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Submitted on 1 Jan 1978
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THE PHONON SPECTROSCOPY OF DOPED
MAGNESIUM OXIDE USING SUPERCONDUCTING
TUNNEL JUNCTIONS
F. Hasan, P. King, D. Murphy, V. Rampton
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-993
THE PHONON SPECTROSCOPY OF DOPED MAGNESIUM OXIDE USING SUPERCONDUCTING T U N N E L J U N C T I O N S
F.Hasan, P.J.King; D.Murphy and V.W. Rampton.
Department of Physios, University of Nottingham, University Park, Nottingham NG7 2RD, UK.
Résumé.- Nous avons fait des expériences sur MgO contenant des ions C r2 +e t M n 3+, utilisant des pho-nons quasi-monochromatiques produits par des jonctions tunnel supraconductrices. Dans les deux cas nous avons observé l'absorption d'énergies de 4,9 cm-1 et 9,5cm-'. Nous pensons que ceci pourrait être attribué aux transitions de l'état fondamental A aux états E et A_ respectivement. L'écart spin-orbite Dserait alors 2,4 cm .
. . 2+ 3+
Abstract.- Experiments have been made on MgO containing Cr ions and Mn ions using quasi-mono-chromatic phonons generated by superconducting tunnel junctions, in both cases, absorptions at ener-gies of 4.9 cm~l and 9.5 cm have been observed. These are tentatively assigned to transitions from the At ground state to the E state and A. state respectively. The spin-orbit splitting D is
then 2.4 cm-1.
INTRODUCTION.- The problem of Jahn-Teller ions in magnesium oxide has been studied for a number of years. Such ions include Cr , Mn and V . These are all strongly coupled to the host crystal lattice and give strong phonon scattering as is shown by ultrasonic attenuation, acoustic paramagnetic reso-nance and low temperature thermal conductivity
mea-2+ surements. References to recent work on Cr in MgO
3+
are given by Bates IM and to work on V in MgO by King and Oates 111. We have made experiments using superconducting tunnel junctions to investigate the scattering of quasi-monochromatic phonons by these ions.
2+ 3+ .
THEORY.- The Cr and Mn ions are isoelectronic and in both cases the cubic crystal field of MgO splits the ground state of the free ion leaving a
E state lowest. The Jahn-Teller interaction and the spin-orbit splitting result in a lowest group of ten states having symmetries A , T , E, T„, A,. There is an excited group of levels having symme-tries T. or I, and E.The relative positions of the-se levels are a function of the spin-orbit splitt-ing parameter D, the Jahn-Teller tunnellsplitt-ing split-ting 6 and the anharmonicity of the Jahn-Teller coupling. The theory has been given by Fletcher and Stevens /3/ and is reviewed by Bates /I/. EXPERIMENTS .- Experiments have been made on five samples of magnesium oxide doped as follows : the first with 800 ppm chromium , the second with 840 ppm manganese, the third with 450 ppm of vanadium, the fourth was nominally pure but known to contain
2+
Cr ;the fifth was titanium doped but had been 2+
shown to contain Cr . All samples were cut and
polished to allow phonon propagation along a < 1 1 0 > direction to be studied.Experiments were made using tin tunnel junctions as phonon generators and alu-minium tunnel junctions as phonon detectors with sine wave modulation at 287 Hz after the manner of Eisenmenger/4/.The temperature was 1.06 K.With the-se junctions the available frequency range is from about 2.9cm to about 10cm .The lower limit is set by the energy gap of the aluminium detector and the upper limit by the reabsorption in the generator of the relaxation phonons at twice the energy gap of tin.In the chromium doped sample all phonons in the range 5cm to 9.5cm appeared to be strongly scat-tered. In the other samples with lower concentrations
2+ 3+
of Cr or Mn two resonant absorptions are seen at 4.9cm and 9.5cm .The traces obtained for the se-cond and fourth samples are shown in figure 1.These absorptions are very weak in the vanadium doped sam-ple but in addition the expected peak in the trace at the onset of detection of relaxation phonons is suppressed possibly indicating an absorption at a-bout.2.9cm~ .
DISCUSSION.-We interpret the resonance absorptions -1 -1 ' 2+ at 4.9cm and at 9.5cm as being due to Cr and
3+.
Mn ions.Evidence from low temperature thermal con-3+ ductivity (Challis and Ghazi/5/) shows that Mn in
. . 2+
MgO is very similar to Cr in MgO.At the tempera-ture of our experiments these resonances must be due to transitions from the ground state A or from sta-tes extremely close to the ground state. The only allowed transition from the ground state is to the E state and we may tentatively assign the resonance at 4.9cm to this transition.The resonance at 9.5 cm is likely to be due to transitions between the
A,and A2 levels, the lowest and highest levels of
the loyest group.
1.2 20 3.0 36
Generator Voltage mV.
1.2 2.0 3.0 L.0
Generator Voltage mV.
I
Fig. 1
:Upper
-
Phonon transmission through nomi-
nally pureMgO
Lower
-
Phonon
transmissionthroughmanga-nese doped MgO.
This transition is forbidden for ions on precise-
ly cubic sites but is allowed for ions on distorted
sites /6/. With this assignment, it is possible to
obtain a value of the spin-orbit splitting
D
=2.4 cm-' which is consistent with earlier work(e.g.
Pate1 and Wigmore 171.) However the assignment of
the 4.9 cm-I resonance to an A 1 to E transition is
nor. consistent with this value. It seems likely that
the problem is more complex because of the distor
tions of the sites due to random lattice strains
and D cannot be evaluated simply by neglecting the
effects of distortions.
The vanadium doped saxple contained a little
chromium and showed
thecr2+
resonances, though on-
ly weakly. We observed no resonance absorption at
7 cm-I corresponding to the doublet-triplet split-
ting of
v3+
as predicted by Buisson and Nahmani/8/.
The possible absorption at 2.9 cm-I has not been
attributed to any ion.
References
/I/
Bates,C.A., Phys. Rep.
25
(1978) 187
/2/ King,P.J. and Oates,S.G.J.Phys. C
2
(1976) 389
/4/ Eisenmenger,W.,Physical Acoustics(Academic
Press) 1?_ (1976) 79
/ 5 /
