The inverse deuterium isotope effect, previously found in K-(BEDT-TTF){sub 2}CU(NCS){sub 2}, is also found in two other BEDT-TTF-based superconductors with different packing motifs and different types of anions. Remarkably, the magnitude of the isotope shift is essentially identical in all three superconductors, ca. +0.26 {+-} 0.06 K. These results, when taken together with the recent results of Lang et al. on the uniaxial pressure derivatives of T{sub c}, suggest that the inverse isotope effect may not have a direct relationship to the pairing mechanism but instead is a reflection of the change in the internal lattice pressure

Geiser, U. W.

Kini, A. M.

Schlueter, J. A.

Wang, H. H.

Ward, B. H.

UNT Digital Library

58510*cg./o/q 3 -On the “Inverse” Isotope Effect in Organic Superconductors:New Findings and ImplicationsAravinda M. Kini, John A. Schlueter, Brian H. Ward, Urs W. Geiser, and H. Hau WangChernistiy and Materials Science DivisionsArgonne National LaboratoryArgonne, IL 60439submitted for publication in theProceedings of the International Conference on the Science and Technology ofSynthetic Metals (ICSM 2000), Badgastein, Austria, July 15-21,2000‘fie submittedmanuscripthas been created bythe University of Chicago ss Operstnr oiArgonne Nationsl Laboratory (“Argonne”) uncle]ContractNo. W-31-109-ENG-38with the U.SDepartment of Energy. The U.S. Governmmretains for itself,and othersacting on its behalfa paid-up, nonexclusive, irrevncstde worldwidtlicense in said ardcle to reproduce, preparrI derivative works, distribute copies to the publicIsnd perfnrns publk)y and display publicly, bj~or on bebeff of dre Government.*work suppofled bY the u-s. Dep~ent of Energy, BES.Matefi~S Sckl’xes, under &Xl~CtW-3 1-109-ENG-38..To be presented at the International Conference‘on the Science and Technology of Synthetic Metals, ICSM 2000,Badgastein, Austria, July 15–21, 2000.On the “inverse” isotope effect in organic superconductors:new findings and implicationsAravinda M. Kini,* John A. Schlueter, Brian H. Ward, Urs W. Geiser and H. Hau WangChemist~ and Materials Science Divisions, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA~Received ; acceptedAbstractThe “inverse”deuteriumisotopeeffect, previouslyfound in K-(BEDT-TTF)zCU(NCS)Z,is also found in two otherBEDT-TTFbasedsuperconductorswith differentpackingmotifs and differenttypes of anions. Remarkably,the magnitudeof the isotopeshift is essentiallyidenticalin a.11three superconductors, cu. +0.26 * 0.06 K. These results, when taken together with the recent results of Lang et al. on theuniaxial pressure derivatives of T., suggest that the “inverse” isotope effect may not have a direct relationship to the pairing mechanism butinstead is a reflection of the change in the “internal lattice pressure”.Keyworak: Organic superconductors,BEDT-TT’For ET, isotope effect on Tc, BCS theory1. IntroductionDespite intense research efforts towards understandingthe nature of electron-pairing mechanism in organicsuperconductors, a consensus has not yet emerged.Experimental evidence has been presented in support ofboth conventional (phonon-mediated, BCS, s-wave orderparameter symmetry) and unconventional (not phonon-mediate~ non-BCS, d-wave order parameter symmetry)mechanisms [1]. Our comprehensive and carefidlyperformed isotope effect studies have revealed twocompeting isotope effects in the ambient pressure organicsuperconductor K-(BEDT-TTF)zCU(NCS)2 — a “normal”isotope shift (lowering of T.) when the largest possiblemass increase of BEDT-TTF (hereafter ET) molecule isachieved by 34Sand 13Clabelling and art “inverse” isotopeshift (icrease in T.) when hydrogen (H) atoms in ETmolecule are replaced by deuterium (D) [2]. While theformer effect is a clear indication for the involvement oflattice phonon modes in electron-pairing, the latter effectcan not be reconciled with any existing theoretical models.In this connection, we have continued our isotope effectstudies to determine whether the “inverse” D isotope effect*Corresponding author: Tel: (630) 252-3468; Fax (630) 252-4470; E-mail: amkini@anl.gov~ Work at Argonne National Laboratory is sponsored by the U. S.Department of Energy, OffIce of Basic Energy Sciences, Division ofMaterials Sciences, under Contract W-3 1-109-ENG-38.is specific to K-(BEDT-TTF).zCU(NCS)2 or is moreuniversal in nature. We have now found that it occurs in atleast two other ET based superconductors, viz., KL,-(ET),Ag(CF&(BDCE) where BDCE is l-bromo-l,2-dichloroethane [3] and @“-(ET)#iF~CHlCFZS03. Further-more, the magnitude of the isotope shift is essentially thesame (+0.26 t 0.06 K) in all three superconductors (withT.’s ranging ffom 2.9 to 9.2 K), irrespective of the donorpacking motifs (K or fY’)or the nature of the counter anions(polymeric or discrete or organometallic). In themeantime, Mi.iller, Lang and coworkers have determineduniaxial pressure coefficients of T. in K-(E~Cu(NCS~ andfi’’-(ET)zSFSCHzCFZSOJby high-resolution measurementsof the coefficients of thermal expansion, and have found alarge negative uniaxial stress effect perpendicular to theconducting planes in both superconductors [4]. These tworesults, when taken together, strongly suggest that the“inverse” D isotope shift may not be directly related to theelectron-pairing mechanism, but rather to the change in“internal lattice pressure”.2. Results and DiscussionAs a continuation of our isotope effect studies [2,3],we have recently determined the effect of D substitution inthe ET portion of ~-(ET)zSFsCHZCFZSOs. The samepreparative, measurement and data analysis protocols asused previously by us were employed, and they aredescribed in detail elsewhere [5]. An “inverse” isotopeshift of +0.27 t 0.06 K was found for this completelyx<Table 1Summary of H-to-D isotope shifts and uni~xial pressure derivatives (perpendicular to conducting planes) in three superconductorsSuperconductor T. (K) ATC(K) (i3TrJ@)L(K/kbar)KL-(ET)&(cF~)@DCE) 2.90t0.04~-(ET)zSF,CHzCF,SO,0.21 + 0.06 —4.34 * 0.05 0.27k 0.06 -5.9t0:25K-(ET)2CU(NCS)2 9.20*0.05 0.30* 0.07 -6,2ko.25organic superconductor. In Figure 1, we graphicallyillustrate the “inverse” isotope effect found so far in threedifferent superconductors, which are of essentially thesame magnitude within experimental uncertainties (see alsoTable 1).In Table 1, we have also included the uniaxial pressurecoel%cients of T= for K-(ET)zCU(NCS)2 and ~”-(ET)2SFSCH2CF2SOS perpendicular to the conductingplanes. These were obtained from high resolutionmeasurements of the coefficient of thermal expansion, u =tl x (Wa), on single crystals of the two superconductors,which show large and highly anisotropic phase transitionanomalies at T, [4]. By combining the thermal expansiondata with the literature results on the specific heat and byuse of the Ehrenfest equation, the uniaxial pressurecoei%cients were obtained. The most notable finding isthat the uniaxial pressure derivatives perpendicular to theconducting plane, i.e., along a* in K-(ET)zCU(NCS)2and cin &’-(ET)ZSFSCHZCFZS03,me rafier l~ge and of nemlythe same magnitude, –5.9 k 0.25 and -6.2 k 0.25 K/kbarrespectively. The essentially similar and large negativevalues of the uniaxial pressure derivative in two differentsuperconductors now provide new insights into why the Disotope effect is “inverse” and is of the same magnitude.In this connection, it is necessary to know what if anychanges in the lattice parameters are caused by the H-to-Dsubstitution in these materials, because it has been arguedthat the inverse isotope shift could be an outcome ofchanges in the lattice parameters and consequently theband structure. Watanabe et al. have showed by four-circlesingle crystal x-ray diffraction (300-12K) studies that theH-to-D isotopic substitution in both K-(ET)zCU(NCS)Zandrc-(ET)zCu~(CN~]Br results in no significant changes inthe lattice parameter perpendicular to the conductingplanes, although measurable changes occur in other latticeparameters at low temperatures [6]. Since C–H(D) bondsin ET-based superconductors are oriented perpendicular tothe planes, and C–D bonds have a smaller zero pointdisplacement compared to C–H bonds, H-to-D substitutionshould result in longer (or softer) intermolecular contactswith the atoms in the anion and with other donor molecules[7]. In other words, the “internal lattice pressure”perpendicular to the planes becomes smaller as a result ofthe H-to-D substitution, and this change is expected to beof the same magnitude irrespective of the anion or thepacking motif within the conducting plane. Since the largenegative pressure derivative of Tc in the directionperpendicular to the planes is the same for the twosuperconductors, it therefore follows that the observedisotope shift is merely an outcome of the lowering of the“internal lattice pressure”.Finally, we note that the “inverse” D isotope effect is aclear indication of the importance of interlayer coupling inlayered organic superconductors, and may not be directlylinked to what mediates electron-pairing. Whangbo et al.have suggested that within the framework of thegeneralized BCS model and the McMillan equation, the H-to-D substitution should result in a “softer” lattice and alarger A (electron-phonon coupling constant), and hence ahi&er Tc [7,8].o-0.2-0.4‘T-0.6-0.8-1#’-’j?-pA.+::: B+s:8+. +.+.+- ++. :::.‘m *::●.2 ■ ■ *Temperature (K)Fig. 1. Representative superconducting transition curves (bya.c. susceptibility) of (A) ~-(ET)2Ag(CF,).(BDCE), (B)13”-(ET)#F,CH,CF$O, and (C) K-(ET),CU(NCS),.Plus symbols = H; tilled squares= D.3. References[1][2][3][4][5][6][7][8]See e.g., J. Wosnitza, J. Low Temp. Phy. 117 (1999)1701; M. Lang, Supercond. Rev. 2 (1996) 1.A. M. Kini et al. Synth. Met. 85 (1997) 1617 andreferences therein.J. A. Schlueter et rd. Physics C, 265 (1996) 163.J. Mtiller et al. Phys. Rev. B, 61 (2000) 11739.J. A. Schlueter et al. Physics C, Submitted.Y. Watanabe et al. Synth. Met, 86 (1997) 1917.J. M. Williams et al. Science, 252 (1991) 1509.M.–H. Whangbo et al. in V. Z. Kresin and W. L.Little, (eds.), Organic Superconductivity, Plenum,New York, 1991, p. 243.DISCLAIMERThis 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.PortionsDISCLAIMERof this document may be illegiblein electronic imageproduced from thedocument.products. Images arebest available original

On the ``inverse'' isotope effect in organic superconductors: New findings and implications

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On the ``inverse'' isotope effect in organic superconductors: New findings and implications

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