Nonlinearelasticpropertiesinhigh-T c superconductorNd 1:85 Ce 0:15 CuO 4 underpressure K.P.Jayachandran,C.S.Menon * School of Pure and Applied Physics, Mahatma Gandhi University, Priyadarshini Hill PO, 686 560 Kottayam, Kerala, India Received12June2001;receivedinrevisedform3January2002;accepted9January2002 Abstract TheelasticityofhightemperaturesuperconductorNd 1:85 Ce 0:15 CuO 4 (NCCO)undertheapplicationofpressurehas been studied theoretically using finite strain elasticity theory. The pressure derivatives of the second-order elastic constantsofNCCOarecalculatedfromtheknowledgeofitssecond-andthird-orderelasticconstants.Wehaveused thedeformationtheoryfortheevaluationofthesecond-andthird-orderelasticconstants.Thestrainenergydensity / is estimatedbytakingintoaccounttheinteractionsofninenearestneighborsofeachatomintheunitcellofNCCO.The energy density / thusobtainediscomparedwiththestrain-dependentlatticeenergyderivedfromcontinuummodel approximation.Thesecond-orderelasticconstantsofNCCOshowthattheelasticstiffnesswithinthe ab-layersislarge comparedwiththebindingforcesbetweenthelayers,whichisexpectedofalayer-likematerial. C 333 islargerthan C 111 , whichmeansthattheanisotropyalongthe c-axisismorepronouncedthanthatalongthebasalplaneofthecrystal.The expressionsfortheeffectivesecond-orderelasticconstantsofNCCOsystemhavebeenderivedinitsstrainedstatein terms of the natural state second- and third-order elastic constants. These expressions are employed to obtain the pressurederivativesoftheeffectivesecond-orderelasticconstants.Theresultsthatthelargerpressurederivative(dC 33 = dp)along c-axisdirectionthanthatalong ab-plane,i.e.(dC 11 =dp),corroboratestheobservationthatthelayersclose-up substantiallyunderhydrostaticpressurewhilechangeininteratomicdistanceinalayerismuchsmallerinNCCO. Ó 2002ElsevierScienceB.V.Allrightsreserved. PACS: 62.20.D;74.25.L;74.62.F Keywords: Elastic anisotropy;Higherorderelasticconstants;Superconductors 1. Introduction Higherorderelasticconstantsandtheirpressure derivatives provide insight into the nature of binding forces between atoms since they are rep- resentedbythederivativesoftheinternalenergyof thecrystal.Indeedtheresponseofamaterialtoan applied stress is always determined by the elastic constantsandtheirpressurederivatives.Theyplay animportantroleinlowtemperaturephysicssince theyareadirectmeasureoftheanharmonicityof thelattice.Moreover,theelasticconstantsofoxide superconductors provide interesting informa- tiononlong-wavelengthacousticphonons,which may enter the basic superconducting mecha- nism[1].Becauseofthisanenormousamountof PhysicaC382(2002)303–310 www.elsevier.com/locate/physc * Correspondingauthor.Tel./fax:+91-481-597-923. E-mail address: spapf@sancharnet.in (C.S.Menon). 0921-4534/02/$-seefrontmatter Ó 2002ElsevierScienceB.V.Allrightsreserved. PII:S0921-4534(02)01229-7