Crystals - Zhang et al. - 2021 - Unknown

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pubs.acs.org/JPCLLetterSuperconductivityandHigh-PressurePerformanceof2DMo2CCrystals##JunliZhang,ZhenCao,XinHe,WenhaoLiu,YanWen,LuigiCavallo,WencaiRen,HuimingCheng,andXixiangZhang*CiteThis:J.Phys.Chem.Lett.2021,12,2219−2225ReadOnlineACCESSMetrics&MoreArticleRecommendations*sıSupportingInformationABSTRACT:Two-dimensional(2D)materialshaveattractedsignificantattentionfortheirabilitytosupportnovelmagneto-electricaltransportandtheiropticalandmagneticproperties,ofwhichtheirsuperconductivityisparticularlyofinterest.Herewereportonthebehaviorofsuperconductivityin2DMo2Ccrystalswhenhydrostaticpressureisapplied,whichhasnotyetbeendescribedintheliterature.Wefoundthatthelocalizationofboundaryatomsdisorder-inducedCooperpairscansuppressthesuperconductingtransitiontemperature(Tc)aseffectivelyasamagneticfieldandcurrent.WeobservedthattheTcinitiallydecreasedasthepressureincreasedto1.75GPabutthenbegantoincreaseasthepressureincreasedfurtherto2.5GPa.Ourdensityfunctionaltheorycalculationsrevealedthatthisbehaviorwaslinkedtothemodulationofthestrengthoftheelectron−phononcouplingandtheelectronproperty,whichwastriggeredbycompressionofthelatticeunderhighpressure.Weattributedtheinflectionpointinthehydrostaticpressure-dependentTccurvetothestructuralphasetransitionofMo2Cfromahexagonaltoanorthorhombicstructure.ThisworkpresentsanewavenueforthestudyofthesuperconductivityofMo2C,whichcanbeextendedtoapplytoother2Dsuperconductorstomodulatetheirelectronicstates.22Two-dimensional(2D)materials,basedonavarietyofgatedMoS2transistorsistuned.Thus,exploringandtuningchemicalcomponents,suchastransitionmetalchalcoge-superconductivityin2Dmaterialsisafundamentalresearch1,23−56,78nides,carbides,nitrides,orcarbonitrides,havetopicofgreatinterestinthefieldsofcondensedmatterphysicsattractedconsiderableattentionsincethediscoveryofandmaterialsscience.9graphene.These2DmaterialshaveseveralappealingRecently,theapplicationofexternalpressurehasbeen10,11properties,suchasanultralowweight,ahighYoung’semployedtomodulatethephysicsofsuperconductors;i.e.,theDownloadedviaUNIVOFNEWMEXICOonMay16,2021at15:04:50(UTC).modulus,highflexibility,opticaltransparency,outstandingcrystallinestructure,electronicstate,andphononfrequencycarriermobility,andalongspin-diffusionlengthsuitableforcanbeeffectivelychangedbyapplyingexternalpressure.ForSeehttps://pubs.acs.org/sharingguidelinesforoptionsonhowtolegitimatelysharepublishedarticles.spintronicsdevices.2Dsuperconductorshavesimilarlygainedexample,studieshaveshownthatthecriticaltemperature(Tc)particularinterestduetotheirattractivephysicalpropertiesinofsuperconductivityandthechargedensitywave(TCDW)canlow-dimensionalelectronicsystems,includingsuperconductingbeincreasedbymorethananorderofmagnitudewhenhighfluctuations,BKT(Berezinsiii−Kosterlitz−Thouless)transi-pressureisappliedto2H-TaSand2H-TaSe.23Besides,the22tions,andquantumphasetransitions(QPTs)atzerosuperconductivitycanbeachievedinsemiconductive2H-12,13temperatures,amongothers.EnormouseffortshavebeenMoSatanultrahighpressure.24Aninversecorrelation2devotedtostudying2DsuperconductivitysincethediscoverybetweenquasiparticlemassandTchasalsobeendemonstrated14−16ofsuperconductivityinPbandSnthinfilms.Forexample,whenhighpressureisappliedtoYBaCuO.25However,the248‑δQPTsbetweenthesuperconductingandinsulatingphasewasbehavioroftransitionmetalcarbides(TMCs)underhydro-discoveredinasuperconductingthinfilmwithathicknessin17staticpressurehasnotyetbeendescribed.Thus,weaddresstherangeof0.3−10nm.A“dirty-boson”modelwas18,19proposedtostudythiseffect.AlargeRashbaspin-splitting,causedbyspin−orbitinteractions,wasalsofoundin2DReceived:January8,2021superconductivemetals.20Asaresult,interfacialsuper-Accepted:February22,2021conductivitywasrealizedatthepolarizedinterfaceofPublished:February26,202113,21LaAlO3/SrTiO3film.Furthermore,studieshaveindicatedthatthein-planecriticalfieldcouldbefarbeyondthePauliparamagneticlimitwhenthesuperconductivityoftheionic-©2021AmericanChemicalSocietyhttps://dx.doi.org/10.1021/acs.jpclett.1c000712219J.Phys.Chem.Lett.2021,12,2219−2225

1TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetterFigure1.MorphologyandmicrostructureofMo2Ccrystal:(a)opticalimage;(b)EDXmap;(c)atom-resolvedHAADF-STEMmicrograph;(d)crystallinestructure;(e)AFMimage;(f)thethicknessoftheMo2Ccrystal.thisknowledgegapbystudyingthehighpressure-relatedphysicsof2DTMCs.ManyTMCs(suchasMo2C,W2C,WC,TaC)aresuperconductorsandcanbereadilypreparedbychemicalvapordeposition(CVD),whichprovidestheidealplatformtoinvestigatesuperconductivityandnewphysicsinalow-dimensionalmaterial.Inthiswork,westudythesuperconductivityof2DMo2Cwhileapplyingexternalpressure.Wedemonstratethattheboundaryatomsdisorder,themagneticfield,andthecurrentcanaffectthesuperconductingbehavior.Specifically,weshowthattheTccanbemodulatedbyapplyinghighhydrostaticpressure.Finally,weemploydensityfunctionaltheory(DFT)calculationstoexplorethisphenomenonincloserdetail.Theprepared2DlayeredMo2Ccrystalshaveaperfecthexagonalshapewithanedgelengthof7μmandasmoothsurface,asseenintheopticalimageinFigure1a.TheuniformdistributionofMoandCatomsintheMo2CnanosheetwasverifiedbytheSTEM-EDXmapping(Figure1b,FigureS1),inwhichthemolarratioofMoandCwasabout2:1.FromtheFigure2.TransportpropertiesoftheMo2Ccrystalatzeropressure.atom-resolvedHAADF-STEMimage(Figure1c),wecon-(a)Temperaturedependenceoflongitudinalresistance(R(T))measuredat20μA.TheinsetistheopticalimageoftheMo2CfirmedthatourMo2Cnanosheetshadahexagonalcrystallinenanodeviceusingthefour-probemeasurements.Thethicknessofthestructureandthattheirgrowthdirectionwasalongthe[100]Mo2Cis9.2nm.(b)NormalizedR(T)curvesoftheMo2C-baseddirection.Figure1dclearlyshowsthearrangementofthedevicewithdifferentthicknessesmeasuredat20μAwithoutaatomsinahexagonalcrystallinestructure,inwhichthesmallermagneticfield.(c)R(T)curvesoftheMo2CdeviceunderdifferentradiusCatomsintercalatedintothelargerradiusMoatomstoperpendicularmagneticfieldsforthedevicewith9.2nmthickMo2C.forma2Dinterfilledstructure.Inaddition,wefurtherverified(d)R(T)curvesoftheMo2CdevicemeasuredatdifferentcurrentsthehighqualityoftheMo2Ccrystalsbytheatomicimageforthedevicewith7.4nmthickMo2C.(Figure1c),wherenoobviousdefectsorimpuritieswereobserved.BasedontheAFMimagesinFigure1e,f,wefoundmagnifiedR(T)curvefortheMo2Cnanosheet,revealingthatthatthethicknessofthethickestMo2CnanosheetwasstilltheMo2CcrystalbecamesuperconductingatTc=4.5K.Thismoderatelyuniformataround11.8nm.apparentsuperconductivityisrelatedtotheCooperpairsthatWefirststudiedthesuperconductivityoftheMo2Cwerecoupledbyelectron−phononinteractionsneartheFerminanosheetbyvaryingthethicknessandmagneticfield.Figurelevel.NotethattheBKTphasetransition(Tϕ)occurredbelow32ashowsthetemperaturedependenceoftheresistance(R(T))Tc(Tϕ

2TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetterFigure3.High-pressuretransportperformanceoftheMo2Ccrystal.(a)R(T)curvesof9.2nmthickMo2Ccrystalunderdifferentpressuresrangingfrom0to2.5GPa.(b)MagnifiedviewofR(T)curvesaroundTc.(c)VariationofTcunderincreasinghydrostaticpressureupto1.75GPa.(d)Schematicstructureofthephase-transitionprocessunderhighpressure.(e)VariationofTcunderincreasinghydrostaticpressurefrom0to2.5GPa,andthecorrespondingcrystalstructure.Figure4.ElectronicstateoftheMo2Ccrystalatdifferenthydrostaticpressures:(a)calculateddensityofstate(DOS),(b)phonondensityofstate(PHDOS),and(c)Eliashbergspectralfunctionα2F(w)forhexagonalMoCundervariouspressures;(d)DOS,(e)PHDOS,and(f)Eliashberg2spectralfunctionα2FfororthorhombicMoCundervariouspressures.2theMo2Cnanosheetsreducedfrom11.8to6.6nm.ThisWealsostudiedtheeffectofamagneticfieldonthephenomenonwasalsoobservedintheCo-basedJosephsonsuperconductingbehavioroftheMo2Cnanosheet.TheR(T)junction,272H-NbSe,28,29andNbS,30highlightingtheeffectcurvesoftheMo2Cunderdifferentmagneticfieldsare22ofthicknesson2Dsuperconductivity.ThesuppressionofpresentedinFigure2c,wheretheappliedmagneticfieldwassuperconductivityinthicker2Dmaterialsmaybeduetotheperpendiculartothedeviceplane.WefoundthattheTcshiftedtoalowertemperature,andthetransitiontemperaturerangedisorder-inducedCooperpairslocalization,theunboundbroadenedasthemagneticfieldincreased.Weproposetwovortex−antivortexpairs(i.e.,asindicatedbytheBKT26,31mechanismstoexplainthisobservation.Thefirstmechanismmechanism),orthereductioninCoulombscreening,involvesthebreakingoftheorbitalpair,wherethemagneticbecausethinnerMo2Ccrystalsaremoredisorderedduetothevorticespenetratethesuperconductors,andthegeneratedhigherratioofboundaryatoms.Here,weattributetheLorentzforcethenbreakstheCooperpairapart.ThesecondsuppressedTcintheMo2Ccrystalstothedisorder-inducedmechanisminvolvesZeemansplitting,wherethestrongCooperpairslocalizationduetothevortex−antivortexpairsmagneticfieldbreaksthepairedelectronsinthespin-singlet.thatappearbelowTc(Tϕ

3TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLettertheself-magneticfieldplayedthesameroleastheexternalhydrostaticpressure.Thetotalenergyoftheorthorhombicmagneticfield.NotethatthesuperconductingstateofMo2CMo2CismuchlowerthanthehexagonalMo2C(Figure3e).waswellmaintainedinFigure2duntilthecurrentincreasedtoThus,thisstructuralphase-transitionprocesscanbereasonably1mA,demonstratingthatthemaximumcurrentunderwhichexpectedinthehexagonalMo2CfromanenergydynamicstheMo2Ccanmaintainzeroresistanceishigh.Ourresultsperspective.Meanwhile,asimilarphasetransitionwasalsoindicatethatMo2CsuperconductorshavebroadpotentialobservedwhenthehexagonalMo2Cwasirradiatedbyan3235applicationsindevicesbasedontheJosephsoneffectandelectronbeaminTEM.Thisphenomenonalsosupportsourfaultcurrentlimiters,amongothers.Wefurtherstudiedthehypothesisthatthestructuralphasetransitionexistsinsuperconductivityofthe9.2nmthickMo2CcrystalathighhexagonalMo2C.pressuretoexploretheimpactofpressureontheelectronicWefurtherstudiedthevariationoftheelectronicstateunderstructure.Thetemperature-dependentresistancemeasuredatvaryingpressureconditionsusingDFTsimulations.IncreasingdifferentpressuresispresentedinFigure3a.WefoundthatthethehydrostaticpressureleadstotheshrinkingofthelatticeTcdecreasedgraduallyfrom4.13to3.84Kwhenthepressureconstant,anditcanfurtherinducethevariationoftheincreasedfrom0to1.75GPa.Figure3cshowsthealmost36electronicproperties.Thepressure-dependentdensityoflinearvariationofTcwithpressureatarateof0.166K/GPainstate(DOS)inFigure4apresentsthistrend:bothvalencethispressureregion.Thisfeatureissimilartothatfoundinbandmaximum(VBM)andconductionbandminimumtraditionalsuperconductors.ThevariationofTcintermsofthe(CBM)haveablueshifttowardthehigherenergeticstatespressurecanbedescribedfurtherbyMcMillian’sformulawhenwiththeincrementofpressure.Moreover,theFermilevelcan33,34Tϕ

4TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLettersuperconductors,suchasthe2Dtransitionmetalcarbides,α()ω={ijj4πyzz[+−()/εε221/2ε]2}1/2transitionmetalnitrides,transitionmetalcarbonitrides,jz121kλ{(3)transitionmetalchalcogenides,amongothers.TheMo2CcrystalsweregrownonaCufoilsubstrateusingwhereε1andε2aretheaveragedrealandimaginarypartsoftheCVDmethoddescribedinthepreviousresearch.3Thehighthefrequency-dependentdielectricfunction,respectively.ThepurityCufoil(99.5%)wasplacedontopofaMofoil(99.5%)phono-spectrumwascalculatedusingthefinite-difference40ofthesamesize,wheretheCufoilactedassubstrateandthemethodthroughanopen-sourcePhonopypackage,andtheMofoilasthesource.Thesamplesweretransferredintoa1in.microperturbationoftheatomicpositionswasperformedonatubefurnaceandheatedto1085°Cunderaflowof200sccm2×2×3supercell.H2.Aflowof0.35sccmCH4wasintroducedasthecarbonsourcetogrowtheMo2Ccrystal,andthewholereaction■ASSOCIATEDCONTENTprocesswascarriedoutatambientpressure.Thegrowthtime*sıSupportingInformationwascontrolledbetween2and50min,fortuningthesizeandTheSupportingInformationisavailablefreeofchargeatthicknessofthepreparedMo2Ccrystal.Thesampleafterhttps://pubs.acs.org/doi/10.1021/acs.jpclett.1c00071.growingwasthenquenchedtoroomtemperaturetoavoidtheElementalmapsandEDXspectrumoftheobtainedimpurityphase.Mo2Csample(PDF)TheMo2CnanosheetwastransferredtoaTEMgridoraSi/SiO2substrateusingthepolymerstampmethodreportedpreviously.3Thecrystalstructureandchemicalcompositionof■AUTHORINFORMATIONtheMoCsinglecrystalswereanalyzedusingatransmissionCorrespondingAuthor2electronmicroscope(TEM,FEITitanThemisZ).Atom-XixiangZhang−DivisionofPhysicalScienceandEngineeringresolvedimageswererecordedbyaprobe-correctedhigh-angle(PSE),KingAbdullahUniversityofScienceandTechnologyannulardark-fieldscanningtransmissionelectronmicroscopy(KAUST),Thuwal23955-6900,SaudiArabia;(HAADF-STEM)withaprobesemiconvergentangleof23.5orcid.org/0000-0002-3478-6414;mradandacollectinganglerangeof60−200mrad.ToavoidEmail:xixiang.zhang@kaust.edu.sathebeamdamage,theatomicHAADF-STEMimageswereAuthorsobtainedbyanimageseriescomposedof20fastexposureJunliZhang−DivisionofPhysicalScienceandEngineeringimagesofthesameregion,followedbyapost-acquisition(PSE),KingAbdullahUniversityofScienceandTechnologyimagedistortioncorrectionandaveragingwiththededicated(KAUST),Thuwal23955-6900,SaudiArabia;Keyscript.ThechemicalcompositionofthesamplewasexaminedLaboratoryofMagnetismandMagneticMaterialsofthebytheenergy-dispersiveX-rayspectroscopy(EDX,BrukerMinistryofEducation,SchoolofPhysicalScienceandSuper-X)attachedonthemicroscopy.Thethicknessoftheα-Technology,LanzhouUniversity,Lanzhou730000,China;Mo2Cwasmeasuredbyatomicforcemicroscopy(AFM,orcid.org/0000-0002-8671-2417AsylumResearchMFP-3D).ThenanodevicesfortransportZhenCao−DivisionofPhysicalScienceandEngineeringmeasurementswerepreparedusingelectronbeamlithography(PSE),KingAbdullahUniversityofScienceandTechnology(EBL,Crestec-9000C).ThetransportpropertiesoftheMo2C(KAUST),Thuwal23955-6900,SaudiArabiadevicesweremeasuredusingaQuantumDesignphysicalXinHe−DivisionofPhysicalScienceandEngineering(PSE),propertiesmeasurementsystem(PPMS).Thehigh-pressureKingAbdullahUniversityofScienceandTechnologycellmodel(QuantumDesignHPC-33)wasusedforhigh-(KAUST),Thuwal23955-6900,SaudiArabiapressureelectricalmeasurements.TheDaphne7373oilwasWenhaoLiu−DivisionofPhysicalScienceandEngineeringusedasapressuremediuminthispressurecell.(PSE),KingAbdullahUniversityofScienceandTechnologyTheDFTcalculationswereperformedatthePerdew−(KAUST),Thuwal23955-6900,SaudiArabia37Burke−Ernzerhof(PBE)leveloftheexchangefunctionals.YanWen−DivisionofPhysicalScienceandEngineeringTheelectronsweredescribedusingtheProjectorAugmented-(PSE),KingAbdullahUniversityofScienceandTechnology38WaveMethod(PAW)implementedintheVASPpackage,(KAUST),Thuwal23955-6900,SaudiArabiawithanenergycutoffof450eV.TheBrillouinzonewasLuigiCavallo−DivisionofPhysicalScienceandEngineeringsampledusing3×3×3k-pointscenteredattheγpoint.The(PSE),KingAbdullahUniversityofScienceandTechnologyprototypicalmodelforthehexagonalstructureandortho-(KAUST),Thuwal23955-6900,SaudiArabia;rhombicstructure,theMo2Cstructurewasconstructedasorcid.org/0000-0002-1398-338Xfollows:thebulkhexagonalstructureMo2CwasrepresentedbyWencaiRen−ShenyangNationalLaboratoryforMaterialsasupercellcontaining2×2×2unitcells.TheorthorhombicScience,InstituteofMetalResearch,ChineseAcademyofstructureMo2CwasconstructedfollowingasimilarmethodSciences,Shenyang110016,China;orcid.org/0000-0003-outlinedinapreviousstudy39becausethecarbonatomsinthe4997-8870crystaldatabasewerehalf-occupied.ThecellparametersandHuimingCheng−ShenyangNationalLaboratoryforatomicpositionswereoptimizedusingthecriteriathatMaterialsScience,InstituteofMetalResearch,ChinesestipulatethattheHellmann−FeynmanforceoneachatomAcademyofSciences,Shenyang110016,China;waslessthan0.001eV/Å.Thereafter,toevaluatethevariationorcid.org/0000-0002-5387-4241oftheelectronicpropertiesduetotheappliedexternalCompletecontactinformationisavailableat:pressure,weconstructedfurthermodelswith1−4%shrinkagehttps://pubs.acs.org/10.1021/acs.jpclett.1c00071ofthecellparameters.Theatomicpositionswerefurtheroptimizedusingthesamecriteria.TheabsorptioncoefficientAuthorContributions#wascalculatedfromJ.L.J.andZ.C.contributedequallytothiswork.2223https://dx.doi.org/10.1021/acs.jpclett.1c00071J.Phys.Chem.Lett.2021,12,2219−2225

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