1 SubstitutioneffectofZnandCuinMgB 2 on T c andstructure S.M.Kazakov,M.Angst,J.Karpinski* SolidStatePhysicsLaboratory,ETH,8093Zürich,Switzerland I.M.Fita,R.Puzniak InstituteofPhysics,PolishAcademyofSciences,Warsaw ,Poland PACS:74.70.Ad Keywords:superconductivity,MgB 2 . *Correspondingauthor,ph:004116332254,karpinski@solid.phys.ethz.ch Abstract The investigation of Zn substitution in MgB 2 polycrystalline samples shows that about 0.1 Zn can be substituted in the structure of Mg deficient samples whi le it can not be substitutedinstoichiometric MgB 2 . Asa resultofthe isovalentZnsubstitutionintotheMg position of Mgdeficientsamples the aand c latticeconstantsincrease byabout 0.17%and 0.2%respectively.Susceptibilitymeasurementsshowaslight loweringof T c byabout0.5 K for 0.05 Zn, which decreases to 0.2 K for 0.1 Zn substitution. Investigation of the pressure derivativeof T c showsd p/dT c =-0.15K/kbarindependentofZncontent.SubstitutionofCu leads to multiphase samples without any changes of lattice const ants, which indicates that CudoesnotenterthestructureofMgB 2 .TheCusubstitutionbroadensthesuperconducting transitionconsiderably,whiletheonsetremainsunchanged. Introduction The recent discovery of superconductivity at 39K in MgB 2 by Akimitsu et al. [1] has stimulated world wide excitement. During the first month after the discovery dozens of papers have been published concerning the synthesis of polycrystalli ne samples and the investigation of basic physical properties. The transition tem perature is close to the limit suggestedtheoreticallyforphononmediatedsuperconductivity.Therefore, aquestionarises regarding the mechanism of superconductivity in this system. The bor on isotope effect measured by Bud’ko et al. [2] indicates that MgB 2 is a phonon-mediated superconductor. According to several authors the high transition temperature is pr obably due to the low mass of boron, which yields high phonon frequencies. This is supported by band calculations by Kortus et al. [3] which show a strong electron-phonon inter action. A