducted in the normal state (Ong, 1990),primarily the Hall angle, cotfl~. Previous studies (Wuyts,1995) have shown that the slope of cot(3,(P) is linearlyproportional to the two-dimensional charge carrierdensity in the CUOZplanes and is therefore seen as amore reliable measure of charge carrier density,especially in light of the anomalous temperaturedependence ofHall number, ~. In the interest of makingsuch a comparison of slopes, inverse Hall mobility,HcotO~, is plotted versus T in Fig. 2. It turns out thatthe ratio of the slopes for the higher-TC samples Hg#l(120 K) and Tl#2 (90 K) is around two (2.03). Thissuggests that Hg-1212 has about twice the charge carrierdensity ofT’I-l212, for samples close to optimal doping(as indicated by theticlose to optimal TC).An independent confirmation of this factor of twocomes from assessing the characteristic temperatureT“ below which the Hall angle data deviates from theusual -T? &n@t&e depe&d&e.. ,In another. study. ...*’q ~-*:.$=-=~.#%$--~@t-*~&@~>*5*%%’? .f: .,*.5>*~(Jm “&.w1998jJ:~$WasJ:hO~.@ & PWPO.~:Pa!.to*ec~gge~,g:%.i$;; $~om;&ng reduced ~nc,>,, .2+. .... ,. ..+’for tie present Hall data, ‘the ratio for this’ qu&ity9.Gapud et al. .r*I 1t 112108642 -V (10’ K2)Fig. 2. Inverse Hall mobi~ity(cot9~times magnetic field) versussquare of temperature for Hg-1212 and Tt-1212.between TI-1212 and Hg- 1212 was found to be alsoaround two (1.9).From these comparative studies, it was found that (1)the irreversibility lines and critical current density ofthe’ 1212’ species overIap when plotted against reducedtemperature, implying that the shift in TCis due mainlyto a change in the order parameter, and that (2) thischange in the order parameter is likely to be due to adoubling of the charge carrier density as a result ofthe cation exchange. This, in turn, opens the possibilitythat electronic structure, namely charge earner densi~,may have a larger influence than lattice an isotropy inproducing high critical temperatures (TC) in thesematerials.ACKNOWLEDGMENTSThis project was partially funded by the U.S. Air Force,the Department of Energy (U.S.), and EPSCOR (U. S.)SQUID time was generously provided by MidwestSuperconductivity, Lnc.REFERENCESGapud, A. A., 1999a. Comparat~~e Stu$ies-of,t12 12’:”Superconductors~Doctoral Dissertation: Univ~si& of.Kansas (Supefiisor: J. Z. Wu), unpublished. “ -‘Sandia is a multipro~ramlaboratoryoperatedby Sandia Corporation,aLockheedMartin Compimy. for the10United Stotes Department of Energyunder contract DE-AC04.94AL85~.Gapud, A.A. et al., 1999b. Giant TC Shift inHgBa2CaCu,0,+6 and TlBazCaCuz07-a Supercon-ductors Due to Hg-Tl Exchange. l%ys. Rev. B 59(1):203-206.Gapud, A. A et al., 1999c. Supercurrent inHgBa2CaCu20ti and TlBazCaCuz074 Epitaxial ThinFilms. Appl. Phys. L.ett. 74(25):3878-3880.Gapu4 A. A. et al., 1999d. The Normal State Hall Effectin’ 1212’ Superconductors. Unpublished.Jin, R. & H. R. Ott, 1998. Hall Effect of YBazCuJO,Single Crystals. Phys. Rev. B. 57(21):13872-13877.Ong, N.P., 1990. The Normal State Hall Effect in High-temperature Superconductors. In Ginsburg, D. M. (cd.)Physical Properties of High Temperature Super-conductomj Singapore, World Scientific: Vol. 2, p. 459.Siegal, M. P. et al., 1997. Synthesis and Properties ofT1-Ba-Ca-Cu-O Superconductors. J. of MaterialsResearch. 12:2825.Siegal, M. P. et al., 1998. Growth and Characterizationof TIBaCaCuO Thin Films. J. of Superconductivity.11:135.Wuyts, B. et al., 1993. Influence of the Oxygen Contenton the Normal-state Hall Angle in YBazCulOx~ Films.Phys. Rev. B 47(9):55 12-5515. plus references therein.Wuyts, B. V.V. Moshchalkov, & Y. Bruynseraede, 1995.Scaling of the Normal-state Transport Properties ofUnderdoped YBa,Cu,O,. Phys. Rev. B. 51(9):6115-6118.Wu, J. Z.; S. L. Yan, & Y.Y. Xie, 1999. Cation Exchange:A Scheme for Synthesis of Mercury-based High-temperature Superconducting Epitaxial Thin Films. Appl.Phys. Letl. 74(10): 1469-1471Yan, S. L. et al., 1998. High Critical Current Density inEpitaxial HgBa2CaCu20X Thin Films. Appl. Phys. Lett.73(20): 2989-2991.

Comparative transport studies of ``1212'' superconductors

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Comparative Transport Studies of '1212' Superconductors

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