Optical spin orientation of excitons in GaSe under longitudinal magnetic field

Solid StateCommunications,Vol. 18,Pp. 889—891,1976. PergamonPress. Printedin GreatBritain

OPTICAL SPINORIENTATIONOFEXCITONS IN GaSeUNDER LONGITUDINAL MAGNETIC FIELD

F. Minami,Y. OkaandT. Kushida

TheInstitutefor Solid StatePhysics,TheUniversityof Tokyo, Roppongi,Mmato-ku,Tokyo 106, Japan

(Received21 October1975by Y. Toyozawa)

The degreeof circularpolarizationof the free-excitonluminescencelinehasbeenmeasuredin GaSeexcitedby circularly polarizedlight at 4.2Kunderlongitudinalmagneticfield. The resultshowsthat the spinrelaxationtime of excitonis field-dependentbutthe spinmemorybeforereachingtheexcitongroundstateis almostunaffectedby the appliedlongitudinalmag-neticfield.

THE OPTICAL orientationof spinsof freecarriersand whereL~(orL) is the intensityof the luminescenceexcitonshasrecentlybeenstudiedin varioussemicon- componentwith thephotonspin parallel(oranti-parallel)ductors.13It hasbeenfoundthatvaluableinformation to thatof the exciting light. In orderto measurethisonthe relaxationkineticsof theseexcitedparticlescan quantity,a X/4 platerotating~at 33 Hz anda linearpolar-beobtainedby this technique.Especially,the studyof izer wasplacedbetweenthesampleand thespectrometer.the degreeof polarizationof luminescencewithvarying Then,the outputsof the two lock-in amplifierssetattransversemagneticfield is a powerfulmethodto deter- 750and66Hz representthe averageintensity(L’ + L )/2mine thelifetime as well asthe spinrelaxationtime,and andthe difference(L~—L)/2, respectively.Theseout-a numberof experimentshavebeencarriedoutby this putswereprocessedby acomputerandthe degreeofmethodon variousmaterials.As to the effectof applied polarizationp was displayedon a chartrecordersimul-longitudinalmagneticfield, however,onlya few works taneouslywith the ordinaryemissionspectraanddif-havebeenreportedon the opticalorientationsof free- ferencesignalspectra.Althoughthemeasurementwaselectronspinsandnuclearspins.46In the present madeon thewholespectralregionof luminescence,incommunication,we reportthe measurementof the de- thepresentcommunicationwe confineourselvesto thegreeof polarizationof excitonluminescencein GaSe free-excitonline (cf. Fig. 1).underlongitudinalmagneticfield. In additionto the In a magneticfield H

1 appliedparallelto thec-axis,first observationof theeffect of the longitudinal field on thedegreeof polarizationof luminescenceis givenby thethe opticalspinorientationof excitons,thefield following expression:

4dependencesof the spinrelaxationtime andthe spin T r / Hmemorybeforereachingtheemitting statehavebeen p(H,) = G + tanh

1~11~‘J , (2)obtainedseparately. T1 + T T1 + T \ 2kT

TheGaSecrystalgrownby Bridgmanmethodwas wherer andT1 arethe lifetime andthespinrelaxationdirectly immersedin liquid He andexcitedby circularly time of theemittingstate,G is a phenomenologicalpara-polarizedlight (A = 570nm) incidentparallelto the meterwhich characterizesthe spinmemorybeforereach-crystalc-axis. Theexcitationsourceemployedwasa c.w. ing the luminescentlevel,g11 is theg-valueof the emittingtunabledyelaser(SpectraPhysicsmodel 370)pumped statein theparallelconfiguration,PB isthe Bohr mag-by anargonion laser.The excitingbeamwaschopped neton,k is the BoltzmannconstantandT is theabsoluteat 750Hz, passedthrougha circular polarizer(Polaroid temperatureof the crystal. In casethat thespinmemoryHNCP37),andthen focusedinto the sample.Theback- is completelylost,we obtainonly the secondterminward-directedluminescencewasanalysedby a 0.75m the right-handside of equation(2). We shallcall the firstsinglegratingspectrometer(Spexmodel 1700).The andthe secondtermsof this equationthespinmemorysignalwasdetectedby an S-20type photomultiplier termand thethermalizationterm.Becausetheyare even(HamamatsuTV modelR453)andfed into two lock-in andodd functionsof H1, respectively,we canobtainamplifiers(PARmodel 121).The degreeof circular thesetwo termsseparatelyby making~ {p (H,) + p(— H1)}polarizationof luminescenceis definedas and~{p (H,) — p(— H1)}.

L~—L Figure 1 showsthemagnitudes.ofthesetwo terms= L~+ L - , (1) thusobtainedfor the exciton line asa function of the

appliedlongitudinal magneticfield. We seethatboth

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890 OPTICAL SPINORIENTATION OFEXCITONS IN GaSe Vol. 18,No. 7

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GaSe 4.2K20 / Excitation 570nm -~

FE 2 10 - S

10 0.5 /600 595 590

7 A(nm)

0 I

0 5 10 15 ‘~0 5 10 15H

3 (kG) H, (kG)Fig. 1. Longitudinalmagnetic-fielddependenceof the . .

magnitudesof~{p(H,)+ p(— H,)) (uppercurvea)and Fig. 2. The ratio of the spinrelaxationtime T1 and the~{p(H,) — p (— H,)} (lower curveb) measuredat the lifetime r of exciton,andthe spinmemoryG beforefree-excitonline center(587.8nm) of GaSeat 4.2K. The reachingthe excitongroundstateasa function of theemissionspectrumof thestudiedsampleis alsoshownin appliedlongitudinalmagneticfield.the inset,wherethe free excitonline is denotedby anarrow. of the intermediateexcitonstatesdiscussedby Bonnot

eta!.8 Sincethedensityof thefreeexciton luminescencevaluesincreasewithH,. The field dependenceof line did not changeby theapplicationof an external~{p (H,) — p (— H,))was foundto coincidewell with magneticfield, theexciton lifetime r is consideredto bethatmeasuredunderthe excitationby linearly polarized independentof H,. Therefore,themagnetic-fielddepend-light. This factconfirmsthat this quantityis dueto the enceof T1/r is ascribedto that of T1. The increaseofthermalizationtermwhich is irrelevantto thespinmem- thespinrelaxationtimewithH1 is explainedby amech-ory. From this thermalizationterm we candetermine anismanalogousto the motionalnarrowingeffect. Whenthefield dependenceof T1 /r by theuseof equation(2). the Larmorfrequencyof excitonspinexceedstherateThe resultis shown in Fig. 2. The valueof g11 —~2.7~was of the modulationof randomlocal field which isusedin this analysis.It is notedthat thespin relaxation responsiblefor the spinrelaxation,the effectivetimeoftime T1 is of thesameorderof magnitudeas the life- interactionbetweentherandomfield and theexcitontime i-. As H, is increased,T1 /r first increases,then is spinis not determinedby themodulationratebut issaturatedaroundH, of 8 KG. Thefield dependenceof reducedbecauseof therapidmotionof the magneticG is alsoplottedin Fig .2. Thiswasobtainedfrom the moment.Thus,the spinrelaxationtimebeginsto slowspinmemoryterm by usingtheH,-dependenceof T1/r. downwhentheLarmor frequencybecomescomparableWe notice that G is almostindependentofH, andabout to themodulationrateof therandomlocal field. From0.4—0.5. theresult of Fig. 2, the lattermodulationrateis estimated

Theflat dependenceof G on H, may bereasonable to beof the order of 1010 sec* The saturationof T11rbecausetime requiredfor the exciton to reachthe lowest at largevaluesof H, may indicatethepresenceof anotherstateafter theoptical generationis consideredto bevery spinrelaxationmechanismwith muchhighermodulationshortcomparedwith T1 (— r). That theabsolutevalue rate.More detaileddiscussionof theH,-dependenceofof G is lessthan unity may bedue to theL—Tsplitting T1/r will be given in a separatepaper.

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