PHYSICAL REVIEW B 72, 235208 共2005兲

Spin multiplicity and charge state of a silicon vacancy „TV2a… in 4H-SiC
determined by pulsed ENDOR
N. Mizuochi,1,2 S. Yamasaki,2,3 H. Takizawa,4 N. Morishita,4 T. Ohshima,4 H. Itoh,4 T. Umeda,1 and J. Isoya1,2
1Graduate School of Library, Information and Media Studies, University of Tsukuba, 1-2 Kasuga, Tsukuba-City,

Ibaraki 305-8550, Japan
2Diamond Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2,

1-1-1 Umezono, Tsukuba-City, Ibaraki 305-8568, Japan
3Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba-City 305-8577, Japan
4Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki-City, Gunma 370-1292, Japan
共Received 15 August 2005; revised manuscript received 7 October 2005; published 28 December 2005兲

In this paper, we unambiguously re-determine the spin multiplicity of TV2a by pulsed electron nucleus double
resonance technique. The TV2a center is one of the most commonly observed defects in 4H-SiC, and its origin
was identified as one belonging to a class of negatively charged silicon vacancy by means of continuous-wave
electron paramagnetic resonance 共EPR兲 and the two-dimensional nutation method of pulsed EPR technique.
However, a model with the spin multiplicity of triplet 共S = 1兲 and the neutral charge state has recently beensuggested. Our result clearly shows that TV2a is a quartet spin 共S = 3 / 2兲 state and thus should be singlenegatively charged 共−1兲.
DOI: 10.1103/PhysRevB.72.235208

PACS number共s兲: 61.72.Ji, 76.30.Mi

I. INTRODUCTION

carbon atoms, the difference between TV2a and V−Si共I,II兲 isvery small. These also indicate that the distortion of TV2a isslightly larger than that of V−Si共I,II兲. Their HF parameters almost correspond to the theoretical values of the negatively
charged vacancy in 4H-SiC.14 In addition, V−Si共I,II兲 and TV2aare different in terms of thermal stability: V−Si共I,II兲 were annealed below 900 °C,4,15,16 but TV2a remained up to higher
temperatures.17,18 In particular, TV2a was observed not onlyin irradiated SiC, but also in as-grown high-purity semiinsulating 共HPSI兲 4H-SiC,8 which is an important base material for wide-band-gap electronics.18 Therefore a Si vacancy labeled TV2a is recognized as a possible candidate forthe carrier-compensation center in this material and is thus avery important defect.
The EPR spectrum of TV2a exhibits ZFS,2,3,5 indicatingthat TV2a is a high spin state 共S ⬎ 1 / 2兲. Initially, the spin
multiplicity of TV2a was assigned to triplet 共S = 1兲, and TV2a
was thus regarded as a neutral Si vacancy.5 Later, from thetwo-dimensional 共2D兲 nutation method of pulsed EPR technique, the spin multiplicity was clearly determined to bequartet 共S = 3 / 2兲. Accordingly, the charge state was reassigned to be a single-negatively charged state 共−1兲. After thisreassignment, however, the spin-1 共S = 1兲 models were reported again based on a pulsed EPR study7 and a continuouswave 共cw兲 EPR study.8 Thus the spin multiplicity or the
charge state of TV2a is currently controversial. In contrast
with TV2a, quartet 共S = 3 / 2兲 and negative charge 共−1兲 havebeen concluded for V−Si共I,II兲.13
It is important to determine the spin multiplicity of TV2afor several reasons. One reason is that the charge statestrongly influences the thermal stability of Si vacancies.
First-principles calculations predicted that negatively
charged Si vacancies could be stable in HPSI and n-type SiC,
while other charge states of the silicon vacancy should not be
stable because of their high formation energies. This naturally explains why Si vacancies are absent in p-type

In SiC, there are two types of fundamental lattice vacancies: the silicon vacancy and the carbon vacancy. Both ofthese strongly affect the electrical properties of this material.
They are observed by electron paramagnetic resonance
共EPR兲 or optically detected magnetic resonance 共ODMR兲.1
Using EPR or ODMR, these two types can be simply distinguished according to the hyperfine 共HF兲 interactions due tothe four nearest-neighbor 共NN兲 atoms of a vacancy. For the
carbon vacancy, the NN atoms are Si, so we can observe theNN 29Si 共nuclear spin I = 1 / 2, natural abundance=4.7%兲 HF
interactions. On the other hand, the NN 13C 共I = 1 / 2, naturalabundance=1.1%兲 HF interactions can be observed for thesilicon vacancy. Among a number of EPR centers reported sofar, those labeled V−Si共I,II兲 and TV2a in 4H-SiC or 6H-SiC
were found to generate the NN 13C HF interactions.2–9 These
three centers have been identified as a silicon vacancy distorted along the c axis 共关0001兴兲 with a C3v symmetry.6,9–12
The g values of V−Si共I兲 and V−Si共II兲 cannot be distinguished in
the X band. Therefore they were originally assigned as asingle center and labeled as V−Si.13 From detailed angularanalyses of the NN 13C HF interactions with respect to crystal’s rotation, they could be distinguished, and V−Si共I兲 andV−Si共II兲 were assigned as the silicon vacancies at the hexagonal and the quasicubic sites, respectively.9 V−Si共I,II兲 are very
close to Td symmetric geometry, while TV2a is relatively distorted, resulting in the different EPR spectra.6,10–12
V−Si共I,II兲 do not exhibit zero-field splittings 共ZFS兲 in the conventional X-band EPR, indicating that the deviation from Td
symmetric geometry is very small. On the other hand, TV2a
exhibits ZFS 共兩D兩 = 35.1 MHz兲. The distortion of TV2a, which
is manifested in the ZFS, is likely to be caused by a perturbation of the crystal field, presumably by the presence of an
accompanying impurity or defect located at some distancealong the c axis 共关0001兴兲.6 As for the HF parameters of NN
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MIZUOCHI et al.

materials.19 The spin-1 model for TV2a, however, calls theabove idea into question. Furthermore, there is the otherquestion of why V−Si共I,II兲 and TV2a are simultaneously observed in thermal equilibrium 共in dark兲.4,6,7 For the coexistence of the −1 and 0 charge states, the Fermi level should bevery close to the ionization level of 共− / 0兲 which was calculated to be located at 1.2–1.3 eV 共Ref. 20兲 or 0.7–0.8 eV
共Ref. 19兲 above the valence-band maximum. On the contrary,
the equilibrium coexistence was observed in a variety ofsample conditions such as different dopant concentrations
and different radiation conditions.4,6,7 Furthermore, so far,except for the suggestion of the triplet state 共S = 1兲 on TV2a,the electronic states of single vacancies which have a highspin ground state 共S ⬎ 1 / 2兲 in semiconductors are limited tothe orbitally nondegenerate 4A2 state 共S = 3 / 2兲, which is not
subject to Jahn-Teller distortion. They are a single-negatively
charged Si vacancy in 3C - , 4H - , 6H-SiC,9,13,21 a singlenegatively charged vacancy in diamond,22 and a neutral Ga
vacancy in GaP.23,24 The interactions between electrons suchas the exchange interaction and the electron repulsion6,20 affect the determination of the ground state. For its elucidation,
the experimental determination of the spin multiplicity of theground state is important. Thus the spin multiplicity andcharge state are related to significant issues that we have toanswer.
In spin multiplicity determination, electron nucleus double resonance 共ENDOR兲 spectroscopy can give crucial information. Treating the first order, the energy levels and the
ENDOR frequencies for a spin system incorporating S and I
are given by22,25
E M S,M I = M Sge␮eB + 共M SAeff − gn␮nB兲M I

共1兲

␯ = 共1/h兲兩M S Aeff − gn␮nB兩,

共2兲

and

respectively, where M S and M I are the magnetic quantum
numbers 共兩M S兩 艋 2S + 1 , 兩M I兩 艋 2I + 1兲 of electron spin andnuclear spin, respectively, ge␮eB is the electron Zeeman
splitting, B is the applied magnetic field, Aeff is the HF splittings, and gn␮nB / h is the frequency of nuclear Zeeman splitting. Obviously, the ENDOR frequencies should be completely different depending on whether M S values are−1 , 0 , + 1 共S = 1兲 or −3 / 2 , − 1 / 2 , + 1 / 2 , + 3 / 2 共S = 3 / 2兲.
In fact, the spin multiplicity of quartet 共S = 3 / 2兲 for V−Si has
been unambiguously determined in this way.13 In this paper,
we also demonstrate such a determination for TV2a using ahigh-resolution pulsed-ENDOR spectrometer.
II. EXPERIMENTThe sample used in our experiments was singlecrystalline n-type 4H-SiC 共Nippon Steel: nitrogen dopant兲with the carrier concentration of approximately 1 ⫻ 1017 /cm3. A sample 1.5 mm thick was cut to a size 共3 ⫻ 15 mm兲
appropriate for our EPR measurements in the X band. The
crystal was irradiated by 3-MeV electrons with the total fluence of 4 ⫻ 1018e / cm2. The sample was placed on a watercooled holder to avoid beam heating and was kept below 330
K during the electron irradiation.

FIG. 1. 共Color online兲 ESE spectrum of the electron-irradiatedn-type 4H-SiC at room temperature taken with the magnetic fieldalong the c axis 共关0001兴 axis兲. The microwave frequency was ␯= 9.599 GHz. The arrows indicate the magnetic field positions
where the ENDOR spectra were measured.

Electron spin-echo 共ESE兲 detected EPR and pulsed
ENDOR experiments were carried out at room temperature
in a dark condition on a Bruker ELEXSYS X-band spectrometer. ESE experiment was performed using a two-pulse sequence 共p1-␶-p2兲 with the duration of microwave pulses p1
and p2 of 20 ns each and delay times ␶ of 1.2 ␮s. Pulsed
ENDOR experiments were performed using a three-pulse sequence 共p1-␶-p2-T-p3兲 with microwave pulses p1 , p2, and p3
20 ns wide and delay times ␶ and T of 800 ns and 16.2 ␮s,respectively. During the time T an intense radiofrequency
pulse 10 ␮s wide was applied to the sample. Spectra wereobtained by monitoring the echo while scanning the radiofrequency. In both experiments, the delay of 1 ms was used
for repeating the pulse sequence in accumulation.
III. RESULTS AND DISCUSSION

The electron spin-echo 共ESE兲 spectrum of the electronirradiated 4H-SiC when the magnetic field was parallel to thec axis 共B 储 c兲 is shown in Fig. 1. In the central part of the−spectrum, the negatively charged silicon vacancy 关VSi
共I,II兲,−9g = 2.0028兴 is observed. On both sides of VSi共I,II兲, there are
EPR signals labeled TV2a. We measured the ENDOR spectraof them and confirmed their spin multiplicities. In the previous study, a TV2b signal was also observed in the continuouswave EPR spectrum of the same sample as the present
study.6 The absence of it in Fig. 1 is due to the difference ofthe spin-relaxation times which affects the ESE signal intensity. First of all, we show the results for V−Si共I,II兲 whose spin
multiplicity of quartet has previously been unambiguously
determined by ENDOR.13
The ENDOR spectrum of V−Si共I,II兲 measured at 342.3 mT
is shown in Fig. 2共a兲. At this field, a strong 29Si HF satellite
due to 12 next-nearest-neighbor 共NNN兲 Si atoms was observed 共see Fig. 1兲. The ENDOR signals were clearly detected and labeled as ␯A1 , ␯A2 , ␯B1 , ␯B2 , ␯C , ␯D, and ␯E.
The observed frequencies are shown in Table I. Figure 3共a兲

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FIG. 2. 共Color online兲 ENDOR spectrum the electron-irradiatedn-type 4H-SiC at room temperature taken with the magnetic fieldalong the c axis 共关0001兴 axis兲. 共a兲 Spectrum of V−Si共I,II兲 at 342.3 mT.
共b兲 Spectrum of TV2a at low-field side 共339.8 mT兲. 共c兲 The spectrumof TV2a at high-field side 共344.8 mT兲.

shows the energy-level splitting for a spin quartet 共S = 3 / 2兲state with one nucleus 共I = 1 / 2兲, which was calculated using
the successive three terms in Eq. 共1兲. As indicated in Figs. 2
TABLE I. Observed and calculated resonant ENDOR frequencies of V−Si共I,II兲 and TV2a.
Obs. 共MHz兲

Calc. 共MHz兲a

V−Si共I,II兲

␯A1
␯A2
␯B1
␯B2
␯C

14.86
15.11
9.10
9.35
6.89

14.88
15.12
9.09
9.33
6.89, 6.97

TV2a

␯A1
⬘
␯A2
⬘
␯B1
⬘
␯B2
⬘
␯C1
⬘
␯C2
⬘

14.76
15.53
9.00
9.79
6.83
7.06

14.77
15.55
8.98
9.76
6.84
7.10

The calculated frequencies were derived from Eq. 共1兲 by using theparameters described in the text.

aFIG. 3. Energy-level schemes under Aeff ⬎ 0 , gn ⬍ 0, and D⬎ 0. 共a兲 Energy levels showing the effects of successive terms in
Eq. 共1兲 and ENDOR transitions at constant magnetic field. 共i兲 Addition of electron Zeeman interaction. 共ii兲 Addition of the first-order
HF interaction. 共iii兲 Addition of nuclear Zeeman interaction. 共b兲
Energy levels with respect to magnetic field and EPR transitions inquartet for D ⬎ 0 at the B 储 c axis.

and 3, ␯A1 , ␯A2 , ␯B1, and ␯B2 were assigned to the transitions of the 兩M S兩 = 3 / 2 manifold, and ␯C was assigned to thetransition of the 兩M S兩 = 1 / 2 manifold. The present data andanalysis are consistent with those reported previously for
V−Si共I,II兲,13 except that we could resolve small splittings ofabout 0.25 MHz between ␯A1 and ␯A2 and between ␯B1 and␯B2. In the previous study, these splittings were not detectable due to a broad linewidth 共0.3–0.5 MHz兲.13 Therefore ourpulsed ENDOR measurements have higher resolution thanthose of the previous CW ENDOR studies. We think theorigin of these splittings is site splittings for the following
reason. We found a signal-intensity ratio of approximately
3:1 for the ␯A1 and ␯A2 lines and for the ␯B1 and ␯B2 lines. As
shown in Fig. 4, the 12 NNN Si atoms are classified into
three types: nine basal and three c-axial Si atoms according
to their locations relative to the NN C atoms. Therefore two
split lines with intensity ratio 3:1 are expected for the relevant HF satellite when B 储 c,9 which is quite consistent with
our observation.26 We calculated the ENDOR frequencies of
the transitions of the 兩M S兩 = 3 / 2 for B = 342.3 mT using Aeff
= 7.99 and 8.15 MHz and gn = −1.1106.25 They correspond

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MIZUOCHI et al.

FIG. 4. 共Color online兲 12 NNN Si atoms of a silicon vacancy.
They are classified into three types: nine basal and three c-axial Si
atoms according to their locations relative to the NN C atoms. The
position of the vacancy is assumed to be hexagonal site in thisfigure as an example. It should be noted that the number of types is
the same as that in the case of the quasicubic site.

well to the measured frequencies of ␯A1 , ␯A2 , ␯B1, and ␯B2
as shown in Table I. The parameters of Aeff = 7.99 and 8.15
MHz are very close to the reported value 共=8.35 MHz兲 for
the HF coupling constant of the NNN 29Si atoms, which wasreported to be isotropic in the EPR measurement.9,13 Using
the same Aeff parameters, we calculated the resonant frequencies for M S = 1 / 2 to be 6.89 and 6.97 MHz. Their differenceis only 0.08 MHz, so the site splitting was not resolved forthe ␯C line. But looking carefully, we can see that the ␯C line
has a small shoulder on the high-frequency side, suggesting
the presence of the site splitting. It should be noted that thesignificant difference in signal intensity of the small splittings excludes the possibility that the small splittings comefrom the difference in Aeff between V−Si共I兲 and V−Si共II兲. In our
samples, the concentration of V−Si共I兲 was almost the same asthat of V−Si共II兲.9 Accordingly, we have consistently reproduced the spin-multiplicity determination for V−Si共I,II兲 evenwith the much improved resolution. The same procedure isapplied to TV2a in the next step. In our measurements, unfortunately, the ENDOR signal of M S = −1 / 2, whose frequency
is estimated to be around 1.1–1.2 MHz, could not be identified. It is because the signal intensity at the low-frequency
region is very weak due to the instrumental requirement andthe spectrum at the low-frequency region is very complicated
due to many superpositions of small frequencies of HF interaction at distant sites.
The ENDOR spectra of TV2a measured at 339.8 and 344.8
mT are shown in Figs. 2共b兲 and 2共c兲, respectively. We observed signals that can be assigned to the transitions of the兩M S兩 = 3 / 2 manifold. They are labeled ␯A1⬘ , ␯A2⬘ , ␯B1⬘ , and␯B2⬘ , respectively. Likewise in V−Si共I,II兲, the small splittings

were observed. Signals that can be assigned to the transitionof the 兩M S兩 = 1 / 2 manifold were observed and labeled ␯C1⬘and ␯C2⬘ . The observed resonant frequencies and the calculated resonant frequencies are summarized in Table I, where
we used Aeff = 7.93 and 8.45 MHz for the calculation. As
shown in Table I, the calculated resonant frequencies correspond well to the observed ones. The Aeff parameters of 7.93
and 8.45 MHz correspond to the reported values 关8.7 MHz
共Ref. 6兲 and 8.35 MHz 共Ref. 12兲兴 for the HF coupling constant of the NNN 29Si atoms, which was reported to be isotropic in the EPR measurement.6,12
Assuming S = 1, the ENDOR frequencies for B = 339.8 and344.8 mT were calculated to be 5.6± 0.2 and 11.4± 0.2 MHz,
respectively, using the reported values of Aeff parameters 关8.7
MHz 共Ref. 6兲, and 8.35 MHz 共Ref. 12兲兴. As seen from Figs.
2共b兲 and 2共c兲, the signal was not observed around these frequencies. Therefore we can conclude that the spin multiplicity of TV2a is not triplet 共S = 1兲 but must be quartet 共S = 3 / 2兲.
It is also notable that the signals labeled ␯A⬘ and ␯C⬘ wereobserved only in the low-field transition and the signals labeled ␯B⬘ were observed only in the high-field transition, as
shown in Figs. 2共b兲 and 2共c兲. This can be reasonably explained by the spin-3 / 2 model. The sign of the HF coupling
constant of the NNN 29Si共ASiNNN兲 can be assumed to be positive as reported previously.13 On the other hand, the ZFS
parameter 共D兲 was reported to be positive from the W-band
EPR measurement at 1.2 K.7 When D ⬎ 0, the transition of
兩S , M S典 = 兩3 / 2 , 3 / 2典 ⇔ 兩3 / 2 , 1 / 2典兩S , M S典 = 兩3 / 2 , 3 / 2典 ⇔ 兩3 / 2 ,1 / 2典 should be observed on the low-field side, while thetransition of 兩3 / 2 , −1 / 2典 ⇔ 兩3 / 2 , −3 / 2典 should be observedon the high-field side, as illustrated in Fig. 3共b兲. Under Aeff
⬎ 0 and D ⬎ 0, it can be derived from Eq. 共2兲 that the signals
labeled ␯A⬘ and ␯C⬘ should be observed only in the low-fieldtransition and the signals labeled ␯B⬘ should appear only on
the high-field side. These also support our conclusion.
In the ENDOR spectrum of Fig. 2, several signals otherthan those of the NNN 29Si atoms were observed. We tentatively consider that they are due to HF interactions at distantsites. As for ␯D , ␯E , ␯D
⬘ , and ␯E⬘ in Figs. 2共a兲 and 2共c兲, they
are tentatively assigned as the signals from third NN 13C HF
interaction. In the case of ␯D⬘ and ␯E⬘ in Fig. 2共c兲, assumingthat Aeff is 1.69 MHz and the sign of it is positive, it isexpected to be observed at 4.5 and 6.2 MHz in high-fieldtransition. The frequency of ␯E⬘ is 4.5 MHz and also corresponds to it. By using the value of 1.69 MHz, the other two
signals were expected to be observed at 1.2 and 2.8 MHz.
However, we could not identify them because the spectrumat the low-frequency region is very complicated due to manysuperpositions of small frequencies of HF interaction at distant sites. The frequencies of ␯D 共6.2 MHz兲 and ␯E 共4.5
MHz兲 in Fig. 2共a兲 also correspond to the calculated frequencies of 6.2 and 4.5 MHz by using Aeff = 1.66 MHz. As for thesignal at 10.3 MHz in Fig. 2共a兲, it may be tentatively assigned to fourth NN Si atoms. Assuming that Aeff is 4.94
MHz, the signals at 5.4 and 10.3 MHz are expected to beobserved. These correspond well to the signals observed at
5.3 and 10.3 MHz in Fig. 2共a兲. Probably, other signals arealso ENDOR frequencies at distant sites. In the previous
study,13 these lines were not observed. Maybe this comes

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from the different sensitivity of the signals due to the different measurement method with the present study.
Previously, Son et al. suggested that TV2a should be atriplet spin state, because its spectrum did not exhibit thecentral 兩S , M S典 = 兩3 / 2 , 1 / 2典 ⇔ 兩3 / 2 , −1 / 2典 transition in thedark.8 In their EPR measurement, however, other signals
stronger than the TV2a signal were observed at around thefield position where the central transition should appear. It istherefore unclear whether the central transition of a quartet
state was really absent or not. As for the measurements underlight illumination, Son et al. proposed the polarization modelwith a triplet ground state and a singlet excited state.8 On theother hand, we have proposed the model to explain little
enhanced 兩S , M S典 = 兩3 / 2 , 1 / 2典 ⇔ 兩3 / 2 , −1 / 2典 transition in any
direction of the magnetic field.6 Thus their experimental results are still consistent with our conclusion.
Orlinski et al. also suggested the spin-1 model for TV2abased on the estimation of the nutation frequency 共␻n兲 ratios
by measuring the pulse lengths needed for the maximum
ESE signal.7 The estimated ␻n ratios were not consistent
with those of our previous nutation experiments,6 resulting in
the controversy. However, it should be mentioned that thoseresults of ours were obtained from 2D nutation experiments.
In principle, the 2D technique gives more accurate and reliable ␻n ratios, especially when the detected ESE signal consists of two or more signals having different ␻ns.27,28
The present conclusion that TV2a should have a quartet
spin 共S = 3 / 2兲 and should be negatively charged is consistent
with the theoretical aspects. The 共0 / + 兲 and 共− / 0兲 levels of
the silicon vacancy were predicted to be 0.2–0.3 eV and1 E. Janzén, I. G. Ivanov, N. T. Son, B. Magnusson, Z. Zolnai, A.

Henry, J. P. Bergman, L. Storasta, and F. Carlsson, Physica B
340-342, 15 共2003兲.
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V. S. Vainer, V. I. Veigner, V. A. Il’n, and V. F. Tsvetkov, Sov.
Phys. Solid State 22, 2011 共1980兲 关Fiz. Tverd. Tela 共Leningrad兲22, 3436 共1980兲兴.
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V. S. Vainer and V. A. Il’n, Sov. Phys. Solid State 23, 2126
共1981兲 关Fiz. Tverd. Tela 共Leningrad兲 23, 3659 共1981兲兴.
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H. J. von Bardeleben, J. L. Cantin, I. Vickridge, and G. Battistig,
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5E. Sörman, N. T. Son, W. M. Chen, O. Kordina, C. Hallin, and E.
Janzén, Phys. Rev. B 61, 2613 共2000兲.
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7 S. B. Orlinski, J. Schmidt, E. N. Mokhov, and P. G. Baranov,Phys. Rev. B 67, 125207 共2003兲.
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0.7–0.8 eV above the valence-band top of 4H-SiC,
respectively,19 which are much lower than the mid gap.
Therefore, from the theoretical viewpoint, the silicon vacancy is expected to be negatively charged in our n-type SiC.
The coexistence of TV2a and V−Si共I,II兲 in our samples is quite
understandable because they were both negatively charged
states of the silicon vacancy. In addition, the spin-3 / 2 model
also explains the high thermal stability of TV2a. According to
the first-principles calculation,19 the formation energy of thesilicon vacancy is higher for the neutral charge state than for
the negatively charged state, resulting in higher thermal stability for the latter state. This will be related to the fact that
the TV2a centers could be present in HPSI SiC even after
1600 °C annealing.18

IV. SUMMARY

The spin quartet 共S = 3 / 2兲 state and the negatively charged
共−1兲 state of TV2a in 4H-SiC have been confirmed by pulsed
ENDOR measurements. After demonstrating the spinmultiplicity determination for the known center V−Si共I,II兲 共S= 3 / 2兲, we applied the same procedure to the TV2a center. We
could observe signals that we assigned to the transitions ofthe 兩M S兩 = 3 / 2 manifold. The observation of the 兩M S兩 = 3 / 2
manifold and lack of observation of the transition of the兩M S兩 = 1 one at the frequencies expected in the case of S = 1
clearly indicate that the spin multiplicity of TV2a is quartet
共S = 3 / 2兲 and not triplet 共S = 1兲.

Baranov, E. N. Mokhov, C. Hallin, and J. L. Lindström, Phys.
Rev. B 66, 155214 共2002兲.
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Overhof, Phys. Rev. B 56, 7384 共1997兲.
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