COOPERATIVE OPTICAL PHENOMENA AT 0,3 KELVIN

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COOPERATIVE OPTICAL PHENOMENA AT 0,3 KELVIN

J. Thorne, S. Bramwell, P. Day, R. Denning, A. James

To cite this version:

J. Thorne, S. Bramwell, P. Day, R. Denning, A. James. COOPERATIVE OPTICAL PHE- NOMENA AT 0,3 KELVIN. Journal de Physique Colloques, 1985, 46 (C7), pp.C7-561-C7-565.

�10.1051/jphyscol:19857100�. �jpa-00224959�

COOPERATIVE OPTICAL PHENOMENA AT 0t3 KELVIN

J.R.G. Thorne, S.T. Bramwell, P. Day, R.G. Denning and A.C. James

Department of Inorganic Chemistry,

Parks Road, University of Oxford,

Abstract

Using a novel helium3 cryogenic system we obtain data on a variety of transition n-etal cchnpounds showing tenperature dependent optical and magneto optical behaviour below 1 Kelvin.

The cryostat (Oxford Instruments, custom built) is a carbon sorb pumped liquid helim-3 system.

pumped helium-4 pot at 1.2 K is used to effect condensation of helium3 which, in turn, is purged by a graphite sorb held at 4.2 K. The helium3 (10 cm3 liquid) has a hold time of -5 hours and can be recycled by heating the sorb to 50 K to cause rewndensation. Temperature measurement utilises a g e m i u m resistance thermometer imnersed in liquid helium three in contact with the sample plate. Base tmprature is -0.32 Kelvin and rises to -0.35 Kelvin with a pulsed laser input of -1m. Laser heating does not normally cause problems.

Since samples are imersed in coolant, t h e m 1 conduction is good. We have investigated true crystal temperatures in caesium uranyl chloride samples, utilising the selectively dispersed luninescence from the two lawest electronic origins which have a separation of 1.6 an-'. The Boltvnann populations of each provide a mlecular thexmmeter. Observation of energy transfer k t w e e n these origins is also possible. This occurs with a t k constant of 1 microsecond by emission of a single phonon in an intramolecular process.[ll

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19857100

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The following examples illustrate the advantages of being able to observe low energy activated phonon processes, wakly magnetically ordered systems, and of preventing t h e m 1 detrapping.

i) Maqnetic orderinq phenomena:

Below the Neel temperature of 0.6 K Cs3MnC15 behaves as a three d k s i o n a l antiferromgnet. Polarised absorption measurements made in this laboratory[2] show that transitions have electric dipole character and arise frcm 6 ~ 1 g r o d state to the

excited states. Rivoal et al. [3] report the low temperature behaviour of the

bands. We show here that a similar temperature dependence is found for the 4 ~ 1 origins (marked A and B below), which are too

to be seen in absorption and are observed only in excitation. The small ground state splitting that this dependence reflects is seen to be a cooperative magnetic effect since in a magnetically dilute material Cs3Mg(Mn %)Cl5 no such temperature effect is seen. In this crystal the bands A and B are shifted to lower energy and appear broadened and doubled. The doubled peaks are tentatively assigned to moncmr and magnetically coupled dimer.

Figure 1

f Temperature Dependence of 1, origlns

3

Kelvin t h e i n t e n s i t i e s of a l l bands v a r i e s approximately a s t h e square of t h e temperature a s predicted[S] f o r a two-dimensional easy plane ferramgnet. Below 2 K the

a t 15840 6' resolves i n t o two ccmponents, a spin wave creation band having a l m s t teinprature independent intensity, and a spin wave annihilation sideband whose i n t e n s i t y increases a s ~ ~ e x p ( - ~ (

, where E( 0

i s t h e zone centre gap i n t h e spin wave spectrun.

W e nckl

report t h e observation of t h e pure exciton band which is revealed only when t h e temperature reaches 0.8 K. The i n t e n s i t y of t h e hot band is indeed seen t o drop t o zero a t f i n i t e temperatures and we may make an accurate estimate of t h e gap i n t h e s p i n wave dispersion. Figure 2 shows the l o w temperature behaviour of t h e band _and the temperature dependence of the derived hot band (absorption a t T minus absorption a t 0.35

The f i t t e d curve has E(O)=0.83 6'.

Figure

T-mpraturr &pmd-no- of hot b-d intmn-Sty

Rb2CrClq

Absorption

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ii) Electronic enerqy miqration in the solid state:

Excitation localisation in ordered and disordered media is of great theoretical and practical interest. In a molecular system the rate of transport of electronic energy is controlled by twu factors, the size of the intemlecular coupling and the local inhomogeneity or energy difference between the two ions involved in the transfer process. If the latter greatly exceeds the f o m r , transport will be possible only with phonon assistance (non-resonant) and will be quenched as the temperature tends to zero.

By definition, resonant transport exhibits no spectral changes. The spreading of an initially localised wavefunction is revealed only by the final trapping of the excitation at a distant site. If the intermlecular coupling is

(<l &l), then to establish whether transport is assisted, we require temperatures less than 1 Kelvin in order to stop phonon participation.

We

show data for the uranyl ion

ion in single crystal caesium uranyl chloride which may be isotopically labelled in the oxygen atom of the uranyl group to produce lattice traps or anti-traps. We create a situation in which

u ~ ~ o ~ ~ + (0.5% abundance, trap depth 10 an-') and

(0.65%, trap depth

5 c m - ' ) are present in a

lattice. The bulk lattice is selectively excited and the fluorescence spectrummonitored with tim. Figure 3 shows the evolution of both trap emissions. At 0.4 Kelvin no back transfer from these trapped sites can occur.

Transfer to traps m y be diffusion or trap-step limited. The final trapping step itself is k n m to be almst independent of temperature at 1o.v temperature, and to depend u p n the cube of the energy defect or trap depth.[6]

Thus single step transfer to

is eight times faster. We conclude that

migration is trap-step controlled, since the deepr trap emission appears more

rapidly. Were it diffusion controlled, each isotopic species would appear in

proportion to its relative abundance in the crystal. The quenching of

phonon-assisted processes at law tgnperature does not inhibit the migratory

passage of the excitation towards trapping sites.

Cs2U02C14

0.35 K

Time resolved Emission

(MICROSECONDS)

Conclusion:

In s u m % y we observe that it is only by conducting optical experiments at such low temperatures that it is possible to answr fundamental questions abut weakly interacting systems.

References:

/l/ Thorne, J.R.G. Denning,R.G. , Barker,T. J. and Grhley ,D. I., J.L&n. in press /2/ Tacon,R.J., Ph.D.Thesis, University of Oxford (1978)

/3/

Riv0a1,J.C.~ Briat,B. and %rre,J.P., J.Appl.Phys. 53 ^{(1982) 2713}

/4/

Janke,E., WX~,T.E.~ 1r0nside~C.N. and Day,P., J.Phys.C: Solid State Physmr 15 (1982) 3809

L

/5/ Gregson,A.K., Day ,P. , Oki ji,A. and Elliott ,R. J. J. Phys.C: Solid State Phys., 9 (1976) 4497

-

/6/ Denning,R.G. , 1ron~ide~C.N. , ^Thorne, J.R.G. and Woodwark,D.R. , ^{Mol .Phys.} , 44

(1981) 209