重要-Phonon focusing in cubic crystals 外文学习材料

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1、PHYSICALREVIEWBVOLUME32,NUMBER415AUGUST1985PhononfocusingincubiccrystalsD.C.HurleyandJ.P.WolfePhysicsDepartmentandMaterialsResearchLaboratory,UniversityofIllinoisatUrbanaC—hampaign,Urbana,Illinois61801(Received25January1985)Thenatureofthephononintensitypatternassociatedwithlow-te

2、mperatureheat-pulsepropaga-tionisexaminedtheoreticallyforawiderangeofcubiccrystals.Ingeneral,hugeanisotropiesinthethermalfluxarepredictedduetophononfocusing.Thephonon-focusingpatternsofcubiccrystalsdividenaturallyintotwomajorgroups,asdistinguishedbythesignoftheiranisotropyfactorE

3、=Cll/C44—Cl2/C44—2.ExperimentalandtheoreticalintensitypatternsofCaF2andSiinthelong-wavelength,dispersionlesslimitareexaminedindetailtoillustratethetypicalfocusing-patternfeaturescharacteristicofpositiveandnegativeA.Forbothregimes,theevolutionofparticularpat-ternstructuresisstudie

4、dsystematicallyastheelasticconstantsarevaried.Investigationoftheflux-intensitypatternandslowness-surfacetopologyforvariouselasticconstantsrevealsdistincttrendsinthephonon-focusingstructures.Basedonthisstudy,wehavedefinedasmallnumberofangulardimensions(oneortwoforeachphononmodeand

5、anisotropyregime)whichquantitativelydeterminetheprincipaldirectionsofsingularflux.Thevariationsoftheseanglesaremappedontothegeneralelastic-parameterspace.Wethuspresentameansforquicklydeterminingtheangularpositionsoftheprinciplephonon-focusingcausticsforanarbitrarysetofelasticcons

6、tants.I.INTRODUCTIONoffocusingpatternsoverawiderangeofelasticconstants.Inadditiontorevealingtopologicaldetails,thisgeneral-Atlowtemperatures,theballisticheatfluxemittedizedsummarymayserveasapracticalreferenceforfromapointsourceinacrystalisstronglyconcentrated—characterizingthehea

7、t-fluxanisotropyinaparticularalongcertaincrystallinedirectionsaneffectknownascrystalwithouthavingtoresorttoadetailedcalculation."'"phononfocusing.Thenatureanddegreeofangularanisotropythatispresentinthecrystalsheat-fluxintensi-II.BACKGROUNDANDMETHODStypatternaredirectlydeterminedb

8、ytheelasticconstantsofthecrystal.Phonon-